SBR performance with an excessive long sludge age

In the UK water industry, the sequencing batch reactor (SBR) has been seen as a technically-economically advantageous option as an alternative to the continuous flow activated sludge process (ASP). The technology has enjoyed a booming period over the last decade, with many installations now operational. These installations appear to have achieved various degrees of success, but not without operational and performance issues. This paper reviews the typical design practices of SBR, and focuses on one particular SBR supplied by Biobubble Ltd within the spectrum of various SBR technologies. Biobubble SBR is designed as a true SBR with discrete phases of fill, aerate, settle and decant. It often guarantees an effluent quality of 15: 10: 2 (TSS : BOD : AmmN) and a low sludge production. Its main operating parameters are distinct in comparison with continuous flow ASP, and indeed other 'mainstream' SBRs. Differentiation can be seen on plant sizing, sequence control and equipment configuration. Studies have been carried out by Anglian Water Services Ltd to gain a better understanding of this type of SBRs. It has been found that the reliable performance is due to the long hydraulic retention time (HRT) of about 70 hours (aerated), which provide high,tolerance against shock loadings. The good settlement is a result of a long settling phase of 1.5 hours and a low fill time ratio (FTR) of about 0.05, creating a feast/famine condition similar to a plug-flow ASP. The low sludge yield of 0.33 kgSS x kgBOD(-1) is a result of a long sludge retention time (SRT) of about 200 days, which enables aerobic sludge digestion in the SBRs.