Composting Innovation

- from Sewage Works to Composting Plant

The rate of adoption of composting - especially of kerbside collected green waste - has been accelerating over the past few years, but finding suitable sites can be a limiting factor. The site chosen for this Enviros designed project - and the extent to which existing infrastructure has been modified and re-used -makes this project a remarkable example of sustainable re-use on a large scale.

To convert a sewage works into a composting plant might seem an unlikely transformation, but draining the sewage treatment tanks and converting them into compost reception areas, windrow slab areas and soon - under a second stage contract which commenced in December - into in-vessel tunnels, has brought many advantages, as well as significant cost-savings.

The Deerdykes Composting Plant, Cumbernauld, Scotland, which has been taking green waste from three local authorities for the past 9 months, lent itself very well to this transformation. The project also attracted Scottish Executive support from the outset and won a grant of approximately £600,000 (0.9M Euros) from the Waste and Resources Action Programme (WRAP) Organics Capital Support Programme, through funding provided by the Scottish Executive.

Deerdykes was built in the 1970s and was originally known as the Cumbernauld Sewage Purification Works. The Kelvin Valley Sewer now serves the area, taking sewage past the site to a much larger and more modern plant downstream.

Clearly, reuse - where possible - is the most sustainable option for redundant civil engineering structures. Enviros Consulting recommended reusing the works which were in good structural condition, despite 30 years of use. In addition to structural elements, the condition of the drainage system was also found to be very good and has needed little more than a thorough cleaning - and only minor modification - for its new role. Most of the original structures are, to varying degrees, being recycled -

• The original activated sludge process plant, with inner walls demolished, has become the compost reception, mixing, and pre-screening area.
• The original rectangular sedimentation tanks make excellent in-vessel composting tunnels with their end walls demolished - once a central dividing wall has been installed - and roofed.
• One of the two circular clarifier tanks has also been used to store sludge press washwater and a gently sloping access ramp has been added to lead vehicles down what were once tanks, but are now composting bays.

The production of premium quality compost from a wide range of organic waste materials requires a highly controlled in-vessel environment. Deerdykes will use a forced-air, in-vessel process, providing an initial sanitisation and stabilisation process over an initial composting period, followed by maturation in the open.

This will be similar to the existing system already in place at the Beddington Farmlands composting plant in Beddington, Surrey - operated by Viridor Waste Management - and also designed by Enviros.

Blowers and fans will circulate air through pipes into the composting tunnels, while a series of probes, set within the composting mass, monitors the temperature. A biofilter will provide a very high level of control on atmospheric discharges and prevent the generation of odours.

It was fortuitous that the site was so well suited to composting. The choice of alternative uses for the site had previously highlighted a number of potential development challenges. The site was not suited to housing development, being within an industrial zone. However, finding an industrial tenant would not have been easy, as several other industrial sites of a similar size were standing vacant nearby. Furthermore, those properties could be utilised for industry without the need for - and cost of - demolition of the substantial existing sewage treatment works structures.

It is often time consuming and expensive to apply for the necessary licences and consents to operate waste processing facilities, such as composting plants. Here again, there were advantages at this site. Discussions with the planning authority were simplified for development as a waste management centre, based upon the previous established use as a waste site.

So, it was not surprising that the Deerdykes site was highlighted when business managers at Scottish Water Waste Services (SWWS) began to look for possible locations for the construction of a new composting facility. The Deerdykes site rapidly became the prime choice to be operated as a profitable venture on a commercial basis. This would also assist the Scottish Executive (SE) and the Waste Resources Action Programme (WRAP), tasked with promoting the development of new waste facilities throughout Scotland.

A feasibility study by Enviros in early summer of 2004, showed that there would be strong initial demand from local authorities for facilities to accept large quantities of green waste, which could be economically processed on the site by windrow composting on open slabs.

The need for this facility was also urgent - with kerb side collections of green (garden) waste due to start in January 2005 - so there was very little time to design and construct the facility to offer to the local authorities on a term contract basis.

SWWS also has a growing number of clients in industries, such as food production, that require additional sludge processing capacity - for dewatering and treating liquid sludges, as well as treatment of industrial sludge arriving on site in cake form. Disposal of sludge cake solids by co-composting has been shown to be successful elsewhere if in-vessel composting technology is utilised to ensure a high degree of control of composting conditions, as well as odours and bio-aerosols. Therefore, sludge press facilities have been included within the initial design stages and subsequent construction contracts.

It has been recognised from the start that the composting industry will develop in Scotland.

Furthermore, there will be a growing demand for the more sophisticated composting techniques required for -

• Sludge co-composting with other (mostly green/garden) wastes;
• In vessel composting, to reduce emissions and, especially, to avoid odours and provide a more controllable environment for the sanitisation stage;
• Accepting waste - which would be subject to the Animal By-Products Regulations - from industry and from the planned segregation and collection of additional organic waste streams, such as the introduction of household kitchen waste collections which are being planned by the unitary authorities.

The Deerdykes Facility will, in time, meet all these requirements.

Conclusion
The project was a first of its kind in Scotland and has provided much needed composting capacity in the region. The existence of this plant should lead to the development of others elsewhere in Scotland.

The project has turned a liability into an asset for the owner - Scottish Water Waste Services - and attracted a grant of approximately £600,000 from The WRAP Organics Capital Support Programme, through funding provided by the Scottish Executive.

It has been shown that sustainable and innovative re-uses can be found for large redundant civil engineering structures. The high quality compost produced is being sold successfully and, as Scottish compost market requirements become more sophisticated, SWWS will progress from the present simple, but effective open windrow systems, to state-of-the-art in-vessel systems to process a wide variety of organic materials and serve premium areas of the compost market.

To view a recent paper presented about the project in November 2005 - click here

For further information -