Milton Keynes Plant Harnesses AD for Energy & WWTW Sludge Digestion
At Anaerobic-Digestion.Com we like to bring you information on all types of AD Plants, and we are able to report that sewage works sludge AD Plant technology is now being developed significantly as the following article describes.
Anglian Water has a new biosolids and energy plant at a Milton Keynes WWTW which treats sludge from the plant and provides energy, as well as sludge treatment, which we understand has set new standards for anaerobic digestion technology, and uses thermal hydrolysis as the pre-treatment stage. The £16M sludge treatment centre uses Cambi technology.
The following is an abridged version of an article which appeared in the October 2007 edition of Water & Wastewater Treatment. See more at the W&WT site here.
Anglian Water (AWS) serves around 5.6M wastewater customers. The region produces around 520,000tonnes of wet sludge per annum and AWS recycles more than 90% of its biosolids (treated sludge) to agriculture.
During 2005, AWS appointed consultant Mott MacDonald and two contractors - GTM (a joint venture between Galliford Try & Imtech Process) and Black & Veatch (B&V) to choose the optimum pre-treatment technology for sludge pasteurisation, and to build or upgrade the chosen treatment sites.
The most significant factors in the exercise were capital cost, quantity of biosolids generated and operational costs.
It was established that advanced digestion technology, as exemplified by the Cotton Valley Sludge Treatment Centre (STC) project in Milton Keynes, could both enhance biogas production and increase digester loading.
Based on the original business plan, the decision for Cotton Valley would have had a simple pasteurisation system built processing 13,700tonnes of dry solids (TDS) per year of mainly indigenous sludge with a typical UK digester loading.
But the whole-life cost exercise showed that installing Cambi thermal hydrolysis process (THP) plant at the site, up-front of the existing digesters, gave best value when treating 20,618TDS of sludge as an enlarged STC. Cambi THP can enable digesters to operate at sustained 6kgVS/mVday organic loading (average UK loading is about 2kgVS/m3/day), and can greatly increase the throughput of existing digesters.
The other advantage of Cambi was that the THP process is proven to produce cake that is typically 8-12% points higher than cakes from other processes. The combined sludge then undergoes thermal hydrolysis pre-treatment at high temperature and pressure, prior to conventional mesophilic anaerobic digestion. The Cambi process works by pressure cooking sludge cake in series of tanks to make a sterilised, non-viscous liquid at about 10% dry solids (DS) that can be fed directly to digesters. Steam is recycled within the process to reduce the overall heat requirement.
Apart from increased digester loading rates, the other major advantage is that the nature of the digester feed sludge is completely changed, so dewatering is greatly improved and biogas production is increased.
The Cambi process is being used in 19 projects so far around the world, treating sludge from 8M people. Two Cambi sites the project team visited were already operating at the high digester loading than the mark II Cotton Valley project required: Dublin Corporation's Ringsend wastewater treatment works (WwTW) and Thames Water's Chertsey WwTW. AWS, through its international company Celtic Anglian Water, is the operator of the Ringsend plant.
B&V was appointed to develop the STC project at Cotton Valley, having previously built Ringsend WwTW, the world's largest Cambi plant, which serves about 1.5M people. "Cambi UK now operates the THP plant at Chertsey as part of an incentivised contract for Thames Water.
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