Water and Effluent Treatment Magazine
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22 WET NEWS OCTOBER 2016 More and more utilities are incorporating additional treatment for high strength liquor in plants using thermal hydrolysis alongside anaerobic digestion. But it is important to consider the benefits of separate treatment for high ammonia waste streams, says ACWA's Bob Hastings. Do not let high strength liquor derail the advantages of advanced digestion INSIGHT Sewage & sludge treatment • A s we look forward to AMP7 it is likely utilities will be encouraged to step up their capability for producing heat and power as part of the wastewater treatment process. In the past few years the use of thermal hydrolysis has made the use of anaerobic digestion in wastewater plants more efficient and much more economically viable, with some plants now producing enough gas to sell back to the grid. However, thermal hydrolysis has disadvantages, particularly because of the high levels of ammonia in the waste streams produced by the process. Unless this is properly treated the economic benefits of advanced digestion can be seriously undermined. Economically viable ACWA was one of the first companies involved in research into the best way to deal with high strength, high ammonia liquor. Since the late 1980s the company has worked with many of the leading utilities in the UK to make their advanced digestion plants more efficient and more economically viable. The company developed its Amtreat technology specifically to deal with the challenge of the high strength liquors produced by advanced digestion. By adding this additional stage specifically to treat wastewater with high levels of ammonia it becomes possible to ensure that the anaerobic digestion process is as stable, and as economically viable as possible. One of the great benefits of using thermal hydrolysis alongside the anaerobic digestion process is that it makes it possible to produce more gas. Not only does this mean wastewater plants will be able to generate heat and electricity to be used on site, oŠen it means wastewater plants will also have the capacity to export additional gas and electricity back to the grid. The more effective the advanced digestion process, the higher the levels of ammonia in the digestate. High strength liquor created by plants using thermal hydrolysis can have ammonia levels as high as 2,500mg per litre, which renders it hard to treat. In wastewater plants using thermal hydrolysis alongside anaerobic digestion there are two sources of high strength liquor. The first stream comes from the pre-thickening process and the second from the post digestion de watering stage. Generally, all three of these high strength sludge streams are collected together and this high strength liquor is fed back into the start of the treatment process. However, the ammonia levels of the collected waste stream can be so high they can derail the wastewater treatment process. Adding high strength liquor to an ASP can also render this treatment less effective – meaning final effluent could have unacceptably high levels of ammonia and the plant may find itself in breach of its consent. A growing problem ACWA has found the difficulties of dealing with high strength liquor in wastewater treatment are increasing and this is likely to get worse. Sewage Works with higher loads due to population increase, may find themselves suddenly unable to meet their targets for ammonia. Restrictions on ammonia levels for treated effluent are becoming increasingly tight with some permitted levels of ammonia as low as just 1mg per litre. The Amtreat system, developed by ACWA has now been fitted at seven wastewater treatment plants in the UK, with three more in development. The system works by collecting the high strength liquor in a Balancing Tank before feeding through to the AMTREAT Process. ACWA has recently worked with United Utilities (UU) at Leigh wastewater treatment plant in Greater Manchester. Leigh acts as a treatment plant for sludge imported from other UU sites across the region. The new Amtreat plant has a processing capacity of 805m 3 per day and is designed to perform full denitrification with a minimum of 95% ammonia removal. Once the ammonia levels have been reduced the treated effluent can be fed back into the head of the treatment process without fear of disrupting the smooth operation of the plant. The Leigh plant is the first to have been built by ACWA to co-ordinate with a thermal Sewage treatment works with higher loads may find they are unable to meet their targets for ammonia