Toward Net Energy-Positive Wastewater Treatment via Anaerobic Codigestion of Organic Wastes at Small Scale

Abstract

Anaerobic codigestion of multiple organic wastes can increase biomethane production and move small-scale wastewater treatment plants (WWTPs) towards becoming net energy producers. Despite this, no study has examined in detail the quantity of organic waste required to make a small-scale plant energy positive. To address this gap in the literature, this study uses data from a WWTP with anaerobic codigestion treating 8,710 m³/d of domestic wastewater from a community of 14,400 people. Using Monte Carlo simulation, codigestion was modeled to produce between 458 and 832 m³ biomethane/day (at 35°C and 1 atm), which equated to 1,180–2,244 kWh/d. In the best case (highest electricity production, lowest facility electricity requirement), a 30% efficient microturbine produced 86.3% of WWTP electrical needs, indicating that the community does not produce sufficient organic waste for the WWTP to be energy positive. Models were most sensitive to the composition of the influent organics, which can be a concern at small-scale WWTPs servicing communities with relatively small amounts of available waste organics. Other considerations, such as the implementation of other renewables, a cost-offset analysis using local electricity costs, and barriers to implementation, are also discussed. This novel study strongly suggests that anaerobic codigestion can be an important component of net energy-positive small-scale WWTPs, but that other renewables are likely required for WWTPs to become energy self-sufficient—a finding not clearly presented in other studies. This approach can be used by other small-scale WWTPs to assess the feasibility of codigestion at their location.