Sludge reuse: the time is now

Mike Rewcastle, technical strategy manager - sludge, for Northumbrian Water, argues the water industry should not be exclusively responsible for the future management of sludge

What to do with sewage sludge has always been a challenge for society, which it is happy to delegate to the water industry and forget about. In its turn, the industry has responded to regulatory direction and developed a system of dealing with sludge generated by sewage treatment operations beginning initially with 'disposal' to local farmland and evolving into 'agricultural recycling' helped along by successive regulations defining treatment criteria and requiring control of added nutrients and contaminants. The practicalities depend upon the quantity and quality of the sludge matching the needs of economically accessible agricultural land such that many larger industrial connurbations have turned to either landfill or incineration as applicable solutions. Until the end of 1998, many coastal areas additionally relied on disposal of sludge at sea.

The recent, long-awaited, major national investment in sewage collection and treatment is achieving great environmental improvements, unprecedented levels of compliance and media praise for clean bathing waters. However, this has been accompanied by a much less publicised quiet revolution, with a huge increase in sewage sludge production being an inevitable consequence of the extra treatment.

The industry could sacrifice some of this success if we fail, collectively, to find equally environmentally sensitive methods to effectively manage this sludge.

Traditionally sludge has been considered a 'waste' requiring disposal, although

greater insight is driving a change in perception and due consideration of the resource value which may be recovered. The earliest manifestation of this was a change in terminology to 'agricultural recycling' and the recycling of nutrients to the soil rather than the 'disposal' of sludge to land. I believe it is timely to widen the debate on the status of sewage sludge within the context of the broader waste industry and European and national strategies on waste and energy. The definition of waste and the objectives of the waste hierarchy require both interpretation and consideration by the industry and its regulators. With waste being defined as 'any substance which constitutes a scrap material or an effluent or other unwanted surplus substance arising from the application of any process', the waste definition better fits the treated effluent which is discarded into the environment, than the sludge which is removed or generated by the processes. The purpose of the waste regulations is to protect the environment from the harmful effects caused by the collection, transport, treatment, storage and tipping of waste. In particular it aims to encourage the recovery and use of waste in order to conserve natural resources. Sludge is recovered from the processes and treatment completes the recovery and prepares the sludge for use.

It is on this latter point where enlightenment in interpretation would be welcomed - once the recovery of waste has been achieved and the use of it made possible as a substitute for other resources, it must at some point cease to be a 'waste' - what else could recovery mean? I would argue this occurs at the end of the recovery process at which point the material has resource value, a specified quality and a negotiable commercial value. In recent years this has led to the development of sludge products which have a specification and are marketed in various ways. The continued definition of these products as wastes can restrict the potential for use.

In his pre-budget statement in November 2002, the chancellor acknowledged the need for a change in perception of waste and an encouragement of recovery of resources with further disincentives to waste disposal. Hopefully this will result in more joined-up regulation from the relevant government departments, jointly encouraging resource recovery and removing unhelpful restrictions.

Considering the recoverable value of sewage sludge as a resource demands an evaluation of the intrinsic properties of sludge that may be exploited and the potential drawbacks involved in its use. Any value largely accrues from the organic matter, which has associated with it:

  • plant nutrients (particularly phosphorus which is a limited natural resource),
  • chemical energy, measured as calorific value, which, as it is continually produced, may be considered as a renewable energy source,
  • in some applications, the mineral (ash) content may be valuable. The potential drawbacks are:
  • the many contaminants that may accumulate in the sludge from numerous diffuse and point sources within a catchment, arising from society's activities,
  • the physical nature of the sludge, which initiates as a liquid and must be handled and transported to suitable treatment facilities and eventual reuse or disposal.

Realisation of the resource value requires the sludge to be in a suitable form and location to exploit opportunities and the potential drawbacks must be controlled so that environmental damage does not occur. It should be appreciated, however, that the industry must deal with sludge and thus the removal off-site and logistical handling is a necessity irrespective of whether the material is disposed of as waste, or recovered as a resource in whatever way.

The consideration of options necessitates complex technical, environmental and economic evaluations and the latter is influenced by regulatory incentives and disincentives such as landfill and other environmental taxes, climate change levy, renewable obligations certificates, enhanced capital allowances and the like. It is within this consideration that a wider interest group ought to be engaged. Because of this complexity there is a danger of being selective rather than comprehensive in any evaluation and most apparently robust methodologies can be weakened by subjective application. It remains a difficult task to establish environmental economic value and to compare one environmental impact against another, let alone judge between an economic aspect and a technical one. It is my view that there is some considerable work to be done in this area and not exclusively by the water industry.

It is important regulations are developed that allow different acceptable methods of sludge management in the interests of society and that regulators, representing different sections of government, integrate activities to encourage the overall stated objectives and do not restrict, intentionally or unintentionally, the sensible development of new practices. At present there is only stated government support for recycling to agriculture as the preferred option and all water companies are committed to complying with regulations designed to protect this outlet and safeguard public health and the environment. However, charged with the responsibility of managing ever-increasing sewage treatment processes on behalf of society, the industry must address the long-term strategic requirements and ensure safe, secure, economic processes that enable sludge to be continually managed such that these processes continue without jeopardy.

Past history, and indeed recent history, has shown that alongside the well-documented recycling benefit of sludge use in agriculture, there are a number of drawbacks that can make the outlet vulnerable. The public health and food safety debate, begun several years ago and partially resolved, continues to create uncertainty with significant impact on operations in 2003. The impact of foot and mouth disease was a salutary lesson and some of the implications of the revision of the EC sludge directive have been evaluated as having a major adverse impact on the viability of the outlet. Coupled with public unwillingness to accept unpleasant odours from sludge spreading, despite reported support for recycling (classic yet understandable nimbyism), a vociferous and well informed scientific opposition to the practice, reinforced by bans in some countries, it would be a brave strategy manager who recommended exclusive reliance on the agricultural outlet.

Wasted potential

Inevitably this leads to consideration of viable alternatives and the introduction of flexibility into strategies. The wider industry contributes to this by developing new technologies and attempting to address these issues. Many technologies which show promise fail to make it through the maze of regulation and acceptance into what is a fairly traditional and conservative industry, and in the sludge treatment arena there are several 'stranded assets' incapable of playing a part in future management. Companies obviously do not want to invest heavily in processes that do not fulfill the strategic aims, hence the need for reliable ways to prove technologies, anticipate regulatory direction and confirm outlet markets before determining actions.

Again, there are opportunities for wider debate and co-operation - from national waste and energy strategies, through local waste planning processes to synergies between different industries in waste and energy management. Good examples include:

  • co-composting of sewage sludge with municipal green waste,
  • use of dried sludge as a substitute fuel in cement manufacture,
  • growing of energy crops (fertilised by sludge) for renewable energy generation,
  • co-combustion in power generation,
  • production of light-weight aggregates from sewage sludge cake and other previously landfilled wastes.

All of these have different attributes - technical, environmental, economic - and attract different levels of support and acceptance by regulators and the public. In most cases, there may exist prejudiced views that need to be influenced by a consistent presentation of the facts. Across Europe, national policies encourage different options, thus it is difficult to determine best practice although clearly some of them have been adjudged to represent sustainable long-term solutions to this crucial issue for society.

At the beginning

As the water industry enters the critically important periodic review process (PR04) which sets the investment needs for the medium-term it is vital sludge management issues receive consideration for the role they play in maintaining and consolidating the environmental improvement made. Stated objectives of this review are the preservation of environmental gains and securing sustainable long-term cost-effective solutions. In the absence of any new quality drivers from legislation, this requires an assessment of the existing asset base to continue to deliver sustainable sludge management in the future, with sufficient flexibility to meet strategic objectives and preserve operability of sewage treatment processes. A move towards sustainable development is necessarily a long-term objective, but one which must have a beginning



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