Water & Wastewater Treatment

TECHNICALLY SPEAKING 35 February 2014 Water & Wastewater Treatment wwtonline.co.uk S ewage sludge is now the key resource for renewable energy generation for most UK water and sewage companies (WASCs). Many use some form of mesophilic anaerobic digestion (MAD) to recover energy from sewage sludge via biogas, which is then used for electricity generation. Some WASCs have begun to deploy advanced anaerobic digestion (AAD) to maximise this route for energy recovery, but it has definitive limits dictated by digester detention and ultimately by CAPEX. However, even after advanced digestion, substantial calorific value and mass for disposal remains. Some WASCs operate sludge incineration, which significantly reduces the final disposal mass, literally down to ash. Initially, incineration used by the UK water industry lacked energy recovery, but recent capacity increases and upgrades have introduced it. The technology usually deployed is steam turbine, with a typical generating efficiency of around 32%. Incinerators also have a high parasitic load. These factors limit the returns from incineration, creating the opportunity for newer technology to improve this position. Incineration replacement UK WASCs can now strategically replace incineration with advanced energy recovery (AER) technologies. They will recover more energy downstream of digestion and simultaneously reduce issues with the disposal of digestate and even recover further value from the AER by- product. Indeed, the application of AER technologies may not be limited to the replacement of incineration, but could also be an alternative disposal route. This alternative technology for WASCs has been developed over the last two decades, through the deployment of full-scale gasification for energy recovery for organic and municipal waste, plus established technology for energy recovery from sewage. In the same period, substantial work has been done to advance pyrolysis as a waste-to-energy technology for organic wastes. For sewage sludge this has included full-scale material recovery of oils from sewage sludge. Alternative processes Like incineration, gasification and pyrolysis offer the potential to minimise the waste mass for ultimate disposal from processing sewage sludge for its sludge treatment centres (STC) and largest treatment works. These alternative technologies also offer the prospect of greater energy recovery and/or lower operating cost than that offered by incineration. At present, the value from these waste products is still uncertain but represents an opportunity for further value recovery. The AER technology profiles assessed by ourselves include: • Gasification of raw sludge with conventional digestion and advanced digestion based on thermal hydrolysis • Pyrolysis of raw sludge with conventional digestion and advanced digestion Incineration is the existing technology for post- digestion energy recovery, that is, unrestricted combustion of sewage sludge. This has a high parasitic load and generates significant heat from combustion. The apparent simplicity of combustion is misleading, as off-gas treatment is quite difficult and sewage sludge incineration is a complex multi-stage process. Gasification and pyrolysis Gasification is not a combustion process, but like incineration it is complex. The organic components react in a gas stream of steam, steam and air or steam and oxygen to form a syngas, whose principal component is initially carbon monoxide. There is a large range of reactor types and applications and the reactor type and gasifying agent selection is dependent on the fuel characteristics and the outputs required from the reactor. A gasifier's outputs include char, syngas and heat. Any syngas produced usually needs cleaning to assure reliable combined heat & power (CHP) – gas engine operation for electricity generation. Pyrolysis is no less complex than incineration or gasification. It is the thermal decomposition of organic material, in which the organic fraction devolatilises to form either a condensate (liquid product) or gas (syngas). The end product required determines the operating temperature range for pyrolysis with liquids, for example, oil recovery, occurring at lower temperatures and syngas being the principal product at higher temperatures. Higher operating temperature also favours production of cleaner syngas. However, as is the case with gasification, some degree of gas cleaning is normally required for reliable gas engine /CHP operation. Virtual model A Virtual Works model was constructed by MWH for a large, OFWAT 'category six' wastewater treatment works (WwTW) of 500,000 population equivalent (PE) with sludge imports. The Virtual Works model is a combined process flow diagram (PFD) including mass balance and associated energy balance, supplemented by a UK industry standard carbon-accounting tool from WRc/ UKWIR (2011: Version 5). It also includes steady state models for grit removal, primary treatment and biological treatment and chemical and transport accounting datasheets. The Virtual Works (creatively called Greendale WwTW) built for this analysis serves 500,000PE with an annual daily average flow of 150Mld and sludge imports. In the first instance, liquid sludge imports of 6.75tDS/day were assumed for the existing works. Upgrades with conventional digestion, advanced digestion, incineration and AER of raw sludge and Now that sludge incineration can now be replaced with alternatives that generate energy and extract other resources from wastewater biosolids, what is the best way to go? MWH technical director Ajay Nair and his team review the options The burning question on energy recovery u 36 Advanced digestion capacity installed at Avonmouth sewage works in Bristol

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