Of all the nutrients measured, ammonium/ammonia concentrations give rise to most concern, due to its toxic propoerties. Ammonium concentrations increase along the Danube, and they exceed target values in half of all samples. Extremely high values corresponding to water quality Class V (poor) of the interim ICPDR assessment have been observed in the River Arges, indicating that untreated or insufficiently treated wastewater from municipalities may be worsening these problems.
The tributaries with the highest concentrations of nitrogen in form of nitrate are the Morava, the Dyje, the upper and middle reaches of the Sio, Iskar, the Russenski Lom, the Arges and the lower reaches of the Prut.
The total nitrogen load upstream of the Danube Delta transported by the Danube River is estimated to be between 537,000 and 551,000 tonnes per annum. This data covers the period 1992-1996, and should only be taken as a rough indicator of the current size of the nitrogen load.
Most Danube countries are working to reduce emissions of nitrogen and other nutrients from municipal wastewater treatment plants.
In Austria 1,432 wastewater treatment plants (including around 850 plants with a population equivalent (p.e.) between 50 and 2,000) were in operation in the beginning of 2002. The plants > 2,000 p.e. reduced the incoming pollution load by 95% for BOD5, by 87% for COD, by 67% for nitrogen and by 83% for phosphorus.
In the Czech part of the Danube River Basin, four of the 12 wastewater treatment plants included in the Joint Action Programme were reconstructed to further reduce nutrient discharges by the end of 2002.
In Slovakia, nitrogen emissions were reduced significantly at three wastewater treatment plants in the Bratislava region.
In Slovenia, technical improvements aimed at nitrogen reduction were made at several small wastewater treatment plants. Only three plants with a population equivalent of more than 100,000 have yet to comply with EU-directives on nitrogen treatment.
In Romania, a total of 1,445 wastewater treatment plants were in operation in 2002, of which about 45% reported satisfactory performance. An additional 15 plants are under construction.
daNUbs was a mulitnational EU research project carried out under the leadership of the Technical University of Vienna. The results from this project include estimates of nutrient inputs into the river network (MONERIS), as well as an assessment of the loads of nitrogen, phosphorus and silica transported via the river network. These results indicate that the nutrient status in the Black Sea has significantly improved since the 1980s.