Techno-economic Assessment of Codigestion of Waste Activated Sludge with Food Waste for Enhanced Bioresource Recovery

Abstract

The co-digestion of waste-activated sludge (WAS) with food waste (FW) at wastewater treatment plants can produce enough biogas to allow for power generation. The current study evaluates the feasibility of the approach and its applicability to local settings in South Africa. The monodigestion of WAS (monodigestion) and codigestion at 20% (codigestion 1) and 40% (codigestion 2) FW concentrations were investigated employing a pilot-scale solar-heated digester. The findings were upscaled considering a local large-scale wastewater treatment plant (with a capacity of 0.42 million m3/day) and evaluated using techno-economic analysis (TEA). The results show that employing combined heat and power generators to convert the biogas to electricity could result in a profitable outcome, enabling the plant to offset its power requirements. Monodigestion and codigestion 1 can offset 22% and 94% of the plant’s power requirements and save $0.5 and $2 million, respectively. Codigestion 2 can offset 196% of the power requirements resulting in $2.3 million in savings and $2.4 million in sales from the excess power. Alternatively, the excess biogas can be sold as compressed natural gas (CNG) with an annual market value of $4 million. The TEA shows a positive net present value, an internal rate of return value higher than the plant’s discount rate, a profitability index greater than 1, and a payback period of less than a year for all cases. The findings show that the codigestion approach is financially lucrative and may benefit the government, stakeholders, and investors.