When draining a reservoir, large, instantaneous volumes of water can come into contact with all sorts of pollutants that have accumulated over the years, including organics, debris and solids. So, it is imperative that a downstream system is in place to capture and process waters, stopping them from reaching and damaging the natural environment. The best way to showcase this is with a real life example.
The Wentwood reservoir refurbishment
Morgan Sindall, the leading construction company, had been appointed by Welsh Water to undertake maintenance and repair works on a Victorian control tower situated within the 42-acre Wentwood Reservoir in South Wales.
The eighteenth-century valves installed within the tower had to be removed and replaced to comply with modern standards set out by the water utility companies. But owing to the original design of the structure, to do so meant fully draining the water within the reservoir. This had to be done to gain access to the base of the tower so that the redundant fittings could be mechanically removed and lifted out.
The reservoir once had a dedicated downstream water treatment house, but this had been offline and inactive for years. A temporary solution was required so that the reservoir waters could be emptied and discharged without polluting the natural environment.
Siltbuster was tasked with supplying a modular, multistage water treatment system capable of treating the peak reservoir discharge of up to 600 m3/hr. The waters contained slow settling solids in suspension which meant the solution had to incorporate an automated chemical dosing system to aggregate the solids and enhance their settlement velocity in the water.
The existing pipework which connected the reservoir to the water treatment house was bypassed and the entire flow was re-routed to the temporary Siltbuster system. So sensitive was the project that treated water monitoring sensors were installed at the outlet point and interlocked with the Siltbuster equipment to ensure that in the event of the treated water quality decreasing, actuated valves would automatically isolate the feed preventing any sediment-rich waters from escaping to the environment. Simultaneously a GSM module would send a SMS text messages to a pre-selected list of users alerting them that the system had auto-shutdown whilst a visual alarm beacon altered staff who were present onsite.
Due to the nature of the project, it was envisaged that during the drawdown a proportion of the water would be free of mineral fractions which cause the dirty/discoloured appearance of the waters. It was during these phases that no pre-treatment chemicals would be required to achieve the strict <60mg/l discharge consent. To this end, Siltbuster installed a turbidity sensor on the plant’s inlet which connected to the chemical dosing pumps. This ensured that pre-treatment chemicals (coagulants & flocculants) were only introduced when the waters required treatment whilst conserving chemical supplies and minimising consumable costs.
With the reservoir’s level now dropping and the treated water discharging safely to the surrounding environment, the material which once burdened the waters in suspension and elevated the TSS levels was captured within each of the 8No. HB50 lamella clarifier’s hoppers. A duty and standby sludge pump arrangement was installed that allowed each settlement unit to be remotely emptied. The slurry was then held and stored within a 30m³ agitated tank whilst a vacuum tank periodically emptied and disposed of the sludge offsite.
All the chemical dosing, monitoring and data logging equipment was housed within purpose built 20ft containers to prevent unauthorised access, vandalism and containment of the bulk chemical supply. These chemical dosing containers, which have been used across Hinkley Point C and in Qatar as part of the 2022 football World Cup, are complete with a triple IBC spill stand and a cut-out self-bunded floor so in the event there is a breach or spill, the chemical solution is contained.
The system treated over 500,000m³ of water and whilst the reservoir was emptied, continued to be installed downstream as a precaution whilst works on the control tower continue.
A Final Word
The important thing is that specialists are engaged with early on in the planning stages of reservoir works. This enables the right amount of preparation to happen to minimise risk to the natural environment. Right now, the spotlight is on environmentalism and sustainability, with the importance of wastewater treatment as one of the focus points.
