Water and Effluent Treatment Magazine
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Where pollutant loads are high, receiv- ing environments are very sensitive, or space is at a pre- mium, they will reliably remove pollutants and protect the envi- ronment and downstream vege- tative features and habitats. 14 WET NEWS OctOber 2017 Giving protection to environ- mental water quality A cross the UK, there has been some welcome pro- gress in implementing Sustainable Drainage Systems (SuDS) to manage flood risk. But when it comes to water quality, there are real dangers that inad- equate drainage schemes may be storing up future problems in the very environments they are designed to protect. Led by the CIRIA's C753, The SuDS Manual, more well- designed management trains are now being included in best- practice drainage schemes that combine a series of components to slow the flow, reduce the vol- ume of runoff and protect against flood risk across a catch- ment. Yet, while good practice in flood risk management moves forward, it is important to keep full sight of the water quality role of SuDS and their potential to deliver repeatable, maintain- able protection especially on sites where there is a higher risk of pollution. Robust legislation has driven the implementation of SuDS for water quality in Scotland while in England, the government's decision to encourage SuDS on major new development via the National Planning Policy Frame- work provides little prescription for water quality improvements. In England and Wales, Envi- ronmental Permitting Regula- tions give the regulators powers to require a permit where there is a higher risk of water or groundwater pollution. However, they are being when the run-off is passed directly into vegetative SuDS devices such as ponds, basins, swales and reed beds, the met- als will accumulate and may infiltrate to ground. Why design a vegetative treatment device to be a habitat for wildlife and then contaminate it with toxic metals and hydrocarbons? Great design can protect hab- itats from gross pollution and improve water quality while delivering amenity and biodi- versity benefits to the local com- munity. Especially for high risk sites, a management train of treatment devices must work in series to manage the pollutants effectively and to keep persis- tent bioaccumulative chemicals out of the environment. The SuDS Manual includes comprehensive guidance for drainage designers on assessing and managing water quality and it promotes the use of a Simple Index Approach for low and medium risk sites. The manual also describes the level of pollution risk for each type of development and assigns a Pol- lution Hazard Index for sus- pended solids, toxic metals and hydrocarbons. SuDS design must include adequate treat- ment stages to deliver a Mitiga- tion Index equal to or greater than the Hazard Index. On higher risk sites, a more complex risk assessment must also be completed, especially where groundwater is at risk. The manual also includes details of manufactured devices. The installation of a manu- factured treatment device, such as a hydrodynamic separator, upstream of a pond, basin or wetland, will remove and retain the sediment-bound pollutants. The downstream habitat is shielded from hazardous pollut- ants, and build-up of solids in the bottom of a pond or basin is prevented. The design capacity of the pond or basin is not com- promised by the build-up of sol- ids and its ability to attenuate stormwater is protected. Manufactured devices give reassurance of measured, pre- dictable and independently-ver- ified performance. Maintenance plans can be set and budgets planned according to manufac- turer recommendations. A hydrodynamic separator can capture several tonnes of pol- luted sediment which is then easily removed, every year or two, by a vacuum tanker. Without such a device, if the solids are allowed to pass for- ward to settle out in the pond or basin, the maintenance interval is unknown, the cost can be high, hard to budget for and require specialist service provid- ers. A hydrodynamic vortex sep- arator such as the SDS Aqua- Swirl is the ideal choice to protect a vegetative device and remove gross pollution where high pollutant loads are expected. A management train including an Aqua-Swirl and a pond will deliver excellent water quality improvements, flow con- trol and biodiversity benefits. Manufactured SuDS devices provide environmental water quality protection that can shield vegetative features on high-risk sites "It simply is not good enough to design a SuDS scheme with discharges to rivers and streams, or to ground where the pollutants will accumulate in the soil and, in time, migrate down to the underlying groundwater" Jo Bradley, SDS inconsistently enforced for high- way outfalls. Regulators mostly rely on voluntary improvements and, while it is encouraging that treatment schemes are being introduced on the highest risk outfalls without permits, such interventions are limited and inconsistent and little monitor- ing is taking place to see if they are adequate. For some developments, pro- viding there is sufficient space for them, vegetative SuDS devices work well to break down pollutants and the water can safely be passed into groundwa- ter or rivers and streams. However, on high risk sites – motorways and trunk roads, haulage distribution centres, airports or recycling centres – the pollutant load in the run-off is higher, the nature of the pol- lutants is more threatening and groundwater is at risk. Equally, some receiving environments may be particularly sensitive to water-borne pollution and in need of special protection. In these cases, it simply is not good enough to design a SuDS scheme with discharges to rivers and streams, or to ground where the pollutants will accu- mulate in the soil and, in time, migrate down to the underlying groundwater. The pollutants of most concern are toxic metals such as copper and zinc which cannot biodegrade and which are contained in the run-off from many high risk sites. The pollutants are attached to silts and sediments and, • INSIGHT Flood risk management Pollutant load in run-off is higher from high risk sites, but a hydrodynamic vortex separator can offer protection, says SDS' Jo bradley.