Prospects and Limitations of Surface Runoff Quality Management in the Republic of Serbia - Page 04

 

Objectives of Stormwater Pollution Control and Management

If conditions in the basin, in its natural state, and after urbanisation are considered, it can be concluded that urbanisation changes the runoff conditions, such as:

  • The increase in impervious areas leads to an increase in runoff velocity and volume of runoff resulting in higher peak discharges and an increase of total runoff volume;
  • Stormwater runoff quality is affected by land use, traffic intensity, air quality and other factors, which usually lead to the deterioration of runoff quality.

The magnitude of pollution effects form urban stormwater runoff depends on the characteristics of the receiving waters and the quantity and rate of contaminant discharge. The objective of storm water management measures is to reduce the impact of urbanisation on runoff dynamics and quality. Due to seasonal and site-specific variation in runoff quality, a single, widely accepted approach/methodology with clearly identifiable measures for environmental protection from pollution carried by stormwater runoff has not been established. Extensive research has been conducted on managing urban runoff, the results of which indicate the following:

  • The pollutant concentrations in stormwater runoff are much higher in first flush, although there are results that show that this is not always the case.
  • Pollutant concentrations and total load within the watershed depend primarily on the rainfall intensity (higher intensity – higher pollution loads).
  • Atmospheric deposition may also affect runoff quality as pollutants in the air are deposited onto land and are then washed away by stormwater. Therefore, runoff quality depends on the length of drought periods (the time interval between two rainfall/precipitation events), or the total time period without precipitation.

It is important to note that the sources of pollution present in stormwater runoff belong to many different types of non-point (diffuse) sources of pollution. Therefore, it is clear that the reduction of pollution caused by stormwater runoff can only be achieved by implementation of a combination of different measures that must include administrative and technical measures to reduce pollution emission from nonpoint pollution sources, including, but not limited to: watershed and city planning measures, erosion control measures, air pollution control measures (motor vehicle emissions reduction and control of NOx from other sources, etc.), source control, treatment of polluted runoff near the place where pollution is generated, runoff retention and infiltration measures, etc.

Additionally, 'end of pipe' treatment is occasionally used to reduce pollutant loads in stormwater before its discharge into the environment. Due to large variations of runoff flow and quality, it is difficult to establish design criteria for treatment devices, and their required efficiency should be determined following careful runoff quality analysis of the catchment area, as well as analyses of immediate and long term impacts of storm runoff on receiving water quality and status.

In developed countries, great attention is paid to urban stormwater management. The current trend is towards establishing a decentralised stormwater management that includes various measures aimed at lowering peak runoff flows and minimising pollutant loads prior discharge into the environment. Low-Impact Development (LID) is a decentralised source control stormwater management approach to site planning, design, and pollution prevention. LID uses a decentralised approach to wastewater management systems that include on-site infiltration through retention basins, stormwater reuse and the release of treated effluent into the environment under conditions which will not endanger the receiving waters.

During the construction and reconstruction of storm sewer systems in already urbanised areas, the implementation of urban runoff quality control measures is limited. Therefore, solutions must be adapted to specific site conditions and realistic stormwater planning and management goals, which should be defined in the decision making process which includes public participation.

The following four objectives define an integrated and balanced approach to reduction of excess runoff volumes and the detrimental impacts of stormwater pollution on recipients (Heaney et al., 1999):

  • Use of pollution prevention measures aimed at proper handling, storage and disposal of hazardous substances that could threaten water quality;
  • Control of nonpoint pollution sources and prevention of contact between pollution and stormwater runoff;
  • Reduction of runoff flows and/or volumes, reduction of pollutant loads and concentrations near the place of its origin;
  • "End of pipe" treatment, or measures on the downstream end of the sewer system involving the use of various devices and processes for stormwater runoff treatment to remove pollution, according to local conditions. A prerequisite for the implementation of these techniques is that the above three groups of measures have already been applied.