In European rivers, oxygen consuming substances decreased over the period 1992 to 2021. Biochemical oxygen demand (BOD) fell to half of the 1992 level, but has remained steady at around 2.8mgO2/l since 2010. Ammonium concentrations fell to 20% of the 1992 level. After 2014, the level stabilised around 110µgN/l. The decrease in BOD and ammonium concentrations is a consequence of the improvement in wastewater treatment. The economic crisis in central and eastern European countries during the 1990s also resulted in decreasing pollution from manufacturing industries.

Figure 1. Biochemical oxygen demand and ammonium in European Rivers
Biochemical oxygen demand and ammonium in European Rivers

Organic pollution of rivers from wastewater, both municipal and industrial, 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 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 by aerobic biological organisms to break down organic matter present in a given water sample at a certain temperature over a specific time period. BOD and ammonium increase with higher loads of biologically degradable organic matter.

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

BOD

In European rivers, BOD levels have generally been decreasing between 1992 and 2021 (Figure 1a), with an average annual decrease in BOD of 0.07mg/l (0.7% per year). The BOD reached its lowest level (2.4mg/l) in 2011 but surpassed 3.0mg/l in the period 2015–2016. A significant decrease is evident at 45% of the river sites, with an additional 4% of the rivers having a marginally decreasing trend. A significantly increasing BOD trend is recorded at 9% of the sites. The shorter, more representative time series of 2000–2021 closely follows the longer one.

Ammonium

Annual ammonium concentrations decreased by 11.5µg/l per year (-2.3%) on average over the period 1992-2021 (Figure 1b). Significantly decreasing concentrations were observed at 72% of the sites, with an additional 4% of the sites showing a marginal decrease. No change has been observed at 22% of the river monitoring sites. A significant increase was evident at 2% of the sites. The shorter, more representative time series of 2000–2021 shows higher concentrations with a similar trend of overall decrease.

Figure 2. Status of biochemical oxygen demand in rivers in European countries
CountryLess than 1.4 mgO₂/l1.4-2.0 mgO₂/l2.0-3.0 mgO₂/l3.0-4.0 mgO₂/lGreater than 4.0 mgO₂/l
Albania (26)0000100
Austria (94)68.118.112.81.10
Belgium (108)39.813.933.35.67.4
Bulgaria (101)25.718.829.714.910.9
Croatia (49)63.38.212.2214.3
Cyprus (24)66.716.716.700
Czechia (792)8.22535.41615.4
Estonia (84)26.251.2191.22.4
Finland (6)5033.316.700
Germany (157)3525.524.810.24.5
Greece (127)46.514.216.58.714.2
Ireland (182)91.28.20.500
Italy (2,454)27.512.233.913.812.6
Kosovo* (49)18.44.18.24.165.3
Latvia (61)1.631.167.200
Lithuania (463)17.140.233.77.11.9
North Macedonia (18)011.138.911.138.9
Poland (3,330)8.424.733.61914.3
Portugal (123)15.42.434.140.77.3
Romania (121)037.219.82419
Serbia (35)5.742.94011.40
Slovakia (16)18.812.562.56.30
Slovenia (20)1000000
Spain (2,604)0.21.156.94.337.5

The current mean concentration of BOD for the period 2019-2021 is 3.0mgO2/l for 25 European countries (11,045 sites), with 71% of the river monitoring sites having a BOD of less than 3mg/l.

Countries with the highest share of river sites in the best quality class (i.e. less than 1.4mg/l) are Slovenia (100%), Ireland (91%), and Austria (68%). The share of monitored river sites with BOD equal to or higher than 3mg/l is particularly high (50% or more) in Albania, Kosovo under UNSCR 1244/99, and North Macedonia. High BOD levels are observed in agriculturally and industrially developed lowlands of Europe, such as the Po valley and lower BOD in the highlands of Europe such as the Alps, and the Dinaric Alps.