The UK Water Industry currently generates approximately 800GWh pa of electrical energy from sewage sludge. Traditionally energy recovery from sewage sludge features Anaerobic Digestion (AD) with biogas utilisation in combined heat and power (CHP) systems. However, the industry is evolving and a number of developments that extract more energy from sludge are either being implemented or are nearing full scale demonstration. This study compared five technology configurations: 1 - conventional AD with CHP, 2 - Thermal Hydrolysis Process (THP) AD with CHP, 3 - THP AD with bio-methane grid injection, 4 - THP AD with CHP followed by drying of digested sludge for solid fuel production, 5 - THP AD followed by drying, pyrolysis of the digested sludge and use of the both the biogas and the pyrolysis gas in a CHP. The economic and environmental Life Cycle Assessment (LCA) found that both the post AD drying options performed well but the option used to create a solid fuel to displace coal (configuration 4) was the most sustainable solution economically and environmentally, closely followed by the pyrolysis configuration (5). Application of THP improves the financial and environmental performance compared with conventional AD. Producing bio-methane for grid injection (configuration 3) is attractive financially but has the worst environmental impact of all the scenarios, suggesting that the current UK financial incentive policy for bio-methane is not driving best environmental practice. It is clear that new and improving processes and technologies are enabling significant opportunities for further energy recovery from sludge; LCA provides tools for determining the best overall options for particular situations and allows innovation resources and investment to be focused accordingly.
Results of our trials demonstrate that the alternating aerobic/anaerobic activated sludge (AAA-CMAS) system is capable of producing effluent of high quality in either fully-loaded or under-loaded conditions. The aeration energy saving and the sludge reduction of the AAA-CMAS system was shown to be of the order of 30% and 15% respectively. Because denitrification process is taking place within the reactor under controlled conditions and not in the sedimentation tank where degree of denitrification cannot be controlled, effluent of the AAA-CMAS system was found to contain much less suspended solids and E. coli counts than that of the conventional system, resulting in further cost saving in chlorination and in back-washing of the filter.
Thermal hydrolysis has proven to be an efficient pre-treatment process for sludge before anaerobic digestion (AD), by thermally enhancing organic matter hydrolysis. Recent research has shown that a new configuration with the existing technology can further enhance the efficiency of the system. The intermediate thermal hydrolysis process (ITHP) has been explored and tested in the Sludge and Energy Innovation Centre pilot plant located at Basingstoke sewage treatment works for a period of 15 months. The pilot facility has allowed operational considerations to be explored and understood to inform the design and construction of full scale. ITHP results showed a volatile solids destruction of 64% and an average overall specific gas production of 503 m3/TDS. Furthermore, techno-economic analysis was used to compare conventional thermal hydrolysis process (THP) with surplus activated sludge (SAS) only THP and ITHP. Data captured from operational sites, laboratory scale experiments and the large scale ITHP pilot plant, was used in the model. The results showed that ITHP offers an excellent solution for energy recovery having the best economic return, but overall the largest CapEx. SAS only THP is the cheapest to build but does not perform as well as conventional THP and ITHP. Conventional THP remains an excellent solution when space and AD volume is constrained.
Thames Water is the largest water company in the UK. The company treats about 400,000 tonnes dry solids (tds) p.a. sewage sludge. The company's current strategic investment plans focus on improving treatment through enhanced digestion and reducing the wet mass of sludge recycled to land. Thames recognises sludge as a resource and is evaluating advanced options to unlock the full energy potential from sludge. Anaerobic digestion has been the foundation of sludge treatment and, in combination with combined heat and power, is the main process for recovering renewable energy in Thames Water and in the UK Water Industry. It is our view that sludge drying is an essential stage for advanced energy recovery process technologies such as pyrolysis or gasification. Efficient use of waste heat is critical to the economic and environmental sustainability of sludge management. Innovative technologies are a key part for the vision to convert a water company into a renewable energy provider.
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