Emissions from composting include odours, volatile organic compounds (VOCs), noise, dust and micro-organisms known as "bioaerosols", which include pathogenic bacteria and fungal spores.
While noise can be controlled through operational measures and dust is unlikely to cause a nuisance, both odour and bioaerosols may affect the public at some distance from facilities. Odour in particular has been the focus of local campaigns against proposed plants.
Emissions can be greatly reduced if composting is conducted in fully enclosed, "in-vessel", systems, but these are more expensive and only a handful operate in the UK.
In August, the Agency issued a position paper on composting and health effects,1 together with studies on the health2 and environmental3 effects.
The position paper covers composting of biodegradable waste. It does not deal with treatment of unsegregated wastes, but says that "where such waste is being composted similar standards will apply as appropriate."
The Agency's position is that it will not license, and will object to planning applications for, any new composting facility within 250 metres of a workplace or dwelling - unless the application is accompanied by a site-specific risk assessment which shows that bioaerosol concentrations can be maintained at background levels at the nearest building.
This rule has been set on the basis of the health effects study carried out for the Agency by AEA Technology. The study examined emissions from two turned-windrow, green waste facilities and from Biffa's in-vessel plant on the Isle of Wight. The latter treats kitchen waste, a small amount of green waste and fines from the production of refuse-derived fuel.
The study found that dust and VOC emissions were low, while noise levels were easy to control. Odour may be a potential problem at some sites, particularly those treating mixed or kitchen wastes.
The main problem highlighted by the study is bioaerosols. There is "limited consensus", it says, on the levels at which health effects occur because of the variable response of individuals and the wide range of organisms that can be present. Furthermore, dispersion modelling is difficult because organisms clump together in larger particles and do not behave like gases, and some organisms lose their viability over time.
Despite these uncertainties, the study lays down "safe" concentration levels for bacteria and fungi. These are often exceeded in "natural outdoor situations where health effects are not generally noted" and thus "may be conservative", it notes.
While bioaerosol concentrations at the study sites generally exceeded the reference levels, indicating that workers will need respiratory protection, "simple" modelling suggested that concentrations dropped below the reference levels within 250 metres.
The Composting Association's protocol for sampling and measurement of airborne micro-organisms does not suggest any such minimum distance. It recommends that, in most cases, sampling need only be carried out at sites within 200 metres of a sensitive receptor - a limit based on dispersal monitoring by the Association in 1998.
AEA's study also recommended that applications for waste management licences and planning permission should be considered in parallel - something the Agency is keen on for other waste facilities, such as incinerators.
The environmental impact study, conducted by Casella Science and Environment, covered much the same ground but also looked at leachate generation.
On bioaerosols, it notes that concentrations in air reduce by 80-90% at 20-40 metres from facilities. But it then adds that field-based and modelling studies are required before "any firm conclusions can be drawn" on their effects.
The Agency's position may eventually be superseded by the European Commission's proposed Directive on biological treatment of biowaste (ENDS Report 314, pp 51-52 ) - but in the meantime it will act as another driver for the industry to invest in in-vessel, rather than open-air, systems (see pp 17-18 ). Some in-vessel systems can be fitted with filters to reduce bioaerosol and odour emissions.
The Agency intends to identify and review the licences and registrations of all existing composting sites which may be affected by its position paper after the HSE has completed its research into dispersal monitoring and health effects.
The Agency appears to have suggested to the Government that similar restrictions should apply to smaller composting sites which are currently exempt from licensing. It has also proposed a charging scheme for registering and inspecting such facilities.