Loch Ascog Water Treatment Works (WTW) - Case Study
Loch Ascog WTW, on the Isle of Bute in Western Scotland, treats water abstracted water from the loch. The main treatment at the works was slow sand filtration followed by disinfection.
The quality of the treated water was not compliant with the National Requirements of The Water Supply (Water Quality) Regulations 2000 in relation to:
- Iron and manganese
- Turbidity and taste
There were two reasons for the non-compliance. First, algae produced in the loch during summer overwhelmed the slow sand filters, requiring the filters to be scraped at too frequent intervals. Second, the loch water contained high concentrations of total manganese and total iron. In addition to the non-compliance of water quality, works' throughput could not be maintained at the required rate.
Scottish Water's solution was to install:
- (a) FilterClear' plant as the first stage of treatment to reduce algae concentration
- (b) Manganese and iron oxidation and filtration plant downstream of the slow sand filtration plant
The performance objective imposed on the FilterCtear™ plant was to reduce the concentration of algae, indicated by chlorophyll a, by at least 80%. Such a removal would allow the downstream processes, comprising the new Mn/Fe removal plant and the existing slow sand filtration plant to operate normally and produce quality-compliant treated water.
The FilterClear'' plant delivered excellent performance, as indicated by the results of the formal Performance Test, presented in the graphs.
FilterClear' reduced chlorophyll a concentration by 83%, allowing the downstream processes to operate normally. Average concentrations of chlorophyll a in FilterClear' effluent was 3.4 mg/l, and the downstream processes further reduced the concentration to less than the measurement limit of 0.4 mg/l.
FilterClear' reduced the total Mn concentration by 94%, but the downstream processes gave little additional removal. The average and maximum total Mn concentrations of FilterClear'' effluent were 18.9 ug/l and 27.2 jjg/l respectively, compliant with the MAC value of 50 ug/l.
FilterClear' reduced the total Fe concentration by 80%, and the downstream processes further increased the removal to 87%. The average and maximum total Fe concentrations of FilterClear' effluent were 34.6 ^g/l and 95 ug/l, compliant with the MAC value of 200pg/l.
FilterClear' reduced the average turbidity of the raw water by 82%, from 7.5 NTU to 0.88 NTU, and downstream treatment further reduced the value to generally less than the measurement limit of 0.3 NTU, giving compliance with the MAC value (for treated water at works) of 1 NTU. Similarly, compliance was obtained in terms of taste.
The high performance of FilterClear'' particularly with regard to removal of algae, manganese and iron brought the quality of the treated water into total compliance and allowed the downstream processes to operate normally under comparatively low loadings. ,