Treatment of human waste in small-scale facilities: a prospective review

A sustainable approach to the small-scale treatment of portable toilet and septic tank waste is in need in order to minimize the associated public health and environmental risks. Treatment of human wastes is highly regulated by legislation in many countries given the high content of organic components and pathogens. In conventional wastewater treatment plants (WWTP), sewage undergoes a series of treatment stages (primary, secondary and tertiary) before effluent discharge. Delivering this approach at a small scale, to meet demands of temporary events, public gatherings or remote locations provides benefits in reducing transport and bulk handling. For a small-scale treatment process to effectively work the process steps need to be simplified and minimized. We review the potential of a new treatment scheme where the first step is the solid-liquid fraction separation, followed by the anaerobic digestion of the solid fraction including energy recovery in a combined heat and power (CHP) unit. The liquid fraction undergoes a series of filtration and disinfection steps to comply with effluent regulations. Digestate from anaerobic digestion is burned on site to provide local domestic/office heating. This approach has a great potential for application in different locations where inputs may be sporadic, such as outdoor festivals, disaster response scenarios and construction sites.