Dynamic versus static composting – the role of agitation during composting process
It is a well known fact that composting is a process driven by the activity of different aerobic microorganisms. Some of them prefer temperatures between 55 and 65 °C, others work better at temperatures below 40°C. All of them need oxygen, which is available free of charge as a part of the fresh air we all need for our life. For you and me it is very easy to get access to fresh air. Just step outside of our house and take a deep breath. But what about the microorganisms, that lives deep inside a compost pile? How to make the fresh air available for them? To answer these questions we need to have a look deep into the compost pile.
In the microcosm of compost the compost material texture generally consists of three parts: solid phase (organic and inorganic particles), liquid phase (water) and gas phase (free air space between particles). Microorganisms are settling on the surface of the solid phase. Everything they need for life and growth they receive via the liquid phase of the system, also the oxygen. Thus it is necessary to take care for a sufficient and complete moistening of the solid phase of the system first. Second step is the adequate fresh air supply via the gas phase. Water and air are sharing the free space between the particles and compete with each other. Keeping the balance in this competition is the basic task of the composting process.
Simplified model of the compost microcosms:
The compost particles (brown) are surrounded by a water film (blue), where the microorganisms live. The oxygen (green arrows) from the free air space (white areas) is transferred into the liquid phase, where the microorganisms make use of it. Carbon dioxide (red arrows) produced by the microorganisms is transferred from the liquid phase into the gas phase. Air volume needs to be exchanged.
Sufficient fresh air supply to the compost material we can achieve in different ways. The simplest solution is using the “Chimney Effect”, which works pretty well in small and middle size windrows with triangular cross section. High temperatures in windrow core create an air flow from the bottom to the top. Already used air with high carbon dioxide content is leaving on top while fresh air is entering into the system from the environment. For larger windrows or windrows of trapezoid cross section and in vessel systems active ventilation is necessary.
There are different composting technologies available in the market. All of them try to follow the basic rule of composting, as described above. The technological range of the available systems starts at simple open air windrow composting technology, which can be combined with under floor ventilation and/or membrane cover systems and ends at sophisticated in vessel systems with automatic process control.
Nevertheless which technology is chosen, the various composting technologies are often classified in dynamic, semi dynamic and static technologies. If we speak about this classification the main characteristic to differ between those classes is the movement (or agitation) of the compost material.
- Dynamic composting technologies are characterized by continuous movement of the compost material. A typical dynamic composting technology is drum composting, where the process takes place in a horizontal cylindrical vessel that rotates around his axle.
- Semi dynamic composting technologies are characterized by frequent agitation (more than one agitation per week), but discontinuous compost material movement (agitation). Among the semi dynamic composting technologies are agitated windrow, agitated bed and agitated lane composting.
- Static composting technologies are characterized by non frequent compost material movement (equal or less than one agitation per week). Typical variations are aerated static pile composting and static tunnel composting.
To understand the role of agitation in composting process we again need to have a look into the microcosm of the compost material. During the process the compost material is compacted by its own weight and by the microbiological activity itself. Microorganisms make use of the organic matter after a while larger particles break down to smaller ones. When the free air space between particles becomes smaller, the competition between air and water went stronger.
If the water can not disappear the space for air becomes less and finally insufficient for free air exchange and causes a lack of oxygen supply. Even forced aeration systems can not longer achieve homogeneous air distribution because larger air channels will build inside the compost material. Large air volumes using these channels causing excessive dry out of the zones around the air channels while other parts of the material will no longer receive any airflow. Anaerobic zones will build causing bad smell and slow down of the composting process.
To avoid this situation frequent agitation is absolutely necessary to maintain the free air space between the compost particles which finally allows fresh air supply to all zones of the of compost material. Therefore agitation must be an integral part of each composting system. Furthermore, agitation can also improve other processing steps of the composting system:
- Initial Mixing: At begin of the process compost material is often very inhomogeneous. Even green waste collected from parks and gardens often contains larger volumes of grass clippings or leaves without any other materials mixed in. For optimal starting conditions the input material has to be mixed carefully. Separate mixing equipment is often too expensive, but agitation can take over this job.
- Distribution of moisture: It might happen that the input mixture is too wet or becomes too wet due to high precipitation. All water not bound for surface moistening of the solid particles will remain in the free space between the particles. Sooner or later this excess water follows the way of gravity down to the bottom of the windrow or vessel. Agitation will redistribute the excess moisture allowing to use the “too much” of water from the bottom zone for remoistening of the surface zones. During the composting process water is evaporated from the compost material because of high process temperatures. This water has to be replaced by irrigation. Homogeneous distribution of moisture can be only achieved by agitation during irrigation.
To have a complete picture we now want to have a look at the real demand for agitation. Obviously agitation is good for the process, but how much? Shall we agitate as often as possible? No! Too much agitation can also have a negative effect on the process and will cause unnecessary operation costs too. The right answer is: As often as necessary.
As discussed above the demand for agitation is created by the need for maintaining the free air space in compost material. It is also well known that the microbiological activity as well as the biological degradation is higher at begin of the process and decreases towards its end. Compaction takes place much faster in the phase of high biological activity. Therefore also the demand for agitation is not constant over the whole processing time. There is a higher demand at begin of the process than at its end. From this point of view technologies with fix agitation frequency do not allow an optimal processing. Also continuous agitation, like practiced in dynamic composting systems, will not allow an adaption of the agitation frequency to the demand. Only systems with flexible agitation frequency, like many semi dynamic technologies, will give us the opportunity to agitate as often as needed.
When it comes to the point to decide, which composting system is suitable for the project, it is necessary to have a look at the agitation system. Static systems without frequent agitation will miss one very important possibility for process control. Dynamic systems do offer this possibility, but it’s worth to have a look at it in detail. The optimal system will offer you to decide, when and how often to agitate … to get the microorganisms working.