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) tochoose 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 quickly became apparent that utilising and maximising green energy generation would be a major driver in this project.

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/m3/day organic loading (average UK loading is about 2kgVS/m3/day), and can greatly increase the throughput of existing digesters.

The question changed from, “How cheaply can we make a class A cake?” to, “How much sludge can we digest efficiently in our existing digestion capacity?” In this case, the answer was 50% more than previously thought.

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. Therefore the quantity of final biosolids would be no more than in the original design. The STC will treat indigenous primary and secondary sludge from Milton Keynes, plus liquid imports and raw cake from the surrounding area.

The combined sludge then undergoes thermal hydrolysis pre-treatment at high temperature and pressure, prior to conventional mesophilic anaerobic digestion. The digested sludge is dewatered to cake and conveyed via dedicated conveyors to a storage bay.

Reaping the benefits

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.

The system operates completely automatically, and requires daytime attendance for routine and preventative maintenance. 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.