**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].

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

#### **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**.

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


#### **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.*
