Comparing the performance of thermophilic and mesophilic anaerobic digestion of pasteurised sewage sludge
Hill, S., Shana, A., Perez, E.R., Fountain, P., Thames Water, UK
(free)
Abstract
Robust performance of the anaerobic digestion process depends on many factors including digester
temperature, substrate quality, organic loading rate, hydraulic retention time and the skill of the operator
involved. This applies to both thermophilic anaerobic digestion (THAD) and mesophilic anaerobic
digestion (MAD) processes.
Historically, THAD processes are predominantly used as a pre-treatment process based on a shorter
hydraulic retention time (HRT). Standalone THAD processes with a longer HRT have also been used
at the lower end of the thermophilic temperature range, but due to the increased heat requirements of
this process its use had been limited. However certain pre-treatment processes such as Thermal
Hydrolysis (THP) and pasteurisation result in the availability of a significant amount of low grade heat
which has to be removed to ensure the effective operation of the MAD process. Using a THAD process
would eliminate the need to remove all or any of this heat.
This paper is an extension of last year’s work presented during the Aquaenviro 2017 conference, which
compared the performance of both thermophilic and mesophilic anaerobic digestion of THP pre-treated
sludge. This paper will now compare the performances of the two processes when fed with pasteurised
sludge as opposed to THP sludge.
The results obtained from the laboratory scale experiments showed that both the THAD and MAD
processes performed similarly; achieving an overall long-term average VSR of 35 – 40% and an
average biogas yield of 275 – 300 m3/tds. Results from dewatering trials show that the polymer dose
for the MAD sludge is 11 kg/tds, whereas for the THAD sludge it is higher at 15 kg/tds. All four digesters
produced similar cake dry solids of around 30 – 31%.
