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1.4 Collection And Reuse Of Stormwater

Technical Description

In developing urban areas, stormwater can be gathered in separate stormwater sewers and reused after simple treatment for washing streets, cooling, watering of gardens, and other purposes requiring nonpotable water. In addition to providing this reuse potential, this technology makes possible the treatment of urban runoff before it enters waterbodies. The simplest form of stormwater treatment consists of open or closed sedimentation basins, retention ponds, or storm sewer, in-line separation chambers for capturing oil products. Stormwater management technologies include provision of grit chambers and swirl separators within sewer mains, and skimmers which capture floatable solids and oil products from the water surface of basins. Infiltration ditches also can be used for filtration of oil products.

Extent of Use

As a rule, in the central and oldest parts of Central European cities, stormwater is conveyed in combined sewer systems to municipal treatment plants, and discharged, following treatment, to receiving waterbodies. In a few newer urban districts, separated sewer systems have been constructed which allow for stormwater reuse. However, this technology is not very popular and not widely used.

Operation and Maintenance

Regular inspections during rainy weather should be conducted to minimise the occurrence of blockages within the collection system. If basins are used, mowing of the grass within the basins, periodic removal of sediments, and cleaning of the outlet pipe are also required. Litter, soil, and leaf removal is required from in-line separators.

Level of Involvement

This technology is best implemented at the municipal government level.

Costs

Costs are different according to local conditions, possibilities for stormwater usage, and existing infrastructure. Because dual collection systems are required for stormwater and wastewater conveyance, the costs may be higher than for combined systems (which may require larger diameter pipe networks to convey the higher volumes of wet weather flows).

Effectiveness of the Technology

The efficiency of this technology when combined with simple pond-based treatment facilities is high, with approximately 80% of oil products and suspended solids being removed from the stormwater. Depending on the area drained by the system, a large volume of water can be saved if stormwater is reused for purposes requiring low quality source waters.

Suitability

Reuse of stormwater can be a favourable option for municipalities with scarce water resources and an high demand for low quality, nonpotable water. The desirability of stormwater reuse is strongly dependent on general environmental quality and pollution within the rainwater catchment area, and may vary depending on the intensity and duration of storms and the length of the period between storms. Stormwater quality also varies within individual storms, with water quality at the beginning of a storm pollution being poor (known as the "first flush" effect). Also, the quantity of stormwater is variable, with periodic high flows during storms and low or no flows between storms. Hence, detention ponds, using biological treatment, are often used as such ponds are most flexible with regard to volume. Nevertheless, it is important that such ponds are of adequate volume to detain between a 2 and 100 year recurrence interval design storm, so that the runoff is not discharged into waterbodies without sufficient treatment. Because water quality of the runoff generally improves during the course of a storm, the degree of impairment at the end of the storm is rather low and this water can be reused after simple treatment or discharged into waterbodies without a significant pollution risk. Investigations of the stormwater flows show that long rains of low intensity create greatest pollution loads to surface waters. Table 2 presents a summary of the principle chemical characteristic of stormwater based on pollutant concentrations reported in the literature. The Table clearly shows that pollutant concentrations are such that treatment should be provided prior to discharging the runoff to natural water courses.

TABLE 2. Typical Pollutant Concentrations in Stormwater in Eastern Europe.

Advantages

Using low quality water for nonpotable needs, instead of potable freshwater resources, augments the available supplies of the latter. Providing treatment of stormwater runoff from urban areas also provides an opportunity to upgrade degraded natural waters that have previously received untreated stormwater discharges.

Disadvantages

Because limited treatment methods of purifying stormwater are used, effluents may remain contaminated with persistent pollutants like heavy metals, and, in consequence, should not be used for gardening purposes if the produce is intended for human consumption. Use of contaminated stormwater may lead to contaminant accumulation in the soil.

Cultural Acceptability

This technology is culturally acceptable. Its use is largely outside of the public eye, and, hence, is mainly of concern to administration staff and workers.

Further Development of the Technology

Better schemes of the assessment of environmental impacts connected with stormwater use for various purposes need to be developed. Given that treatment is often needed, improved methods of purifying stormwater, that are inexpensive and simple to install and operate, should be developed.

Information Sources

Rolands Bebris, Ministry of Environmental Protection and Regional Development, 25 Peldu Street, 1494 Riga, Latvia, Tel. (371-7) 227145, fax: (371-7) 820442, e-mail: BEBRI@VARAM.GOV.LV.

Anna Egle, V/U "Meliorprojects", 11 Novembra Bulvaris 31, LV-1494 Riga, Latvia, Tel. (371- 7) 228734.

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