Stormwater – helping to tackle Australia's water crisis
Box 2 | Removal of pollutants from stormwater
A number of methods have been developed to remove pollutants from stormwater. The processes involved in the treatment methods can be classified as physical, biological or chemical processes.
Physical processes
There are two general methods by which pollutants can be physically removed from stormwater. Sedimentation basins are designed to temporarily detain and reduce the flow of stormwater, which facilitates the settlement of sediments and other contaminants at the bottom of the basins. Basins are particularly effective in the removal of larger contaminants from stormwater, but are less effective in removing the smaller contaminants such as clay particles. The rate at which the sedimentation basins remove pollutants from the water is dependent on the flow of the water, the size of the sediments as well as the retention time of the stormwater in the basins.
The second physical process used to remove pollutants from stormwater is filtration. Filtration involves the separation of materials from the stormwater by passing the water through a filter. This method is useful for the removal of suspended solids and pollutants bound to sediment in the water. However, the filtered materials accumulate in the filter and reduce the flow through the filter. Regular maintenance of the filter is required to retain the effectiveness of the filtration process.
Biological processes
Bacteria play an important role in wetlands and swales constructed as stormwater treatment systems by removing nitrogen, organic matter and dissolved metals. Under anaerobic conditions – conditions which are typical in sediments, where little or no oxygen is available – bacteria act as nitrogen-removing agents. They do this by converting nitrate (NO3) to nitrogen (N2). However, an excess of nitrogen from fertilisers and animal manure in stormwater can cause undesirable growth of bacteria, algae and plants. Bacteria and other micro-organisms also degrade organic matter like garden clippings suspended in the water by oxidation, releasing carbon dioxide gas. In addition, their role in converting sulfates to sulfides helps with removal of dissolved metals.
Plants used in stormwater treatment can indirectly remove pollutants by promoting bacterial activity. They also take up metals and other nutrients from stormwater.
Chemical processes
Chemicals can be added to stormwater causing some of the pollutants dissolved in the water to become insoluble. For example, aluminium- and iron-containing chemicals can be added to make phosphorus compounds commonly found in commercial fertilisers insoluble in water. Alternatively, chemicals can be added to lower the pH of the water, making it more acidic, and reducing the solubility of some pollutants. The solids formed can generally be removed from the stormwater either by filtration or sedimentation.
Posted June 2008.