Aarhus University Seal / Aarhus Universitets segl

No. 194: Impact assessment of wastewater discharge in Øresund

Carstensen, J. & Christensen, J.P.A, 2021. Konsekvensvurdering af spildevandsudledning i Øresund. Potentielle effekter for havmiljøet. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 38 s. - Teknisk rapport nr. 194. http://dce2.au.dk/pub/TR194.pdf

Summary

In May 2020, the Copenhagen Municipality gave HOFOR permission to discharge 160,000 m3 raw sewage over five days from Strandvængets Pumpestation to the discharge point U4 located in Kongedybet outside of Copenhagen Harbor. In this report, the potential environmental effect of this discharge in Øresund is assessed, given that the permission had been utilized. Øresund does not comply with good ecological status according to the Water Framework Directive at present, which is primarily due to excessive nutrient inputs from land, primarily nitrogen.

 

The discharge of 160,000 m3 raw sewage with high concentrations of nutrients and organic matter is estimated to increase inputs to Øresund with 96 tonnes COD, 52 tonnes BOD, 8,7 tonnes TN and 1,3 tonnes TP. This contribution is less than 1% of the annual nutrient inputs and slightly more for COD and BOD. The additional contribution corresponds to a doubling of the daily inputs from point sources to Kongedybet for COD, TN and TP, and a 10-doubling for BOD over the 5-day period.

 

The discharge from Strandvængets Pumpestation will not have any significant ecological imprint on Øresund as a whole, where the cumulative inputs over longer periods are more important. However, slightly increased concentrations of nutrients and chlorophyll as well as slightly reduced water transparency are expected in Kongedybet, but these temporary effects are relatively small compared to the natural variability over time. In the vicinity of the discharge point U4, enhanced algae production and reduced transparency are expected at levels similar to the spring bloom. Such short-term blooms and reduced transparency will not have an effect on eelgrass and macroalgae in the area. Similarly, the input of readily degradable organic matter in the raw sewage can lead to oxygen depletion in the vicinity of the discharge point. A short-term drop in oxygen concentrations will most likely affect benthic fauna to a minor degree, unless anoxic conditions develop with release of the toxic hydrogensulfide. In such case, and since the discharge from Strandvængets Pumpestation is not permanent, it is expected that the benthic communities will recover within a number of years.

Øresund receives large inputs of nutrients and organic matter every year, most of these from scattered sewage outlets; the cumulative effect of these causing eutrophication and an impoverished environment. The short-term discharge from Strandvængets Pumpestation alone has therefore a relatively minor effect on the marine environment in Øresund, which is primarily constrained to the vicinity of the discharge point. However, this temporary discharge contributes nutrients and organic matter to an ecosystem that is already receiving excessive inputs and not fulfilling ecological targets. In the event that an extraordinary discharge is inevitable, it is recommended that the discharge is executed in late autumn or early winter when recreational activities and nutrient uptake by algae are low.