Stormwater run-off has been underestimated as a source of pollutants - and it is not just water companies who are responsible for river water quality. As the Water Framework Directive requires a more integrated approach, Chris Williams of Hydro International discusses a new low-cost solution to sediment control.
The overriding requirement being that all rivers achieve "good" status by 2015.
Also, there is a common consensus that this be achieved by a combined approach of:
- Quality objective setting
- Dest control through best available techniques
Much of the river and aquifer quality degradation can be attributed to diffuse pollution deriving from roads where their drainage infrastructure either directly connects to a receiving watercourse or discharges to an aquifer through percolation.
Removing pollutants at the earliest stage in the treatment process offers advantages for cost and water quality in the same way that near-source stormwater attenuation offers a best-value solution to flood water management.
Combining storage, control and treatment also provides a "treatment-train" approach advocated by national sustainable drainage manuals.
Recent work in the US has examined the volume and type of pollutants in stormwater, and developing a small-footprint, low-maintenance filtration solution that can be targeted at identified sources of contamination.
The technology could offer a solution to meeting the WFD requirements for controlling pollution as close as possible to its point of entry into the drainage system.
Achieving control of the particulate matter and sediments in stormwater run-off has been identified as critical where the run-off source has a high potential pollutant load. Industrial locations, roads, car parks, service stations, public works storage and waste-recycling areas pose the greatest risk.
Settlement ponds to control the sediments would be impractical in most of these locations. Even if space could be found, the cost of the land and maintenance would be prohibitive.
Pollutants entering the stormwater system due to surface run-off are many and various. Monitoring in the US has identified some of these pollutants as suspended and other solids, phosphates, ammonia and nitrates, phenols, aluminium, cadmium, copper, chromium, lead and zinc. Some of which are included in the list of priority substances that form Annex X of the WFD.
The sources of contamination can be complex. Pollutants can be incorporated by several different mechanisms. They can be collected from the atmosphere by rain or as wind-transported dust fall subsequently washed from the surface as run-off. One source of zinc is from vehicle tyres.
Since these studies, lead has been largely removed from petrol and the concentrations have dropped markedly. Metals and particularly heavy metals are still generally of high interest because of their persistence and possible toxicity when accumulated in ecological systems.
Work published in 2005 demonstrated the association of metal pollutants with particles in urban run-off. And it looked at the distribution of metal concentration with particle size, as well as the presence of metals in dissolved and colloidal bound forms.
Overall, this showed that filtration of particles from the stormwater samples, even at relatively large sizes (20µm) removed many important pollutants such as total phosphates, most heavy metals such as lead, copper and cadmium, also zinc, but was less effective in removing nitrates.
Comments on the use of settlement ponds for this run-off indicated that small ponds could be expected to remove particles down to 20µm while larger ponds could achieve 5µm or a little better. The disadvantage of ponds is the large land area needed to achieve high-level particulate removal.
New technology initially developed under the US Environment Protection Agency's Small Business Innovation Research Programme and further developed by Hydro International is demonstrating that a small footprint filtration device can provide effective filtration and treatment for specific classes of stormwater pollutant.
The device, known as the Up-Flo Filter, has been field-tested as well as undergoing full-scale laboratory trials, and has demonstrated "excellent correlation" between the two types of trial.
The technology is a low-cost solution to near-source sediment and pollution control. It can combine course screening, sedimentation and high-rate filtration in a single device, offering an entire treatment train approach.
The design enables a choice of different filtration media to be used appropriate to local pollution conditions. It has low head loss and operates without power. The system of bagged filtration media ensures minimum maintenance, the sump can be cleaned of floatables and sediment with a standard gully cleaner so whole life costs are low.
The advantages are that the whole area of the filter and medium is used for more effective filtration than down-flow filtration, and the longer life of the media reduces costs.
Also, the high filtration rate per square metre enables high flow rates, and thus lower footprint than conventional pond or vegetative treatment systems. This allows development areas to be maximised without compromising the environment.
The drain-down water level washes pollutants out of the filter media into a sump, preventing blinding and improving filter life for reduced maintenance. And it allows the filter media to dry out between storm events, which improves the effectiveness and longevity of the system.
The field and laboratory trials show that particles are effectively removed, with at least 50% of the particles in the 1-10µm range removed, and 95% in the 30-250µm segment.
Now low-cost, high-rate filtration separation technology offers a solution that meets the combined approach requirements of the WFD, in a footprint little bigger than a conventional manhole and gully pot; and variable in size to fit the flow regime.
It can take the benefits of a source control approach with relatively low-cost modification to the existing sewerage infrastructure, and make it more tailored to local problems.
By combining Up-Flo's better and more economical filtration of a range of particulate matter and associated pollutants, with vortex control provided by the Hydro-Brake Flow Control, full use can also be made of the storage capacity of the existing infrastructure.
This could save millions in new plant and improve river or aquifer quality to meet the requirements of the WFD, within a short time frame.
Chris Williams is managing director (Europe) of Hydro International. T: 01275 878371