The effects of thermal hydrolysis and ammonia concentration on digestion rates at Basingstoke STW. Searching for the sweet spot

Panter, K.1, Fountain, P.2, Shana, A.2, 1Cambi, UK, 2Thames Water, UK


Thames Water owns and operates 8 thermal hydrolysis sludge centres and is planning more. It has operational and R&D experience of most of the ways of implementing thermal hydrolysis process (THP).  This paper focusses on the observed differences in biogas yield (m3/ton dry solids) of digesters that receive THP sludge and particularly the effect of ammonia. It is proposed that biogas yields of over 520m3/tds (equivalent to 70% VSR) are possible in digestion projects where sufficient digester volume exists for lower loaded lower DS% feed. Hydraulic retention time (HRT) is generally not the limiting constraint and counter-intuitively it may be better to aim for shorter retention time digestion with lower levels if ammonia inhibition. The work is from two full scale projects and further lab work is proposed to back up the findings.

The paper reviews previously published results from Long Reach STW and then focuses on the start up and conversion from conventional digestion to post THP digestion during start up of THP at Basingstoke STW during the spring of 2018. Biogas rates were measure before and after acclimation to high ammonia digestion. Bacterial populations were also studied. The main conclusion from the work is that the digestion rates at low ammonia below 1500mg/l were close to 70% VSR with a biogas yield of 522m3/tds. Subsequently the loading rates were increased, and ammonia rose to 2700mg/l resulting in much higher overall biogas production from the given volume of digesters but the biogas yield dropped to 425m3/tds which is the normal expected biogas yield following THP digestion at high DS%. This suggests approximately 18% inhibition due to ammonia and/or less VFA scavenging from acetate oxidizer organisms in high DS%/ high ammonia digesters.

When deciding the sizing and implementation of THP on digesters that are currently conventionally loaded the designer has some flexibility depending on desired DS% of Cake, need for Class A, need for more digestion capacity and biogas yield requirement that can lead to reduced capital and operating costs.

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