The role of soluble P & EPS on dewatering performance

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: (1) the impact that high concentrations of soluble phosphorous on the monovalent to divalent ions on the sludge and its effects on dewaterability, and (2) 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.