Sustainable wastewater treatment thanks to codigestion

Can wastewater treatment become more sustainable?

While the goal of treating wastewater is to minimise the impact on the receiving environment, the processes themselves are usually powered by electricity generated by fossil fuels. In particular, aeration for BOD reduction and biosolids management contributes to a high carbon footprint and, potentially, low sustainability aspect.

However, things are improving with energy costs driving reduction targets. A number of benchmark studies and installations demonstrate significant sustainability improvements.

Energy efficiency at treatment facilities is usually aimed at efficient aeration, low energy pumping and possibly solar and wind power installations. However, the greatest opportunity lies in the waste-to-energy program. This typically involves utilising anaerobic digestion for sludge stabilisation and producing biogas, with cogeneration used to produce electricity. There are several examples of cogeneration using biogas around Australia.

Consulting company GHD has been involved in a number of cogeneration projects both in the industrial sector and for municipal applications. The company is currently working with SA Water on a codigestion project at SA’s largest plant, Bolivar Wastewater Treatment Plant (WWTP). Codigestion involves tanker offloading of high-strength waste to feed the anaerobic digesters, supplementing the existing WWTP sludge, which consequently produces more biogas to convert into electricity.

SA Water has been very progressive in establishing cogeneration facilities at its WWTPs. Trials at Glenelg WWTP (capacity of 60 ML/day) have led to a full-scale installation, which has boosted energy production to a point where the electricity self-sufficiency of the plant now lies at 72%. At Bolivar WWTP (150 ML/day), cogeneration and electrical production are currently at 63% self-sufficiency.

The current project aims to boost this even more with a permanent installation to receive additional external wastes. This will not only provide benefits to the plant operation’s bottom line, but will also likely reduce the waste going to landfill. It will also deliver a significant economic benefit for the state through cost-effective disposal for certain types of trade waste and lower operational costs factored into water rates — a win-win situation.

GHD has also worked on codigestion plants in the USA, at Ithaca WWTP (38 ML/day) in New York and at Hill Canyon Treatment Plant (50 ML/day) in California. The latter recently upsized cogeneration (700 kW) and installed solar panels (150 kW). To further boost biogas production, Hill Canyon now receives a variety of high-strength waste streams in its codigestion process and, as a result, the plant has now achieved 100% self-sufficiency of its energy usage.

*Mitch Laginestra is GHD’s Service Line Leader – Wastewater Treatment & Recycling. Ross Carter is a project manager at SA Water.