Costain had been subcontracted to install drainage throughout the Nine Elms gaswork redevelopment site on the South Bank London, which is being cleared as part of the plan to deliver 20,000 new homes by 2022. However historical contamination meant that it was polluted by petroleum hydrocarbons and the water generated by excavation work needed to be treated before it could be released to sewer.
The Overall Objective
Costain was using a gravity based hydrocarbon and solids removal system. However, gaswork hydrocarbon contamination is quite complex, with some of the hydrocarbons floating, some sinking and some remaining near neutral buoyancy. Gravity based systems therefore only remove some of the contaminants. This put a huge pressure on the expensive granular activated carbon (GAC) vessels in the last part of the system, which had to remove all of the remaining contaminants from the water.
As a result, the carbon vessels were prematurely failing. Each vessel holds two tonnes of carbon, costing £1,500 per tonne to replenish. The costs were mounting up with the carbon needing to be replaced every 15 days on average.
Siltbuster mobilised one of its D20 DAF units, employing it within the existing treatment set up. The installed system incorporates a 2-stage chemical dosing process adding and mixing a coagulant, followed by a flocculant to aggregate the contaminants into larger particles. This use of the DAF allowed for a much greater removal efficiency of the contaminants, reducing the load on the carbon vessels, ultimately saving Costain money.