On-farm composting methods: large scale passive aeration


Growing concerns relating to land degradation, threat to eco-systems from over and inappropriate use of inorganic fertilizers, atmospheric pollution, soil health, soil biodiversity and sanitation have rekindled the global interest in organic recycling practices like composting. The potential of composting to turn on-farm waste materials into a farm resource makes it an attractive proposition. Composting offers several benefits such as enhanced soil fertility and soil health – thereby increased agricultural productivity, improved soil biodiversity, reduced ecological risks and a better environment. The following technology is part from FAO's publication on-farm composting.


Large Scale Passive Aeration

Windrow Composting

Turned Windrows:

Windrow composting consists of placing the mixture of raw materials in long narrow piles or windrows which are agitated or turned on a regular basis. The turning operation mixes the composting materials and enhances passive aeration. Typically the windrows are initially from 3 feet high for dense materials like manures to 12 feet high for fluffy materials like leaves. The width varies from 10 to 20 feet. The equipment used for turning determines the size, shape, and spacing of the windrows. Bucket loaders with a long reach can build high windrows. Turning machines produce low, wide windrows.


Windrows aerate primarily by natural or passive air movement (convection and gaseous diffusion). The rate of air exchange depends on the porosity of the windrow. Therefore, the size of a windrow that can be effectively aerated is determined by its porosity. A light fluffy windrow of leaves can be much larger than a wet dense windrow containing manure. If the windrow is too large, anaerobic zones occur near its centre which release odours when the windrow is turned. On the other hand, small windrows lose heat quickly and may not achieve temperatures high enough to evaporate moisture and kill pathogens and weed seeds.

For small to moderate scale operations, turning can be accomplished with a front-end loader or a bucket loader on a tractor. The loader simply lifts the materials from the windrow and spills them down again, mixing the materials and reforming the mixture into a loose windrow. The loader can exchange material from the bottom of the windrow with material on the top by forming a new windrow next to the old one. This needs to be done without driving onto the windrow in order to minimize compaction. Windrows turned with a bucket loader are often constructed in closely spaced pairs and then combined after the windrows shrink in size. If additional mixing of the materials is desired, a loader can also be used in combination with a manure spreader.

A number of specialized machines have been developed for turning windrows. These machines greatly reduce the time and labour involved, mix the materials thoroughly, and produce a more uniform compost. Some of these machines are designed to attach to farm tractors or front-end loaders; others are self-propelled. A few machines also have the capability of loading trucks or wagons from the windrow.

It is very important to maintain a schedule of turning. The frequency of turning depends on the rate of decomposition, the moisture content and porosity of the materials, and the desired composting time. Because the decomposition rate is greatest at the start of the process, the frequency of turning decreases as the windrow ages. Easily degradable or high nitrogen mixes may require daily turnings at the start of the process. As the process continues, the turning frequency can be reduced to a single turning per week.

By the end of the first week of composting, the windrow height diminishes appreciably and by the end of the second week it may be as low as 2 feet. It may be prudent to combine two windrows at this stage and continue the turning schedule as before. Consolidation of windrows is a good wintertime practice to retain the heat generated during composting. This is one of the advantages of windrow composting. It is a versatile system that can be adjusted to different conditions caused by seasonal changes.

With the windrow method, the active composting stage generally lasts three to 9 weeks depending upon the nature of the materials and the frequency of turning. Eight weeks is a common period for manure composting operations. If three weeks is the goal, the windrow requires turning once or twice per day during the first week and every three to five days thereafter.

Passively Aerated Windrows:

The method, passively aerated windrow system, eliminates the need for turning by supplying air to the composting materials through perforated pipes embedded in each windrow. The pipe ends are open. Air flows into the pipes and through the windrow because of the chimney effect created as the hot gases rise upward out of the windrow.

The windrows should be 3–4 feet high, built on top of a base of straw, peat moss, or finished compost to absorb moisture and insulate the windrow. The covering layer of peat or compost also insulates the windrow; discourages flies; and helps to retain moisture, odour, and ammonia. The plastic pipe is similar to that used for septic system leach fields with two rows of 1/2-inch diameter holes drilled in the pipe. In many aerated pile applications, the pipe holes are oriented downward to minimize plugging and allow condensate to drain. However, some researchers recommend that the holes face upwards. Windrows are generally formed by the procedures described for the aerated static pile method. Because the raw materials are not turned after the windrows are formed, they must be thoroughly mixed before they are placed in the windrow. Avoid compacting the mix of materials while constructing the windrow. Aeration pipes are placed on top of the peat/compost base. When the composting period is completed, the pipes are simply pulled out, and the base material is mixed with the compost.This method of composting has been studied and used in Canada for composting seafood wastes with peat moss, manure slurries with peat moss, and solid manure with straw or wood shavings. Manure from dairy, beef, swine, and sheep operations has been used.




Created date

Mon, 23/10/2006 - 12:30

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