**Fabiana Passos \* , Thiago Bressani-Ribeiro , Sonaly Rezende and Carlos A. L. Chernicharo**

Department of Sanitary and Environmental Engineering, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627, Belo Horizonte 31270-901, Brazil<EMAIL_ADDRESS>(T.B.-R<EMAIL_ADDRESS>(S.R<EMAIL_ADDRESS>(C.A.L.C.)

**\*** Correspondence<EMAIL_ADDRESS>Tel.: +55-31-34093593

Received: 31 January 2020; Accepted: 20 May 2020; Published: 1 July 2020

**Abstract:** Rural sanitation is still a challenge in developing countries, such as Brazil, where the majority population live with inadequate services, compromising public health and environmental safety. In this context, this study analyzed the demographic density of these rural agglomerations using secondary data from the Brazilian Institute of Geography and Statistics (IBGE). The goal was to identify the possibilities associated with using small-scale upflow anaerobic sludge blanket (UASB) reactors for sewage treatment, mainly focusing on biogas production and its conversion into energy for cooking, water heating and sludge sanitization. Results showed that most rural agglomerations lacking the appropriate sewage treatment were predominant from 500 to 1500 inhabitants in both northern and southern Brazilian regions. The thermal energy available in the biogas would be enough to sanitize the whole amount of sludge produced in the sewage treatment plants (STPs), producing biosolids for agricultural purposes. Furthermore, the surplus of thermal energy (after sludge sanitization) could be routed for cooking (replacing LPG) and for water heating (replacing electricity) in the northern and southern regions, respectively. This would benefit more than 200,000 families throughout rural areas of the country. Besides the direct social gains derived from the practice of supplying biogas for domestic uses in the vicinity of the STPs, there would be tremendous indirect gains related to the avoidance of greenhouse gas (GHG) emissions. Therefore, an anaerobic-based sewage treatment may improve public health conditions, life quality and generate added value products in Brazilian rural areas.

**Keywords:** anaerobic treatment; bioenergy; energy assessment; rural sanitation; sludge; wastewater

### **1. Introduction**

Sanitation is closely related to public health, environmental safety and life quality worldwide. Particularly in developing countries, sanitation has been debated in terms of human rights, highlighting situations of extreme violation [1]. In Brazilian rural areas, the Federal Sanitation Policy (Law n◦ 11,445/2007) determines social inclusion and the reduction of regional inequalities, seeking to provide adequate conditions of environmental health to rural populations and small isolated urban centres. In this context, sanitation plans, programs and projects in areas occupied by low-income populations should be given priority. Moreover, solutions attending to indigenous people and traditional populations should be compatible with their social and cultural characteristics. Finally, the referred legislation indicates the unity and articulation of different institutional agents, as well as the development of their organisation, technical aspects, management, and financial and human resources capacity, considering local specificities [2].

Nevertheless, despite the legislation, sparsely populated areas, characterized by low demographic densities, are commonly neglected due to the principle of economy of scale. The rural invisibility to public policies results in a precarious health situation and a wide regional inequality. In addition, rural agglomerations and/or communities in urban areas but distant from the urban sanitation infrastructure are examples of possibilities for developing collective small-scale sustainable sewage treatment plants (STPs).

In this context, an anaerobic sewage treatment has been investigated and applied in developing countries, such as Brazil. In fact, this country has the largest number of installed upflow anaerobic sludge blanket (UASB) reactors treating sewage in the world. In a recent publication, anaerobic-based STPs using UASB reactors accounted for 667 systems among the 1667 systems acknowledged (i.e., 40%), comprising systems serving from 5000 to 1 million inhabitants [3]. Moreover, an investigation of 2734 STPs in six Latin American and Caribbean countries showed that, besides Brazil, UASB reactors have been extensively used in Mexico, Colombia, Dominican Republic and Guatemala. Such anaerobic reactors represented up to 20% of the total number of STPs for all of the assessed countries [4]. It is worth mentioning that full-scale UASB reactors have also been successfully applied in India [5].

The advantages and potentialities of this technology are related to several aspects, noteworthy are the low sludge production and implementation and operation costs, compared with conventional aerobic (e.g., activated sludge) or physicochemical processes [6]. Moreover, an anaerobic sewage treatment generates biogas, which may be converted into electric or thermal energy for use in the STP itself or in the nearby community. The energy conversion process and its application depend on many factors, such as the STP size, energy policies and subsides, climatic conditions and socio-economic local characteristics. In general, biogas use in small-scale STPs in rural areas is designated to thermal energy conversion, such as water heating, cooking and sludge sanitization. For small- and medium-scale STPs (PE > 2000; PE < 100,000, where PE represents population equivalent), biogas use for the cogeneration of electricity and heat is generally not feasible, due to energy costs, lack of incentive programs for energy recovery from biogas and the poorly developed market for combined heat and power (CHP) engines [7].

The present study aimed at characterizing the potential for biogas generation in Brazilian rural agglomerations which are currently unattended by sanitation services. These agglomerations were organized in three categories using secondary data from the Brazilian Institute of Geography and Statistics (IBGE): (a) rural areas in urban area; (b) isolated rural areas with large settlements and; (c) isolated rural areas with small settlements. The goal was to identify the possibilities associated with using small-scale UASB reactors for sewage treatment, mainly focusing on biogas production and its conversion into energy for cooking, water heating and sludge sanitization, considering the different climate conditions amongst Brazilian regions. Additionally, carbon emissions were also assessed for the proposed technological flowsheets.

#### **2. Material and Methods**

#### *2.1. Identification of Rural Agglomerations: Secondary Data*

The most recent Brazilian demographic census was used to identify and analyze sewage sanitation infrastructure in the rural areas [8]. Secondary data collected aimed at classifying situations that lack an appropriate treatment, such as rudimentary pit, ditch or direct discharge into rivers, lakes or the sea. The available data of rural agglomerations were further reclassified by the National Program of Rural Sanitation (PNSR) [9], in order to better match the demographic densities of the different Brazilian localities/agglomerations. After the reclassification, four categories of rural households were adopted, identifying them with each specific sanitation demand and possible biogas uses, as shown in Table 1.

**Table 1.** Rural household classification and recommended sanitation solutions according to the National Program of Rural Sanitation.

Note: For this study, only "A", "B" and "C" were considered. Isolated stands for agglomerations far from the central core of the municipality (urban area).

As can be seen, Table 1 summarizes the description and recommended sanitation solutions considered for each population category. Category A comprises agglomerations located in peripheric regions of an urban area and, therefore, technological options for sewage treatment may be carried out as in cities. In those cases, UASB reactors may be developed as a decentralized option for recovering possible by-products for local use, such as biogas. Categories B, C and D are isolated or far from urban areas. Agglomerations classified as category B have a more urban-like lifestyle, while C and D have an agricultural economy and fewer services (e.g., transportation). For category B, an option for sanitation solutions may be decentralized systems comprising UASB reactors, while STP management may be conducted by a group of users that benefit from valued by-products. Agglomerations from category C may also use collective STP systems with UASB reactors when settlements are nearby, while a familiar approach would be applied for isolated houses. Individual solutions may require external technical support from public services, although collective solutions may possibly be managed through a local auto-organization approach. In fact, a recent study is being conducted in an *urban* occupation without wastewater treatment services, showing the possible scenarios of users and residents that manage the system and benefits from generated by-products, such as biofertilizer. Category D consists of isolated and dispersed occupation, leading to individual sanitation solutions. Therefore, this category was not considered for implementing UASB reactors and biogas recovery.

For this study, the surveyed agglomerations considered all populations from 500 to 3000 inhabitants associated with categories A, B and C, as they can be potentially served by decentralized small-scale STPs Moreover, data were analyzed according to the geographic region and, therefore, separated in two groups: northern region (North, Northeast and Centre-West) and southern region (South and South-East), as illustrated in Figure 1. These were chosen to discuss the different uses for the potential biogas produced, based on the different climate conditions amongst Brazilian regions. The average annual temperature in the southern region is around 20 ◦C, while in the northern regions it raises to around 28 ◦C [10]. Of course, the wide variety of local geographic conditions alongside the country is implied in the variations in such values. In any case, anaerobic digestion in UASB reactors has been successfully carried out throughout the country [3].

A previous study developed by our group [11] identified that the highest environmental, economic and also social gains of biogas recovery are associated with its primary use for sludge sanitization, as this allows the production of safe biosolids that can be used for agricultural purposes. This practice, besides contributing for closing nutrient (nitrogen and phosphorus) cycles, also plays a role for reducing the demand on chemical fertilizers, and for avoiding sludge transportation and disposal in landfills. Further potential uses of biogas produced in small anaerobic-based STPs are for cooking (an attractive alternative for all Brazilian regions, North and South) and for water heating, a choice especially appealing for the South Region, where cooler temperatures prevail. These recommended biogas uses according to each population category are summarized in Table 2.

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**Figure 1.** Groups of Brazilian regions considered for the distribution of the rural population under the categories A, B and C (detailed in Table 1). **Figure 1.** Groups of Brazilian regions considered for the distribution of the rural population under the categories A, B and C (detailed in Table 1).

A previous study developed by our group [11] identified that the highest environmental, economic and also social gains of biogas recovery are associated with its primary use for sludge **Table 2.** Recommended biogas end uses based on population categories and geographic regions.

#### *2.2. Energy Assessment and Carbon Emissions Evaluation*

summarized in Table 2.

**Geographic Region Population Category Recommended/Potential Biogas End Uses** North A, B and C Sludge sanitization Cooking South A, B and C Sludge sanitization Water heating *2.2. Energy Assessment and Carbon Emissions Evaluation* Biogas potential applications and end use depend on the amount of biogas generated, which is primarily a direct function of the STP size. Biogas production and energy recovery options for small-scale STPs using anaerobic reactors were estimated as follows. The equations used to perform the calculations were adapted from Soares et al. [12] (Equations (1)–(10)) and the parameters used are summarised in Table 3.

**Table 2.** Recommended biogas end uses based on population categories and geographic regions.

Biogas potential applications and end use depend on the amount of biogas generated, which is primarily a direct function of the STP size. Biogas production and energy recovery options for small-scale STPs using anaerobic reactors were estimated as follows. The equations used to perform **Table 3.** Parameters used for determining the biogas production and energy recovery potential in small-scale sewage treatment plants (STPs) (adapted from [7,8]).

Note: 1 LPG: liquefied petroleum gas; 2 DS: dry solids.

The energy potential of methane (ECH4-potential) was calculated in terms of unitary methane yield (YCH4), daily per capita sewage generation (Qpc), person equivalent (PE) and the lower calorific value of methane (LCVCH4), as in Equation (1).

$$\text{E}\_{\text{CH4-potential}} \text{ (M) d}^{-1}) = \text{Y}\_{\text{CH4}} \times \text{Qpc} / 1000 \times \text{PE} \times \text{LCV}\_{\text{CH4}} \tag{1}$$

The thermal energy potential (Eth-potential) was calculated in terms of the energy potential of methane and the thermal efficiency of boilers (Eboilers), as in Equation (2).

$$\text{E}\_{\text{th-potential}} \text{ (MJ d}^{-1}) = \text{E}\_{\text{potential-CH4}} \times \text{E}\_{\text{boilers}} / 100 \tag{2}$$

Daily sludge production in UASB reactors (Psludge-UASB) was calculated in terms of the daily dry sludge production per capita (DSPE) and the person equivalent (PE), as in Equation (3).

$$\text{LP}\_{\text{sludge-UASB}} \text{ (kgDS d}^{-1}) = \text{DS}\_{\text{PE}} / 1000 \times \text{PE} \tag{3}$$

The mass of water in sludge (Mwater) was assumed using the daily sludge produced in reactors (Psludge-UASB), the sludge specific mass (γ) and the excess sludge concentration (C), as in Equation (4).

$$\text{M}\_{\text{water}}\,\text{(kg)} = \text{P}\_{\text{sludge-UASB}} / (\gamma \times \text{C} / 100) \tag{4}$$

The daily energy demand for sludge sanitization (Eth-sludge) was calculated in terms of the daily sludge produced in reactors (Psludge-UASB), sludge specific heat (Hs), the difference between the sludge temperature and the sanitized sludge temperature (∆s), the mass of water in sludge (Mwater), water specific heat (Hw) and, the energy loss through the walls of the sanitizing tank (ELsanit-tank), as in Equation (5).

$$\text{E}\_{\text{th-sludge}}\text{ (MI d}^{-1}) = \text{[(P}\_{\text{sludge-ULSB}} \times \text{H}\_{\text{s}} \times \Delta \text{s}) + (\text{M}\_{\text{water}} \times \text{H}\_{\text{W}} \times \Delta \text{s})] \times (1 + \text{EL}\_{\text{sanit-tank}}/100)/1000 \tag{5}$$

The daily surplus of thermal energy (Eth-surplus) was calculated in terms of the daily thermal energy potential (Eth-potential) subtracted from the daily energy demand for sludge sanitization (Eth-sludge), as in Equation (6).

$$\mathbf{E}\_{\text{th-surplus}} \text{ (MJ d}^{-1}) = \mathbf{E}\_{\text{th-potential}} - \mathbf{E}\_{\text{th-sludge}} \tag{6}$$

The daily water heating potential (Wpotential) was calculated in terms of the daily surplus of thermal energy (Eth-surplus), the daily thermal efficiency of boilers (Eboilers), water specific heat (Hw) and the difference between the tap water temperature and the temperature of a bath (∆w), as in Equation (7).

$$\mathbf{W\_{potential}} \text{ (m}^3\text{ }^\text{water}\text{ d}^{-1}) = \text{[(E}\_{\text{th-surplus}} \times \text{E}\_{\text{boilers}}/100)/(\text{H}\_\text{W} \times \Delta\text{w})]/1000\tag{7}$$

The daily use of biogas for cooking (BUcooking) was calculated in terms of the daily surplus of thermal energy (Eth-surplus), the lower calorific value of methane (LCVCH4), the daily biogas consumption for cooking (BCcooking) and the methane content in biogas (%CH4), as in Equation (8).

$$\text{BU}\_{\text{cooling}} \text{ (MWh} \text{ d}^{-1}) = (\text{E}\_{\text{th-surplus}} \times 1000 / \text{LCV}\_{\text{CH4}}) / (\text{BC}\_{\text{cooking}} \times \text{\textdegree } \text{\textdegree C}\_{\text{CH4}}) \tag{8}$$

The montly avoided CO2 emission due to replacement of LPG for cooking (Avoided CO2 cooking) was calculated in terms of the surplus of thermal energy (Eth-surplus), the lower calorific value of methane (LCVCH4), the lower calorific value of LPG (LCVLPG) and the CO2 equivalent emission factor for LPG burn (EFLPG), as in Equation (9).

$$\text{Avoided CO}\_2\text{cooling (kgCO}\_2\text{ month}^{-1}) \colon \text{E}\_{\text{fl-surplus}} \cdot \text{30} \times \text{LCV}\_{\text{CH4}} \text{LCV}\_{\text{LFC}} \times \text{EF}\_{\text{LFG}} \tag{9}$$

The monthly avoided CO2 emission due to electricity replacement for water heating (Avoided CO2 heating) was calculated in terms of the surplus of thermal energy (Eth-surplus) and the CO2 equivalent emission factor for the electricity generation in Brazil (EFelec), as in Equation (10).

$$\text{Avoided CO}\_2\text{ heating (kgCO}\_2\text{ month}^{-1}) = \text{E}\_{\text{dh-surplus}} \times 30 \times \text{EF}\_{\text{elec.}} \tag{10}$$

For small volumes of produced biogas (1–5 Nm3 d −1 ), directly burning them after the hydrogen sulphide (H2S) removal is the most traditional biogas use for domestic applications, such as for heat production especially for cooking. In population agglomerations with agricultural practices, biogas could also be routed for sludge sanitization. These technological arrangements are illustrated in Figure 2. A simplified desorption column followed by a biofilter was considered in the flowsheet for the H2S and CH4 abatement due to the presence of these gases in the anaerobic effluent, therefore, no additional energy gains were achieved because methane is not recovered. The idea in this case is just to avoid greenhouse gas and odorous emissions. *Energies* **2019**, *12*, x FOR PEER REVIEW 6 of 13 For small volumes of produced biogas (1–5 Nm3 d−1), directly burning them after the hydrogen sulphide (H2S) removal is the most traditional biogas use for domestic applications, such as for heat production especially for cooking. In population agglomerations with agricultural practices, biogas could also be routed for sludge sanitization. These technological arrangements are illustrated in Figure 2. A simplified desorption column followed by a biofilter was considered in the flowsheet for the H2S and CH4 abatement due to the presence of these gases in the anaerobic effluent, therefore, no additional energy gains were achieved because methane is not recovered. The idea in this case is just

to avoid greenhouse gas and odorous emissions.

**Figure 2.** Flowsheet of the proposed biogas uses for small-scale STPs (adapted from [12]). **Figure 2.** Flowsheet of the proposed biogas uses for small-scale STPs (adapted from [12]).

These gaseous management schemes were also evaluated in terms of carbon emissions using the tool "Sulphide and Carbon Emission Avoidance and Energy Recovery in STPs" [23]. The tool estimates the corresponding amount of methane supposed to be emitted into the atmosphere by anaerobic-based STPs, which is then converted to a CO2 equivalent, allowing the assessment of the carbon footprint of the STP. These gaseous management schemes were also evaluated in terms of carbon emissions using the tool "Sulphide and Carbon Emission Avoidance and Energy Recovery in STPs" [23]. The tool estimates the corresponding amount of methane supposed to be emitted into the atmosphere by anaerobic-based STPs, which is then converted to a CO2 equivalent, allowing the assessment of the carbon footprint of the STP.

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

#### *3.1. Rural Agglomerations in Brazil 3.1. Rural Agglomerations in Brazil*

The secondary data gathered from the demographic census [8] show that rural agglomerations lacking an appropriate sewage treatment were predominant from 500 to 1500 inhabitants, for all three categories analyzed (A, B and C) in both groups (northern and southern regions), as depicted in Figure 3. This population range embraces more than 5.0 and 2.5 million inhabitants in the northern and southern regions of Brazil, respectively, accounting for more than 92% of the Brazilian rural population gathered in categories A, B and C. This is a clear indication of the need for appropriate sewage treatment solutions for such small settlements. The secondary data gathered from the demographic census [8] show that rural agglomerations lacking an appropriate sewage treatment were predominant from 500 to 1500 inhabitants, for all three categories analyzed (A, B and C) in both groups (northern and southern regions), as depicted in Figure 3. This population range embraces more than 5.0 and 2.5 million inhabitants in the northern and southern regions of Brazil, respectively, accounting for more than 92% of the Brazilian rural population gathered in categories A, B and C. This is a clear indication of the need for appropriate sewage treatment solutions for such small settlements.

A clearer picture of the population distribution in the whole assessed range (500 to 3000 inhabitants) considering the three categories is shown in Figure 4. Most of the population live in "rural areas of urban extension" (category A—Table 1), totalling almost 5.3 million inhabitants, followed by the population

that live in "isolated rural areas where small settlements prevail" (category C—Table 1), accounting for around 2.4 million inhabitants. It is worth mentioning that approximately 94% of the population in category C is located in the northern region. A much lower population contingent (less than 600,000 inhabitants) lives in "isolated rural areas where large settlements prevail" (category B—Table 1). Bearing these numbers, one can realize that the population living in all three categories of such small settlements could potentially benefit from using the by-products generated in sustainable small-scale anaerobic-based STPs, especially biogas (for cooking, water heating and/or sludge sanitization), sanitized sludge and treated effluent (both for agricultural purposes), as further discussed in the following section. *Energies* **2019**, *12*, x FOR PEER REVIEW 7 of 13 **Figure 3.** Distribution of the Brazilian rural population of the northern and southern regions according to intervals and categories. Categories A, B and C as shown in Table 1. A clearer picture of the population distribution in the whole assessed range (500 to 3000

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**Figure 3.** Distribution of the Brazilian rural population of the northern and southern regions according to intervals and categories. Categories A, B and C as shown in Table 1. **Figure 3.** Distribution of the Brazilian rural population of the northern and southern regions according to intervals and categories. Categories A, B and C as shown in Table 1. (for cooking, water heating and/or sludge sanitization), sanitized sludge and treated effluent (both for agricultural purposes), as further discussed in the following section.

population living in all three categories of such small settlements could potentially benefit from using the by-products generated in sustainable small-scale anaerobic-based STPs, especially biogas (for cooking, water heating and/or sludge sanitization), sanitized sludge and treated effluent (both **Figure 4.** Distribution of the Brazilian rural population of the northern and southern regions according to categories. Categories A, B and C as shown in Table 1. **Figure 4.** Distribution of the Brazilian rural population of the northern and southern regions according to categories. Categories A, B and C as shown in Table 1.

#### for agricultural purposes), as further discussed in the following section. *3.2. Proposed Flowsheet for Sewage Treatment and by-Products Recovery/Use 3.2. Proposed Flowsheet for Sewage Treatment and by-Products Recovery*/*Use*

summarized in Table 5, specially focussing on potential biogas uses.

In order to better exemplify some of the environmental, economic and social gains associated with the use of by-products (biogas, sludge and water) generated in small-scale anaerobic-based STPs, we considered the treatment and by-product end uses flowsheet depicted in Figure 5. Although there are other possibilities of destination/uses of such by-products, depending on many factors (e.g., economic activities nearby the STP, applied process units, STP size, etc.), we carried out the study considering only the alternatives schematically represented in Figure 5 and further summarized in Table 5, specially focussing on potential biogas uses. In order to better exemplify some of the environmental, economic and social gains associated with the use of by-products (biogas, sludge and water) generated in small-scale anaerobic-based STPs, we considered the treatment and by-product end uses flowsheet depicted in Figure 5. Although there are other possibilities of destination/uses of such by-products, depending on many factors (e.g., economic activities nearby the STP, applied process units, STP size, etc.), we carried out the study considering only the alternatives schematically represented in Figure 5 and further summarized in Table 4, specially focussing on potential biogas uses.

**Table 4.** By-products' (biogas, sludge and water) end uses considered in this study.

the study considering only the alternatives schematically represented in Figure 5 and further

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**Figure 5.** Schematic flowsheet representation of the small-scale anaerobic-based STP and by-products' end uses considered in the study. Source: Adapted from [24] **Figure 5.** Schematic flowsheet representation of the small-scale anaerobic-based STP and by-products' end uses considered in the study. Source: Adapted from [24]

#### **Table 5.** By-products' (biogas, sludge and water) end uses considered in this study. **Geographic Population Population Equivalent By-Products end Uses** *3.3. Potential Uses of the Biogas Produced in Small-Scale Anaerobic-Based STPs in the Northern and Southern Regions of Brazil*

**Biogas Sludge Effluent**

#### **(Inhabitants)** A 3,021,261 3.3.1. Use of Biogas for Sludge Sanitization

**Category**

**Region**

fertilized.

North Sludge sanitization Cooking Agriculture Fertirrigation B 272,638 C 2,277,349 **Total 5,571,248** South A 2,259,180 Sludge sanitization Water heating Agriculture Fertirrigation B 300,177 C 153,760 Thermal sludge sanitization can be achieved by means of a boiler fed on biogas and a simple heated concrete tank. This tank should be preferably fed once a day, in order to avoid the need of a big biogas holder. The sludge needs to be heated to 70 ◦C for 30 min (pasteurization) by means of a heat exchanger installed in the tank. After the sanitization process, the sludge can be routed to simple dehydration units (e.g., drying beds) or it can be directly spread on the agricultural land to be fertilized.

**Total 2,713,117** *3.3. Potential Uses of the Biogas Produced in Small-Scale Anaerobic-Based STPs in the Northern and Southern Regions of Brazil* 3.3.1. Use of Biogas for Sludge Sanitization Thermal sludge sanitization can be achieved by means of a boiler fed on biogas and a simple The results presented in Figure 6 show that the thermal energy generated by a boiler fed on biogas is much higher than the demand for sludge sanitization (less than 30% of the available energy), considering the typical parameters presented in Table 3. Therefore, the surplus of thermal energy (more than 70%) can be used for other purposes, such as for cooking (northern region) and/or for water heating (southern region), as discussed in the following section. *Energies* **2019**, *12*, x FOR PEER REVIEW 9 of 13

**Figure 6.** Potential thermal energy generation from biogas and supplied demands (sludge sanitization and other uses—cooking/water heating). **Figure 6.** Potential thermal energy generation from biogas and supplied demands (sludge sanitization and other uses—cooking/water heating).

#### 3.3.2. Use of Biogas for Cooking and Water Heating 3.3.2. Use of Biogas for Cooking and Water Heating

and transportation of LPG if biogas is used for cooking [14].

The results presented in Figure 7 show that the direct use of biogas (after attending the demand for sludge sanitization) would allow more than 200,000 families to cook without the need of another external source of heat in the northern area of Brazil. This means that an equivalent population of approximately 800,000 inhabitants, or close to 15% of the total rural population that live in northern Brazil (categories A, B and C altogether), could be supplied with the generated biogas. This The results presented in Figure 7 show that the direct use of biogas (after attending the demand for sludge sanitization) would allow more than 200,000 families to cook without the need of another external source of heat in the northern area of Brazil. This means that an equivalent population of approximately 800,000 inhabitants, or close to 15% of the total rural population that live in northern

addition, biogas is considered a clean and renewable source of energy, therefore, its use in the replacement of LPG would represent remarkable environmental gains due to the extremely high CO2 emission factor of the latter (a petroleum-derived gas). Finally, this study considered agglomerations with no infrastructure in terms of sanitation. Therefore, the recovery of biogas for cooking is an added benefit of having a sewage treatment solution, which can also foster the implementation of new decentralized STPs. As a matter of fact, a recent study on a rural household in Costa Rica found that a family would save the equivalent of USD 26/month with the acquisition

Likewise, the results depicted in Figure 7 indicate that the surplus of thermal energy (after attending the demand for sludge sanitization) would be enough to produce almost 5000 m³ of hot water per day (50 °C). Considering a family with four persons and a consumption of 30 liters of hot water per bath/shower, the amount of produced hot water would be enough to supply almost 40,000 families per day in the southern region of Brazil (around 160,000 persons, or approximately 6% of the total rural population living in this region). This also has an associated positive social impact, as

electricity usually represents a large share (~50%) in the monthly bill paid by families.

Brazil (categories A, B and C altogether), could be supplied with the generated biogas. This possibility of biogas use is of great importance for this region, since the delivery (and costs) of liquefied petroleum gas (LPG) is a matter of concern. In this case, biogas can be supplied at much lower costs than LPG when considering the acquisition and transportation for delivering the fuel. In Brazilian rural areas, there are no gas pipelines and generally road infrastructure is inadequate. In addition, biogas is considered a clean and renewable source of energy, therefore, its use in the replacement of LPG would represent remarkable environmental gains due to the extremely high CO2 emission factor of the latter (a petroleum-derived gas). Finally, this study considered agglomerations with no infrastructure in terms of sanitation. Therefore, the recovery of biogas for cooking is an added benefit of having a sewage treatment solution, which can also foster the implementation of new decentralized STPs. As a matter of fact, a recent study on a rural household in Costa Rica found that a family would save the equivalent of USD 26/month with the acquisition and transportation of LPG if biogas is used for cooking [14].

Likewise, the results depicted in Figure 7 indicate that the surplus of thermal energy (after attending the demand for sludge sanitization) would be enough to produce almost 5000 m3 of hot water per day (50 ◦C). Considering a family with four persons and a consumption of 30 liters of hot water per bath/shower, the amount of produced hot water would be enough to supply almost 40,000 families per day in the southern region of Brazil (around 160,000 persons, or approximately 6% of the total rural population living in this region). This also has an associated positive social impact, as electricity usually represents a large share (~50%) in the monthly bill paid by families.

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**Figure 7.** Thermal energy surplus (after sludge sanitization) and families attended with heat for cooking (northern region) and for water heating (southern region). **Figure 7.** Thermal energy surplus (after sludge sanitization) and families attended with heat for cooking (northern region) and for water heating (southern region).

#### 3.3.3. Avoided Emissions of GHG 3.3.3. Avoided Emissions of GHG

According to the results presented in Figure 8, a remarkable negative carbon footprint would be achieved if the surplus biogas (after its main use for sludge sanitization) was used for cooking in the northern region of Brazil. In this case, approximately 6.1 Gt CO2eq·y−1 (per capita of 1100 kg CO2eq·PE−1·y−1) would be avoided to be emitted into the atmosphere. Such a CO2eq reduction relates to the replacement of LPG by biogas. This may be compared with the per capita GHG emission in the Brazilian energy sector, which is approximately 60 kg CO2eq·PE−1·y−1. Although a much lower avoidance of GHG emissions would be achieved with the use of the According to the results presented in Figure 8, a remarkable negative carbon footprint would be achieved if the surplus biogas (after its main use for sludge sanitization) was used for cooking in the northern region of Brazil. In this case, approximately 6.1 Gt CO2eq·y −1 (per capita of 1100 kg CO2eq·PE−1 ·y −1 ) would be avoided to be emitted into the atmosphere. Such a CO2eq reduction relates to the replacement of LPG by biogas. This may be compared with the per capita GHG emission in the Brazilian energy sector, which is approximately 60 kg CO2eq·PE−1 ·y −1 .

surplus thermal energy from biogas for water heating, there would still be a contribution for neutralizing the overall STP carbon footprint, avoiding the emission of approximately 8 Mt CO2eq·y−1 (per capita of 3 kg CO2eq·PE−1·y−1). In this case, the CO2eq reduction is associated with the replacement of electricity by biogas, considering the emission factor of the Brazilian electric matrix and the energy consumption of an electric shower (Table 3). Although a much lower avoidance of GHG emissions would be achieved with the use of the surplus thermal energy from biogas for water heating, there would still be a contribution for neutralizing the overall STP carbon footprint, avoiding the emission of approximately 8 Mt CO2eq·PE−1 ·y −1 (per capita of 3 kg CO2eq·PE−1 ·y −1 ). In this case, the CO2eq reduction is associated with the replacement of electricity by biogas, considering the emission factor of the Brazilian electric matrix and the energy consumption of an electric shower (Table 3).

**Figure 8.** Avoided greenhouse gas (GHG) emissions due to the use of biogas instead of liquefied petroleum gas (LPG) for cooking (northern region) and for water heating (southern region).

Biogas has been used in rural areas in different regions worldwide and with different feedstock supplies to the anaerobic reactors. As may be observed, in most cases, biogas use in rural areas worldwide is applied to individual/family biodigesters treating agricultural and manure feedstock.

*3.4. Comparison with Biogas Use in Other Rural Contexts*

3.3.3. Avoided Emissions of GHG

**Figure 7.** Thermal energy surplus (after sludge sanitization) and families attended with heat for

According to the results presented in Figure 8, a remarkable negative carbon footprint would be achieved if the surplus biogas (after its main use for sludge sanitization) was used for cooking in the northern region of Brazil. In this case, approximately 6.1 Gt CO2eq·y−1 (per capita of 1100 kg CO2eq·PE−1·y−1) would be avoided to be emitted into the atmosphere. Such a CO2eq reduction relates to the replacement of LPG by biogas. This may be compared with the per capita GHG emission in the

Although a much lower avoidance of GHG emissions would be achieved with the use of the surplus thermal energy from biogas for water heating, there would still be a contribution for neutralizing the overall STP carbon footprint, avoiding the emission of approximately 8 Mt CO2eq·y−1

cooking (northern region) and for water heating (southern region).

Brazilian energy sector, which is approximately 60 kg CO2eq·PE−1·y−1.

**Figure 8.** Avoided greenhouse gas (GHG) emissions due to the use of biogas instead of liquefied **Figure 8.** Avoided greenhouse gas (GHG) emissions due to the use of biogas instead of liquefied petroleum gas (LPG) for cooking (northern region) and for water heating (southern region).

#### petroleum gas (LPG) for cooking (northern region) and for water heating (southern region). *3.4. Comparison with Biogas Use in Other Rural Contexts*

energy consumption of an electric shower (Table 3).

*3.4. Comparison with Biogas Use in Other Rural Contexts* Biogas has been used in rural areas in different regions worldwide and with different feedstock supplies to the anaerobic reactors. As may be observed, in most cases, biogas use in rural areas worldwide is applied to individual/family biodigesters treating agricultural and manure feedstock. Biogas has been used in rural areas in different regions worldwide and with different feedstock supplies to the anaerobic reactors. As may be observed, in most cases, biogas use in rural areas worldwide is applied to individual/family biodigesters treating agricultural and manure feedstock. For instance, China has the highest number of household biogas plants in the world, with 19% of the total population in rural areas (0.9 billion people) using biogas. However, an anaerobic digestion application relies on biodigesters fed with animal manure and agricultural residues [25]. Similarly, rural biodigesters developed in the Latin America region have also been used for treating animal manure, with some applications in agricultural residue and cooking grease. However, no data were found with biodigesters fed with sewage, not even co-digested with other substrates [26]. Nonetheless, both reviews identified similar bottlenecks, such as the low anaerobic biodegradability of lignocellulosic biomass, low temperatures in the winter season (~10 ◦C), low understanding of proper biogas use and limited management and technical support. Some of them such as the interest and involvement of the local population, as well as technical support and management, are also possible bottlenecks that rural STPs with UASB reactors may have.

#### **4. Final Remarks**

The overall balance of biogas production and thermal energy generation that could be achieved via the implementation of small-scale anaerobic-based STPs to attend to the Brazilian rural population grouped in categories A, B and C (around 8.3 million inhabitants) is extremely relevant and should not be neglected by designers and policy makers. The thermal energy available in the biogas would be enough to sanitize the whole amount of sludge produced in the STPs, making this material (biosolid) available to small farmers or even to encourage the practice of family farming nearby the plants. Besides contributing for closing the nutrient (N and P) cycles and lowering the production costs of agricultural products, there would still be a huge indirect benefit derived from the destination shift of this material, nowadays simply transported and disposed of in landfills.

Moreover, the surplus of thermal energy (after sludge sanitization) would be sufficient to attend to the demand of more than 200,000 families in the northern region with biogas for cooking (replacing LPG), and around 40,000 families in the southern region with biogas for water heating (replacing electricity). Again, besides the direct social gains derived from the practice of supplying biogas for domestic uses in the vicinity of the STPs, there would be tremendous indirect gains related to the avoidance of GHG emissions, especially when biogas is used to replace LPG. In this case, we estimated negative (avoided) GHG emissions equivalent to 6.1 Gt CO2eqy −1 ·y −1 .

Likewise, an anaerobic treatment process may benefit small communities not only due to biogas and biosolids production but also with water reuse for agriculture. In this case, a simple post-treatment system (e.g., polishing pond) can meet the disinfection standards for restricted irrigation purposes [27]. Additionally, nitrogen that remains in the effluent can be considered a bonus for land irrigation. Therefore, an anaerobic sewage treatment can be faced as a low-cost technology that generates added value by-products and may improve public health conditions and life quality in Brazilian rural areas.

**Author Contributions:** All authors obtained and discussed the results and wrote the paper. S.R. specifically contributed to analyzing the secondary data from the Brazilian demographic census, while T.B.-R., F.P. and C.A.L.C. led the part on biogas recovery in UASB STPs. All authors have read and agreed to the published version of the manuscript.

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

**Acknowledgments:** The authors acknowledge the support obtained from the following Brazilian institutions: Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq; Fundação de Amparo à Pesquisa de Minas Gerais—FAPEMIG; Instituto Nacional de Ciência e Tecnologia em Estações Sustentáveis de Tratamento de Esgoto—INCT ETEs Sustentáveis and the National Health Foundation (Funasa), that promoted the National Program of Rural Sanitation (PNSR).

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


**3. Basics of Electroplating of Nickel**

Electroplating of nickel involves the passage of current between two electrodes (anode and cathode) immersed in an electrolyte containing nickel salts to cause dissolution of Ni2+ ions in the anode to be deposited at the cathode. Equation 1 and 2 show the anodic and cathodic reactions that take place during nickel deposition. The anode is mainly a nickel plate while the cathode is any metal or material that needs to be protected or decorated. The nickel ions deposited at the cathode are replenished by those formed as the result of the dissolution of the anode. According to Faraday's law of electrolysis, the amount of nickel dissolved at the cathode is equal to the amount of nickel deposited at the cathode, which is directly proportional to the product of current and time [13].

Anodic:

such as high hardness, excellent corrosion resistance, thermal stability, wear resistance, and self-lubrication properties [3]. The nanoparticles incorporated into the matrix include those of

Electrodeposition is one of the surface modification techniques that are used to fabricate nickel nanocomposite coatings. This technique has several advantages over the other processing methods which include low cost, simplicity of operation, versatility, high production rates, industrial applicability, and few size and shape limitations [7]. However, co-deposition of nanostructured inert particles using electrodeposition has its own challenges. Agglomeration of particles in the electroplating bath, inhomogeneity in the distribution of particles in the matrix, and low content of particles in the coatings are some of the drawbacks associated with this process [8]. These problems compromise the quality of the coatings and result in poor performance during application. Therefore, proper process development and optimization are

Many researches have been conducted in an attempt to address these limitations. Additions of chemical agents into electrolytic solutions to aid co-deposition of the particles have been found by many researchers to reduce agglomeration of particles and increase their incorpo‐ ration in the matrix. These additives disturb the electrostatic stabilization of the particles and hence promote their suspension in the solution [9]. Pulse current electrodeposition is another method that has been employed to enhance co-deposition and improve uniform distribution of particles. This type of plating has three independent variables for controlling co-deposition as compared to direct current plating which only has one variable [10]. Ultrasonic energy has also been used to improve the inclusion of particles into the metal matrix. It enhances mass transport of particles to the cathode for co-deposition, reduces the thickness of the diffusion

**2. Electrodeposition of nickel composite/nanocomposite coatings**

Electrodeposition of composite coatings has been studied and researched by several scholars over the past few decades. The focus of the research is centred around the development and fabrication of advanced and novel surface coatings that can withstand both physical, chemical, and mechanical deterioration under service conditions. In order to produce these high-quality coatings, the mechanism of co-deposition of reinforcement particles with the matrix, optimi‐ zation of the process operating parameters, and the properties of the resultant deposits need

Electroplating of nickel involves the passage of current between two electrodes (anode and cathode) immersed in an electrolyte containing nickel salts to cause dissolution of Ni2+ ions in the anode to be deposited at the cathode. Equation 1 and 2 show the anodic and cathodic

metal oxides, carbides, nitrides, borides, polymers, and carbon-based materials [4–6].

required to counteract the limitations.

206 Electrodeposition of Composite Materials

to be fully understood.

**3. Basics of Electroplating of Nickel**

layer, and disperses the particles in the electrolyte [11,12].

$$\text{Ni} \rightarrow \text{Ni}^{2+} + \text{ } \text{2e}^{-} \tag{1}$$

Cathodic:

$$\text{Ni}^{2+} + \text{2e}^{-} \rightarrow \text{Ni} \tag{2}$$

There are many bath solutions that have been developed for producing different types of nickel deposits. The ones that have gained more usage include the Watts and sulphamate baths. These plating baths are reinforced with surfactants and other additives to improve the quality of the deposits such as brightness, surface morphology, and other functional properties.

### **4. Mechanism of Co-deposition**

Many models have been developed to understand the mechanism of co-deposition of particles into metallic matrix. These models predict the processes that are involved during particle codeposition and their adsorption rate into the coatings. The processes include electrophoresis, mechanical inclusion, adsorption, and convective-diffusion. One of the most used and accepted model by scholars is Guglielmi's two-step adsorption model. In the first step of particle incorporation, the particles are loosely adsorbed on the cathode covered by a cloud of metal ions. Strong adsorption of particles follows in the second step with current density playing a key role in the particles to be strongly adsorbed on the cathode. The strongly adsorbed particles are embedded into the growing metallic layer [14]. The author related the volume fraction of co-deposited particles (α) to the volume percent of particles suspended in the plating bath (*C*) with the Langmuir adsorption isotherm as shown in equation 3

$$\frac{C}{a} = \frac{M i\_{\ o}}{n F \rho\_{\text{m}} v\_{\ o}} \exp(A - B) \eta \left(\frac{1}{k} + C\right) \tag{3}$$

where is the atomic weight of the electrodeposited metal *i*o the exchanging current density, the valence of the electrodeposited metal, *F* the Faraday constant, ρ*m* the density of electrodepos‐ ited metal, the overpotential of electrode reaction, *i* = *io* exp(*Aɳ)* and *k* the Langmuir isotherm constant, mainly determined by the intensity of interaction between particles and cathode. The parameters *vo* and *B* are related to particle deposition, and both play a symmetrical role with the parameters *io* and *A* related to metal deposition [15]. Ref. [16] improved Guglielmi's model by using three modes of current density to differentiate the reduction of adsorbed ion on the particles. This new model involved three steps, where in the first step particles are convectively forced to the surface followed by loose adsorption and then irreversible incorporation of particles into the matrix by reduction of adsorbed ions. Ref. [17] incorporated a third-order polynomial equation to further improve Guglielmi's model. This corrective factor will help account for the effects of adsorption and hydrodynamic conditions. Many other models have been developed which involved statistical approach to predict the chances of particles being included into the deposit. However, all these models cannot predict the effect of particles on electrocrystallization and are limited to specific conditions.

The manner of incorporation of particles into metal matrix depends mainly on the electrode‐ position process parameters. Some of the most important parameters include the speed at which the bath is stirred, the applied current density, and electrolyte composition. Bath agitation serves as a medium that assists particles to be transported to the cathode, while applied current density and electrolyte composition are responsible for the formation of ionic cloud around the introduced particles. There are three possibilities for particles to be incor‐ porated in to a metal matrix [18]:

A schematic diagram of particle incorporation into a metal deposit is shown in Figure 1. The manner in which particles are incorporated into the coating determines the quality of the resulting deposits. Coatings containing uniformly distributed and truly embedded particles exhibit superior properties than the other manners of incorporation.

**Figure 1.** Schematic representation of different co-deposition possibilities


**1. Introduction**

Self-organization of molecular structures refers to the spontaneous assembly of individual building blocks into ordered and thermodynamically stable nanostructures by noncovalent interactions, resulting in the formation of dynamic and responsive materials. The noncovalent (reversible) interactions include electrostatic attractions, metal/ligand complexes, π/πstacking, ionic interactions, hydrogen bonding, or hydrophobic effects without the application of external energy. Therefore, self-assembly is a low-cost and high-yield process [1–6] that can be activated by external stimuli, which involves changes in temperature, pH, ionic strength, radiation, the addition of other molecules, or their combination [2, 7–9].

© 2016 The Author(s). Licensee InTech. 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. © 2018 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.

Supramolecular polymer networks can be formed by polymer blends (PBs) in which the reversible and highly directional noncovalent interactions are randomly distributed across the polymer chains [6]. The resulting supramolecular structure displays polymeric properties in dilute and concentrated solutions that follow well-established theories of polymer physics [9]. These networks exhibit enhanced material properties compared to the individual polymers [1], and the reversibility of noncovalent interactions provides self-healing properties to these materials [6, 10].

conformation reversal to a reordered-renatured state [11–19, 21–23, 27, 29, 30]. In saline solu-

Viscoelasticity of a Supramolecular Polymer Network and its Relevance for Enhanced Oil Recovery

moieties on the xanthan trisaccharide side chains, reducing repulsive forces between polymer chains. The side chains collapse down to the backbone causing conformation transition to a rigid rod-like shape of reduced hydrodynamic size stabilized by hydrogen bonding [23]. This conformation increases the intra- and intermolecular attraction and the xanthan chains tend to adopt a more rigid ordered conformation [13, 14]. The stability of xanthan gum is attributed to its ordered conformation which is stabilized by salts. Thus, the optimum functionality of

Xanthan gum is prone to self-association at low concentrations and has been used as a building block to generate self-assembled structures [24, 25]. The onset of macromolecular selforganization has been reported at concentrations ≥0.1 wt% [8]. Parallel packing or side-by-side associations stabilized by hydrogen bonds facilitate the formation of three-dimensional networks [8, 19, 26]. Previous research has also demonstrated the self-organization of xanthan gum through polyelectrolyte complexation with other polysaccharides, such as chitosanxanthan, β-lactoglobulin-xanthan, α-galactosidase-modified guar gum-xanthan, and sodium caseinate-xanthan, among many others [8, 26, 31]. Another example of self-aggregation through hydrophobic interactions is the intermolecular binding of xanthan gum-carob gum. These self-organizations take place in both the ordered and disordered conformations of the

Advantages of xanthan gum include non-toxicity, high water solubility, salinity tolerance, stability over a broad range of pH values, thermal stability against hydrolysis provided by its ordered conformation, high shear stability, high viscosities but a significant shear-thinning behavior on shearing at a low concentration, slight variations in viscosity with changes in temperature, availability, the ease of processing, and low manufacturing costs [11, 17, 18, 22, 26, 34, 35]. The main downside of xanthan gum is high sensitivity to microbial attack. Salt-tolerant aerobic and anaerobic microorganisms can degrade the xanthan gum chains leading to loss in solution viscosity. In practical applications, biocides are added to the xanthan gum solution to suppress the

Partially hydrolyzed polyacrylamides, HPAMs (**Figure 1(b)**), with degree of hydrolysis ranging from 25 to 35%, are the most used polymers for enhanced oil recovery (EOR) [35, 37]. HPAM is highly sensitive to mono- and divalent cation environments because of the shielding effect of the negative charges on the polymer backbone. Thus, the static repulsion between the HPAM lateral groups fades and the polymer structure collapses, which drastically reduces the viscosity of the polymer solution [35]. Divalent cations (e.g., Ca2+) can also induce the self-aggregation of HPAM through polyion-cation complexation and depending on the concentration of Ca2+ inter- and intrachain complexation takes place [36]. The molecular conformation and viscosity of HPAM solutions are also affected by the degree of hydrolysis, pH, temperature, molecular weight, pressure, and solvent quality [35, 36]. HPAM is susceptible to mechanical degradation under high shear due to chain scission reducing the polymer chain size and molecular weight, which in turns reduces the viscosity of the polymer solution. Advantages of HPAM are low manufacturing costs and resistance to bacterial attack [36].

xanthan macromolecule depending on the ionic strength [8, 20, 32, 33].

and Ca2+) condense on the ionized carboxyl

http://dx.doi.org/10.5772/intechopen.77277

97

tions, monovalent or divalent cations (e.g., Na+

xanthan gum requires the addition of salt [17].

growth of degrading microorganisms [11, 17, 36].

This study focuses on the formulation and characterization of a supramolecular polymer network based on reversible interactions among the main- and side chains of a mixture of xanthan gum, HPAM, and HMPAM or associating polymer (AP) in brine. Xanthan gum and HPAM are the most commonly used polymers in the field for enhanced oil recovery (EOR) [11]. **Figure 1** displays the chemical structures of these polymers.

Xanthan gum is a high-molecular weight (≈2–3 × 106 g/mol) anionic polysaccharide derived from the fermentation of the bacterium *Xanthomonas campestris*. Its chemical structure contains a cellulose backbone consisting of five monosaccharides to form a pentasaccharide repeating unit. The cellulosic backbone is substituted at C-3 on alternate β-1,4-D-glucopyranosyl residues with charged trisaccharide side chains of β-D-rhamnopyranosyl β-1,4-D-glucuronopyranosyl, and α-1,2-D-mannopyranosyl. The two charged functional groups, COO− , are found in the α-Manp and β-GlcAp residues (**Figure 1(a)**) [8, 11–25].

Xanthan gum is a stereoregular polymer and displays order–disorder transition in solution that depends on polymer concentration, ionic strength, and temperature [11–14, 17, 19, 24, 26]. A high polymer concentration and an increased ionic strength support ordered structures, while elevated temperatures favor disorder structures [11]; therefore, xanthan gum is a thermally responsive biopolymer [27]. At low temperatures, the native state of xanthan gum shows an ordered and rigid conformation as a single helical or a double-strand helix stabilized by inter- and intramolecular hydrogen bonds. The ordered conformation of the xanthan molecule is semi-flexible with a hydrodynamic length ranging from 600 to 2000 nm and a hydrodynamic diameter of 2 nm [20]. At elevated temperatures (≥60°C) [12, 16, 24] and low ionic strength, the native state, which displays an order state, transitions to a more flexible disordered-denatured state [13]. Optical rotation measurements have demonstrated that xanthan gum dissolved in distilled water at 25°C displayed the nonordered conformation [24]. However, a decrease in temperature (<40°C) [16, 24, 28] and the simultaneous increase in the ionic strength cause

**Figure 1.** Generic chemical structures of (a) xanthan gum, (b) HPAM, and (c) associating polymers (**Figure 1(a)** was adapted from reference [12] and **Figure 1(b)** and **(c)** were adapted from reference [13]).

conformation reversal to a reordered-renatured state [11–19, 21–23, 27, 29, 30]. In saline solutions, monovalent or divalent cations (e.g., Na+ and Ca2+) condense on the ionized carboxyl moieties on the xanthan trisaccharide side chains, reducing repulsive forces between polymer chains. The side chains collapse down to the backbone causing conformation transition to a rigid rod-like shape of reduced hydrodynamic size stabilized by hydrogen bonding [23]. This conformation increases the intra- and intermolecular attraction and the xanthan chains tend to adopt a more rigid ordered conformation [13, 14]. The stability of xanthan gum is attributed to its ordered conformation which is stabilized by salts. Thus, the optimum functionality of xanthan gum requires the addition of salt [17].

Supramolecular polymer networks can be formed by polymer blends (PBs) in which the reversible and highly directional noncovalent interactions are randomly distributed across the polymer chains [6]. The resulting supramolecular structure displays polymeric properties in dilute and concentrated solutions that follow well-established theories of polymer physics [9]. These networks exhibit enhanced material properties compared to the individual polymers [1], and the reversibility of noncovalent interactions provides self-healing properties to

This study focuses on the formulation and characterization of a supramolecular polymer network based on reversible interactions among the main- and side chains of a mixture of xanthan gum, HPAM, and HMPAM or associating polymer (AP) in brine. Xanthan gum and HPAM are the most commonly used polymers in the field for enhanced oil recovery (EOR)

Xanthan gum is a high-molecular weight (≈2–3 × 106 g/mol) anionic polysaccharide derived from the fermentation of the bacterium *Xanthomonas campestris*. Its chemical structure contains a cellulose backbone consisting of five monosaccharides to form a pentasaccharide repeating unit. The cellulosic backbone is substituted at C-3 on alternate β-1,4-D-glucopyranosyl residues with charged trisaccharide side chains of β-D-rhamnopyranosyl β-1,4-D-glucuronopyranosyl,

Xanthan gum is a stereoregular polymer and displays order–disorder transition in solution that depends on polymer concentration, ionic strength, and temperature [11–14, 17, 19, 24, 26]. A high polymer concentration and an increased ionic strength support ordered structures, while elevated temperatures favor disorder structures [11]; therefore, xanthan gum is a thermally responsive biopolymer [27]. At low temperatures, the native state of xanthan gum shows an ordered and rigid conformation as a single helical or a double-strand helix stabilized by inter- and intramolecular hydrogen bonds. The ordered conformation of the xanthan molecule is semi-flexible with a hydrodynamic length ranging from 600 to 2000 nm and a hydrodynamic diameter of 2 nm [20]. At elevated temperatures (≥60°C) [12, 16, 24] and low ionic strength, the native state, which displays an order state, transitions to a more flexible disordered-denatured state [13]. Optical rotation measurements have demonstrated that xanthan gum dissolved in distilled water at 25°C displayed the nonordered conformation [24]. However, a decrease in temperature (<40°C) [16, 24, 28] and the simultaneous increase in the ionic strength cause

**Figure 1.** Generic chemical structures of (a) xanthan gum, (b) HPAM, and (c) associating polymers (**Figure 1(a)** was

adapted from reference [12] and **Figure 1(b)** and **(c)** were adapted from reference [13]).

, are found in the

[11]. **Figure 1** displays the chemical structures of these polymers.

α-Manp and β-GlcAp residues (**Figure 1(a)**) [8, 11–25].

and α-1,2-D-mannopyranosyl. The two charged functional groups, COO−

these materials [6, 10].

96 Polymer Rheology

Xanthan gum is prone to self-association at low concentrations and has been used as a building block to generate self-assembled structures [24, 25]. The onset of macromolecular selforganization has been reported at concentrations ≥0.1 wt% [8]. Parallel packing or side-by-side associations stabilized by hydrogen bonds facilitate the formation of three-dimensional networks [8, 19, 26]. Previous research has also demonstrated the self-organization of xanthan gum through polyelectrolyte complexation with other polysaccharides, such as chitosanxanthan, β-lactoglobulin-xanthan, α-galactosidase-modified guar gum-xanthan, and sodium caseinate-xanthan, among many others [8, 26, 31]. Another example of self-aggregation through hydrophobic interactions is the intermolecular binding of xanthan gum-carob gum. These self-organizations take place in both the ordered and disordered conformations of the xanthan macromolecule depending on the ionic strength [8, 20, 32, 33].

Advantages of xanthan gum include non-toxicity, high water solubility, salinity tolerance, stability over a broad range of pH values, thermal stability against hydrolysis provided by its ordered conformation, high shear stability, high viscosities but a significant shear-thinning behavior on shearing at a low concentration, slight variations in viscosity with changes in temperature, availability, the ease of processing, and low manufacturing costs [11, 17, 18, 22, 26, 34, 35]. The main downside of xanthan gum is high sensitivity to microbial attack. Salt-tolerant aerobic and anaerobic microorganisms can degrade the xanthan gum chains leading to loss in solution viscosity. In practical applications, biocides are added to the xanthan gum solution to suppress the growth of degrading microorganisms [11, 17, 36].

Partially hydrolyzed polyacrylamides, HPAMs (**Figure 1(b)**), with degree of hydrolysis ranging from 25 to 35%, are the most used polymers for enhanced oil recovery (EOR) [35, 37]. HPAM is highly sensitive to mono- and divalent cation environments because of the shielding effect of the negative charges on the polymer backbone. Thus, the static repulsion between the HPAM lateral groups fades and the polymer structure collapses, which drastically reduces the viscosity of the polymer solution [35]. Divalent cations (e.g., Ca2+) can also induce the self-aggregation of HPAM through polyion-cation complexation and depending on the concentration of Ca2+ inter- and intrachain complexation takes place [36]. The molecular conformation and viscosity of HPAM solutions are also affected by the degree of hydrolysis, pH, temperature, molecular weight, pressure, and solvent quality [35, 36]. HPAM is susceptible to mechanical degradation under high shear due to chain scission reducing the polymer chain size and molecular weight, which in turns reduces the viscosity of the polymer solution. Advantages of HPAM are low manufacturing costs and resistance to bacterial attack [36].

Hydrophobically modified polyacrylamides (HMPAMs) are water-soluble polymers containing both hydrophobic groups (e.g., methyl or ethyl acrylates, alkyl vinyl ethers, styrene comonomers, or alkyl acrylamides) and weakly charged pendant groups (e.g., carboxylic acid pendant groups) directly attached to the polymer backbone (**Figure 1(c)**). The hydrophobic groups can be distributed in a block-like fashion, randomly, or discretely distributed along the backbone. In aqueous solutions, these hydrophobic groups aggregate through intra- and intermolecular associations that increase the hydrodynamic volume of the polymer, which increases the viscosity of the polymer solution [36, 38]. The alkyl chain length and charge density determine the hydrophobicity of these polymers [39]. The presence of counter-ions in the aqueous media reduces the repulsion between charged groups within the polymer chain, which favors inter- and intrachain hydrophobic interactions [39, 40]. Benefits of HMPAM include salt tolerance and the enhancement in solution viscosity in the presence of low-molecular-weight electrolytes because of the shielding of intramolecular Coulombic attractions rather than the intermolecular hydrophobic interactions [36]. The inter- and intramolecular hydrophobic interactions are dynamic and reversible. Therefore, under high shear rate, these associations are interrupted and the viscosity of the solution decreases, but as soon as the high mechanical shear is removed, the hydrophobic groups re-associate and the viscosity of the solution returns to its original value [36].

were mixed using a magnetic stirrer under strong mixing conditions to prevent the formation of lumps (e.g., fish eyes) at room temperature. The viscoelastic behavior, DMA, and DTMA of the baseline polymers and SAP were characterized through oscillatory rheology at 25°C using a Bohlin Geminin HR Nano Rheometer manufactured by Malvern (Worcestershire, UK) equipped with a parallel-plate measuring geometry (gap: 1000 μm) and a solvent trap to prevent drying effects during measurements. The amplitude sweep was performed in the strain range from 1 to 1000% at a fixed frequency of 1 rad/s, while the frequency sweep was run from 0.01 to 100 rad/s at fixed strain within the linear viscoelastic (LVE) range as determined from

Viscoelasticity of a Supramolecular Polymer Network and its Relevance for Enhanced Oil Recovery

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**Figure 2** displays the frequency sweeps as the curve functions of the loss factor (tan*δ*), storage modulus (*G*'), loss modulus (*G*"), and complex viscosity (|*η*\*|) for the baseline polymers B, HPAM, and AP at concentrations of 0.2, 0.2, and 0.1 wt%, respectively, prepared in 2.1 wt% brine. The frequency sweeps (**Figure 2**) demonstrate that all polymers follow the characteristic shear-thinning flow behavior of polymers, thus viscosity decreases with increasing shear rate. At the same salinity (2.1 wt% brine), temperature, and angular frequency range, polymer B at 0.2 wt% solution concentration shows a significantly higher viscoelasticity than the HPAM at 0.2 wt% solution and the AP at 0.1 wt% solution. The behavior of polymer B (i.e., xanthan gum) results from its molecular-ordered aggregation behavior in saline solutions.

Regarding the loss factor (tan*δ*), which is useful to describe the viscoelastic behavior of materials in the low shear range (*ω* ≤ 1 s−1) [12, 45]; **Figure 2** shows that polymer B behaves different from the HPAM and AP polymer solutions. Polymer B exhibits a flow transition from a viscoelastic liquid behavior (tan*δ* > 1) to a viscoelastic weak-gel behavior (tan*δ* < 1) as angular frequency increases. At low angular frequencies (i.e., *ω* ≤ 1 rad/s), the polysaccharide molecules are entangled with neighboring macromolecules forming aggregates stabilized

**2.1 4.2 8.4**

**Figure 2.** Frequency sweeps as curve functions of tan*δ*, *G*', *G*', and |*η*\*|for baseline polymers prepared in 2.1 wt% brine.

NaCl 1.72 3.45 6.9 MgCl2 0.04 0.09 0.18 CaCl2 0.33 0.65 1.30

SO4 0.01 0.02 0.04

the preceding amplitude sweep.

**Salts Brine compositions (wt%)**

Na2

**Table 1.** Brine compositions (wt%).

The goal of this exploratory research was to formulate a supramolecular polymer network with enhanced properties (i.e., viscoelasticity, mechanical stability, salt tolerance, and thermal stability), taking advantage of the synergistic combination of the beneficial properties of the individual polymers in the blend. The formation and characterization of the supramolecular polymer network was demonstrated through frequency (time) and temperature-dependent oscillatory rheology [4, 5]; the use of other analytical techniques for structural characterization of the supramolecular system is beyond the scope of this work.

In this chapter, the formulation and characterization of the supramolecular self-assembled polymer network (SAP) is first described, and then the effect of ionic strength on the viscoelastic behavior of the SAP is discussed. The next section describes the structural strength of the SAP through dynamic mechanical analysis (DMA), followed by the evaluation of the SAP thermal stability by dynamic mechanical thermo-analysis (DMTA). In the following section, the long-term thermal stability of the SAP prepared in brines having different ionic strengths is discussed. Finally, the performance of the SAP in displacing heavy oil employing conventional sad-pack displacement tests at simulated oil reservoir conditions is presented.