Biochemical oxygen demand in rivers

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Water

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'Biochemical oxygen demand' is a measure of how much dissolved oxygen is being consumed as microbes break down organic matter. A high demand, therefore, can indicate that levels of dissolved oxygen are falling, with potentially dangerous implications for the river's biodiversity.

High biochemical oxygen demand can be caused by:

• high levels of organic pollution, caused usually by poorly treated wastewater;
• high nitrate levels, which trigger high plant growth.

Both result in higher amounts of organic matter in the river. When this matter decays, the microbiological activity uses up the oxygen. Biochemical oxygen demand is therefore one of the main parameters used in the Urban Wastewater Treatment Directive for controlling discharges. Unsurprisingly, large rivers - where wastewater plants are more likely to be located - registered higher levels of oxygen demand than smaller rivers. Improvements in wastewater management saw biochemical oxygen demand fall in all sizes of river during the early 1990s. However, levels have begun increasing slightly more recently in all but the smallest rivers.

Despite this recent trend, the picture is far from bleak. As the Figure shows, over 80% of rivers in northern Europe have a biochemical oxygen demand of under 2 mg O2/l, which indicates a relatively clean river. This figure drops to under 40% for southern Europe, however, where over 20% of the monitoring stations registered readings of over 5 mg O2/l, which indicates relatively high pollution.