How to Make Compost in Commercial Quantities by the Windrow Process

THE COMPOSTING PROCESS

The open-air windrow system is a proven and effective method for stabilising and sanitising green waste materials from civic amenity sites, kerbside collections and landscaping contractors. It is said that the method has been used since Roman times. The composting process is typically completed in 10-16 weeks, depending on the required end product, and in the UK most if not all green waste composting is carried out in compliance with the compost quality standard PAS 100.

Each stage of the composting process needs to be managed effectively and consistently to achieve the desired standards, and is monitored by the Organics Recycling Association for compliance with the requirements of the standard, before PAS 100 certification can be awarded to each composting site.

RECEPTION, SORTING AND SHREDDING

The most common problem is that contaminates arrive in the green waste, it is essential to ensure a clean feedstock as part of the process. Getting that right then sets the plant up to be capable of producing the required high quality of compost. The first stage in achieving this must take place during collection, and before material arrives at the composting site. Operators must work closely with the waste disposal section in the local authorities and with the collection contractors’ customers to make sure they can effectively train their “suppliers” in the supreme importance of delivering clean material.

After the green waste is delivered, the contaminates still present must be time consumingly, and rigorously, hand-picked from the stockpile before shredding.

Even when such care is taken plastic contamination remains as one of the composting industry’s greatest problems. Compost contaminated with plastic litter is simply not attractive to customers, although it may otherwise have all the properties of a good compost. Bits of variably coloured plastic look awful in the soil, but can also damage the health of wildlife when ingested. Many composters insist that the contaminates be removed before any further treatment takes place, although fine screeing can be used after the compost has matured and before it leaves site, most operators like to ensure the best feed quality, rather than take the plastics etc out at the end.

This is due to the fact that after the material has been shredded, removal is both expensive and relatively ineffective.

A growing number of local authorities collect garden waste at the kerbside in bags. In this case the bags cannot be plastic types. The solution has been to use bags that are fully ‘degradable’ during the composting process. A standard for compostable materials like the bags for compost collection is available (BS EN13432:2000) and using it makes it easier for buyers to identify suitable bags. To prevent contamination of the final compost products, the solution adopted by a number of the compost operators is to only accept waste collected in bags that comply with the standard. The responsibility for this is the supplier’s and they must provide evidence of compliance.

TURNING

Turning the windrows during the composting process is essential in order to get air into the material to maintain biological activity and achieve the correct temperatures for sanitisation. The composting plant operator for instance, usually turns the material at least eight times during the windrow process, with rigorous monitoring of both temperature and moisture content throughout.

MONITORING & TESTING

Comprehensive monitoring and testing is undertaken at several stages of the process to ensure both process control and end product quality. Thorough monitoring is essential to demonstrate PAS 100 compliance, and to ensure that appropriate temperature ranges and moisture content are maintained. The composting plant operator monitors temperature on both sides of the windrow on a daily basis, testing up to ten locations for each windrow. Windrows must maintain a temperature of at least 55°C for three consecutive days prior to turning. If the temperature of a windrow exceeds 75°C, it must be turned to cool it down. Temperatures are measured and logged multiple times daily so that a full record for each windrows progress is maintained, and records of each batch are routinely kept.

It is critical that the moisture content is controlled so that the composting process is not prevented due to loss of compost structure, and ensuing lack of aeration or microbial activity. If moisture is present at above 60% the windrow must be turned to allow moisture to escape. Exceeding this level could make it difficult for air to get in by removing the open fibrous nature of the compost and allowing it to bulk down into a mass with very little airflow for composting to occur in, causing the material to become partly anaerobic, and produce odours. If moisture is below 40 per cent, water needs to be added, otherwise the rate of degradation slows down considerably.

SCREENING

When the composting process is complete, the resulting soil conditioner is screened, typically in a trommel screen to produce a range of products for the end markets. Oversize material is re-introduced into windrows with fresh feedstock to undergo further degradation and add structure to kerbside-collected feedstocks.

PRODUCT COMPLETION AND DISPATCH

A well operated and controlled green waste composting process produces a high-quality soil improver with a good nutrient content that is used to improve or replace soils in land restoration, landscaping, agriculture and gardening projects, and if it is made to PAS 100 it may be saleable to garden centres and the big supermarket chains, as a garden soil improver.

The compost made is often also supplied in bulk loads to sites, or as a bagged product under a number of brand names when the site is PAS 100 certified. The company routinely conducts a rigorous end product testing procedure, incorporating physical, chemical and micro-biological testing of samples. This ensures that the product achieves the required standards.

SITE MANAGEMENT

If operators are to produce high-quality composts, then site management plays a crucial role; one that is as important as the composting process itself. Areas that need to be carefully managed and assessed are cleanliness, health and safety, record¬keeping and the infrastructure of the site.

Daily site inspections typically include checking the cleanliness of yard areas, drainage systems, notice boards, mud/debris on access roads and the public highway, cleanliness of feedstock, odour monitoring, litter monitoring, pest infestations, weather conditions, birds and scavengers and dust checks during turning.

Management must ensure that the expected standards are adhered to by providing good operator training, and that the successful operation of these plants depends on the availability of verified data waste data.

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