Water and Effluent Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/283328
24 WET NEWS MARCH 2014 Stewart Tattersall, of MWH, Peter Barratt, of Anglian Water, Mike Kirk, of Essex & Suffolk Water, and Jeanette Sheldon, of South East Water, have been working together to review the biological iron removal process used in the UK during the past 20 years. Here's what they discovered. The challenge: High iron concentrations removal The plants comprise pressurised filters for biological iron removal and wash water recovery, where practised, is returned downstream of the biological iron stage, as shown here at Powdermill H igh iron concentrations in groundwater are expe- rienced in many parts of the UK. The Prescribed Concen- tration Value (PCV) in the Water Supply (Water Quality) Regula- tions 2010 for iron is 0.2mg/l but generally water companies' tar- get concentrations are 0.05mg/l. However, groundwater con- centrations can reach 20mg/l in certain areas, making the water source unsuitable for conven- tional single stage treatment. This means alternative sources of raw water have to be found. Another approach is the con- cept of a biological treatment plant. This is not new as there are already more than a hundred biological installations in Europe, not only for iron removal, but also for the removal of manganese, ammonia and nitrate. The technology for these installations constructed in the 1990s was brought into the UK by French companies either owning the water companies, or as process contractors based in the UK. In the UK, iron in the soluble form is generally removed from groundwater by physico-chemi- cal oxidation. Generally, the pH of the raw water needs to be raised above 7.2 and suitably enriched in dissolved oxygen. The pH can be adjusted by aera- tion with the partial elimination of carbon dioxide, followed by removal of the insoluble ferric hydroxide on filters operating at around 6m/h; however, there is the case where the raising of the pH would result in the calco- carbonate equilibrium being surpassed and scale formed. To circumvent this, a strong oxidant, e.g. chlorine, can be dosed upstream. A disadvantage of the physico-chemical process is at iron concentrations greater than 5mg/l, clarification is o—en required to reduce the load on to filters, with the consequent increase in capital and operating costs. Biological iron removal is an alternative process that has been in operation for the past 20 years at 15 or so plants in the eastern part of the UK, treating groundwaters from 0.15mg/l to 20mg/l in a single stage. To identify the most cost- effective approach, Stewart Tat- tersall, of MWH, in collaboration with Anglian Water, Essex & Suf- folk Water and South East Water, reviewed their dedicated biolog- ical plants. Some of these plants also have a biological manganese second stage, rather than the more conventional physico- chemical filtration stage equipped with catalytic media. References are made to the asso- ciated removal of ammonia in the biological stage, which is an advantage in obviating the need for breakpoint chlorination. The plants examined were: • Anglian Water: Pettistree • Essex & Suffolk Water: Walpole • South East Water: Pow- dermill n • INSIGHT WATER TREATMENT The ammonia reduction across the iron filters is about 65% on a 95%ile basis and 90% on average A 95%ile iron concentration at the outlet of the Fe filters of 19.4µg/l and 16.7µg/l post manganese filters