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Aggregate level assessment

Organic pollution of rivers from waste water, both municipal and industrial, as well as diffuse runoff from agriculture, negatively affect aquatic ecosystems, causing loss of oxygen and changes in species composition (i.e. deterioration of ecological status). Severe organic pollution may lead to the rapid de-oxygenation of river water, high concentration of hazardous ammonia and disappearance of fish and aquatic invertebrates. In addition, it can have negative effects on the use of the water for human purposes such as drinking, bathing and recreation. Without treatment, organic pollution is slowly diluted and degraded naturally along the river course. Biochemical oxygen demand (BOD) and ammonium are key indicators of organic pollution in water. BOD is the amount of dissolved oxygen needed (i.e. demanded) by aerobic biological organisms to break down organic matter present in a given water sample at a certain temperature over a specific time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20°C. Ammonium is toxic to aquatic life at certain concentrations in relation to water temperature, salinity and pH. BOD and ammonium increase with higher loads of biologically degradable organic matter.

Key sources of organic pollution are municipal waste water; industrial waste water, especially from paper or food processing industries, and agricultural emissions, especially from surface runoff of silage, manure and slurry from intensive livestock farms.