Biochar-Assisted Wastewater Treatment and Waste Valorization

*Abhishek Pokharel, Bishnu Acharya and Aitazaz Farooque*

### **Abstract**

Biochar is the solid byproduct of pyrolysis, and its cascading use can offset the cost of the production and its use in application such as soil remediation. A wide variety of research on biochar has highlighted its ability to absorb nutrients, metal and complex compounds, filter suspended solids, enhance microorganisms' growth, retain water and nutrients as well as increasing the carbon content of the soil. Besides, sustainable biochar systems are an attractive approach for carbon sequestration and total waste management cycle. The chapter looks into such cascading use of biochar in wastewater treatment for recovering nutrients and improving the efficiency of activated sludge treatment and anaerobic digestion for producing biosolid with enhanced soil amendment properties.

**Keywords:** biochar, wastewater treatment, activated sludge treatment, anaerobic digestion, nutrient recovery, waste valorization

### **1. Introduction**

Today, the global population continues to grow by 83 million annually and is predicted to be 9.8 billion in 2050 [1]. This increase in population will lead to higher demands of food, water, and energy, which have already been constrained due to the competing needs for limited resources in many parts of the world [2]. The challenges presented by climate change, pollution, and developing economy are posing significant pressure on food, water, and energy systems [3]. Efficient and integrated management of energy, food, and water resources could help address several of the biggest global challenges, such as climate change, sustainable economy, food security, environmental and social security [4, 5]. In the future, we will need increased food production, clear water sources, as well as alternative energy options with minimum resource utilization and ideally decreasing environmental impacts [6]. Work is underway to improve the food production chain as well as develop new technologies for renewable energy. So far less focus has been given to the water, especially to the management of the wastewater. There is a need for shifting the paradigm in the case of wastewater management from treatment and disposal to reuse, recycle, and resource recovery. With growing water scarcity and the fact that uncontrolled disposal of wastewater to the freshwater system is causing depletion of the system also stresses toward a change in mindset about wastewater management. This approach will prevent detrimental impacts on human health and ecosystem caused by the current handling methods. The next step toward a sustainable future will be wastewater treatment serving multiple purposes of treatment and recovery of resources like water, nutrient, and energy. The efficient wastewater management approach will see a cascaded benefit in other sectors including production of fertilizers such as nitrogen and phosphorus. Phosphorus is obtained from ore called phosphate rocks. The quality and accessibility of currently available phosphate rock reserves are declining, and the cost to mine, refine, store, and transport them is rising [7, 8]. Similarly, the production of nitrogen and other mineral fertilizers is energy intensive as well as contributes to environmental pollution [3, 9]. The nitrogen fertilizer can leach to nearby water bodies leading to the phenomenon of eutrophication. The richness of nutrients in the water results in excessive growth of macroalgae and could lead to anoxic events and loss of aquatic system. Recovery of these nutrients from wastewater helps to close the cycle and reduce the amount of chemical fertilizer, directly contributing to the sustainability of food production.

One of the first indications of intentional nutrient recycling is documented 5000 years ago in rural Asia, where human excreta was used for fertilization of fields called "night soil" [10]. In the nineteenth and twentieth century with the industrial revolution, the population density became high, which gave rise to "Sanitation Revolution," a transition from land-based to water-based disposal of human wastes. This disposal system changed the nutrient cycle from reuse to complete discard. Following the Industrial and Sanitation Revolutions, the Green Revolution that reformed agriculture largely abandoned organic fertilizers and put forth the mineral fertilizers [10, 11]. Furthermore, owing to the excessive population growth, producing enough food with only organic sources of plant nutrients has become impossible. Therefore, the need for mineral fertilizers is a true fact. Thus, many urban areas have dedicated wastewater treatment plant to remove the nuisance of human waste. But, it is becoming evident that future changes, particularly those associated with urbanization and population growth-related increase in volume of wastewater, add more stress to the wastewater system performance [12].

The greater dependency on fossil fuels in every sector is heavily contributing to global warming and climate change [13]. As an alternative, abundant biomass could play an essential role in reducing the dependency on fossil fuel as well as contribute toward sustainable development. Pyrolysis of biomass produces biochar and bio-oil. The bio-oil could be used as fuel to substitute the petroleum products with some upgrading that includes catalytic esterification and hydrogenation. The biochar could be used for energy and soil application [14]. Soil application helps in sequestration of carbon dioxide and subsequently supports food production. At present, the biochar application in soil remediation is not cost-effective. The financial feasibility could be improved by developing a cascaded use of biochar, as discussed in this chapter. The inherent properties of biochar make it suitable for (a) recovering nutrients from the wastewater, (b) improving the activated sludge treatment to reduce the energy use for aeration and to improve the settling ability of sludge, (c) increasing the energy recovery from sludge through anaerobic digestion, and (d) enhancing the quality of the biosolids for soil application. There are reports of biochar application having agronomic benefits in fertilizer management, yield, and soil biota [15–20]. Biochar, as a sound absorbent, also holds promise for low-cost wastewater treatment as an alternative to activated carbon [21–24]. The integrated use of biochar in wastewater treatment addresses the current issues with the management of wastewater. However, the benefit of using biochar varies with its type and characteristics, which depends on the biomass, and the pyrolysis conditions [25].

This chapter provides insights on the use of biochar in a wastewater treatment process to enhance the treatment as well as recover valuable byproducts. The chapter will discuss biochar production and properties, mechanisms involving removal of organic and inorganic compounds from the effluent phase, and role in activated sludge treatment and anaerobic digestion.

**223**

*Biochar-Assisted Wastewater Treatment and Waste Valorization*

**2. Biochar properties for wastewater treatment**

mine the effectiveness of biochar in wastewater treatment.

relatively low polarity and C/N ratio [30, 34, 37].

anion exchange capacity in biochars [38].

Biochar is a carbon-rich solid material produced from biomass through a thermochemical process called pyrolysis. During pyrolysis, lignin, cellulose, hemicellulose, fat, and starch in the feedstock are thermally broken down forming three products: biochar (solid), bio-oil (partly condensed volatile matter), and non-condensable gases (CO2, CO, CH4, and H2) [26, 27]. The bio-oil and gases can be captured to produce energy and depending on the feed valuable coproducts like wood preservatives, food flavoring, adhesive, or biochemical compounds [28]. The yield of biochar and the properties, however, depends on the pyrolysis condition. Slow pyrolysis at moderate temperature (350–500°C) and slow heating rate results in higher yield (30%) of biochar than around 10% or less yield with fast pyrolysis (600–700°C and fast heating rate) or gasification (temperature 700°C or above) [29]. The feedstock type and pyrolysis condition used during the production of biochar notably change the physiochemical properties such as surface area, polarity, atomic ratio, pH, and elemental composition [25, 30, 31]. These properties deter-

Biochar has wide applications in water and wastewater treatment because of its distinctive characteristics, for example, adsorption capacity, specific surface area, microporosity, and ion exchange capacity [30, 32]. The removal mechanisms of different pollutants are governed by their interactions with various attributes of biochar, which depends on pyrolysis temperature and feedstock type [33]. Pyrolysis temperature greatly affects the properties of biochar. The increase in pyrolysis temperature results in higher carbon content, hydrophobicity, aromaticity, surface area, and microporosity in biochar [34]. Similarly, the pH of the biochar increases with increasing pyrolysis temperature due to enrichment of ash content in the biochar [35, 36]. High-temperature (>500°C) biochar has low polarity and acidity due to loss of O- and H-containing functional groups [34]. Lower pyrolysis temperature (<500°C) facilitates partial carbonization, thus yielding biochar with smaller pore size, lower surface area, and high O-containing functional groups [36]. Lower temperature biochar contains a higher content of dissolved organic carbon,

Biochar often compromises of both positively and negatively charged surfaces (zwitterionic) [34, 35]. The negatively charged functional groups contribute to cation exchange capacity (CEC) whereas anion exchange capacity (AEC) is also exhibited by O-containing functional groups (oxonium heterocycles) in biochar [36, 38]. Oxygen (O) containing alcohol, carbonyl, and carboxylate functional groups are generally believed to contribute to biochar cation exchange capacity because they carry a negative charge and serve as Lewis bases for the sorption of cations. Whereas, it is believed that oxonium functional groups contribute to pHindependent anion exchange and that both pyridinic functional groups and nonspecific proton adsorption by condensed aromatic rings contribute to pH-dependent

Biochar derived from woody biomass and crop residues has a higher surface area compared to that of solid municipal wastes and animal manure [30]. Apart from the usual pyrolysis method, different engineering methods have been developed and used to expand biochar's applications. Engineered biochar is the derivative of biochar that is modified by physical, chemical, and biological methods to improve its physical, chemical, and biological properties (e.g., specific surface area, porosity, cation exchange capacity, surface functional group, pH etc.) and its adsorption capacity [37, 39, 40]. Some of the modification includes anaerobic digestion of feedstock before pyrolysis, steam/gas activation, pyrolysis using microwave heating, ball milling, magnetic modification, chemical modification using hydrogen

*DOI: http://dx.doi.org/10.5772/intechopen.92288*

*Applications of Biochar for Environmental Safety*

contributing to the sustainability of food production.

a cascaded benefit in other sectors including production of fertilizers such as nitrogen and phosphorus. Phosphorus is obtained from ore called phosphate rocks. The quality and accessibility of currently available phosphate rock reserves are declining, and the cost to mine, refine, store, and transport them is rising [7, 8]. Similarly, the production of nitrogen and other mineral fertilizers is energy intensive as well as contributes to environmental pollution [3, 9]. The nitrogen fertilizer can leach to nearby water bodies leading to the phenomenon of eutrophication. The richness of nutrients in the water results in excessive growth of macroalgae and could lead to anoxic events and loss of aquatic system. Recovery of these nutrients from wastewater helps to close the cycle and reduce the amount of chemical fertilizer, directly

One of the first indications of intentional nutrient recycling is documented 5000

years ago in rural Asia, where human excreta was used for fertilization of fields called "night soil" [10]. In the nineteenth and twentieth century with the industrial revolution, the population density became high, which gave rise to "Sanitation Revolution," a transition from land-based to water-based disposal of human wastes. This disposal system changed the nutrient cycle from reuse to complete discard. Following the Industrial and Sanitation Revolutions, the Green Revolution that reformed agriculture largely abandoned organic fertilizers and put forth the mineral fertilizers [10, 11]. Furthermore, owing to the excessive population growth, producing enough food with only organic sources of plant nutrients has become impossible. Therefore, the need for mineral fertilizers is a true fact. Thus, many urban areas have dedicated wastewater treatment plant to remove the nuisance of human waste. But, it is becoming evident that future changes, particularly those associated with urbanization and population growth-related increase in volume of

wastewater, add more stress to the wastewater system performance [12].

The greater dependency on fossil fuels in every sector is heavily contributing to global warming and climate change [13]. As an alternative, abundant biomass could play an essential role in reducing the dependency on fossil fuel as well as contribute toward sustainable development. Pyrolysis of biomass produces biochar and bio-oil. The bio-oil could be used as fuel to substitute the petroleum products with some upgrading that includes catalytic esterification and hydrogenation. The biochar could be used for energy and soil application [14]. Soil application helps in sequestration of carbon dioxide and subsequently supports food production. At present, the biochar application in soil remediation is not cost-effective. The financial feasibility could be improved by developing a cascaded use of biochar, as discussed in this chapter. The inherent properties of biochar make it suitable for (a) recovering nutrients from the wastewater, (b) improving the activated sludge treatment to reduce the energy use for aeration and to improve the settling ability of sludge, (c) increasing the energy recovery from sludge through anaerobic digestion, and (d) enhancing the quality of the biosolids for soil application. There are reports of biochar application having agronomic benefits in fertilizer management, yield, and soil biota [15–20]. Biochar, as a sound absorbent, also holds promise for low-cost wastewater treatment as an alternative to activated carbon [21–24]. The integrated use of biochar in wastewater treatment addresses the current issues with the management of wastewater. However, the benefit of using biochar varies with its type and characteristics, which depends on the biomass, and the pyrolysis

This chapter provides insights on the use of biochar in a wastewater treatment process to enhance the treatment as well as recover valuable byproducts. The chapter will discuss biochar production and properties, mechanisms involving removal of organic and inorganic compounds from the effluent phase, and role in activated

**222**

conditions [25].

sludge treatment and anaerobic digestion.
