When brown is the new green
When regenerating brownfield sites, one option is to manufacture topsoil using organic materials. This can make perfect financial sense, writes Richard Swannell
Brownfield regeneration remains key to the government’s plan for creating sustainable communities, with as much as a third of the 66,000 hectares of brownfield land identified by the National Land Use Database readily available for development. With brownfield regeneration, however, comes the issue of restoring the land to a high quality as quickly and cost effectively as possible; especially if it is contaminated.
Given increasing concern over global warming, there is also the issue of ensuring that the regeneration process, and resulting development, doesn’t cost the earth – or leave the developer out of pocket.
One of the issues facing developers of brownfield sites is ensuring they have topsoil that is able to meet their landscaping and building requirements.
Traditionally, developers looking to source good-quality topsoil have chosen to import it, disposing of the site’s original subsoils and mineral wastes. Not only is this costly, but the diverse geological landscape of the UK gives rise to extremely variable topsoils, which are not universally suited to all landscaping needs across all regions. Disposing of original subsoils and mineral wastes off site also represents significant financial and environmental costs.
One alternative is to manufacture topsoil in-situ by mixing organic materials with existing subsoils. For example, high-quality compost produced from recycled municipal garden waste – such as grass cuttings, pruning and leaves – is a green, organic material that can be used to manufacture an ideal topsoil for effective plant growth. Not only is it free of plant and weed seeds, but quality compost produced to BSI PAS100 (British Standards Institution Publicly Available Specification) is lighter than imported topsoil and therefore less costly to import.
The Waste & Resources Action Programme (Wrap) has been working with partners to investigate and demonstrate the benefits of using locally sourced, quality compost on brownfield sites through a series of pilot and trailblazer projects. Initial findings are impressive as they reveal that the compost not only helps create a soil that improves nutrient content, moisture retention and plant growth, but can also help significantly improve the project’s bottom line.
The following two projects highlight some of the main benefits of using high-quality compost in regeneration projects: the Polkemmet-Heartlands topsoil recycling scheme trials; and the regeneration of the Royal Ordnance Factory at Chorley.
The Polkemmet project, which aims to convert a former open-cast coal mine in central Scotland into a championship-level golf course, provides important evidence of the growth benefits of using high-quality compost to manufacture topsoil.
Initial laboratory trials involved developing and testing manufactured topsoils created using different ratios of crushed colliery waste from the site and quality compost, which was imported. Field trials were then used to establish the effectiveness of the soil mixtures in growing high-quality grass for golf course fairways.
The key assessment criteria for creating the optimum soil mix in the laboratory included percolation rate, water retention and nutrient status. And the successful trials showed that a mix of 75:25 colliery waste to compost had the required physical and chemical properties.
The subsequent field trials then showed that this manufactured topsoil was able to effectively support the growth of a range of grasses.
The main benefits of the trials resulted from reusing the colliery wastes from the site, which included:
- Reduced transport costs as the majority of materials were already present on site
- Reduced disposal costs as the materials were recycled and not disposed
- Reduced materials costs as the compost was generally less expensive and more readily available than imported topsoil
Results from the trials also provided a cost comparison for producing 1,000 tonnes of manufactured soil compared with importing 1,000 tonnes of natural topsoil.
The figures suggested that a cost saving of at least £10 per tonne could be made by manufacturing the topsoil in situ using compost. There was also evidence of an ongoing reduction in maintenance costs as a result of the compost being weed-free.
The Chorley project demonstrates the positive financial impact of using compost as an in-situ soil-improver. The factory, based on 265 hectares of land, produced explosives between 1936 and 1990.
As part of a drive to restore natural areas in the North-west of England, BAE Systems led the redevelopment of the site, transforming it into the village of Buckshaw, which also includes a business park.
There was no topsoil available on the site and a significant need to import this resource to accommodate all the landscaping requirements, such as playing fields, parks, natural woodlands and roadside verges. Insufficient natural topsoils were available in the region to meet this need, although the site retained a significant quantity of useable sandy clay loam subsoils.
These were deficient in organic matter, nitrogen, phosphate, and potassium, and highly alkaline (pH up to 11), rendering sustainable plant growth impossible. But mixing these subsoils 50:50 (by weight) with 7,000 tonnes of locally produced PAS 100:2005 compost generated a high-grade topsoil. This is now supporting healthy tree growth.
The project demonstrated that, by adopting an in-situ soil-manufacturing approach, more than £100,000 was saved.
But, as the Polkemmet trials suggest, the benefits of using high-quality compost do not stop once the initial soil development has been completed.
Compost contains slow-releasing nitrogen and phosphorus, which will help to offset future requirements for inorganic fertilisers.
For those who may be concerned about the quality of recycled compost, the BSI PAS 100 certification provides a welcome level of security and reassurance as only approved manufacturers are able to use the mark. Customers using and specifying PAS 100:2005 compost can therefore be confident that the product is consistent, safe, reliable and sustainable.
Manufacturing topsoils using compost also encourages the growth of a sustainable market for recycled mineral and plant wastes, as the raw materials for high-quality compost are produced in most local-authority areas from garden waste.
Many local authorities now provide a door-to-door waste collection and recycling service, which includes gathering and recycling green garden waste – much of this is already being used to generate high-quality compost.
By diverting more municipal organic waste from landfill and producing high-quality compost, we are also helping to meet the government’s challenging Landfill Directive targets, which are:
- By 2010 reducing biodegradable municipal waste landfilled to 75% of that produced in 1995
- By 2013 reducing biodegradable municipal waste landfilled to 50% of that produced in 1995
Wrap is currently supporting further trailblazer projects throughout the UK to develop this market, examining areas such as the potential for manufactured topsoils to sequester carbon – thereby helping to reduce global warming.
Richard Swannell is director of organics at Wrap. To find out more about how PAS 100 compost can help reduce the cost of brownfield regeneration projects, call 0800 100 2040, or visit www.wrap.org.uk/composting