This report has been compiled on request of the Swedish Environmental Protection
Agency to facilitate the discussion with the EU Commission. The EU Commission has
announced that it will take Sweden to the European Court of Justice for failing to ensure
proper treatment of urban waste water according to the Urban Waste Water Treatment
Directive (Directive 91/271/EEC). In Sweden natural nitrogen removal (retention) in
waterbodies is considered as part of the treatment of emissions, when transported to the
Nitrogen retention is a well-known phenomenon that includes several natural
biogeochemical processes, which permanently remove nitrogen from the water. The
effect may be considerable in areas with many lakes. Sweden has 92 000 lakes larger than
1 hectare. It is rather normal with 30-70% nitrogen retention in Swedish lakes and rivers.
The main process for natural nitrogen retention is denitrification, which is the same
process that is applied for biological treatment in waste water plants.
Natural retention is hard to measure, however, and has to be estimated based on several
assumptions like so many other fluxes in nature. In Sweden a model system has been
developed for large-scale calculation of nutrient transport, including retention, from land
to the sea, with relatively high geographic resolution. The system couples field-scale
models with catchment models and is scientifically documented and reviewed. It has been
applied since 1997 for international reporting to HELCOM. The catchment model (HBVNP)
is tuned and evaluated against monitored time-series of measurements where such
are available. The nitrogen retention that is calculated with HBV-NP is composed of
nitrogen that is permanently transferred to the atmosphere and sediment, and which
therefore will not further contribute to the eutrophication of water systems.
Both calculations and measurements show that the retention is largest in the summer,
especially in areas with many lakes and high loads. The retention capacity of lakes differs
geographically; in the northern part of the country it is low, while the lakes in the
southern part of the country are more effective as nitrogen sinks. In Southern Sweden the
mean retention is 30-40 kg ha-1 lake yr-1. In total about 30 000 tonnes nitrogen is reduced
annually in lakes and rivers, and 70% of this is reduced in southern Sweden. For
emissions in the interior of the country, the load is reduced considerably during the
transport through rivers and lakes, especially for south-central Sweden where the
retention in the lakes is high. The accumulated retention can be over 80% in certain areas.
It is difficult to evaluate the model results, since nitrogen retention cannot be measured
directly and is integrated for large areas and waterbodies. It is common to use other
variables to judge the credibility of the retention calculations. The result of the HBV-NP
model is continuously evaluated against time series of observations in watercourses, both
discharge and nutrient concentration, when it is used operationally. The model is
evaluated both visually and statistically. The agreement for discharge and water balance is
normally good, while the nutrient concentration can deviate more from observations.
Sensitivity studies show that the model is relatively robust. When the model is compared
to other models or budget calculations, the deviations can normally be explained by
different assumptions or input data. The HBV-NP model has about the same precision as
other similar models, nevertheless, the water discharge normally shows better accuracy.
For source apportionment calculations, the retention in the flow paths of the landscape is
accumulated for the emissions from specific urban waste water treatment plants
(UWWTP). The emissions that pass through many lakes have only small impact on the
sea. Natural nitrogen retention in rivers and lakes reduces the Swedish UWWTP:s
contribution to the coast with 3200 tonnes per year, which corresponds to 18% of their
total emissions. However, the retention varies substantially between different parts of the
country. Most of the UWWTP with large emissions are located along the coast and in
southern Sweden, where 90% of the natural retention occurs. In the interior of northern
Sweden there are few UWWTP and the retention capacity is low, and thus, the amount of
reduced nitrogen is also low.
Everything indicates that the calculated nitrogen retention for Sweden is of the right
magnitude and that the large-scale pattern for Sweden is correct, even if deviation can
occur for small specific areas and for different time-periods