Scientific study seeks to survey safety of sewage sludge

Heavy metals, sewage and mud may sound more like the key components of a rock festival, but are in fact the subjects of a study looking into the sustainable disposal toxin-rich sludge.

Since the 1990s, Defra has been conducting experiments to see how the poisonous heavy metals which accumulate in sewage sludge might impact on soil quality when the waste is spread on agricultural land.

Disposing of treated sludge in this way has a number of environmental benefits as it replaces valuable organic matter and avoids the need for incineration or landfilling, but there is also the potential danger of toxins contaminating land.

The tests were designed to calculate the effect of cadmium, zinc and copper in sewage sludge on micro-organisms in the soil.

The research team has now filed and interim report, which can be viewed on the Defra website, which suggests that the metals could have an impact on soil quality over the long term.

It also warns against over-egging the danger, as it points out that it would take over 100 years to reach the deliberately high concentrations of metals used in the experiment.

It also says that in practice the concentrations of metals reaching the sewage system has fallen considerably in recent years.

The long timescale over which the effects were projected means that there is good time, before there is any significant risk of damage occurring, to properly investigate the issues arising from the experiments so far.

It is important that this work is taken forward carefully to ensure that policy on sewage sludge spreading is based on sound science, said Defra.

Further work is now planned to understand the mechanisms behind the identified impacts and how these could be influenced.

Sam Bond

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