Options for the Disposal and Reuse of Wastewater Sludge, Associated Benefit, and Environmental Risk

Abstract

To protect human and environmental health, wastewater treatment is one of the important activities in urban and industrial areas. Urbanized increasing population with industrialization demands more amount of wastewater treatment. Despite wastewater treatment’s positive impact on human and environmental health, it also produces sludge as a by-product of the process. Characteristics of the sludge mainly depend on the source of wastewater and the process applied for its treatment. Domestic sludge generally contains a large number of pathogenic bacteria carrying biodegradable compounds. Characteristics of industrial sludge vary greatly. It may contain biodegradable, non-biodegradable, toxic compounds, heavy metals, etc. The sludge may be in the form of liquid or semisolid with 0.25–12% solids. Thus, the handling and disposal/reuse of sludge may become a complex task due to its large volume and infectious and/or toxic nature. This chapter analyses the characterization and quantity estimate of the sludge produced during the application of various municipal and industrial wastewater treatment options. Current practices for the disposal and reuse options such as anaerobic digestion for biogas production, composting to utilize as a fertilizer, brick production, filler material, and bioplastic production will be reviewed and the suitability of each option in terms of benefit and risk will be critically analyzed.

1. Introduction

Wastewater contains a lot of solid and liquid waste discarded after use and defined in various combinations based on their sources of generation. The wastewaters from residential apartments, colonies, and institutions are termed domestic, wastes from agricultural fields, mining effluents and industrial are termed as industrial wastewater [1].

With the increase in population coupled with urbanization and industrialization, the quantity of wastewater is also increasing. To cater to the increasing demand for water for urban, industrial, and agricultural purposes, the need for its treatment, call for water reuse, and resource recovery are growing exponentially. Wastewater treatment is one of the important activities in urban and industrial areas in order to protect human and environmental health and to eliminate or neutralize these pollutants. Though wastewater treatment has a positive impact on human and environmental health, it also produces sludge as by-product of the process. The sludge may be in the form of a liquid or a semisolid, with 0.25 to 12% solids that come from households and industries [2]. This sewage sludge may cause various environmental and health problems such as nutrient leaching, loss of soil biodiversity, emission of GHGs, and pathogenic outbreak [2]. Challenges related to managing the sludge thus produced are cost related to handling and transportation, disposal methods, strict regulations, and environmental threats [3]. A clean and sustainable environment can only be built by introducing technologies that are environment friendly and economically feasible. Hence, the treatment of wastewater as per quality standards is no longer the only objective of WWTPs; they also need to focus on its discharge management units. Sewage sludge contains insecticides, cleansing agent, oils, fats, grease, thinners, paints, etc. The high fat content of the sludge may be utilized for biofuel productions such as biodiesel, biohydrogen, bioethanol, and bio-oils [4].

Therefore, the chapter will include the characterization and quantity estimate of the sludge produced during the application of various municipal and industrial wastewater treatment options. Current practices for disposal and reuse options such as anaerobic digestion for biogas production, composting to utilize as fertilizer, brick production, filler materials, or bioplastic production will be reviewed, and the suitability of each option in terms of benefit and risk will be critically analyzed.

2. Wastewater generation, sludge production, and treatment

2.1 Global scenario of wastewater and sludge generation

According to a research carried out by Qadir et al. [5], around 380 billion m³ of wastewater is generated annually across the world. And if we consider the annual growth of population and development, the daily wastewater generation can hike up by 24% by the end of 2030 and 51% by 2050. Asian countries have contributed largely to wastewater generation with 42% (159 billion m³) of wastewater globally and hence need attention [5]. Due to population growth and rapid urbanization, immediate attention is required to manage wastewater.

2.2 Indian scenario of wastewater and sludge generation

India is the second most populated (1.38 billion) country in the world with 900 million people living in rural areas and 483 million in urban areas (around 35%). In rural regions, the wastewater produced is approximately 39,604 MLD (72,368 MLD for the years 2020–2021; NITI [6]). Urban areas have higher waste generation due to higher water needs for flushing and sewage drainage as compared to rural areas.

According to a report published by the Central Pollution Control Board (CPCB), sewage generated is about 72,368 MLD with treatment capacity of only about 20,235 (≈28.0%) MLD. 52,133 MLD (≈72.0%) of domestic sewage from cities and towns is being disposed of without treatment, thus making it the biggest source of pollution of water bodies (NITI [6]). Total 1631 STPs are installed/proposed in India with a total treatment capacity of 36,668 MLD. Out of this, 1093 (68%) STPs are operational, 5% are under construction, and 2% are non-operational. Figure 1 describes the current status of STPs in India as per a report from CPCB [7]. Around 39,55,000 metric tonnes of dry sludge is generated every year after complete treatment of sewage [8].

Figure 1.

Current status of STPs in India as per CPCB [7].

2.3 Treatment technologies adopted in India

In India, different treatment technologies such as activated sludge process (ASP), up-flow anaerobic sludge digester (UASB), oxidation pond (OP), and advanced technologies like SBR and MBBR are adopted for the treatment of sewage [7]. Figure 2 presents the capacity distribution of technologies adopted for domestic wastewater treatment in India.

Figure 2.

Wastewater treatment technologies adopted for domestic wastewater treatment in India.

2.4 Sewage classification and characteristics

Sewage can be broadly classified in three ways: domestic sewage, industrial sewage, and storm sewage. Domestic sewage is the water from houses and apartments, containing 99.9% water by weight and < 0.1% of a wide variety of dissolved and suspended impurities [1]. It contains high concentrations of organic matter and nutrients (phosphorus and nitrogen) [9]. Water discharged from industries having chemical compounds during various industrial processes is called “Industrial Sewage” [10].

Components of wastewater based on their source of origin are presented in Table 1.

Nature of sludge	Sludge components	Environmental impact
Domestic	Pathogens	Waterborne/communicable disease
Domestic; Industrial	Suspended solids	Anaerobic layer formation in aquatic environment
	Biodegradable organics	Oxygen depletion and biological degradation
	Nutrients	Eutrophication
	Dissolved inorganic solids
Industrial	Refractory organics	Carcinogenic
	Heavy metals	Carcinogenic

Table 1.

Components of wastewater based on their source of origin.

The size and capacity of WWTPs are determined by assessing the total volume of sewage generated from nearby areas connected to sewer systems in terms of inflows and infiltration [5]. The degree of treatment depends upon environmental conditions and effluent discharge standards prescribed by government/local bodies. Stream standards include amount of DO, coliforms, turbidity, acidity, and heavy metal contents, intended to maintain the existing water quality of streams where it has to be disposed of [5].

“Sludge is a byproduct of STP having organic compounds, macro and micronutrients, trace elements including toxic metals, microorganisms, and micro pollutants” [11]. Micro/macro-nutrients are the source of plant nutrients, whereas organic constituents serve as soil conditioner [12]. Sewage sludge is neutral to slightly alkaline in nature with high organic matter and high concentrations of N, P, Ca, and Mg [9].

Characteristics of the sludge thus produced mainly depend on the source of wastewater and the process applied for its treatment. Domestic sludge generally contains a large number of pathogenic bacteria along with biodegradable compounds. Characteristics of sludge generated in different STP processes are presented in Table 2.

Sludge-Producing Processes	Physical Characteristics of Sludge	Digestion process involved
Primary Settling tank sludge	Gray slimy liquid Settleable solids around 50–60% of the total applied solids Offensive odor	Chemical conditioning followed by dewatering Can be readily digested due to high volatile solid content (60–80%)
Chemical-Precipitation sludge	Dark-colored slime with objectionable odor, but less from sludge obtained from PST	Decomposition at a slower rate Its density is increased by long residence time in storage
Activated sludge process	Brown or dark flocculants Brown—under-aeration in good condition without offensive odor Dark—it may be approaching a septic condition.	This can be thickened by floatation or centrifugation with or without chemical addition It will digest readily alone or mixed with fresh sewage solids
Trickling filter	Brownish, flocculants, and relatively inoffensive when fresh	Sludge digests readily
Aerobically digested	Brown to dark brown with a flocculent appearance	Well-digested sludge
Anaerobically digested sludge	Dark brown to black in color with lots of gasses	Primary sludge produces two times CH4 than activated sludge
Composted sludge	Dark brown to black varying on the basis of bulking agents used. Inoffensive odor	Can be used as a soil conditioner by mixing with bulking agents such as saw dust and wood chips.

Table 2.

Characteristics of sludge generated in different unit operations and processes in an STP [11].

Raw (untreated) sewage has approximately 90 gram per day per capita suspended solids, and about 60% solids are removed in PST, leaving behind 4–5% solid, and the remaining suspended solids are either oxidized in a secondary tank or amalgamated in the biological mass [13]. The amount of solids thus produced depends on the sludge age, and the volume of sludge depends on its water content and the volume of the solids [4].

2.5 Sludge generation and treatment

The STPs are designed based on the influent characteristics, primarily total suspended solids (TSS), biochemical oxygen demand (BOD), and fecal coliform (FC). The preliminary treatment process along with the primary sedimentation tank is referred as primary treatment. It is designed to remove 60–70% of the suspended solids (organic and inorganic) and 30–40% of BOD (organic) associated with it. The sludge from PST is gray and slimy with offensive odor and can be digested by employing simpler operations [11].

In secondary treatment, microorganisms decompose the organic matter and more than 85% of both suspended solids and BOD is removed. The treated effluent from WWTPs usually contains BOD suitable for disposal. Aerobic systems such as stabilization ponds, ASP, SBR, and MBBR and anaerobic systems such as anaerobic ponds and UASB are secondary treatment processes. Sludge from the activated sludge process approaches the septic conditions rapidly and can be digested alone and/or along with primary sludge [11].

Tertiary treatment includes chemical precipitation and membrane technologies. Tertiary treatment methods can remove >99% BOD but are used in special cases due to their cost of operation. Chemical precipitation with metal salts (FeCl₃) and nitrification-denitrification is commonly used to remove phosphorus and nitrogen, respectively, from sewage in tertiary treatment methods [11].

“Primary sludge” is the TSS settled in PST, secondary sludge is the mixed liquor settled in SST by gravity, “return sludge” is a part of secondary sludge going into aeration tank, and “excess sludge” is wasted sludge from SST [4]. Sewage after chemical precipitation produces chemical sludge [14]. In biological treatment, 1–2% of BOD/COD is converted into solids, making sewage sludge [15].

The sludge from PST and SST is passed through a sludge-thickening process in sludge thickeners. If the thickened sludge is put through the digestion process anaerobically to produce CH₄, it may be termed anaerobic digester, and if digested aerobically, it is called aerobic digestion [2]. The digested sludge will have to be dewatered using sludge drying beds (centrifuge/filter press/natural solar drying beds) [11]. Figure 3 explains the various types of sewage sludge generated through various unit processes of sludge treatment taken from Metcalf and Eddy [11].

Figure 3.

Types of sewage sludge generated through various unit processes of sludge treatment [11].

Anaerobic digestion has various advantages over other methods. It leads to recovery of methane, nutrients, and dying-off pathogens due to relatively long detention periods. Use of larger closed tanks may lead to increase in capital cost. Further sensitivity of micro-organisms involved in anaerobic digestion toward small environmental changes is the major con of using this facility. The residue liquid from the system has very high oxygen demand, suspended solids, and high concentration of nitrogen. Anaerobically digested sludge produces about twice as much methane gas as does waste-activated sludge [8].

In aerobic digestion, biological degradation of organic matter takes place in the presence of oxygen. In this process, microorganisms (sludge) are oxidized to CO₂, H₂O, and ammonia. Aerobically digested sewage can be dewatered easily on drying beds. The pH of the system is required to be maintained as pH drop may occur when ammonia is oxidized to nitrate and the alkalinity of the sewage is insufficient [3]. Long-term aeration of the waste-activated sludge creates a bulking material difficult to thicken [9].

Treated wastewater can be viewed as a resource for energy, nutrients, and water, which is a much undervalued resource in India [16]. However, the main challenge toward generating a common statement about waste management is quantifying it in terms of volumes of wastewater generated, collected, treated, and reused at different scales.

3. Sludge management

The methods used to treat and dispose of sludge are sludge thickening, sludge digestion or stabilization, conditioning, dewatering, drying, incineration, and ultimate disposal, as described in Figure 4.

Figure 4.

Process of treatment and disposal of sewage sludge.

Anaerobic digestion is regarded as a major and essential part of a modern WWTP [4]. Three types of anaerobic digesters are being used: single-stage process (single-chamber bioconversion of sludge), double-stage process (separate acidogenic and methanogenic chambers), and temperature-phased anaerobic digestion (combination of a thermophilic unit prior to a mesophilic unit) [4]. Table 3 provides details about sludge treatment methods along with processes involved and their impact on sludge mass and volume.

Unit processes	Unit operations	Impact on sludge
Sludge Thickening	Gravity	Reduction in sludge volume
	Floatation
	Centrifugation
Stabilization of sludge	Alkaline	Stabilized sludge
	Anaerobic digestion	Stabilized sludge with reduced mass
	Aerobic digestion
	Thermal aerobic digestion
	Composting	Stabilization of sludge followed by recovery of useful products
Conditioning of sludge	Chemical	Improvement in dewatering conditions
Dewatering of sewage sludge	Centrifuge	Reduction in sludge volume
	Sludge drying beds
	Lagoons
Incineration	Multiple hearth incineration	Resource recovery and reduced volume
Lands Application of bio solids	Land application	Beneficial use + disposal
	Dedicated land application	Disposal + land reclamation
	Landfilling	Disposal

Table 3.

Sludge treatment methods along with processes involved and their impact on sludge [11].

Thermal drying and incineration can reduce the volume of sludge by carbonizing organic constituents in sludge. But thermal drying is very costly due to its high energy requirement. In incineration, organic matter gets destroyed, the heavy metals get mixed with ash, and its efficiency is based on the degree of dewatering to reduce moisture content that is applied on the sludge prior to incineration. Sewage sludge ash has very high P₂O₅ content compared to commercial superphosphate [12].

The ash can be used as a raw material for the manufacture of construction materials, namely, bricks, tiles, pavers, and cement. The energy required to heat the sludge for moisture removal can be achieved using oil, natural gas, coal, and even electricity [3]. To use the ash as a phosphate fertilizer, it is needed to be extracted and also checked for heavy metal content to be within safe limits as per standards.

4. Reuse practices commonly adopted

The sludge generated from the different WWTP units, namely, PST, SST, and others, has become a problem for mankind due to the unavailability of land for disposal, high population growth, and very fast urbanization/industrialization. Therefore, it is indeed a requirement to develop long-term solutions by recycling and reusing the sludge thus produced to achieve a zero-waste strategy.

4.1 Biogas

Anaerobic digestion of sludge produces fuel-rich biogas as a by-product, which can be utilized to meet the energy and fuel demand, making WWTPs self-efficient. Biogas is a combination of methane (50–75%), carbon dioxide (25–50%), and other gases [17]. It can be collected from an anaerobic digester tank and converted to electrical energy and heat energy [12].

Biogas can also be upgraded to a relatively larger fraction of methane. It can be used for domestic purpose, electricity generation, and transportation in place of compressed natural gas (CNG). CO₂ and H₂S produced in the process can be recovered by using activated bio-char as adsorbents or biologically using CO₂-fixing microalgae or sulfur-reducing bacteria (SRB) [9]. Bio-electro-chemical systems may also be employed for upgradation of biogas through the electro-methanogenesis process, in which CO₂ gets converted to CH₄ [18].

In a global context, around 10,100–14,000 TWh (1 TWh = 10⁸ KWh) biogas can be produced using currently available substrate ranges, and the energy thus produced by consuming almost all resources gives 6–9% of the total energy consumption globally and reduces 23–32% of the world’s coal consumption [9].

4.2 Bio-hydrogen

Bio-hydrogen is an intermediate product of anaerobic digestion (AD), having a higher calorific value than biogas. It is considered green energy as its combustion only generates water without GHG emissions [19]. This process is known as dark fermentation by avoiding methanogenic activity and controlling the operating parameters of the AD system, namely, pre-treatment of inoculum, short HRT, and acidic pH [20]. Until now, the achieved yield of hydrogen production is very less (<6%). Very few studies have been conducted to study yield improvement by pre-treating substrates using calcium peroxide and nitrous acid. It can also be improved by the co-fermentation of sludge with other materials that can reduce the C/N ratio of the sludge [21].

4.3 Bioplastic

Bioplastic is made from the union of microorganisms, with macromolecules, namely, starch, cellulose, and protein [22]. It is biodegradable in nature and thus not a threat to humans. It is even used in producing postoperative sutures (medical surgical equipment). The high investment cost of these products has made them uncommon for regular use and manufacturing. The bio-synthesis of micro-organisms as an energy storage component produces compounds such as polyhydroxyalkanoates (PHA) [22].

The bioplastic derived from only biomass materials is often called all-bioplastic and others are part-bioplastics. “All-bioplastics” are “protein plastics” (soybean fiber, cellulosic, and algae-based resin), and “part-bioplastics” mainly contain “starch bioplastic,” modified with starch and cellulose [23]. All-bioplastics are also called biodegradable bioplastic, and they have poor water and moisture resistance [18]. PHA is a biodegradable plastic, with all good features except high cost of the raw material [24]. Bioplastics are broadly used in making packaging products such as shopping and trash bags, bottles, labels, packaging films, cushioning, fibers of synthetic clothes, children’s toys, and home interior furnishings and décor items [22].

Conventional plastic has petroleum residues as the raw material, which is going to be exhausted someday, but the source of bioplastic is organic biomass, which is inexhaustible. Bioplastic is biodegradable in nature and can be broken down as water and carbon dioxide. It has very low CO₂ emission, thereby reducing the temperature of Earth [22]. As it is completely biomass-based plastic derived from starch, cellulose, and protein, it does not contain any organic toxic substances. High cost, lack of technology and market, and lesser customer awareness have undervalued its use on a larger platform.

The global bioplastic production capacity was 2.11 million tonnes in 2018 and is expected to exceed 2.6 million tonnes in 2023 [22]. The market price of per kilogram of bioplastic PHA is approximately six times higher compared to petroleum-based plastic [5]. However, replacing petroleum-derived plastics with bioplastics does not necessarily solve the plastic waste issue. To make the bioplastic use an effective solution, it is need to study its recycling, reuse, and the carbon footprint gathered in throughout life cycle.

4.4 Bio-fertilizers

The sludge from anaerobic digestion process is used to obtain biogas and waste is left in the form of slurry, termed “digestate” [25]. This digestate may be used as a fertilizer for plants as a source of macro/micro nutrients. Bio-fertilizer produced from the digested sludge may become a substitute of chemical fertilizers. Bio-fertilizer improves the fertility of soil and provides the option for waste disposal and resource recovery, thus solving environmental issues associated with waste [25].

The fertilizer of sewage sludge gives rise to the problem of bio-accumulation of heavy metals in agricultural soil in the topsoil and can be transferred to the food chain in a magnified way [18]. Because the higher doses of sludge application on ground have higher heavy metal concentration instead of comparatively lower doses, its intermittent uses with additional analysis of its exposures will be a great way to deal with its negative impacts.

4.5 Syngas

Syngas is different from biogas as biogas is formed during the biological degradation of organic mass in anaerobic conditions (CO₂ + CH₄), whereas syngas is composed of carbon CO, CO₂, and H₂ when coal or biomass is gasified [26]. In thermochemical treatment, sludge is fed into a reactor, where it is partially oxidized at 300–900°C (pyrolysis), and syngas is produced along with tar and other products. Various useful products can be derived from this syngas, namely, fertilizers, synthetics, and fuels [17]. Despite the high cost of production and complexity in operating procedure, this technology has ranked among the most advanced technologies to convert biomass to energy due to a large yield. Gasifying agents such as air, steam, and oxygen were used to produce different types of syngases. Air is the most commonly used gasifying agent [19].

4.6 Compost

Composting is an efficient and cost-effective method for treating and reusing sewage sludge post-digestion and used as soil amendments. The compost properties are controlled and modified by using bulking agents such as high moisture content, lesser porosity, and low C/N ratio [25]. Composting could reduce polycyclic aromatic hydrocarbons (PAHs), but biodegradation processes of sludge can form toxic intermediary products, causing soil toxicity, leading to environmental stress and reduction in soil microbial activity [18]. In this process, the organic matter is turned into a stabilized product, which can be applied as a form of returning organic matter to soils, which acts as a carbon sink. The safety and efficacy of sewage sludge composting should be monitored carefully in terms of microbial indicators such as community structure, diversity, and composition.

4.7 Bio-oil

Sewage sludge can be recycled as a jet fuel (hydrocarbons C₈-C₁₆) by pre-conditioning and processing through pyrolysis at temperatures 450–700°C to produce a bio-oil [27]. This is a two-stage process of hydrodeoxygenation and hydrocracking in a batch reactor under high pressure (autoclave). This fuel may meet the jet fuel specifications in terms of calorific value, viscosity, density, and freeze point; however, it fails in terms of smoke release, flash point, and total acid number [27]. The conversion of sewage sludge into jet fuel can be a sustainable pathway for energy production and a promising route for sewage sludge management [28].

4.8 Construction materials

Use of dried sludge as a clay substitute to produce an engineering quality brick can be a suitable option of sludge reuse. The proportion of sludge in the mixture and the firing temperature are the two key factors affecting the quality of bricks [10].

Low organic matter sewage sludge is also used in manufacturing concrete mix alterations [29]. According to various researchers, strength is inversely proportional to sludge content when greater than 10% mixing is done; higher the sludge content, greater the strength loss. Though its use in the manufacture of construction materials solves a very small portion of the problem, but this method is assumed to be safe for human health and environment [1]. The by-products obtained from sludge recycling and processing are summarized in Table 4.

By-product of the sludge treatment process		Remarks/significance/process	References
Energy source	Biogas (60% CH4)	1 m^3 biogas = 23.3 M/m^3 energy value	[9]
Cooking fuel	Biogas	Source of cooking fuel.	[9]
Transport fuel	CBM and LBM	Upgraded from Biogas	[12]
Liquid anaerobic digestate		Struvite precipitation (with phosphate and ammonium ion recovery)	[9]
Dried digested sludge	Bio-fertilizer	Land spreading within the safe limits.	[25]
Electricity and heat		Energy is recovered from incineration of dried raw or digested sludge	[17]
Production via syngas	Bio-methane	Steam catalytic/gasification and pyrolysis of dried sludge to produces syngas feedstock for producing bio-methanol	[12]
Phosphorus recovery	Biological Phosphorus	Obtained thermo-chemically from ash or tar obtained while gasification	[1]
Biofuels	Bio Oil; Biodiesel; Bioethanol	Fast pyrolysis; Transesterification of dried sludge; Fermentation	[2, 12, 18]
Carbonaceous materials	Bio-char; Hydro-char	Pyrolysis; hydrothermal carbonization	[12]
Green hydrogen	Biosyngas	Gasification/pyrolysis + bio-methane	[26]

Table 4.

By-products obtained from sludge recycling and processing.

5. Limitations and risks associated

Limitations and challenges while dealing with reuse practices are maintaining the quality standards with precise monitoring in order to reduce the pollution risk. In this regard, the source and impact of contamination needs to be checked regularly by employing risk assessment studies. In this study, environmental systems, exposure pathways, and the recipients of the pollution loads including human populations should be considered and analyzed for exposure. When the bio-solids are released to the soil, they do not need to meet the water quality standards. Figure 5 explains the potential risks imposed on humans and the ecological environment on utilizing sewage sludge as a resource.

Figure 5.

Potential risk imposed on humans and the ecological environment on utilizing sewage sludge as a resource.

6. Conclusions

Wastewater sludge reuse and recycling prior to disposal serves as the basis of sludge management. The energy and resources that can be obtained using sewage sludge could be a long-term solution for disposal, resource recovery, and future energy needs. The application of these steps on a practical ground will be a fascinating approach for ensuring the long-term sustainability and lowering the overall carbon footprint. This chapter provided a brief discussion of the possible options for the treatment of sewage sludge and its reuse options after smaller amendments on soil as fertilizer, compost, concrete production, biogas, and bioplastics. Wastewater treatment plants generate a certain level of carbon footprint and wastes; therefore, it is necessary to reduce the carbon footprint and manage the waste in order to keep the world safe.