Dr Margi Lennartsson
The Henry Doubleday Research Association (HDRA) is an organisation that deals with organic horticulture in its widest sense – from domestic gardening to allotments, landscaping, and commercial organic production. One of our key concerns is the composting of organic waste for use in urban horticulture. Over the past decade we’ve seen the dawn of a new era for waste management. Gone are the days when we would think it acceptable to collect our waste and then just dump it in a big landfill site, or even pump it into the sea. Instead, we’ve seen recycling plants, including composting facilities, established all over the country.
The first big step towards a greater emphasis on composting came in 1990 with the publication of the government White Paper ‘This Common Inheritance,’ which set very clear targets for recycling. Over the years these targets have been updated and expanded. A second milestone came in 1999, with the introduction of the EU Landfill Directive, which sets clear limits on how much material can be sent to landfill, with targets based on landfill volumes in 1995. Thus, by 2010 the UK must landfill no more than 75 per cent of the volume of waste disposed of through that route in 1995, 50 per cent by 2030, and 35 per cent by 2020. This represents a dramatic shift in our reliance upon landfill. The EU Landfill Directive also refers specifically to biodegradable waste, with the implication for the UK that by 2010 we will have to divert three and a half million tons of biodegradable waste per annum from landfill. At the moment we divert about one million tons, so although we already have seventy big composting sites across the country, this is really is just the beginning: we still have a long way to go before we can be confident of achieving the targets.
There are various ways of disposing of biodegradable waste which avoid one of the drawbacks of landfill – the consequent free emissions of methane, a very potent greenhouse gas. One option is anaerobic digestion, which can generate some usable energy, but this method still leaves a residue which has to be disposed of somehow. Incineration is another well-supported option, but an imperfect one, since wet biodegradable waste is not particularly fitted to incineration, as it reduces the calorific value of the waste. Thus, the third option, composting, looks to many (including HDRA) as the most cost-effective way of moving forward.
Actually, there are a number of different composting options from which to choose. There are on-site options, including home composting, (i. e. composting material in our own domestic gardens), community composting, and on-farm composting. And then there are centralised composting units, either open-air systems, which are the most common at present, or the in-vessel systems that are beginning to emerge. Let’s examine these options in detail.
Many people, and especially urban horticulturalists, are very familiar with domestic on-site composting in the garden, in which biodegradable waste is collected from the garden and placed on a compost heap along with selected kitchen vegetable waste. Most local authorities are involved in active initiatives to promote this sort of composting, because it’s obviously one of the best ways of diverting biodegradable waste. No transport at all is involved, since the waste stays at home, and households create their own compost, which can then replace some of the inputs that would otherwise be used in the garden. Local authorities are also getting a lot of help in encouraging more domestic composting, for example through the efforts of HDRA and other organisations in promoting composting education programmes: how to set them up, how to get people involved, and how to present the message. There is also the Master Composting Manual, a guide for local authorities on successful implementation of home composting schemes. There are good functioning examples of campaigns to emulate, such as Rugby Borough Council’s ‘Rugby Rotter’ and the Coventry Composter Newsletter. And there are large amounts of practical information available to the public on home composting, such as that produced at HDRA.
An alternative centralised option is open-air windrow composting. Not everyone can compost their own domestic waste at home. There are always going to be some wastes that have to be collected at the curbside and brought to central sites for composting. At present these facilities are usually located outside the city boundaries, or even in the rural areas. The material is fed through a large shredder in which it is macerated to make it uniform and more accessible for micro-organisms. These facilities do not have to be large-scale, although many are. Smaller-scale equipment is available for use in parks and by garden landscapers, with the shredder simply mounted on the back of a tractor.
After shredding, the material is placed in what are called windrows – long heaps, usually around 2 metres high and 3 metres wide. The material stays in the windrows for about twelve to sixteen weeks, and it is during this time that the composting takes place. The material has to be turned regularly, because composting is an aerobic process and requires air. When the material is macerated, the micro-organisms within it start to break it down, generating heat – composting is basically decomposition under elevated temperatures. If the material were just left undisturbed the temperature could easily climb to 70 or even 80 degrees centigrade very quickly. But the most effective temperature for the composting process is around 45 degrees, so to maintain that temperature the material is turned regularly, around once a week, sometimes by dedicated equipment that straddles the compost heap, turning it as it moves down the row.
After the composting process has been completed, the material is normally moved into an even bigger pile, where it remains for another month or two, or perhaps even longer, as it goes through its maturation phase. After that, it is passed through a large sieve or screen, which sorts it into different size fractions, with the big material taken out, and the woody stuff going back for further composting. And thus, almost by magic, at the end of this process you have wonderful, humus-rich compost.
Within urban settings, the in-vessel system may be more appropriate. Here, the composting process takes place indoors, mainly to control the emissions. With a well-run outdoor system you can control odours quite well, but there are times when any collection of waste will give off odours, and that’s obviously unacceptable within an urban area. Hence in Holland, for example, with its very high population density, in-vessel systems have been developed and deployed much more extensively.
When you compost indoors, instead of turning the material, you normally force or suck air through it, to keep it aerated. Any air leaving the unit is filtered and cleaned before it leaves, so in environmental terms it is a very clean system. But it is also a very complex system, which makes the handling of the waste much more expensive.
No matter how it is made, it is the finished compost that provides the connection with urban agriculture, and indeed agriculture as a whole, for compost is really one of the most useful resources upon which agriculture can draw – especially organic production. Compost can be put to many uses, and distributed through a variety of market channels.
Most of the compost produced by centralised units, and of course all the compost produced in domestic units, is used by the domestic sector. This compost is suitable for use as a soil improver without any further treatment: it can be dug in before vegetables are sown or incorporated into the soil around the roots of newly planted trees. It can also be mixed according to balanced formulae with things like bark, or other wood fibre, to make a growing medium for container-grown plants: the unblended compost is rarely suitable for direct use as a potting compost. Compost can also be formulated with fibrous materials, such as bark, for use as a peat replacement.
If we consider the statistics on compost usage and likely supply, however, it is clear that once the targets are reached, very large volumes of compost will be generated that will very soon satisfy these domestic sector markets. Although every householder should be encouraged to use compost, we know that this market is not big enough to absorb all the compost that will become available. Indeed, it has been estimated that if all biodegradable waste were to be composted then the domestic sector market would be able to absorb only 2 per cent of the material produced. There must be further expansion, therefore, in the use of compost by the landscaping industry in urban parks and gardens. And, more importantly, some of this material will eventually go to agriculture. Agriculture is the secure, large-volume market for compost, and HDRA is working hard to promote its use on farms.
The horticultural benefits of compost can be summarised as follows. Compost is decomposed humic material with nutrients, and it is most widely used to improve the structure of soils. Heavy clay soil can be lightened with it, improving water infiltration and the crumb structure of the soil. In sandy soil compost binds the sand with other particles and increases water retention. All plant production – food production – is dependent on having a well-structured soil, and it is here, in the promotion of food production, that one of the important benefits of compost lies. Compost can also supply plant nutrients, both short-term and long-term, and therefore act as an effective replacement, or at least a partial replacement, for artificial fertilisers, and one that is more sustainable, given that artificial fertilisers very energy demanding.
But perhaps the biggest benefit of all lies in the microbiological activity which compost supports. Compost is full of microbes – a great diversity of organisms – and adding these microbes is good for the soil. But these microbes are in the compost in the first place because it contains energy, the sources upon which these organism rely to be active. Adding this energy source – the organic matter – to the soil therefore helps to increase the biological activity within the soil itself, with consequent improvements resulting to the structure of the soil, because active microbes secrete substances that help glue soil particles together. Furthermore, a microbially active soil can actually control some pests and diseases.
Nevertheless, some barriers remain to the greater use of compost in food production. As regards its use in agriculture, while this is potentially the largest market, there are problems related to the costs associated with using compost. Compost is a low-value product, and the main value associated with it is the saving in environmental costs derived by not sending its source material to landfill. But moving compost long distances away from the points of production will mean incurring new transport costs. We should therefore encourage the use of compost for food production within urban and peri-urban areas, as well as in the traditional urban markets detailed above, in order to keep these costs to a minimum.
Note: this chapter is a transcript of a lecture delivered by Dr Margi Lennartsson, Director of International Research at the Henry Doubleday Research Association. It has been edited by Richard Wiltshire.