Water & Wastewater Treatment

TECHNICALLY SPEAKING 36 Water & Wastewater Treatment February 2014 wwtonline.co.uk in combination with conventional digestion and advanced digestion have all been modelled in this analysis. Cost of treatment Each model was used to derive the unit cost of treatment for the whole sludge processing train and unit cost of treatment adjusted for benefit, both independent of sludge throughput, for example, as £/tonne dry solids (£/tDS) of sludge processed. Each model also generated the sludge processing energy demand and energy generation, independent of sludge throughput, for example, kWh/tDS. The costs included: • Power • Carbon fees • Chemicals • Sludge disposal • Staffing • Services • The benefits available included: • Power import cost defrayed by in situ generation • Power exports, where generation exceeds total site demand • Renewable Obligation Credits (ROCs) • Reductions in carbon fees • Reduction in sludge disposal costs AER carbon emissions were based on the full-scale study of sewage sludge gasification by Takahashi in Japan in 2007, where gasifier emissions were found to be 55% of sewage sludge emissions under combustion in a fluidised bed incinerator. The power demand and generating potential of each of the options is presented in Figure 1. Power generation The AER options are able to generate more power than incineration options, which appears to be due to the greater electrical power generating efficiency associated with gasification and pyrolysis. When gasification is compared to pyrolysis, there is an advantage to pyrolysis for electricity generation associated with better syngas quality and quantity potential. In part, this may be attributed to a higher parasitic load for the gasification options examined. Taking this factor into account, gasification fares better when combined with digestion and especially with advanced digestion if a low carbon outcome is sought. Similar to combining incineration with digestion, the combined technologies maximise the low parasitic load of biological treatment, while using the brute force of incineration or gasification to release most of the remaining energy that digestion is unable to access. Gasification does this more efficiently than incineration, but also may produce a better outcome than the non-optimised AAD-gasifier system in this study. It has waste heat that could be used better than in this study in combination with AAD. This is a separate investigation that remains to be undertaken and reported on. Risk management The final unit cost of treatment (UCT) presented in figure 2 can only be secured if the operating risks underpinning them are adequately managed. If energy production recovery practice becomes sub-optimal, a UCT set as a target will not be maintained. The great advantage of Virtual Works based modelling is that the model can be used to describe the entire operational envelope associated with a UCT. This means that a UCT has context which can be presented to an operator, allowing them to manage the risks to attaining the UCT. For example the impact of changes in the primary settling tank (PST) removal efficiency or the impact of a lower dewatered cake DS concentration. Outcome Based on a MWH Virtual Works model for the fictitious Greendale WwTW, we found that advanced energy recovery (AER) by gasification offered better energy recovery prospects and lower operating costs than incineration when paired with digestion or advanced digestion. This appears to arise from the better overall energy balance from the non-combustion process. The range of gasification technologies available is large and at present we suspect that there are further synergies that can be derived for a digestion or advanced digestion/ gasification AER system, with regard to recovering heat for digester and/or thermal hydrolysis process heating. The second AER option, Pyrolysis, offers further advantages over the gasification options considered here. These arise from production of a better syngas product than gasification, favouring more effective gas engine/CHP power generation. For carbon footprint reduction, raw sludge pyrolysis or AER technology paired with AAD provides the best outcomes. Another tool that MWH has developed to help understand the impact of resource inflation is the Median Water model, based on a fictitious UK water company and associated asset base representing the median of all UK WASCs. Using this tool we have been able to value the operational saving returns arising from systematic deployment of the leading AER/combined AER solutions at the largest sewage treatment works and all sludge treatment centres. Taking a typical UK WASC, this represents at least £35M/yr. Clearly questions surround any new technology and application. However, notwithstanding the cost and competition for available capital, there is potential for advanced energy recovery technologies, certainly at some of the larger treatment plants with regional sludge treatment centres attached. We see a strong future for extracting maximum energy from wastewater, whilst also retaining facets of other benefits such as nutrient recovery. nnn Figure 1. Energy recovery potentials: power demand and generation per tDS sludge Figure 2. Cost and benefit scenarios modelled for imagined Greendale sewage treatment works 2012

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