Chapter 26, the 2015 SuDS Manual (CIRIA publication C753, Ref 1) presents the Simple Index Approach (SIA) and Mitigation Indices for discharges to surface and groundwater (Tables 26.3 and 26.4). This useful Guidance provides for the adoption of SuDS Proprietary Treatment Devices (PTD), especially DiBT certified devices (Ref 2).
Green Infrastructure SuDS components are rightly seen as the primary option for designers due to their proven amenity and biodiversity value. PTD differ in many ways to green infrastructure and natural soils, offering some distinct and easily proven performance advantages.
Advantages Proprietary Treatment Devices for SuDS
Primary Treatment Devices (PTD) are particularly useful for meeting the challenges of highly contaminated road and commercial yard area runoff (Ref 3). It could be argued that PTD, like permeable pavements, contribute to biodiversity and even amenity perhaps by delivering a consistently clean water discharge into the wider water environment. However, PTD clearly do not provide the same level of amenity or biodiversity as the Green Infrastructure.
One particular feature of certified PTD is that they can deliver certainty of treatment process especially where assessed over long time cycles. Properly certified PTD (e.g DiBT) do not cause episodic releases of heavy metals for example when road salts enter the drainage system. Green Infrastructure such as wetlands and ponds can release high episodic concentrations of metals with inflows of stormwater containing road salt. (Refs 4,5). The precise scientific basis for some of the SUDS Manual’s Indices assessments for Green Infrastructure is in places uncertain or unknown.
Key Issues to consider when comparing Proprietary Treatment Devices
The SuDS Manual suggests Designers should assure themselves that manufactured PTD meet certain standards. 3P Technik supports this and urges designers to ask relevant key questions as to out-turn project and certified performance. These questions might include:
1. What is the Total Suspended Solids/Particle Size Distribution (TSS/PSD) range against which a % removal rate is stated?
Some devices present a high % removal efficiency, but of sediments in the particle range of least relevance, often up to several 10s of mm. 95% removal of over 550 microns (continuous deflection devices for example) is in no way comparable to a 3P HydroSystem, which removes 92% of sub 200 microns in lab test with silica sand. The critical particles are often sub 64microns. (Frustratingly, the TSS ranges are often not stated in the SuDS Manual itself, other than in Ch 14 for proprietary treatment devices).
2. Does the device address dissolved contaminants? (Around 1/3 third of some metals are truly dissolved in road runoff)
Many devices do not remove any of the dissolved metals load whatsoever. Therefore, devices like 3P HydroSystem which do remove dissolved metals do not need to treat as much of the total flow to have a similar treatment performance when compared to a device unable to remove dissolved metals from the incoming stormwater flow.
3. Is it proven that the PTD retains bound contaminants when road salt enters the device during winter runoff periods?
This is the often overlooked critical issue where metals are a key target contaminant for removal. Few PTD have this capability. DiBT certified devices do have this capability. Manufacturers should be challenged as to whether they have data proving metals are not just retained temporarily. Many simple chemically active filters only retain temporarily. Many solid separation devices will release metals under low flow winter conditions. Disastrous consequences can result for the receiving waters.
4. Is it proven that the PTD can cope with physical washout of sediments?
Where TSS is retained, it should not be capable of being remobilised by peak or exceedance flows above the design treatment flow rate. By-passes may operate in this case. This would be the case with some green infrastructure elements too, something which is not often critically reviewed or understood within green infrastructure designs. But some basic designs risk allowing all retained sediments to be “washed out” during large storm events. This is true of many basic UK termed “silt traps”, many of which will contain no significant amount of silt particles whatsoever.
5. Beware particle separator PTD claiming to remove “silts” without certification. Challenge them on what the actual grain sizes involved in their treatment processes and performance assessments are.
It is useful for designers to have an understanding of particle sizes when considering the performance of devices. Many countries are increasingly focussing just on the particles finer than sand, i.e. the silts and clays. This is because most of the mass of metal contamination is bound to the readily available exchange sites on the finer mineral particles. These finer particles are also the most bio-available and ingestible. Larger particles of minerals do not have the number of exchange sites at their particle surface, per gramme of TSS.
| Particle Type | Size (mm) | Size (microns) | Pollutant Carrying Potential |
|---|---|---|---|
| Medium GRAVEL | 6 – 20 | 6000 to 20,000 | None |
| Fine GRAVEL | 2 – 6 | 2000 to 6,000 | None |
| Coarse SAND | 0.6 – 2 | 600 to 2000 | Low |
| Medium SAND | 0.2 – 0.6 | 200 to 600 | Low to Medium |
| Fine SAND | 0.06 – 0.2 | 60 to 200 | Low to Medium |
| Coarse SILT | 0.02 – 0.06 | 20 to 60 | Medium to High |
| Medium SILT | 0.006 – 0.02 | 6 60 20 | High |
| Fine SILT | 0.002 – 0.006 | 2 to 6 | High |
| CLAY | < 0.002 | <2 | Very High |
6. Some PTD manufacturers claim TSS removal down to sub 64 microns, or even finer sizes, but state no blocking occurs. They should be asked to explain where have the retained particles gone.
The UK still has a few devices on sale that either can’t be maintained or result in the product being disposed of frequently once blocked, e.g. expanded plastic/polystyrene pieces in a bag. In some cases manufacturers state their PTD never blocks up but they may not then be able to explain where or how the retained TSS has in fact been retained.
7. Can the PTD be safely maintained, evacuated or backwashed? How can maintenance be carried out to remove the retained particles from the environment?
It is clearly common sense to only deploy PTD that, having stored sediments in a chamber where they cannot be washed out, can easily and economically be removed by normal techniques such as gulley suckers etc.
3P Postion Paper
Contact us for more information about UK SuDS Manual Mitigation Indices for the 3P HydroSystem.
References:
- The SuDS Manual C753. CIRIA. 2015.
- Plant for treatment of rainfall runoff containing mineral oil for infiltration 3P Hydrosystem heavy traffic. Z-84.2-4. 11 May 2010, DiBT.
- C. Dierkes, A. Schumann and G. Hyett, Performance of an innovative treatment device for runoff from roads with high traffic densities. 11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008.
- Tromp et al, Retention of heavy metals and poly-aromatic hydrocarbons from road water in a constructed wetland and the effect of de-icing. Journal of Hazardous Materials 203– 204 (2012).
- M. Backstrom, S. Karlsson, L. Backman, L. Folkeson, B. Lind, Mobilisation of heavy metals by de-icing salts in a roadside environment, Water Research 38 (2004) 720–732.