Dear Muthukrishna In the ...

Dear Muthukrishna In the absence of an effective chemical treatment, biofilms can only be prevented by removing nutrients that support their growth or having a low water temperature. The first approach is used in some European countries where drinking water is distributed without a residual. This concept is referred to as biostability and the availability of nutrients was expressed in terms of the assimilable organic carbon (AOC) or biodegradable dissolved organic carbon (BDOC). I had experience of AOC and it was generally accepted that an AOC of 10 microgrammes per L was necessary for biostability. It would be equivalent to a very good quality groundwater that is not under any direct influence from a surface water. The success of biostability depends on having very extensive treatment process capable of achieving significant nutrient removal and maintaining an intact distribution system. It would require significant upgrading to the capacity of an existing treatment works. Using temperature to prevent biofilms is not a practical option. A number of options are available to limit biofilm growth. Generally, smooth surfaces support less biofilm compared to the rough surfaces of corroded metal pipes; the latter have cracks and crevices that can provide a nice location for growth and protect organisms from potentially adverse conditions. The shorter the residence time of water the more likely is it possible to maintain an effective residual. Also, the higher the velocity, the greater the surface shear stress and the thinner the biofilm. Don’t ignore the contribution of sediments. They can offer a good surface for biofilms. Where they are found, sediment can be removed by flushing (water mains) and draining and cleaning treated water storage reservoirs). Devices (called "pigs") are sometimes used to mechanically scrape biofilms from pipe surfaces, although their success is open to question. Using crushed ice is viewed as a desirable option. Including a disinfectant will make cleaning more effective. Effective control of water treatment can help to reduce the amount of particles entering the network and so reducing the amount of sediment accumulation.