What is the best scale for implementing anaerobic digestion according to environmental and economic indicators?

Wastewater treatment plants (WWTPs) are the most commonly used environmental management systems to ensure that water pollution is properly managed. The costs associated with energy consumption and sludge management are two elements that penalize the operation of WWTPs. Anaerobic digestion (AD) unit showed to be a good alternative that allows the valorization of the organic load into bioenergy. However, not all WWTPs incorporate this technology in the sludge line that is associated with a minimum scale plant in order to ensure steady and profitable operation of the unit. With the decentralized context taking more strength one question arises: Is the AD technology viable in smaller plants? In this study four different sludge lines with different plant sizes were evaluated from an environmental and economic point of view. The sludge lines range between 25,000-1,000,0000 of equivalent inhabitants and the main difference between them is that small WWTP has no AD unit. According to the results, environmental impacts of the AD technology do not depend directly on the size of the plant. It is for this reason that not only medium and large plants bring environmental and economic benefits, but also smaller ones, as long as the necessary premise of biogas valorization into bioenergy is met. AD technology can be improved with the addition of agrowaste that can enhance the organic load in the anaerobic digestor and improve the yield of biogas production and the eco-efficiency, in such a way that the premises of technological, economic and environmental viability are fulfilled.