Dr Richard Coulton of Siltbuster outlines the main types of on-site water pollution, and how to treat them.
Under the Environment Agency’s Regulatory Policy Statement (RPS), construction companies likely to discharge anything off site other than clean water need a Bespoke Discharge Permit from the Environment Agency.
This comes in the context of the Environment Agency recently prosecuting a Leeds-based house building company £120,000 for illegally polluting a watercourse, by not controlling silty run-off.
It should be noted that the new sentencing guidelines remove any economic benefit derived by committing the offence, always making it cheaper to prevent the pollution incident. “It should not be cheaper to offend than to take appropriate precautions” Judge Francis Sheridan said at the trial of Thames Water, fining them £20m for polluting the river.
Furthermore, all companies – even those that don’t need such a permit – must have a site water management plan in place prior to starting any work. That plan needs to show how they will minimise and deal with any problem waters generated during the construction process. The most common sources of onsite water pollution are silty waters, oil and concrete wash water.
Around 40 per cent of construction industry water pollution involves silt-laden water run-off. When a construction site strips the topsoil, it takes away the land’s strongest defence against erosion, vegetation. The resultant surface has no protection from rainfall and run-off, so silty water enters drains and watercourses, potentially blocking the gills of fish and smothering
If silty water does arise it needs dealing with – and with expensive tankering costs, people must master on-site treatment techniques. Lagoons or settlement tanks are possible solutions – but these take up a lot of space. The smarter option is using lamella clarifiers, which are up to 20 times more space efficient. They use gravity to settle the particles out of the silty water, making it safe to then be released. If the waters have very fine slow settling particles, such as clays, a chemical dosing stage can be added to aggregate the particles and increase settling rates.
Hydrocarbons are commonly encountered on construction sites in the form of petroleum- based substances such as petrol, diesel, kerosene and oils.
To minimise this pollution, oil spill kits and refuelling vehicles can be provided in desig- nated areas. However, in certain cases, such as remediation of contaminated sites, oil polluted water may be hard to avoid. In such cases an oil water separator will be needed to remove free floating oil and light non-aqueous phase liquids from water through separation by flotation.
Concrete wash water
On a volume by volume basis concrete wash water has the potential to cause much harm as it can’t be easily diluted. This is because the pH of concrete wash water is incredibly high – typically 12 to 13, which is the equivalent of oven cleaner.
This highly polluting water needs to have its suspended solids removed and its pH reduced to an acceptable value. The solids removal is relatively straight forward, however choosing the best pH adjustment method is less clear-cut.
Dilution isn’t practical or cost effective, while mineral acids are dangerous to handle and can easily overshoot the target resulting in acidic water. Citric acid is also easy to overdose, and it increases the water’s ‘biochemical oxygen demand’ above acceptable limits.
Carbon dioxide is by far the best neutralising agent. It’s virtually impossible to acidify water using CO2, it has no hazardous by-products, is easy to store and is the most cost-effective.