Severn uses glucose injections to reduce nitrate levels

The experiment, carried out in conjunction with the Environment Agency and the Drinking Water Inspectorate, was designed to test and model the effects of injecting glucose into an aquifer. Glucose is thought to speed up the natural denitrification process, by boosting the population of denitrifying bacteria.

According to Dr Elise Cartmell, project manager at the WRc, the technique is safe: “In our nine-month trial, the use of a glucose solution with a concentration of 40mgC/l was found to reduce the concentration of nitrates significantly, by 20-25mg/l as N. Some undesirable intermediaries such as nitrites and nitrous oxides were formed as a result of the denitrification process, but these can be kept to a minimum as long as the amount of solution used is carefully controlled.”

Under the Drinking Water Supply (Water Quality) Regulations (1989), set in line with European legislation, nitrate levels in public water supplies must not exceed 50mg/l (NO3), or 11.3mg/l as N.

Dr Cartmell said: “Some water companies with a nitrate pollution problem have stayed with the traditional route of improving removal at the treatment works, such as Anglian Water. However, this technique could be used to ensure that nitrate levels do not exceed current limits before the water reaches the works.”

In many aquifers in southern England, nitrate levels are increasing at a rate where the limit will soon be exceeded, dramatically increasing water treatment costs. To try and reverse this trend, Wessex Water has announced plans to pay farmers up to 40/ha/yr to convert to organic farming (see Water & Waste Treatment June 1999, p 6).

However, even if the use of nitrate-rich fertilisers is reduced, nitrate levels could still increase, as Dr Cartmell explained: “It is difficult to prevent nitrates leaching from the soil once applied, and in some areas, the travel time from the soil to the aquifer can be as much as 50 years.”

The latest research was carried out by the WRc on sandstone cores taken from a landfill site at Burntstump, owned by Hansons Waste Management. Hansons provided a grant for the project under the landfill tax credit scheme. Severn Trent is now planning to use the technique at its Newport borehole, which like Burntstump, is located above a sandstone aquifer.

Dr Cartmell said: “Sandstone is probably better suited to this method of treatment than chalk, because there tend to be fewer fissures, so the water moves slowly enough for the technique to be effective.”

The technique has already been used to treat groundwater supplies in the USA and France. French scientists have, according to Dr Cartmell, tried injecting glucose into chalk aquifers, but this has not yet been tried on a large-scale in the UK.