How biowaste can bolster the soil

A growing number of local authorities are looking towards MBT solutions to treat residual waste. MBT processes often produce organic MSW fines – the small organic size fraction that remains after either the recyclable materials have been removed or high energy fractions recovered – and there is potential for these fines to be used in remediation of poor-quality soils.

In 2002, ADAS was commissioned by Composting Research to research the development of a product suitable for use in land restoration. The work forms part of the composting research project, a £3.4M initiative funded by the Landfill Communities Fund, which is carrying out trials to grow bioenergy crops on land restored with compost.

The project aims were to establish the effect of commercial land application rates on the physical and chemical properties of the soil, and understand their effect on plant establishment and yield.

The first step was to manufacture a product from the organic fraction of household residual waste. To achieve this, waste was pulverised in a slowly rotating dano drum, which was then passed through a 25mm screen. The undersize formed the composting feedstock and this was mixed in equal proportions with green waste prior to composting in batch tunnels, followed by windrow composting.

After this, the material was screened and the sub 10mm fraction formed the product. To date, three 1,000-1,500 tonne batches of the MSW fines/green and kitchen waste derived compost have been used to restore three sites. The first site at Collyweston, Northamptonshire, was laid down in August 2005.

This had been backfilled with inert material and had a heavy clay sub-soil. Compost was applied at rates between zero and 1,200 tonnes per hectare in replicated trial plots.

Plotting for growth

The composts were mixed with the native, inorganic soil forming materials in-situ – subsoil clay to produce a manufactured topsoil capable of supporting plant growth. Each plot was super-imposed with four different vegetation sub-plots, of which three – Willow, Miscanthus and Reed Canary grass – are biomass crops with high energy potential. The remaining sub-plot was sown with an amenity grass seed mixture.

The second site at Ketton, also in Stamford, was restored using the same methodology in spring 2006. It also has a heavy clay subsoil, but over an alkaline base following the landfill of cement kiln dust.

Similarly, an area of Maltby Colliery, a coal spoil site, was restored in spring 2007.

Here, coal shale was mixed with compost to manufacture the topsoil and the highest application rate of compost was increased to 2,000 tonnes per hectare.

Signs of promise

The results to date are promising, showing significantly beneficial effects, as compost application rates increase. Results after the first year show that biowaste additions increase topsoil fertility, the size and activity of soil microbial populations, and improve the soil physical properties of porosity and aeration.

Overall, the beneficial effect of biowaste addition to soils was substantiated by increased yield, with ryegrass herbage showing a significant increase.

Local authorities have made great strides in reducing reliance on landfill and returning derelict land to use. By using a region’s waste to develop its own land and locating CHP plants near to areas of newly fertile soils, biomass can be produced for a more sustainable solution.

Peter Davies is senior composting consultant at ADAS