AWWA MTC61132

This paper presents the findings of pilot study work which was carried out primarily to establish cleaning regimes for a submerged microfiltration installation treating water that has seasonally high organic loadings. In parts of the United Kingdom it is illegal to supply water containing more than one Cryptosporidium oocyst in 10 litres, yet there have still been outbreaks with suggested links to supplies where detected oocyst concentrations were significantly below this limit. One such supply was from an 18.5 mgd plant where treatment was ozonation (for color reduction) followed by slow sand filtration. Due to the political sensitivity of the situation it was imperative to provide as sure a barrier as possible against oocysts, quickly and economically. Submerged membrane microfiltration was the chosen solution, placed after the existing slow sand filtration. A pilot filter was set up at Invercannie to prove the process and to confirm the cleaning regimes that should be applied on the full scale plant. Due to the fast track program, the pilot testing was carried out in parallel with construction. The membrane feedwater was ozonated and slow sand filtered, but still contained Total Organic Carbon (TOC) in the range 0.75 mg/l to over 7 mg/l. Such levels of TOC can lead to severe organic or biological fouling on membranes, unless the cleaning regime is highly effective. Ozonation in the process train is also thought to exacerbate the fouling potential by the breakdown of the natural organic matter to assimilable organic carbon. Normal backwashing with air and water had to be enhanced by the use of a maintenance clean also known as a chemically enhanced backwash (CEB). Here sodium hypochlorite is introduced during a modified backwash. The membrane is allowed to soak in the sodium hypochlorite solution for 10 minutes before the cell is rinsed twice with membrane filtered water. In order to meet the performance criteria of 28 days between cleaning in place (CIP) washes the Chemically Enhanced Backwash frequency required varied between one every 2 days with 25 mg/l chlorine up to once per day with 150 mg/l chlorine. This is a demanding environment for the membranes, as they are frequently exposed to very high levels of chlorine. If chlorine levels were not kept high enough during the (approximately) daily chemically enhanced washes, the membrane's performance fell below the specified requirements. The membranes must operate with feedwater temperatures down to 4 °C, at an external area flux of 43.1 L/(m².h) (25.3 gal/ft²/d) with a trans-membrane pressure not exceeding 85 kPa (12.3 psig). In addition, the significant levels of chlorine in the CEBs led to the formation of trihalomethanes (THMs), although a clear correlation between the two was not always evident. The full scale plant has operated successfully since commissioning. The TMPs at full scale have been closely comparable to those found for the corresponding time of year in the pilot plant. Resistance at full scale has been greater. At the time of writing, the full scale plant had yet to operate over the full winter period. The data gained from pilot Runs 17 and 18 from January to March 2004 do however give confidence that appropriate Chemically Enhanced Backwashes, with chlorine levels up to 150 mg/l if necessary, should enable satisfactory operation to be maintained. Includes 2 references, tables, figures.