Addressing sludge concerns

Three UKWIR projects evaluate the risks of contaminants in sludge

Project A: Priority hazardous substances, trace organics and diffuse pollution (Water Framework Directive) WW17C.

From May 2002 to March 2004, UKWIR will invest £330,000 in project reference WW17C. This study will provide technical and financial information to assist in planning for the Water Framework Directive (WFD) and, in particular, the 'initial characterisation, pressures and economic analysis' which is scheduled for completion in 2004.

The objectives are to identify sources and routes into WwTWs of a range of priority substances and to determine the efficiency or ability of WwTWs to treat them. Approximately 50 chemical substances will be studied in detail. Selection has been based on legislation linked directly or indirectly to the WFD. The substances included are the WFD priority substances and priority hazardous substances, substances subject to statutory control under the Dangerous Substances Directive and those included in the draft sewage sludge directive. The latter are included owing to the strong interrelationship between the concentrations of substances in effluent and sludge. The sludge pathway will be included in fate studies and considered in assessment of treatment options.

Awaiting results

The techniques being used are a literature review of over 2,000 peer-reviewed papers and reports, fate modelling studies and a field study involving sampling and chemical analysis of crude sewage, final effluent and sludge from a range of WwTWs catchments. The first year report, due in May 2003, will compare and contrast the findings from these three activities.

Figure 1 illustrates the sources and pathways of pollutants to sewers and environmental receptors. Point source discharges from industry are controlled by water utilities via trade effluent consents and thus diffuse sources are increasingly identified as the most significant of substances covered by the WFD. Substances entering WwTWs from diffuse sources such as runoff, domestic, commercial, institutional and the service industry are not consented and vary considerably in magnitude. They are, therefore, difficult to isolate, quantify and regulate. In addition, sewerage systems are readily accessible with numerous connections on private property and via manholes and gullies.

A hierarchical approach has been adopted in the project to provide clarity. This approach can be illustrated by consideration of pollution from runoff. The main sources of pollutants in urban runoff entering sewers include:

  • transport and vehicle-related pollution (including vehicle washing and garages),
  • degradation of roofing and cladding materials, construction materials,
  • atmospheric deposition, amenity activities.
The sources of pollutants from transport and vehicle-related pollution include:
  • losses from vehicle lubrication systems,
  • vehicle exhaust emissions,
  • degradation of automobile tyres and brakes,
  • road maintenance,
  • accidental spillages on roads, etc.

By calculating the relative contribution from the different sources and their precise origins, the project will identify and appraise options for remedial action or treatment and make recommendations on the most sustainable solutions.

Project B: Organic contaminants in sewage sludge applied to agricultural land: A critical evaluation of the proposed limit values for organic contaminants in the EU working document on sludge and development of a screening process to identify priority pollutants in sewage sludge.

This recently published report of work, funded by water companies and undertaken by Professor Kevin Jones of Lancaster University has concluded the relevance of proposed organic contaminants standards in the third consultation document are questionable. The value of adsorbable organic halogens (AOX) as a parameter of contamination is questioned - the test covers a wide range of compounds having differing properties. It provides little, if any, meaningful information about risks posed from applying sludge to land.

The majority of sludge will not exceed the limit value for polychlorinated biphenyls (PCBs) and Professor Jones recommends TEF-rated PCBs should be considered as a part of the toxicity equivalent (TEQ) value reported with polychlorinated dibenzodioxins and dienzofurans (PCDD/Fs). This recommendation is consistent with an earlier EU review of organic contaminants which questioned the value of monitoring PCBs. PCBs have been heavily proscribed by EU directives and as a result the concentrations observed in sludges and soils has, and continues, to decline. A minority of sludges are expected to exceed the TEQ limit value and given the toxic properties of PCDD/Fs and PCBs the situation regarding TEQ should be monitored. Beyond control

While a majority of sludges are expected to exceed the proposed EU limit values for linear alkylbenzene sulphonates (LAS), nonylphenol ethoxylates (NPE) and poly aromatic hydrocarbons (PAHs), the report concludes LAS and NPE do not accumulate in the soil and are not thought to pose a problem a present levels and hence the limit values suggested are difficult to justify. Furthermore, there is implied criticism in the report of the proposed limit for PAHs. These compounds are known not to accumulate in the foodchain. Another interesting conclusion is that sewerage undertakers can do little to control the levels of these compounds in sewage sludge, since their sources are domestic, commercial and atmospheric rather than in industrial discharges.

One of the report's recommendations is that sludges are screened for polybrominated flame retardants - compounds present in plastics, particularly the surrounds of computers and similar electrical goods - which could accumulate in the food chain, and, it is suggested, work should be undertaken to determine the precise position. Overall, this report will be welcomed by the sewerage undertakers and government alike since it seriously questions the wisdom of the commission in proposing some unnecessary standards. To this extent, it is further evidence confirming the report on organic contaminants in sludge commissioned in 2001. Project C: Monitoring and control of potato rot.

A Ministry of Agriculture, Fisheries and Food (MAFF) link project on the monitoring and control of potato brown rot has recently been reported. This three-year project costing approximately £250,000 was a collaborative effort involving industry sponsors Potato Processors Association, British Potato Council, Haith Industrial, Degussa, AEC, Greenvale AP, McCain Foods (UK), Wm Morrison Supermarkets, UKWIR and the Plant Health & Seed Inspectorate at MAFF (now Department of the Environment, Transport and the Regions - DEFRA). Contractors to the project were ADAS and the Central Science Laboratory at York.

Spreading concern

The need for this project arose when brown rot was found in some potatoes and tomato crops in the south east of the country. The organism was also found to be present on woody nightshade on the banks of a limited number of watercourses. Whilst it was known the import of infected material and the subsequent washing of potatoes was the source of the infection, there was concern the organism might over-winter on woody nightshade, that irrigation of potatoes with infected river water might cause further infection of potato crops and that the disease could become endemic in the UK.

The objectives of this research were to develop suitable monitoring methods, methods of disinfection and to determine whether the sewage treatment process afforded an additional barrier to the spread of the organism, either by processing effluent discharge to watercourses or returning the solids from potato washing to agricultural land. A new test for the organism has been developed at the Central Science Laboratory which overcomes the problems of the existing test on wastewaters. The new test is specific, highly sensitive and quick. It has been shown to reliably detect low pathogen populations in potato washings and sewage effluents, even in the presence of millions of other non-pathogenic bacteria.This development ensures waters can be tested before being used for irrigation if necessary.

Trials on pilot sewage sludge treatment plant at Leeds University showed that a surrogate for Ralstonia solanacearum, Erwina carotovora, was effectively destroyed by the sludge treatment process and tests of full-scale WwTWs showed the organism was not present in sewage effluents, thus discharging of potato processing wastes to sewer provides another defensive barrier to the release of the organism to watercourses.

As part of the project, specific sets of guidelines were developed and directed to potato processors, those using irrigation water and to water companies accepting processing effluents.

Combining skills

This project demonstrated the benefits to be gained from pooling the expertise of a wide range of sponsors who were able to bring their skills to the table. Chris Rowlands of Severn Trent Water, who represented the water industry on the steering group for this project explained: "The project was extremely successful in achieving its technical objectives and tapping into the expertise of many organisations. Over and above this, it also gave such a wide and disparate range of stakeholders the opportunity to understand the pressures faced by the various organisations and extremely valuable in terms of networking opportunities. I hope the contacts made will help us in the future whenever the need arises."



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