Water Sensitive Urban DesignSelecting a Treatment
The selection and implementation of structural treatment measures involves six steps. These are: (Click on the titles below to view the detail)
- Determine treatment objectives: Establish the pollutants of concern in the catchment (e.g. litter, sediments, and nutrients) and the level of pollutant retention required.
- Develop treatment train: Assess the treatment processes required and appropriate measures and ordering, including any pre-treatment requirements (e.g. screening of coarse sediments or flow control).
- Site identification: Identify potential sites and site constraints (e.g. slopes and soil types).
- Short-list potential treatments: Identify all applicable treatments.
- Compare potential treatments: Compare all potential treatments for removal efficiency, maintenance requirements, social impacts and costs.
- Detailed design: Complete detailed design of the optimal treatment.
This selection guide reviews the first five steps of the above process. The detailed design process requires further, more site-specific information evaluation and is outside the scope of this guide. This guide demonstrates a methodology for selecting and ranking treatment options using litter as the target pollutant.
Physical Screening or Rapid Sedimentation Techniques.Typically retained contaminants: gross pollutants and coarse sediment. Treatment Measures under this category are grassed swales and Litter Traps.
Fine Particle Sedimentation and Filtration Techniques.Typically retained contaminants: Fine sediment and attached pollutants. Treatment Measures under this category include: Swales, Infiltration Trenches, Porous Paving, Bio-Retention Systems, Rain Gardens.
Enhanced Sedimentation and Filtration, Biological Uptake, Adsorption onto Sediments.Typically retained contaminants: nutrients and heavy metals. Treatment Measures under this category include Bio-Retention Systems, Wetlands.
1. Determine treatment objectives
The stormwater pollutant profile of any catchment area is determined largely by the area's land-use and stormwater management. For example, human derived litter can be a problem in commercial areas, whereas sediment run-off is often more prevalent in developing urban areas.
To isolate the pollutants of concern in any catchment, the designer needs to closely examine receiving water degradation in light of the area's land-use and current management practices. The performance objectives (see table below) set out in Chapter 2 of the Urban Stormwater Best Practice Environmental Management Guidelines (CSIRO 1999) are a guide to the typical pollutant load reductions required to contribute to State Environment Protection Policy (SEPP) compliance. The draft Australian Runoff Quality guidelines (http://www.arq.org.au) provides assistance for decision making in selecting, integrating and locating stormwater treatment measures.
Stormwater quality performance objectives
|Pollutant||Receiving water objective:||Current best practiceperformance objective:|
|Post construction phase:|
|Suspended solids (SS)||Comply with SEPP (eg. not exceed the 90thpercentile of 80 mg/L) (1)||80% retention of the typical urban annual load|
|Total phosphorus (TP)||Comply with SEPP (eg. base flow concentration not to exceed 0.08 mg/L) (2)||45% retention of the typical urban annual load|
|Total nitrogen (TN)||Comply with SEPP (eg. base flow concentration not to exceed 0.9 mg/L) (2)||45% retention of the typical urban annual load|
|Litter||Comply with SEPP (eg. no litter in waterways) (1)||70% reduction of typical urban annual load (3)|
|Flows||Maintain flows at pre-urbanisation levels||Maintain discharges for the 1.5 ARI* at pre-development levels|
|Suspended solids||Comply with SEPP Effective treatment of daily run-off events (eg. >4 months ARI).||Effective treatment equates to a 50%ile SS concentration of 50 mg/L.|
|Litter||Comply with SEPP (eg. no litter in waterways) (1)||Prevent litter from entering the stormwater system|
|Other pollutants||Comply with SEPP||Limit the application, generation and migration of toxic substances to the maximum extent practical|
|1 An example using SEPP(Waters of Victoria1988), general surface waters segment.
2 SEPP schedule F7-Yarra Catchment-urban waterways for the Yarra River main stream.
3 Litter is defined as anthropogenic material larger than five millimetres.
In order to protect receiving waters, treatments may be required to reduce the impact of one or more of the following pollutant categories:
- gross pollutants: trash, litter and vegetation larger than five millimetres;
- coarse sediment: contaminant particles between 5 and 0.5 millimetres;
- medium sediment: contaminant particles between 0.5 and 0.062 millimetres;
- fine sediments: contaminant particles smaller than 0.062 millimetres;
- attached pollutants: those that are attached to fine sediments - specifically, nutrients, heavy metals, toxicants and hydrocarbons; and/or
- dissolved pollutants: typically nutrients, metals and salts.
The treatment measures considered in these guidelines have been assessed according to their trapping efficiency for each pollutant category.
The overall treatment effectiveness of a measure is a function of its pollutant removal rate and the volume of run-off treated. A high flow by-pass is generally designed into treatment measures for protection from large flood flows that could damage the device or scour and transport previously collected pollutants downstream. The maximum flow rate at which a treatment measure is designed to operate effectively is termed the design flow.
Selecting the design flow is a trade-off between the cost and space requirements of the device (a higher design flow will generally require a larger facility with additional costs) and the volume of water that could potentially by-pass the measure and avoid treatment. Figure 1.1 plots the volume of mean annual run-off that would be treated at, or below, the design flow rate for a range of design standards, for several hypothetical catchments with different times of concentration, using Melbourne rainfall data. For regions outside Melbourne there is a procedure to determine the appropriate relationship (Wong et al. 1999).
The plot in figure 1.1 shows that the curves are relatively independent of the time of concentration of the catchment, and also that the incremental benefit of increasing the treated volume of run-off diminishes beyond a design flow rate of the 2 year ARI. Further, the plot suggests that generally the optimum operating range falls within a design flow rate of between 0.25 and 1.0 year ARI discharges.
2. Develop A Treatment Train
Many pollutant treatments, particularly those targeting fine pollutants, require a number of measures used in sequence to be effective. Figure 2.1 illustrates a relationship between pollutant type and treatment processes. There is a clear relationship between pollutant size (gross to dissolved) and the appropriate process that can be employed to retain the pollutant. The treatment types in Figure 2.1 show the size range of pollutants that each treats effectively. By knowing the target pollutants, appropriate treatment measures can be selected.
The figure also illustrates the approximate hydraulic loading rate for effective operation of the various treatments. The hydraulic loading rate is a function of the treatment process (either screening, sedimentation, enhanced sedimentation, filtration or biological uptake) and can be used to approximate the area required to install a facility given the design flow. This is useful to assess the space requirements for the various treatment measures.
The treatment train approach is particularly important when a treatment measure requires pre-treatments to remove pollutants that may affect the performance of the treatment measure. For example, wetland systems are often employed to protect receiving environments from the impact of excessive level of nutrients and heavy metals. However, wetlands will perform poorly if gross pollutants (eg. Litter) and coarse sediments are not removed prior to the wetland treatment. It is therefore important to select and order treatment measures appropriately to ensure that wetland systems are protected from gross pollutants and coarse sediments.
By taking this 'treatment train approach', the most effective sequence of the treatments can be determined. The WSUD Engineering Procedures provides advice on deign detail on WSUD Treatment Measures.
Scale of Treatment
Inter-relationship between site - precinct - regional stormwater management measures
Stormwater treatment can be broken down into three overlapping categories as demonstrated in the following table. Many treatment measures can be 'sized' to suit the land area available.
|Site Elements||Precinct Elements||Regional Elements|
The diagram below is an example of a stormwater treatment train with pre-treatment measures for the protection of the downstream wetland system.
The previous diagram illustrates a stormwater treatment train, which incorporates the use of swales and infiltration trenches, a bio-retention system and wetlands. In this system, the grassed swale has been applied to the local streets in the estate, in place of conventional nature strips. The swales are constructed in combination with the infiltration trench beneath the swale, and act as the primary treatment measure removing gross pollutants (eg. Litter) and coarse sediments. The conventional median strip in the main boulevard has been modified to incorporate an innovative drainage system-the secondary treatment measure. This treatment measure or Bio-retention system removes fine sediments and filters adsorbed pollutants (eg. Nitrogen and Phosphorous). The final component of this treatment train is the tertiary measure-the wetland system which removes very fine particulate matter, in addition to biological uptake of pollutants (eg. Heavy metals and Nutrients).
3. Site identification
3.1 Locating a Treatment
When determining the location for stormwater treatment measures, many factors must be considered. One fundamental question is whether to adopt an 'outlet' or a 'distributed' approach.
The traditional outlet approach involves constructing a single large treatment at the catchment's outlet. Although this 'single site' approach offers obvious maintenance advantages, it has the disadvantage of needing to treat very large volumes of water at a location sometimes far from the pollutant's source.
An alternative is the distributed approach. Here, a number of smaller and potentially different treatments are installed throughout a catchment.
A distributed approach to stormwater pollution treatment has many advantages over the outlet approach. These include:
- improved protection: water quality protection may be distributed along a greater length of the waterway, thus protecting immediate downstream waterway reaches;
- localised treatment: specific targeting of treatments may be directed at highly polluted sites;
- distributed risk: the distributed approach has a lower risk of overall system failure, as the failure of any single treatment will not usually significantly impact on the total treatment system performance;
- improved removal efficiencies: distributed treatments are typically located in areas of lower flow. Lower flow velocities, volumes and higher pollutant concentrations in the stormwater at these sites, leads to higher operating efficiencies;
- staged implementation: individual sites may be brought into operation in stages; and
- lower total cost to the community.
Typically, a distributed treatment approach will incorporate a range of structural treatment types. To ensure optimal pollutant removal efficiency, a treatment train approach should be considered during each step of the design process, particularly where pre-treatment needs may be an issue.
3.2 Site Constraints
The characteristics of a particular site can limit the choice of treatment measures suited to the area. These constraints fall broadly into two categories - physical and social.
Physical site constraints can make construction difficult or impossible, and maintenance expensive if not addressed adequately. Factors to consider include:
- topography - e.g. steep slopes
- soils and geology - e.g. erosivity, porosity, depth to bedrock or instability
- groundwater - e.g. geochemistry and water table depth
- space - limited open space, proximity to underground services. (e.g. gas, power)
- environmental - significant flora and/or fauna species, heritage values
Social constraints include issues of health and safety, aesthetics and impacts on recreational facilities. Factors to consider include
- odour problems
- visual impacts
- physical injury - resulting from unauthorised access to structures;
- contamination - infection, poisoning or injury caused by trapped pollutants or algal blooms
- vermin - e.g. mosquitoes, rats.
Many social issues can be addressed simply during the treatment design stage. This may involve development of occupational health and safety procedures for operations and maintenance staff, installation of warning signs, fencing around dangerous areas and so on.
4. Short-list potential treatments
A short list of potential treatment techniques that meet the requirements for the target pollutants and site constraints should be developed.
Various treatment techniques are described in Treatment Measures.
Specific pollutant retentions should be compared to performance objectives required.
5. Compare potential treatments
Having established a short list, the treatment measures should be reviewed in detail to determine the best options. Factors to consider include maintainability and operability, pollutant retention, head requirements, cost and secondary benefits. These considerations are further described below.
5.1 Maintenance and operation
A poorly maintained treatment measure may not only perform badly; it may become a flood hazard or a source of pollution itself. Treatment measure operation and maintenance requirements vary widely. When assessing the treatment measure's maintainability and operability, the following issues should be considered:
- ease of maintenance and operation - the selected treatment should be easy and safe to maintain and operate
- extent of maintenance - ensure the maintenance requirements are within the operator's capability
- access to the treatment site - consider the ease of site access, when reviewing the treatment's maintenance requirements
- frequency of maintenance - ensure that resources are available to carry out maintenance at the required frequency
- debris and pollutant clearing - during clearing, the treatment should not require direct human contact with debris and trapped pollutants (automated clearing optionsare preferred)
- disposal - consider the disposal requirements of any waste from the treatment process.
5.2 Pollutant Retention
A closer look at the treatment measure's pollutant retention is required at this stage. Depending on issues of maintainability, operability, cost and head requirements, the overall pollutant retention efficiency for each specific target pollutant should preferably be as high as possible. Pollutant retention can be determined using tools such as MUSIC and STORM.
5.3 Secondary Benefits
Certain treatment measures provide incidental benefits beyond the primary goal of removing the target pollutants.
Some treatment measures demonstrate the potential to remove pollutants other than the primary targets they were selected to remove, e.g. a litter trap that also removes sediment. Other treatment types provide added benefits such as aiding flood control, ecological enhancement or provision of an educational resource. All such benefits need to be considered when selecting a treatment measure.