Thomas, D., Severn Trent Water Ltd, UK(free)
At Loughborough Sewage Treatment Works (95,000 population equivalent) a new process – 48hr fermentation of 6% of the RAS flow – has been installed as part of an Enhanced Biological Phosphorus Removal (EBPR) conversion to meet a 2 mg/l total-P consent under the Urban Wastewater Treatment Directive (UWWTD). The EBPR capacity at the site had been assessed and found to be “poor-to-good”. RAS fermentation was proposed as the technique to improve EBPR activity, potentially increasing the biological removal by 50 to 60% and reducing the demand for ferric sulphate to ensure consent. Pilot trial and computer modelling results have been confirmed in full-scale results during the first 4 few months of commissioning. Before conversion, Loughborough was a five lane ASP (built circa 1974) operated in a conventional anoxic/aerobic configuration, with the anoxic fraction comprising 7% of the total volume. The site comfortably achieved a 20/30/5-10 (BOD/SS/AMM) consent. In order to comply with requirements of the Urban Waste Water Treatment Directive (UWWTD), the works is now subject to an additional total phosphorous (P) limit of 2 mg/l with accompanying total iron consent of 3.5 mg/l in the final effluent. There are two main options for achieving the P consent; chemical dosing and/or modifications to the biological process. In terms of modifying the process, Enhanced Biological Phosphorus Removal (EBPR) offers potentially significant revenue cost savings over ‘conventional’ chemical P removal, while also providing a more environmentally sustainable option. However, successful EBPR operation is strongly influenced by the wastewater characteristics. Provisional screening tests indicated that the wastewater at Loughborough was on the lower side of typical strength for a domestic wastewater, potentially lacking sufficient volatile fatty acids to facilitate adequate biological phosphorus release and subsequent uptake to meet the new limit. To establish what degree of EBPR could be achieved, a test rig was installed on site to simulate the EBPR process. It was operated for a period of 200 days in order to trial various configurations. Following completion of the tests, the test reactor showed that a 15/15/70 anoxic/anaerobic/aerobic split of the 68m long ASP lanes would be the most effective arrangement, with the plant operation in a standard “A2 /O” configuration as shown below in Figure 1.
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