Composting at Bryn Pica

August 2005, saw the opening of Amgen Cymru’s purpose built green waste composting facility at Bryn Pica. The facility, built at a cost of £250,000, is designed to handle a maximum 20,000 tonnes of green waste per year to produce a compost end product that can be used for soil conditioning. The facility consists of a concrete hardstanding area of 5,000m², which accommodates two 50m by 30m open sided sheds under which the green waste is processed.

What do we mean by green waste? Green waste refers to all compostable organic garden material such as grass cuttings, leaves, hedge and tree prunings etc although certain types of specified weeds, like Japanese Knotweed, are excluded from being composted under law. The material received at Bryn Pica consists of the green waste collected as part of the Local Authority’s kerbside recycling scheme and also that which has been dropped off at the five Community Recycling Centres by members of the public for recycling

The composting process is a simple and natural one similar to that practised by many gardeners at home, although to handle the inputs involved at Bryn Pica and to speed up the process, large vehicles and items of machinery are used. The composting process can be split into several stages, which will be described in further detail in turn.

Reception and acceptance of the green waste at the facility is the first stage in the process. Lorries delivering the green waste follow strict reception and acceptance procedures to ensure the material being delivered is suitable for composting and does not contain items that are not degradable or which could damage the machines used later in the process. Such things that the composting attendant is looking for are certain types of weeds like Japanese Knotweed, Giant Hogweed and Himalayan Balsam. If any of these are present then that load of green waste will be rejected, as it is an offence under law to compost these weeds due to the virulent nature in which they can spread. Other materials they will look out for are plastics in the form of bottles or bags which do not degrade and affect the end quality; metal cans, pieces of scrap garden waste, stones, bricks or pieces of concrete which again affect end use quality and can cause significant damage to machinery.

Following acceptance, the next stage of the process is blending and shredding. A large throughput shredder is used to breakdown the woody and green material into smaller pieces which aids the composting process. At this stage it is also important to ensure the right mix of woody / green leafy material is shredded together. This is known as the carbon / nitrogen ratio. Too much of one or not enough of the other hinders the composting process, as does the lack of moisture. To avoid this we add water to the shredded material as it leaves the shredder.

The shredded material is then allowed to naturally biodegrade in a controlled manner over time. This occurs in elongated triangular piles called windrows. The windrows are monitored for temperature, moisture and oxygen content to ensure satisfactory conditions for the microbes responsible for composting to thrive. Regularly, the windrows are turned using a loading shovel to allow the introduction of oxygen and to mix the windrow material around to ensure it is all composted. Furthermore, usually during turning, water is again added to achieve the optimum moisture content. Ideally temperatures within the windrow should be greater than 65ºC to kill off any harmful pathogens. This heat is generated as a result of the action of the microbes breaking down the shredded material. This windrowing stage usually lasts 8 weeks depending on the conditions achieved.

The final stage in the process is screening and maturation. When it has become evident from the monitoring that all microbial activity has slowed down, we then screen the windrowed material to produce products of varying quality and particle size. Like shredding, large throughput machines are used. These machines contain adjustable plates with predetermined size holes in them that allow the material to be separated. The finer material that is able to pass through the holes is termed the undersize whilst the larger material unable to pass through but which passes over the plates is called the oversize.

The finer undersize material can be used for soil conditioning whilst the coarser oversized woody material can either be used as mulch for weed suppression and moisture retention in flower beds or be re-shredded and reintroduced back into the composting process.

Currently, most of the end product produced is used at Bryn Pica to enhance the quality of our restoration material. As the site was once an opencast coal mine, the material we have at our disposal to cap off the waste consists generally of poor quality shales, sandstones and some clays.

Whilst within Rhondda Cynon Taf, this soil conditioner has been used on land reclamation schemes such as Coed Ely and civil engineering works, like the Porth Relief Road. Departments within RCTCBC such as Parks, Allotments and Cemeteries are also recognising the benefits of the material for their own uses, with notable use of the material now being made within Aberdare Park.

Additionally, material has also been supplied to charitable organisations such as Groundwork Trust & Fernhill Community Trust for use on projects within RCTCBC.

To allow us in the future to be able to sell the material commercially, Amgen is currently in the process of attaining the British Standard PAS 100 for our composted end products. PAS 100 has been developed in recent years to provide a degree of confidence to end users that products bearing this mark are fit for use and meet various standards and levels of quality assurance.