*Article* **Adsorptive Recovery of Cd(II) Ions with the Use of Post-Production Waste Generated in the Brewing Industry**

**Tomasz Kalak 1,\*, Jakub Walczak 1 and Malgorzata Ulewicz 2**

**Abstract:** Post-production waste generated in the brewing industry was used to analyze the possibility of Cd(II) ion recovery in biosorption processes. Brewer's grains (BG), which are waste products from beer manufacturing processes, are a promising material that can be reused for biosorption. The biomass contains appropriate functional groups from fats, proteins, raw fibers, amino acids, carbohydrates and starch, showing a strong affinity for binding metal ions and their removal from wastewater. The biosorbent material was characterized by several research methods, such as particle size distribution, elemental composition and mapping using SEM-EDX analysis, specific surface area and pore volume (BET, BJH), thermogravimetry, electrokinetic zeta potential, SEM morphology and FT-IR spectrometry. Initial and equilibrium pH, adsorbent dosage, initial metal concentration and contact time were parameters examined in the research. The highest biosorption efficiency was obtained at a level of 93.9%. Kinetics analysis of the processes and sorption isotherms were also carried out. Based on the conducted experiments, it was found that this material has binding properties in relation to Cd(II) ions and can be used for wastewater treatment purposes, being a low-cost biosorbent. This research studies are in line with current global trends of circular and sustainable economies.

**Keywords:** water quality; brewing industry; residual brewer's grains; biosorption; Cd(II) ions; kinetics

#### **1. Introduction**

Cadmium (Cd) is one of the most toxic and harmful metals. It exists naturally in water, air, soils and foodstuffs. The most common forms of the metal are pure cadmium oxide (87.5% Cd), cadmium sulfide (77.6% Cd), otavite, cadmium carbonate (61.5% Cd) and greenockite (CdS). Cadmium is released into the environment as a result of many industrial processes, such as production of alloy, cadmium-nickel batteries, pesticides, fertilizers, plastics, pigments and dyes, smelting, electroplating, mining, refining and textile operations [1–3]. Cadmium has toxic properties in relation to living organisms and is a problem for the environment and food chain when it is present in excess amounts. The World Health Organisation recommends the maximum contaminant level (MCL) of Cd(II) in drinking water at up to 0.005 mg/L [4].

The brewing industry occupies a strategic economic position in the global economy and is open to technological development and scientific progress. In 2019, global beer production amounted to approximately 1.91 billion hectoliters [4]. The following countries are the leading producers in the world (in million liters in 2017): China (39.788), USA (21.775), Brazil (14.000), Mexico (11.000), Germany (9.301), Russia (7.440), Japan (5.247), Great Britain (4.405), Vietnam (4.375), Poland (4.050). Beer is ranked fifth in the ranking of the most consumed beverages in the world after tea, carbonated drinks, milk and coffee, with an average consumption of 23 L per person per year [5–7].

**Citation:** Kalak, T.; Walczak, J.; Ulewicz, M. Adsorptive Recovery of Cd(II) Ions with the Use of Post-Production Waste Generated in the Brewing Industry. *Energies* **2021**, *14*, 5543. https://doi.org/10.3390/ en14175543

Academic Editor: Gabriele Di Giacomo

Received: 30 June 2021 Accepted: 1 September 2021 Published: 5 September 2021

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**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/).

Increasing production in the brewing industry also brings with it millions of tons of waste, which is both an ecological and economic problem. Huge amounts of brewer's grains (BG) each year lead to environmental degradation, but also to the loss of valuable material that could be used as animal feed, fuel, raw material for methane production and many other purposes. Due to its valuable composition, one of the directions of reuse may be the extraction of ingredients such as amino acids, fats, proteins, polysaccharides, fiber, enzymes, vitamins, flavor compounds or phytochemicals, which can be reused as functional ingredients in food or pharmacological production [8]. Another alternative solution may be the use of brewer's grains (BG) to treat wastewater from heavy metals and other toxic pollutants through adsorption processes. Due to the presence of many functional groups (e.g., carboxyl, amide, amine, etc.), it is possible to bind metal ions in chemical and physical reactions. The biosorption process is one of the most promising and alternative techniques because of its low operating costs, mainly due to the use of cheap biowaste, the ease of obtaining materials, minimizing significant amounts of waste and high efficiency in removing metal ions from wastewater. The costs of water treatment by adsorption are difficult to determine as they depend on many factors. However, it can be assumed that they are in the range of about USD 5–200 per m3 and are significantly lower compared to other commercial methods of metal removal from wastewater of about 10– USD 450 per m3) [9]. In 2018, beer production in Poland amounted to approx. 40.93 million hectoliters [10]. Assuming that per 100 hectoliters of beer produced, there is an average of approx. 15–19 kg of the by-product in the form of brewer's grains (BG) with a dry matter content in the range of 35–40% [11], it can be estimated that in 2018, from 6.139 to 7776.7 tons of this waste were generated in Poland. Such amounts can be considered significant, which offers the possibility of their potential use in biosorptive processes for removing metals from wastewater.

The aim of this study was to determine the efficiency of the biosorption process of post-production waste in the form of brewer's grains in relation to Cd(II) ions. In addition, the purpose was also to characterize the physicochemical properties of the biosorbent using selected analytical methods as well as to study sorption kinetics, equilibrium and isotherms.

#### **2. Experimental Procedure**

#### *2.1. Materials and Methods*

2.1.1. Brewer's Grains Preparation

In this study, samples of the brewer's grains (BG) (Figure 1) were obtained as a result of a beer production process in the brewing industry in a Polish factory (Greater Poland voivodeship). The brewing effluents collected from the production process were wet biomass; therefore, they were dried in a laboratory dryer (BINDER Gmbh, Tuttlingen, Germany) at a temperature of 60 ◦C to constant weight and then placed in polyethylene containers and cooled in a desiccator. Then, the dried BG were ground and screened through a sieve in order to select grains with a diameter less than 0.212 mm. The samples prepared in this way were used for analyses. All chemical compounds used in the study were pure for analysis. Distilled water was used in the research.

#### 2.1.2. Brewer's Grains Characterization

Particles of brewer's grains (BG) with a diameter less than 0.212 mm were used in these studies. At the beginning, the physicochemical properties of the biomass were analyzed with the use of selected analytical techniques such as particle size distribution, elemental composition and mapping using SEM-EDS analysis, electrokinetic zeta potential, specific surface area and pore diameter (BET adsorption isotherms), pore volume and pore volume distribution (BJH), thermogravimetry (TGA, DTG), SEM morphology, and FT-IR analysis. The detailed description of the methods are presented as supplementary material (SM ethods).

**Figure 1.** Sample of brewer's grains.

#### 2.1.3. Biosorption Process of Cd(II) ions

The efficiency of the Cd(II) biosorption process on brewer's grains (BG) was determined by shaking at room temperature (23 ± 1 ◦C). A standard solution (Cd2+, 1 g/L, Sigma-Aldrich) was used in the experiments. Biosorbent samples (1–50 g/L) and a Cd(II) stock solution at a concentration of 10 mg/L (volume of the solutions V = 20 mL) and at an initial pH 2–5 were placed in conical flasks and shaken at rotational speed 200 rpm for 60 min until equilibrium was reached. NaOH and HCl solutions (both 0.1 M) were used to adjust the pH of the Cd(II) stock solutions. After the biosorption processes, the solutions were centrifuged for 15 min (4000 rpm) for phase separation. The concentration of cadmium(II) ions in the solutions after biosorption was analyzed using an atomic absorption spectrophotometer SpectrAA 800 (F-AAS, at a wavelength λ = 228.8 nm for cadmium, Varian, Palo Alto, USA). The measurements were repeated three times at room temperature (23 ± 1 ◦C) and normal pressure and the results are presented as mean values of all measurements. Measurement errors were calculated, and confidence intervals are presented in tables and figures. The biosorption efficiency A (%) and the biosorption capacity *qe* (mg/g) were calculated on the basis of Equations (1) and (2):

$$A = \left[\frac{\text{C}\_0 - \text{C}\_\text{f}}{\text{C}\_0}\right] \times 100\% \tag{1}$$

$$q\_{\mathfrak{c}} = \frac{(\mathbb{C}\_0 - \mathbb{C}\_{\mathfrak{c}}) \times V}{\mathbf{m}} \tag{2}$$

where: *C*0 and *Ce* (mg/L) are initial and equilibrium metal ion concentrations, respectively, and *V* (L) is the volume of solution and m (g)—mass of BG.

Kinetics and isotherms were studied using pseudo-first-order and pseudo-secondorder, Langmuir and Freundlich models according to Equations (3)–(6).

$$q\_t = q\_t \left(1 - e^{k\_1 t}\right) \tag{3}$$

$$q\_l = \frac{q\_e^2 k\_2 t}{1 + q\_e k\_2 t} \tag{4}$$

$$q\_{\mathfrak{c}} = \frac{q\_{\max} \mathcal{K}\_L \mathcal{C}\_{\mathfrak{c}}}{1 + K\_L \mathcal{C}\_{\mathfrak{c}}} \tag{5}$$

$$q\_c = K\_F \mathbb{C}\_c^{\frac{1}{\pi}} \tag{6}$$

where *qt* (mg/g) is the amount of Cd(II) ions adsorbed at any time *t* (min.), *qe* (mg/g) is the maximum amount of Cd(II) ions adsorbed per mass of BG at equilibrium, *k*1 (1/min) is the rate constant of pseudo-first-order adsorption, *k*2 (g/(mg·min.)) is the rate constant of pseudo-second-order adsorption, *qmax* (mg/g) is the maximum adsorption capacity, *KL* is the Langmuir constant, *Ce* (mg/L) is the equilibrium concentration after the adsorption process, *KF* is the Freundlich constant and 1/n is the intensity of adsorption.

#### **3. Results and Discussion**

#### *3.1. The Process of Beer Production and Post-Production Brewer's Grains*

Four main raw materials are used in the production of beer: water, barley malt, hops and yeast. Water is necessary primarily for the malting of barley, mashing of malt and production of malt wort, but also for cooling processes, in the boiler room and for washing equipment and packaging. The quality of water in brewing should be appropriate, hence it is subjected to treatment processes. Water quality requirements are specified in the Regulation of the Minister of Health on the quality of water intended for human consumption [12]. The general scheme of beer production is shown in Figure 2. Initially, wort is produced in many different processes. First, malt is ground mechanically to allow enzymes to break down insoluble substances into soluble compounds. The next and most important stage of wort production is mashing, in which insoluble malt components (cellulose, starch) at an optimal temperature and under influence of enzymes pass into the aqueous solution, creating a soluble extract in the form of minerals, proteins and sugars. Then, in the wort filtration process, solids are separated into so-called brewing waste or spent grains. During the boiling of wort, resins and essential oils are released from hops, protein-tannic compounds are precipitated, enzymes are decomposed and the wort becomes acidic and colored. Hops and hot sludge in the settling vat are removed from the hop wort followed by cooling and aeration. In the fermentation process, sugars are converted by yeast enzymes into C2H5OH and CO2. In the last stage of fermentation, maturing and conditioning at low temperatures take place, in which yeast cells draw energy from the decomposition of spare substances still present in the wort. The yeast flocculates and sinks to the bottom of the tank. At this stage, the taste of the beer is finally achieved. At the end, beer is filtered to remove particles that could cause turbidity. The finished beer is poured into bottles or cans and sold [8].

The amount of brewer's grains produced is difficult to estimate. Producers keep their data secret because for the protection of intellectual property and the fear of inspection by organizations responsible for waste management. On the other hand, production processes are varied, and depend on technologies used and the size of brewery. Based on the literature, it is assumed that an average of 15–19 kg of by-product in the form of brewer's grains with a dry matter content of approx. 35–40% is produced per 100 hl of produced beer [11,13].

#### *3.2. Characterization of the Biosorbent*

Pursuant to the Act on Waste and the Regulation of the Polish Minister of Environment on waste recovery outside of installations and devices, the brewer's grains are classified as waste with the code 02 07 80 and are defined as bagasse, post fermentation must sediments (trub) and decoctions [14,15]. The brewing waste is brown or light brown in color and has a characteristic grain or bread aroma. Moist biomass contains approx. 18–23% of dry matter consisting mainly of barley, barley malt, wheat, rice, maize and hops. The most important nutrients are proteins, fats, crude fibers, crude ash, starch, carbohydrates, phosphorus, calcium, lysine, methionine, potassium, sodium, magnesium, B vitamins and others depending on the origin of the biomass. It is estimated that the total amount of nutrients is approx. 66% of dry matter, starch and sugars-approx. 14% of dry matter and protein substances-approx. 26% of dry matter. According to the Regulation of the Polish Minister of Environment on waste recovery outside of installations and devices, the method of recovering brewer's grains (BG) as waste is the R14 recovery process, stating that the waste can be used for 'feeding animals-in accordance with the rules of feeding individual species' [16]. Feeding livestock with BG waste has many benefits, such as elimination of costs related to storage and disposal, nutritional components having a good effect on health and condition of animals, as well as lower costs of feeding animals compared to the

use of commercial feed. There are also other applications, such as use as a raw material for the production of various feeds, the use of modified BG for the production of edible mushrooms, the use as biomass for gasification processes in the production of methane in biogas plants and the use as an additive to fossil fuels in power plants as well as processing into solid fuel (briquettes, pellets) [13]. Another alternative application of post-production BG may be the removal of heavy metals from aqueous solutions in biosorption processes.

**Figure 2.** General scheme of beer production.

In preliminary experiments, BG waste was characterized in terms of physical and chemical properties using a number of methods. The analysis of particle size distribution using a laser diffraction method showed the presence of one peak with a particle size of 67.8 nm (Figure S1). The strength of the material and the rate of hydration are properties that depend on particle size distribution. Moreover, this parameter serves as an important indicator of particle quality and performance in many different processes. It should be noted that the laser diffraction method has limitations, in that only particles capable of being suspended in solution can be analyzed. Due to the complexity and nature of the BG mixture, the larger and heavier particles fall to the bottom of the solution. Smaller particles are able to dissolve faster in solution and form a more stable suspension compared to larger ones. Therefore, it was possible to measure only particles suspended in the solution. Based on the results, it can be stated that the BG particles are not homogeneous. In accordance with the literature data, as the biosorbent particle size decreased, the surface area of particles and the number of active sites on the sorbent increased, which resulted

in an increase in cadmium ion removal efficiency [17,18]. Hence, it is suggested to use biosorbent fractions with the smallest particle size in experiments.

Elemental analysis was carried out using the SEM-EDS method and the results are presented in Figure 3 and Table 1. The biomass mainly contained elements, such as: O, C, P, Mg, Si, Ca, K, Na, S and oxides: CO2, P2O5, MgO, SiO2, CaO, K2O, Na2O, SO3. The content of oxides was not the result of direct measurements, but the result of stoichiometric calculations based on the estimation in the EDS microanalyzer. The measurement in this method was carried out pointwise on the surface of the biomass sample. Therefore, the content of elements and their amounts may slightly differ in different places of the sample due to complexity of the BG mixture and its inhomogeneity.

**Figure 3.** The EDS spectrum of brewer's grains.

**Table 1.** Elemental composition of BG waste (EDS microanalyser).

As a result of BET analysis, specific surface area (4.218 m2/g), pore volume (0.0046 cm3/g) and pore diameter (4.386 nm) were determined (Figures 4–6). The BET adsorption isotherm, which depends on the pore size and the intensity of adsorbent-adsorbate interaction, slightly resembles the type III isotherm in that it is slightly convex towards the pressure axis. The isotherm indicates cooperative adsorption, which indicates that previously adsorbed molecules can contribute to increasing the adsorption efficiency of other free molecules. On the other hand, low adsorption efficiency may occur in the case of low relative pressure and weak interaction at the adsorbent-adsorbate interface. In any case, the isotherm becomes convex into the pressure axis in a situation where, after a single adsorption, the interaction between adsorbent and adsorbate promotes adsorption of further metal ions [19].

**Figure 4.** Linear form of the BET adsorption and desorption isotherm of brewer's grains.

**Figure 5.** Pore size distribution in brewer's grains during adsorption.

**Figure 6.** Pore size distribution in brewer's grains during desorption.

Thermogravimetric analysis was carried out at a temperature ranging from 29 to 600 ◦C (Figure 7). As a result of these measurements, a gradual loss of weight of the biomass material with increasing temperature was observed. The first phase of decomposition of the BG pomace occurred in the temperature range from 30 to 120 ◦C, and the second in

the range of about 180–500 ◦C, as shown by the TGA and DTG curves. In the first stage there was a slight decrease in weight equal to approx. 1.8%, which may have resulted from evaporation of adsorbed water from the pomace sample. In the next phase, greater weight loss (about 38%) was observed, which was a consequence of the pyrolysis process and the evaporation of most of the volatile substances (e.g., CO2). At a temperature of 355 ◦C, the spectrum shows a very intense peak (DTG) that relates to the breakdown of lipids, carbohydrates and proteins. Similar research results have been published in the literature [20,21].

**Figure 7.** Thermogravimetric curves of brewer's grains.

The analysis of electrokinetic zeta potential was carried out by combining electrophoresis and measuring the velocity of particles in liquid with red laser (λ = 633 nm) based on the Doppler effect (LDV). Determining this parameter is important for many industrial activities, including wastewater treatment in terms of environmental protection. It makes it possible to determine the influence of surface charge on the metal ion sorption process and to estimate its efficiency [22]. According to the literature, aqueous slurry is more stable at higher zeta potential values, as electrostatically charged particles repel each other, overcoming the tendency to aggregate and agglomerate [23,24]. These studies showed that pH values had an influence on the surface charge, which gradually decreased from −3.01 mV (pH 2.01) to −32.1 mV (pH 6.93) (Figure 8). The isoelectric point (IEP) is set at the zero point (no electrical charge) and represents the equilibrium balance between negative and positive ions (the system is the least stable). At the IEP point, the particles of the suspension are characterized by the lowest solubility, viscosity and osmotic pressure. In this analysis, the electrostatic charge did not reach the isoelectric point (IEP) and took only negative values below the IEP. This means that there is an advantage of positively charged particles over negatively charged ones on the biomass surface in the pH range from 2.0 to 7.0.

Brewer's grains were analyzed using SEM morphology and images are shown in Figure 9A,B. Generally speaking, the brewer's grains consist of particles of varying size, including larger, elongated particles and smaller spherical particles placed between them. Shape irregularities occur with both smaller and larger particles. The structure is not homogeneous, and developed flat surfaces are visible. Some particles have a fibrous structure, suggesting they are derived from the same barley grains or embryonic tissue. The remaining particles are covered with small spherical projections that represent the characteristic formations on the outer cells of the seed coat. The visible spherical material is less cohesive, has a looser structure, and the particles are not densely packed. Such particles have uneven surfaces and edges and can chip off easily. A similar structure to this biomass was described by other authors [25–30].

**Figure 8.** The change in zeta potential with equilibrium pH.

**Figure 9.** SEM images of BG residues: (**A**) magn.: ×10,000, scale bar: 2 μm, (**B**) ×10,000, 2 μm.

#### *3.3. FT-IR Analysis*

FT-IR measurements of brewer's grain residues before and after the sorption process were carried out and Figure 10 shows the spectra. The following experimental conditions were used to examine the samples: BG dose 25 g/L, initial concentration of Cd(II) ions 10 mg/L, initial pH 4, T = 23 ± 1 ◦C, contact time 60 min. The description of the FT-IR peaks is presented in Table 2. The spectra were analysed before and after the biosorption process in terms of differences in shape, intensity of bands, frequency and possible interaction of functional groups with Cd(II) ions. Figure 10 shows that after the sorption process, the intensity of the bands shifted towards lower transmittance values, and their location remained at the same wavelengths or was slightly shifted. These changes can be identified as follows: 3284.83 (shift to 3289.81 cm−1), 2923.72 (shift to 2919.92 cm−1), 2852.02 (shift to 2851.84 cm−1), 1643.84 (shift to 1633.69 cm−1), 1515.88 (shift to 1539.91 cm−1), 1428.13 (shift to 1427.82 cm−1), 1032.92 (shift to 1032.7 cm−1). The occurring phenomena of peak shifts can be explained by the interaction and bonding of Cd(II) ions with functional groups of compounds present in BG residues, probably due to the process of ion exchange. This shifting could occur due to the formation of a bond between Cd2+ with oxygen and nitrogen.

During the biosorption process with BG sorbent, the mechanism of ion exchange between Cd(II) ions and various metal cations (e.g., Mg2+, Ca2+) probably took place. The presence of various metal cations was determined by the SEM-EDS method (Table 2). It is assumed that ion exchange between protons or ionizable cations on the surface of the biosorbent participated in binding of Cd(II) ions at initial pH 4.0.

**Figure 10.** FT-IR spectrum of BG residues before and after Cd(II) ions adsorption.

Zeta potential studies showed that BG particles contained a lot of negative charges on their surface (Figure 8). It is therefore possible to assume that electrostatic attraction could also occur during the biosorption of Cd(II) ions. In these studies, at initial pH 4.0, the BG sorbent had the highest sorption capacity, where zeta potential ranging from −16 to −17 mV was observed. This phenomenon could have been a consequence of the presence of a greater number of localized electrons, which could attract more Cd(II) ions on the BG surface [31].

According to this research, the probable mechanism of Cd(II) biosorption can be explained by ion exchange reactions based on the Equations (7)–(9):

$$\text{Cd}^{2+} + \text{BG}\_{\text{m}} - \text{Ca} \rightarrow \text{BG}\_{\text{m}} - \text{Cd} + \text{Ca}^{2+} \tag{7}$$

$$\rm{Cd}^{2+} + \rm{BG}\_{\rm{m}} - \rm{Mg} \rightarrow \rm{BG}\_{\rm{m}} - \rm{Cd} + \rm{Mg}^{2+} \tag{8}$$

$$\text{Cd}^{2+} + \text{BG}\_{\text{m}} - \text{H} + \text{H}\_{2}\text{O} \rightarrow \text{BG}\_{\text{m}} - \text{Cd}^{+} + \text{H}\_{3}\text{O}^{+} \tag{9}$$

where: BGm is the brewer's grains residues matrix.

It should be mentioned that high sorption efficiency of Cd(II) ions at the level of at least 90% was observed in this study under various experimental conditions. Chemical properties of cadmium, which facilitate the attraction of the sorbent to active sites, promote satisfactory results. Additionally, the electronegativity constant (Pauling's) of Cd(II) is 1.69, which is responsible for electrostatic attraction [32]. High sorption capacity of Cd(II) may be also influenced by such factors as first hydrolysis constant p*KH* 10.1 (negative logarithm of hydrolysis constant), ionic radius of Cd2+ (1.54 Å) and ionization energy (207 kcal/g/mol). These parameters favor easier sorption on the brewer's grains surface [33].

**Table 2.** FT-IR peaks of BG residues.

#### *3.4. Biosorption Process of Cd(II) Ions*

#### 3.4.1. Impact of Adsorbent Dosage

The impact of BG dosage on biosorption efficiency of Cd(II) ions was investigated and Figure 11 shows the results. The ffollowing conditions were used during the research: initial concentration of Cd(II) ions 10.0 mg/L, pH 2–5, agitation speed 200 rpm, T = 23 ± 1 ◦C, contact time 60 min. In general, increasing sorbent dosage up to 50 g/L increased the biosorption efficiency. The results clearly show that maximum sorption was reported at a dose of 10 g/L (93.9%, initial pH 4) and the lowest efficiency was reported at the initial pH 2. Further increasing the BG dose was not necessary as no further improvement was observed, and optimization of the process was achieved. Besides, biosorption capacity decreased from 8.4 mg/g (dose 1 g/L, pH 5) to 0.04 mg/g (dose 50 g/L, pH 2) (Figure 11B). The parameter was the highest at the smallest doses (1 g/L), while Cd(II) removal (%) was the lowest, which means that the saturation value was reached. This phenomenon can be attributed to the increased surface area of the BG biosorbent, availability of more sorption sites, and it may result from completed interactions of some factors. The decrease in biosorption capacity in the subsequent reactions can be attributed to aggregation or overlapping of active sites, resulting in a reduction in the total surface area of the biosorbent. The results obtained in this way can be explained by the fact that biosorption capacity depends on both the concentration of adsorbed Cd(II) ions and biomass dosage [36,37].

**Figure 11.** The impact of BG dosage on biosorption efficiency (**A**) and biosorption capacity (**B**) of Cd(II) ions.

#### 3.4.2. Impact of Initial Concentration of Cd(II)

Biosorption behavior under different initial concentration of Cd(II) ions was studied and the results are presented in Figure 12. After analyzing previous research results, it was decided to apply the following experimental conditions: initial concentration of Cd(II) ions (2.5–50 mg/L), adsorbent dosages 1–50 g/L, initial pH 4, contact time 60 min, rotation speed 200 rpm, T = 23 ± 1 ◦C. An increase in adsorption efficiency was observed in the case of initial concentrations of 1 mg/L and 50 mg/L. In other cases, there was no upward

or downward trend, and the results remained stable. The highest removal equal to 93.43% was achieved for the biomass dosage of 10 g/L and initial concentration of 10 mg/L.

**Figure 12.** The impact of initial concentration of Cd(II) ions on BG residues removal efficiency.

#### 3.4.3. Analysis of pH Profile

The influence of initial pH on the efficiency of the process was investigated and the results are presented in Figure 13. The applied conditions of the experiments were as follows: initial concentration of Cd(II) ions 10.0 mg/L, BG dosage 1.0–50 g/L, pH range of 2–5, contact time 60 min, rotation speed 200 rpm, T = 23 ± 1 ◦C. After analysis of the results, it was found that the best performance was obtained at pH 4.0. Maximum sorption was observed for the following biosorbent doses: 1 g/L (84.2%, initial pH 5), 2 g/L (89.99%, pH 4), 3 g/L (91.35%, pH 4), 4 g/L (90.86%, pH 5), 5 g/L (92.62%, pH 4), 10 g/L (93.87%, pH 4), 15 g/L (93%, pH 5), 20 g/L (87.2%, pH 4), 25 g/L (87.47%, pH 4), 50 g/L (81.82%, pH 4). In all cases an increase at pH 2 and 3, and a decrease in adsorption at pH 5 were demonstrated. At pH 2 the formation of cadmium chloride probably occurred, which may make adsorption more difficult. The adsorbent dosage of 1.0 g/L revealed the lowest efficiency. As seen in Figure 13B, the highest biosorption capacity was observed as follows: 8.4 mg/g (1 g/L sorbent dosage, pH 5), 4.56 mg/g (2 g/L, pH 5), 3.22 mg/g (3 g/L, pH 5), 2.37 mg/g (4 g/L, pH 5), 1.87 mg/g (5 g/L, pH 5), 0.93 mg/g (10 g/L, pH 4), 0.59 mg/g (15 g/L, pH 4), 0.43 mg/g (20 g/L, pH 4), 0.35 mg/g (25 g/L, pH 4), 0.16 mg/g (50 g/L, pH 4). On the basis of the obtained results, the relationship between equilibrium pH and the highest biosorption efficiency obtained at initial pH 4.0 was investigated. It was shown that equilibrium pH increased to pH 6.4–7.3. Hence, a change in pH after the sorption processes in the examined systems was noticeable. To conclude, the highest adsorption efficiency was observed with the biosorbent dose of 10 g/L at initial pH 4.

The cation exchange mechanism took place during binding of Cd2+ ions in an aqueous solution. The surface of brewer's grains and functional groups could be protonated with a large number of H+ ions. As a consequence, the number of negatively charged active centers decreased, and the number of positively charged centers increased. In an aqueous solution, Cd2+ ions compete with H+ ions which, through electrostatic repulsion, interfere with adsorption of positively charged Cd2+ ions. As the pH increased to 3–4, the surface of the brewer's grains became more negatively electrostatically charged. There was a deprotonation of acid groups and initiation of ion exchange by increasing electrostatic affinity, which influenced the binding of more Cd2+ ions. Based on the literature, in the pH range of 2–5, Cd exists in the ionic form, while at a higher alkaline pH, Cd precipitates as hydroxides. Therefore, in these studies, the maximum sorption efficiency was obtained at pH 4. At pH 5, a decrease in sorption was observed, which could result from the competition of hydroxyl ions in active sites. Other forms of cadmium, such as Cd(OH)+ and Cd(OH)2, can interfere and slow down the sorption process. The influence of many factors (including pH) on Cd(II) sorption mechanism has been presented in the literature [38].

**Figure 13.** The impact of initial pH on removal efficiency (**A**) and biosorption capacity (**B**) of Cd(II) ions.

#### 3.4.4. Studies of Contact Time

The influence of contact time on biosorption was investigated, and the results are presented in Figure 14. Our previous research helped to establish the following optimal experimental conditions: initial concentration of Cd(II) 10 mg/L, initial pH 4.0, BG dosage 10 g/L, rotation speed 200 rpm, T = 23 ± 1 ◦C. Based on the results it was shown that the best biosorption efficiency was noticed within the first 5–10 min, and no changes up to 60 min were observed. Thus, there was no need to carry out further experiments on longer times of the process. After 10 min. the sorption efficiency was 93.34% and remained at this level.

**Figure 14.** The impact of contact time on the Cd(II) ions biosorption efficiency.

#### 3.4.5. Pseudo-First-Order and Pseudo-Second-Order Kinetic Models

The kinetics analysis of the biosorption of Cd(II) ions onto BG residues was performed. Pseudo-first-order and pseudo-second order models were used for calculations. The model parameters were estimated and the results are shown in Table 3 and Figures S2 and S3 (Supplementary Material). The biosorption process was very quick, and equilibrium was achieved after 5–10 min. In the case of the pseudo-second reaction model the correlation coefficient was *R*2 = 0.999, which means that the biosorption process better fits the kinetic model indicating possible ion exchange and sharing of electrons between cadmium ions and BG surface [39]. It is assumed that biosorption may be the rate-limiting step in

the process. Such parameters as contact time, initial and equilibrium concentration of Cd(II) were experimental variables. As brewer's grain particle sizes increased, *K*1 values decreased. During biosorption diffusion probably occurred, and ion exchange appeared in the process. Cd(II) ions could be attracted by the active sites of the BG waste surface, which was related to an increase in coordination number with the surface. The effect was the phenomenon of adhesion to the biosorbent surface and the formation of chemical bonds [40].

**Table 3.** Parameters of pseudo-first-order and the pseudo-second-order rate equations.

3.4.6. Isothermal Studies

The analysis of Langmuir and Freundlich isotherms were performed in relation to biosorption of Cd(II) using the BG biosorbent. The isotherm parameters were calculated and are presented in Table 4 and Figures S4–S13 (supplementary material). Based on the data, it can be assumed that the Freundlich model better describes the biosorption process. Other investigators, such as Khajavian et al. [41], Kaparapu and Prasad [42], and Naeem et al. [43], reported that the biosorption processes fits the Freundlich model better than the Langmuir model. The *R*2 coefficient, related to dimension separation factor or the equilibrium parameter, determines whether the biosorption system is favorable or unfavorable. The *RL* parameter determines the shape of isotherms in the following way: irreversible (*RL* = 0), favorable (0 < *RL* < 1), linear (*RL* = 1), unfavorable (*RL* > 1). In the present study, the obtained data were in the range of 0 < *RL* < 1, which translates into favorable biosorption for the removal of Cd(II) ions [40]. The Langmuir equation contains the *KL* parameter, which is the energy constant related to biosorbent and the binding energy of a solute as well as biosorption heat. It defines the interaction between adsorbate and sorbent surface, and the higher the values of the *KL* parameter, the stronger interaction between them [44]. The Freundlich isotherm is associated with the relationship between the concentration of metal ions dissolved in a liquid at equilibrium (*Ce*) and the concentration of dissolved ions on the surface of a biosorbent (*qe*). The determined isotherm parameters (*Kf*-biomass sorption capacity index, *n*-sorption intensity index) indicate the ease of separation of Cd(II) ions in the solution and biosorption on BG. In this research, the highest biosorption efficiency was 93.9% (initial pH 4.0, biosorbent dosage 10 g/L, initial concentration of Cd(II) 10 mg/L, contact time 60 min, agitation speed 200 rpm, T = 23 ± 1 ◦C). A comparison of biosorption capacity of BG for the removal of Cd(II) ions with different adsorbents is presented in Table 5, where *qmax* of BG is comparable or even greater than that of various adsorbents published in the literature.

**Table 4.** Langmuir and Freundlich isotherm parameters for biosorption of Cd(II) using BG residues.

**Table 5.** A comparison of biosorption capacity of BG for the removal of Cd(II) ions with different adsorbents in the literature.

#### **4. Conclusions**

Brewer's grains obtained during processing in the brewing industry were examined for the removal of Cd(II) ions from aqueous solutions. The physicochemical properties of the biosorbent material were determined using several analytical methods. Biosorbent dosage, initial concentration of Cd(II) ions, initial pH and contact time were factors examined in terms of their influence on biosorption. The results revealed that the maximum biosorption efficiency was equal to 93.9% under the following conditions: initial pH 4.0, biosorbent dosage 10 g/L, initial concentration of Cd(II) 10 mg/L, contact time 60 min, rotation speed 200 rpm, T = 23 ± 1 ◦C. In general, the average process efficiency was above 80% under different experimental conditions. The possible biosorption mechanism may involve ion exchange and electrostatic attraction. The FT-IR analysis revealed slight shifts of peaks and changes in the intensity of bands, which can indicate Cd(II) ion binding by various functional groups. The kinetics analysis showed that the pseudo-second order kinetic model and the Freundlich model fit the experimental data better.

In conclusion, it is assumed that post-production brewer's grains are capable of removing Cd(II) ions with high efficiency. These results may be an opportunity for industrial use of brewer's grains to remove heavy metals as well as improve water quality, which is in line with current global trends in sustainable development and the circular economy.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/en14175543/s1. Figure S1: Particle size distribution of brewer's grains, Figure S2: Pseudo-firstorder isotherm for adsorption of Cd(II) ions with brewer's grains, Figure S3: Pseudo-second-order isotherm for adsorption of Cd(II) ions with brewer's grains, Figure S4: Langmuir kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 1 g/L), Figure S5: Langmuir kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 5 g/L), Figure S6: Langmuir

kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 10 g/L), Figure S7: Langmuir kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 30 g/L), Figure S8: Langmuir kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 50 g/L), Figure S9: Freundlich kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 1 g/L), Figure S10: Freundlich kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 5 g/L), Figure S11: Freundlich kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 10 g/L), Figure S12: Freundlich kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 30 g/L), Figure S13: Freundlich kinetic isotherm for the biosorption of Cd(II) with brewer's grains (BG dosage 50 g/L).

**Author Contributions:** Conceptualization, T.K.; methodology, T.K.; validation, T.K.; formal analysis, T.K.; investigation, T.K. and J.W.; resources, T.K. and M.U.; data curation, T.K.; writing—original draft preparation, T.K.; writing—review & editing, T.K.; visualization, T.K.; supervision, T.K.; project administration, T.K.; funding acquisition, M.U. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research did not receive a specific grant from any a funding agency in the public, commercial or not-for-profit sectors.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

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

#### **References**


*3.11. Selective BTK Inhibitors*

Bruton's tyrosine kinase (BTK) is a member of the Tec family of tyrosine kinases involved in the high-affinity receptor for IgE (FcεRI)-dependent mast cell production of cytokines and degranulation [84,85], and in the IgE-mediated activation of human basophils [86]. BTK inhibitors could be useful to treat pathological mast cell responses of asthma [87].

A Phase II RCT [25] reported that orally administering remibrutinib (LOU064) 100 mg to inadequately controlled asthmatic patients did not induce an improvement in trough FEV1 and in morning and evening PEF compared to PCB. Changes in the ACQ5 score, in the asthma symptom score, and in the number of puffs of SABA taken daily were not different between remibrutinib and PCB groups [25].


*Article* **Comprehensive Analysis of Housing Estate Infrastructure in Relation to the Passability of Firefighting Equipment**

**Pavel Vrtal, Tomáš Kohout \*, Jakub Nováˇcek and Zdenˇek Svatý**

Department of Forensic Experts in Transportation, Faculty of Transportation Sciences, CTU in Prague, Konviktská 20, 110 00 Prague, Czech Republic<EMAIL_ADDRESS>(P.V<EMAIL_ADDRESS>(J.N<EMAIL_ADDRESS>(Z.S.)

**\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** The article focuses on the assessment and evaluation of the passability in densely populated parts of cities with multi-storey housing estates, in terms of the operation of the integrated rescue system (IRS) in the Czech Republic. The aim of the research is to minimize the arrival times to conduct the intervention as efficiently as possible. The presented problem is caused by unsystematic development of housing estates and the emergence of secondary problems in the form of inability to reach the place of intervention by the larger IRS vehicles. The vision presented in this document presents a systematic approach to improve the serviceability of individual blocks of flats. The main aim is to ensure passability, even for the largest equipment, such as fire engine ladders. Detailed mapping of the selected sites by drones, construction of their digital model, and subsequent virtual verification of the passability by specific vehicle models on identified access roads was performed. The results obtained by this procedure can then be implemented in the navigation of the fire safety forces and facilitate their arrival at the site of intervention. At the end, specific ways are presented in which the whole system can be modified to be able to intuitively change and choose individual access routes in real time, based on the current situation in the area.

**Keywords:** fire rescue service; smart cities; housing estates; swept paths; digital model

## **1. Introduction**

The resilience of each city is determined by the level of risk management or the management of potential critical situations. In order to be effective in its function, the city itself must have the necessary comprehensive resilience to the potential threats, collapses, or functional limitations. One of the most important aspects of any city is the provision of transport infrastructure for the operation of the IRS. Specifically, this involves ensuring accessibility in densely populated areas of the city with high-rise prefabricated housing [1].

The history of housing estates in the Czech Republic varied and the architectural character of these localities has evolved considerably over the years. From an urban, architectural, and technical point of view, the largest problems are presented by the largerscale housing estates. These were built mainly in the 1970s and 1980s and were almost exclusively constructed with prefabricated apartment buildings. An example is Prague, the capital of the Czech Republic. There are more than 600,000 flats in apartment buildings, in which almost 90% of the population of the capital city lives [2]. At the same time, the available statistics [3] show that almost half of Prague's population is concentrated in housing estates. There are currently 16 housing estates on the territory of the capital city with at least 10,000 inhabitants. Such large housing estates pose a number of traffic and transport challenges that are not currently thoroughly solved [1].

If we look at the housing estate as a system, it is possible to notice its gradual and continuous development, which in many aspects can be seen as a positive change (development according to the needs of residents), but also in many aspects as a negative one (unsystematic reconstruction of the area, and deviation from the originally intended

125

**Citation:** Vrtal, P.; Kohout, T.; Nováˇcek, J.; Svatý, Z. Comprehensive Analysis of Housing Estate Infrastructure in Relation to the Passability of Firefighting Equipment. *Appl. Sci.* **2021**, *11*, 9587. https:// doi.org/10.3390/app11209587

Academic Editor: Leon Rothkrantz

Received: 9 September 2021 Accepted: 11 October 2021 Published: 14 October 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/).

transport and pedestrian links). The increasing level of motorization, and with it the ever-increasing demand for parking spaces [1,4], are not insignificant factors. For this reason, traffic at rest is accumulating in places that are not allocated for this purpose in the first place. Typical examples include inappropriate parking at crossroads, at the mouth of slip roads, or where parking is prohibited by traffic signs. Inappropriate parking reduces the efficiency of the transport links in the area and not only significantly reduces the traffic service but also worsens the safety and accessibility of the area.

The inappropriate construction layout of the area, together with the violation of road traffic rules, generates a significant problem, namely, the restriction of the passage of the IRS units during the intervention. Significant is the deterioration in time efficiency during passage through the area; thus, a delayed possibility of rapid intervention. These problems mostly affect the Fire Rescue Service (FRS), which has the largest vehicles within the IRS fleet. The length of fire tankers is almost 10 m, and in the case of a fire engine ladder truck, the vehicle is more than 10 m long [5,6]. Removing obstacles or finding an alternative detour route leads to significant time delays, which consequently results in increased risks to the person in danger or damage to the corresponding property.

The purpose of this article is to present an approach that can be used to solve the most important problems that limit the movement of the FRS through existing residential areas and then to point out the most important aspects that must be taken into account for a systematic evaluation. By applying once-off area-based data collection, it is possible to build a digital model of the area in which the quality of individual access roads can be assessed. On the basis of this analysis, it is possible to define a scheme of the key emergency routes along which the time losses are minimised and the passability of the FRS vehicles is ensured. Finding the most suitable access routes to each high-rise building allows the most effective and safe intervention. Additionally, effectivity can be increased by implementing the findings in the currently used software platforms by the FRS, e.g., dispatch and navigation [7]. A secondary objective is to provide examples of how to modify the whole clearance assessment approach using modern technologies in such a way that it can operate online in the future and intuitively assess risks in real time.

In the following chapters, current methods and procedures related to the passage of the FRS vehicles in urban areas and the methods aimed at mutual communication of the intervening vehicles are discussed. Furthermore, the methods used in the analysis of area in close proximity to the intervention target are discussed, followed by specific examples, justification, and the outputs of a pilot project. The next part is focused mainly on the visualization and creation of models related to the FRS ladders and their subsequent application in the search for the optimal solution within the area. The paper concludes with suitable methods to support procedure automatization in the course of the following years.

The following paragraphs describe the research method of the proposed project.

#### **2. The Current Approach**

The current approach to the problem of firefighting vehicles throughput, route planning, or effective arrival times focuses primarily on the routes between the fire station and the general area near the location of the fire. Several scientific publications have dealt with this topic, and several of them are summarised in the following paragraphs. Notable is a publication from the city of Changsha (China) [8,9] that is addressing the arrival times of firefighting units within the city. The premise in this study is that at least one fire tanker and one fire ladder truck should arrive at a given location within 240 s of a fire being detected. However, problems caused by congestion and a low density of fire stations in city centres make this assumption very difficult.

A similar approach was applied in the city of Nanjing (China) [10] in a study to verify the accessibility of different parts of the city in relation to the effective travel times of existing fire stations. The assessment of the suitability of fire station locations and their time of accessibility to the fire scene was analysed using the FC2SFCA method, which focuses on specifically defining the service boundary from each fire station and determining from which stations it is appropriate to lead the response to different parts of the city. The paper furthermore describes the comparison of the previously mentioned method with the older E2SFCA method and assesses the differences and precision of each procedure. The method shows that the coverage near fire stations or in the city peripheries is far more effective than coverage in the city centre, where high traffic density is a particular problem.

Publications from Pennsylvania State University (USA) [11] discuss the arrival times to the location of an incident together with a main focus on the impacts and economic losses associated with the time lost by the emergency services in reaching the fire location due to traffic congestion.

Other research related to integrated emergency services [12] is concerned with improving the efficiency of passage through light-controlled intersections, where emergency vehicles share their positional information with traffic lights. Research conducted in Saudi Arabia uses a mobile application that links the vehicle data with map data and the traffic light sensors. In the event that an IRS vehicle arrives at a traffic light, the vehicle's lane switch to the clear phase. In this way, traffic from other traffic directions does not accumulate at the crossroads and the traffic flow is smooth, without significant delays.

Research at Bangladesh University [13] focused on dynamic changes in the signal depending on the current position of the IRS vehicles. The proposed system works on a cloud centric Internet of Things (IoT) principle. It collects information about the location of vehicles and signals at traffic-light-controlled crossroads. The acquired information is evaluated, and the system subsequently changes the phases on the traffic lights to enable a priority pass-through of the IRS vehicles.

Another research study was trying to achieve similar results by solving crossing problems using a complex RFID-based system [14]. The system is designed to be able to detect both image and sound from the IRS vehicles and thus be able to change the order of phases at light-controlled intersections. This enables to the IRS vehicles an easier crossing of these incriminated points.

The research projects also aim to address the interaction between vehicles and their environment. A paper published by the University of Žilina (Slovakia) [15] presents

cooperation between individual vehicles and sensor data, helping to increase the safety and improve the communication skills of individual road users. The use of vehicle-tovehicle (V2V) and vehicle-to-everything (V2X) methods is intended to help warn drivers of approaching emergency services in time to create space for mutually safe crossing.

It could also be mentioned that there is a research focused on modelling buildings and their subsequent servicing by firefighting vehicles. Research from the National Central University of Taiwan [16] focuses on simulating fires on individual floors of high-rise buildings and their subsequent firefighting strategies. The proposed method involved the exploration of a 3D geometric network model (GNM) based on a building information model (BIM) to simulate firefighting. The operation of firefighting ladders during a response was also simulated in a 3D virtual environment. Finding the ideal location for the fire trucks to stop was supported by a methodology that can be used to determine the ideal location of the truck placement before the actual arrival of the engine in the fire area. At the same time, future research considers the use of real-time traffic sensors data to predict traffic conditions and adapt the optimal routing of the intervention line.

Similarly to the abovementioned strategy of deploying fire trucks around burning buildings, the research published by Belgian, Dutch, and Chinese academics [17] focuses primarily on water refilling of fire trucks in the process of fighting oil fires. The procedure uses the eM-plant simulation tool to model the firefighting process. In the context of the so-called chemical accident and to prevent its escalation, strategies are also considered to allocate fire trucks according to the distance from the hydrants and to divide fire trucks according to their number.

Improving the strategy of serving a given area is also discussed in a publication from the Netherlands [18]; it aims towards the optimisation of the allocation of firefighting vehicles to the location of an accident from the perspective of the emergency control room, based on historical dispatching problems where multiple vehicles from different locations are dispatched to the location of an incident and a decision has to be made as to which fire engine will be on site first. In this paper, a method to dispatch fire trucks is described in a way that strikes the right balance between rapid response to the actual fire and also good coverage of any simulated incidents.

After reviewing the currently available publications, it can be concluded that areas in close proximity to the actual intervention site are not being addressed in greater detail. However, these areas may have the highest loss times when the IRS vehicle is not able to access the actual location of the intervention. Currently, the intervention of the IRS forces is guided by the fastest possible route to the place of the critical accident, but without any knowledge of the actual situation in the target area. The innovation of the presented concept lies in the application and combination of knowledge from the Road Safety Audit (RSA), new methods of detail analysis of the area, especially by using aerial photogrammetry, and subsequent simulation in a virtual environment.

#### **3. The New Approach**

To further elaborate and verify the abovementioned conclusions, a pilot project in cooperation between the CTU in Prague, Faculty of Transportation Sciences, and the Prague 11-Chodov district was carried out. This project enabled the conversion of the theoretical intentions related to the solution of housing estates from the perspective of the passage of the FRS into real practice. This project has also enabled to define a methodological procedure that contains generalised guidelines to implement proposed solution in other localities. Figure 1 below shows the generalised steps of the procedure. The aim is to meet all the input requirements raised by the IRS units and local municipalities and to minimise any adverse effects on the residential needs. Marked boundaries around the steps, either solid or dashed, represents their nature. While the parameters marked with solid lines are determined and defined once during the evaluation. The dashed marked are iteratively adjusted over time in relation to the actual situation and development within the target area.

**Figure 1.** Diagram of the generalised procedure steps.

The target objective of the current project was to create a simplified offline digital model of the location, in which it would be possible to verify and evaluate the operation of the IRS during fire or other emergency situation. In the context of interaction between the FRS dispatching centre and the FRS commanders, this method can be used to prevent traffic complications caused in areas in close proximity to the target site and to avoid an increased arrival time due to inappropriately chosen routes for intervention.

#### *3.1. Primary Inputs*

In the beginning, it was necessary to assess all aspects related to the target area. Specifically, this involved assessment of the location and its relations within the town, identification of the key roads, and evaluation of their significance on the rest of the surrounding area. Additionally, it was necessary to define the appropriate crossings with the surrounding parent road network for the initial arrival of firefighting equipment. Linked to this is the necessity to assess the impact of traffic volumes on these roads, traffic flow mix, and junction capacity. These present an important factor that influences the initial choice of routes for intervention into the area. Furthermore, the assessment of the area from the point of view of the resident's demands was performed. The aim was to minimise the interference with the existing capacity of parking areas and at the same time with areas intended for leisure activities (parks and playgrounds). Lastly, it is necessary to consider the experience of the firefighters who serve the area, as they are aware of the most significant issues that limit the passage of their equipment.

#### *3.2. Dataset Extension*

In order to create a sufficiently detailed digital model of the area and to enable verification and simulation of the passability, low altitude aerial photogrammetry with a drone was used. The primary objective of the measurements was to obtain a digital three-dimensional spatial model with sufficient accuracy and detail. In contrast to classical measurement methods, the digital image correlation method was used, to facilitate the speed of data acquisition and processing [19]. The advantage of its application is that it does not require the explicit identification of individual points but uses image recognition algorithms to automatically identify the key features among the images, link them, and subsequently reconstruct the spatial information. Firstly, the Structure from Motion technique (SfM) was utilized. The SfM method uses a number of unordered images that depicts a static scene or an object from arbitrary viewpoints and attempts to recover camera parameters and a sparse point cloud that represents the 3D geometry of a scene [20]. The second stage consists of a further reconstruction of the geometric scene. This was carried by the Dense Multi-View 3D Reconstruction algorithm (DMVR), which was run on an already-aligned image set. The algorithm operates on a pixel base, thus enabling it to obtain a higher level of detail than the SfM [20]. The scale, transformation, and validation of the resulting model

were performed with signalised points measured with the GNSS station. The resulting accuracy was 2.5 cm in position (X,Y) and 3 cm in height (Z). In this way, the resulting model was aligned with cadastral maps and it was possible to clearly define individual parcels intended for potential remediation. Figure 2 show a part of the assessed area in which the pilot project was carried out. Figure 2 depicts positions of captured images (over 3000 photographs were taken) together with the necessary overlap for successful reconstruction. Blue areas that are represented with the highest frequency represent the highest overlap of each image. The number of overlapping images then decreases to lower values in the extreme areas of the site. At the same time, the figure below also shows the statistics of the performed measurements.

**Figure 2.** Camera locations and image overlap with parameters.

The photogrammetric outputs are similar to laser scanning but with significantly lower time demands and corresponding costs, and only a slightly lower accuracy (RMS of 2.1 cm). For the purposes of this study, a textured triangular polygon mesh of the area was created together with a detailed orthophoto with a resolution of 2 cm/px [19,20]. Such detail is essential, especially in places where road width ratios reach very small values that border on the minimal required passage profile of the FRS vehicles.

The advantage of this approach is also the fact that the resulting model corresponds to the current situation at the time of measurement, e.g., shows parked vehicles, dustbin placements, fences, etc. Furthermore, it is complemented by the textures of the environment for effective presentation and orientation; therefore, it tells about the actuality and related deficiencies much more accurately than could be obtained from a panorama view, satellite imagery, or classical aerial maps.

In order to assess the passability, it is not necessary to work directly with a 3D model of the environment. The initial site analysis can be performed on a 2D basis (sufficiently detailed orthophoto is available). However, the use of a 3D model is necessary for evaluation of the clearance at locations where there are features that restrict the clearance profile of fire engines. These elements are, for example, unmaintained vegetation, road signs, streetlights, or bridge structures.

An illustration of the digital model is presented in Figures 3 and 4. Figure 3 shows the resulting spatial model, mesh, together with the location of each captured image. This presents a significant advantage due to the possibility to easily retrieve detailed photographs of the selected area during the subsequent analysis. Figure 4 shows the same model with textures.

**Figure 3.** Digital model of the area with individual images.

**Figure 4.** Digital model of the area with textures.

Another way to expand the input of the evaluated area was through implementation of the RSA principles. The site inspection enables to detect potential critical locations and problems that might not be apparent at first look from the digital model or to somebody without traffic safety knowledge. Examples include high kerb edges at the entrances to emergency routes that could be used by the FRS, inadequate placement of road signs, or traffic-related safety issues.

Due to the specific geometric requirements of the FRS equipment, especially fire engine ladder trucks, which have the highest spatial demands, it is necessary to evaluate the passability of the vehicle through the area for the purposes of optimal intervention routes. Furthermore, the passability assessment enables verification of the efficiency of the selected routes. The passability can be easily verified with use of specialized swept path software. However, since almost every firefighting vehicle is unique in its parameters and differs from factory vehicles, the general vehicle data sheets cannot be used without adjustments. The same issue is with use of the predefined models designed for the verification of passability. A typical example of limited applicability of using already constructed models is the fact that each vehicle has different additional equipment, e.g., in the form of a pull-out ladder or a rescue basket. On the other hand, the basic precondition is that the high-rise firefighting equipment serving the area remains unchanged for several years (15–25 years). Therefore, the assessment or proposed changes can be related to the demands of specific fire vehicles that are serving the evaluated area.

As a result of incomplete technical documentation of the fire engine ladder truck used in the project (affected by the unique accessories mentioned above), it was necessary to create a model for subsequent simulations. A wide range of measuring methods can be effectively used to collect the data concerning the car body, its add-on components, and its dynamic characteristics, e.g., curb radius. The combination of the above-obtained parameters can then be suitably used in the creation of digital models, which can then be used to construct swept paths to verify the passability of the site under consideration. The chosen reference vehicle is shown in Figure 5.

**Figure 5.** Photo of a fire engine ladder truck [5,6].

#### *3.3. Relations and Model Definition*

The standardised vehicle models and freely available vehicle databases very often represent an idealized vehicle, which in its dimensions covers approximately 85% of the fleet of the same or similar type of vehicle or vehicles defined by national standards [21]. Common practice for passability assessments is to use the garbage truck model. However, due to significant differences in the dimensions and characteristics of the vehicle swept path during cornering, this leads to a situation where the vehicles that play the most crucial role in saving lives are omitted. Therefore, during the project, a simplified model of an FRS ladder truck was created and subsequently used to the assessment. Figure 6 shows the resulting simplified model defined in the AutoCad Vehicle Tracking software. For the purposes of model creation, it was necessary to determine not only the characteristic dimensions of the vehicle but also the actual turning radius, wheel angle, etc. The characteristic dimensions were determined with photogrammetric and geodetic measurements. Figure 7 shows the captured images of the ladder truck, reconstructed spatial model, and dimensions of the selected parts of the truck (length, height, ladder dimensions, etc.).

**Figure 6.** Final digital model of a fire engine ladder truck created in AutoCad Vehicle Tracking.

**Figure 7.** Photogrammetric measurements of the characteristic dimensions of the FRS ladder truck in Photomodeler Scanner.

Figure 8 shows the differences between the garbage truck model and FRS ladder truck.

**Figure 8.**

In order to obtain a real swept path of the reference FRS vehicle, a research experiment was conducted with an FRS vehicle (Figure 9). In this experiment, the full turn of the FRS vehicle at maximum wheel travel was monitored by drone. The speed of the vehicle was determined based on the FRS experience at approx. 5–10 km/h. The footage was subsequently rectified to enable direct identification of wheels trajectory and the vehicle contours. Thus, it was possible to determine the actual turning radius and wheel angle.

**Figure 9.** An example of a calibration experiment of the swept path for vehicle model design.

Ackermann's steering geometry was subsequently used for verification of the model validity, specifically by calculating the steering angle. The principle is based on a fixed axle where only the wheels rotate and is shown in Figure 10 [22].

$$\alpha\_l = \tan^{-1} \left( \frac{L}{R - \frac{T}{2}} \right) \tag{1}$$

$$\alpha\_0 = \tan^{-1} \left( \frac{L}{R + \frac{T}{2}} \right) \tag{2}$$

**Figure 10.** Ackermann's steering geometry [22].

By substituting the observed turning radius into the equation, a value for the wheel angle was obtained and compared with the experimentally measured value for the wheel angle. This verified that the determined vehicle turning radius corresponded to the real vehicle.

The resulting simplified model showed that vehicles that are very similar to firefighting vehicles in terms of their parameters (length, width, and height) actually have significantly different trajectories. Figure 11 shows a comparison of the swept paths of a municipal waste vehicle (blue) and a fire engine ladder truck (red). This difference in their manoeuvrability is closely related to the different turning radius or steering angle of the wheels. The different behaviour can be seen especially in the initial phase of the turning manoeuvre, due to the greater body overhang of the fire engine ladder truck compared to a conventional truck.

**Figure 11.**

By using digital models of firefighting equipment and a digital model of the area, it was possible to verify the passage through the selected area in the current layout; this enabled to define the critical locations where potential bottlenecks may occur and which require subsequent structural modifications. These basic outputs became the basis for subsequent comprehensive research outputs. The most frequent problems affecting the passability are high kerb lines, which cannot be driven over into the vicinity of buildings; incorrectly implemented parking places, which interfere with the crossroad areas; inappropriately placed vertical traffic signs, which prevent parking of vehicles with the overlap of the bow onto the pavement; or inappropriately placed solid obstacles, e.g., concrete posts, railings, etc. The main objective is to eliminate the identified problems as soon as possible. Priority is given to problems that can be solved easily, at a low cost, or with low corresponding administration. However, the general idea is that the measures must be performed systematically and in coordination between all involved partners.

Most prefabricated buildings in the area have several possibilities from which a rescue operation can be performed, but not all roads are suitable for the passage of larger firefighting equipment, e.g., spatial or load limitations. The most efficient solution logically seems to be the use of roads that were designed to enable firefighting operations at the time the area development. However, there are cases when during time the purpose of these roads changed, some of the connections disappeared, or parts of them are currently used for different purposes than the one for which they were originally established. Examples include the creation of parking places, flower gardens, partial or total redesign, etc. Despite this fact, these roads can still serve as a basis for the area serviceability.

Based on the available infrastructure in the evaluated area, several options were proposed to ensure the accessibility of every individual building in the project. At the same time, it was possible to determine the theoretical time delays of the individual intervention routes depending on the travel time to each location. Figure 12 illustrates the serviceability options for the area and the design of the main or alternative emergency route. Furthermore, this solution identifies the locations where it is possible to perform only a one-way passthrough or where it is possible to turn around or pass another vehicle. The digital model also provides information about the vertical distance of the road from the face of the building. This distance significantly influences the overall reach of the fire

ladders during an intervention and thus the course of the actions. It is essential to always find an area in front of the building where the distance is as small as possible, but at the same time, the area is wide enough to allow the support legs of the vehicle to be extended as far as possible to achieve the required stability and is able to withstand the load of the vehicle.

**Figure 12.** Schematic representation of the serviceability proposal for the evaluated area together with details of the selected area.

Each entrance road to a particular block of flats has different loss times and different levels of complications depending on the occupancy of the roads by parked vehicles. For this reason, it can be presumed that time losses of an intervention vary during the day and directly correspond with the actual situation. During working hours, when most of the residents are away from their homes, the time-losses are lower in comparison to the evening, when parking capacity is considerably saturated and vehicles are parked even in places not set up for this purpose. For this reason, it is desirable to supply and accordingly modify the proposed emergency routes with online information, e.g., in the form of camera footage or by using data from traffic sensors. The advantage of the proposed approach is that the potential bottlenecks or critical locations can be determined in advance. Thus, the monitoring is designed with consideration of the specifics of the evaluated area. This leads to the possibility to select the most appropriate access route to individual buildings in a significantly more effective way than through a simple empirical experience.

#### *3.4. The Validation of Proposal and Project Outputs*

A mathematical description of the presented problem of FRS passability through housing estates can be determined by two basic premises. The first premise is the attempt to minimize the time of arrival of the FRS to the location of the fire, similarly as in case of optimization of serviceability of larger areas. The second premise works with the hypothesis of maximisation of the sum of probabilities for effective FRS intervention, such as maximisation of the probability of collision-free passage through the area in such a way that there are no elements limiting passage along the route, such as the spatial layout of the roads. Minimalisation is the probability that potential traffic excess in the form of a bottleneck will occur on the determined access route. The third factor is the probability of effective intervention directly at the place affected by the fire in terms of the technological possibilities of FRS vehicles. This is in close relation to the specific parameters of the target building/object. Parameters such as the maximum possible height available by means of a fire truck ladder or the minimum perpendicular distance from the extinguished object must be considered here.

However, there is a significant difference between the optimization of serviceability of larger area, e.g., part of the city, and the blocks with high-rise buildings, where only few potential routes are possible, special infrastructure is already in place, and only few access points exist. Therefore, it is very difficult to perform specific mathematical verification of the proposed routes, as each FRS intervention is very unique and different complications may occur. It is only possible to obtain a theoretical verification, but this lacks many variables and the result is thus highly biased. The general verification procedure of the proposed changes is related to the experimental determination of the transit speed of each route. For this reason, it is necessary to perform control measurements directly in real traffic with specific cars. In the authors' opinion, a general mathematical prescription for verifying the passage of FRS vehicles through a specific location cannot be made explicit using a clearly defined mathematical apparatus. Each such passage is unique and is always influenced by a number of variable parameters.

The research found that the FRS may not be the only end-entities benefiting from the research. Other involved parties can benefit from the research and use this for the city's development and to increase its safety. One of the key users is the dispatch of the IRS, which can implement the information in the navigation of the emergency services based on the actual place of the emergency. At the same time, it enables to inform the responding units in advance about the current traffic situation, possible risks, or most effective access routes. With the help of the online digital model of the area, it is possible to provide reliable information about the state of the site, both in terms of traffic and the possibilities of the vehicle placement. At the same time, it is possible to provide information where it is possible to pass another vehicle or the most suitable places for staging fire trucks so that the intervention can be carried out from the closest possible distance.

The identified service routes are also directly applicable to the fire truck crews. The Czech FRS is currently using a map-based application for localisation and best utilisation of the hydrants in the area of intervention. The services routes are prepared and implemented using GIS systems. This approach enables to add this information within the application to improve the information available to the fire truck crew. Furthermore, this approach also enables to have an up-to-date overview of the individual service routes in the area in case the monitoring system is active. A similar approach is reported in Portuguese research, which also implements a decision-making system in GIS applications for the crew of the FRS in order to reach the fire location as quickly as possible and thus reduce the consequences [23].

Last but not least, the GIS data can be used to inform the municipality administration about critical areas and suitable passage routes for the IRS. The benefit is a safe and systematic development of the area in the future, as it is known which roads are necessary or where the problems arise; also, it enables to mitigate potential disruption of links in the service of the IRS in cases of road closures.

#### **4. Discussion**

While the methodological procedures applied in the previous chapter can be modified to achieve a direct response to the actual situation in the locality, the practical implementation is limited in several ways. The changes outlined in this chapter summarise how to effectively supplement the digital model of the area and improve its serviceability based on online information.

The most important part of the project, which currently contains the most data, is the digital model of the location. To be able to talk about a real virtual copy, which is similar in its characteristics to the vision of the Twin City model [1], it is necessary to add elements and systems capable of real-time monitoring of the city's traffic processes thanks to sensors directly connected to this virtual model. This includes, for example, data from sensors used to calculate the current occupancy of parking places [1]. If it is clear that the capacity of the parking places is not fully utilised, there will most likely be no inappropriate parking of vehicles at crossroads and no significant disruption to the flow or throughput of traffic through the site. In the event that the parking capacity is fully occupied, inadequate parking can be expected even at locations that are crucial for passability of the IRS (crossroads, fire roads, etc.). In these cases, the use of camera monitoring and subsequent systematic control seems to be an ideal solution [24]. When monitoring a lot of similar locations, it is possible to use induction loops or detectors working on a similar principle as a support tool that automatically informs the control centre of a potential issue.

Another approach is the potential utilisation of drones, which can be effectively used for detailed and periodical analysis of the assessed locations in a very short time. Verification of critical places or areas could be done similarly to the small parcel delivery system introduced by Amazon in recent years. Figure 13 illustrates a docking station mounted on a lamppost to serve as both a take-off point, charging station, and a connection point for the dispatch centre controlling the area. The drones provide an ideal means to scan a selected area in a short time frame and send the acquired data to a central control station before the actual firefighting operation takes place. In this way, it would also be possible to optimise in advance the route option during the intervention, thus avoiding any significant time delays [8]. However, the possibility of using drones in built-up areas is limited. Currently, such a data collection option cannot be implemented, as legislation and data protection do not allow the use of drones over built-up areas [25,26].

**Figure 13.** Drone docking stations on lamp posts [27].

Another way to obtain the necessary data for the assessment and analysis of selected locations can be through lidar or ultrasonic detectors. These can be mounted on garbage trucks that visit the sites several times a week. The collection of these data could be used to update the input data and to update the area model itself.

#### **5. Conclusions**

The effort to systematically evaluate the target areas of intervention in densely populated parts of cities with multi-storey housing estates has very specific goals—serviceability by the IRS, reduction of the arrival times of the IRS, and to enable the passage of large FRS vehicles. The research presented methods for obtaining the necessary data to evaluate the current condition of the locality and to design remediation measures. Primarily, these were the application of various photogrammetric and geodetic measurements to obtain outputs that were subsequently used for modelling in specialised software. This outlined a suitable procedure for mapping the locality and for obtaining the specific parameters of the FRS vehicles. This contribution presented a novel methodological approach that contains generalised guidelines for their practical implementation. The methodology enables to create a digital model of the location, in which it would be possible to verify and evaluate the operation of the IRS during fire or another emergency situation. Furthermore, the approach was validated through a pilot study in cooperation with municipal administration and the FRS. The pilot project has made it possible to consider all the specifics of the selected location and proposed a comprehensive solution in the form of a methodological procedure that can be used repeatedly for the analysis and subsequent design of measures for high-rise housing estates.

The new methodology approach of the area management also helps during the future development of the area, as it is possible to determine which roads or areas in the locality are crucial to ensure the serviceability by the IRS or which presents the highest risk or time delays. For this reason, it is also possible to plan a systematic remediation of the weak spots. The vision of the conceptual design was primarily to create a digital 3D model in which individual variants of the firefighting equipment's arrival to the intervention

site can be simulated, verified, or tested, depending on the actual situation in the area. By integrating it into the current navigation systems used by the FRS, it is possible to optimise the arrival time and minimize the loss of life or the level of damage to properties. At the same time, the methodological approach presented by the research can be further developed by extension of the spectrum of the measured data or by implementation of other approaches to collect these data; for example, using sensors, camera surveillance systems, or detectors, as discussed in the previous chapter.

The safety and ideal serviceability of existing urban areas is a continual process, which needs to be periodically revised and improved due to the area development. It is essential to apply new procedures and use modern technological possibilities to facilitate the work of the emergency services to improve the resilience of the urban area in relation to the potential loss of life and property.

In this paper, we define approaches and methods for the comprehensive analysis of housing estates in terms of FRS vehicle passability. The paper presents a detailed outline of the scientific approach of each step of the proposed solution itself. We are aware that the problem addressed does not only affect housing estates in the Czech Republic but is a problem occurring worldwide. From a historical point of view, this problem is reasonably expected; for example, in all post-communist countries of Central and Eastern Europe, where in the past similar housing estates were constructed from uniform prefabricated materials. As a result of this conclusion, we believe that the presented approaches and methods, while performed only in the Czech Republic, and thus with consideration to its specifics, are applicable with only slight adjustments globally. This statement is also supported by the set of technological resources and software available to academics and professionals globally. This research and its outputs provide a clearly defined and unified set of methods applicable to the analysis of any housing estate across the world's countries.

**Author Contributions:** Conceptualization, P.V., T.K. and J.N.; methodology, P.V., T.K. and J.N.; software, P.V.; validation, P.V., T.K. and J.N.; formal analysis, P.V., T.K. and J.N.; investigation, T.K.; resources, P.V., T.K. and J.N.; data curation, J.N.; writing—original draft preparation, P.V. and T.K.; writing—review and editing, P.V., T.K. and J.N.; visualization, P.V. and Z.S.; supervision, P.V., T.K. and J.N.; project administration, P.V., T.K., J.N. and Z.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

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