Oxygen consuming substances in rivers
Concentrations of biochemical oxygen demand (BOD) and total ammonium have decreased in European rivers in the period 1992 to 2012 (Fig. 1), mainly due to general improvement in waste water treatment.
Is organic matter and ammonium pollution of rivers decreasing?
Biochemical oxygen demand (BOD) and ammonium are key indicators of organic pollution in water. BOD shows how much dissolved oxygen is needed for the decomposition of organic matter present in water. Concentrations of these parameters normally increase as a result of organic pollution caused by discharges from waste water treatment plants, industrial effluents and agricultural run-off. Severe organic pollution may lead to rapid de-oxygenation of river water, high concentration of ammonia and disappearance of fish and aquatic invertebrates. Some of the year-to-year variation can be explained by variation in precipitation and runoff.
The most important sources of organic waste load are: household wastewater; industries, such as paper or food processing; and silage effluents and manure from agriculture. Increased industrial and agricultural production in most European countries after the 1940s, coupled with a greater share of the population connected to sewerage systems, initially resulted in increases in the discharge of organic waste into surface water. Over the past 15 to 30 years, however, the biological treatment (secondary treatment) of waste water has increased, and organic discharges have consequently decreased throughout Europe. See also CSI 024: Urban waste water treatment.
Present concentrations per country
See the WISE interactive maps for information displayed by countries on BOD in rivers and ammonium in rivers
In 2012 (or the latest reported year), countries with an average BOD concentration in the lowest category (less than 1.4 mg/l) are Slovenia (1.0 mg/l), the United Kingdom (1.2 mg/l), France (1.3 mg/l) and Ireland (1.4 mg/l).
In 2012 (or the latest reported year), countries with an average ammonium concentration in the lowest category (less than 40 µg/l) are Norway (11 µg/l), Finland (26 µg/l), the United Kingdom (32 µg/l), Sweden (39 µg/l), Slovenia (40 µg/l) and Ireland (40 µg/l).
Overall trend in BOD and total ammonium
In European rivers, oxygen demanding substances have been decreasing throughout the period 1992 to 2012 (Figure 2). Total BOD concentration decreased by 1.6 mg/l from 1992 to 2012. By using the filter in figure 2 the river BOD trends for the individual countries are illustrated.
The average yearly decrease in BOD is 0.08 mg/l (-2.9 % per year). A significant decrease is evident at 62% of river stations, with an additional 6% of stations showing a marginally decreasing trend (see Rivers - BOD - statistical analysis). On the other hand, a significantly increasing BOD trend is recorded at only 3% of the stations, with marginally increasing BOD at an additional 1% of the stations. Countries where more than 60% of the stations show a negative trend in BOD concentrations are Ireland (100%), Luxembourg (100%), Slovenia (92%), Slovakia (87%), France (81%), the United Kingdom (75%), Denmark (74%), Austria (66%), Bulgaria (66%) and Lithuania (63%).
Likewise, the relative trend calculation for ammonium shows that the average ammonium concentration decreased by 231 µg N/l in the period 1992–2012 (Figure 3). By using the filter in figure 3, the river ammonium trends for the individual countries are illustrated.
The average yearly decrease is in ammonium is 11.6 µg N/l (-3.5 % per year). Significantly decreasing concentration trends have been observed at 59% of the stations, with an additional 5% of stations showing a marginally decreasing trend (see ). A significantly increasing trend is evident at 3% of stations and a marginally increasing trend at 1% of stations. Countries where more than 60% of the stations show a negative trend in ammonium concentrations are Luxembourg, the former Yugoslav Republic of Macedonia, Slovenia and the United Kingdom (all 100%), Germany (92.6%), Lithuania (88.5%), Ireland (75%), Poland (75%), France (71.7%), Bulgaria (71.6%), Belgium (70.4%), Norway (70%) and Austria (66%).
The decrease is mainly due to improved sewage treatment resulting from the implementation of the Urban Waste Water Treatment Directive and national legislation. The economic downturn of the 1990s in central and eastern European countries also contributed to this fall, as there is an ongoing decline in pollution from manufacturing industries. This suggests that either further improvement in waste water treatment is required or that other sources of organic pollution, for example from agriculture, require greater attention, or both.
BOD and total ammonium time series and trends per geographical region
The largest absolute decrease of BOD from 1992 to 2012 has occurred in southeastern European rivers (61%), where concentrations are at their lowest level to-date. They are still, however, the highest in Europe (about 3.1 mg O2/l). The largest yearly decrease is evident in western Europe (3.4% per year). Concentrations in northern European rivers (represented by rivers of Finland only) are the most stable (less than 2 mg O2/l), with an average yearly decrease of 0.8%. The largest proportion of rivers with a negative BOD trend is in western Europe. Since BOD records are traditionally low in the north, the decreasing trends are less pronounced there. However, the share of rivers with an increasing trend is relatively high both in the north and the east.