Case Study – Moving Towards Carbon Neutral Wastewater Treatment Plants: The Case for Combined Heat and Power

 Wastewater treatment plants (WWTPs) require electricity for operation of pumps, mixers, dosing equipment, aerators and other process equipment, as well as for ancillary services including lighting and offices. Those WWTPs that have anaerobic digestion (AD) systems to treat water also require heat for the digesters (heating improves digester efficiency and ensures optimal volatile solid stabilization) and sludge drying. Electricity, diesel and/or gas is typically used to provide the necessary energy inputs. Wastewater treatment facilities often account for more than 20% of municipal electricity consumption1 and are among the top three users of energy (and associated costs) in municipal operations.

 At the same time, WWTPs generate methane from the process of AD. Methane is an energy source that can be recovered to provide some or all of the plant’s energy requirement. In combined heat and power (CHP) processes, the methane gas can be combusted to generate electricity in a turbine. The residual heat can also be recovered for use in the plant (heating of the digesters). CHP thus provides an efficient way of recovering the maximum energy value from the methane stream, which may otherwise be vented (released into atmosphere) or flared (burnt with an outside flame). Most WWTPs in South Africa that have digesters generally vent the biogas without any treatment. Through providing on-site heat and power, CHP can result in energy and associated cost savings, improve the reliability of the plant and minimise the impacts of power outages on plant operation, which is particularly relevant in South Africa. There is the potential for WWTPs to gain energy neutrality, generating up to 100%of the plant energy needs, although in practice they tend to produce less than 60% of energy demand. This potential, however, depends on the plant configuration, treatment processes and the capacity of the CHP unit.