The often conflicting demands of regulation and the need to reduce costs can be turned into a win:win situation by investing in the right mix of technologies for sludge treatment. This paper will discuss the work completed by Entec UK Ltd in conjunction with Thames Water Utilities Ltd in developing their sludge strategy and, in particular, a review of the most appropriate technologies to meet their business drivers. Amongst the main drivers were to maximise renewable energy production and deliver sustainable solutions, so technology had to be the key to achieving both objectives. The short-listed or leading technology would need to be commercially proven, technically robust and in line with Thames Water Utilities Ltd’s Strategic Direction Statement.
Of particular interest was the carbon footprint of the technologies. However this would be fairly meaningless in isolation and would therefore need to be investigated as part of a complete process route from production to the final recycling/disposal outlet. The approach taken therefore took two stages; the first to identify suitable technologies, the second to identify the best use of these technologies in combinations defined by the process routes.
The first stage looked at 34 established and emerging technologies and was able to rank them against Thames Water Utilities Ltd’s drivers. The leading technologies for each process stage were then chosen to create the process routes for review in the second stage. The second stage reviewed and compared these process routes for; general advantages and disadvantages, location and size considerations, energy balance, carbon footprint and financial impact. The routes were divided into outlets for recycling to agriculture and thermal destruction that suited the diversity of sludge production in the Thames Water Utilities Ltd operational area that varied between high density-conurbation and constrained-rural. The results of this analysis implied that using a process route of; thermal hydrolysis, anaerobic digestion, dewatered cake to agriculture presented the best fit to Thames Water Utilities Ltd’s requirements for its existing sludge to land operations.