Effect of natural organic matter composition on performance of conventional water treatment processes

Abstract

The presence of natural organic matter (NOM) in surface water sources poses various challenges in drinking water treatment processes.Conventional coagulation is being applied in most of the treatment plants in Uganda and other similarly developing countries despite the differences in nature and characteristics of NOM in raw water sources. It is intended mainly for removal of colour and turbidity at these plants.On the other hand, enhanced coagulation may not be solely sufficient for treatment ofraw water from sources with either high iron II or algae content and thus the need for pretreatment with oxidants. The use of some oxidants like chlorine, which is a common practice, leads to formation of disinfection by products (DBPs) reported to beCarcinogenic.In this study, the effectiveness of conventional water treatment methods in removing NOM from water sources with NOM of different characteristics was evaluated.Two surface water sources, lake (Lake Victoria) and swamp (Nabajjuzi swamp) for Buwama and Masaka waterworks, respectively, were selected for the study. NOM in both raw water sources was characterized and samples taken from the different stages of the treatment train. A laboratory trial experiment for relocation of chlorination dozing points on swamp water in Masakawas also done.Results from the study indicated that the NOM in both raw water sources was non-humic, relatively hydrophilic, less aromatic and of low molecular mass. With reference toalgae analysis, lake water (Buwama) comprised averagely of 2.3105±8287cells/mL of algae with more species compared to 2.6×104±1581cells/mL of swamp water (Masaka) with less species.Buwama algae species comprised ofMicrocystis, Merismopedia, Planktolyngbya, Anabaena, Chlorella, Chlorococcum, Struarastrum, Ulothrix, Navicula, Nitzschia unlike Masaka with onlyMicrocystis, Anabaena, Chlorella, Chlorococcum.On the other hand, average raw water TOC and DOC concentrations were, respectively, 10.4±1.5 mg/L and 9.8±1.5 mg/L for swamp raw water; and 8±2.4 mg/L and6±1.3 mg/L forlake raw water.Seasonal variance in NOM concentration was observed for lake water with an increase in NOM especially in the dry season unlike swamp water whose variance was minimal. 100% algae removal at both water treatment plants alongside NOM removal of 63.3% and 74.4% was achieved in Masaka (swamp source) and Buwama (lake source)respectively. The low pH and alkalinity corresponding to 5.3±0.2 and 11±2.5 mg/L for the swamp raw water in Masaka formed a basis for the application of soda ash so as to improve NOM removal during treatment. The THMs present in treated water for Masaka were high (111±40ppb) and above acceptable standards (80ppb) while those of Buwama (50±14ppb) were within the acceptable limit according to USEPA. On the contrary, both Masaka and Buwama treatment plants met the THMs standards for WHO (200ppb). Despite 100% algal removal at both water treatment plants and low THM levels in the lake, the high algal content and species presentare known for generation of toxins likemicrocystins during oxidation. From the laboratory experiments carried out on swamp raw water, the relocation of chlorine application from before clarification to after clarification achieved 87% decline in THMs formed during treatment.