Part of Solent Protection Society’s activities this year has been to investigate the efficiency of coastal Waste Water Treatment Works (WWTWs). Our research revealed that the increased level of nitrates in Solent waters is, in part, caused by the discharge of effluent from WWTWs.

Although clearly much improvement is required to waste water treatment, a substantial benefit from these plants, which have been developed since the 1970s, is the capture of Biogas which is as a “green” fuel. The capture of biogas is a great benefit as not only is it a free green fuel but it also prevents the release of methane which has a global-warming potential 25 times more potent than carbon dioxide.

Digestate as fertiliser

So Biogas is a renewable energy source, mainly consisting of methane and carbon dioxide, created by the anaerobic fermentation by bacteria of organic waste containing carbohydrates. Biogas can be produced from many sources of raw materials such as agricultural waste, municipal waste, plant material, sewage, green waste and food waste.

A Biogas Plant is an anaerobic digester that treats farm waste and energy crops. A Biogas Plant can be fed with energy crops such as maize, silage or biodegradable wastes including sewage sludge and food waste. Using anaerobic digesters, (air-tight tanks with different configurations), the micro-organisms transform biomass waste into biogas and digestate. Digestate is the remaining inorganic matter not transformed into biogas which can be used as an agricultural fertiliser.

Higher quantities of biogas can be produced when wastewater is co-digested with other residuals such as that from the dairy, sugar, and brewery industries. For example by mixing 90% of wastewater from a beer factory with 10% cow whey, the production of biogas can be increased by 250% in comparison to the biogas produced solely from brewery wastewater.

There are four large Waste Water Treatment Works operated by Southern Water in the Solent area, at Budds Farm Havant, Millbrook Southampton, Peel Common near Fareham and Woolston Southampton. Southern Water is increasing its capacity to generate electricity from biogas with the installation of new Combined Heat and Power (CHP) engines.

We welcome these cost effective developments which will no doubt assist the UK in meeting its reduced CO2 targets.