One of the first UK stormwater treatments of its kind has helped a developer ‘do the double’ on a new project.
Fairview New Homes used Hydro International’s Up-Flo filtration technology to incorporate SuDS with two levels of treatment into the surface water drainage for a new access road in Faversham, Kent.
Environment Agency planning requirements sought two levels of treatment for the commercial but a SuDS approach using completely natural features was not possible. Fairview and consultant Infrastructure Design then worked with Hydro International to develop a solution that combined a custom-modified Up‑Flo filtration unit with Hydro StormBloc modular storage and infiltration blocks.
Jim Brand, the developer’s maintenance manager, explained: “Working closely with our engineering consultants and Hydro, we were able to put together a fit-for-purpose solution which met the maintenance requirements of Kent Highways, while satisfying SuDS principles.”
The access road is needed to service retail and light commercial units as part of the mixed-use redevelopment of the Eurocentre wholesale distribution centre, on a site which is bounded by the London to Dover main railway line. While much of the access road offers natural flow to a swale for treatment, around 100 metres of the road was below surrounding land.
“We needed to provide polishing treatment to remove finely dispersed pollutants, as it was adjacent to a source protection area,” pointed out Phil Tomes of Infrastructure Design. “To create sufficient fall for a swale, we would have had to try and install it some three metres below ground level, which was not possible.
“We had to look at engineered solutions and the Hydro Up-Flo solution was ideal. The unit could be located in a chamber at sufficient depth to provide effective settlement, screening and filtration treatment for the water.”
Rainwater from the road passes initially through a bypass separator (the first level of treatment) before entering the concrete chamber containing the Up-Flo Filter, which acts as a sump where sediments are allowed to settle. Any floatables are controlled by baffles on the filter module inlet. The water flows up through the fine sand filter medium in the six filter modules until it discharges through the outlet. The Up-Flo unit thus provides the essential second level of treatment, removing coarser sediments and floatables and fine suspended solids.
“Kent Highways specified maintenance requirements before agreeing to adoption of the installation,” continued Tomes. “Kent preferred to avoid their crews having to physically enter the chamber through a manhole cover in order to change the filter medium bags in the Up-Flo unit.”
To accommodate that requirement, the consultant turned to Hydro for some input. “We responded to the need by innovating a design modification, which demonstrates the versatility of the Up-Flo Filter unit,” said Marcus Mumford of Hydro’s design team. “The modification can now be used by other Hydro customers to meet similar requests in future.
“We adapted the design by mounting the modules containing the filter bags on a platform that could be raised and lowered with a screw-operated penstock gate mechanism. The design enables the filter bags to be exchanged at approximately ground level. Maintenance intervals are currently estimated as yearly or longer.”
To meet the surface water discharge constraints for the site, the final discharge from the filtration unit has to be infiltrated into the ground. Without room for a separate soakaway, a Hydro modular construction StormBloc storage infiltration tank was used. Because it is loadbearing, it could be easily sited under public space in a shallow excavation and landscaped over. It provides around 180 cub. metres of storage attenuation and infiltration, without occupying valuable surface area.
Although used extensively in the US, the Up-Flo filter is still relatively new to the UK. It is described as having a gentle action to facilitate sediment settlement, due to the rising water flow through the filter unit system.
The drain-down system eliminates virtually all possibility of blinding and clogging. Bagged filter media from the modules is exchanged manually and a standard gully vacuum unit employed to remove sediment from the sump.
During any extreme rainfall events, a bypass system, with baffles to exclude floatables, allows a temporary overflow to discharge. As well as its availability in concrete vault design which can be upsized to meet a range of maximum flows, other Up-Flo configurations are available, including single packaged units suitable for manhole-sized installations, and separate filter units for retrofit. Filter media can also be selected to meet specific water quality and pollutant requirements.
