*Review* **The EU Training Network for Resource Recovery through Enhanced Landfill Mining—A Review**

**Daniel Vollprecht 1,\*, Lieven Machiels 2 and Peter Tom Jones 3**

**\*** Correspondence<EMAIL_ADDRESS>Tel.: +43-3842-402-5110

**Abstract:** The "European Union Training Network for Resource Recovery Through Enhanced Landfill Mining (NEW-MINE)" was a European research project conducted between 2016 and 2020 to investigate the exploration of and resource recovery from landfills as well as the processing of the excavated waste and the valorization of the obtained waste fractions using thermochemical processes. This project yielded more than 40 publications ranging from geophysics via mechanical process engineering to ceramics, which have not yet been discussed coherently in a review publication. This article summarizes and links the NEW-MINE publications and discusses their practical applicability in waste management systems. Within the NEW-MINE project in a first step concentrates of specific materials (e.g., metals, combustibles, inert materials) were produced which might be used as secondary raw materials. In a second step, recycled products (e.g., inorganic polymers, functional glass-ceramics) were produced from these concentrates at the lab scale. However, even if secondary raw materials or recycled products could be produced at a large scale, it remains unclear if they can compete with primary raw materials or products from primary raw materials. Given the ambitions of transition towards a more circular economy, economic incentives are required to make secondary raw materials or recycled products from enhanced landfill mining (ELFM) competitive in the market.

**Keywords:** enhanced landfill mining; NEW-MINE; waste treatment

### **1. Introduction**

Although landfill mining, "the process for extracting minerals or other solid natural resources from waste materials that have previously been disposed of by burying them in the ground" [1], has been investigated since 1953 [2], the interest in this topic only really started in the 1980s in the USA [3] and the 1990s in Europe [4]. The main reasons for Landfill Mining towards at the end of the 20th century comprised landfill remediation and landfill volume recovery [5]. Between 2017 and 2015, several national (research) projects such as the "Closing the Circle" project (Flanders/Belgium) [6], the LAMIS project (Austria) [7–10], the MINERVE project (Wallonia/Belgium) [11] and the TönsLM project (Germany) [12] were conducted, which focused more on material and energy recovery. These projects covered different process steps ranging from geophysical exploration via exploration to dry and wet mechanical processing.

In summary, these studies demonstrated that using state-of-the-art technologies can yield concentrates of metals, inorganic-nonmetallic materials and combustibles. The metal concentrates can already be sold at the market, since standard pyrometallurgical recycling routes exist, which makes this fraction a key economic driver for landfill mining [13]. However, the inorganic-nonmetallic and combustible fractions are more challenging, while the remaining fine fraction represents the final sink for contaminants [14] that have to be removed from the circular economy [15]. In detail, recycling of inorganic-nonmetallic materials even from fresh construction and demolition waste is mainly as aggregate [16],

**Citation:** Vollprecht, D.; Machiels, L.; Jones, P.T. The EU Training Network for Resource Recovery through Enhanced Landfill Mining—A Review. *Processes* **2021**, *9*, 394. https://doi.org/10.3390/ pr9020394

Received: 21 December 2020 Accepted: 18 February 2021 Published: 22 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

which is a low-value application that is, furthermore, associated with a quality decrease due to the negative impact on the attached cement paste on concrete properties [17]. Similarly, energy recovery from calorific fractions of the waste may either occur by classical waste incineration processes [18] or after mechanical processing to refuse derived fuel (RDF) in co-incineration plants [19]. Nevertheless, even in the latter case the waste supplier has to pay for the material instead of obtaining revenues. Finally, regarding the fine fractions it has to be mentioned that 78% of excavated soils, most of them not contaminated, are landfilled in Austria [20] due to low landfilling costs and low material values. Therefore, it seems unlikely that contaminated soil-like materials from landfills would be recycled.

For these reasons the question arose if innovative technology might be an option to increase the economic feasibility of landfill mining projects. Consequently, in 2013 the term "enhanced landfill mining (ELFM)" was defined as "the safe conditioning, excavation and integrated valorization of (historic and/or future) landfilled waste streams as both materials (waste-to-material, WtM) and energy (waste-to-energy, WtE), using innovative transformation technologies and respecting the most stringent social and ecological criteria" [6]. To investigate the potential of such "innovative transformation technologies" and to cluster the existing European expertise in the field of landfill mining, the European Enhanced Landfill Mining Consortium (EURELCO) was founded in 2014 [21]. From this consortium the "EU Training Network for Resource Recovery through Enhanced Landfill Mining (NEW-MINE)" was developed and received funding from the European Union in 2016. For a period of four years, 15 early stage researchers (ESRs) investigated the exploration of and resource recovery from landfills as well as the processing of the excavated waste and the valorization of the obtained waste fractions using thermochemical processes (Figure 1).

**Figure 1.** Approach of the NEW-MINE project (www.new-mine.eu accessed on 16 November 2020).

The NEW-MINE project yielded a significant output of scientific publications, ranging from geophysics via mechanical process engineering to ceramics. Although there is already a review article that links the first NEW-MINE results with previous investigations [22], there is no comprehensive review assessing the entire research output of NEW-MINE with

respect to its practical applicability in waste management systems, which is the aim of this review article.

This article is structured in the following way: Firstly, the methods for literature review and discussion of the project results are given. Secondly, the project results are summarized referring to the individual publications. In this context also the state-of-theart before the project and the publications referring to the NEW-MINE publications are shortly summarized. Thirdly, in the Discussion section, the practical applicability of the NEW-MINE results in waste management systems is discussed. Finally, a summary of the project results and the challenges regarding their practical application is given.

### **2. Materials and Methods**

Based on the publications listed on the NEW-MINE website (www.new-mine.eu accessed on 16 November 2020), additional publications of the authors were identified using Google Scholar and searching for the names of the NEW-MINE researchers. In a second step papers citing the NEW-MINE publications were identified and set into the context of NEW-MINE. The focus was on peer-reviewed publications, but certain conference proceedings were selected in those cases when the contained information was not found in a peer-reviewed article. Additionally, the search term "NEW-MINE" was used to find publications referring to this project. In the Google Scholar searches between dozens and few hundred articles appeared which were checked for consistency to the review topic. Publications by authors with similar names who were obviously other persons than the NEW-MINE researchers were excluded as well as publications referring to "new mines", but not to the NEW-MINE project. The entire literature research was conducted in November and December 2020. Finally, the results of the NEW-MINE project were checked for practical applicability based on the experience of the authors in waste management and on personal communications with colleagues over the last years. In this context, some theoretical reflections on resource classification and varying terminology in interdisciplinary research were added.

### **3. Results**

### *3.1. Innovative Landfill Exploration & Mechanical Processing*

### 3.1.1. Landfill Exploration

Landfills have been investigated long before the NEW-MINE project with respect to environmental problems arising from the landfilled waste [23–25]. With respect to the recovery of resources from landfills, the task for geophysical exploration is different: The landfill as a potential anthropogenic deposit shall be explored with respect to the quality, quantity and "bonitaet" (=geological factors influencing the mining) [26] of the contained secondary raw materials [27]. In contrast to pollutant-related exploration tasks, the resource-related exploration task aims not for the identification of individual objects or leakages but for the estimation of the average material composition. Before the onset of the NEW-MINE project only few studies focused on these kinds of exploration, e.g., for a distinction between landfilled foundry sands and iron-rich materials using magnetic methods [28]. Parallel to the NEW-MINE project, but not referring to it, another research group distinguished between metal enrichments and plastic enrichments in a landfill using a combination of electromagnetic and magnetic methods [29].

The publications within the NEW-MINE project include both applied studies focusing on the exploration of landfills with respect to the material composition and fundamental studies focusing on data processing of geophysical sensors.

The first applied study [30] investigated the suitability of electromagnetic induction measurements and ground penetrating radar to characterize the geometry and electric properties of waste layers. It was appreciated by scientists of two other EU project on ELFM, the SMARTGROUND project who cited it with respect to electromagnetic measurements in their study on ELFM in the UK [31], and the RAWFILL project who cited it with respect to geophysical exploration in general [32]. The second applied study [33] used magnetic total-

field measurements to estimate the bulk magnetic susceptibility of the Hollabrunn landfill, Austria, by inverse modelling and validated the resulting susceptibility by manual sorting of drill-core samples. In parallel, the magnetic susceptibility of reference materials was determined by laboratory analyses and the resulting bulk susceptibility of the landfilled waste mixture was computed using a weighted mean. The differences between the bulk susceptibility derived from inverse modelling of the field data (0.06 to 0.11 SI) and the values obtained from computing from the values from the reference materials (0.01 to 0.05 SI) highlight the challenges to determine the iron content in landfills from magnetic data. This publication has not yet been cited, although two publications released in 2020 have addressed magnetic exploration in the context of landfill mining [34,35].

Consistent with the overall character of the NEW-MINE project and the requirements of the Marie Skłodowska Curie Actions, the share of fundamental research was dominant which is also reflected in the higher number of publications regarding the basic aspects of geophysical exploration. For example, a probabilistic inversion of electromagnetic data using the so-called Kalman ensemble generator, was introduced [36]. This approach has already been already cited twice by other authors [37,38] and was further extended to a joint inversion of direct current resistivity and small-loop electromagnetic data [39], which was published in a mathematical journal, *Algorithms*, highlighting the interdisciplinarity of the NEW-MINE project. Finally, offset parameters were incorporated into the probabilistic inversion framework to estimate systematic errors in electromagnetic measurements [40].

In summary, the role of geophysics in post-NEW-MINE projects (e.g., RAWFILL) is higher than in pre-NEW-MINE projects (e.g., TönsLM), and post-NEW-MINE projects refer to the results of NEW-MINE.

### 3.1.2. Mechanical Processing

Mechanical processing of landfilled waste was the main focus in pre-NEW-MINE projects with dry methods being investigated in the LAMIS project [7] and wet methods in the TönsLM project [12].

Based on a comprehensive study on state-of-the-art processing of excavated waste from the Halbenrain landfill, Austria, using an existing mechanical-biological treatment (MBT) plant [41], a novel processing concept using a ballistic separator as first processing step for untreated excavated landfilled waste was developed and tested at the Mont-Saint-Guibert landfill, Belgium [42]. The NEW-MINE approach to use state-of-the-art MBT was addressed outside NEW-MINE in the context of the problematic character of the fine fraction [43], its biological treatment to increase the value of the fine fraction [44], and regarding landfill mining in general [45].

A second key innovation in this field reached by NEW-MINE was the demonstration of the feasibility of near-infrared (NIR) sorting for the beneficiation of heavy fractions from dry-mechanical processing [46]. The authors could enrich the inert materials in these fractions from values between 85.6 to 98.8 wt% in the input to a range between 97.7 and 99.6 wt% in the output, by ejecting the plastics: this represents an important step towards a possible recycling of the inert fraction. Another finding, i.e., the impact of attached defilements on sorting performance, was subsequently considered in Austria's largest waste management project, ReWaste 4.0 [47]. The applicability of results from mechanical processing of ELFM materials to "fresh" waste highlights the relevance of NEW-MINE results in this field. Within NEW-MINE, the influence of surface roughness and surface moisture of plastics on NIR sorting was investigated [48], and the findings were used outside NEW-MINE for material flow characterization in sensor-based sorting systems using an instrumented particle [49] and for a study of the influence of material alterations and machine impairment [50]. NEW-MINE learnings on the effects of throughput rate and input composition on sensor-based sorting [51] were subsequently used within the above mentioned ReWaste 4.0 project [47,52].

Finally, the challenge of the fine fraction, which has been identified in pre-NEW-MINE projects, e.g., in the LAMIS project [53], was firstly summarized in two review publica-

tions [54,55]. A post-NEW-MINE study on contaminants in landfills [56], despite using a misleading soil definition [57], added further information on this topic. Other publications [44,58] also addressed the observation that the fine fraction makes up the majority of the landfilled material, but used correct terminology. Further post-NEW-MINE studies on leachate characteristics [59] and composition of the fine fraction [60] added further information to these fields. The need to further process the fine fraction to minimize re-landfilling was mentioned by a publication on geophysical exploration of landfills [61]. The problem stated within NEW-MINE that currently no valorization options for the fine fraction exist because of the possibility of contamination of this fraction [43] was later considered by an Estonian [62] and by an Indian study [63]. The same statement was also extracted from the second review [55] but with the suggestion to tackle this problem by further processing [44]. In contrast, a Hungarian group extracted the contradicting statement that fine fractions can be considered as a relevant source of metals, calorific fractions, inert fractions and soil-like material recovery [64]. The explanation that the amount of the fine fraction increases over time due to humification processes was mentioned in an Indian study on the application of Fourier-transformed infrared spectroscopy for its characterization [65]. The need for a deeper understanding of the physico-chemical properties of the fine fractions including their distribution across the different grain size fractions was also applied to shredder fine fractions [66]. Within the field of landfill mining, the general observations regarding the fine fractions were considered by authors of the SMARTGROUND project [67].

Then, the fine fraction < 90 mm obtained by ballistic separation was characterized [68] and further separated into a light fraction, a heavy fraction, ferrous and non-ferrous metals and a fine fraction < 4.5 mm using windsifting, magnetic separation, eddy-current separation and screens [69]. The heavy fraction was investigated with respect to the utilization as construction material and the light fraction with respect to the use as refuse derived fuel (RDF) [70]. The fine fraction < 4.5 mm was further processed and characterized with respect to the relationship between mineralogy and leachability indicating the stable mineralogical bonding of many contaminants (e.g., Pb as metal and Pb-Ca phosphate and Zn as Fe-Zn alloy, ZnS and ZnSO4) [71].

In summary, novel research results obtained within NEW-MINE, especially using sensor-based sorting, can be applied to areas beyond ELFM.

### *3.2. Thermochemical Conversion and Solar Energy Storage*

### 3.2.1. Thermochemical Conversion

Most pre-NEW-MINE landfill mining projects covered the value chain from exploration via excavation and mechanical treatment to the production of concentrates of secondary raw materials, i.e., metals for pyrometallurgical recycling, mineral wastes for the production of construction materials and combustibles for energy recovery. These concentrates are still wastes (and not products) but state-of-the-art recyling and recovery routes do exist, which is why the actual recycling or recovery was not part of most of these projects. With respect to the combustibles these recovery routes include among others:

Consequently, previous projects investigated the combustibles with respect to the (co-) incineration in existing facilities considering not only calorific properties but also the content of heavy metals [7] as these restrict the valorization options of RDF by coincineration [19]. For those cases in which the combustibles fulfil the requirements for coincineration in the cement industry, the inorganic constituents of the RDF are incorporated into the mineral phases of the clinker contributing to its desired hydraulic properties, which justifies to address the share of RDF used on a material level as "recycling index" [73]. In contrast, for those cases in which contaminant concentrations exceed limit values for

RDF, energy recovery in MSWI plants is the state-of-the-art option. MSWI yields bottom ashes whose recycling is restricted by limit values for total concentrations and/or leachable concentrations of contaminants [74,75]. The chemical composition and leachability, as well as the element-specific limit values significantly vary between individual EU member states. Table 1 presents the ranges of concentrations relative to ranges of limit values for material used in road construction, showing that the total contents of Cd, Cu, Cr, Pb and Zn and the leachable contents of Pb are most problematic.

**Table 1.** Ranges of total (mg/kg) [76] and leachable (mg/L) [76,77] contents and respective limit values (CH, DK, SWE, FIN, FRA, AUT, D, NL, BEL for total and DK, ESP, SWE, FIN, FRA; AUT, GER; NED, BEL for leachable contents) for recycling of MSWI bottom ashes.

Following the pathway of classic incineration for those calorific fractions exceeding the threshold values for co-incineration would, therefore, result in the need to re-landfill significant proportions of the resulting bottom ashes. Disposal of MSWI bottom ash at increasing scarcity of landfill volume and the need for landfill aftercare for up to more than 100 years [78] is associated with significant costs, which will represent a massive financial and environmental burden for governments and the public in the future.

This is why within NEW-MINE, despite sharp criticism [79], gasification and pyrolysis were investigated as alternative thermochemical conversion processes within NEW-MINE with the background idea to use the produced syngas and to couple these processes to plasma technology to convert the resulting ashes directly into a vitreous slag, which might be favorable compared to MSWI bottom ashes with respect to mechanical and leaching properties. As it was not possible within the project to test plasma technology, vitreous slag from previous tests was used for further experiments, but also MSWI bottom ash was melted to obtain a similar slag.

Within NEW-MINE, the light fraction obtained by mechanical processing of excavated waste from the Mont-Saint-Guibert landfill was subjected to pyrolysis (N2 atmosphere, 900 ◦C, 1 h) and the resulting char was subjected to steam gasification using thermogravimetry analysis (TGA) to study the reaction kinetics [80]. This publication was cited as one of the few studies on gasification kinetics of MSW chars [81] and regarding the observed increase in activation of the char by gasification [82]. Furthermore, the study was cited with respect to the energy savings, low NOx emissions and pollutant reduction of steam gasification compared to conventional thermochemical conversion technologies [83]. In a second study, a sample of the fine fraction (<10 mm) and two samples of combustibles (with different share of defilements) were used for a similar experimental setup [84]. The latter publication was cited in a review on co-incineration of organic waste and coal [85] and with respect to the higher production of CO and CO2 in pyrolysis when using finer waste fractions in a publication on the prediction of gaseous products from RDF pyrolysis [86]. Two further results originated from this NEW-MINE publication: (1) pyrolysis of landfilled

waste yields less oil and gas than pyrolysis of fresh waste and (2) gasification of char from pyrolysis of landfilled waste yields waste fuel with a higher reactivity compared to the same process using fresh waste, which is explained by the catalytic effect of the metals present in landfilled waste [87]. Further studies citing this publication are on pyrolysis of pre-treated trommel fines [88]. In the third study on this topic, it was shown that steam co-gasification of landfilled waste with biochar increases H2 yields [89]. This publication was mentioned in a review article on waste-to-energy technologies [90] and with respect to the use of the syngas from gasification for ammonia synthesis [91].

The second issue addressed within NEW-MINE regarding gasification was the cracking of tars from the produced syngas which would restrict its utilization options. At first, a review on the state of the art regarding the role of plasma in syngas tar cracking [92] was published, which was cited outside of NEW-MINE by two other review articles [93,94] mentioning the efficiency, but also the short lifetime and high costs of this technology, as well as a novel warm gas tar cleaning processing called OLGA. In a next step, naphthalene was used as a model tar molecule and Corona plasma-aided thermal cracking was demonstrated successfully for its removal at 800 ◦C [95]. This study was cited with respect to tar generation and conversion kinetics [96], regarding the required high temperatures of 1100 ◦C which represent a disadvantage compared to catalytic reforming [97], and as an example for thermal tar cracking [98]. Finally, two CaO-rich catalysts doped with Sr were synthesized from mussel shells and successfully tested for tar cracking in the syngas [99].

Another NEW-MINE publication focused on further aspects of the pyrolysis process. Lab-scale pyrolysis tests at 400 to 700 ◦C with the above-mentioned light fraction of the Mont-Saint-Guibert landfill revealed the enrichment of polycyclic aromatic hydrocarbons (PAH) in the condensable pyrolysis products, highlighting the need to further treat this output fraction [100]. Although the main focus of thermochemical conversion technologies within NEW-MINE was on the treatment of the combustibles, thermal treatment (400/450 ◦C, 30 min) was also used to assess the quality of the nonferrous metals obtained from mechanical processing of the excavated waste from Mont-Saint-Guibert landfill [101].

### 3.2.2. Solar Energy Storage

In contrast to waste incineration and co-incineration, which produce energy and even contribute to 3.7% of the German end energy consumption [72], gasification requires external energy. The use of energy from renewable sources for this purpose is environmentally favorable, but requires energy storage. Consequently, within NEW-MINE, MgO-stabilized SrO was synthesized and successfully tested for heat storage via the SrO/SrCO3 cycle [102]. This research was considered outside NEW-MINE with respect to radiation propagation in a heat exchanger transforming solar radiation into high-temperature heat [103], thermochemical energy storage in the CaO/CaCO3 [104] and MnAl2O4/MnAl2O4−x [105] system and two reviews on thermochemical energy storage [106,107]. The most recent NEW-MINE publication in this field deals with thermochemical heat storage via the CuO/Cu2O redox cycle and describes the synthesis of granules with a gravimetric energy storage density in the range of 470 to 615 kJ kg−1 [108].

In summary, NEW-MINE publications on thermochemical conversion and solar energy storage focused on the organic constituents of gasification chars and of oxide/carbonate systems for solar energy storage and created a novel state-of-the-art. However, the fate of inorganic pollutants during gasification was not addressed and the intended production of vitreous slags via plasma processing that might immobilize these pollutants was not realized in the project.

### *3.3. Benefication of Products from Thermochemical Conversion*

### 3.3.1. Melting and Vitrification

Since the intended plasma gasification of ELFM materials was not realized within NEW-MINE, three groups of input materials were selected for further processing, i.e., (1) MSWI bottom ashes, (2) vitreous slag from plasma gasification ("Plasmastone") from

a pre-NEW-MINE project and (3) synthetic vitreous slags mimicking the plasmastone composition. The aims of the beneficiation of these materials comprised the recovery of metals and the beneficiation of the nonmetallic fractions for specific applications in civil engineering beyond the state-of-the-art applications of MSWI bottom ashes as low-strength aggregates [109]. Thermal treatment of MSWI bottom ashes within NEW-MINE aimed for the recovery of metals and beneficiation of the mineral fraction. For this purpose, two heating technologies were investigated.

Firstly, a submerged arc furnace (SAF) and an electric resistance furnace (ERF) were used to melt MSWI bottom ash obtained from a Dutch MSWI plant at 1500 and 1400 ◦C, respectively. These experiments yielded an Cu-Fe alloy and a vitreous CaO-Al2O3-SiO2 slag, which might be used for the production of glass ceramics or inorganic binders [110] and were linked to the application of SAF technology in other fields [111].

Secondly, microwave heating was used to study the dielectric properties from the above mentioned MSWI bottom ash from a Dutch MSWI plant and revealed that MSWI bottom ash absorbs microwaves with low losses until 320 ◦C but above this temperature pyrolysis enhances the dielectric loss [112]. This study was cited with respect to microwave cladding, an emerging surface modification technique [113]. In a next step, flash microwave vitrification was realized at 2.45 GHz within 1.5 min. The advantage of this technology for vitrification is that the cooling rate is naturally so fast, due to the inherent cold environment, that no specific efforts are required for quenching [114].

### 3.3.2. Alkali Activation

The next group of studies within NEW-MINE dealt with the further benefication of vitrified MSWI bottom ashes. In one study, the vitrified bottom ash was subjected to alkali activation (1 M NaOH and 2.5 M NaOH and sinter crystallization (800 ◦C or 900 ◦C), thereby yielding porous (70 vol%) glass ceramics with a compressive strength of 3 MPa, which might be used as an alternative to lightweight concrete [115]. Another group of studies addressed the production of inorganic polymers by alkali activation. Compared to geopolymers, which are the main subgroup of alkali activated materials, inorganic polymers differ with respect to their chemical composition.

A synthetic plasmastone analogue was used to study the impact of solid-to-liquid and K2O/SiO2 ratio and the type of activation solution on the resulting inorganic polymers. Adjustment of these process parameters yielded inorganic parameters with a compressive strength of up to 119 MPa [116]. This study was cited in a review article as an example to valorize vitrified MSWI bottom ash [117]. Based on these findings, strategies to increase the volumetric stability of inorganic polymers were developed [118]. This work was cited in the context of alternative cementitious materials for radioactive waste encapsulation, taking into account that strengthening of inorganic polymers was observed due to radiationinduced iron oxidation [119]. One approach to increase the volumetric stability is the addition of CaO-rich materials, which was demonstrated in another study [120].

### 3.3.3. Sintering of Glass-Ceramics

The last group of studies refers to the sintering of vitreous slags from plasma processing of MSW or from MSWI vitrification to glass ceramics, which might be used as building materials such as aggregates and panels. The most-cited publication of NEW-MINE [121] dealt with the production of porous glass ceramics from the vitreous slag obtained by plasma processing of MSW. It was demonstrated how increasing the firing temperature of glass ceramics can trap chromium in stable pyroxene group minerals and adding borosilicate glass can decrease the leaching of Cr, Mo and V as boron acts as a network former. This paper was cited in publications on the preparation of glass-containing foams from geopolymers [122] and vitrified MSWI bottom ash [123] in which the formation of wollastonite and the freezing of the microstructural evolution were mentioned. Other papers cited this publication with respect to the recycling of glass waste into foam glass [124–129], porous waste glass for lead removal in wastewater treatment [130], lead stabilization through

alkali activation and sintering of Pb-bearing sludge [131], utilization of waste glass for the production of sulphuric acid resistant concrete [132], mechanical and alkali activation of MSWI fly and bottom ashes for the production of low-range alkaline cement [133] and foam glass-ceramics [134], inorganic gel casting for manufacturing of boro-alumino-silicate glass foams [135], porous glass-ceramics derived from MgO-CuO-TiO2-P2O5 glasses [136], alkali activation of coal and biomass fly ashes [137], nickel-based catalysts for steam reforming of naphthalene utilizing MSW gasification slag as support [138], production of porous glass ceramics from titanium mine tailings and waste glass [139], porous bioactive glass microspheres [140], Al-SiO2 composites [141], glass-ceramic foams from alkali-activated vitrified MSWI bottom ash and waste glasses [142]. Another study used vitrified MSWI bottom ash as input material to obtain similar porous glass ceramics [143] and was cited by some of the publications that also cited the first study.

In a next step, dense glass with good mechanical and environmental properties (low leaching due to stable incorporation of heavy metals in stable hedenbergite, wollastonite and iron oxides) were obtained from a mixture of "Plasmastone", recycled soda-lime glass and kaolin clay by cold pressing and fast heat treatment (1000 ◦C, 40 ◦C/min) [144]. This publication was referred to as a case study for the influence of crystallization time on glass ceramics [145].

The crucial linkage between mineralogy and technical and environmental performance was also investigated in a further study in which low temperatures (800 ◦C) were sufficient to immobilize heavy metals in a stable matrix using spent borosilicate glasses and additional electromagnetic shielding functionalities were obtained by magnetite formation [146]. This approach was cited in a study on the substitution of feldspar by waste glass for porcelain production [147] and in a paper on cheap pore-generating agents for ceramics [148]. Finally, the promoting effect of firing in nitrogen atmosphere on the stabilization of pollutants and novel functionalities was investigated [149]; the paper was cited in studies on the recycling of iron-rich inorganic wastes into functional glass-ceramics [150] and on the production of glass ceramics from MSWI bottom ash and coal fly ash by melting and sintering [151].

In summary, the NEW-MINE publications on beneficiation of products of thermochemical conversion of (excavated) MSW, i.e., vitrified MSWI bottom ashes and vitreous slags of plasma gasification, yielded novel insights into the relation between (crystalline/vitreous) structure and (mechanical/leaching) properties of waste-derived glass ceramics and inorganic polymers. Although these specific approaches to address MSWI residues might not be realized in practical waste management, the underlying mechanisms are also relevant for the mobility of heavy metals in other mineral wastes and allow the eco-design of their properties and, consequently, enhanced recyclability.

### *3.4. Multi-Criteria Assessment*

### 3.4.1. Life Cycle Assessment

Already before NEW-MINE, comprehensive methods for landfill mining projects have been developed. With respect to ecological and socio-economic criteria, this approach was based on utility analysis and then transferred into a utility-net present value chart [9]. In contrast, within NEW-MINE, for the ecological assessment life cycle assessment (LCA) was used, although it was not applied to landfill mining, but to the impact of sanitary landfills themselves [152]: this approach yields important information regarding the alternative to ELFM, i.e., state-of-the-art landfill aftercare. This LCA publication was cited with respect to the site selection for landfills [153,154] and in a study which yielded the controversial statement that "recycling metals except gold had more negative environmental impacts than mining" [155]. Further citing studies deal with environmental impacts of combustion-based energy production [156], composting on closed landfill sites [157], bioreactor landfills [158], suppression of methane generation [159], modelling of landfill gas production [160], organic waste enrichment [161]. Within NEW-MINE, LCA was also applied to compare smelting of MSWI bottom ash with the state-of-the-art mineral processing approach and revealed that the energy demand for smelting processes overrules, with respect to global warming, the positive effect of secondary raw material production [162].

In summary, LCA demonstrated the environmental impact of all scenarios of landfill management. Landfill aftercare requires a high effort for gas and leachate treatment, while traditional landfill mining with MSWI yields mineral fractions of MSWI bottom ashes which cannot substitute for high-value primary raw materials due to the leaching of pollutants. In contrast ELFM, with the production of vitreous slags–either via plasma gasification of municipal solid waste (MSW) or via smelting of MSWI bottom ashes, yields more valuable secondary raw materials which are characterized by a lower leaching, but their production requires more energy.

### 3.4.2. Techno-Economic Assessment

Based on a comprehensive study on existing economic assessment methods [163] it has been suggested within NEW-MINE to follow learning-oriented approaches. Subsequently, an economic and environmental assessment of a possible ELFM project in Sweden has been conducted and revealed that none of the scenarios is economically profitable, but of all them result in net avoided emissions due to the recovery of metals [164]. In a next step, more than 500,000 landfill mining scenarios were evaluated with respect to their net present value and the influencing factors were identified via global sensitivity analysis. It was found that 80% of these scenarios show negative results and that revenues from avoided landfill management costs are more important than revenues from resource recovery [165]. This study was cited in a review paper on the impact of landfills on the environment [166] in a study on the environmental and economic assessment of ELFM in Tehran [167] and with respect to geophysical exploration of landfills [35].

In summary, NEW-MINE results confirm that ELFM is ecologically favorable, but economically not profitable, which demonstrates the need for an integrated ecological and economic assessment [9,35]. This means, that for those cases in which this integrated assessment shows a positive effect of ELFM, economic incentives would be required to make secondary raw materials or recycled products from ELFM competitive in the market.

### 3.4.3. Sociological Assessment

Before NEW-MINE, landfill mining was investigated mainly from technological, environmental and economic perspectives. However, it is well known that large infrastructure projects often fail due to public resistance [168]. Therefore, within NEW-MINE, also stakeholders' perspectives on ELFM were investigated [169] and stakeholder needs were identified [170]. The latter study was cited in a study on the characterization of excavated waste [171]. Finally, stakeholder archetypes for ELFM were defined: The Engaged Citizen, the Entrepreneur, the Technology Enthusiast, the Visionary and the Skeptic [172]. In summary, the importance of the involvement of stakeholders in ELFM has been demonstrated and fundamentals for corresponding strategies have been laid.

### **4. Discussion**

### *4.1. Disciplinarity and Interdisciplinarity*

Within the NEW-MINE project the entire process chain, from landfill exploration via excavation and mechanical processing to thermochemical conversion and beneficiation of conversion products, has been investigated. This broad scope constituted a unique feature of the NEW-MINE project and, correspondingly, many publications do not address the landfill mining itself, but waste treatment technologies, which may also be used for waste from other sources than landfills. Some publications, e.g., dealing with energy storage, are only loosely linked to landfill mining. This broad scope enabled the interdisciplinary collaboration of researchers, for example between geophysicists and waste scientists [33], waste scientists and polymer scientists [48], experts in mechanical and thermal waste treatment [84,89,101] and in high and low temperature waste mineralogy [121,144]. However, the majority of the publications can be allocated to a single scientific discipline. With

respect to the number of publications citing the NEW-MINE publications, no significant difference between disciplinary and interdisciplinary publications is visible. Although most disciplinary publications were cited by researchers from other disciplines (e.g., [165] by [35]), most of them were cited by colleagues from the authors' discipline.

Considering that the overall research question of the project was to investigate if innovative technologies increase the economic and environmental feasibility of landfill mining, more interdisciplinary cooperation would be required. For example, if the purity of an inert fraction obtained by sensor-based sorting is increased from 86 to 98% [46], what is then the impact on the economic, environmental and societal assessment of ELFM? This and several similar questions demonstrate the need for further interdisciplinary research in the field of ELFM for which NEW-MINE could have been an excellent starting point.

### *4.2. Recycling Waste from Landfills*

Although a lot of reasons for landfill mining have been identified and excavation of landfills for environmental remediation has been practiced already for decades, it is the recovery of resources from landfills, which fostered the landfill mining projects in the last decade including the NEW-MINE project This is clearly indicated in the project's name, i.e., "EU Training Network for Resource Recovery Through Enhanced Landfill Mining". During the NEW-MINE project a lot of effort was devoted to treat the excavated waste by mechanical, thermochemical and chemical methods. The final aim of these treatments is to obtain either a secondary raw material, which may substitute for a primary raw material in an industrial process, or a product, which substitutes for a product with the same functionalities but is made from primary raw materials. Regarding the production of secondary raw materials, NEW-MINE demonstrated that e.g., the 2D fraction of ballistic separation can substitute for fossil fuels in co-incineration plants. In contrast, the use of the inert fraction as raw material for the production of concrete [70] and the use of the fine fraction as raw material for the production of compost soil [71] fails, due to the exceeded limit values for total and leachable contaminants.

Regarding the production of a product, it has been demonstrated that e.g., glassceramics [144] fulfil the mechanical and leaching requirements for building products. Apart from the problem that several countries, e.g., Austria, have also limit values for total contents, it has to be mentioned that these products have to compete with products with the same functionality but are made from primary raw materials. Consequently, comparative economic and environmental assessments are required to investigate the production costs and environmental impact of e.g., lightweight concrete made from primary raw materials (e.g., AiriumTM) and waste-derived glass-ceramics that compete for the same application. With respect to raw materials, the same applies: e.g., recycled aggregates compete with natural aggregates (with the problem of decreasing quality with increasing number of recycling cycles [173]), but also additional challenges occur: The RDF that can be produced by ballistic separation or windsifting has a negative value, i.e., the producer has to pay the cement plant to incinerate it. Therefore, within the Halbenrain case study in NEW-MINE, the lightweight fraction, which could have been delivered to an RDF production plant was re-landfilled, as this was the cheaper option. The reason for this paradox is that combustible waste must not be landfilled in Austria and incineration is an expensive process, considering also the taxes that have to be paid. Therefore, co-incineration plants can demand negative prices for RDF, as it is still cheaper than incineration. Only for the specific case study of ELFM, re-landfilling of combustibles is allowed and represents the cheapest option.

In summary, even if secondary raw materials or recycled products can be produced from ELFM materials, it remains unclear if they can compete with primary raw materials or products from primary raw materials. First observations for Austria suggest that this may often not be the case. Given the ambitions of transition towards a more circular economy, economic incentives are required to make secondary raw materials or recycled products from ELFM competitive in the market.

### *4.3. Waste Management Meets Mining Economics*

Finally, as is already evident in the project name "NEW-MINE" in which terminology from mining economics ("resources") and waste management ("landfill") are used, it represents a good example to apply a figure illustrating the different terminologies from mining economics, waste management and process engineering [27] to MSW landfills and ELFM (Figure 2).

**Figure 2.** Location of MSW landfills in waste and resource terminology (Reprinted with permission from ref. [27]. Copyright 2020. Daniel Vollprecht).

Often, in the recycling business, the term "waste" is avoided due to its negative connotations. Instead, the terms "residue", "anthropogenic resource", "by-product" or "secondary raw material" are used, but they have a different meaning.

Waste is precisely defined in the European Waste Framework Directive [174] as "any substance or object which the holder discards or intends or is required to discard". Consequently, the material in an MSW landfill is waste, as the (former) holder discarded it. If the material is recycled, it reaches the end-of-waste at the moment when it substitutes for a primary raw material. Considering the technologies investigated within NEW-MINE, this means that the material remains a waste until the waste-derived glass ceramics or inorganic binders finally substitute for conventional construction ceramics or cement, respectively, and this only if they fulfil the technical and environmental requirements for the competing products made from primary raw materials.

A by-product is only a substance or object whose further use is certain [174], i.e., there cannot be a by-product in a landfill, since if the further use would have been certain, it could not have been disposed of. In contrast to "waste" and "by-product" which originate from waste management, the term "residue" stems from process engineering and refers to "the part that is left after the main part has gone or been taken away" [175]. In some MSW landfills also residues can be found, as industrial waste has not always been disposed of in separate landfills. Similar to "residue" also "secondary raw materials" are defined by the Great Soviet Encyclopaedia as "materials and articles which, after complete initial use (wear), may be used repeatedly in production as starting material" [176].

The terms "resource" (reasonable prospects for eventual economic extraction in the foreseeable future) and "reserve" originate from mining economics (current economic extraction possible) and have been introduced into waste management [13]. If current

economic extraction is possible, the material will be a secondary raw material for a subsequent industrial process, i.e., "anthropogenic reserve" and "secondary raw material" are synonyms. With respect to ELFM, only a small fraction of the excavated materials within NEW-MINE, i.e., the metal fractions from magnetic and eddy current separation or from smelting processes, fall in this category. Some other fractions, e.g., the combustibles, might be classified as "anthropogenic resource" if RDF prices rise significantly above zero. However, the results for the fine fraction do not justify a classification as anthropogenic resource, as no reasonable prospects for their purification exist [71].

In summary, the quantity of resources that can be recovered from MSW landfills can be significant, if valorization options for the inert fraction and the combustibles become feasible. However, quality and "bonitaet" [26] of anthropogenic resources from MSW landfills represent significant challenges for future ELFM projects.

### **5. Conclusions**

The NEW-MINE project (2016–2020) was a European research project investigating the entire process chain of ELFM. Within the NEW-MINE project in a first step concentrates of specific materials (e.g., metals, combustibles, inert materials) were produced which might be used as secondary raw materials. In a second step, recycled products (e.g., inorganic polymers, functional glass-ceramics) were produced from these concentrates at the lab scale. However, even if secondary raw materials or recycled products could be produced at a large scale, it remains unclear if they can compete with primary raw materials or products from primary raw materials. Given the ambitions of transition towards a more circular economy, economic incentives are required to make secondary raw materials or recycled products from ELFM competitive in the market.

**Author Contributions:** Conceptualization, D.V.; methodology, D.V.; investigation, D.V. and L.M.; writing—original draft preparation, D.V.; writing—review and editing, L.M., P.T.J.; visualization, L.M.; supervision, P.T.J., L.M.; project administration, P.T.J., L.M.; funding acquisition, P.T.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** The NEW-MINE project has received funding from the European Union's EU Framework Program for Research and Innovation Horizon 2020 under Grant Agreement No 721185.

**Acknowledgments:** The authors thank the entire NEW-MINE team for the fruitful collaboration within the project.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

### **References**


**5. Discussion**

The best performing network architectures using frequency analysis input data (FFT and STFT) require a small size of the pooling layer as well as a small size of the receptive field. The extreme case of *e* = 1 and *p* = 1 even results in a simple scaling of the frequency analysis of each sensor signal and averaging across the data points of all sensor signals. The actual classification is performed in the fully connected layer using those averaged frequency analysis data points. This raises the question of whether a CNN is necessary or if a Multi-Layer Perceptron (MLP) neural network is sufficient for solving the classification task. Therefore, a MLP neural network consisting of only a fully connected layer with 128 neurons was trained using the FFT input data. The resulting classification accuracy on the test dataset was 87.27 %, 72.97 % on the mass dataset and 68.37 % on the tire dataset. This demonstrates that the convolutional operation adds robustness to the performance of the neural network.

While promising results for diagnosing defective dampers using Convolutional Neural Networks are presented in this paper, limitations for a real-world implementation still exist, which will be discussed in the following paragraphs.

There are only data of one specific vehicle used in this paper's investigations. All networks were trained and tested using data of this vehicle. Recording training data for every unique vehicle configuration with different damper defects during the development phase seems challenging for an actual implementation. Therefore, portability of a trained classification system with high generalization capabilities for the diagnosis of different vehicles is desirable.

Robustness is a critical aspect for a real-world application. Even though the network architectures showed a robust behavior for tire and mass variations, further robustness analysis is necessary because there is a great variety of different circumstances during the usage of a vehicle.

Real damper defects might not occur in a switching manner, but the loss of damping forces might increase gradually over a long period of time. A classification process for the damper's health state might therefore not detect a minor defect. This can be encountered in two ways: Adding additional

classes or predicting a continuous score for the health state of each damper. However, both approaches raise the need of additional training data.

## **6. Conclusions**

This paper analyzed the suitability of using Convolutional Neural Networks for the diagnosis of automotive damper defects using driving data of the longitudinal and lateral acceleration as well as yaw rate and wheel speeds. The classification performance using different pre-processing methods was analyzed. Using detrended time-signals as well as frequency analyses such as FFT or STFT showed the best results.

The analysis of the network architecture showed that the size of the receptive field and the size of the pooling layer needs to be chosen according to relevant oscillation frequencies of the input signal when using time-related detrended input data. The analysis of the trained kernel weights demonstrated that a frequency analysis is performed by the CNN for detrended time-signal input data.

Using FFT input data results in the overall best classification performance. A small size of the pooling layer performs best and the size of the receptive field can be chosen arbitrarily. The trained kernels perform a comparison of the amplitudes for several frequency differences.

Using STFT input data results in a similar classification performance as using FFT input data. The investigation of the trained kernels showed that the time information is not used by the CNN. Therefore, the STFT pre-processing does not result in any benefit. The reduced frequency resolution compared to a FFT pre-processing even decreased the robustness regarding unknown characteristics in the testing data such as additional mass or changed tires.

Table 3 shows the performance of the best network configurations of the three investigated pre-processing methods. The number of Floating Point Operations (FLOPs) for the execution of the model as well as the number of tuneable parameters are an indicator of the possibility of an implementation on the Electronic Control Unit (ECU) of a vehicle. However, since a specific value for the computing power of an automotive ECU cannot be found, the authors are not able to judge about the real-time implementation. The selected network configurations were chosen with regard to the performance of the three different datasets. The best classification accuracy results from using FFT input data with less network parameters than when using STFT input data.

The software and data of this paper are available online [53] (see the Supplementary Materials).

**Table 3.** Comparison of best performing network configurations

**Supplementary Materials:** The software and data is available online at [53]. https://github.com/TUMFTM/ Damper-Defect-Detection-Using-CNN/.

**Author Contributions:** T.Z. (corresponding author) initiated this paper. His contribution to the overall methodology of the proposed approach was essential and he performed the analyses presented in this paper. T.Z. supervised the master thesis of T.H.-P. who made essential contributions to the implementation of the proposed approaches. Both implemented this paper. M.L. made an essential contribution to the conception of the research project. He revised the paper critically for important intellectual content. M.L. gave final approval of the version to be published and agrees to all aspects of the work. As a guarantor, he accepts responsibility for the overall integrity of the paper. Conceptualization, T.Z.; methodology, T.Z. and T.H-P.; software, T.Z. and T.H-P.; validation, T.Z. and T.H-P.; formal analysis,T.Z. and T.H-P.; investigation, T.Z. and T.H-P.; resources, T.Z.; data curation, T.Z.; writing–original draft preparation, T.Z. and T.H-P.; writing–review and editing, T.Z.; visualization, T.Z. and T.H-P.; supervision, T.Z. and M.L.; project administration, T.Z.; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript.

*J. Sens. Actuator Netw.* **2020**, *9*, 8

**Funding:** The work described in this paper was supported by the basic research fund of the Institute of Automotive Technology from the Technical University of Munich. This work was supported by the German Research Foundation (DFG) and the Technical University of Munich (TUM) in the framework of the Open Access Publishing Program.

**Acknowledgments:** We would like to thank the vehicle manufacturer for supplying us with a research vehicle.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **Appendix A. Implementation Details**

The CNN were implemented in TensorFlow 1.9 using Python 3.5. During training, we use Adam optimizer [54] with a mini-batch size of 128. The optimizer minimizes the sum of a Sparse Softmax Cross Entropy plus the sum of weight decay (L2 regularization). Training is stopped after 650 epochs or if the accuracy with validation data has not significantly improved for 50 epochs (Early Stopping). Kernels and weights are initially set following He Initialization scheme [55] and the bias terms are initially set to a small constant (0.01). We use Rectified Linear Unit (ReLU) activation functions for every layer except for the output units, which are linear neurons.


*2.2. Theoretical Framework: Towards Global TWC*

In our analysis, we conduct macro-level research on TWC from the perspective of inter-country relations and select four different proximity forms of geographical, economic, organizational, and colonial, to analyze the general mechanism which affects the intensity of global TWC (Figure 1).

**Figure 1.** The theoretical framework for the formation of global TWC.

2.2.1. The Relations of Proximities and Global TWC

Geographical proximity. The influence mechanism of geographical proximity on TWC is mainly manifested in three aspects: water resources endowment, geographical position, and natural environment difference between state actors. Countries with favorable water resources endowment tend to have weaker willingness to cooperate, such as countries with a high water supply and low water demand. Conversely, countries with a low water supply and high water demand will show a high willingness to cooperate. In terms of geographic position, as Tobler's [70] first law of geography revealed, the distance between things in space is inversely proportional to the closeness of relationships, and the closer geographical distance between countries or the direct existence of basin links is more

conducive to communication and interaction. Besides the factor of geographical distance, the actor in the same transboundary basin should be taken into account. Geographical proximity will also affect countries' understanding of the environment and further affect their diplomatic behavior, prompting them to formulate foreign policies consistent with the environment [71].

Economic proximity. The effect of economic proximity on TWC is mainly manifested in the economic foundation, market demand, and attention of domestic society. The economic foundation is the basis of each actor's behavioral ability. Although some countries have a high willingness to cooperate, their economic development level is relatively weak, and they are unable to undertake international responsibilities or take effective actions in cooperation, which restricts the development of TWC between themselves and other countries or international organizations. Market demand emphasizes the attraction of other countries' water markets to one country. Positive demand, negative demand, and potential demand have different degrees of influence on the country's TWC cooperative willingness and ability. Trade flows and trade-based interdependence among countries also contribute to the promotion of peace and cooperation among countries [72]. The domestic social environment, interest groups, and other factors will affect national policymakers in formulating foreign policy [73]. Furthermore, the pursuit of seeking economic benefits on TWC drives interest groups to lobby the government to water cooperation [74].

Organizational proximity. As Boschma pointed out, organizational proximity includes the similarities that participants are connected by sharing the same reference space and knowledge [60]. Additionally, it often means the same space of relations based on the effective interactions of various nature [75]. It includes a relation of similarity and a relation of membership. Organizational proximity mainly affects the formation of TWC from three aspects: international water law, information exchange, and international regime. International water law refers to a series of treaties or practices reached among the international actors to solve international water resources development and protection. Strictly speaking, there is currently no unified international water law [76]. However, international water law emphasizes the goal of equitable development and sustainable use of transboundary water resources, regulates the rights and obligations of international actors, and helps to safeguard and enhance the cooperative ability of all actors. The information exchange helps to ensure the effectiveness of communication among the actors in the organization. The higher the level of information exchange and the more formalized and institutionalized the process, the more conducive to the smooth flow of tacit knowledge, which could influence the actors to reach a consensus on cooperation. The international regime is a series of principles, norms, rules, and decision-making procedures formed around the expectations of actors in a given field of international relations. Neoliberal institutionalists, such as Keohane, emphasized that an effective regime can promote official contacts and establish cross-governmental communication networks [77]. The more international regimes established between countries means that they not only have more options for cooperation channels in dealing with specific issues, but can also avoid the possibility of losing cooperative participation due to the failure of a single rule.

Colonial proximity. Colonial proximity also affects the formation of TWC mainly from three aspects: historical relations, international habitus, and conceptual cognition. Historically, the close relationship between the colonizers and the colonized was derived from the decolonization policy adopted by the colonizers after World War II, which emphasized that the actions taken by colonial countries in the process of the collapse of the colonial empire were aimed at maximizing their interests. Compared with historical relations, international habitus emphasizes the current behavioral tendencies of the countries. The habitus of the actor derives from its long-term practice, is acquired through lasting experience in its social status, and is the practical logic of its action [78]. Although in the post-colonial system, countries have gained sovereign independence and equality in the sense of international law [79], under the influence of international habitus, the former colonial powers still actively dominate international affairs and reshape the international order by their strength. Additionally, the former colonies, influenced deeply by their politics, economy, as well as culture, not only maintain direct contact and cooperation with the former colonial powers in many affairs, but also have been indirectly affected in the practice of cooperation with other countries. Conceptual cognition is another factor that affects TWC between former colonial powers and colonies, as well as between colonies. Lu [80] pointed out that some former colonial powers, out of compensation for their historical responsibilities, strengthened rectification justice or political reconciliation in their interactions with former colonies, so as to carry out cooperative activities with a nature of assistance in many fields, including water cooperation. From the perspective of constructivism, since countries that have been colonized by the same colonial power may have similar language, norms, and culture, they can help countries to shape identity [72]. Therefore, these countries can form more effective communication and promote water cooperation activities in dealing with water affairs.


**1. Introduction**

Path planning and tracking are the main tasks studied for unmanned vehicles, especially unmanned aerial vehicles (UAVs), unmanned ground vehicles, and unmanned underwater vehicles [1–4]. UAVs, which have been used extensively in defense industry and academic studies in recent years, perform tasks such as surveillance, target tracking, search and rescue, and payload transportation [4–7]. The obstacles and their positions in the region where UAVs will operate play an important role in the effective operation of UAVs [8]. Establishing a safe path by determining the risky areas in military operation and natural disaster areas, following the path that has been generated, and releasing the payloads to the predefined regions are critical for the successful performance of the mission [9]. In this study, a new path planning and tracking algorithm based on metaheuristic optimization is developed for the payload hold–release task by avoiding the obstacles at the target points defined around the planned path.

A path planning and tracking is required for the UAV to safely reach the target location from the starting location depending on certain restriction conditions such as minimum flight distance and time [10]. UAVs may be exposed to inconvenient land and weather conditions while performing critical tasks. UAVs try to overcome this problem with their maneuverability and altitude capabilities [1]. This situation causes the UAV to consume more energy [11]. In the presence of obstacles and constraints, optimal path planning is required for the UAV to safely follow the specified path with minimum energy and time consumption [12,13]. The UAV path planning problem is a complex optimization

**Citation:** Belge, E.; Altan, A.; Hacıo ˘glu, R. Metaheuristic Optimization-Based Path Planning and Tracking of Quadcopter for Payload Hold-Release Mission. *Electronics* **2022**, *11*, 1208. https:// doi.org/10.3390/electronics11081208

Academic Editors: Luis Hernández-Callejo, Sergio Nesmachnow and Sara Gallardo Saavedra

Received: 12 March 2022 Accepted: 8 April 2022 Published: 11 April 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

problem that requires efficient algorithms to solve. This optimization problem can be solved with classical algorithms as well as by using quite efficient metaheuristic algorithms. Simple path planning for UAV is performed with the Voroni diagram algorithm [14], the probabilistic roadmaps algorithm [15], the A\* algorithm [16], and rapidly discovered tree-based algorithms [17]. However, since the kinematic and dynamic constraints of the UAV are rarely considered, these algorithms are generally not preferred in practical applications. In recent years, various approaches have been proposed for the autonomous path planning of the UAV, including meta-heuristic optimization algorithms. In [18], modeling of the battery performance of the UAV is emphasized. A multi-variable linear regression model has been created for the minimum energy consumption of the UAV on the specified path. The generated energy consumption model is used as a fitness function in the optimization algorithm. The performance of the proposed algorithm has been verified with various scenarios for the minimum energy consumption of the UAV. In [19], a path planning algorithm based on k-degree smoothing is proposed to define the coordinated path planning of the UAV in a safe and efficient manner. In the study, a kdegree smoothing method that aims to obtain a safer path using the ant colony optimization (ACO) algorithm [20] is presented. The proposed algorithm draws attention with its slow convergence speed and high probability of being stuck to local optima. In order to deal with these problems, a hybrid optimization algorithm obtained by combining maximum– minimum ACO (MMACO) and Cauchy mutant (CM) operators is recommended in [21]. As paths with faster convergence speed and better solution optimization are preferred in practical applications, swarm-based bio-inspiring optimization algorithms with low computational complexity and high computational speed are used extensively. In [22], an improved particle swarm optimization (PSO) algorithm has been proposed to achieve faster convergence speed and better solution optimization in the path planning of the UAV. The performance of the algorithm has been tested on various UAVs under many environmental constraints with Monte Carlo simulations. In [23], the 3D path planning problem of the UAV in the presence of obstacles is solved with the grey wolf optimization (GWO) algorithm [24] and the performance of the proposed algorithm is compared with metaheuristic algorithms such as PSO, the whale optimization algorithm (WOA), and the sine cosine algorithm (SCA). In the literature, metaheuristic optimization algorithms play an important role in solving different engineering problems, as well as path planning and tracking [25–27].

UAVs may encounter various obstacles while performing specified missions by sticking to a path. In [28], an obstacle avoidance algorithm based on ellipsoid geometry is proposed for the UAV to remain loyal to its original path and avoid the obstacles in its environment by creating waypoints in the presence of obstacles that obstruct the UAV flight path. The search for an avoidance path in the proposed algorithm is based on the use of ellipsoid geometry as a limited region containing the obstacle. Considering the geometry of the defined obstacle, a limited ellipsoid zone is created, and new crossing points are calculated within this zone. A convolutional neural network (CNN) approach based on depth estimation using molecular camera data to enable the quadrotor UAV to independently avoid collisions with obstacles in unknown and unstructured environments is presented in [29]. In [30], a new algorithm has been proposed that analytically calculates the path efficiently and effectively to create an environment map with a path without collision. In the developed algorithm, an initial path is created by the intersection of two 3D surfaces. Each obstacle position is shaped around the obstacles by adding a radial function to one of the two surfaces. The developed algorithm ensures that the intersection between deformed surfaces does not intersect with obstacles. The algorithm provides that the safe path is created in real time in the UAV's path tracking. In [31], every point in the motion environment is scanned with the 2D lidar on the UAV and the position of the UAV is estimated using the point cloud correction method. Unlike many studies with lidar, the effects of motion on the point cloud have been taken into account. In the proposed algorithm, point cloud features obtained by laser radar are extracted and a clustering is made based on relative distance and density. A robust nonlinear flight controller framework with

six dimensional force and torque estimators that includes a model predictive controller (MPC)-based trajectory plan that considers the trajectory planning problem as an optimal control problem with nonlinear obstacle avoiding limitations is proposed in [32].

The ability of the UAV, which has a payload transportation system, to move around the reference trajectory and hold payloads from a certain point and to release payloads to specified targets with minimum error makes the UAV important for critical missions. In releasing the payloads to the specified targets, the UAV should be able to determine the path on its own or stay loyal to the specified path. In [33], neural-network-based real-time UAV control is performed in order to release the payloads to the marked targets by following a certain path with minimum error. The controller structure includes feature extraction and selection stages. In order for the UAV to release the payloads on the predetermined coordinates with the highest accuracy, the full mathematical model of the UAV, as well as the model of the payload transportation system, is needed. In [34], both the dynamic model parameters and the payload transportation system model of the UAV are handled together with the controller approach based on the law of feedback linearization. It is stated that the stabilization of the UAV, especially when releasing payload, is improved with the proposed controller approach. The controller scheme robust to payload changes in various weights is presented in [35]. The proposed controller provides the stabilization of the UAV in the suspended position by compensating the weight changes in the UAV with payload transportation system. In [36], an optimization-based controller algorithm has been developed for the UAV moving around a certain trajectory to make minimum oscillation at maximum payload. It is emphasized that the developed algorithm performs optimal control, especially in maneuvers.

In this study, a new metaheuristic-optimization-based path planning and tracking algorithm with a very high convergence speed is proposed to the UAV with payload transportation system in order to plan a path by avoiding obstacles under constraints such as mass uncertainty, unknown parameters, and unmeasurable external disturbances and to release the payloads to the target points with minimum error while staying loyal to path. The proposed algorithm is robust as it copes with unknown system dynamics and adverse environmental factors. The main contributions of this study are that the new hybrid Harris hawk optimization (HHO)–GWO algorithm for path planning is proposed, the new path planning and tracking control strategy is developed together, and the path-tracking performance of the quadcopter in payload hold–release mission has been analyzed. In addition, the positional error due to the mass uncertainty can be minimized by the proposed control strategy, as well as the energy function. The results of the study are shown that the mass uncertainty and energy of quadcopter during payload hold–release mission have been minimized using the new proposed path planning and tracking algorithm.

The remainder of this paper is organized as follows. The dynamic model of the quadrotor UAV used in the study is given in Section 2. The proposed controller approach for the path planning and tracking of the UAV is introduced in Section 3, including GWO and HHO algorithms. The generated maps are presented in Section 4. The results obtained with the proposed model are discussed in Section 5. Finally, in Section 6, the main results of the study, and future work are highlighted.

## **2. Dynamic Model of Quadcopter**

Quadcopter is an underactuated type of UAV with four motors and six degrees of freedom (three translational and three rotations) and capable of landing and taking off in limited areas [37]. The evaluation of translation and rotation dynamics together in the motion control of a quadcopter is an important control problem. In the solution of this control problem, it is very important to take into account the non-linear parameters in the dynamics of the quadcopter. The main components of the quadcopter, Euler angles (roll, pitch, yaw), body frame, and global frame are illustrated in Figure 1.

**Figure 1.** The main components of quadcopter.

The following parameters:

are assumed in the model of quadcopter [38]. Position changes during quadcopter flight are measured in the frame, accelerometer, and gyro values are measured in the body frame. For this reason, it is necessary to define the transformations between body and coordinate systems. In this study, *cos*(.), *sin*(.), and *tan*(.) are represented by *c*(.), *s*(.), and *t*(.), respectively. Considering these transformations, the velocity expression in the frame is obtained by using the velocity in body frame as:

$$
\begin{bmatrix}
\mathcal{X}^G\\\mathcal{Y}^G\\\mathcal{Z}^G
\end{bmatrix} = \begin{bmatrix}
c(\boldsymbol{\uprho})c(\boldsymbol{\uptheta}) & c(\boldsymbol{\uprho})s(\boldsymbol{\uprho})s(\boldsymbol{\uptheta}) - c(\boldsymbol{\uprho})s(\boldsymbol{\uprho}) & c(\boldsymbol{\uprho})c(\boldsymbol{\uprho})s(\boldsymbol{\uptheta}) + s(\boldsymbol{\uprho})s(\boldsymbol{\uprho})\\s(\boldsymbol{\uprho})c(\boldsymbol{\uptheta}) & s(\boldsymbol{\uprho})s(\boldsymbol{\uptheta})s(\boldsymbol{\uptheta}) + c(\boldsymbol{\uprho})c(\boldsymbol{\uprho}) & c(\boldsymbol{\uprho})s(\boldsymbol{\uprho})s(\boldsymbol{\uptheta}) - c(\boldsymbol{\uprho})s(\boldsymbol{\uprho})\\-s(\boldsymbol{\uptheta}) & c(\boldsymbol{\uptheta})s(\boldsymbol{\uptheta}) & c(\boldsymbol{\uprho})c(\boldsymbol{\uptheta})
\end{bmatrix} \begin{bmatrix}
\mathcal{X}^b\\\mathcal{Y}^b\\\mathcal{Z}^b
\end{bmatrix} \tag{1}
$$

where *X*˙ *G*, *Y*˙ *G*, *Z*˙ *G* velocities (m/s) (*X*,*Y*, *Z*) in global frame, *φ*, *θ*, *ψ* (roll, pitch, yaw angles), (rad), and *x*˙ *b*, *y*˙ *b*, *z*˙ *b* velocities (*X*,*Y*, *Z*) in the body frame [38,39]. The equations of motion of the quadcopter consist of two main components, dynamic and kinematic. Dynamic components explain the motion of the quadcopter according to Newton's second laws, while kinematic components explain the quadcopter's transformation equations. The rotational kinematics of the quadcopter describe the relationship between the angular rate and Euler angles. According to this rotation kinematics, since the angular rate is given in the body frame and the Euler angles are given in the frame, the relation between each other is obtained as:

$$
\begin{bmatrix}
\dot{\phi} \\
\dot{\theta} \\
\dot{\psi}
\end{bmatrix} = \begin{bmatrix}
1 & s(\phi)t(\theta) & c(\phi)t(\theta) \\
0 & c(\phi) & -s(\phi) \\
0 & \frac{s(\phi)}{c(\theta)} & \frac{c(\phi)}{c(\theta)}
\end{bmatrix} \begin{bmatrix}
p \\ q \\ r
\end{bmatrix} \tag{2}
$$

by using the transformation matrix, where *p*, *q*, *r* roll, pitch, yaw rates (rad/s) and *φ*˙, ˙ *θ*, *ψ*˙, and (rad/s) time derivatives of Euler angles, respectively [38,40]. Translational *s* describes the linear motion of all forces acting on the quadcopter during flight according to the

coordinate frame. Equations of motion resulting from translational *s* of the quadcopter are obtained as in Equation (3), according to Newton's second law:

$$\begin{bmatrix} \ddot{X}^G\\ \ddot{Y}^G\\ Z^G \end{bmatrix} = \begin{bmatrix} \frac{1}{m}(-[c(\phi)c(\psi)s(\theta) + s(\phi)s(\psi)]u\_1 - K\_{dx}\dot{X}^G) \\\ \frac{1}{m}(-[c(\phi)s(\psi)s(\theta) - c(\psi)s(\phi)]u\_1 - K\_{dy}\dot{Y}^G) \\\ \frac{1}{m}(-[c(\phi)c(\theta)]u\_1 - K\_{dz}\dot{Z}^G) + \mathcal{g} \end{bmatrix} \tag{3}$$

where *X*¨ *G*, *Y*¨ *G*, *Z*¨ *G* accelerations (m/s2) *X*,*Y*, *Z* in the coordinates, *m* mass of quadcopter (kg), *Kdx*, *Kdy*, *Kdz* drag coefficients, *X*˙ *G*, *Y*˙ *G*, *Z*˙ *G* velocities *X*,*Y*, *Z* in the coordinates, and *u*1 is total thrust of all motors, respectively [41]. The rotational *s* of quadcopter describes the relationship:

$$
\begin{bmatrix}
\dot{\Phi} \\
\dot{\theta} \\
\dot{\Psi}
\end{bmatrix} = \begin{bmatrix}
\frac{\left[\left(f\_y - f\_z\right)qr - f\_r q (w\_1 - w\_2 + w\_3 - w\_4) + lK\_T u\_2\right]}{f\_x} \\
\frac{\left[\left(f\_z - f\_x\right)pr + f\_r p (w\_1 - w\_2 + w\_3 - w\_4) + lK\_T u\_3\right]}{f\_y} \\
\frac{\left[\left(f\_x - f\_y\right)pq + K\_d u\_4\right]}{f\_z}
\end{bmatrix} \tag{4}
$$

between the second derivatives of Euler angles (*φ*¨, ¨ *θ*, *ψ*¨) (rad/s2) on each axis depending on the square of its motor speeds (*w*1, *w*2, *w*3, *w*4) (rad/s), namely, torques, and *Jx*, *Jy*, and *Jz* (kg m2) quadcopter moments of ineartia on each axis. *u*2 refers roll control input, *u*3 describes pitch control input, *u*4 indicates yaw control input, *KT* is the thrust coefficient, *Kd* is the drag torque proportionality constant, and *l* is the arm length of quadcopter (m) as in Equation (4) [38,41]. The quadcopter moments of inertia on each axis and mass of quadcopter are expressed [41]:

$$I\_X = I\_Y = \frac{2(m\_c + m\_l)R^2}{5} + 2l^2 m\_{ll} \qquad I\_{\overline{z}} = \frac{2(m\_c + m\_l)R^2}{5} + 4l^2 m\_{ll} \qquad m = 4m\_{ll} + m\_c + m\_l \tag{5}$$

where *mc* is the center mass of quadcopter (kg), *R* is the radius of center mass (m), *mm* is the motor mass (kg), and *ml* is the payload mass (kg). In this study, the total mass in the system model of the quadcopter is changed during the payload hold–release mission depending on the weight of the payload carried, and the moment of inertia in each axis is directly related to this mass change. To summarize, the dynamic and kinematic model of the quadcopter with six degrees of freedom is represented as Equations (1)–(4). The relationship between motor speeds and control variables is defined as:

$$
\begin{bmatrix} u\_1 \\ u\_2 \\ u\_3 \\ u\_4 \end{bmatrix} = \begin{bmatrix} K\_T & K\_T & K\_T & K\_T \\ 0 & -lK\_T & 0 & lK\_T \\ lK\_T & 0 & -lK\_T & 0 \\ K\_d & -K\_d & K\_d & -K\_d \end{bmatrix} \begin{bmatrix} w\_1^2 \\ w\_2^2 \\ w\_3^2 \\ w\_4^2 \end{bmatrix} \tag{6}
$$

Note that the control variables are directly proportional to the squares of the motor speeds.


Meet the editor

Dr. Vassil Mihaylov received his medical degree from the Medical University, Sofia, Bulgaria, and completed his general surgery residency at the Military Medical Academy, Sofia. He completed a transplant fellowship at Queen Elizabeth Hospital, Birmingham, UK, after which he returned to Sofia as a part of the HPB and transplant team at the Military Medical Academy. Dr. Mihaylov obtained his Ph.D. with a thesis on the surgical

aspects of liver transplantation. He was appointed Associate Professor of Surgery in 2017. He has published more than 100 papers in peer-reviewed journals and has given many scientific presentations at international meetings. He is a Fellow of the American College of Surgeons and a member of several professional organizations including the Bulgarian Transplant Society, the European Society for Organ Transplantation (ESOT), International Liver Transplantation Society (ILTS), Society of Laparoscopic Surgeons, and International Hepato-Pancreato-Biliary Association (IHPBA). His clinical and research interests include hepatobiliary and pancreatic surgery, minimally invasive surgery, and liver transplantation.


**Author details**

Alina Porfire\*, Marcela Achim, Cristina Barbalata, Iulia Rus, Ioan Tomuta and Cecilia Cristea Faculty of Pharmacy, University of Medicine and Pharmacy "Iuliu Hatieganu", Cluj Napoca, Romania

\*Address all correspondence to<EMAIL_ADDRESS>
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**85**

*Pharmaceutical Development of Liposomes Using the QbD Approach*

2009. Available from: https://www.ich. org/fileadmin/Public\_Web\_Site/ICH\_ Products/Guidelines/Quality/Q8\_R1/ Step4/Q8\_R2\_Guideline.pdf [Accessed

[10] Raman NVVSS, Mallu UR, Bapatu HR. Analytical quality by design approach to test method development and validation in drug substance manufacturing. Journal of Chemistry.

[11] Pramod K, Tahir MA, Charoo NA, Ansari SH, Ali J. Pharmaceutical product development: A quality by design approach. International Journal of Pharmaceutical Investigation.

[12] Chowdary KPR, Shankar KR, Kumar PS. Recent research on QbD approach in formulation development – A review. International Journal of Computer Science Trends and Technology. 2014;**4**(1):282-292

[13] Yu LX. Pharmaceutical quality by design: Product and process development, understanding, and control. Pharmaceutical Research.

[14] Sylvester B, Porfire A, Muntean DM, Vlase L, Banciu M, Luput L, et al. Optimzation of prednisoloneloaded long-circulating liposomes via application of Quality by Design (QbD) approach. Journal of Liposome

SS. Application of quality by design approach to optimize process and formulation parameters of rizatriptan loaded chitosan nanoparticles. Journal of Advanced Pharmaceutical Technology

[16] De Beer TR, Wiggenhorn M, Hawe A, Kasper JC, Almeida A, Quinten T,

Research. 2018;**28**(1):49-61

[15] Shirsat AE, Chitlange

& Research. 2015;**6**(3):88-96

01-2019]

2015;**2015**:1-8

2016;**6**(3):129-138

2008;**25**(4):781-791

*DOI: http://dx.doi.org/10.5772/intechopen.85374*

[1] Rayaprolu BM. Quality by design: A brief introduction. Journal of Pharmacovigilance. 2015;**3**(4):e142

[2] Tefas LR, Rus LM, Achim M, Vlase L, Tomuta I. Application of the quality by design concept in the development of quercetin-loaded polymeric nanoparticles. Farmácia.

[3] Sangshetti JN, Deshpande M, Zaheer Z, Shinde DB, Arote R. Quality by design approach: Regulatory need. Arabian Journal of Chemistry.

[4] Lionberger RA, Lee SL, Lee L, Raw A, Yu LX. Quality by design: Concepts for ANDAs. The AAPS Journal.

[5] Yu LX, Amidon G, Khan MA, Hoag SW, Polli J, Raju GK, et al. Understanding pharmaceutical quality by design. The AAPS Journal. 2014;**16**(4):771-783

[6] The International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) Guidelines [Internet]. 2019. Available from: https:// www.ich.org/products/guidelines/ quality/article/quality-guidelines.html

[7] FDA Guidances [Internet]. 2019. Available from: https://www.fda.gov/ Drugs/GuidanceComplianceRegulatory Information/Guidances%20/default.htm

[8] EMA Reflection Paper: Chemical, pharmaceutical and biological

information to be included in dossiers when Process Analytical Technology (PAT) is employed [Internet]. 2006. Available from: http://www.ema.europa. eu/docs/en\_GB/document%20\_library/ Other/2009/10/WC500004890.pdf

[9] ICH harmonised tripartite guideline. Pharmaceutical development Q8(R2).

2018;**66**(5):798-810

2017;**10**:S3412-S3425

2008;**10**(2):268-276

**References**

*Pharmaceutical Development of Liposomes Using the QbD Approach DOI: http://dx.doi.org/10.5772/intechopen.85374*


*3.4. Pathogenesis and Immunological Implications*

The exact mechanism that triggers HS is not fully understood. Feasibly, the formation of immune complexes is strongly suspected (Gell and Coombs type III reaction) and the cellmediated reaction (Gell and Coombs type IV reaction) may contribute to the development of this challenging disease.

Some cases showed positive skin tests [3,7], high serum total IgE levels [7], high milk-specific IgE antibodies [13,14] or circulating immune complexes [3]. A significant increase of histamine and ECP in BAL several hours after a milk OFC was reported [10]. In one case report, deposits of immunoglobulins, complement, fibrin and milk protein antigens diffusely scattered were described on immunofluorescence studies of lung tissue biopsies [2]. It is probable that a cause concurring to HS is the aspiration of milk, in particular among patients with an uncoordinated swallowing mechanism, tracheal/esophageal anomaly or gastroesophageal reflux. However, in the paper of Boat et al., this condition was ruled out [7]. Concerning the data on delayed immunity, in some cases a delayed skin test response to intradermal test [3,7] or a lymphocyte response [8] was reported. Other authors postulated that milk antigens might trigger an immune complex reaction resulting in multiorgan abnormalities, such as pulmonary, gastrointestinal and renal ones. In fact, pulmonary and gastrointestinal signs and symptoms were frequently associated [11] and granular immuno-deposits have been demonstrated along the glomerular basement membrane in a child with crescentic glomerulonephritis and PH [13].

Most studies, mainly the oldest ones, characteristically found high titers of precipitins (likely immunoglobulins of class G) against bovine milk proteins in the patients' sera, by using the Ouchterlony double-immunodiffusion technique [4,5,7,11]. However, it is not sufficiently explicable why some children develop precipitating antibodies to ingested protein and other children do not. Moreover, it is not known if these precipitins play a causative role in the disease. Children with precipitins usually have an increased incidence of recurrent respiratory tract diseases, anemia and hepatosplenomegalia. However, precipitating IgG antibodies to milk are not pathognomonic of the disease, since they have been found among around 1% of healthy children in the absence of clinical manifestations [4], and in 4% to 6% of children with chronic disorders, including celiac disease, cystic fibrosis, IgA deficiency, Down's syndrome, Wisckott–Aldrich syndrome and Hurler's syndrome [7]. Additionally, methods used for detecting precipitins are obsolete and reports on them are timeworn. Therefore, the role of these antibodies should be critically considered.

In other more recent reports, high values of specific IgG to milk proteins were found using an immunoenzimatic technique [14]. In one case report [15] CM IgG4 was found to be elevated. Even in these cases, the role of this specific CM IgG is not clear.