4.2 What types of composting methods exist?

Composting encompasses various methods tailored to transform organic materials into valuable compost efficiently. These include windrow composting, aerated static-pile composting, passive aerated composting and in-vessel composting. Each method presents unique features and considerations in the pursuit of sustainable waste transformation.

Windrow composting

In this method, the material to be composted is formed into long piles, typically triangular or trapezoidal in section and 1.25 − 2.5 m in height, with a width-to-height ratio of roughly 2 to 1. These piles should be large enough to retain heat and ensure that thermophilic conditions are reached but also porous enough to allow oxygen to flow to its core. Regular turning of windrows is necessary to maintain porosity and allow oxygen into the core of the windrow.

Please note that larger windrows have a higher capacity to hold compost mixture, which can lead to quicker achievement of the required temperature for pathogen inactivation. However, it also requires more effort to turn. On the other hand, smaller windrows are easier to turn by hand, but they may take longer to reach the necessary temperature for pathogen inactivation. Therefore, windrows turned by machinery should be larger, while windrows turned manually should be smaller to reduce the required effort.

Aerated static-pile composting

This method allows for composting without turning, wherein the compost material is stacked in piles of about 2 m in depth and then covered with a layer of finished compost or suitable material to reduce heat loss. The piles are aerated by blowing air into them via pipes placed beneath them. This method eliminates the need for manual compost turning, as the aeration process is enough to ensure proper decomposition. Aerated static-pile composting is known to be more efficient than turned windrow composting, as it requires less time to complete. However, this method is more costly and requires a reliable power source, sound maintenance systems, and an adequate supply chain.

Passive aerated composting

A passive aerated compost box allows air to flow through the compost without turning. It consists of a box with holes on the sides and a perforated pipe or a layer of sticks at the base. The pipe or sticks create an air space under the compost, which helps to aerate the pile.

In-vessel composting

In-vessel composting is where the material to be composted is placed in enclosed reactors with systems to control temperature, moisture, and odours. However, commercial in-vessel composters can be expensive and relatively complex compared with other techniques. It is usually used in colder regions or for composting organic food waste.