Nowadays, wastewater treatment plants should not be considered as just treatment systems, but rather as a factory at which value-added by-products can be produced without any extra cost. In this context, activated sludge bacteria can produce special lipids known as polyhydroxyalkanoates (PHAs) that are potential bio-plastics; they also accumulate neutral lipids known as triacylglycerols (TAGs) that are biodiesel precursors. One can exploit these types of cellular lipids from mixed microbial communities readily available in wastewater on which they grow and make value-added by-products from waste stream in treatment plants.