Task 2 – Literature review of environmental and ecological impacts
August 2008

Project partners: SNIFFER, SEPA, NIEA, Scottish Government, Forestry Commission, Health Protection Scotland

Use of the report
The technical report has been developed through a collaborative project, managed and facilitated by SNIFFER and has involved the members and partners. It provides background information, within the confines of the project brief, to support and inform member organisations and others.
Whilst the document is considered to represent the best available scientific information and expert opinion available to the consultant at the stage of completion of the report, within the confines of the specification given, it does not represent the final or policy positions of SNIFFER or any of its partner agencies, and it recognises that the historic practices regarding sewage sludge recycling  discussed are not current practice within the UK.

Background to research

Although there is a great deal of research and scientific data on sewage sludge application to land, much of this relates to modest application rates on agricultural land.  In recent years there has been a substantial increase in the amount applied to forestry, and to former opencast coal sites in the UK for purposes of land restoration.  Application rates of sewage sludge have been considerably higher than traditionally practiced, and sewage sludge has been applied using different techniques.  There is concern that poorly managed practices could result in risks to human health, water, air and soil quality and to biodiversity.  Public and political interest is high and this project will address the urgent need to review this activity and develop decision support systems and guidelines to ensure that the activities will not affect public health or adversely affect the environment.

Objectives of research

In detail, the project aims to:
The objective of the report is to fulfil the first aim in relation to the environmental and ecological effects of spreading sewage sludge on non-agricultural land.

Key findings and recommendations

The majority of the available literature and research identified relates to the application of sewage sludge to agricultural soils and due to significant difference in the soils, the sewage sludge, application techniques and rates involved, it is often difficult to extrapolate these findings to the use of sewage sludge in forestry and land reclamation.

The application of sewage sludge improves the physical properties and fertility of soil.  The potential impact on soil physical properties is dependent on the original texture and organic matter content of the soil and improvements in structure are not linear with application rate.  

There is potential for negative impacts on soil physical properties due to the blockage of pores by the sewage sludge itself or the associated microbial growth, and the potential formation of crusts.

Although a number of the pollutants within sewage sludge have the potential to accumulate within the soil there is no evidence of direct impact on soil physical properties.  However, soil quality may be affected indirectly through negative impact on soil organisms.

Improvements in the soil physical structure associated with sewage sludge amendments can decrease soil leaching through reduction of preferential transportation routes, and reduce runoff and sediment loss through improved infiltration and structural stability.

Leaching of sewage sludge applied pollutants to groundwater, except nitrogen, appears to be limited.  However, the depths of incorporation involved at some land reclamation and forestry sites requires detailed consideration of the depth to groundwater and soil properties below the incorporation depth.

For the majority of pollutants considered there appears to be greater potential for transport to surface water, dissolved in runoff or attached to particles carried in runoff or by wind, than to ground water via leaching.

Application of sewage sludge often causes an increase in the soil population, its activity and its mineralization capacity, with the level of response being dependent on the original composition of the soil.  However, application of sewage sludge at high rates which contain significant concentrations of certain pollutants has been shown to have a negative impact on soil biodiversity and its functioning.

Application of sewage sludge can have positive impacts on biodiversity of fauna and flora through addressing potential deficiencies in soil fertility, plants and animals.  Excess levels of pollutants have however been shown to negatively impact the biodiversity of terrestrial and aquatic flora and fauna. The potential for bioaccumulation through the food chain of pollutants applied to the soil via sewage sludge application for forestry and land reclamation appears to be minimal.

The implications of pollutants associated with sewage sludge on biodiversity requires further research, especially in relation to potential additive toxic effects through interaction of components within sewage sludge and long term exposure impacts

The literature shows that the application of sewage sludge to land can lead to the emission of ammonia and the major greenhouse gases, carbon dioxide, methane and nitrous oxide.  In comparison to other outlets for the disposal of sewage sludge such as combustion the application of sewage sludge in land restoration and forestry may represent a carbon sink.

There is potential for the other components of sewage sludge (e.g. heavy metals and organics) to be lost from the soil to air via volatilisation, however it is believed the impact on air quality should be minimal compared to background levels present in the air.

These conclusions are based on the rates and method of application utilised in agriculture further research is required in relation to the rates and application methods associated with land reclamation and forestry.

Sewage sludge application in land reclamation and forestry can be appropriate and beneficial, provided appropriate guidelines are followed to minimise environmental and ecological damage and maximise the potential benefit.  The application of sewage sludge to land may require reconsideration at a later date in light of any new information regarding potential environmental and in particular ecological impacts of the constituents of sewage sludge and/or sewage sludge as a whole.

Key words: Sewage sludge, Environment, Forestry, Land Reclamation, Biodiversity

Copies of this report are available from the Foundation, in electronic format on CDRom at 20.00 + VAT or hard copy at 35.00, less 20% to FWR members.

N.B. The report is available for download from the SNIFFER Website 

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