4.2.7. Syngas

Syngas consists mainly of a mixture of carbon monoxide, carbon dioxide, and hydrogen that can be used as combustion fuel (heat energy value 8–14 MJ/kg or 10–20 MJ/Nm3), or converted to liquid fuels using a biological or chemical process [80]. Syngas can be used to produce synthetic petroleum through the synthesis of Fischer–Tropsch, or through the gasoline methanol process. Conversely, the anaerobic bacteria can transform the syngas carbon monoxide into ethanol, with average yields of 340 L of ethanol per ton (municipal solid waste, agricultural waste, animal waste, etc.) [81]. The combustion of biomass (or syngas) in the presence of excess oxygen supply results in full oxidation and the production of hot flue gasses usually used to produce steam to drive electric turbines for electricity production with an output of approximately 30% [82].

#### **5. Strategies and Mechanisms for Recovery of Renewable Energy Products from Waste Water**

To produce the various valuable energy products from the wastewater, it has to be digested with the aid of a variety of microorganisms to produce the specific type of energy product. Even after digestion, the remaining sludge is treated again with various physical and chemical methods to obtain more energy from the wastewater and it's left over materials. A broad variety of energy products and value-added compounds can be extracted from wastewater e ffluents; energy in electricity form can somewhat minimize electricity scarcity [77]. There are various techniques involved in the production of energy products from wastewater namely, MFC, bio-electrochemical system (BES), biochemical, chemical, and biological (aerobic and anaerobic digestion of e ffluent) [58]. The key methods used to produce energy from the waste water are:

#### *5.1. Anaerobic Digestion to Produce Biogas*

This is a biological process that involves using microorganisms to transform the organic waste into valuable products. Anaerobic treatment of liquid waste or wastewater provides the ability to rapidly minimize the organic content of the waste while reducing the energy usage of the treatment process and the production of microbial biomass or sludge [29,71]. AD, as shown in Figure 3, is a complex process that involves and carries a variety of reactions (in absence of oxygen) such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis [2]. AD is a very useful process which is applicable on a wide variety of waste e ffluents (sewage sludge, industrial wastewater, domestic wastewater, etc.) for their conversion to useful products especially into various energy forms, i.e., biohydrogen and methane [55]. The conversion of organic compounds into sludge in wastewater generates a by-product which needs further treatment or disposal. Reduction in sludge and energy consumption are the two attributes

which make it economically attractive for municipal and industrial waste streams to consider direct anaerobic pretreatment of wastewater. AD is a ffected by various factors like temperature (25–35 ◦C), pH (~7), moisture, carbon source, nitrogen, and C/N ratio [2]. AD of sewage sludge used in treatment plants is very useful now because of lower disposal costs, and it is ecofriendly, too. Direct anaerobic treatment may also provide excess energy for relatively warm wastewaters which contain significant degradable organic compounds [47]. However, even with low-strength wastewaters, the energy savings that can be achieved by avoiding most of the aeration costs are significant. Anaerobic treatment effluents, however, are often not suitable for direct discharge into the receiving waters without further treatment that may require aerobic polishing. Nonetheless, this treatment scheme can be explained by the reduced aeration demand and the production of sludge in aerobic treatment following anaerobic pretreatment. The average ambient temperature of the wastewater has an e ffect on anaerobic treatment design quality [83]. Some wastewaters of low and medium strength are relatively cool (<20 ◦C), and the energy needed to heat them to mesophilic temperatures is significant and not economical. Wastewater with a temperature of 20 ◦C and a COD of 20 g/L, and biogas generation produces around the same amount of energy needed to increase the liquid's temperature to 35 ◦C. So, treatment at ambient temperatures is only feasible for wastewaters of low and medium strength. Successful anaerobic treatment of wastewater as low as 15 ◦C is feasible, but application of anaerobic digestion should not be taken into consideration below 12 ◦C [84]. On the other end, there are many industrial wastewater sources that are very warm and can be considered as mesophilic (food processing) use, and in some cases, thermophilic (distillery waste) anaerobic digestion.

**Figure 3.** Showing stepwise anaerobic digestion process.

## *5.2. Fermentation to Bioethanol*

It is well known that bioethanol is developed as a renewable liquid fuel. Bioethanol can be used alone or combined with traditional liquid fuels to form either Gasohol or Diesohol. Bioethanol is usually produced under anaerobic conditions through fermentation of simple sugars, such as glucose and fructose. Several yeasts, including, for example, *Saccharomyces* sp., and other bacteria including *Zymomonas* sp., undergo this fermentation [6,50]. A variety of industries, such as sugar, food processing, meat, and pulp and paper, have developed carbohydrate (glucose, fructose, lactose, etc.) rich wastewater that can be converted into bioethanol through fermentation. However, the current challenges are using waste streams in which organic carbon is not present as simple sugars through the use of chemical or biological pretreatment or novel microorganisms that ferment a wider range of organic substrates [53]. Currently, there is extensive work focusing on pretreatment procedures for cellulolytic. The small yields of ethanol obtained in fermentation (typically 10% (*v*/*v*)) currently need subsequent energy intensive distillation. Conventional ethanol plants will expend more than 30 per cent of bioethanol fuel's heat energy during the distillation process [49,85,86].

## *5.3. Microbial Fuel Cells*

Fuel cells transform chemical energy into electrical energy. Microbial fuel cells work using bacteria that oxidize organic matter in wastewater to transfer electrons to an anode from where they pass to the cathode through a circuit to combine protons and oxygen to form water. Electricity is produced by the di fference in potential coupled to electron flow [58]. Microbial fuel cells have become an emerging technology and a number of these have been successfully operated with both pure cultures and mixed cultures, which have either been enriched by sediment or activated sludge from wastewater treatment plants [87]. Wastewaters of very di fferent characteristics can be used: Sanitary waste, wastewater for food production, dairy manure, swine wastewater, and corn stove. This technology may e ffectively use bacteria already present in wastewater as catalysts to produce electricity while treating wastewater simultaneously, but its advancement is hindered by low power output and high material costs. To date, microbial fuel cells have not been used in large-scale applications, but are used to produce energy for BOD sensors, robots, and small telemetry systems [59,88].

#### *5.4. Combustion, Gasification, and Pyrolysis*

The heating of sludge in the presence of a small supply of oxygen contributes to gasification and syngas output. In order to generate pyrolytic oil, biochar, and non-condensable gases, combustion and pyrolysis require a fully inert atmosphere at moderate to high temperatures (300–900 ◦C). Bio-oil can also be used as a liquid fuel or converted into a synthetic gas (CO and H2), whereas biochar, non-condensable gases, and bio-oil can also be used through combustion to produce electricity and heat [80,89]. Gasification involves the thermochemical conversion of organic compounds through partial oxidation at high temperatures (650–1000 ◦C) to optimize gaseous products (CO, H2, CO2, and light hydrocarbons), particularly synthesis gas (CO and H2) [81]. Depending on the gasifying agen<sup>t</sup> and temperature, the energy content of the natural gas ranges from 4–28 MJ/Nm. Additionally, the biomass contained in wastewater, such as microbial and algal biomass, and other biomass, can also be gasified into energy products, including heat, steam, electricity, syngas and liquid fuels, and biogas [2,90]. If the heat energy is also captured and combined heat and power (CHP) are given, the output can be improved up to 50% and up to 80%. The viability of applying combustion or gasification has to do with moisture content, and the practical problems of tar formation, mineral content, over bed burning, and bed agglomeration. The feedstock must be relatively dry; with 40 to 50 percent maximum moisture content [82]. Co-pyrolysis of sewage sludge with other non-biodegradable waste such as polythene and plastic waste was also performed at 525 ◦C in a stirred batch reactor under N2 atmosphere. This potential synergetic strategy resulted in better yield of H2 and CH4 as compared to individual pyrolysis of sludge, and that can be used as gaseous fuel. This combined feasible managemen<sup>t</sup> provides an alternative for both residues to be generated in a better output [91]. Dryers may be used in the design, but there is a direct trade-o ff between the amount of energy in the feedstock and the amount of energy spent on drying. Since gaseous and solid phase contaminants are potentially generated from the application of these technologies, there are many possible negative environmental consequences, including heavy metals, dioxins, furans, and NOx gases [92]. However, evidence suggests that technological interventions can control these emissions and use of combined cycle gas turbine, the generated gas can be diverted to various end uses such as direct combustion for heat and electricity generation.

#### *5.5. Incineration of Sludge for Energy Production*

Sludge is one of the useful byproducts from wastewater treatment plants predominantly consists of 75% mud, and 25% solid matter, but for any further use, it must be treated to remove the pathogens. The residual sludge can also be used for the manufacture of di fferent energy products from it after wastewater treatment [22,30]. Typically, bio-solids are comprise of huge quantities of water that can be collected by using dewatering machines for further use either as biofertilizer or by incineration for heat and electricity generation. This technique is used by various Municipal Solid Waste (MSW) organizations or waste managemen<sup>t</sup> firms to dispose of the bio-solids and extract the energy from it to fund their operational expenses [44]. Thanks to its global e ffect on waste minimization, resource optimization, and renewable energy production, energy recovery from wastewater and sludge in contemporary to wastewater managemen<sup>t</sup> remains assured. Figure 1 shows the focus energy conversion methods, which shows the sludge conversion pathways to biogas, heat, and electricity. Commercially incinerated bio-solids with several hearth furnaces (MHF) and fluid bed furnaces (FBF) [47,53]. MHF burns bio-solids and allows the hot air to dry incoming bio-solids, reduces moisture, and increases the e fficiency of MHF for heat generation, while FBF is a modern and e fficient technology that provides continuous operation without any multi-stage device that eventually increases the efficiency of the overall process and technology of incineration [93]. Residual gas pollution is a challenge in both of these incineration technologies, but can be addressed with the use of advanced scrubber systems to make these technologies more e fficient and environmentally friendly [94].

#### **6. Advancements and Integrated Technologies to Recover Energy Products from Waste Water**

Anaerobic wastewater treatment is currently the most commonly used method for extracting energy from wastewater. The energy is harvested as methane production. Removal of the energy lost due to heat dissipation during energy conversion from di fferent reducing matters to methane, energy consumption to sustain microbial activity, and residual reducing matters of wastewater after treatment, 80% of the chemical energy found in the original reducing matters can be transferred to methane [24,95]. In view of the fact that only about 35% of methane's chemical energy can be converted into electricity through the combustion cycle, the overall e fficiency of energy recovery is about 28%. If more e fficient CH4-driven chemical fuel cells are created, this number will theoretically increase to 40%. Our future perspectives should be production of more and more energy products to minimize the cost of overall wastewater treatment processes [43,75]. While streams and technologies can be matched individually, the integration of technologies and waste streams holds the greatest promise in achieving long-term energy security while maximizing wastewater treatment. There are many examples where this worked. Thus, our aim should be to design an approach in such a way that wastes that we generate can be reconverted into pathogen free resources without any cost and without polluting our environment. Some of the steps which may be taken are as follows:

#### *6.1. Scale up of MFCs*

MFCs utilized now are designed for lab scale purposes, but to know the utility of MFC, these should be scale up to a practical level so the energy recovery can be enhanced and optimized [57]. The restriction in scaling of MFCs are high scaling cost, pH bu ffers, high internal resistance, high material cost, and low efficiency of mixed cultures on an electrode and these limitations may be overcome by reactor engineering, biological employment and material development [63].

## *6.2. Digitalization of Process*

The parameters and techniques used in a process should be monitored to an extent to achieve a high yield of by-products and chemical compounds from the treatment of wastewater and effluents [12]. In case of electricity generation electro-chemical parameters, i.e., electric current, indicators, and electrode potential, are some useful parameters which can be monitored for optimum electricity production [37,68].
