Treatment MeasuresLitter Traps : Litter Trap Types
More Information
The decision of which type (and brand) of trap to select is a trade-off between the life cycle costs of the trap, the expected pollutant removal performance in regard to the values of the downstream waterbody and any social or political considerations.
Life cycle costs Vs. Pollutant Removal Performance
For further information on any of the below Litter Traps simply click on the title:
| In-Line | Self Cleaning Screen |
|---|---|
| Floating Trap | Sediment Trap |
Relevant Article
Removal of suspended solids and associated pollutants by a CDS gross pollutant trap by T. A. Walker R. A. Allison T. H. F. Wong R. M. Wootton http://www.catchment.crc.org.au/pdfs/technical199902.pdf
In-Line Litter Traps
(Overview of Urban Stormwater: Best Practise Environmental Management Guidelines, Section 7.7 Primary Treatment)
Gross Polutant Traps
Typically a gross Pollutant trap is a sediment trap with a trash rack (see below). They generally consist of concrete lined wet basin upstream of a weir, with the trash rack located above the weir. This type is a combination of two types for the purpose of collecting pollutants that would otherwise pass one treatment or the other.
TUGGERNONG LAKE, CANBERRA
Litter Collection Baskets
A litter collection basket is typically steel sheet basket with holes. The basket is situated below the invert of the inlet pipe. Water falls into the device and all particles greater than the hole size are retained. As litter is trapped the effective pore size is reduced and smaller particles are trapped. A drop in the channel bed from inlet to outlet is required to allow the waters to pass into and through the basket. Above the trap point (basket) an access port must be available to 'clean' the basket. The infrastructure cost are medium to high along with the maintenance costs, but is a simple and effective litter trap. This type of Litter Trap is useful in areas of high litter loads, can be retro fitted to existing drainage systems and has minimal visual impact. In comparison this trap has high installation costs and is limited by sufficient channel bed incline, it can get blocked and cause upstream flooding or re-suspension of the pollutants and can be a possible source of odours.
Boom Diversion Systems
This treatment type typically consists of a vertically hinged boom located in the path of the entering stormwater. Target litter: Syringes. In low flow conditions the boom diverts all the flow into a screened collection chamber. During High flow conditions the boom only diverts floating items, with the majority of the flow by-passing the trap. The system can be retro fitted to existing drainage lines. Maintenance of cleaning is required typically monthly and are currently being tested in Victoria (US:BPEMG 1999)
Release Nets
This treatment is simply netting attached over the outlet of a pipe. The net is typically shaped as a cylinder and length is dependant upon the catchment. If the design flow is exceeded or the netting is full (weight determined) the net can detach from the drain, with the net opening being chocked by a short tether. Its trapping ability is dependant upon the pore size of the netting used. It visually unattractive and could be subject to vandalism but has simple, low installation costs, volume and pore size of netting can be changed at any time, Easy to maintain, involving no manual handling of pollutants.
Trash Racks
Typically installed in drainage channels, the trash rack consists of vertical or horizontal steel bars spaced between 40-100mm apart. As trash builds smaller particles can be trapped and is manually cleaned.
Cup and Saucer Creek, Sydney
Return Flow Litter Baskets
Water passes through the labyrinth, exiting near the inlet weir. Water leaving the labyrinth collection basket produces a hydraulically driven barrier that diverts incoming water in the collection basket. The force of the return flow from the water exiting the collection basket drives the barrier. The process operates in all flows except flood conditions, in which the flow bypasses the system.
Hydraulically Operated Trash Racks
Stormwater is filtered through a series of vertical screens prior to flowing under a fixed baffle, then over a weir. The sluice gate is operated during flood conditions to enable floodwaters to pass through the device. This type of treatment has minimal head loss, with high flow by-pass that avoids pollutant scouring. In contrast this trash rack involves intensive maintenance, requires a large area and is aesthetically obtrusive.
Floating Traps
Flexible Booms
This comprises of a set of partly submerged floating booms located across a waterway. The boom collects floating debris as they travel downstream. This mobile trap can enhance the aesthetics of downstream waterways along with single location collection that is able to rise and fall with the flow. This type of trap is limited by its capture efficiency, fragility of booms, difficulty of maintenance and it can aesthetically obtrusive.
Floating Debris
This type of trap has evolved from the flexible boom, floating trap. The changes involve more ease for maintenance and enhanced material retention capabilities. The booms of typically polyethylene have submerged skirts protruding into the water that deflect floating debris into the collection chamber. A one way gate at the opening of the collection prevents the escape of trapped debris, with a sliding gate at the downstream end providing access for cleaning.
Sediment Traps
Sediment Settling Basins and Ponds
This type of structure is designed to trap coarse sediments. They typically take two forms; a tank usually constructed of concrete or a pond typically constructed as an excavated hole. The enlargement of the channel causes sedimentation to occur due to the subsequent reduction in flow velocity.
This type of trap is simple in both construction and design. Minimal fine sediment removal, large area requirements, the potential for pollutant breakdown in wet sump limit the sediment traps' applicability and it could be a source of sediment/pollutants during a high flow event due to scouring.
Circular Settling Tanks
A cylindrical tank, it is divided up into two chambers; the upper diversion chamber and the lower retention chamber. Waters are directed into the lower retention chamber by weirs. The waters exit the retention chamber through an outlet riser pipe, with the sediment collecting at the base of the retention chamber. During flows in excess of design capacity the diversion weirs are topped and flow is diverted from the retention chamber.
The trap is able to protect material from flood scouring, suitable for targeting specific problem areas with retrofitting ability and retains a high proportion of sediments. High initial costs and the likelihood of the inlet pipe to be blocked by gross pollutants limits this treatment type.
(After CSR Humes 1997)
Hydrodynamic Separators
This type of separator induces a vortex upon the entering stormwater. The system relies upon this vortex to separate the sediments from the waters. Two design categories are currently present. The first is a collection chamber at the base of the separator that is periodically cleaned. The second is the incorporation of a separate line that pipes the separated sediment and pollutants to sewer. The second category is essentially self-cleaning.
This treatment type has a high removal rate with minimal maintenance for cleaning. It is limited due to cost, lack of performance data and as the flow velocity falls so does the removal rate.
(Storm King®, after Hydro Australasia)
Self Cleaning Screen
Circular Screens
A circular screen separates pollutants and litter trapping them in a catchment sump. The circular motion serves to push the trapped litter down the trap into the sump, thus keeping the screen clean. These types of traps are considered to be efficient but have a high installation cost and moderate maintenance cost with a 2-3 monthly clean out. Monitoring of a CDS device (as shown below) yielded a 90% of sediment in the sump was smaller than the screen mesh size by Walker et al (1999).
Diagram Provided by CDS TECHNOLOGIES 2002
Downwardly Inclined Screens
This treatment comprises a downwardly inclined trash rack with a pollutant holding shelf at its base. The water falls between the trash rack bars whilst the pollutants remain. The flow of water pushes the stranded pollutants down the rack onto the shelf for future collection. The Racks are typically between 20-45 degrees inclination from horizontal.
(after Baramy 1997)