*7.7. Economic Benefits*

Sludge-to-energy systems reduce the need for more costly and polluting power sources, such as fossil fuels. However, those who run waste-to-energy operations will directly benefit from selling the gas and solid digestate [54].

#### **8. Barriers in Recovery for Renewable Energy from Wastewater**

The interrelationship between energy and organic wastewater content should facilitate energy recovery operations from di fferent sources, including wastewater treatment plants wastewater [106]. Combining the anaerobic digesters with biosolid incineration for electricity generation from wastewater utilities will reduce energy consumption through 5 to 85 per cent [83]. But still there are many challenges that must be took in consideration for e fficient utilization of wastewater to transform it into energy related products and other valuable products. Recovery of energy from the wastewater depends upon its organic content, and any deviation in it further adds to uncertainties in its utilization in the reactors for energy production [105]. Low temperature is another crucial challenge in the operation of anaerobic digesters because most of the microorganisms work in ambient temperatures, i.e., 15–35 ◦C and if there is any change from this range, the organism will not work e fficiently and the kinetics of overall process falls down and reduce the production of energy components, i.e., CH4 and H2 [65,100]. Additionally, it will make it di fficult to perform the anaerobic digestion to produce the valuable energy products from the wastewater. Wastewater is mainly composed of di fferent organic materials and nitrogenous wastes. Almost all biodegradable matter in wastewater is converted into methane, but there is the chance of forming nitrous gas (potent greenhouse gas) during the partial nitration process from the nitrogenous wastes, and it may also reduce pH and oxygen levels that are important for survival of various microorganism inside the digester [2,107]. Sometimes phenolic substances are inhibitory to microbes, and grea<sup>t</sup> care should be taken in the selection of microbes for production of desired energy products. Moreover, the complexity of wastewater due to the introduction of new chemicals and substances from various anthropogenic activities is another growing challenge. Such activities not only change the uniformity of wastewater, but also make energy recovery from wastewater an uphill task [108,109]. Potential barriers to hydropower generation at wastewater treatment plants include a lack of excess heat, flow rate variations, turbine failure due to blockages, or particulate matter present in wastewater, especially in raw sewage. A resource recovery process is not cost e ffective due to excessive operational cost. While various technologies have been explored in the academic arena for the recovery of water, electricity, fertilizers, and other wastewater products, none of these have

ever been implemented on a large scale because of technological immaturity and/or non-technical bottlenecks [110]. In order to treat wastewater more effectively and recover the essential energy-related products, all these problems and concerns have to be tackled in a very comprehensive way in order to solve all the wastewater-related issues besides developing more efficient and ecofriendly technologies.
