Rus, E.1, Fountain, P.2, Mills, N.1, Shana, A.2, Molokwu, O.1, and Asaadi, M.3.
Crossness and Beckton wastewater treatment plants have experienced issues with dewatering of their final digested sludge, requiring double the polymer demand for floc formation than that expected and achieving up to 25% less dry solids in the final cake. This paper looks into two possible root causes for the poor dewatering performance observed onsite: the impact that high concentrations of soluble phosphorous on the monovalent to divalent ions on the sludge and its effects on dewaterability, and the effects of erratic feeding of the digesters and the bacterial enhanced extra-polymeric substances excretion due to shock loads.
Analysis and lab scale dewatering done on digested sludge samples from other 9 THP sites across the UK showed a clear relationship between soluble P and monovalent to divalent ratio, which also showed a relationship with polymer demand and final cake dry solids. Nevertheless, both Crossness and Beckton have seen an improvement in dewaterability with no change or increasing monovalent to divalent ratios, which goes against the observed trend. Furthermore, a set of lab scale digesters seeded with Crossness and Beckton digested sludge and fed with hydrolysed sludge from the two sites under controlled conditions (constant feeding and similar iron dosing) show an improvement in dewaterability with time compared to site. This suggests other factors may be driving this improvement.
Extra-polymeric substances were considered a possible second factor having a negative impact on dewaterability. Both bound and soluble protein related extra-polymeric substances showed a weak relationship with polymer demand. Beckton site showed higher concentrations with higher polymer demand. This was not the case with Crossness.
The results seem to show that both factors explored here play a role in digested sludge dewaterability. Nevertheless, other factors are also most likely having an impact on polymer demand and final cake dry solids and further work to explore other causes is recommended.
Thames Water have built a number of Thermal Hydrolysis Plants (THP) to meet new capacity demands, improve efficiency and reduce operating costs. Two of these plants, Beckton & Crossness, located in East London utilise a two stream Cambi B12 THP system, 6 x 4,000m3 digesters, 3 x 2MW CHP units and 5 x Bucher press dewatering units. The guarantee for the final stage dewatering cake quality was 38% dry solids (DS). Initially the plant dewatering achieved 38%DS and better, but after a couple HRTs the cake DS dropped and the polymer consumption increased. This poor dewatering timed with the loss or washout of the seed sludge which had come from a different STW. During this period volatile solids destruction (VSD) and biogas production remained stable and as expected (Rus et al., 2015). The dewatering issue seemed to affect the rate of release of water as well as polymer demand and cake DS. The nature of the dewatering problems is very typical of dewatering issues seen on other Biological Nutrient Removal (BNR) sites. Beckton and Crossness were not designed to operate in BNR mode; nevertheless it was subsequently found that both sites were operating effectively as BNR. However, initial lab investigations suggested that the poor dewatering performance cannot be explained by the BNR \ sludge chemistry issue aloneATERING PERFORMANCE