**1. Introduction**

Wastewater treatment plants (WWTPs) are integral part of our society. Lately, WWTPs and its management have become an important issue in the world and also listed in many of sustainable development goals [1]. The WWTPs were initially designed to remove the pollutants depending on the flow variability to meet the certain discharge standards. However, emerging concepts and practices in WWTPs field have extended its application to energy recovery, reuse and nutrient recycling. Also, majority of WWTPs around the world are not designed with these multiple functions in mind and depend only on conventional technologies to solve these problems and fails to strike the balance between demand and supply of water. Especially, this is quite evident from the water scarcity clock which shows that there are about 2 billion of world population still living in water scarce areas [2]. Moreover, the changing dynamics of world such as accelerating energy dependent lifestyles, sudden global pandemics, economic fragmentations, climate change patterns and other major environmental concerns have affected the selection of suitable

WWTPs. Since many WWTPs have life cycle of 50–100 years, or even longer, the selection of WWTP will affect the development of that particular area. Several studies have shown that selecting and deciding the WWTPs are mostly based on their technical, economic, environment and social aspects [3]. Though extensive researches are being conducted and major strategies have been formulated, the search for long lasting technology in the wastewater treatment field is still on-going.
