This paper presents an independent review of a long-term survey undertaken to monitor the residual biogas potential (RBP) of various digested sewage sludges from a number of water company sludge treatment centres (STC) in England.

The RBP test is unique to the UK and was designed to determine the residual biodegradability of digestate samples by measuring the quantity of biogas produced by the digestate sample under specific conditions. Although suitable for most biodegradable organic materials, the test was originally developed with a focus on non-sewage sludge, being developed for inclusion in the PAS 110 specification, which formed a key element on the UK Government's anaerobic digestion protocol. However, the methodology is directly applicable to sewage sludge.

The survey was undertaken to provide evidence regarding the stability of the digestate, in-line with the associated improvement programme requirements which are typically detailed in the Environmental Permits for the individual STCs. The data presented covers 56 STCs, including results for conventional mesophilic anaerobic digestion (CAD), Pasteurisation (PAS), acid phase digestion (APD) (EH, EEH, and HpH/Helea), and THP (Cambi and Biothelys) anaerobic digestion (AD) technologies.

The paper summarises the performance of sewage sludge AD plants in relation to the RBP test. It goes on to discuss the applicability of the RBP test in this exercise, and then sets out what an equivalent upper tier limit would logically be, when comparing the PAS 110 limit for non-sewage sludge.