Magnox Takes Second Siltbuster pH Adjustment Unit

25 Jul 2013

Siltbuster Ltd has just sold a second of its patented PMPU20 pH adjustment units to Magnox, the management and operations contractor responsible for 10 nuclear sites and one hydroelectric plant in the UK. The two units are being used to reduce the pH level and soften the water emanating from flooded basements that have been backfilled with crushed concrete. The first unit was purchased by Magnox North and deployed at the former Chapel Cross Nuclear Power Station in 2010. The second unit has just been purchased by Magnox Southern, following successful deployment of a hired PMPU20 at the former Bradwell Nuclear Power Station.

Commenting on the sale, Siltbuster’s Regional Engineer Kristian Downs said: “we have sold a dozen of these units over recent months, reflecting a growing trend for treating concrete washwater with CO2 rather than the more traditional method of using mineral acids.”

Whilst crushed concrete is inert when placed above the water table, if below the water table, lime exposed on the surface of the concrete particles dissolves, raising the pH of the water and increasing its hardness, due to the calcium in the lime. “We have seen pH values as high as 13.6 in the water seeping out of saturated crushed concrete backfills – similar to that of domestic oven cleaner, and look at the health warnings that come with that!”

Traditionally high pH waters have been treated using strong mineral acids, such as hydrochloric or sulphuric acid. However this is difficult and dangerous to handle. Unless the dosing is carefully controlled, it can result in acidic water being discharged instead. From an environmental perspective, this is as bad as not treating the water in the first place.

He continued: “Our PMPU20 units use carbon dioxide for pH adjustment instead. The big advantages are that our units are readily available across the UK, are easy to handle on site and ensure that the pH of the treated water does not drop below 6 (the typical lower limit set for safe discharge to the environment) and cannot drop below 5.8). Adding CO2 can increase the suspended solids concentration in the treated water, but our system addresses this by not only reducing the pH of the water but also softening it, by removing any excess calcium present. To do this we carry out the pH adjustment in two steps, separated by an intermediate clarification stage. It looks a bit like the ‘Push Me Pull U’ from Doctor Do Little, hence the unit’s name PMPU.”

The unit initially uses carbon dioxide to reduce the pH to around 12 through an automated control system, maximising the precipitation of calcium carbonate from solution. This is then removed by the integrated clarifier, before a second carbon dioxide pH adjustment stage is used to further reduce the pH to a level suitable for discharge, typically pH 7 neutral. This second pH adjustment stage also converts any residual calcium carbonate solids into dissolved calcium bi-carbonate, resulting in excellent treated water quality, containing little or no suspended solids.

Siltbuster, whose various treatment systems are used throughout the construction industry, regularly comes across this problem where crushed concrete has been used below the water table or on sealed piling mats/working platforms due to rain water infiltration. It has also encountered it in runoff from lime stabilised soil.

“Contractors don’t always realise that whilst the runoff from these sites may look crystal clear, the elevated pH means that the water is highly toxic to aquatic life,” adds Downs. “We’re extremely pleased that using our range of PMPU single skid mounted systems is becoming regarded as best practice across the industry.”

Siltbuster Ltd is the leading supplier of water treatment solutions to the construction industry.

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