Treatment MeasuresLitter Traps : Overview
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PRIMARY TREATMENT - physical screening or rapid sedimentation techniques. Typical retained contaminants: gross pollutants and coarse sediments
The removal of large pollutant matter such as urban litter is a primary treatment method for urban stormwater runoff. There are many differing types of gross pollutant traps that are commercially available, but all have the purpose of detaining for future collection (removal) of litter and sediment above 5 millimetres in size. This removal of large pollutants enables downstream treatment of stormwaters' to occur more efficiently and effectively.
Selecting a litter trap for stormwater installations can be a confusing task with many claims by vendors and numerous issues to consider. This section provides assistance to stormwater managers for the selection of litter traps for stormwater so those funds are spent wisely and that the most appropriate traps is installed in each location.
An unfortunate natural litter trap on a Melbourne waterway
Concern for the impacts of stormwater litter and gross pollutants (litter and organic material) have increased in recent times. There has been some research carried out in Melbourne on its characteristics and transport mechanisms, some relevant findings are:
- Approximately 100,000 m3 (including 1 billion litter items) of gross pollutants reach Melbourne's waterways annually;
- Stormwater gross pollutants are composed of approximately 20% litter (plastic, paper and metal) and 80% organic material (such as leaves and twigs);
- The majority of gross pollutants are carried during times of the highest flows;
- Less than 20% of litter is transported as floating material, the remainder is either entrained in the flow or sinks; to the creek bed;
- Commercial areas such as shopping centres contribute the most amount of litter to the stormwater system; and
- Ranges of techniques are available for removing litter. The most effective strategies involve a combination of non-structural measures (eg. education programs and source controls) and structural treatments.(eg. Booms, traps, baskets)
The location for a litter trap is also a complex issue and readers are referred to the Urban Stormwater Best Practice Environmental Management Guidelines (Stormwater Committee, 1999) for guidance.
In addition, to investigate litter loads from different areas for the purpose of selecting a location for a trap, readers are referred to the decision-support-system for determining effective trapping strategies for gross pollutants developed by the Cooperative Research Centre for Catchment Hydrology (CRCCH, 1998).
References
- Urban Stormwater - Best Practice Environmental Management Guidelines, Victorian Stormwater Committee, 1999, CSIRO Publishing
- A decision support system for determining appropriate trapping strategies for gross pollutants, 1998, Cooperative Research Centre for Catchment Hydrology, Research Report 98/3