Dealing with Algal Toxins and Dissolved Organics in Drinking Water

Several types of cyanotoxins found in surface water bodies are recognized as having human health effects, whereas taste and odor affect the palatability of water and give rise to public complaints. Conventional water treatment unit operations may be effective in removing the cyanobacteria cells, but cyanotoxins and dissolved organics are not targeted for removal by them. Special treatment units need to be introduced to deal with these substances and attention paid to the process design as many operational issues may be encountered. We used a water supply project in the Eastern Province of Sri Lanka as a case study to investigate the performance of unit operations in water treatment plants for which the source is shallow surface water sources with high inflows of nutrients. The present case study was designed to evaluate the efficacy of prechlorination, activated carbon adsorption, dissolved air flotation (DAF), filtration and disinfection in removal of cyanotoxins, and taste and odor causing dissolved organic substances from the source water. Raw water quality was evaluated using algal concentration, algal toxin concentration, and chemical oxygen demand. To evaluate the efficacy of treatment operations, the sequence of initial unit operation was changed on each day as with prechlorination and with powdered activated carbon (PAC), with prechlorination and without PAC, without prechlorination and with PAC, and without prechlorination and without PAC. In addition, laboratory analysis was done to obtain adsorption isotherms using three types of different PAC. The primary findings of our study were that PAC was effective in removal of Microcystin and chemical oxygen demand (COD) but needs to be optimized by providing sufficient contact time, and prechlorination does not improve the performance, whereas postchlorination is effective in removing any traces of Microcystin left after PAC. (C) 2012 American Society of Civil Engineers.