Factsheet - Cool composting: a fresh approach

Cool Composting: a fresh approach

factsheet Centre for Alternative Technology Publications

CAT has been developing a fresh approach to composting…

If you're a keen gardener looking for a reliable alternative to the traditional 'hot' heap, or a keen 'green' householder looking to cut your contribution to landfill and global warming, then the high fibre method is just what you need.

Figure 1 left: A mixture of kitchen waste and crumpled card being tipped into a container: the essence of the new 'high-fibre' system described in this factsheet.

Figure 2 above: Healthy cabbage plants growing in high-fibre compost. On the left, pure compost; on the right, half compost, half soil.

T

here is a great tradition of making compost for the garden. It was developed originally by enthusiastic gardeners, whose aim was to produce the highest quality compost as quickly as possible. To do this they would usually bring in materials from outside such as straw and animal manures, make up a carefully layered heap with selected garden wastes such as weeds and grass clippings, and turn the steaming pile every few days, adding water if necessary. The result was a product with

almost magical powers to stimulate healthy and vigorous plant growth.

This tradition continues, and detailed instructions on how to do it can be found in any general garden book. But this factsheet is not about that kind of composting. It is aimed not just at gardeners, but at householders who want to use their gardens to reduce the amount of organic waste going to the dustman. They will still get compost – very good compost – but the main aim is to

reduce the environmental impact of household wastes, with great compost as a bonus end-product. This means that bringing in extra materials from outside doesn’t really make sense. But without them the traditional methods do not work very well and the compost does not heat up as it is supposed to do. This factsheet introduces a completely new approach that works without heating.

We call it Cool Composting. Its differences from the traditional approach are summarised in figure 3.

Figure 3: Two agendas for home composting The Traditional Gardener…

The Cool Composter...

Is trying to make very high quality compost at (almost) any cost

Is trying to reduce the environmental impact of household waste

Is prepared for a complex task with regular monitoring and intervention

Wants something simple, self-regulating, with minimum intervention

Uses intensive batch processes

Uses minimum-effort continuous processes

Focuses largely on garden wastes

Focuses on kitchen and household wastes, but ideally wants to process all organic wastes

Will combine on-site materials with others brought in from outside if necessary

Only uses resources arising in the house and garden

Favours classical high-temperature microbial composting

Finds that ambient-temperature composting with a wide range of decomposers is most appropriate

2 Composting Waste organic matter can become either a problem or a resource,

What is compost?

If you walk through a wood, you will usually find the ground is covered with leaves.

Remove the top layer and you will find leaves broken up into small parts. Beneath these you will find a soft crumbly brown material which merges into the soil. What you are seeing is the fundamental process of composting from beginning to end. It starts with recognisable, structured matter derived from living organisms, goes through various states of ‘decay’, and ends in a remarkably homogeneous dark substance often known as humus.

our organic wastes together in a big pile so that everything is nice and tidy, and so that we can collect the product easily.

depending on what happens to it.

Why do it?

the environment rather than into

In nature, the generation of humus serves to recycle the mineral nutrients for plants to use again, and maintains an open, crumbly soil structure. It can do the same for householders: feed their plants and soils and maintain healthy communities of soil organisms, while turning wastes that would otherwise be smelly and unpleasant into a kind of distilled goodness. The finished compost helps vegetables, flowers and houseplants grow The process does not happen all on its marvellously well and to resist diseases. own. It is the result of being attacked by a Making your own compost saves having to huge variety of organisms known as buy potting media, fertilisers and soil decomposers who use dead organic matter improvers, and avoids the environmental as food. Decomposers are not trying to impacts of manufacturing and transporting produce humus, any more than a fire is trying these products. But its greatest environmental to produce ashes: these are simply the contribution is in stopping organic waste products left at the end – in this case, very leaving the household. Most of the weight of valuable ones. The same fate would befall fresh organic matter is water, and trucking any material derived from living things if it this to a distant landfill is not a sensible use of were left in a wood. Dead bodies, tree trunks, fuel. Worse, when this material gets into a grass clippings, banana skins, dung, paper, landfill the normal compost-breakdown wool, tea leaves…all would be consumed by process cannot occur. Instead of turning to the decomposers. The breakdown is not just carbon dioxide and water plus compost, it physical, but goes right to the finest molecular turns into carbon dioxide and methane plus a details. Most of the original substance would kind of putrid soup. Methane is a greenhouse be used for energy by the decomposers and gas about twenty times more potent than ‘breathed out’ as carbon dioxide and water carbon dioxide. Some of it can be captured vapour, literally disappearing into thin air. and used for fuel – but not all of it.

What gradually emerges is a much smaller Meanwhile the mineral content of the material volume of stable organic matter, resistant to cannot be returned to the soil. Worse still, the further breakdown, incorporating a variety of liquids that emerge, far from the benign rich minerals in a relatively concentrated state. fertilisers they could be in a garden situation, are mixed with industrial and other wastes Composting is simply an applied form of and become toxic leachates that can pollute this natural process. Basically we keep all rivers and ground water.

Home composting ensures that it turns into something good that helps something bad that damages it.

The benefits of home composting apply to people with gardens. What about those who have none? In this case what ought to happen, and does in a few areas, is that the Council will collect separated organic waste and compost it on a larger scale. Centralised composting of this kind can be carried out on a fairly local scale (a few tonnes per week) or on a large industrial scale (hundreds or thousands of tonnes per week). This is good; it achieves the aim of turning waste to resources rather than resources to waste, and in fact some householders with gardens might prefer to contribute at least some of their more difficult waste to a centralised system – see figure 10. But the raw material still requires handling and transport, so the optimum situation is for green householders to deal with their own organic wastes as much as possible in their own gardens.

Understanding compost If you intend to take up the challenge and compost your own household waste it is handy to understand some of the principles that make composting work. On this and the next few pages the bar at the bottom will tell you about the theory while the main text gets on with practical recipes. To make things easier for beginners, we have introduced a new terminology based rather whimsically on bread and cheese. You might find it helpful to read the section on the two pages below before going on.

Composting: a helpful culinary analogy

<Bread> categories & nicknames

Go off if kept; potentially smelly

Low; rich in lnitrogen High; little nitrogen

Each of these simple principles: complementary materials, equal proportions (with perhaps a bit more bread), mixed with a texture that is not too fine or too coarse, digestible ingredients, and don't worry about getting things exactly right – applies to composting. In fact the analogy is so close we have given the nicknames <bread> and <cheese> to the two main sorts of ingredients for composting. The table to the right explains more about the two categories.

Can be kept indefinitely if dry

Thirdly, the ingredients must be digestible. Slabs of overcooked, unleavened bread too hard to bite or chew do not make a good sandwich. Soft bread is best, but crackers or <Ryvita> are ok for some tastes.

<Bread>

Secondly the ingredients need to be combined in the right way. A block of bread and a block of cheese eaten one after the other is no good, even if the proportions are right. Neither is putting the ingredients in a blender and eating the resulting baby food. They need to be combined fairly intimately while still retaining their identity, and thin layers of each is a very effective way to achieve this.

Stiff, dry, rough, light.

Usually dull colours

Firstly the proportions of bread and cheese need to be right. Too much bread is dry and tasteless. Too much cheese is equally unpleasant in a different way. Equal volumes is roughly right, although it is usually better to have somewhat more bread. But it works well within quite a broad range: there's no need to be too fussy.

<Cheese>

The creation of a tasty bread-and-cheese sandwich is an excellent guide to what will and will not work in composting. Both practices, sandwich-making and compost-making, work best when they draw on the complementary qualities of light, open-textured, carbohydrate materials and dense, moist, protein-rich materials.

If you are ever in doubt about what to do in composting, ask yourself this: what makes a good cheese sandwich?

Soft, moist, dense.

Often intense colours

Physical Storage C:N quality qualities ratio

The bread and cheese sandwich

Very Soft: <Sliced Bread> Intermediate or mixed: <Ryvita> Very hard: <Ship's biscuit>

Composting 3

Chemical make-up of compost materials

Please everyone, relax. Remember we are trying to minimise waste through composting, not produce 'perfect' compost at any cost. In any case TV compost is a fraud: it is always meticulously riddled to look good on the camera, or in some cases is simply commercial compost out of a bag. This gives everybody a false idea of what is a reasonable standard. Home compost is nearly

Function in compost

Grass clippings, vegetable trimmings, sappy plant material, cooked kitchen wastes, animal manures, human urine and faeces

Complex molecules, proteins, major mineral nutrients and trace elements, often non-cellulose carbohydrates and fats also

Provides nitrogen and moisture

Cooked staples: Bread, potatoes, rice, pasta, corn, millet

Straw, dried bracken, paper, cardboard, used tissues

Structural carbohydrates, mostly cellulose and hemicellulose

Provides ready energy and carbon, temporary structure for gas exchange, decomposer habitat, buffers moisture

Dried breads, crispbread, stale toast

Tree leaves, hardwood sawdust, shredded woody waste

Cellulose/hemicellulose plus tannins and lignin

Provides long-term structure for gas exchange, slow-release carbon, buffers moisture

Uncooked grains, hard-tack, overcooked bread

Wood, bark, twigs, fibrous stalks, softwood sawdust

Ligno-cellulose

Provides permanent structure for gas exchange

Accessibility to decomposers

Process category

High.

Break down easily

'Soft' wastes

Rich protein foods: Cheese, yoghurt, meat, pulses, fish, eggs

Equivalents for compost

This kind of minimum-effort system has a lot going for it, and it is the basis of our improved 'Cool Composting' approach. It is strikingly different from compost demonstrations on TV, where the garden gurus make a great fuss about getting the heap to heat up, and always seem to produce a smooth soot-like powder in no time at all.

Home composters often feel they have failed if their compost does not heat up, takes a long time and ends up lumpy and twiggy.

high-fibre method best

Dietary analogies

contents of dead pots and window boxes and so on, then simply hope for the best. In principle this is sound (a bit of everything, plenty of both <bread> and <cheese>) and sometimes it works rather well, especially if the container is a large one and the householder is in no hurry to retrieve the product. This is reminiscent of the ‘garden dump’ that many gardeners have behind the shed or at the back of the shrubbery for really problematic wastes: cleared brambles, old fence posts, rubble, clods of soil. Many years later, obliged to clear out the dump for some other project one is amazed to find that perfect black loam has emerged from such unpromising raw material. This is compost alchemy at its finest, and it is a comforting fact that, as in the woods, any organic material will turn to humus in the end.

Low. Resistant to breakdown, require specialist decomposers

Stack method best

There are two extreme possibilities for the would-be composter: one is a system that requires a lot of skill, time and effort and produces reliably outstanding results. The other extreme demands very little from the householder, in return for erratic performance and sometimes total failure. The first is the garden enthusiast’s approach, the second is what 99% of composting householders actually do. We are looking for a middle way that gives reliable results without needing more skill and care than most householders are prepared to give.

'Hard' wastes

This is the Rolls-Royce process.

Unfortunately (quite apart from the effort involved, which is considerable) it does not really fit the patterns of modern life. In order to achieve heating a heap needs to be above a critical size (about half a cubic metre) and has to be constructed all at once. But it is unusual to have large quantities of both <bread> and <cheese> available at the same time: both gardens and households tend to generate small amounts of both continually, with occasional gluts of one or the other.

We cannot stockpile household <cheese> because it will go off: it has to be processed promptly in a continuous rather than batch process. For these reasons serious classical Classical hot composting: composters brought in ready-made supplies the pursuit of perfection from outside – traditionally straw and animal A big pile of carefully-layered <bread> and o manure. The other drawback of the classical <cheese> will heat up to 60 or 70 C in a matter of days (see box below). This is caused system is that it does not process woody waste unless it is shredded. Traditionally, by the accumulation of metabolic heat from bacteria and fungi reproducing very fast and woody waste was simply burned to get rid of it, and other difficult-to-compost wastes consuming the easily available materials. such as autumn leaves were given separate After a week or so they either cook themselves to death or run out of air or easily piles of their own. accessible food, so activity declines and the Modern enthusiasts of the classical system temperature starts to drop. Then the heap is accept that there is no particular reason for supposed to be ‘turned’ or mixed up, and haste. They commonly do only one turn, then possibly watered, and another heating cycle leave the system to cool down, whereupon it commences. This might happen three or four times, and usable compost could be ready in is colonised by the rest of the decomposer community and becomes the kind of cool, under six weeks. biodiverse system described in figure 7.

Such a process is used in large-scale central composting units and is the traditional method The opposite pole: recommended in most garden textbooks. Its the indiscriminate dump posh name is 'thermophilic' (i.e., heat-loving) For many householders with compost bins composting, but we call it 'classical', or and small mixed gardens the default pattern 'traditional' composting. The heating kills most is simply to put everything in as it arises – of the weed seeds and pathogens in the raw kitchen waste, hedge clippings, weeds, material. leaves, prunings, twigs, lawn mowings,

Composting Practice

4 Composting always lumpy when it comes out of the heap, and does need riddling if you want the compost for 'fine' purposes such as potting medium. As for heating, it is not vital; very few people achieve it anyway, and its supposed benefits are simply not worth the extra trouble. And what's the hurry?

There are however some real, if minor, drawbacks to the minimalist system. One is that the timing is often wrong, with huge gluts of only one sort of material, then another, arriving at random. The result is a heap too dry in some parts and too wet in others.

Particularly in the winter there is little garden waste, only constant additions of kitchen scraps, usually leading to a soggy smelly section of the heap completely deficient in oxygen (see below). Turning might help this a bit, but in practice almost nobody gets round to turning, even the real enthusiasts – and you might get some bitter complaints about the smell. And in a seriously shrubby garden the compost will often become a depressing mixture of slime and twigs.

bottom with a tap from which a rich brown fluid could be drawn. This fluid is particularly suitable for use as a houseplant fertiliser and is a distinctive feature of wormeries (figure 4a). Worms are supplied with the container, often with peat-based bedding and special ‘worm food’ and a source of lime. Most of the appropriate worm species need lime, and adding it also serves to counteract acidity. If all goes well the initial worm population multiplies into a large permanent colony.

There is no significant heating, which would, anyway, kill the worms.

Unfortunately the failure rate for these wormeries has been rather high. Among the problems encountered are that • an average household’s kitchen waste can easily be added at a greater rate than the worms can deal with, leading to anaerobic conditions; • in the enclosed space, anaerobic bacteria and their toxic products can kill off the worms, since there is nowhere for them to escape; • many common items (such as citrus peel and onions) are ‘not allowed’;

Figure 4a: A wormery set-up – early designs

Wormeries: a special case In between the perfectionist classical system and the minimalist 'dump' is a relatively new development: the wormery. Wormeries were introduced as self-contained composting systems for patios, balconies or garages to deal with food scraps, using deliberately introduced worms instead of relying only on natural microbes. Wormeries address a problem which classical composting did not: how to deal with small amounts of <cheese> constantly generated in the house. Early designs were usually about the size of dustbins (in fact most were modified dustbins).

They usually had a false floor and sump at the

Figure 4b: A modern stacking design wormery set-up

How and why it works Most home composters simply want a good recipe to follow, and we have plenty of these in the main text. But home composting sometimes throws up situations you didn't anticipate. In these cases it is often helpful to have some understanding of the underlying biological and physical processes.This understanding also explains why the <bread> and <cheese> principles work. It's not difficult!

Given that the transformation of dead organic matter into compost depends on decomposer organisms, it stands to reason that you will get the best results if you provide the conditions they prefer. Although there is a huge diversity of decomposers (see figure 7 for a general overview) and they seem so alien in many ways, the ones we want to encourage are biochemically very similar to people, and need oxygen, water and the right sorts of food. Without these they will fail to thrive and eventually die. Let's look at each of these factors in turn.

Oxygen: Oxygen is plentiful in air but in a closed space with oxygen-breathing creatures it can be used up very quickly. This situation is very common in poorly-structured compost heaps, and can have drastic consequences. As the level of oxygen falls the activity of oxygen-breathing organisms drops off and a completely different range of organisms can start to multiply. These are mostly oxygen-hating bacteria (known as

'anaerobes' – the same ones as are found in landfills) that cannot grow in an oxygenrich environment. Now they seize their chance. From a composting point of view the most significant thing about them is that they produce a wide variety of chemical substances which are smelly, poisonous, or environmentally damaging. Not only do the anaerobes fail to break down the original material efficiently, they severely inhibit the efforts of those that can. If they take over they are very bad news for composting, so the oxygen-poor (or 'anaerobic') conditions that permit them to flourish are to be avoided if at all possible.

In order for fresh oxygen to enter it, the heap must have a porous sponge-like structure with holes running right through it. The most common reported failures in composting are where the raw material (often grass clippings or food waste) slumps into a kind of vegetable porridge with no holes at all. If you poke this stuff your nose will immediately tell you the anaerobes have taken over! In order to overcome the problem it is normal practice to add some other material which is stiff enough to maintain a network of galleries throughout the heap, rather like pit-props in mines.

This can operate on quite a coarse scale, using say twigs or hedge trimmings or crumpled cardboard, or on a finer scale using shredded woody waste or (finer still) sawdust.

Composting 5

• if the ‘juice’ is not withdrawn regularly it can build up and drown the entire system, producing dreadful smells; • retrieving finished compost from the bottom of the container is difficult without tipping the whole lot out; it is a messy business and the worms have to be separated and restored for the next batch; • fruit flies are almost inevitable and are disconcerting in indoor situations.

biscuit>. Households with gardens would need to attend separately to these. In this case, why bother with the hassle of a separate wormery?

In summary, a single wormery will not satisfy the requirements of the householder with a garden of any great size. If you have to run several to cope with your waste there are better and cheaper systems available.

(see figures 10 & 11). The sizes and relative proportions of these two compost heaps would depend on the size of the household, and the size and nature of the garden (see figure 12).

A Hybrid Approach

The system we propose for dealing with 'soft wastes' is in effect a hybrid of the standard methods: • it is like a classical system in that it uses In some respects poor performance is not a large container with roughly equal Is There Another Way? surprising because the system breaks the proportions of <sliced bread> and It is surprising that there is so little systematic <bread> and <cheese> rules: too much <cheese>; research on home composting. It seems to be <cheese>, not enough room for bulky generally assumed that everything that can be • it is like a wormery in that much of the work <bread>, so not enough structure to keep the done has been, and that the only choices are is done by worms in a continuous process, texture open. Theoretically the activity of the and most of the raw material is household from among existing methods. Our research worms could serve to provide air channels, rather than garden waste; at CAT in the last few years has shown that but in a small closed space one single great improvements are still possible, making • it is like the dump in that there is minimum anaerobic pocket can produce enough foul management or maintenance: no efforts to home composting accessible to a much wider chemicals to kill all the worms in the bin. encourage heating; no control of moisture; range of households.

Once the worms are in decline the system can no turning. rapidly nosedive into an impossibly smelly What are we really looking for? We want goo.

The method introduces an important a system that can operate continuously, with novelty, that is, deliberate use of waste paper small amounts being added all the time, but Significantly, the latest generation of and cardboard as a source of <sliced bread>. which can handle occasional gluts. It should commercial wormeries have re-thought the be based on a small number of easily-learned In fact the system recommends that system and produced stacking designs that householders incorporate in their compost rules. It should involve a minimum amount of produce thin horizontal layers and plenty of heap all the waste paper and cardboard that effort. It should be able to deal with the ventilation, as in a classical heap, and also maximum range of household organic waste. is unsuitable for recycling. These materials making harvesting much easier (figure 4b). have many important functions. They supply And it should reliably produce usable They are much better, but they are still too easily accessible carbon; mop up surplus compost. In view of past limitations in home small to absorb more than a fraction of a composting, this seems an unlikely Holy Grail, moisture; reduce acidity; keep the heap well typical household’s organic waste. You would but surely one worth seeking. Consideration ventilated; break down rapidly leaving almost usually need quite a few, and if you have a no residues; and seem to stimulate the activity of the biological processes, observation of garden it would probably be better to have a household waste output, and subsequent of decomposer organisms, possibly by larger and more robust system outside. providing a variety of protected habitats. In trials, all suggest that an improved ‘dump’ is probably the best model, but should probably fact they seem to be a far better all-round One obvious aspect of wormeries is that be split into two streams – one for soft wastes source of <bread> than the traditional they do not deal with any significant quantity only, producing fine compost fairly quickly, ‘brown’ wastes of the garden, and using them of garden wastes, and certainly no <ship's and one for a mixture of hard and soft wastes reduces still more the volume of solid waste

Water: Most decomposers do not actually live in water (although some do) but need films of water on the material they are working on, and a humid atmosphere. Much organic matter has a high water content anyway, and the normal processing by decomposers produces water as well (along with carbon dioxide) so in principle there should be plenty. The problem is that there is often too much: if the galleries through the heap are filled with water, oxygen cannot diffuse through quickly enough and the anaerobes will take over, turning the water into a witch’s brew that kills most decomposers. On the other hand if the galleries are too open water can evaporate and be lost from the heap, eventually drying it out so much that the decomposers cannot function. A balance has to be struck, and fortunately there are a number of practical tricks you can employ to strike it without having to take samples or measure anything. The most effective is the half-lid system described in box 3 on page 9. See also the box at the foot of page 7.

Food: Finally, let’s look at the nature of the raw material that will serve as food for the decomposers. It is helpful here to think about what we ourselves need as food, and why. An agreeable and balanced meal is usually a combination of plain carbohydrate energy-foods and rich protein/mineral/vitamin foods, and this is the basis of many traditional culinary pairs such as bread and cheese, fish and chips, rice and dhall, pasta and meat sauce, tortillas and beans, egg on toast. Exactly the same principle applies to composting, and for the same reasons. Like us the decomposers need both the plain and plentiful elements carbon, oxygen and hydrogen (mostly as highenergy carbohydrates) to provide metabolic fuel, and the scarcer elements like nitrogen, phosphorus, iron and potassium to make proteins, nucleic acids and thousands of other special-purpose molecules. As with food, the raw materials for composting tend to fall into two recognisable classes, one containing high-energy carbon and not much else, the other containing plenty of nitrogen and other specialpurpose elements. We have given these two classes the nicknames <bread> and <cheese> (see pages 2-3).

6 Composting

Box 1: A note about printing inks In the old days printing inks contained toxic heavy metals such as lead and cadmium, and there is still a widespread belief that the inks are poisonous.

They are not any more. In fact the heavy metal content is lower than it is in most garden soils, so by adding paper and cardboard you are actually improving matters and diluting any existing heavy metals in your garden.

Figure 5: Typical paper and cardboard waste items suitable for composting.

Items which do not naturally have irregular shapes are shown 'scrunched'. destined for the dustbin. Because paper and cardboard are made largely of cellulose fibres, and because these seem to play a role similar to roughage in the diet, we have called this the ‘high-fibre’ or HF system.

The most common examples of compostable paper and card (all in the <sliced bread> category) are: • used tissues; • kitchen towels; • other crumpled papers; • cereal boxes; • other cardboard packaging; • toilet roll spindles; • egg boxes.

From an environmental point of view it is best to recycle, rather than compost, flat paper, such as newspapers, magazines,

office paper and junk mail, and anyway these are difficult to compost without a lot of preparation. That leaves material which is not flat, but bulky and unsuitable for recycling, and – gratifyingly – ideal for composting (see figure 5). Thus the two main forms of household waste paper are complementary, one suitable for recycling, the other for composting. Having said this, balled newspaper does work extremely well in HF systems, so if there is a shortage of other sources newspaper can be used, at the cost of a little extra effort and some loss from the recycling stream.

Paper and card for composting can be collected separately from kitchen waste or put into the same kitchen bin – (after all, they are all going to get mixed together eventually – see figure 1 on page 1).

If collected separately it is important to put them into the compost regularly and not save them up for months. It is a good habit to take both bins to the compost at the same time. It is all very simple, but there is one vital process: scrunching. Compost loves irregular, air-filled shapes. Some paper is already in an ideal form – used tissues, paper towels and egg boxes for example – but it is important to give cardboard boxes a quick squeeze to crumple them a bit (see figure 5). Neatness is the enemy here: do not carefully flatten boxes and put them on the heap in a tidy stack.

Abuse them first! There is no need to tear or shred them unless they are very large. As a rule of thumb you are looking for scrunched balls of <bread> with a volume between 0.1 and 1 litre (between the size of an egg and the size of a melon, perhaps fist-size being a good medium) mixed with cells of <cheese> of a similar or slightly smaller size range.

There are however some cardboard materials that should not be used because they have plastic laminations which do not break down. Milk and drinks cartons are the main examples, but many frozen food packets are also laminated. The plastic is quite inert and non-toxic, and the cardboard in these items gets broken down eventually in the

The logic of <bread> and <cheese>

As it happens, the materials we are calling <bread> often have the stiffness to act as 'pit-props' keeping the insides of the heap accessible to fresh oxygen from outside.

So, generally speaking <bread> can supply both carbon/energy and oxygen. As it also happens, the materials we are calling <cheese> usually contain a good deal of moisture and the stickiness to conserve it, so they can supply both nitrogen (and other minerals) and water. This means that, in principle, mixing wastes from just these two categories will provide all the necessary ingredients all in one go. This makes the composter's life a lot easier! The table summarises the effects on composting of a shortage of the main four ingredients, and what can be done to remedy them.

BUT there is a complication as far as practical composting is concerned, in that some kinds of <bread> are easily accessible to decomposers and break down readily, while others are resistant to decomposers and take a long time to break down. This makes a practical difference because in order for the <bread> and <cheese> to help each other effectively, they should decompose at roughly the same

rate. That is why in the table on page 2-3 we distinguish three categories of <bread>.

One category, which we call <sliced bread> is rather soft and breaks down at the same rate as <cheese>. Together, <sliced bread> and <cheese> make up a functional category called 'soft wastes' which are best dealt with by any of the conventional systems or by our High Fibre method. The other two categories of <bread> make up the 'hard wastes'. What we call <ship's biscuit> is the really tough and resistant branches and twigs, while <Ryvita> is an intermediate category. Hard wastes are best processed separately, or 'exported' for processing elsewhere.

A note about <ship's biscuit> These days we don't have any common carbohydrate food that is completely inedible! But in the days of sailing ships the staple food was a dense, dry, rock-hard cooked bread that would keep for months in its dry condition. It was known as hardtack or ship's biscuit and could only be eaten after prolonged soaking. This is the closest culinary analogy to really tough woody wastes.

Composting 7

The invertebrates are essential to the HF system: it doesn’t work without them. They are all likely to be in your garden anyway, so in most cases they just turn up, although it takes a while for the populations to build up in the heap. Once you have got a good mixed population going there is usually no further problem for new batches of compost. It’s the beginning that is most difficult, a danger that the cheese will putrefy and set everything off in the wrong direction. To ensure a good start it is best to overdo the bread a bit to make sure there’s plenty of air, and seed the heap with a good batch of worms from a similar heap, or failing that, get worms from a fishing-tackle shop or from suppliers of wormery systems. The commonest kind is the small stripy brandling or tiger worm Eisenia foetida, although Dendrobaena worms are equally good. You might find that small white worms become common; these are 'enchytraeids' or potworms and more tolerant of acidic conditions. They do the job but not as well as the larger red worms.

compost. But removing the plastic from the finished compost can be a bit of a nuisance, so it is best to keep this problem to a minimum. In the fullness of time we shall have to bully manufacturers not to put plastic on their packaging; it has no function apart from making the printing look sharp and glossy. As for printing inks, contrary to a persistent urban myth, they are not a problem (see box 1).

What do you actually do?

The basic method is extremely simple. A layer of crumpled cardboard and paper is put in the bottom of a container, say 20-30cm deep, ideally with some worm-rich compost from a previous batch, or someone else's system. Then the heap is simply fed with 'soft wastes' as they arise (see table in pages 2-3).

For typical households this generates approximately equal volumes of <bread> and <cheese> in suitable layers or cells.

Otherwise the ‘sandwich’ rules are followed, for example grass clippings should not be introduced in layers thicker than 15cm. If there is more than this, extra crumpled paper/cardboard should be introduced.

Over the next few months, the organisms and biological processes summarised in figure 7 swing into action, and a zone of fine dung or 'frass' builds up from the bottom. As more material is added at the top, the zone of furious decomposer activity moves up with it.

In a mature heap you will usually find a heaving layer of worms about six inches below the surface. In fact it is a good check that everything is OK to dig down a little way from the top and observe the visible activity.

A correctly functioning High Fibre heap should be visibly lively.

Eventually your compost container will fill up and need emptying. But the top will still have undecomposed material from the last contribution, while the finished compost will be at the bottom (see figure 6). There are various things you can do about this, and to some extent this depends on the design of your container, which we will now discuss.

Figure 6: Typical profile of a High Fibre compost heap. At the top, fresh material just added – note cereal box. Below, dark crumbly humus.

Containers Making compost, either by the classical method or the HF method, does not absolutely need a container. But without some kind of enclosure the stuff gets scattered about by wind and animals. Almost any kind of

Applying the <bread and cheese> principles Constituent

Water

What happens if there is not enough?

Goes slowly or stops

Remedy First Aid

Long-term

Add water

Leave cover off permanently Use more <cheese>

In HF systems you will find ants and woodlice rather than worms

Several litres through rose of watering-can over several days; or better, leave cover off during rain

Nitrogen

Goes Slowly

Mix in moist <cheese>

Use more <cheese>

Carbon

Smelly

Mix in <sliced bread>

Use more <sliced> or <Ryvita>

Oxygen

Smelly

Turn the heap Mix in dry <bread>

Use more <bread>

Smells of ammonia, like a farmyard or stable Smells putrid or sour

such as fresh grass clippings or plate waste

Balled cardboard is good and <Ryvita> is often easier to mix in

especially wetter kitchen wastes such as tea leaves/coffee grounds, or freshly-mown grass clippings You can also add urine diluted10:1 through the watering-can rose

If persistent, occasional layers of <Ryvita>. Regular addition of <ship's biscuit> gives a permanent solution but very coarse compost

8 Composting

Box 2: Limitations and drawbacks of the HF system • Sometimes it just doesn’t ‘boot up’ spontaneously.

Comment: Use plenty of worms as a starter kit and keep the lid off to begin with.

Best to start in the summer.

• It’s not as fast as ‘real’ classical composting.

Comment: True, but it’s faster than failed classical composting! And what’s the hurry?

• Because it does not heat up it does not kill weed seeds.

Comment: Even classical composting does not achieve complete weed kill. Slightly weedy compost is normal with most systems, and is not usually fatal. The ‘dual system’ described later addresses this problem.

• By adding paper and card the volume required is increased, meaning a bin costs more to build or buy and takes up more space.

Comment: True – and the recommended low/flat profile of the container increases the footprint even more. This is a genuine disadvantage in a small garden. On the other hand you are processing the most bulky part of your household waste and greatly reducing the size of your bin bag, and once it's really booted up the rate of volume reduction is high.

container can work, although designs vary in such qualities as size, shape, appearance, durability, ease of use, cost etc. People are bound to ask which is the best sort of container, but that depends on your circumstances. It does not seem to matter very much, but better results are obtained with containers that are: • large (300 litres to 1000 litres); • low/flat rather than tall and thin; • open at the top and bottom, or with a partial cover such as that provided by an old carpet.

The material of which they are made, and whether they have holes in the side or not,

• Some plastic associated with the cardboard feedstock always gets into the heap and does not degrade. It has to be removed from the compost before use.

Comment: Yes, it does: another genuine disadvantage, although perhaps a minor one. Instead of removing twiggy bits you remove bits of plastic. Experience reduces this problem over time (see figure 8).

• The selection of compostable paper and card is yet another category in separation of rubbish at source i.e. in the kitchen or other rooms.

Comment: Yes, but one quickly learned.

• The emphasis on kitchen waste could lead to greater problems with fruit flies and rodents, especially with open containers.

Comment: In principle yes, but the difference is not usually noticeable. Where these are serious problems special containers might be needed, and at CAT we are currently working on these (see boxes 9 & 10).

• If woody wastes are included they are not fully decomposed in the harvested compost. If they are not included what happens to them?

Comment: Woody wastes are always problematic. Various possibilities are discussed in box 5.

does not noticeably affect the results. As far as convenience of compost harvesting is concerned, it depends whether you want to retrieve small amounts occasionally. In this case it is handy to be able to access the bottom of the heap where mature compost develops. For this purpose containers with a coal-bunker-like hatch are useful, although there is a tendency for uncomposted material from above to fall into the resultant gap. Being able to remove a whole side is usually more convenient. Generally it is better to harvest on approximately a yearly cycle, removing the recently-added layer and the worm-rich layer beneath it, then taking out the finished material and stacking it under cover or in

Figure 7: Some food chains in the compost heap Recalcitrant carbohydrates

Fungal-feeding invertebrates: mites, nematodes

Fungi

Micropredators

Detritivores

e.g., epigeic worms, woodlice, millipedes, springtails, mites, slugs, ants, insect larvae

Simple sugars

Dead organic matter

Bulk ingestion

mites, nematodes

Bacteria and actinomycetes

Bacteria-feeding invertebrates: mites, nematodes

Protozoa and rotifers

Anaerobes

Key:

Top carnivores:

centipedes, staphylinid beetles, pseudoscorpions

Anaerobic microbes Living tissue Dead matter, faeces, cadavers Anaerobic pathways in absence of oxygen

Aerobic microbes Macro-invertebrates Micro-invertebrates

Dominant in thermophilic compost systems Active in low-temperature compost systems

bags for later use, then replacing the top layers in the container to start the next batch.

The same process applies to ‘lift-off’ conical bins. For neatness’ sake, if you have the space it is very useful to have a pair of containers.

When one is full, the top layers are simply forked into the adjacent bin and the finished material covered with carpet or tarpaulin (see figure 12b).

A new method for woody waste Observation of ‘dumps’ and the minimalist chuck-it-all-in school of composting has suggested a hitherto-unexplored possibility: a system designed to optimise the beneficial qualities of 'dumps', with containers designed

What happens when the materials are combined ‘correctly’ is that the structural rigidity of the <bread> maintains a complex pattern of air spaces which allows the decomposer organisms to breathe, and as it breaks down, supplies them with energy. The <cheese> provides nitrogen-rich food, and moisture which fills the spaces with humidity without waterlogging. So we have air, water and a balanced diet, and the decomposers are able to multiply quickly. In a large heap constructed all at once, the heat generated by all the metabolic activity cannot escape fast enough and the temperature rises. This favours fungi and bacteria (thick-bordered boxes) and excludes larger organisms. If the heap is small or assembled over a longer period the heat escapes and the temperature, as in most natural situations, is not much higher than the surroundings. This situation favours a full range of decomposers. The larger ones physically break up or ingest the material, mix it about and carry smaller ones around with them. The microbes absorb soluble nutrients or exude enzymes to break down the raw materials chemically. Some middle-sized ones feed directly on the microbes. Others feed on them. All the large ones produce dung which provides accessible food for the microbes.

If there is not enough oxygen the whole system collapses and the anaerobes take over.

Composting 9

Box 3: To cover or not to cover?

It is traditional either to have a lid on the container or to cover the compost with a piece of board or carpet. This is usually explained as keeping the heat in and the rain out. Too much rain could cause waterlogging and nutrient loss through leaching. A lid could also reduce smells; prevent rats, squirrels or birds getting in; and simply look neater. It seems obviously advantageous. Yet many systems with lids dry out, while many systems without lids perform extremely well because rain keeps them moist. Furthermore, lidded systems produce huge swarms of fruit flies which launch themselves into the user’s face when the lid is removed. Unlidded systems also have flies, but they leave in small numbers and are not so noticeable. With cool compost systems there is little heat to retain, so there is no advantage here in lids, and in the case of the 'slow stack' system (see figure 11) we positively encourage rainwater. So for fast systems is it lids or no lids?

The answer sounds unlikely and eccentric: half a lid. This allows water freely into only part of the compost mass.

It stretches the habitat diversity within the container, with one part extra wet, another extra dry. Different organisms favour their own ideal conditions, so this simple expedient favours biodiversity in the heap. But more importantly it means that whatever the weather and outside conditions, there will always be a ‘perfect’ zone for the most important invertebrate decomposers, the brandling worms. This zone will move around the heap but there will always be a large population of worms ready to recolonise other parts as soon as conditions become favourable. It means that rain can be left to provide moisture, and nutrient losses are reduced.

A very simple way of producing the half-lid effect is to put a plastic sheet over part of the compost pile and keep it weighted down with an old carpet.

Box 4: A special case: 'Grassboarding' Usually, grass goes into the compost along with food and paper, but in some gardens there is an immense glut of grass clippings in the summer, enough to completely upstage the modest offerings of the house. Grass is very <cheesy> and must be combined quickly with a good supply of <bread> or it will go off in a particularly disagreeable way, so you need an extra supply of <bread>. The very best method, which we use routinely at CAT, is to combine the grass clippings as they arise with equal volumes of used paper towels, which have the perfect shape. This produces exquisitely clean compost in a few months, and is ideal for many institutions which have supplies of both grass and paper towels. You might find it difficult to obtain such large quantities of used paper towels, but you can use corrugated cardboard instead, readily obtained from a variety of sources and easy to stockpile ready for use. Roughly ripped and crumpled, the pieces of cardboard are simply put into a large open container and the grass is poured on top. A bit of agitation helps the grass to fall into the various voids and crannies. The aim is to avoid any concentration of grass greater than a few litres. It is reminiscent of the need to avoid a critical mass when handling, say, plutonium, although the consequences of failure are not quite so drastic!

This system is rather fast and gives clean, weed-free compost. The only disadvantage is that corrugated cardboard cartons nearly always have parcel tape on them, and this needs to pulled out of the finished compost.

It is worth remarking here that balled single sheets of newspaper work even better, but preparing them is time consuming and it seems illogical to use something which has the correct shape for recycling while neglecting another waste stream that does not.

Figure 9: Grass clippings and corrugated cartons. The frass layer can be seen clearly at the bottom, together with (unaviodable?) skeins of parcel tape.

Figure 8: HF compost after riddling. Scale is given by film canister in the foreground. Most rejects are plastics, although there are always oddities. The object at top right is synthetic fabric.

to match. The worst problem with unshredded woody wastes is their openness and tendency to dry out. This is the opposite of ‘normal’ composting, where the main problem is oxygen supply. Here there is no danger of anaerobic conditions, but what is needed is: • compression to reduce the open voids; • sources of nitrogen; • reservoirs of moisture.

These conditions can be met by placing into a tall, narrow, open-topped container the following: • woody waste of all kinds up to 2cm diameter sticks; • sappy weeds; • weeds or dug-up plants with root balls; • waste sods and turf; • spent composts; • leaves; • shredded material.

10 Composting Box 5: What about woody waste?

Woody waste (<ship's biscuit>) can be a minor nightmare for the composter simply because it takes so long to break down. Although theoretically it is a good source of <bread> it is completely out of sync with the breakdown of <cheese> so does not help much in other compost processes. The amount to be dealt with varies a lot.

Some gardens have virtually no woody waste, some have huge, even overwhelming, quantities, building up faster than they can be dealt with. Occasionally (as with, say, removing a tree or a hedge) there will be a sudden glut. What is to be done? Here are some options, not mutually exclusive: Make a bonfire and burn the stuff…

Comment: This is the traditional way and is a lot of fun. It generates potash-rich ash.

Today it would annoy the neighbours, and possibly be illegal, as well as generate a lot of carcinogens and lose a lot of nutrients and organic matter. It is not an environmentally sound solution.

Use it as a fuel in the house…

Comment: Again illegal in smokeless zones, but at least it’s doing something useful, and you get the ash. If you do not have the right type of stove or fireplace, you could give it to someone who has.

Take it to a civic amenity site where it will be shredded and composted on your behalf…

Comment: Probably better, although if you are taking a small amount a long way in a motorised vehicle you could be doing more environmental harm than good.

Break it into small pieces and give it to the dustman…

Comment: This sounds thoroughly un-ecological, but the fate of very heavy woody waste (small branches and upwards) in a landfill is not so dire as that of soft organic wastes. Under landfill conditions <ship's biscuit> will stay intact for thousands of years. Although this uses up some landfill space it could be argued that it locks up

Box 6: To shred or not to shred?

Electric shredders are cheap and easily available, and deal with lighter items of woody waste, turning them (in our culinary composting jargon) from <ship's biscuit> to <Ryvita>. Environmental purists might query the extra energy cost involved, but this is negligible compared with most other energy uses in modern life and counts as a very valuable use of energy (like the battery charger or the light in the bike shed). Shredders are OK. Shredding reduces the apparent volume of waste immediately by eliminating the very large gaps inevitable in a pile of raw woody waste. On a finer scale, it smashes the pieces of wood into irregular shapes, increasing the surface area thousands of times, opening the material up to attack by a variety of decomposers. The resulting close texture prevents the drying out that affects simple habitat piles between showers of rain.

Shredded woody waste varies quite a bit depending on the original materials. Green wood and hedge clippings contain a fair amount of <cheese> and will break down fairly quickly in a pile on their own, about the same as autumn leaves. Dead wood takes longer, but can be accelerated by occasional doses of diluted urine administered through the rose of a watering-can. It can also be incorporated into the other fast methods of composting, which help it to break down. For classical composting it makes reasonable <bread>, but if used in the high-fibre system it will still not break down as quickly as the other components and will generate a rather coarse compost.

Shredded material is usually attractive to look at and pleasant to handle. It can be used immediately as a weed-suppressing mulch around, say, soft fruit, and is particularly good on paths between vegetable or flower beds.

some carbon that won’t be going back into the atmosphere, so paradoxically it is an environmentally friendly thing to do.

Shred it… …and use it as a mulch, or leave it to compost on its own or incorporate it as <bread> into the main compost system (see box 6).

Live with it…

With small pieces you can keep putting it through the main compost system, riddling it out and recycling it. Comment: This is tedious.

Make a ‘habitat pile’…

This is simply leaving it to rot in big piles in its own good time, excellent for stimulating wildlife in larger gardens, particularly beetles, lichens and fungi. The woody wastes will take between three and ten years to break down, and you will be putting more on top all the time, so the pile can get frighteningly big. This is sometimes called the ‘slow bonfire’ and it certainly is slow, partly because it’s resistant <ship's biscuit> anyway, partly because the gaps between the pieces are so large that they dry out, and partly because there is no source of <cheese> to provide nitrogen. On the other hand the fine humus that finally emerges at the bottom is an exceptionally good weed-free general compost.

Put it onto the dump…

The ‘garden midden’ where all the worst things are put that you can’t be bothered to sort out. One of its crucial features is the presence of soil and spent composts or weeds you didn’t dare put in the main compost heap. These supply moisture and nitrogen, and any <ship's biscuit> which is in contact with them breaks down much faster.

Use the slow stack system. (See figures 10 & 11)

Box 7: Smells HF compost does not smell if it is working well.

Smells will arise if there is an anaerobic pocket somewhere. This is usually due to a large amount of cooked waste being put into the heap in one huge blob – say after a big meal with a lot of leftovers, and perhaps it sat in the bin under the sink for rather a long time until you got round to emptying it. Such situations are bound to arise and we cannot eliminate them entirely: one reason perhaps for siting the compost heap discreetly down the end of the garden! But the problem can be minimised.

Empty the kitchen bucket regularly.

Line the bucket with newspaper or a proprietary recycled-paper bag.

Put all or at least some of your collected paper and cardboard into the same bin when it arises; this mops up extra moisture and reduces smells dramatically.

When you put it into the compost heap spread it about a bit; it is handy to keep a trowel or hand fork nearby for the purpose. If the stuff is in a paper bag, bash it about a bit.

If available, a layer of grass clippings will prevent any further smells.

In other words virtually all garden wastes except grass clippings, and even these could go in if required. Between the recommended ingredients for the HF system and this ‘slow stack’ system, we have covered more or less all the organic wastes in the house and garden. This is what the green householder wants. It suggests a near-ideal compromise between the exacting perfectionism of the

Box 8: Activators Some compost makers swear by the use of special ‘activators’. We have found these unnecessary with the HF method, but they might speed up the slow system. Many activators are simply concentrated sources of nitrogen, so of no use to the HF system which already has plenty. If any such thing is needed, what the late great Lawrence Hills used to call ‘Household Liquid Activator No.1’, a.k.a. human urine, is a free and hygienic source of nitrogen better used in the garden than consigned to the sewers. This is effective on the slow heap. If fresh it can be put on neat. If stored it should be diluted at least 10:1 and administered through a watering-can rose. One other really good activator is vacuumcleaner dust, full of human skin and busy mites! So empty the hoover bag into the compost.

classical method and the laid-back inefficiency of the dump method: that there are two parallel compost streams, one using soft and accessible wastes largely drawn from the house, running on a one-year cycle or less and producing relatively clean and fine compost for use in ‘fine’ applications (e.g. potting up); the other running on a two- or three-year cycle and consuming everything else, generating a coarse but acceptable compost for use in rougher applications (e.g. soil improvement). Both would entail very simple procedures: all the operator needs to remember is the distinction between the two systems, which is easy, then to feed and forget. No further attention should be necessary.

Composting 11 Figure 10: The ideal system for the home garden

Key: Garden wastes: Kitchen waste

Nonrecyclable paper and cardboard

High Fibre method Fast, fine compost

Weeds Hedge trimmings Prunings Autumn leaves Spent composts Turf and sods Twigs

Urine (optional!)

Grass clippings

High-nitrogen wastes

High-carbon wastes

Principal pathways

Heavy woody waste

Slow Stack method Slow, coarse compost

Variant pathways

Export to

Habitat pile

• CA site for thermophilic composting • Landfill • Energy from waste

Figure 11: A summary of the recommended systems compared with classical systems Classical system

High fibre system

Slow stack system

Characteristic

Hot

Cool (Ambient)

Cool (Ambient)

Speed

Superfast

Fast in summer Moderately fast in winter

Moderate to slow

Principal organisms

Fungi and bacteria Thermophilic species

Invertebrates and microbes Mesophilic and psychrotrophic species

Invertebrates and microbes Mesophilic and psychrotrophic species

Size

Minimum 500l

Any size

Large size favoured

Aspect ratio height to width

1 Batch

<1 Continuous feed

>1 Continuous feed

Cycle length

~2 months

~1 year

2-3 years

Weed seed kill

Effective through heating

Ineffective, but none introduced

Effective through time

Feedstock

Soft wastes with modest amount of <Ryvita>

Soft wastes, mostly household plus grass clippings

All garden wastes and adventitous soils

Purpose

To generate weed-free, fine compost in a short time

Process soft wastes with minimum effort; generate fine compost

Process woody and problem wastes with minimum effort

Management

Rigorous, critical

Feed and forget

Feed and forget

An Ideal System For The Large Garden The HF system is ideal for kitchen wastes, grass clippings and waste paper, and will produce fine compost on a yearly cycle largely free of weeds (apart from the odd tomato seedling). It needs a relatively low, flat container to avoid the danger of compression. This container should be strongly constructed and if necessary ratproof. Woody waste (hedge clippings, prunings, twigs, autumn leaves, cabbage stalks) is much more resistant to breaking down and benefits from both compression and the admixture of loose, sappy materials.

We recommend therefore that all garden waste except grass clippings go into a separate, tall container. Earth and spent composts are particularly valuable, as are shredded wastes if they have no other pressing use in the garden. Urine can also be added. This system takes longer to produce a somewhat coarser compost, and we call it the ‘slow stack’ method (figure 11).

As a counsel of perfection then, we advocate two separate ‘Cool Compost’ streams with differently-shaped containers, one a ‘Laurel’ format, the other a ‘Hardy’ format (figure 12a overleaf). You can go

further, and add extra containers for seasonal gluts such as grass clippings (figure 12c).

We have designed a modular system to make these containers from recycled plastic, but they can be home-made. The low, squat ‘Hardy’ shape is easily constructed with lashed-together elements like old pallets or a stack of lorry or tractor tyres. The ‘Laurel’ tower shape is harder and the DIYer should look out for old doors. Having said this there is no need to go to too much trouble: the slow stack system should work perfectly well with large pallets.

12 Composting Figure 12: Three examples of the 'combinations' made for the new modular container system designed at CAT. Top: 'Laurel and Hardy' with a low bin for soft wastes and a 'tower' for hard wastes (see figures 10 & 11).

Middle: The 'compact twin' system for households with mainly soft wastes, giving a convenient annual alternation.

Bottom: The 'Ziggurat' for households with both hard and soft wastes and also large quantities of grass clippings, which are dealt with in the middle container following box 4 and figure 9. The coffee mugs indicate scale.

Box 9: Rats and mice Some people – and some local authorities – are extremely concerned that compost heaps will attract rodents, particularly rats.

Others are not. It is hard to take an objective view. Most of the recommendations in this paper assume that rats are not a serious problem. But what if they are?

Making a well-ventilated but rodent-proof container is not easy – especially if you believe, as we do, that the best system has only a partial lid and no base! But let’s think about it logically. Rats can get in through the top (usually by climbing up the walls) or the bottom (burrowing) or through the sides in an ‘undefended’ heap. Some plastic models have both a lid and a base with only small ventilation holes. However a determined rat will gnaw through a few millimetres of the softer plastics with no trouble. We have succeeded in excluding rats from the bottom by lining with expanded metal lathing, available from building suppliers, or by placing the container securely on flat paving slabs with narrow gaps for drainage. As for the top, use a lid if there is a perceived problem, although the half-lid supplemented with a strong half-inch mesh grille would be better.

Box 10: Flies If you compost kitchen waste there are invariably fruit-flies in the compost. These are like house-flies but much smaller, laying their eggs in decaying vegetable matter. They are quite harmless, and in fact their larvae, growing and feeding in the compost, are all part of the decomposer ecosystem. The problem is an aesthetic one: in a closed container, flies hatch out, cannot escape and build up in large numbers. The result is a great cloud of flies in your face the next time the lid is removed.

One of the advantages of a container with no lid (or a part-lid, as described in box 3) is that fruit flies can leave as they hatch out, so do not accumulate and are much less noticeable. We have also found that a complete layer of grass clippings on the surface seems to reduce the number of flies to near zero. This is a promising method, since the flies are mainly a problem in the summer when grass clippings are available. It is simply a matter of timing your offerings to the heap so that you have clippings ready to spread on the surface over kitchen waste and paper.

You might also notice even smaller flies, rather like midges. These are fungus-gnats and also harmless. All the flies are food for the birds, who will swoop by and take them.

© 2001 The Centre for Alternative Technology Machynlleth, Powys, SY20 9AZ Wales, UK Tel. 01654 705980 Fax. 01654 702782 Email: info@cat.org.uk www: http://www.cat.org.uk Written by Peter Harper.

The information is provided in good faith and believed correct at the time of writing, however no responsibility is taken for any errors.

Our publications are updated regularly; please let us know of any amendments or additions which you think may be useful for future editions.

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