Regnbetingede udløb fra kloaksystemer Summary and conclusionsThe quality in our recipients including streams, lakes and coastal areas have in the recent years been improved. Still, in many cases the objectives for the recipients are not met. Therefore limitations of the point sources are required. Among these point sources, not only one, but several sources are contributing to the poor water quality. Among those sources are sewer overflows. Limitations of wastewater discharges from municipal wastewater treatment plants continued up through the seventies to the nineties followed by combined sewer overflow (CSO) control and stormwater control. Because of the significant reduction in pollution from the municipal wastewater treatment plants the points source pollution requires more attention in order to improve our receiving waters. Despite the improvement of municipal wastewater treatment plants, still only 45 % of the streams in Denmark meet the objectives. In 1997 the Danish Ministry of Environment and Energy reported the fraction of urban runoff of the total pollution from point sources was 8 % for BOD, 15 % for total nitrogen and 8 % for total phosphor. Urban runoff may reach the receiving waters either as discharges from stormwater overflows in the areas operated by separate sewer systems or as combined sewer overflows. Discharges from combined sewers do not only contain surface runoff, but rather a mixture of runoff from municipal sludge, resuspended sludge from the sewer and runoff from surface areas and therefore contains pollutants from all sources. Runoff from separate sewers primarily contains pollutants from surface areas (roofs, roads etc.) and resuspended matter from the sewer pipes. Stormwater and CSO discharges have impact on both water resources and human health. The impact on human health restricts the opportunities for the use of the recreation areas. The types of impact on the receiving waters discussed in this report include those caused by physical, aesthetic, microbiological, chemical and biological processes. The main purpose of this project is to examine the literature concerning the stormwater and CSO. The report presents the gathered information on the following three points:
Summarizing the effects Increasing flow The physical effects can be divided into two major effects, increasing flow and flooding. Increased flow from the CSO and stormwater overflows results from increasing urbanisation (increased flow due to less infiltration and larger areas to drain). This increasing flow results in erosion, morphology change and habitat washout. Erosion is a naturally occurring process in waters with changing water velocities. Natural and unaffected streams will have a tendency to reach equilibrium, where the erosion and deposition forces are in balance. Although erosion in streams is a natural process the intensification of this process in urban areas causes increasing concern. Many have stated that the increasing erosion yield caused by urbanisation is a wide spread problem and lead to a number of problems. The washouts of fine sediment can affect the bentich habitats and cause the streams to go into an unsteady equilibrium. The increased flow and washout of bentich material will cause morphology changes affecting the depth and width of the stream. This morphology change can be caused either gradually or as a result of one major runoff. Ecological damages are caused by excessive erosion and flow. Several have documented the change in species diversity due to the increasing urbanization. The change in the recipient would in many cases be irreversible. Suspended soils resuspended by erosion may cause a number of direct and indirect environmental impacts, sunlight reduction, filling of spawning grounds, abrasion of gills and other sensitive tissues, reduced of visibility etc. Flooding Flooding in streams is a negative effect in relation to the use of the areas close to the streams. Flooding has in general been reduced in the Danish streams due to regulation and flooding caused by CSO, and stormwater overflows is not regarded as a problem, but can occur in small creeks. Regarding to the environment flooding is positive, because it will smooth the discharged variations and contribute to a removal of nutrients. Aesthetic effects Aesthetic effects in the receiving waters caused by overflows manifest themselves in visible leavings of matter from the sewer systems. This pollution occurs from separate as well as combined sewers but is more visible from combined sewers. Sightings in Counties in Denmark document problems downstream emissions points in about 20 % of the cases. Aesthetic pollution is important reduce while the general perception of the receiving waters decrease while polluted with toilet paper etc. from the sewers. Installing a sieve, which is maintained properly, can easily reduce aesthetic pollution. Microbiological pollution Microbiological pollution effects on human health and biomass are primarily associated with CSO and to a lesser degree with stormwater. Both stormwater and CSO convey high loads of faecal bacteria, which are typically described by concentrations of the indicator bacteria E. Coli. In 1997 only 4 beaches in Denmark where closed for swimming due to discharges of stormwater and CSO, and can therefore be considered as a local problem. Nutrients The problems regarding the nutrients in the recipients are well described in Denmark and the study of the literature has not discovered new facts. Oxygen depletion Reduction of dissolved oxygen and the contaminated biomass accumulation are typical discharges of oxygen demanding substances. Traditionally dissolved oxygen depletion has been divided into 2 effects, one occurring because of the oxygenation of easily degradable organic matter and ammonium. The delayed depletion of oxygen comes from the oxygenation of the sediment. However there seems to be a significant uncertainty concerning the causal connection. Extensive documentation exist describing moderate oxygen depletion due to CSOs in Denmark. Extensive oxygen depletion has only been reported from streams with generally poor oxygen conditions. There has been no documentation of fish kills. Sublethal effects can occur after CSO’s and stormwater overflows. Ammonia and hydrosulphurous Elevated levels of ammonia and hydrosulphurous may cause toxicity impacts. However, no proof of ammonia or hydrosulphurous effects in the recipients has been found after stormwater and CSO contamination. Substances strange to the environment Toxicity may be associated with both CSO and stormwater but the concentration of both metals and organic pollutants are typically higher in stormwater. The toxicity can both be acute and accumulative, with accumulation in the sediment especially from heavy loaded roads and highways. Toxicity may come in many forms and can be caused by many pollutants especially the metals, PAH’s and pesticides. It has been proved in many cases that the most severe toxicity is associated with stormwater and to a lesser degree CSO, this can be explained by the adsorption of the contaminants to organic matter from the CSO. The new environmental strange substances, as PAH’s and the estrogenic substances, require more attention and the relationship between the substances and stormwater and CSO has to be established. Fish exposed to estrogenic substances have in several cases been known to change sex. Several countries have been disturbed over the increasing problems connected with these substances, but none have taken measures yet, because of the lacking knowledge on the subject Unidentified effects There have been many reporting of unidentified effects on species in recipients after CSO and stormwater overflows including fish kills and loss of sensitive species. Whithout further documentation it is said that most of the fish kills, and loss of species are caused by acute contamination of the recipient. There have not been reported fish kills in Denmark due to CSO and stormwater overflows (accidents not included). Laws and rules in other countries Legislation and practice in other western countries have proved not to differ a lot compared to Denmark. Countries that use their recipients as drinking water supply have off course harsher procedures in the order to protect their drinking water supplies from stormwater overflows and CSO. Measures to reduce the effects from CSO and stormwater overflows As it appears from the text above one of the most profound effects from CSO and stormwater overflows is the increasing flow in the streams caused by the discharges from the CSO and stormwater overflows. Measures to reduce this are obvious. Reducing the impervious areas connected to the sewers system (local seepage) and storing (basins etc.). A large number of measures –in some cases rather exotic-exist there can be applied to reduce the pollution from stormwater and CSO. Several measures have proven to be able to reduce the pollution from stormwater and CSO significantly. Both storage and satellite treatment (vortex, flocculation etc.) have proven to be effective in capturing the pollution connected to the sediment. The best solution is in many cases a combination of both storage and satellite treatment. There exists a large number of data series in other western countries, which can be used to predict more accurately retention of several pollutants in retention basins. Summarising the conclusions:
It seems that focus must intensify on discharges from separate sewers including increasing flows and toxicity.
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