**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

*Options for the Disposal and Reuse of Wastewater Sludge, Associated Benefit… DOI: http://dx.doi.org/10.5772/intechopen.109410*

**Figure 1***. Current status of STPs in India as per CPCB [7].*

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].

#### **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.

#### **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**. 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].

**Figure 2***.*

*Wastewater treatment technologies adopted for domestic wastewater treatment in India.*


#### **Table 1.**

*Components of wastewater based on their source of origin.*

"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**.

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].


#### **Table 2***.*

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

#### **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].

**Figure 3***.*

"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].

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**.

*Options for the Disposal and Reuse of Wastewater Sludge, Associated Benefit… DOI: http://dx.doi.org/10.5772/intechopen.109410*

**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 (singlechamber 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.

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


#### **Table 3***.*

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

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.
