Water & Wastewater Treatment

38 | august 2014 | WWt | www.wwtonline.co.uk In the know Technically speaking: anaerobic technology Is it time to switch the air off on activated sludge? Future use of anaerobic processes will go beyond treatment of solids to encompass the entire flow and the latest pilot work highlights the advantages. O ne hundred years on and whilst the significance and impact of the activated sludge process is beyond doubt it is perhaps timely to think about what next. The challenge going forward is how to upgrade works to meet tighter discharge consents without a commensurate increase in energy, chemicals and ultimately costs. To date this has mainly focused on the last two areas with widespread use of forced aeration to upgrade existing treatment facilities. This occurs across all scales of operation as demon- strated by the recent uptake of aerated constructed wetlands on small sewage works 1 . The process efficacy of the ap- proach appears clear, and its cost and chemical use are attractive on sites with existing horizontal flow wet- lands. However, the approach raises questions linked to its place within the overall philosophy of small works (low energy, low maintenance). For instance, a recent energy audit on an active site showed the energy demand to be equivalent to that of an acti- vated sludge process on a population equivalent basis (approximately 2.5 Wh/PE). Accordingly alternatives are required at all scales or a continuing trend of increasing energy use will be unavoidable. Is anaerobic the answer? For many people the answer lies in anaerobic technology. Anaerobic processes unlock the true resource potential of sewage by converting the carbon content into methane whilst decoupling the nutrient removal proc- esses, enabling recovery and the use of innovative treatment technologies. Future consideration for the use of anaerobic processes goes beyond the use of anaerobic digestion of solids to encompass the entire flow. Such thinking is not new but has tradition- ally been accompanied with negative perceptions due to the relatively high half saturation constant of anaerobic communities, leading to low meth- ane yields. However, recent work is now chal- lenging the validity of such percep- tions and assessing the potential of modern bioreactors such as anaerobic membrane bioreactors (MBRs) and high-rate expanded granular sludge blanket reactors (EGSB). For instance, recent pilot-scale work performed by Cranfield on a sewage works in the north of England has shown an an- aerobic MBR treating settled sewage to be effective down to 6°C without loss Professor Bruce Jefferson ProFessor oF Water engineering, CranField university Challenge: how can we meet tighter consents without a commensurate rise in energy?

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