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Better mixing maximises anaerobic digestion process

09 September 2009

Maximising the benefits of an anaerobic digestion system by achieving a higher gas yield, reducing foaming and eliminating maintenance problems were all achieved following the installation of a Vaughan Rotamix system at Basildon sewage treatment works.

Anglian Water Services Ltd (AWS) at Basildon are responsible for the treatment of waste water and sewage for a residential and business community of over 300,000 and during excess conditions also assist another major treatment works in Tilbury.

Gas compressor mixing systems were originally installed on the two anaerobic digesters, receiving indigenous sludge from the site where it was digested by bacteria to reduce solids and provide a rudimentary cleansing of the sludge mass. The digesters harness the Mesophilic bacteria to volatise organic mass with the sludge, to produce methane. The methane is stored in gas bags and then used to heat the digester boilers, which in turn heat the pasteurisers and ultimately provide heating for their office buildings at the Basildon site.

Prior to P&M Pumps-System Mix Ltd installing the two Vaughan Rotamix Anaerobic Digester mixing systems, AWS were experiencing increasing process problems due to poor mixing in the digester including numerous operational and biological issues. These key issues were high grit deposition and thus reducing active volume of digesters threatening to significantly reduce output outside compliance requirements. Also, the compressors required high levels of maintenance and were proving costly to keep running, furthermore, as deposition increased, the maintenance issues escalated.

In addition, foaming issues were common and although the gas mixing process was not the primary cause it certainly exacerbated the problem.  Not only did the rag mass reweaving within the digester impair their efficiency, it also caused blockage problems in the heat exchangers and further downstream pumps and systems.

Approximately 2 years ago AWS decided to replace the gas compressor type mixers with the ROTAMIX systems and to date they have been delighted with the results. A spokesman at Basildon STW site confirmed, “In the two years since installing the Vaughan Rotamix systems, we are confident that we have seen an increase (although this has not been quantified) in the gas yield from the digesters. He continued, “In addition, we think the system has contributed toward there being a reduction in foaming problems, although we are dosing anti-foam which we were not doing prior to the new mixer installation, so overall we have reduced operational costs.’’  Furthermore, AWS confirm that since installing the ROTAMIX system the digester temperature range from upper sludge level to tank floor has remained a constant ± 1°C, mixing has improved, volatisation of solids is better, gas production has increased and the constant rag issues have been significantly reduced, even eliminated.

When asked about the features they particularly appreciate AWS commented that the externally mounted pumpset and ‘in-situ’ adjustable design make accessibility simple. The overall performance of the VAUGHAN Chopper pump has significantly reduced maintenance expenditure. In fact, after carrying out the yearly inspection on digester pumps 1 and 2, which had been running for 12 months 24/7, all wet end chopping components were showing no wear.  AWS feel that once the two systems are optimised and the operational philosophy is modified they will potentially realise at least 4 to 5 years normal operation before any replacement spares are required.

SYSTEM MIX and P&M PUMPS have worked with the UK’s major water utility companies for over 15 years in various anaerobic digestion applications and during the last 10 years have witnessed a very positive change in the way that sewage sludge has been considered as an alternative source of energy. During this time they have both provided specialist knowledge and expertise in selecting the ROTAMIX mixing systems and are confident that the majority of installations have resulted in up to 100% performance over the life of the plant.

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