Annex III - Construction of compost heaps

Introduction

This annex provides some general information regarding the assembly of materials, construction and features, and the management of compost piles. Further guidance on composting should be gained by consulting individuals with knowledge and local experience.

ASSEMBLING AND PROPORTIONING MATERIALS

There are several ways to assemble raw materials in compost heaps. Materials can either be mixed together in a homogeneous mass before composting or layered in the pile during heap construction. Regardless of the method used, the pile must have the correct C/N ratio and moisture content.

If using wastes which are high in either carbon or nitrogen then they must be blended to balance the ratio. Use Table I in the main text to become familiar with the C/N ratios of numerous raw materials. Proportioning wastes to obtain the desired C/N ratio (between 25-35) is done by visually estimating their quantity and character. This requires some experimentation and experience, Box 6 provides some guidance to initially estimating the ratios of some mixtures. There is less need to mix wastes if their ratios are between 20 and 30.

Blending to achieve proper moisture content also applies to wet and dry wastes. Green, fresh vegetation should be spread out in the sun for several days before composting to reduce moisture contents, then stored under cover until needed. Woody and stalky materials, as well as bark and sawdust, must be kept moist in piles or bins and soaked in water for several days before incorporation into the compost pile. The moisture content can be controlled during heap construction by sprinkling with water

Materials may be assembled in layers in various ways. The common approach is to alternate layers of high and low C/N ratio wastes. Carbon-rich layers are twice as thick as nitrogenous layers. One approach suggests placing 20 cm layers of carbonaceous wastes alternating with 10 cm of nitrogenous materials until the desired height is reached (Poincelot, 1975).

A coarse textured high carbon waste (stalks, brush, etc.) should be laid as a base layer to a thickness approximately 150-220 mm. Layers can be constructed using individual raw ingredients (proportioning them so all wastes are distributed throughout the pile) that result in a C/N and moisture balanced heap. For example, a pile can be constructed with 100 mm of coarse dry wastes, 75 mm green weeds and leaves, 50 mm manure, and a sprinkling of urine soaked earth/wood ash and repeating this sequence 7-10 times. Limit the depth of any individual plant material to 100 mm or of any manure to 50 mm. Premix wastes with poor aeration quality (e.g. slurries) with wastes which are coarse (e.g. straws, grasses). The addition of a small quantity of fertile soil is often recommended to provide more beneficial organisms to the pile.

Alternately, batches of properly balanced wastes (straw, grasses, manure) are mixed together with some previously finished compost and soil (and water if it appears too dry) and laid on a pile making a 200 mm layer. Mixed batches are added in 200 mm layers until the desired height is reached.

A covering of straw, finished compost or soil is recommended for the top of the piles to control fly breeding, shed rain, and insulate against heat loss.

COMPOST HEAP FEATURES

There are several features which can be included during heap or pit construction to enhance the composting process. These are structures for aeration and drainage, insulating walls, and protective covers which are discussed below. Whether these features are included is dependent on the available labor, materials, time, and climate.

Aeration

Constructing the compost heap on a loose structured base or an elevated grid or platform enhances the air flow through a pile between turnings (Figure 2). To construct a platform, pairs of bricks are laid at 600 mm intervals and covered with brush. If termites are a problem a platform 150 mm off the ground can be built with bricks and a grid of bamboo poles. The bases of the brick columns can be put in buckets kept filled with water to keep termites off the columns.

Figure 2 - Methods of aeration for compost piles

To ensure aeration of a heap built directly on the ground, a base can be constructed of drain pipes or shallow trenches ( 150 mm wide by 100 mm deep) spaced at 600 mm intervals and covered with a wire mesh or layer of coarse material such as brush or stalky material. Alternatively a base layer of brush and branches can be used to promote aeration of the pile.

Air vents can also be built into a pile or pit (see Figure 3). During construction poles are inserted vertically after about 0.5 m of material is laid. When the heap has reached its final height, the pole is then pushed back and forth to make a hole approximately 100 mm in diameter. Pole vents should be shaken several times a week to keep them open. Air vents should not be placed closer than 600 mm to an edge of the heap with a 1.2 m maximum distance between vents. Other methods to enhance aeration between turnings are incorporation of poles or stalks during construction which are removed a few at a time over several days or inclusion of a 10-20 cm diameter perforated metal or wire vent in the heap (Minich and Hunt, 1979). Incorporating stalky material from plants such as Typha (cattails), Phragmites, or bamboo also helps aerate the heap (BOSTID, 1981).

Figure 3 - Stockade with inserted hollowed bamboo poles

Drainage

Good drainage is important for a compost site and for the individual stacks and pits. Conditions can turn anaerobic if water is not drained away from the base of the heap. Pits (which are prone to become anaerobic) generally need to be equipped with drainage channels and are best if lined with cement, bricks, or masonry, on high rainfall sites a roof may be needed. In addition, drainage structures also aid in ventilation of the heap. For stacks above ground where drainage is poor, gutters can be excavated around the base to facilitate flow of water away from the heap.

Insulation

This is of interest to subtropical areas such as Nepal, China, and northern India. Organic material is an excellent insulator, but additional insulation is necessary if conditions are cold, windy, or if the heap is smaller than one cubic meter. Insulation allows for a larger volume of the composting mass to reach the highest temperatures. Particularly on small piles, where moisture and heat loss are significant, walls constructed of wooden boards or corrugated metal are recommended.

More appropriate for larger piles is a 25-50 mm covering of soil, compost, hay, or a mud plaster. A plastic sheet (perforated with 25 mm holes at 150 mm intervals) or mats of woven grass or leaves on top of a heap helps insulate and keep in heat while allowing for air circulation.

Protection

A covering or roof constructed over the pile to reduce exposure to direct sun and heavy rains is required in some climates (Figure 4). Direct sun dries out a pile too quickly and excessive rains may water log the pile. A roof should be situated approximately 50150 mm above the top of the heap to allow for ventilation, angled to shed water, and easily disassembled and reassembled to allow for turning of the pile. If mechanized turning is to be used the roof must be sufficiently high to permit access.

Figure 4 - Removable cover on compost bin

COMPOST HEAP CONSTRUCTION

The shape and size of a compost heap is dependent on the quantity of waste and the climate. Described below are some types of construction which may be appropriate for nursery compost operations. These are: below ground - pits/trenches, and above ground - bins and windrows. The maximum height of a heap should not exceed 1.5 m to minimize compaction and retain porosity; widths generally do no exceed 2.4 m. Length is determined by the space available and the quantity of wastes to be composted.

Pits/Trenches

Composting in pits or trenches tends are needed for the anaerobic process because air circulation is limited. Since excavation is required, below-ground composting is initially more costly (in labor and funds) than other non-mechanized methods. Subsequently pits are cheaper to operate because they are not normally turned as often as above-ground stacks; more frequent turning, however, would keep the pit from becoming anaerobic. The benefits of using pits are that they are better insulated for use in windy or subtropical climates and less space is needed for compost operations as they are turned in place. Pits are not recommended for composting where the water table is high or during the rainy season because of potential drainage problems. A roof or rounding of the top of the heap in a pit may allow composting during rains. !

The minimum size for a compost pit is about I m³ which can be accommodated by a hole measuring 1.5 x 0.9 x 0.6 m in depth. The deeper the pit, the more likely it will become anaerobic.

Figure 5 illustrates a typical trench construction of a pit 8 x 3 x 1 m which should accommodate approximately 24 m³ of raw materials producing from 8-15 m³ of finished compost. A lining of concrete or masonry and installation of tiled drainage trenches are optimal. If not lined, then the walls and bottom should be tamped, unlined pits tend to crumble and are often difficult to maintain.

Figure 5 - Details of a compost pit

During filling, compost pits need additional attention to balance the moisture content. Since turning is infrequent, moisture is conserved and care must be taken that the moisture content does not exceed 50%. Trenches can be filled by dividing them into sections and filling each section separately (see Figure 6). The last partition or section is left empty. When turned, wastes are transferred first into the empty or vacated section and then each subsequent section is moved into its vacated neighbor. Completion of two turnings (at week 2 and 5) should produce finished compost in 12-16 weeks under good conditions.

Bins

Bins can be constructed to contain compost heaps above ground when smaller quantities are composted. Several compost bin designs are illustrated in Figure 7. Wire mesh or stockade type bins can be easily moved and reconstructed during turning and also allow the system to be mobile. Compost bins in the subtropics should be constructed with solid walls to protect against heat loss since its relatively small size may not provide enough insulation. Good insulating materials for wall construction would be wooden planks, corrugated iron sheets; cinder blocks can also be used but do not function to insulate as well as other materials. The walls are built on at least three sides and the front either left open or equipped with a removable cover. In hot tropical climates insulated sides are not needed so a wire mesh, stockade, or woven fence can be used to contain the pile. Wooden sided bins under a galvanized roof were constructed in Sabah (Bowen and Chow 1984).

Monitoring of bins may reveal that open-sided bins need frequent irrigation because the sides are vulnerable to drying from exposure to sun and wind. Experience, monitoring, and weather may dictate the need for an insulating blanket or cover on top of a small mass to minimize losses.

Figure 6 - Partly built pit compost showing layers of ingredients

Figure 7 - Examples of compost bins

Windrows and Stacks

Free-standing compost heaps generally are used in production of large quantities of compost. Large quantities are assembled in long windrows the length of which is dependent on space available, the size of the production, and the climate. Assembly of a heap 20 x 2 x I .5 m requires about 80 m' or 20 tons of waste, producing about 6-8 tons of finished compost. During hot summer months, stacks require a minimum size of 2.3-3.8 m³ and in cooler temperatures the minimum size should range from 5.5-8.5 m³ (Gotaas, 19563. For windrows, the height of vertical sides of the heap should be lower during the hot season (approximately 1.2 m) and can be higher in the cool season (up to 1.8 m); if sides are sloped (30 degrees) then the heights should not exceed 1.8 m and 2.1 m in the hot and cool seasons respectively. The width of windrows generally does not exceed 2.5 m, but if a highly mechanized system is used, widths may be as much as 20 feet. The tops of the heaps are typically pointed or rounded in wet seasons and in dry seasons may even have a depression on top to intercept rainfall.

Windrows can be assembled either in layers or, alternatively, if there is available labor, space, and equipment, the (properly balanced) wastes can be premixed before assembling into a homogeneous pile. If turning is to be done infrequently, then the moisture content should not exceed 50%. Turning, as previously described, should incorporate the material near the outside into the inside of the pile. The frequency is dependent on materials, physical characteristics, and the time schedule determined by the nursery manager as to when finished product is needed. The final turning may be included with moving compost to a storage area. Figure 8 shows a layout of a compost system where separate piles are moved and combined during turning events. This may be convenient if there is enough space at the compost site.

Figure 8 - Layout of compost unit in depot using stacks

The physical turning of windrows can be done manually or mechanically. Manual turning of relatively large quantities of compost is time consuming and needs significant amounts of labor. Simple technologies for turning exist and include methods that use horse or oxen dozers or plows. Turning by mechanized methods is either done with modified farm tractors, front-end loaders, and bulldozers or with specially designed windrow turners (Golueke, 1981).

If centralized compost making facilities are to be installed it may be necessary to consider investment in special machinery. Self-propelled, windrow turners have been used where thousands of tons of compost are produced annually. In Kenya, a commercial-scale project composts 5,000 ton/year of tea wastes using a windrow turner. These turn, shred, blend, and aerate windrows simultaneously, types differ predominantly in the way they agitate material and where the pile is reformed (either behind the moving machine or adjacent to the original pile) (Golueke, 1981). Windrow turners can be obtained which operate on varying windrow widths from 3.6-6 m and up to 2.4 m high. These all have a capacity to turn up to 3000 tons/hour, so are generally suited to very large operations. Smaller models turning up to 100 tons/hour are also available. Some benefits other than the ability to work large quantities of waste include that only a single operator is required and no shredding or grinding is required prior to windrow assembly.

COMPOST HEAP MANAGEMENT

Monitoring

A compost heap should be monitored periodically to assess the temperature and moisture conditions. These help indicate the health and progress of the heap. Three to seven days after construction the pile should be examined to ensure that the temperature has risen and that the pile is not too wet or dry. Frequency of monitoring can only be determined from experience through initially daily measurements. If turning has to be frequent, daily monitoring may have to continue to ensure optimal interior conditions are maintained.

The rise in temperature is necessary to kill pathogens. It is also an indication of aerobic microbial activity. Temperature can be monitored by inserting a long thermometer near the center of the mass. If above 60 °C, the pile needs turning and mixing in order to dissipate heat, redistribute high nitrogen wastes, and to aerate it. Alternatively, to assess temperature the insulation or top layers can be peeled back to expose the center of the heap; steam should rise from the pile. (A bad smell is a sign of fermenting and anaerobic conditions.) Another way of testing is to insert a stake of metal or wood about 300-600 mm into the heap and leave it for approximately 10 minutes. Afterwards the portion of the stake which was in the pile should feel very hot, but not too hot to hold and it should also appear moist.

If temperatures have not increased after the first several days to a week, then the moisture status should be checked to determine whether conditions are too wet or dry or the C/N ratio too high. The structure of the material should be loose and crumbly, not packed and lumpy. A handful which is squeezed should exude small droplets of water (like a damp sponge). If the wastes are too wet, then they will not only appear soggy, but squeezing a handful will readily produce water and be smelly. If this is the case, the pile should be turned and rebuilt with the addition of straw-like materials to absorb additional moisture.

If moisture does not appear to be the problem, then the addition of nitrogen-rich materials such as fresh green vegetation or manures will likely initiate the temperature increase. If the heap is too small it may not have enough material to generate and retain high temperatures, in this case the pile should be made larger. Monitoring should continue frequently until problems are resolved.

Turning/Aeration

Turning of a compost heap restores aerobic conditions and by maintaining the thermophilic conditions speed decomposition. Frequency of turning depends on availability of labor, time until compost is needed, climatic conditions, and the progression of microbial activity within the pile. Turning should occur only during the thermophilic stage. Usually a pile is turned a minimum of three times to ensure that all material has resided inside the pile and been exposed to maximum temperatures. Frequent turning causes a loss of nitrogen, so if a nitrogen-rich compost is desired less turning and a longer composting period is recommended (Minich and Hunt, 1979).

Turning results in short-term temperature decreases which, in large volume heaps, will recover within hours. The physical turning of the pile requires that the materials on the outside (typically a layer 150 mm thick) be placed to the interior of the pile during reassembly If possible, material is removed in an orderly fashion, keeping those wastes separate which have been near the outside and inside of the pile. The heap is then rebuilt making sure the outer material is placed near the center of the mass. Sometimes separate bins, piles, or pits can be constructed adjacent to each other so wastes can be transferred from one to the other with less time and effort.

When materials are turned it should be done so that there is more air incorporated into the pile on reassembly. The material should be fluffed up as much as possible during this process. Sprinkling the material with water may be necessary if it appears dry. If wet, dried wastes may be mixed in, keeping in mind the maintenance the proper C/N ratio.

Piles which have been constructed on platforms for aeration may require less frequent turning due to the increased air flow. During cooler weather turning should be limited because temperature recovery is slow, retarding composting If the pile becomes soaked by rain it should be turned, however, in heavy rains it is better to let the pile become anaerobic instead of water logging the entire pile by turning.

In trenches, the last section to be filled is turned into the section left empty upon filling (See Figure 6). The material is moved as 150 mm thick layers, then mixed, and watered lightly. Vent holes should be reconstructed as the sections are filled. Gaining access to a pit for compost turning is difficult and awkward. Using of long handled tools and placing boards across the pit to stand on is recommended.

Maturing and Processing

After composting has been completed, it is often necessary to allow the composted material to mature or ripen further. This breaks down any remaining waste fragments into

a humus high in lignin and microbial protein. For nursery use, compost must be fully mature with a C/N ratio below 15. This process may take up to several weeks or months.

When maturing is complete, the color of the material will be a dark brown to black with a smell both earthy and pleasant. Growth of young plants in the compost to "bioassay" the product can help asses the maturity of the compost and determine whether further composting or maturing is necessary (Chen et. al., 1992).

As a final step for nursery use, the dried (less than 40% moisture content) finished product should be passed through a wire mesh to provide a homogeneous, fine-grained compost. Any organic fragments not passing through the screen can be composted in the next batch.