THE MAGAZINE FOR THE DRAINAGE, WATER & WASTEWATER INDUSTRIES
MAIN FEATURE
Type of component* TSS Metals Hydrocarbons Oil water separator Stormwater filter Retention basin
0.8 0.8 0.5
0.6 0.6 0.5
0.9 0.7 0.6
Automatic closure device at outlet from drainage network
Table 12: Mitigation indices for SuDS components selected (sources: SPEL Class 1 Separator product literature,; CIRIA C753 Table 26.3)
*Remember that after the first SuDS component in the treatment train, only half of the mitigation indices for other downstream components can be used in the calculation.
The first step in designing a SuDS management train for a waste management site would be to determine what pollutants would be present on the site and their expected concentrations. These could include metals, hydrocarbons, chemicals and organic compounds.
Once the pollutants have been identified, suitable filters, treatment media or other manufactured devices can be specified. Surface water from any waste processing facility should be treated before it enters any vegetative SuDS features.
For our imagined site, which is processing mixed household waste at a recycling centre, there are risks of leaks and spillages from the oil tank, battery recycling, green waste, discarded paints and household pesticides and more. An oil/water separator will capture oil spillages but spillages of other substances, and pollution from dissolved metals must be captured using additional devices. This could be a special type of stormwater filter, using treatment media to capture the dissolved pollutants.
The SuDS management train should conclude with a vegetative device such as a retention basin where natural processes and micro- organisms can break down any residual pollutants, and the last suspended particles in the runoff can settle on the bottom of the basin. These devices will need to be monitored and maintained, and regular inspections will ensure that any problems are identified and rectified quickly.
Since waste treatment sites can have a high risk of fire, another important water quality issue for waste treatment sites is how the water used to fight a fire would be captured and treated. There needs to be a closure device at the outlet of the site to prevent contaminated water from leaving the area.
Informed decisions
The design of SuDS management trains is a multi-faceted exercise. Beyond controlling the quantity of water leaving a development, designs must preserve water quality and optimise opportunities to add amenity value and boost biodiversity.
Where the risk of pollutants in surface water runoff is low, the SuDS management train may consist only of natural components. For sites with a medium risk of pollutants, surface water should be treated before it reaches detention features such as ponds, basins or proprietary below-ground systems. Pre-treatment using natural or manufactured components may be possible, depending on circumstances. Surface water from high-risk sites with high pollutant loading should be treated by a suitable manufactured
FOLLOW US
device before it is discharged into vegetative SuDS components.
Differences in the mitigation indices and the sizing of SuDS components to meet the treatment flow rate and the hydraulic (maximum) flow rate also need to be taken into account. Designers also need to have an understanding of the risk of the captured pollutants being flushed out during exceedance events and the pros and cons of the alternative options available.
Design choices should also take maintenance requirements and costs over the lifetime of a development into account, not least because of the need to create asset management strategies and agree commuted sums where SuDS are to be adopted. No system will perform as designed from a water quality or quantity perspective if its components are not properly inspected and maintained. This applies to both vegetative and manufactured elements.
Financial viability is an unavoidable issue. Current regulations regarding SuDS and commercial viability vary in Scotland and Wales. England has yet to decide; a consultation on the implantation of Schedule 3 is expected this year. For restricted brownfield sites, such as the one considered above, it could make sense to allow some room for manoeuvre, depending on the social, economic and environmental impacts of the planned development.
If we are to put together all the pieces of a SuDS jigsaw to the best effect, there must be early conversations between a broader range of stakeholders. Professionals across the supply chain will need to upskill and share knowledge. Some manufacturers of proprietary SuDS systems provide CPD seminars to help SuDS practitioners prepare for the recent changes and those round the corner.
NEXT MONTHS MAIN FEATURE:
Contact Ian Clarke to get involved
ian@nodigmedia.co.uk
February 2024 | 11
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