THP Digestion plants at Crossness and Beckton are in their final stages of commissioning. The decision was made to run a set of chemostats in parallel with the full scale sites in order to better understand the digestion and dewatering of the sludge. The work was carried out by Thames Water Innovation. A bench scale trial consisting of 10L chemostat digesters which were set up to compare the site stability parameters and dewatering with a controlled environment. The addition of iron sulphate into the chemostats showed a clear improvement in dewaterability of the digested sludge with increasing doses of iron.
Crossness and Beckton STWs are Thames Water’s two largest sites with a total of about 5.5millions PE. Until 2014, the sludge produced on those sites was mainly processed through the Sludge Powered Generators implemented on both sites after the sludge to sea disposal ban in 1998. The two SPGs allowed for significant sludge volumes reduction through combustion of raw cake but also for some energy recovery from sludge through heat recovery and electricity generation (steam turbines). Based on the recognised success of Thermal Hydrolysis Process (THP) as sludge pre-treatment to Anaerobic digestion on various sites across the UK (e.g. Chertsey, Cotton Valley, Riverside, Cardiff), Thames Water decided to implement this technology on 5 other major sites, including Beckton and Crossness STWs. Due to the lower overall OpEx costs and higher electrical generation per TDS of the THP process compared to the SPGs, the strategy on these sites was to maximise the throughput of the THP plants but keeping the SPGs operational in order to treat the remaining sludge. The commissioning of Crossness and Beckton started respectively in the summer and autumn of 2014
The work done to support the THP commissioning phase at Crossness and Beckton STWs showed that the presence of soluble phosphorous may partly be causing a worsening on the dewatering of digested sludge by altering the cation balance during digestion. Iron dosing did show benefits in dewatering. Nevertheless, the bench scale digesters or chemostats did show better dewaterability and lower polymer demand than site under the same iron dose conditions. Therefore, something else is believed to be contributing to the dewatering issues. Investigations are still under way to confirm what this might be.
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