Fate of phosphorus during anaerobic digestion of sludge from an EBPR plant

As part of their ongoing commitment to produce safe sewage sludge for recycling to agricultural land, Anglian Water are expanding their enhanced sludge treatment operations at four key sites across the Eastern region. The project at Whitlingham STW uses thermal hydrolysis as a pre-treatment to anaerobic digestion as the main sludge treatment process. As well as producing an enhanced treated sludge, the thermal hydrolysis process facilitates increased volatile solids destruction, and subsequent biogas production. The hydrolysed and digested sludge will be dewatered to sludge cake for recycling to agriculture. The biogas will be used for energy generation for use on-site.

As a consequence of the increased volatile destruction, the dewatering liquors will contain elevated levels of ammonia and phosphorus, which will be recycled to the main works for treatment. This works is an EBPR plant. Therefore, it is essential that any phosphorus removed from the main sewage flow by the EBPR is not simply recycled back via the sludge treatment stream, and that the levels of recycled phosphorus and ammonia do not exceed the capacity of the works to treat it. This paper investigates the fate of phosphorus during anaerobic digestion in order to establish the amount of phosphorus, which is likely to remain in a dissolved form and therefore be recycled back to the EBPR plant, as well as the levels of recycled ammonia, so that the requirements for any side-stream treatment can be established.