**1. Introduction**

All living beings could not live without water. Humans require water for not only to sustain their life, but also to accomplish their day to day activities. But now, a day's pure water gets out of reach of humans because of the addition of various harmful and toxic pollutants in water sources. Beside this basic necessity of the present world, the managemen<sup>t</sup> of resulting effluent or wastewater is another challenge [1,2]. While technologies for the recovery of wastewater resources have been discussed extensively by the scientific community in recent decades, their large-scale implementation in municipal wastewater treatment facilities (WWTPs) still requires serious consideration. This can be demonstrated mainly by technical and non-technical reasons for doing so. Wastewater managemen<sup>t</sup> plays a significant part in sustainable urban planning [3]. It is a well-known worldwide reality that the energy demand is increasingly growing due to rapid population growth that has also increased the rate of generation of wastewater in the last decades. To accomplish both of these obligations, utilization of wastewater should be done in such a manner so that the process used would treat the wastewater along with the production of some cherished products which can be reutilized further [4,5]. Use of waste water for energy generation is economic, as this does not require expensive phenomenon. While several emerging technologies contribute to the wastewater resource recovery challenge, biological approaches

give the greatest promise to recover essential resources from effluent in an efficient manner. The article will generally concentrate on various methods of resource recovery from domestic and industrial wastewater [6]. The next generation of Domestic Wastewater Treatment Plants (DWWTP) targets energy efficiency and the complete use of wastewater for energy generation. There are also increasing concerns to extract useful products, especially renewable energy, from various forms of waste and wastewater from various industrial effluents [7]. Moreover, the fossil sources are very limited and may deplete in the coming future, so alternative sources of energy have to be developed. Therefore, the best approaches include the use of wastewater for production of energy products like bioethanol, biogas, biodiesel, etc., which further can be transformed into electricity [8]. Such energy recovery approaches may help mitigate wastewater sector electricity consumption and show promising areas for renewable energy policy implementation. Our analysis looks only at energy usage and future savings; while very significant, the economics of energy recovery mechanisms of wastewater treatment plants are reserved for a separate examination.

## **2. Characteristics of Wastewater**

The characteristics of wastewater significantly affect the treatment approach to be pursued, as well as the reactor design selection process. For such characteristics, the most important are concentration for suspended solids, organic strength (BOD or COD), temperature, pH, and inhibitor presence [9]. Many reactor designs can damaged by suspended solids and accumulation of grits. For this purposes, liquid waste or wastewater is considered to have a concentration of suspended solids below 1000 mg/<sup>L</sup> with small quantities of grit (inorganic non-soluble solids), often removed by simple pretreatment. Defined as such, wastewater can be graded as low, i.e., below 1000 for industrial, agricultural (including flushed manures), and pulp and paper, medium, i.e., 1000–10,000 for food processing, canning, citrus processing, milk processing, juice processing, and brewery, and high, i.e., 10,000–200,000 for ethanol production, distillery, biodiesel production, petrochemical, and slaughter house concentration [10–12].
