AEROBIC COMPOSTING TYPES
Aerated (Turned) Windrow Composting
The organic waste is laid out in rows of long piles called “windrows” and aerated by turning the pile periodically by either manual or mechanical means. Windrow size is very important: Where the windrow is too large, anaerobic zones occur near its centre. These 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.
There are a number of specialised machines for turning windrows that reduce the time and labour involved considerably, mix the materials thoroughly, and produce a more uniform compost. Some of these machines attach to farm tractors or front-end loaders, others are self-propelled. A few machines can also load trucks and wagons from the windrow.
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 N mixes may require daily turnings at the start of the process but even then it can be difficult to control odour and leachate. As the process continues, the turning frequency can be reduced to a single turning per week.
With ever tightening regulatory controls globally, consents for windrow systems near built up environments are becoming harder to obtain and windrow composting is fast becoming recognised as fit for processing green (yard or garden) waste only.
Liquid waste in the form of leachate is released during the composting process. This can contaminate local ground and surface-water supplies and should be collected and treated.
Windrow composting often requires large tracts of land, sturdy equipment, a continual supply of labour to maintain and operate the facility, and patience to experiment with various materials mixtures and turning frequencies.
With the windrow method, the active composting stage generally lasts three to nine weeks depending upon the nature of the materials, the frequency of turning and local climatic conditions. In dry arid climates or very wet (high rainfall) periods maintaining the correct moisture content can be difficult.
Aerated Static Pile Composting
In aerated static pile composting the aerated bunkers can be enclosed in a building, or can be outdoors, depending on the climate and the location. Organic waste is mixed together in one large pile instead of rows. To aerate the pile, layers of loosely piled bulking agents (e.g., wood chips, shredded newspaper) are added so that air can pass from the bottom to the top of the pile. The piles also can be placed over a network of pipes that deliver air into or draw air out of the pile. Air blowers are activated by a timer or a temperature sensors. Some aerated static piles are placed indoors with proper ventilation (e.g. tunnel composting).
Aerated static piles are suitable for a relatively homogenous mix of organic waste and work well for larger quantity generators of yard trimmings and compostable municipal solid waste (e.g. food, paper) including local governments, landscapers and farms. This method does not work well for composting animal by-products or grease from food processing industries.
Since there is no physical turning, the static pile method requires careful monitoring to ensure that the outside of the pile heats up as much as the core. Air channelling is a common problem resulting in areas that get too hot or too cold, too wet or too dry. Process air will need to be treated via a biofilter and volumes may be very large. Very large buildings may also be needed to provide complete enclosure of the composting process. Static pile composting generates leachate, which will require treatment prior to disposal or agricultural use.
Build-up of moisture (leachate) in the pile will generally react with CO2 and volatile acids lowering the pH of the material and slowing the composting process. As material degrades it also has a tendency to slump or compact that negatively influences airflow.
Static pile composting typically requires equipment such as blowers, pipes, sensors, and fans, which can involve significant costs and technical assistance. However having a controlled supply of air enables construction of large piles, which requires less land than windrow.
When the pile has been formed properly and where the air supply is sufficient and the distribution uniform, the active composting period is completed in about three to five weeks.
Organic materials are fed into a drum, silo, concrete-lined trench, or similar equipment where the environmental conditions including temperature, moisture and aeration are closely controlled. The apparatus usually has a mechanism to turn or agitate the material for proper aeration. In-vessel composters vary in size and capacity.
In-vessel composting can process large amounts of waste without taking up as much space as the window method. In addition, it can accommodate virtually any type of organic waste (e.g. meat, animal manure, small animal mortalities, biosolids, food scraps). Some in-vessel composters can fit into a school or restaurant kitchen while others such as a HotRot 3518 can be as large as a school bus to accommodate large food processing plants and modular/ scalable to process municipal waste.
In-vessel composting works aerobically so produces very little odour and minimal leachate. In HotRot units the surplus moisture is removed in vapour and there is no leachate.
In-vessel composters cost more up front to establish and generally require technical assistance to operate properly, however this method uses much less land and manual labour than other forms of composting.
Conversion of organic material to compost can take as little as 8 days in HotRot units due to its ability to support a much higher active microbial biomass. However once the compost exits the vessel it still requires 2-4 weeks for the microbial activity to stabilise and the pile to cool.