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1.1       Organic matter and ammonium in rivers

The discharge of large quantities of organic matter containing microbes and decaying organic waste, either from agriculture or as waste water from households or industrial effluents, may result in reduced chemical and biological quality in river water, reduced biodiversity of aquatic communities, and microbiological contamination that can affect the quality of drinking and bathing water. Organic pollution leads to higher rates of metabolic processes that demand oxygen. This can result in the development of water zones without oxygen (anaerobic conditions), which has profound direct impacts on the ecosystem. The transformation of nitrogen to reduced forms under anaerobic conditions, in turn, leads to increased concentrations of ammonium, which is toxic to aquatic life above certain concentrations, depending on water temperature, salinity and acidity. Organic matter in water is measured as biochemical oxygen demand (BOD), which is the amount of dissolved oxygen which is required for the aerobic decomposition of the organic matter present in the water. BOD is expressed in mg of O₂/litre, while ammonium concentration is expressed in mg of NH₄-N/litre.