GB2224021A - Conjoint composting - Google Patents

Abstract

A process for preparing compost from sewage solids comprising a first stage for aerobic absorption of sewage solids, water and a source of nitrogen into cellulosic material, and a second stage comprising compressing the initial mixture into a compressed aerobic mass and subjecting the compressed mass to aerobic composting at a temperature of at least 70 DEG C for at least 3 days. The C : N ratio in the first stage is from 30 : 1 to 40 : 1 and the C : N ratio of the final product is less than 20 : 1. The C : N ratio, the pH and the moisture content are monitored in the first stage and one or more of a source of water, a source of acidity or alkalinity and a source of nitrogen may be added. The second stage may be conducted in a heap having a covering of thin plastic sheet material which defines vents and air inlets.

Description

HENSBY BIOTECH LIMITED CONJOINT COMPOSTING This invention relates to the conjoint composting of sewage solids with other materials. Raw municipal or industrial sewage is normally separated by conventional techniques to give a relatively pure effluent, that can be discharged into the nearest water course, and a sludge that is a potential pollutant and must be disposed of in an environmentally sensitive way in order to avoid pollution of land, air or sea. To facilitate disposal, the sludge may be subjected to one or more pre-treatments before disposal.

At present the main disposal routes for sewage sludge are by dispersing at sea, tipping on landfill sites or incineration. In addition, both treated and untreated forms may be used on agricultural land. Thus a simple, and theoretically desirable, technique involves applying raw sludge to the soil, for instance by spreading or injecting, immediately prior to ploughing.

Although this is widely used and convenient where there is ready availability of agricultural land to receive the sludge, in practice there are severe limitations on the extent to which raw sludge can be utilised in this manner. Thus the sludge will often have to be treated to increase its solids content prior to distribution on land, for instance as relatively dry cake, and such treatments can render the sludge more convenient for disposal by other techniques. For instance raw sewage sludge typically has a dry matter content of 1-6%. It can be subjected to various concentration techniques, for instance by the addition of polyelectrolyte conditioners, to give sludges or cakes of higher solids content (for instance 20-30% dry matter content).

Unfortunately, the anaerobic conditions that can occur can give rise to serious odour problems, for instance when the . cake is finally broken open for transport and spreading. Since the technique usually has to be carried out in urban areas where there are difficulties in disposing of raw sludge, the odour problem can be a significant disadvantage. Various techniques of converting the raw sewage sludge to sludges of higher solids content, and even to cakes, are known and often include anaerobic digestion of the sludge.

It is known that various types of sewage can be composted and that if the composting is conducted effectively there is substantially no odour problem. In U.S.A., sewage cake is frequently composted with municipal refuse but the method suffers from the disadvantage that is is necessary to convert the sewage initially into cake and the municipal refuse (which is an essential component of the composting mix) can provide pollutants.

Pressures on existing sludge disposal routes are increasing and public awareness of environmental factors of sewage sludge disposal is also increasing. Thus lower cost methods of composting are necessary to make this a viable route for sewage disposal.

GB1498938 discloses a process of composting a slurry of organic waste material with vegetable matter and flowing the drained liquor in a recirculating flow through the organic material in which anaerobic composting reaction is taking place.

An article in the Journal of the Soil Association (1972), 17, 27-31 by Major W.H. Gifford describes a method of anaerobic decomposition of straw and sludge from anaerobic digestion tanks. The process- comprises building a heap of lightly spread straw and spreading slurry over the straw with turning after 8 to 10 weeks and then leaving for a further period of 6 weeks. The need to avoid compression of the heap is emphasised. This process again is unsatisfactory as the length of time taken for the composting process will increase production costs to an unacceptable level.

A wide variety of other composting processes are, of course, well known and they range from, for instance, the commercial production of mushroom compost by composting of horse manure, straw and water to the amateur production of garden composts by the composting of garden and other domestic refuse.

In Farmers Weekly, 22nd January 1988, a process is described in which chicken manure, 20 tonnes straw and 100,000 litres sewage sludge are formed into a mix, with the sewage sludge being added gradually to the straw over a period of one week, the mix is put into windrows (elongated stacks) at which the temperature rises to around 80 C, the stack is mixed and aerated every 3 days and after 2 weeks the straw/sludge mix has been completely reduced to a compost that is stable, easily transported, suitable for storage or immediate use and has a small fertiliser value.

Although such a technique can, with an optimum combination of straw quality, sewage sludge quality, and chicken manure quality, give a useful product, the described process is not commercially operable in the manner described since there is a very high risk that the conditions necessary for proper composting will be lost during the process or may never even start. If this happens then, instead of a sweet smelling compost, the product will be a very large volume of foul smelling anaerobic slime, with the result that the disposal problems of the sewage sludge would have been vastly increased.This is a particularly serious risk because the straw can be of variable physical and chemical content (for instance depending upon the source and the weather conditions during harvesting), the sewage sludge can be of variable solids and chemical content (for instance depending upon the origin of the sludge), and the ambient weather conditions can be variable and can significantly affect the composting conditions.

It would therefore be desirable to be able to devise a process by which sewage solids could be commercially composted to produce reliably an agriculturally useful compost.

In the invention compost is prepared from sewage solids by a process comprising a first stage comprising aerobically absorbing sewage solids, water and a source of nitrogen into cellulosic material comprising structured cellulosic material in amounts intended to give an initial mixture having a C:N ratio of from 30:1 to 40:1, pH greater than 7.5 and a moisture content of 70-80%, a second stage comprising compressing the initial mixture into a compressed aerobic mass and subjecting the compressed mass to aerobic composting at a temperature of at least 700C for at least 3 days and continuing the composting until the C:N ratio in the final product is less than 20:1, and monitoring the C: :N ratio, the pH and the moisture content at one or more occasions from near the end of the first stage and, in response to the monitored values, adding one or more of a source of water, a source of acidity or alkalinity and a source of nitrogen.

The structured cellulosic material must have sufficient structure that it maintains sufficient porosity in the compressed mass that aerobic composting conditions exist in the compressed mass. The structured cellulosic material is generally a dried, fibrous, cellulosic material typically having a solids content above 80% and usually above 90% and often above 95% by weight. Generally it is material that has been harvested or otherwise collected and stored, the moisture content of the stored material being sufficiently low that substantially no rotting has occurred during storage. The preferred material is cereal straw, especially wheat straw, but other materials that can be used include seed husks, bracken and corn cobs.Other cellulosic material can be incorporated (usually in minor amounts) with the structured cellulosic material, the total amount of cellulosic material being such as to give the desired C:N ratio of the 30:1 to 40:1. Examples of such material include rape and sugar beet waste. The C:N ratio in the total cellulosic material that is used will usually be above 40:1 and often above 45:1, but usually below 60:1.

The sewage solids and water that are aerobically absorbed into the structured cellulosic material are generally provided as raw sewage sludge, but if desired additional water can be added and/or sewage sludges (or even cakes) of higher solids content can be added. Thus, although the process is preferably operated as a way of utilising raw sewage sludge as most or all of the sewage solids, it is also possible to apply the process to the utilisation of other types of sewage solids, generally in the presence of additional water. The solids content of the raw sewage sludge (or other blend of water and sewage solids) is generally in the range 1-10% and often is in the range 3-6%. The sewage may be municipal sewage, food waste slurries or animal waste slurries.If it is municipal sewage, it is preferred that the sewage should initially have a relatively low content of heavy metals or should have been pre-treated to reduce the heavy metal content.

Sufficient of a source of nitrogen must be incorporated with the structured cellulosic material (and optionally other cellulosic material) to give, in theory, a C:N ratio of from 30:1 to 40:1, most preferably about 35:1. If the level of nitrogen drops far below this level then it limits and slows down the composting process, and if the nitrogen level is too high, excessive loss of nitrogen in the form of ammonia occurs. The preferred source of nitrogen is chicken litter (which may also introduce some cellulosic material) but other materials that can be used include brewers grains, sugar beet waste, other vegetable materials having high nitrogen content, other organic materials having a high nitrogen content (e.g. urea) and inorganic compounds containing available nitrogen, especially as ammonia, e.g. ammonium salts.

The proportions of the various materials, and of any other additives that are incorporated should be such that the mix resulting from the aerobic absorption will, in theory have a pH above 7.5 and preferably above 8 since the high pH is necessary in order to facilitate break-down of the structured cellulose. The pH is usually below 8.5 or 9, preferably from 8.2 to 8.5. The materials should also be selected so that the moisture content of the mixture is, in theory, between 70 and 80%. If the amount is above 80% there is a high risk of water-logging of the mix during subsequent stages, so that the conditions become anaerobic. If the amount is below 70% there is usually insufficient water to promote adequate composting of all the components of the mix.Preferably the proportions are selected with the intention that the moisture content should be in the range 75 to 80%, often around 75 to 77%.

The aerobic absorption stage involves maintaining contact of the water and sewage solids and the source of nitrogen with the cellulosic material while the whole mix is aerobic. The process usually takes 1 to 10 days, often 3 to 7 days, and is preferably as short as possible.

Provided substantial compression is not applied, and provided the cellulosic material has a reasonably strong structure, suitable aerobic conditions will prevail if the cellulosic material is formed into a loose heap having a height generally no greater than about 2 metres.

If the cellulosic material is formed of long and/or rigid straw it may be desirable to fragment the straw during formation into the heap. Sewage sludge is then pumped onto the heap, generally in at least 2 or 3 stages of increasing quantities, generally over a period of at least 3 days. Preferably substantially no run-off of the sludge is allowed and the heap is generally mixed at least once to ensure even wetting. Alternatively water is pumped onto the heap accompanied by other sewage solids, or dry sewage solids could be incorporated into the heap before application of the water. If there is any run-off of aqueous medium, preferably it is recovered and pumped back onto the heap.Any other way of bringing the cellulosic material, the water and the sewage solids into contact for sufficient period for the aqueous medium to compost the cellulosic material aerobically to a sufficient extent for the cellulosic material to absorb all the aqueous medium may be employed.

During this stage of composting the temperature in the heap rises and reaches e.g. from 40 to 700C typically 550 to 650C, at the maximum point although higher temperatures may occur. The initial balanced carbon to nitrogen and moisture levels in the mix result in a rise in pH during this initial stage generally to 8.0 to 8.5 or more. This high temperature and high pH enables rapid alkaline hydrolysis of the straw, thus enabling greater absorption of large quantities of liquid sludge.

In order to initiate composting during the aerobic absorption stage (and to promote subsequent composting) it is preferred to incorporate a composting inoculant into the initial mixture of cellulosic material, sewage solids, water and source of nitrogen. A convenient composting inoculant is often horse manure, which is typically present in an amount of 1-10% based on the weight (including moisture) of the straw of the other cellulosic material, typically around 5%.

An especially preferred inoculant is compost from a previous process. This compost may be an end product of the previous process but is preferably taken from a partially composted stage of the previous process, most preferably the stage when the compressed mass is undergoing aerobic composting. The amount of compost that can be incorporated in the initial mixture can be any convenient amount but it is usually adequate to use 1-10% based on the weight of straw or other cellulosic material. Instead of, or in addition to, using these relatively large amounts-of natural inoculants, synthetically prepared inoculants may be used, generally in smaller proportions, typically 0.001 to 1% by weight based on the straw.

As soon as all the aqueous medium has been absorbed in the structured cellulosic material to give the desired moisture content (generally within 2 to 7 days of the start of the process) the resultant initial mixture is then compressed under conditions that permit aerobic composting and the attainment of a temperature of at least 700C for at least 3 days. If the initial mixture is not compressed then, as is shown in the article by Giffard, the composting will be very slow. As a result of compressing however the process is greatly accelerated and high temperatures, often above 750C are readily attained. The precise degree of compression that is optimum depends upon the structure of the cellulosic material and on the amount of aqueous medium.For instance if wheat straw or other highly structured material is used much higher pressures can be applied than if a weak straw, such as barley straw, is used, since the latter is more liable to collapse so as to give a non-porous and potentially anaerobic mixture. The bulk density of the compressed aerobic mass is generally in 3 the range 0.25 to 1 tonne per m3, most preferably around 0.4 to 0.6 tonnes and especially about 0.5 tonnes per cubic metre. If, in any particular process, it is found that anaerobic conditions start to develop then a lower degree of compression must be applied. If the desired high temperatures are not achieved rapidly, for instance within 1 to 3 days, then a higher degree of compression should be applied (unless that leads to anaerobic conditions).The temperature in the mass at this stage is usually at least 750C and often 80-90"C within 1-3 days of compression and should remain at such a temperature for at least 3 days.

Aerobic conditions must prevail throughout the compressed aerobic composting process. Air can be forced into the mass, for instance through a slatted or other porous base or through inlet pipes, especially if the mass is in a composting vessel. Preferably however the compressed mass is in a heap and natural air draught is then usually adequate to maintain aerobic conditions. The heap can be any desired shape but is preferably a windrow (i.e. an elongated heap 1-4m wide and 1-4m high.

Typically within 24 to 48 hours of compressing the mass and forming it into a windrow, the temperature in the windrows rises. The bulk density is chosen so that the most efficient heat insulation is achieved whilst aerobic conditions are retained throughout the windrow, and inside the windrow maximum temperatures of up to 800C or more are reached while the outer portion of the windrow (the outside 10cm) remain at ambient temperatures.

In order to kill the various human, animal and plant pathogens present in the mixture, it is essential to expose the entire mass to moist heat, generally at above 550C for at least 3 days. In order to ensure that all of the contents of the windrow have reached this temperature, and to provide aerobic conditions, the windrow is turned in air and recompressed at least once.

If the windrows are turned too often, the structure is broken down and there is less effective aeration in the windrow. Preferably the windrow is turned 2 to 5 times, often 3 times over a period of from 10 to 14 days. The turning thoroughly mixes the compost and allows effective killing of the pathogens. Tests may be carried out for the presence of salmonellite organisms at each stage.

Such tests are used as an indication of the microorganism safety of the material.

Even though the proportions of water, sewage and cellulosic material, and the contents of each, will have been selected to give the desired proportions in the initial mixture, variations do occur due to, for instance changes in ambient conditions, variations in the method of turning and compressing, and variations in the materials that are used. These variations can lead to disaster in the commerical operation of the process and an important feature of the invention therefore is that the C:N ratio, the pH and the moisture content are monitored on one or more occasions during the process and the results utilised to adjust the conditions prevailing in the compressed aerobic mass.It is not necessary to measure quantitatively all three parameters (C:N ratio pH and moisture content) at every instance and indeed qualitative estimation of the properties (e.g. moisture content) may be adequate. The monitoring is generally conducted near or at the end of the aerobic absorption stage and preferably also on one or more instances during the compressed aerobic stage.

Nitrogen can be added in order to attain the correct carbon to nitrogen ratio by the addition of more nitrogenous material, for example, chicken litter, between the turning steps. If the moisture is too low, more sludge or water may be added, and if the moisture content is too high additional turns are applied to the windrow as water is lost as steam during turning.

Although this causes the temperature to drop, the temperature recovers in a few hours. If the pH is too low then the addition of additional nitrogenous material will generally raise the pH adequately (as a result of production of ammonia) or lime may be added or, if the pH is too high, then acidic material may be added.

The optimum mOisture, C:N and pH values at any particular time will depend upon the particular materials being used, must be kept in balance, and can be determined by routine experimentation and generally by interpolation between the theoretical values in the initial mix and in the composted product after 10-14 days composting at a temperature above 70 C.

These values and preferred values are generally as follows: Initial Mix Preferred GaFosted Preferred Initial Mix Composted C:N 30:1-40:1 34:1-36:1 < 20:1 10:1-20:1 Moisture % 70-80 75-77 65-78 70-75 pH > 7.5 8.3-8.8 7.0-8.0 7.4-7.8 1.5-2.5 1.7-2.1 2-4 2.5-3.5 At this stage the compost is suitable for some applications. However for most applications the moisture is too high, so that the compost is difficult to handle and expensive to transport. In addition, the high pH which reflects the high ammonium level can be injurious to plant life. The carbon to nitrogen ratio may also be too high and if this is applied to the soil the compost will affect plant growth unfavourably as it traps nitrogen in the soil preventing uptake for plant growth.

These problems are overcome by providing a third stage which is a period of maturation to reduce moisture, pH and carbon to nitrogen ratio to acceptable levels. The third stage comprises allowing further aerobic composting which may be either in a windrow or in a loose heap. This maturation period also results in the temperature of the compost dropping, generally due to the microorganisms diminishing the water content and/or the nitrogen, so that the process is self-limiting. The third stage may take from between 7 and 28 days or more.

The third stage also effects the removal of any products of anaerobic composting. Anaerobic processes produce volatile fatty acids and so testing for the absence of acetic acid determines that maturation is complete.

It will be appreciated from the foregoing that when the compressed aerobic composting is being conducted in a heap, it is important to maximise the degree of aeration (at any particular level of compression) and to minimise the amount of surface cooling, so as to minimise the amount of turning that is necessary in order to maintain aerobic conditions and to ensure exposure of the entire mass to temperatures for killing pathogens. Also it is important to achieve adequate control of conditions within the heap even though environmental conditions may vary widely.

It is known to provide heat insulation for preparation of mushroom compost from Scientia Horticulture, 20(1983) pages 53-59, which teaches the use of fibre glass, 10cm thick contained in an envelope of polythene sheet to provide heat insulation.

However, such an insulation layer will be costly, requires storage space and also adds an undesirable risk of possible glass-fibre contamination ot the compost.

Also the described arrangement and shape is liable to tend to give anaerobic conditions in parts at least of the heap.

According to a preferred aspect of the invention the compressed aerobic composting is conducted in a heap that has a covering of thin plastic sheet material that substantially prevents exposure of the outer surfaces of the heap to the atmosphere but that defines one or more vents towards the highest point of the heap for the escape of hot gases and one or more inlets distributed around substantially the entire base of the heap for the entry of air into the heap.

It is unnecessary to provide full insulation of the heap and such a covering produces a reduction in the temperature gradient between the inside and the outside of the heap by preventing direct contact with the air and weather conditions. This also enables greater control of the composting conditions inside the covering and gives improved aeration, without incurring vast additional cost. The covering may be provided by a single sheet of plastic material that has, for instance, had apertures cut in it to define the vents or, more usually, the covering may be provided by two or more sheets of plastic material that are laid on the heap with spacing between them in such a way as to define, for instance, the one or more vents. The plastic is preferably black or clear sheet that is resistant to ultra-violet light.If the plastic is not resistant to ultra-violet light it becomes brittle within a few days of exposure to daylight and is easily damaged. Typically the thickness of the sheet is below 0.5mm, preferably below 0.25mm and most preferably below 0.lem and is generally greater than 0.005mm, preferably from 0.01mm to 0.05mm and most preferably around 0.025mm.

It is desirable that the combination of one or more inlets around the base of the heap and vents at the top of the heap should be such as to create a venturi effect within the heap, and thus the cross-sectional area of the inlets should be considerably greater than the cross-sectional area of the vent or vents. The shape of the heap should be such that "dead spots" at which there is little or no air flow are avoided and so preferably the heap has a peaked top that may be the top of inwardly inclined side walls or that leads from substantially vertical side walls, and preferably there -is a vent provided along the peak and there is a gap in the covering along substantially the entire length of the base of the side walls between the bottom of the covering and the ground or other platform on which the heap has been built.Generally the heap is an elongated windrow in which event the covering preferably defines a single vent along the peak and an inlet along each side between the bottom of the covering and the ground.

The technique reduces the amount of turning that is required during the compressed aerobic composting stage, both from the point of view of achieving aerobic conditions and from the point of view of exposing all the mass to the desired high temperatures. It also renders the process much less susceptable to variations in weather and to water logging due to rain.

The processes of. the invention thus lead, for the first time, to a viable commercial process that can repeatably and regularly result in the conversion of raw sewage sludge or other sewage and straw or other structured cellulosic material to produce an agriculturally useful compost irrespective of variations in the straw or sewage or ambient weather conditions, and without causing unacceptable environmental pollution problems.

The resultant compost is high in organic content and as such can be used as any conventional compost but is particularly useful when mixed with sub-soil to produce top-soil, or as a peat replacement.

Throughout this specification the carbon values are determined by placing a sample in a baffle furnace to obtain a value for organic matter content, and this is followed by calculations to give the carbon value, the nitrogen values are obtained by Kjeldahl anaylsis and the solids content and water content values by weighing the sample before and after drying to constant weight in a microwave oven.

EXAMPLE The following procedure is followed to produce 50 tonnes of sewage compost: 40 Heston bales of wheat straw are broken open and fragmented, using front end loaders, and pushed into a heap. To this heap c. 3 tonnes of chicken litter and c. 1 tonne of horse manure are added. The heap is then mixed, again using front end loaders. The mixed heap is levelled off to a height of not greater than two metres.

The sewage sludge is then pumped on to the heap in increasing quantities over the next five days. No run-off of the sludge is allowed. The heap is turned using front end loaders each day to ensure even wetting and thorough mixing of the raw materials.

The exact proportions of raw materials used depends upon their chemical analyses. The moisture, total nitrogen content and carbon:nitrogen ratio of all of the raw materials are analysed during and near the end of the sewage addition. Adjustments in the amounts of sewage and chicken litter are made, as necessary, to achieve values of: Moisture 75-77% Nitrogen 1.9% Nitrogen/tonne straw > 8 kilo Carbon:Nitrogen 35:1 The mix is formed into windrows using a front end loader and a windrow former. The windrows are square in cross section and are 2 metres high by 2 metres wide. The compost is compressed during windrow formation to a bulk density of c. 1 tonne per 2 cubic metres.

The windrow is turned at least 3 times over the next two weeks. This turning process is accomplished by using Traymaster turning machines. It is analysed during the composting, generally during the turning and, if necessary sewage and/or chicken litter are added or extra steam is allowed to escape during the turning process.

At the end of a two week period the compost has a moisture of c. 73% a pH of c. 7.5, a nitrogen level of c.

2% and a C:N ratio of < 20:1.

The compost is then left for 4 weeks to mature. At the end of the maturation process, a stable compost is produced having a C:N ratio of 11:1, a water-content of 70%, a pH of 7.2, a nitrogen content of 3.0%.

Claims (21)

1. A process for preparing compost from sewage solids comprising: a first stage comprising aerobically absorbing sewage solids, water and a source of nitrogen into cellulosic material comprising structured cellulosic material in amounts intended to give an initial mixture having a C:N ratio of 30:1 to 40:1, pH greater than 7.5 and a moisture content of 70 to 80%, a second stage comprising compressing the initial mixture into a compressed aerobic mass and subjecting the compressed mass to aerobic composting at a temperature of at least 70 0C for at least 3 days and continuing the composting until the C:N ratio in the final product is less than 20:1, and monitoring the C::N ratio, the pH and the moisture content at one or more occasions from near the end of the first stage and, in response to the monitored values, adding one or more of a source of water, a source of acidity or alkalinity and a source of nitrogen.

2. A process according to claim 1 in which composting is initiated during the first stage by incorporating a composting inoculent into the initial mixture of the cellulosic material, sewage solids, water and source of nitrogen.

3. A process according to claim 2 in which the inoculent is one or more of horse manure, compost from a previous process, preferably at the second stage, or other natural or synthetically prepared inoculents.

4. A process according to claim 2 or 3 in which the inoculent is horse manure which is present in an amount of from 1 to 10% based on the weight (including moisture) of the cellulosic material, preferably around 5%.

5. A process according to any preceding claim in which the first stage is for 1 to 10 days and the resultant mix has a pH above 8.0 and below 9.0.

6. A process according to any preceding claim in which the mix resulting from the first stage has a water content of 75 to 80%.

7. A process according to any preceding claim in which following the first stage, the mixture is compressed to a 3 bulk density of 0.25 to 1 tonne per m , preferably from 3 0.5 to 0.6 tonne per m3.

8. A process according to any preceding claim in which, during the second stage, the temperature inside the compressed mass is 80 to 900C for 1 to 3 days.

9. A process according to any preceding claim in which during the second stage, the compressed mass is turned 2 to 5 times over a period of 10 to 14 days.

10. A process according to any preceding claim in which the sewage is supplied as raw sewage sludge or other blend of water and sewage solids and has a solids content of 1 to 10%.

11. A process according to any preceding claim in which the cellulosic -material is a dried fibrous, cellulosic material having a solids content of above 80%.

12. A process according to any preceding claim in which the cellulosic material is one or more of cereal straw, seed husks, bracken, corn cobs, rape and sugar '. beet waste.

13. A process according to any preceding claim in which the total amount of cellulosic material has a C:N ratio of below 60:1 and above 40:1.

14. A process according to any preceding claim in which the source of nitrogen is one or more of chicken litter, brewers grains, sugar beet waster or ammonium salts.

15. A process accroding to any preceding claim in which there is a third stage during which further aerobioc composting is conducted until the compost contains substantially no acetic acid.

16. A process according to claim 1 in which the sewage sludge or other blend of water and sewage solids has a solids content of 3 to 60%, the cellulosic material has a solids content of above 90% and a C:N ratio above 45:1, and the nitrogen source is chicken litter.

17. A process according to claim 1 in which the product resulting from the first stage has a pH of 8.2 to 8.5 and a water content of 75 to 77%.

18. A process according to any preceding claim in which the second composting stage is conducted in a heap having a covering of thin plastic sheet material that defines one or more vents towards the highest point of the heap and one or more air inlets distributed around substantially the entire base of the heap for entry of air into the compressed mass.

19. A process according to claim 18, in which the covering of thin plastic sheet material is provided by two or more sheets that are laid on the heap with spacing between them defining the one or more vents and air inlets.

20. A process according to claim 18 and 19 in which the plastic sheet material is substantially resistant to ultra-voilet light and is below 0.5mm, preferably below 0.25 mm thick.

21. A process according to claims 18 to 20 in which the compressed mass in the second stage is placed in a heap which is an elongated windrow and the plastic sheet material is 0.01mm to 0.05mm thick and defines a single vent along the peak and an inlet along each side between the bottom of the plastic sheet material and the ground.