Water and Effluent Treatment Magazine
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20 WET NEWS SEPTEMBER 2016 A range of instruments from ABB have been fitted to Thames Water's pioneering thermal hydrolysis plant at its Basingstoke treatment works. The instruments are generating a wealth of data that is being used measure performance and identify ways of refining it further. Inset: The FSM4000 AC magnetic flowmeters are being used to both measure and control the flow of sludge between the tanks ONSITE RENEWABLE ENERGY Breaking new ground in biogas production • A two-strong team at Thames Water has been testing a THP process at Basingstoke treatment works. Getting the 'best possible picture' of what was happening was essential. projEcT SpEcS • Test an experimental plant based on an adapted thermal hydrolysis process • Carry out tests and experiments to generate the data needed to fine-tune the process • Provide a combined package of flow, pressure, temperature and level instrumentation How the intermediate thermal hydrolysis process works at Basingstoke STW S pending two years fine- tuning the efficiency of a pioneering sludge treatment process might not appeal to everyone, but for Ester Rus Perez and Aurélien Perrault of Thames Water, it has been an exciting journey of discovery that promises to break new ground in generating energy from sewage. Based at Thames Water's Basingstoke treatment works, the two-strong team are involved in testing an experimental plant based on an adapted thermal hydrolysis process (THP). Extensively fitted with instruments from ABB Measurement and Analytics, the plant is helping to generate a wealth of data that is being used to further the team's understanding of the process and identify ways of refining it further. THP is an established process used in conjunction with mesophilic anaerobic digestion (MAD) to treat and dewater sewage sludge. In a conventional THP process, sludge first passes through a hydrolysis phase, which breaks down large molecules. During this process, steam is injected into the sludge to maintain a high temperature and pressure that helps to break down material such as cell walls and very large molecules. This in turn makes it easier for bacteria to digest the sewage and produce extra gas. AŠer hydrolysis, the sludge is then treated in the anaerobic digester where bacteria digest part of the organic matter. Together, these processes produce a sterile, relatively easy to dewater "cake" that can be sold as fertiliser; together with biogas, which can be used to generate electricity and heat. Although the THP process offers greatly improved efficiency in the conversion of organic matter into biogas compared to MAD by itself, around 50% of this energy-rich sludge is still recycled to land. As well as representing lost potential energy, this residual sludge may also require hundreds of lorries to transport it to agricultural sites where it can be used as soil amendment. The plant at Basingstoke takes this process one step further by incorporating an additional intermediate stage into the process. Known as intermediate thermal hydrolysis (ITHP), this process incorporates an additional MAD stage before the thermal hydrolysis, allowing for additional organic matter degradation and energy recovered from sludge. The ITHP process was discovered by chance by Achame Shana, Thames Water's lead technical consultant on waste treatment and recycling processes, when searching for a solution to the problem of post-digestion stage odours. His idea was to remove the pathogens that were causing the odour by running the digested sludge through a THP reactor and then back through another digester. In doing so, he noticed an increased amount of biogas was being produced as the organic matter was further broken down. From this, the idea of ITHP as means of maximising biogas production was born. In initial tests, the ITHP approach proved to offer improved efficiency, with an overall volatile solids reduction of 66 to 68% as compared with 55 to 60% from a conventional THP process. The Basingstoke plant has been built to test the viability of the ITHP approach at larger scale. Built in 2013, the plant was under test until the end of 2015. As a pilot plant, the facility is heavily instrumented to help gather as much operational data as possible. In order to test the limitations of the process, experiments are run under a variety of different operating conditions. "Before the ITHP process can go to commercial scale, there is a lot of work to be done in testing how it works, how well it works and what can be done to make it work better," says senior research engineer Ester Rus Perez, the plant manager for the ITHP pilot plant. "This includes running the plant under both normal and non-standard conditions, in order to see how far we can push the process to get even better results. "Over the past two and a half years, we have carried out a wide range of different tests and experiments in order to generate the data that we need