Embracing the natural cycle
Colin Brade and Aidan Cumiskey of Monsal look at the positive benefits of using biosolids in agriculture and why the water industry should do more to promote organic recycling
The alternatives to beneficial recycling are the waste disposal routes of incineration, gasification and landfill. These pose many environmental, technical and public acceptance problems and do not contribute to sustainable development. Despite all of this, over the last few years, the long-term viability of the agricultural route has been questioned and the resulting uncertainty poses a major risk to the water utilities in their biosolids recycling operations and associated sludge treatment processes. This in turn has a negative effect on investment in treatment technology by water utilities and sludge technology companies who need to have an opportunity to realise a return on their capital investment projects.
This uncertainty has arisen as a result of specific sector and general public concerns relating to farming and food production practices in the UK. The recent outbreaks of BSE in cattle, Salmonella and Campylobacter in poultry, and Escherichia coli type 0157 in meat products have increased public awareness of food hygiene and the food chain as a major route for pathogen transmission. Whilst there is little evidence of disease in man or animals resulting from sludge applications to land, the need for more fundamental research was acknowledged by the industry to allow these concerns to be fully addressed.
Worldwide there is a huge amount of research into wastewater sludge and its use on land, relating to both risks and benefits. In the US the Environmental Protection Agency (USEPA) Code of Regulations part 503, which cover use of biosolids in land applications, was based on an 11-year study costing $15M and covering 14 pathway risk assessments from pathogens and metals. In the UK a major research programme was commissioned by UK Water Industry Research (UKWIR) in 1998, with an annual budget of about £3M funded by its member water companies and authorities plus additional funding from other sources including the Water Environment Research Foundation (WERF), American Water Works Association Research Foundation (AWWARF), Department of the Environment, Transport and the Regions (DETR), Ministry of Agriculture, Fisheries and Foods (MAFF), Department of Health (DoH) and the Environment Agency (EA).
The UK research is only a part of the work being undertaken in Europe and in October 2002 the European Commission organised a Researching the Sludge Directive workshop in Brussels. It was reported that the workshop found: "Overall, the research studies demonstrate that sewage sludge can be used safely in agriculture in the EU and contributes to the pursuit of sustainability, with minimal risk to the environment or the food chain. Sewage sludge makes a very minor contribution to metal levels and organic contaminants in soil and plants. The risk of pathogen transfer to the food chain can be substantially minimised through treatment and cropping and harvesting restrictions. Conversely, the benefits are compelling for using the nutrient, organic matter and essential elements found in sludge to improve soil quality and stimulate plant growth."
The results of the UK research and other related studies were presented in London on January 9, 2002 at the Chartered Institution of Water and Environmental Management (CIWEM) conference, Biosolids the Risks and Benefits - an Update on the Latest Research. Although much of the work has been separately published this was possibly the first opportunity for so much of it to be reviewed in public by the experts involved. Whilst most of the presentations dealt with risk assessment, Dr Chris Rowlands in his presentation of the joint work undertaken by ADAS, SAC, WRc and UKWIR, Biosolids Benefits to Soil Quality and Fertility, focused on the positive benefits of biosolids use. Dr Rowlands was clear in his view that: "With all the research undertaken and risk analysis in place, the industry should now be taking a proactive approach rather than the conservative and defensive position it has taken in the past, extolling the benefits offered by biosolids."
The authors were left with the clear view that, subject to adherence to the Code of Practice, the use of biosolids in agriculture represents best environmental practice in most cases, contributes to sustainable development and provides many direct benefits to the farmer and consumer with negligible risk.
In the US, where various lobby groups have campaigned over many years against sludge recycling, the USEPA 503 regulations which have been in use since 1993 have provided a firm scientific basis so that biosolids can be positively and confidently marketed. Assisted by the implementation of these regulations the percentage of facilities using sludge to land has increased from 35% in 1988 to 54% in 1997.
The US regulations classify sludge as Class A (below pathogen detection level) and Class B (conventional) in a similar way to the new UK Code of Practice's enhanced and treated sludge classification. In the US there is a large asset base in recycling technologies including anaerobic digestion, chemical stabilisation, composting and heat drying amongst others. Already 12% of biosolids treatment facilities in the US now produce Class A sludge and that number is increasing.
Where does that leave us? Well, the UK also already has a large asset base in recycling technologies - mostly mesophilic anaerobic digestion supported by thickening, dewatering and thermal drying facilities. It has long been established that proper treatment of sludge to reduce microbiological hazards, odour, volume and mass is essential in the production of biosolids suitable for use on land. Existing technologies with refurbishment and enhancement as necessary, have the capability to meet the requirements for high quality biosolids and in particular to satisfy the new UK Code of Practice classifications for treated and enhanced sludges, and also the requirements of the forthcoming upgrade of the European Directive on Sludge to Land 1986.
With sensible refurbishment incorporating technical improvements existing plants can be expected to provide many years of service at low cost with a high level of operability and maintainability. Furthermore, existing manpower, familiar with conventional sludge treatment processes, will also be comfortable in operating these plants.
Anaerobic digestion, pre-pasteurisation, thermophilic operation and thermal drying provide a suite of environmentally friendly modular processes which can complement each other and be combined to meet particular site requirements. Biological processes based on the intensification of naturally occurring pathways are inherently environmentally friendly and minimise risk and costs. The right combination of modular process units in the overall treatment flowsheet will allow energy consumption to be minimised and thus contribute further to environmental sustainability.
In the US there is increasing activity in the application of two-stage digestion, some incorporating a thermophilic stage. This promotes pathogen destruction and offers the prospect of enhanced process stability, gas production and solids destruction. Others are looking at bolt-on thermal pre-pasteurisation or chemical stabilisation, both of which can provide an immediate short-term fix. All of these process options can be achieved with relatively low capital investment, however revenue costs vary considerably with biological processes coming up trumps. The lesson from the US is that the regulatory and market needs can be addressed using existing assets. The industry needs long-term sustainable solutions and with pressure on de-manning, robust biological processes start to sound very attractive when faced with the alternatives.
On a broader note the industry can do more to promote recycling - a solution which many in the water industry believe is right in the long-term. At the 65th European Biosolids and Organic Residuals Conference, Dr Norman Lowe announced an initiative by the EA which will investigate how partnerships can be developed to promote recycling of organic wastes, recognising the success of the US approach. This initiative aims to protect the practice of recycling of organic wastes to land for the future. This will involve a change from, "you can recycle if you wish - that is up to you and if you do these are the rules" to, "we all think that recycling is good and we will help you to do that but these are the rules which have to be met as a minimum". Whilst this may initially seem like a minor change in policy, the effect is very positive in favour of recycling.
The practice of recycling sludge to land following effective treatment to stabilise and reduce pathogens in line with the new Code of Practice will in most cases provide best environmental practice and will contribute to sustainable development in the long-term. Public confidence can be reassured with the industry taking a proactive and positive approach to promote and provide high quality biosolids for beneficial use.
The authors would like to thank the following for their contributions in
the preparation of this article: Dr Tim Evans, Tim Evans Environmental; Dr Peter
Matthews, EA/Monsal and Dr Chris Rowlands, Severn Trent Water.