VARIATION OF PH OF LIME TREATED SLUDGE CAKE DUE TO STORAGE AND ITS IMPACT ON COMPLIANCE
Peter Winter, Manocher Asaadi and John Coles-Carr
RWE Thames Water I&D
(free)
Thames Water recycles approximately 125,000 tonnes dry solids of cake to agricultural land annually. In 2003, 14% of this was lime treated cake, produced at three sludge centres in the Southeast of the Thames Water area. In this paper, the variation of pH of lime treated raw cake during storage and its impact on compliance and ammonia release are evaluated. Dry solids and volatile matter, total and ammoniacal nitrogen and neutralising value were monitored as part of the project.
Different lime dose rates were assessed to achieve pH of no more than 11.5. Lime dosed cakes were then monitored over time to assess the quality of the stored product.
Cake samples were taken from full-scale dewatering facilities to ensure a representative product. The samples were limed and monitored over a period of 30 days. On average, lime doses of 10% and 12% lime (CaO) on dry weight basis raised the pH to 11.5 and above and maintained total or near total Escherichia coli kill throughout the storage period. Cake with a relatively high proportion of surplus activated sludge required higher lime doses, typically 13 to 20% on dry wt basis, to achieve pH of around 11.5 compared to the other sludges under investigation.
The investigation showed that a decrease in pH during storage did not compromise the compliance of the stored cake and there was no significant increase in the E.coli levels in the limed product even at pH of around 9.
KEY WORDS Compliance, Lime, pH, Sludge Cake, Storage
