The Government has pledged to substantially reduce the amount of waste material that goes to landfill sites by 2010. Dick Bilborough looks at composting as an option for dealing with biodegradable waste.

The problem of waste is here to stay. The hierarchy of desirable options for dealing with waste starts optimistically with "prevention", then cheerfully accepts "minimisation", further allows for "reuse" and "recycling", reluctantly acknowledges "energy recovery" and ends despairingly with "disposal" (ie landfill). In the next few years it seems likely that there is going to be rather more disposal than prevention. In the UK, municipal solid waste (MSW) is growing at around 3 per cent a year and even assuming heroic efforts at waste prevention and minimisation, there will still be a huge amount of material that needs to be dealt with every year.

The traditional UK solution of landfill (more than 80 per cent of MSW in 1998), even properly engineered landfill, is being slowly but inexorably reduced by the Landfill Directive and something else will have to be done. This directive, which must be brought into UK law by July 2001, aims to reduce the amount of biodegradable material that can be landfilled to 75 per cent of the 1995 figure by 2010. Further, it aims to reduce that figure to 50 per cent by 2013 and to 35 per cent by 2020. In addition, all material to be landfilled will have to be pre-treated to reduce methane emissions from the tip. Many EHOs will not be sad to see fewer landfill sites in their local authority areas as they can generate tremendous environmental health problems through traffic, noise, dust and seagulls and can not be described as sustainable.

WASTE STRATEGY 2000
The Government's Waste Strategy, published in May 2000, confronted this issue and decided to split the problem into three. One third of MSW would continue to landfill, one third would be recycled (mainly composted) and one third would be incinerated. This last option received most of the media attention, with Friends of the Earth proclaiming that 165 new incinerators will be needed and Michael Meacher, the Environment Minister, apparently retreating from his own department's strategy and making it clear he would not be approving many incinerators at planning appeals. Fortunately, between the disappearing landfill option and the (probably) stillborn option for new incinerators, there is the reuse and recycling option which, instead of burying potential resources or producing problematic emissions and ash, actually creates a beneficial material which has potential uses for landscaping, horticulture and agriculture.

Composting is, of course, only suitable for the biodegradable fraction of MSW, but this is exactly what is left when as much paper, plastic, and other non-biodegradable waste as possible has been removed for recycling, or has been separately collected. There will always be a residue of material that can not be composted, or which has been screened out of the compost, and this will continue to be landfilled. The actual provision of these future compost installations will be by companies holding long-term waste contracts from local authorities and there is an issue around gaining planning permission, as currently few local plans have anticipated this requirement.

At present (1998 figures) only around 1.5 per cent of UK household waste is composted - although the amount is increasing rapidly. Composting is strongly encouraged in the Waste Strategy document, which requires 25 per cent of MSW to be composted or recycled by 2005, 30 per cent by 2010 and 33 per cent by 2015. The following questions then arise: what sort of composting is suitable; and what are the environmental health issues involved?

OVERVIEW OF COMPOSTING SOLUTIONS
Basically, composting comes in two types, outdoors and indoors. In its simplest form green waste can be placed in windrows four meters high, turned with a 360o loader every week for four months and screened. Some good compost will result together with a reject pile containing the plastic and so on, which must be landfilled. A well mixed aerobic compost pile will not smell, although the sight of steam rising from it may persuade the neighbours otherwise. However, it is another story if the pile goes anaerobic, due to lack of turning or excessive rainfall. Unfortunately, it only takes a few odour incidents to sensitise the neighbours and dealing with a perceived smell nuisance is a real problem for all concerned. For a site near to housing, a windrow compost operation is always going to present the risk of nuisance occurring.

To deal with the environmental issues around windrow composting the logical step is to do the composting indoors. The simplest indoor systems are basically windrows in troughs. These use a travelling mixer and are usually enclosed in a building with odour control. These work, but require a large flat area, a very large building and a large biofilter. Most of the more advanced indoor systems have taken the next logical step and put the composting process in a container. These are generically known as in-vessel systems. Typically, the vessel can be the size of a hook lift container or be a large concrete tunnel. The advantage of this type of system is that complete control of the temperature and humidity of the composting material can be achieved. However, such control is at the expense of a complicated system of air blowers and heat exchangers. From an environmental point of view, these systems are to be preferred to outdoor systems for a number of reasons. The odour can be contained and dealt with and the composting process is not visible. Also, since the temperature in the vessel is maintained at the optimum, the compost stabilisation process is speeded up to just two or three weeks, which means that sites require less space.

Almost all in-vessel systems require a maturation stage afterwards where the stabilised compost is left in windrows for several weeks with occasional turning. During this period pathogen die-off continues, the volume decreases and the quality improves. In principle, these maturation windrows can be outside since there is very little risk of odour nuisance from them. A further type of composting system, the silo cage, combines attributes of indoor windrows with those of the second in-vessel, mechanised approach. One example is the continuous flow system where the cages are fed daily from the top and the material passes slowly vertically down the cage. Temperatures are checked throughout the process and the end product arrives at the bottom as a high quality, stabilised, pasturised compost. The whole process, including mixing and feeding the cages, is enclosed in an agricultural-type building and requires a relatively small amount of space. Compared with windrow and in-vessel systems, the silo cage approach requires a far lower energy input and is therefore more sustainable.

AN ANSWER FOR SEWAGE SLUDGE
The composting solutions mentioned have all referred to MSW but composting is equally appropriate for sewage sludge for which an alternative solution is desperately needed. The Landfill Directive will effectively end landfilling of sewage sludge, except in emergencies, and few sewage works have the large amount of sludge needed to make an incinerator a feasible option. Windrow composting of sewage sludge cake mixed with straw, green waste or wood chips will produce an excellent product, but as with MSW, there will always be a risk of odour nuisance. In addition, windrow system cannot be relied on to kill pathogens, especially if there is a long wet spell, and this will prejudice the use of the product.

In-vessel systems however, are very suitable for sewage sludge. Indeed during 1999, trials of the silo-cage composting system showed pathogens reduced to below detectable levels. Silo cage systems can also achieve the higher levels of enhanced treatment required by the matrix agreement in one process - thereby being more efficient and economical. In the future a high quality end product will be a must if composting is going to be seen as a viable solution. New rules governing the application of sewage sludge to agricultural land will mean that only waste that has been treated to an enhanced level will be permitted. Even if more incinerators are built, there will be a lot of compost around and it is reasonable to predict that a buyers' market will emerge. Only a quality product will command a reasonable selling price, and this should encourage operators to run a compost site well so that they can issue quality assurance certificates with their products.

Sustainability is the name of the game and composting, which harnesses the very processes which Nature has used for millennia to recycle nutrients, is its very essence. Using an in-vessel system allows this advantage to be obtained without causing unacceptable and uncontrollable problems for the people living near the site.

Dick Bilborough is managing director of Teg Environmental, which has developed the in-vessel silo cage system of thermophilic composting. For further information contact: Teg Environmental, Crescent House, 2-6 Sandy Lane, Leyland, Preston, Lancashire, PR5 1EB. Tel: 01772 422220. Fax: 01772 422210.
E-mail: rcb@tegenvironmental.freeserve.co.uk Web: www.tegenvironmental.com
Considerable interest has been shown in this system by local authorities (at the LARAC show) and visits have been made to Teg's demonstration site in Preston, Lancashire, to see the process in action.
Readers may be interested in purchasing the WHO Environmental Health Pamphlets on Solid Waste and Health, available from the publications department for £7.60 each. Tel: 020 7827 5882.