Turning Sludge into Nutri-Bio

Sludge is a by-product of the natural sewage purification process that takes place at all Anglian Water’s sewage treatment works. Sludges produced from small works in the countryside are taken to strategically placed Treatment Centres, usually situated on large works, and turned into nutri-bio. Only two methods of treatment are commonly employed, digestion and liming.

Digestion
The digestion process consists of heating the sludge to a temperature of between 32 - 38 degrees and holding it there for about 12 days. The bacteria within the digester produce three things; carbon dioxide, water and methane. They do this by breaking down the volatile organic compounds within the sludge. The methane gas is burnt in special engines to produce heat for heating the digesters, making it self sufficient in energy requirements. The process of digestion achieves the conventional treated product standard of a two-log reduction. The liquid is then allowed to cool exposed to he open air for a further period of 21 days before being dewatered to around 25% dry solids (CAKE) when it is sampled for its nutrient content prior to being recycled to agriculture.

Liming
Lime treated cake is dewatered liquid sludge to which Calcium Carbonate is added to raise its pH above11.5. This effectively kills pathogens due to ammonia release and creates an inhospitable climate for pathogenic re-growth within the cake. The final product from this process is not a liming agent, since the NV is around 4%, (a ‘sweetener’).

HACCP
All sludge treatment, Biosolids production and application to land processes operate under a quality assurance scheme known as the HACCP (Hazard Analysis using Critical Control Points) regime. All these processes are strictly regulated by the Environment Agency.