aeration time, can lead to a lack of oxygen in the upper layer of compost if the aeration time is too short.
Positioning the oxygen sensor High air speeds during composting can impact negatively on how well ammonia and moisture is removed. Particularly at higher temperatures when the oxygen consumption is less, a lower air speed can be used, provided the compost structure allows. Measuring the oxygen concen- tration can reveal what is really going on in the upper compost layer. I often come across systems where an oxygen sensor has been inserted through the wall at front of the bunker. The problem with this position is that the compost slowly settles and become softer during the process due to the combustion as fermenta- tion progresses. This places too much pressure on the sensor probe which can consequently bend. The best position for a compost sensor is 30 cm below the surface of the compost layer, as because of the ‘stop and go’ ventilation system this is the layer that is most sensitive to the lowest oxygen concentration. In closed compos- ting systems with constant aeration, the optimal position is the return duct – the same as in phase II tunnels. In countries where the winter temperature drops below 0 degrees, oxygen measurement sensors are not used in bunkers. The experience gained during spring, summer and autumn is enough to assess the correct aeration time during winter months.
Bunkers, compost piles and transport After the bunker process is another time when oxygen supplies to the compost are not attended to carefully enough. At many plants, after it has been removed from the bunker, and before filling in the tunnel, the compost is placed on a concrete floor in a pile that often is too wide, so that natural aeration cannot introduce enough oxygen into the very centre. One solution is obviously not to create a too wide pile. This enables a natural chimney process that carries oxygen to the compost, without the risk of the centre acidifying. The ‘storage’ time of compost after phase I is also important for an optimal process in phase II. The real reason for many companies to place the compost in a pile or windrow ‘just for a while’ before it is filled in the tunnels, is to build up microorganisms. The compost usually cools sharply after it has been removed from the bunkers. If the compost temperature in the piles is regularly too high (after bunker and before tunnel) cooling the compost from 80 degrees to
Ensure the suction hoses of a measuring system are straight so no water can accumulate. Otherwise, the pump will suck false air.
around 75 degrees before emptying the bunker is advisable. However, caution should be taken not to do this using too high air speeds for too long, as in this phase of the process it is easy to remove too much ammonia from the compost, which diminishes its contents and creates ‘poor’ compost. The compost cools further as the bunker is emptied so usually the compost temperature ultimately arrives at a value between 55 and 65 degrees. When the compost temperature in the pile significantly exceeds 60 degrees, there is a greater risk of an oxygen deficiency occurring in the centre of the pile, and the compost will have difficulty becoming active in the tunnel.
Phase I compost often has to be transported in closed trailers, which creates an oxygen-poor environment in the compost. On arrival, I strongly recommend mixing the load with 2% phase II compost to compensate for any micr- oorganisms destroyed in transit by lack of oxygen. The compost must also be allowed to ‘rest ‘for at least 6 hours before it is filled into the tunnel so that enough microorganisms can be built up to ensure an optimal phase II process. If storing the compost on a concrete floor for some hours is impossible because it causes odour nuisance, it is important to level for long enough (up to 15 hours) to allow the microorganisms to recover and reach sufficient levels.
In my next article we will talk about oxygen in fase II and III.
MUSHROOM BUSINESS 11
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