Fermenting RAS to Improve Enhanced Biological Phosphorus Removal
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.
