Physiological toxicity, cell membrane damage and the release of dissolved organic carbon and geosmin by Aphanizomenon flos-aquae after exposure to water treatment chemicals

Abstract

Reducing the level of dissolved organic carbon is an increasingly important goal in the treatment of drinking water. Dissolved organic carbon (DOC) compounds react with chemical disinfectants to form undesirable by-products, and DOC provides a substrate for bacterial growth. Chemicals used in water treatment processes and surface water reservoir management can cause the release of dissolved organics from photosynthetic aquatic organisms, and specific organic compounds can impart taste, odour and sometimes toxicity to treated water. Thus, chemical treatment of water may exacerbate the problem of dissolved organic carbon loading. The effects of several chemicals used at different stages of the water treatment process were studied on a nitrogen-fixing strain of the cyanobacterium Aphanizomenon flos-aquae. Chemicals included chlorine, potassium permanganate, aluminum sulphate, ferric chloride, calcium hydroxide, hydrogen peroxide and copper sulphate. Physiological effects on Aphanizomenon, as quantified by its ability to fix nitrogen, were compared with cell membrane damage (manifested as potassium release) and the release of organic cellular components, including DOC and the specific odour compound, geosmin. Aluminum sulphate and ferric chloride did not cause either physiological toxicity, cell membrane damage or the release of DOC at treatment usage concentrations. Calcium hydroxide at low levels and hydrogen peroxide caused physiological toxicity, but only limited membrane damage, DOC and geosmin release. Chlorine, copper sulphate and potassium permanganate caused physiological damage with concomitant release of both DOC and geosmin at concentrations lower than those currently used in water treatment. Consequently, these last three chemicals create potential water treatment problems by releasing DOC and specific problem compounds.