Windrow Composting
Windrow Composting, also referred to as Open Air Windrow Composting (OAW) and Open Windrow Composting (OWC), is a means of treating Green Waste and uses naturally occurring microbes which feed on organic material and require oxygen. Open Air Windrow Composting is therefore a form of Aerobic Digestion and by processing the organic material the treatment process produces a compost material.
Overview
OAW is a relatively inexpensive way of treating Green Waste e.g. twigs, leaves, glass cuttings and branch prunings etc. and is generally not suitable for Food Waste because of the legislative requirements to deal with Animal By-product Material - which generally requires management in In Vessel Composting or Anaerobic Digestion treatment processes. The process to turn the original material into the compost type material through processing in windrows takes around 12 to 16 weeks. OAW sites typically process 20,000 to 30,000 tonnes per year, but can operate at smaller and larger scale.
The Process
For the process to produce a soil conditioner to be used by farmers and others it must meet End of Waste Criteria and, in the majority of the UK, this is demonstrated by meeting the Quality Protocol for compost and the associated Quality Standard PAS100. The statutory guidance can be found here[1][2].
Stage 1
Green Waste is delivered to the site (which has to have an Environmental Permit) and large items of contamination such as plastic, metal etc. are removed. The material is then shredded to produce a uniform particle size which has a structure to allow air to pass through it during the treatment process.
Stage 2
The shredded material is then placed into piles in long rows known as ‘windrows’, in the open air. The composting process is kick-started by naturally occurring micro-organisms already in the waste. They break down the material, releasing nutrients (and producing heat and some steam) and in doing so they increase the temperature up to around 65oC needed to kill pathogens and weed seeds and meet the criteria for the final product quality specification. There are three discreet phases in the overall process[3] which is set out in more detail in PAS100 but which is typically:
- Sanitation - temperature must be held above 65°C, for at least 7 days, and turned at least twice
- Stabilisation - temperature maintained above 55°C, managed for 10 weeks, and turned at least 6 times
- Maturation - Post-screening (stage 4 below) the product is generally stored for at least 4 weeks before it can be used
Stage 3
Each windrow is individually monitored and when the required temperature has been reached and held for the required time at each stages it is then turned to mix the material and ensure an even distribution of the composting process and to introduce sufficient oxygen to ensure the process does not turn anaerobic. The turning of the windrows may occur up to 8 times or more before a stabilised compost is produced. The turning process can be done with simple tractor/shovel type machines, or dedicated turning machines that introduce some additional mechanical breakdown of the material.
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Stage 4
Stabilised material is screened, and any remaining contaminants removed, to produce a range of product grades for various end uses such as soil conditioning/a soil improvement material providing it meets the Quality Standard PAS100 which dictates the process standard to ensure that it meets End of Waste Criteria. Any large materials that have not fully broken down in the process (such as larger branches) are taken from the screening process and re-introduced back into the process as outlined above.
The different grades of compost can be used for different purposes. Some grades are used as agricultural soil conditioners, others as mulches and others are suitable for landscape gardening or finer grades can be used on golf courses. In some cases the material is blended with other material to produce other products for horticultural use (for example sand)[4].