Editorial: greening our waste
Dr Susanne Charlesworth
A lack of practical uses for garden and kitchen waste collected from homes means that a proportion of this ‘green’ compost goes to landfill. Around three million tonnes of green waste is sorted by households for recycling each year and the figure continues to rise. Much of this compost has been spread on farmland (around 47per cent), and the finer compost is eventually re-used in gardens as a peatless compost - but the problem lies with the coarser compost (with particles more than 25 mm) which appear to have no obvious use.
Our research for WRAP (Waste & Resources Action Programme) has looked at the potential of coarser grades of both green and mixed green and food composts for use in sustainable drainage (SUDS). SUDs include a variety of devices among them porous paving systems, grassed rooftops, constructed wetlands, swales (grassed ditches), vegetated barriers, retention and detention ponds. This approach allows large volumes of water to drain slowly and naturally rather than being forced through networks of concrete pipes. It also means waste materials in the water are absorbed and processed - with vegetation and microbes making use of the nutrients available in what were toxic materials - enabling water which flows through the sustainable drainage system to be filtered rather than contaminated.
While such approaches have been embraced for more than 25 years in the US, Sweden, France and for over 15 years in Scotland, the take-up has been less than enthusiastic in England and Wales because of concerns over costs and management. Future cities will need to look new technologies such as these, as well as to the distant past and the efficiency of natural processes, if they are to remain viable and human places to live in. Previous research has claimed that just a 10 per cent increase in green spaces in cities is enough to mitigate the expected impact from climate change.There are real advantages to the use of coarse grades of compost for water quality improvement purposes: the open spaces within the compost allows for easy progress for storm water, and allows for increased oxygen levels which encourages more diversity in the microbial population.
As vegetated devices, the plants in swales trap pollutants in their stems and leaves and also take them up systemically, therefore improving water quality. Stormwater infiltration into the soil improves water quality by trapping associated pollutants in the soil but also by treatment in the biofilm which is found naturally within the soil - the systems of bacteria, fungi, protists and animals. Biofilms are complex and dynamic systems, and as they grow and reproduce they can biodegrade pollutants such as oil. The construction of swales and filter strips (grassed slopes) usually requires careful excavation and use of topsoil before grass seeds are sown or turf is laid. If compost is to be used in a SUDS system, to either wholly or partially replace topsoil, it must perform at least as well in terms of dealing with pollution.
In our studies we looked at how green compost and mixed compost performed by applying pollutants, as well as looking at the potential for developing biofilm in the composts. In the tests of performance for removing pollution and contaminants, the mixed compost appeared to perform least well, while green compost and topsoil were similar in their potential ability. Microbiological analysis of the composts revealed that mixed compost had the highest numbers of bacteria and fungi while topsoil had the least. This is reflected in their ability to deal with oil as a pollutant in the leaching columns.
This experiment was very small scale and added the equivalent of a month’s oil loading in a typical urban environment every two weeks, so double the normal amount, but the composts were still able to degrade between 65 per cent and 80 per cent of the oil added. If a swale were installed in an area likely to suffer significant oil pollution, like a lorry park in a motorway service area, it would seem that mixed compost would be better able to degrade this excess oil than either green compost or topsoil.
Field trials are needed, but the coarse grades of compost tested here appear to demonstrate potential to replace some of the topsoil currently used in SUDS devices. It does provides a means of tackling pollutants and could therefore be used in other SUDS devices, such as brown roofs (similar to green roofs, but using locally sourced waste material) and porous paving. Nothing would be more damaging to the successful growth of a culture of recycling than the knowledge that garden and kitchen waste could just end up in landfill. Using compost for SUDS is an ideal platform with genuine sustainability credentials and provides a market for a material which was once considered a wasted waste.
Dr Susanne Charlesworth is a reader in urban physical geography at Coventry University