Rise and Fall in Digesters Fed with Thermally Hydrolysed Sludge

Thermal hydrolysis pre-treatment (THP) of sludge has significant advantages - but care does need to be taken in how digesters are fed and mixed.Historically people have fed conventional digestion using fill and spill: sludge is fed into the digester and then there is a riser pipe leading to a limpet box (overflow chamber) where sludge overflows out from the digester. While this does work for THP digestion, there are some unique aspects relating to sludge density which can cause significant operational issues.Thames Water and some other UK Water companies have had issues with feeding THP digesters. This paper concludes that in nearly all cases the problem is one of rapid rise and fall in sludge density. THP sludge at 5-6% DS is not necessarily more viscous than conventional sludge at 2-3% DS but crucially is producing 2-4 times more biogas per specific volume than conventional sludge; in addition the rate of biogas production follows the rate of feed very closely so that inconsistent feed (and/or mixing) gives very different biogas production and fluctuating sludge densities.This paper considers what is different about the material properties in THP digestion –• What can happen – what problems are caused to operations?• The science behind the problem – why is it happening?• Why it happens at some places and not at others?• Look at several proposed alternatives of how to avoid the problem?