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Soil fertility experiments will help ensure sustainable use of sewage sludge on land

DEPARTMENT FOR ENVIRONMENT, FOOD AND RURAL AFFAIRS News Release (News Release ref : 419/07) issued by The Government News Network on 14 November 2007

An investigation into the effects on soil quality and fertility of the heavy metals present in sewage sludge has published interim findings today. The results will help inform the sustainable recycling of organic material to land to improve soil quality.

Spreading treated sewage sludge on agricultural land provides organic matter and nutrients and maintains soil carbon, as well as offering what is in many circumstances the best environmental option for recycling of sewage sludge. The alternative disposal routes are landfill or incineration, both of which waste a potentially valuable resource.

Experiments started in the mid 1990s to examine the effects of cadmium, zinc and copper in sewage sludge on soil micro-organisms. The aim was to investigate long term impacts, and the metal-rich sludge was applied at elevated rates over 4 years to establish treatments that exceeded the maximum permissible soil metal concentrations detailed in the Sludge Use in Agriculture Regulations. In operational practice, it would take longer than a 100 year period to reach these concentrations.

Headline results show that under certain conditions metal-rich sludge could impact upon some microbial groups, with implications for soil quality over the long term. The significance of the results will be considered in light of several factors, including:

* The intensity of the sludge application in the experiments would not happen in the field, and the results need to be interpreted with caution and in light of modern practice;

* The amount of metals entering the sewerage system has declined sharply in recent years, and the sludges used in the experiments do not represent current quality. The results therefore represent the potential impacts under a worst-case scenario; and

* 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.

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

The interim results are published at:
http://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&ProjectID=10677&FromSearch=Y&Publisher=1&SearchText=SP0130&SortString=ProjectCode&SortOrder=Asc&Paging=10#Description

Notes to editors

1. A Review by an Independent Scientific Committee on the Rules for Sewage Sludge Application to Agricultural Land - Soil Fertility aspects of potentially toxic elements (MAFF, 1993) recommended that "further research was needed to examine the effects of heavy metals from sewage sludge on soil micro-organisms."

2. The experiment was set up in 1994 to determine the effects of sewage sludge additions to land on soil microbial populations and on soil function. Soil micro-organisms are the portion of soil responsible for the breakdown of organic materials and nutrient cycling. The trials have no relevance for health and safety aspects of sewage sludge recycling.

3. The Defra research project was co-funded by the Scottish Executive, Welsh Assembly Government, Environment Agency and UK Water Industry Research.

4. The regulations are: The Sludge (Use in Agriculture) Regulations 1989 (and the 1990 amendments thereto). These are supported by a departmental Code of Practice for Agricultural Use of Sewage Sludge (latest version 1996), which recommends a tighter soil limit for zinc.

5. Since the trials began, changes in industrial practices and strict controls at source have led to considerable reductions in the concentrations of metals found in sludge from sewage works. The main inputs of metals now come from diffuse sources such as run-off and household use rather than industrial sources.

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