AD operators inevitably want to achieve higher gas yields, reduce foaming and eliminate the potential for maintenance problems.

In addition to Rotamix floor-mounted nozzle systems, System Mix can incorporate (as an option), additional high level nozzles which act as ‘scum layer breakers’ and non-chemical foam suppression. ‘Foambuster’ uses the digestate to spray coarse heavy droplets onto the foam, either continuously, or as required, using a dedicated chopper pump and nozzle.  This can also be achieved by designing the system to accommodate the additional flows and pressures required using the existing Rotamix mixing system with the pump operating with a VSD to maintain efficiency. This enables AD operators to apply a virtually universal solution to the problem of how to effectively suppress foam.

In a pilot scheme System Mix provided a system for evaluation on one of five digesters at Anglian Water’s, Great Billing sewage treatment works. As this digester incorporated three roof mounted nozzles one was modified by reducing its length and adding a plume distributor plate, allowing just over 10% of the recirculation flow to act as the foam buster. Work had to be carried out without the need to empty the digester and with minimum impact on Great Billing’s sludge handling capacity.  In extreme circumstances during 2012 the digester consumed a 1m3 capacity IBC of de-foamer every 2 or 3 days. Following installation of the trial system Anglian Water reports savings of £20,000. This equates to a payback of less than two months.