*2.1. Sources of Wastewater*

On the basis of the source from which the wastewater is being generated, there are various types of wastewater, some of them are listed below:


#### *2.2. Features and Pollutnats of the Wastewater*

Wastewater is generally characterized on the basis of physical (color, odor, and turbidity) and chemical (pH, alkalinity, biochemical oxygen demand (BOD), chemical oxygen demand (COD), dissolved oxygen (DO), total organic carbon (TOC), total dissolved solids (TDS), total suspended solids (TSS), conductivity, nitrogen, phosphorus, heavy metals, volatile solids (VS), oil, fats, grease and gases), etc. Di fferent types of sources, along with their typical properties, are listed and discussed in Table 1.


\* BOD after 5 days.

**Table 1.** Characteristics and sources of some wastewater effluents.

Most of the wastewater contains a chemical, biological matter, and other objectionable matter that di ffer from source to source from which it generates. Industrial e ffluent includes a significant quantity of harmful chemicals and heavy metals, i.e., zinc, copper, nickel, lead, cadmium, arsenic, antimony, mercury, etc. [19], with lower biological content. Wastewater from households contains lower levels of chemicals comparatively to industrial wastewater, but high levels of organic matter, whereas agricultural wastewater includes high levels of chemicals in the form of pesticides, weedicides, fertilizers, etc., and biological substances like algae, fungi, bacteria, etc. [5,20]. Waste water consists of 70% organic compounds and 30% inorganic compounds, along with a variety of gases. Organic compounds are mainly carbohydrates, fats, and proteins, whereas inorganic matter consists of heavy metals, nitrogen, phosphorus, sulphur, and chloride, etc. Hydrogen sulfide, methane, ammonia, oxygen, nitrogen, and carbon-dioxide are commonly dissolved gases present in wastewater [21]. Biologically, wastewater consists of di fferent types liverworts, seedy plants, ferns and mosses, bacteria, fungi, algae, and protozoans along with various types pathogens are also found in wastewater, which comes from the human beings su ffering from various diseases [20,22].

#### **3. Treatment Methods of Wastewater**

Various types of pollutants present in the wastewater can be removed by using di fferent strategies. Various treatment methods are used on the basis of source and location for wastewater treatment. Primary treatment can reduce BOD by 20–30% and suspended solids by as much as 60% [23]. This step includes reduction of oil, grease, fats, sand, and coarse solids. Secondary treatment will minimize BOD and total suspended solids by up to 85 percent. This step includes degradation of dissolved contents of the sewage within a biological degradation of system, as shown in Figure 1. The last step of secondary treatment is the removal of biological matter from the treated water with very low levels of organic material and suspended solids [24]. Microbes in the wastewater consume food in the form of organic matter, turning it into carbon dioxide, water, and electricity. Tertiary treatment can remove up to 99 percent of sewage impurities. Some operators add chlorine as a disinfectant before discharging the water. Sometimes nitrogen and phosphorus removal are done by tertiary treatment. Tertiary treatment uses advanced equipment and technologies to further eliminate or discard contaminants or particular pollutants [25].

**Figure 1.** Schematic representation of waste treatment plant.

On the basis of type of matter present, ultrafiltration, sedimentation, sand filtration, etc., are physical methods used to treat the wastewater or industrial e ffluent as shown in Figure 2. In the chemical treatment method, chlorine is the most widely used chemical that acts as oxidizing agen<sup>t</sup> to kill the bacteria that decompose the water. Another disinfecting oxidizing agen<sup>t</sup> called ozone

is used to purify wastewater [10]. Biological treatment methods use biological agents like plants and microorganisms in this way to remove the harmful pollutants. Biological wastewater treatment is done by oxidation bed or aerated systems, and post precipitation. It can be classified into various groups such as aerobic, anaerobic, and anoxic systems or suspended growth and attached growth according to the growth mechanism of microorganisms [26,27].

**Figure 2.** Different methods of wastewater treatment.

#### **4. Wastewater as a Source of Renewable Energy**

Wastewater produced by various sources is usually full of various minerals, nutrients, and organic matter, and these act as a wonderful source of metabolites for the growth and development of various microorganisms, algae, and plants which can be utilize to produce various renewable energy products. Organic matter rich wastewater, when decomposed in an oxygen-free environment, especially deep in a landfill, releases methane gas. Wastewater which is rich in organic matter when it decomposes, particularly deep in a landfill in an oxygen-free environment, releases methane gas [28]. This methane can be collected and used, instead of released into the atmosphere, to generate heat and electricity. Most wastewater treatment systems include different stages and eventually create solid sludge that is treated by thermal hydrolysis to increase the amount of methane it can generate. The processed waste then enters in an anaerobic digester, which ends up breaking it down. The resulting product is a methane-rich gas, or biogas, which can be used for on-site energy needs, or further refined and used instead of natural gas [29]. However, the solid wastewater residues produce a nutrient-rich "digestate" that can be used as a biofertilizer for soil conditioning and to improve plant production. Usually, the concentration of reducing matter in wastewater is expressed as the COD, which indicates how much oxygen is needed to oxidize the reducing matter [30]. A typical wastewater has a 0.5 kg/m<sup>3</sup> COD value and theoretically has the ability to generate 1.47 to 10<sup>7</sup> joules of energy per kg COD oxidized to CO2 and H2O, and the wastewater energy density is 0.74 to 10<sup>7</sup> J/m3. Heidrich et al. [31] have recently statistically calculated the internal chemical energy of wastewater measured at 1.68 × 10<sup>7</sup> J/m<sup>3</sup> for wastewater combined with household wastewater and industrial wastewater, and 0.76 × 10<sup>7</sup> J/m<sup>3</sup> for pure household wastewater. Accordingly, a fair estimation of the theoretical energy density in wastewater is in the order of 10<sup>7</sup> J/m3, which is five times the energy used to treat wastewater, and based on this data, the USA has an approximate capacity of 1.2 × 10<sup>15</sup> J/day, 4.4 × 10<sup>17</sup> J/year of wastewater renewable energy production [32–34].

#### *4.1. Approaches to Determining Energy Potential from Wastewater*

Process streams must be differentiated according to their capacity as energy sources according to the following input streams, process intermediates, and energy outputs in order to relate different technical options for energy recovery from wastewater.
