Should we compost all our household waste?
Waste treatment is one of the most typical sources of disagreement in societies, with composting being perhaps the most controversial. There are those who are fervently in favour and others violently opposed – both at the individual family level and perhaps even more so at municipality level. Is it only a matter of foul smells and convenience, or could there also be health risks involved?
Aerobic or anaerobic decay?
The basic idea of composting is that organic material is decomposed by aerobic, oxygen-consuming processes. Microbes break down litter, dead vegetation from the previous year and other organic material in the environment aerobically (i.e. by using oxygen), if the conditions allow. The most important component is oxygen (air) to support the process. There are other microbes that can break down organic materials in anaerobic or oxygen-poor environments. It is not difficult to detect the difference between aerobic and anaerobic breakdown – your nose will tell you which process is going on. Under anaerobic conditions, many foul-smelling compounds are produced, e.g. reduced sulphuric compounds. These are also toxic, although the sensitivity of our nose to detect them is so good that it is possible to detect their smell long before the concentrations are high enough to cause any harm.
The major problem in composting is to keep the conditions optimal for aerobic decomposition. This means two simultaneous requirements: the moisture should be high enough to allow microbes to operate, but at the same time it must allow the penetration of oxygen through the whole mass, and achieving both at the same time is not simple. If the compost is too dry, the waste will not break down, if it is too compressed and wet, the breakdown becomes anaerobic. Therefore kitchen waste cannot be composted alone, but some dry and porous material such as peat or wood cuttings has to be added. Large-scale composting does not succeed without mixing, and this means work that many consider unpleasant. Kitchen waste also attracts rats, and therefore the site must be carefully closed.
Another possibility is to deliberately strive for anaerobic breakdown under moist conditions. This has to be done in closed containers to prevent the escape of smelly compounds and to collect methane gas that is produced. As much as 50 % or even more of all the carbon in the original waste can be converted to methane. However, because this type of waste management requires special containers, it is more expensive than composting. On the other hand, methane can be used as a source of bioenergy. Therefore while composting consumes energy and releases carbon dioxide that is a greenhouse gas, anaerobic fermentation produces methane that can be used to replace fossil fuels, and it is therefore more climate-change neutral.
Any health problems?
Large-scale composting is associated with environmental health problems, and those are worst in the immediate vicinity of the composting plants. There is relatively little scientific information on the exposures and health consequences of composting. There is somewhat more information on occupational problems in the workers at composting plants.
Scores of chemicals evaporate from biological waste (food waste, plants, tree leaves etc.). Both the spectrum and the concentrations of chemicals vary over the different stages of the process, and are highly dependent on the source material and the conditions. In particular, foul-smelling compounds are produced, if the composting reaction becomes anaerobic and its pH is low. When this unsuccessful compost is mixed, the compounds are released into the environment. The foul-smelling compounds include carboxylic acids (especially butyric acid and its derivatives), reduced sulphur compounds (e.g. dimethyl sulphide, dimethyl disulphide, methyl mercaptane), terpenes, and acetoin.
The concentrations of chemicals released from compost vary hugely, and they depend on the chemical and the site of compost. Terpenes have been detected as much as 800 m distance away from municipal compost sites. Weather conditions (temperature, humidity, wind etc.), have a clear influence on the releases, and the source material composted may be quite different in different cases.
The odour problem has been assessed mostly as an inconvenience factor, but if it is very prominent and recurrent, it may be also a health factor, and contribute to headaches and insomnia.
Industrial-scale composting is associated with another problem: bioaerosols (dusts) containing bacteria and moulds. People are exposed to these especially when working at the compost site, but bioaerosols may spread with the winds to houses in the immediate vicinity. Studies in Germany have shown that at a distance of 200 m downwind of a municipal composting plant, moulds and bacteria have been detected at 100 to 1000-fold levels. Bioaerosol concentrations were elevated up to a distance of half a kilometre and these were associated with self-reported respiratory symptoms in nearby inhabitants. The reported symptoms were quite similar to those observed in occupants of mouldy buildings.
Much more research on bioaerosol formation and distribution needs to be done before one could recommend composting process for large-scale treatment of biological waste. Information is needed both on chemical and microbiological factors and their effects on nearby populations. The effects may be highly temperature-dependent, and therefore studies performed in one country may not be applicable to another country with different climate conditions.
Aerobic or anaerobic treatment?
An important new aspect of the risks and benefits of composting is climate change. Composting in effect is the emission of carbon dioxide, in other words microbiological "burning" of organic material without utilising the energy produced. The only useful aspect of the energy production is that the heat produced will kill most microbes in the compost, and the soil produced is safer to use.
The other alternative, anaerobic treatment of biological waste would seem to be a much more sound solution for several reasons. Most of carbon will be converted to methane which can be collected as a biogas and used for energy production. The closed system will not produce foul smelling or toxic compounds to the environment and emissions of bioaerosols will also be much lower. After secondary composting of the remaining sediment, this can be used as a fertilizer in a similar manner to regular compost (the essential limitation in both is the content of heavy metals such as cadmium). In many parts of Europe, this kind of biogas has already been successfully used as a form of bioenergy.
Composting garden waste is a good way of improving soil quality. However, municipality-level composting causes many environmental and health problems. It also loses valuable organic material that could be used for energy production. Therefore anaerobic treatment of biological waste should be preferred even if the initial investment is more expensive.
Notes and references
- Terpenes are a number of hydrocarbons and their mixtures that are present in wood oils, resins, and turpentine. Some of them smell pleasant such as menthol, many have a specific strong odour.
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