Upgrading conventional treatment processes for water quality improvement - a pilot study

Cheng Ching Lake Water Works (CCLWW), located in southern Taiwan, draws its raw water from a eutrophic lake, which is treated by conventional processes, including pre-chlorination, coagulation, sedimentation, filtration, and disinfection. As the taste- and odor-causing compounds from algae cannot be completely removed by conventional treatment processes, and the hardness is high, there are complaints about water quality. In searching for suitable processes to upgrade the existing facilities, a two-year pilot plant test was conducted. In total 13 processes were tested, which could be divided into three groups. The main feature of the first group was the modification of the existing conventional treatment processes, which included eliminating pre-chlorination and incorporating GAC. The second group incorporated pre- and post-ozonation, GAC bed, with or without pellet softening, into the conventional processes. The third group featured membrane processes, mainly nanofiltration (NF) and its various pretreatment processes, such as microfiltration (MF) or ultrafiltration (UF). The results show that although the first group has higher removal rates of dissolved organic and disinfection by-product precursors, as measured by NPDOC and THMFP, respectively, than those from the existing full-scale plants, the improvement in taste and odour was not adequate. For the second group, the dissolved organic parameters and biostability of the finished water were further improved, and half of the total hardness could be removed by pellet softening. However, earthy and fishy odours still could be detected occasionally by flavour profile analysis. Generally speaking, the third group with processes involving NF could produce the highest quality finished water: no matter organic, inorganic, organoleptic parameters, or biostability were concerned.