Treatment MeasuresLitter Traps : Selecting a Litter Trap

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Design flows

Every litter trap should be designed with provision for a high flow by-pass system. The purpose of the by-pass is to protect the operational integrity of the trap during floods, ensure the trap causes no flooding and to prevent excessive scour of collected pollutants from within a trap.

The trap should be designed for between Q 3-months and Q 1-year, with the operation of the by-pass once these flows are exceeded (refer to Best Practice Guidelines for more details). A rule-of-thumb method for approximating more frequent flows from Q-5 values (which should be available for most minor drainage systems) has been developed for Melbourne, these are:

Q -3 months = 0.20 x Q -5 years,
Q -6 months = 0.33 x Q -5 years, and
Q -1 year = 0.50 x Q -5 years.

* note that these relationships are only valid for Melbourne rainfall conditions

Size of the unit (footprint, depth)

The size of litter traps varies considerably and this will need to be accommodated by the potential location for the trap. Things to consider when assessing the size of traps include:

the require footprint,
the depth of excavation (to the bottom of the sump in some cases) - rock can substantially increase the installation costs, and
The location of any services.

Hydraulic impedance/ requirements

Some litter traps require particular hydraulic conditions in order to operate effectively, for example some traps require a drop in the channel bed for operation. Requirements such as these can affect which traps may or may not be suitable in a particular area.

Other considerations are possible upstream impacts on flow and the hydraulic gradeline due to the installation of the trap. This can increase the flooding risks and all traps should be designed to not increase the flooding risk during high flows. Therefore if a trap increases the flooding risk above acceptable limits it may not be considered further.

Other construction issues

For each specific location there will be a number of other considerations and points of clarification that may sway the decision on which trap may be the most suitable, these include:

Does the cost of the trap include supply and installation or just supply - if so how much is installation likely to cost?
Are specialist equipment required for installation (eg. special formwork, cranes or excavators) and what cost implications do this have?
Is particular below ground access required, will ventilation and other safety equipment be needed - at what cost?
Will the trap impact on the aesthetics of an area - will landscape costs be incurred after the trap installation - if so how much?
Are there conflicts with other services at the site (eg. sewer, water, power or phone lines) and what are the cost implications of these?
Will the trap be safe from interloper or misadventure access? (Child access)
Do the lids/covers have sufficient loading capability (particularly when located within roads) - what is the cost of any increase in load capacity and will it increase maintenance costs?
Will the trap be decommissioned (eg. after the development phase) and what will this cost - what will remain in the drainage system?
Are there tidal influences on the structure and how will they potentially affect performance or construction techniques?

POLLUTANT REMOVAL EFFICIENCIES

The removal rate of litter is the primary function of a litter trap and should be estimated from previous independent testing and compared between different types of traps.

Target litter removal rate

To objectively assess various pollutant traps criteria need to be develop that outline the aims of the litter trap. These can range from reducing:

just floating visible litter,
a proportion (eg. 70%) of all litter,
a proportion (eg. 70%) of all litter and organic material, or
just one component of the litter (eg. sharps).

Melbourne Water generally has the objective of either reducing 70% of the litter load in a catchment, or capturing litter greater than 20 mm with treatment of all flows up to at least Q-3 months. These objectives may vary depending on the beneficial uses and threats to a receiving water body.

Litter trap removal rates

There are many claims by vendors on their respective removal rates for litter and other constituents. It is recommended to check any claims, ensure testing is independent and refer to the Best Practice Guidelines for removal rates estimates when no data is available.

Additional pollutant removal

A litter trap will be one component of a strategy to improve stormwater quality. A Stormwater Management Plan (SWMP, developed for each Local Government area) should identify the threats to waterways, the pollutants and remedial works to reduce the threats. The selection and location of a litter trap should always be consistent with and compliment the objectives set out in the SWMP.

With the SWMP in mind, it is important to recognise that some litter traps have the additional benefit of reducing other non-litter pollutants such as organic material, and some sediment. Should the SWMP identify these as causing a threat to waterways then preference may be given to those traps.

Contrary to the above point is the possibility of a litter trap releasing pollutants during dry weather flows (ie., it collects pollutants during storms and then trickle flows flush some pollutants from the trap - potentially in a changed form). This can be of particular concern with devices that retain pollutants in a wet sump for extended periods. Careful consideration of any performance studies and consultation with owners of existing traps is the most efficient way to explore this issue.

ADDITIONAL CONSIDERATIONS

The selection of a litter trap can also depend on social and political considerations. These should be taken into account on a case by case basis. Influences may include:

Potential odour concerns at a location,
Likelihood of pests and vermin such as mosquitoes or rats,
Impact on the aesthetics of an area, and
Political boundaries for funding.

Maintenance equipment requirements

Is special maintenance equipment required? eg. large cranes, vacuum trucks or truck-mounted cranes. Does this equipment need to be bought or hired - at what cost?
Are there overhead restrictions such as power lines or trees?
Does the water need to be emptied before the pollutants - if so how will it be done, where will it be put and what will it cost?

Construction additions for maintenance

Are road closures required and how much disturbance will this cause?
Are special access routes required for maintenance (eg., access roads or concrete pads to lift from) - and what are these likely to cost?

MUSIC Parameters

The bypass flow will be determined by the design parameters, with the bypass flow being around Q-1 to Q-3. (see Design Flows)