DAF – A Cost-efficient Approach for Hydrocarbons on Brownfield Sites
13 November 2018
Siltbuster has saved Costain approximately £17,000 per week by changing the way it handles hydrocarbon contamination at the former Nine Elms gaswork site on the South Bank, London. Showcasing a new approach, Siltbuster has reimagined how the construction sector should be working on remediation projects
Costain had been subcontracted to install drainage throughout the redevelopment site, 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.
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 was tasked with reducing the load on these activated carbon vessels, minimising the overall operational costs. The Siltbuster team recommended a primary DAF treatment system upstream, to maximise the removal of contaminants earlier on in the process.
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. The DAF creates ‘white-water’ by dissolving air under pressure. As the pressure is released when the water is returned back into the DAF unit, micro-fine air bubbles are formed, which attach themselves onto the contaminants and float them off as a waste sludge. 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.
Francesca Loader, Costain Site Engineer, comments: “We were having a number of issues with poor effluent quality. This was causing us programme delays because we had to keep stopping the works. We tried several methods prior to the DAF but it was only once this was installed that the quality of the water improved dramatically allowing us to complete our works. The hire cost of the DAF unit was lower than the cost of swapping out the GAC vessels, especially as having this in place meant no downtime was required.”
Chris Radegonde, Technical Sales Engineer at Siltbuster, concludes: “The removal of hydrocarbons is a key problem on old gaswork sites and a real area where contractors can save money if they think a little differently. People often think that the implementation of a higher performance system upstream is going to be a large expense, and that it is better to keep changing the carbon vessels instead. This is simply not the case. Taking into account the cost of the carbon, and how often it needs to be replaced, it makes commercial sense to reduce the load on them as much as possible.
“The staged treatment of gasworks related contamination upstream of the carbon vessels easily pays for itself by reducing carbon consumption. Contractors who utilise this more efficient process will maximise their competitiveness, becoming a more cost effective contractor for developers to work with.”
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