New kit to ensure compliance
Paul Fisher explains how a new immunodiagnostic testing kit will help the industry comply with future sewage sludge regulations
Application of sewage sludge to agricultural land offers an immediate win-win scenario. Sludge producers, such as water utilities, gain from a low-cost recycling route, while farmers receive consistent, economic supplies of organic fertiliser. Environmental policy has encouraged this practice in preference to landfill or incineration, which have environmental drawbacks, hence the EU's Sewage Sludge Directive 86/278/EEC. This is implemented through the Sludge (Use in Agriculture) Regulations (1989) in the UK and is currently under governmental review. Although sludge is a useful soil conditioner, supplying valuable organic matter and nutrients, sludge can also contain potentially harmful substances, including pathogens, leading to moves to place further controls on its use.
According to Richard Smith, senior process engineer for Severn Trent Water, tightening of statutory controls on the agricultural use of sludge has been the subject of wide consultation and review, and changes to the regulations are expected to come into force during 2004/2005. The review is expected to introduce a set of microbiological standards for sewage sludge, with maximum allowable concentrations of viable pathogens and log reductions across treatment processes. "In the case of the pathogenic bacteria E.Coli and Salmonella a maximum concentration or viable cell count will be set for treated and enhanced treated sewage sludge," he explained, "placing the emphasis for compliance testing on water companies."
An estimated 10,000 pathogen tests are likely to be necessary across the UK each year. Crucially, however accurate, enough tests are not available for this purpose. Existing testing methods rely on microbiological techniques developed originally for assessing water samples. Although tried-and-tested, this approach is does not provide accurate viable bacterial count data due to the high levels of particulate matter contained in sewage sludge, which make accurate assessment difficult. To overcome these problems Edinburgh-based biotechnology company MicroScience Technologies (MST), a leading industrial biotechnology demonstrator project funded by the DTI's Bio-Wise programme (www.dti.gov.uk/biowise), is developing a novel and commercially viable immunodiagnostic testing kit.
The kit provides an accurate, fast and low-cost means of detecting and counting E.Coli and Salmonella sp. present a sewage sample, thereby providing the clarification necessary to ensure sludge applications to agricultural land comply with impending legislation. Used routinely in clinical and food quality diagnostic operations, MST is working towards transferring the technology to the water sector.
At the heart of the testing kit are manufactured biological molecules called monoclonal antibodies (MAbs). Specific particular MAbs recognise and bind tightly to specific micro-organisms. "One antibody equals one bacteria present, thus forming the basis for detection and counting," explained MST's director of science, Doctor Chris Oldsfield. The kit consists of a graduated plastic sample tube, which contains a pair of mounted square-mesh 'grids', about the size of a ten pence piece, coated with MAbs that recognise specific micro-organisms. Workers taking samples need no special training. The tube just needs shaking and any E.Coli and Salmonella present will bind completely and irreversibly to the antibodies. He added: "The grids are then developed, like photographs, in the laboratory using a fluorescent dye, allowing individual bacteria to be counted using an optical scanner. Grid manufacture involves 'printing' antibodies onto the grid using inkjet printer technology. This helps to keep production costs low and will aid eventual commercialisation. Although the science is very exciting, the focus has been on building a commercial product that will provide a necessary advance in bacteria testing."
MST is partnered in the project by Severn Trent Water, which will be benchmarking the kit under field conditions this autumn. Severn Trent Water produces 160,000 tonnes of dry solid sludge each year. Around 60% of this total is applied to agricultural land, making the company a potential end-user of the eventual technology.
Smith pointed out it is hoped the greater accuracy and improved lab turn-around from this potential new testing approach could have major implications for the water industry. "As a major user of analytical services, any efficiencies gained can improve our business performance significantly. Existing treatment of raw sewage, such as mesophillic anaerobic digestion, already ensures minimal microbial activity in sludge applied to agricultural land. However, under the new regulations we will still need to verify we are compliant."
Severn Trent Water will test prototype kit this autumn under a range of field conditions where performance across a range of different sludge types will be assessed. Smith concluded: "For example, consistency and bacteria load varies significantly between raw and treated sludge. Also the influence of pH on the performance of these kits is an area that requires in-situ validation, especially in situations where quicklime has been used. If ease-of-use in both the field and laboratory are positive and performance meets expectation, then MST's technology is likely to be adopted widely. Businesses will be able to comply more easily at an acceptable cost in the new regulatory environment."