Aeration and oxygen
Far before modern aeration systems were applied, the traditional windrow system was used worldwide and still is today. Compost was, and still is, also composted in piles. These windrows or piles are not always placed on aerated floors, so no air is blown mechanically through the mass. In this case the composter depends enti- rely on the natural flow of air through the com- post, also called the ‘chimney effect’. Before the well-known aeration systems were applied in bunkers and tunnels, a lot of experimentation was done injecting oxygen into the windrows, as in windrows and piles patches of anaerobic com- post are likely to form very easily, especially if the mass is not mixed and turned well enough. There are still companies that turn the compost with loaders to aerate it. But all too often this is just moving the pile around, rather than actually lifting and mixing it - something that it is impossible to do perfectly with a loader. People soon realised that one of the most critical influential factors for a good composting process is oxygen in the compost. Homogeneity and structure also play a very important contributing role. This ultimately led to the development of bunkers and tunnels with the associated machi- nes and installations. The better the mass is mixed with the correct proportions of raw mate- rials containing carbon and nitrogen, the more oxygen the composting process needs, especially at the beginning of this process. Aerated floors have an excellent effect on the final result, especially on the homogeneity of the compost, provided the mass is also mixed per- fectly. For instance, I have seen significantly bet- ter results for compost that is aerated constantly in phase I tunnels than compost that is pulse aerated at intervals in phase I bunkers on the same composting plant with the same compost. A positive effect of constant aeration in phase I tunnels is that the very high ammonia concen- trations accelerate the process. These concentra- tions can be as high as 10,000 to 12,000 ppm during the closed phase I tunnel process. Moreover, the oxygen content is more uniform. So what about composting in windrows without aeration? The results achieved in bunkers and tunnels can also widely differ, depending on the quality of mixing and filling and the levels of aeration at different times during the process, but also on the cold start calculation and how well optimised water management is. In bunkers and tunnels we are dealing with large volumes of compost, while with windrow composting the windrows are about 2 metres wide and 2.10 metres high, which means that good natural aeration can be reasonably expected, provided the compost is not compacted too densely by the compost turner and the compost is not too wet.
Windrows in combination with bunker composting in the US.
Mixing chicken manure without spreading it evenly is directly related to a lower compost quality.
Organising windrow and pile composting
There are also many various sub-systems within windrow and pile composting, all of which affect compost quality. The ultimate quality has a cor- relation to the following factors: If used, how is the pre-wet process organised? Are machines used to pre-wet? How well are the raw materials mixed, and are machines used? Are the wind- rows/piles sheltered by a roof to protect the com- post from negative weather conditions on site? How well can water be applied and drained away? Are the floors slightly inclined so that leachate can run off the windrows/piles? How often is the compost turned by the compost turner? How
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