Overview of environmental issues and their management
Odour problems associated with composting and related organics processing facilities can be traced to problems with one or more of the following four processes: process control; containment of odorous areas; odour control technology; and siting (Giggey et al. 1995). Some of the odour compounds associated with such facilities are listed in Table 2. Although various feedstocks contain a variety of compounds, in many cases they will not be released as odour during processing provided the process conditions are optimised (Goldstein 2002; Gage 2003) (see examples, Appendix B).
Most analyses of composting odours have focused on sulfur compounds, nitrogen compounds and volatile organic compounds (Table 2). Ammonia is commonly associated with unpleasant odour from composting and related organic processing facilities, mainly because it can be easily distinguished from other composting odours (Miller 1993).
Under aerobic conditions the main gaseous product of composting and mulching is carbon dioxide, and the organics are characterised by an earthy or woody odour. The most common gas compounds contributing to odours from aerated static pile composting of organics containing biosolids include dimethyl sulfide, dimethyl disulfide, dimethyl trisulfide, carbon disulfide, and benzothiazole (Fisher et al. 1986; Goldstein 2002). These chemicals can be toxic, although in open-air (aerobic) composting situations they are not present in high enough concentrations to be considered a health risk (Clark et al. 1983). However, presence of these chemicals in the atmosphere can lead to local amenity impacts (Miller 1993).
Under anaerobic conditions - when the biodegrading materials do not receive sufficient air - methane is generated, and this is accompanied invariably by the production of strong and foul odours. These odours are caused by the generation of ammonia, volatile amines (when the degrading organics have a high nitrogen content), hydrogen sulfide and volatile organic compounds (Goldstein 2002).
Table 2: Odour compounds
Description of smell
Detection limit for a particular odour panel
Volatile fatty acids
The absence of odours does not necessarily indicate that the process has not turned anaerobic: odours may be diminished or removed during diffusion of the biogas mixture through fresh compost, odour scrubbers or soil containing biological organisms (Farell 2001; Wilson 2002). However, the presence of unpleasant odours is a good indicator that the process has turned anaerobic.
If calm conditions are likely to occur frequently, the topography and consequent drainage flows of air can have a profound effect on the dispersion of odours, the extent and intensity of odours and, consequently, the impact on local amenity. Calm conditions are most likely to occur in the morning and evening. Locations likely to cause least dispersion are those that have a predictable air drainage flow and no sea breezes or other winds to disturb the stable wind conditions (Walker 1993). In this regard, the worst times of the year for odour dispersion are likely to be late autumn and winter.
High peak odour emissions at composting and related organics processing facilities generally occur during mixing and aeration procedures, such as preparation of the feedstock, and during turning of biodegrading organics (Bidlingmaier 1993). Rapidly biodegrading organics (i.e. Category 3 organics in Table 3, Section 3), such as food and animal organics, may already be giving off odours when they are received at the facility or soon after receipt. Other less biodegradable organics, such as Category 1 organics, are less likely to generate odour when received at the facility.
Odour impacts on local amenity from composting and related organics facilities can arise from poorly managed stockpiles of raw organics and/or organic products (Haug 1993). For example, large stockpiles require more ongoing management and maintenance to ensure that the potential for odour emissions and water pollution from anaerobic conditions is mitigated. Poorly managed stockpiles can turn anaerobic because of lack of aeration through the piles. If excessive moisture comes into contact with the stockpiles it can cause leachate to be generated and drain from stockpiles, potentially causing water pollution (see text later in this section titled 'Water pollution').
The two important pieces of legislation for minimising and controlling odour are the Protection of the Environment Operations Act 1997 (POEO Act) and the Environmental Planning and Assessment Act 1979 (as amended). An overview of the legislative context in relation to odour is available in the Draft Policy: Assessment and Management of Odour From Stationary Sources in NSW (EPA 2001b).
Composting and related organics processing facilities may be sources of particles (or particulate matter) in the atmosphere. Particulate matter may be classified by the particle shape or phase (such as fibres and aerosols), their physical behaviour in air (such as suspended in air or deposited from air); their chemical species, biological activity (such as bioaerosols) and size (including PM10, PM2.5, total, and inhalable dust) (Environment Agency 2003). The highest concentrations of particulate matter from composting and related organics-processing facilities occur during pre-treatment (shredding and mixing) of fresh organics and the turning of biodegrading organics, and can be higher in summer and when organics are dry (Tolvanen et al. 1998). Unsealed access roads and earthmoving equipment can also be sources of particulate matter at composting and related organics-processing facilities.
Composting and most organics-processing activities rely upon the inter-related activities of a diverse range of microorganisms to convert organics into stabilised organic products. Thus high concentrations of bacteria and fungi are likely to be present in the process feedstock, during processing, and in the final products (Swan et al. 2002). It is possible in the absence of control measures that pathogens such as Legionella longbeachae, Aspergillus fumigatus, Mycobacterium tuberculosis and Hantavirus infections may be aerosol transmitted from composting and related organics and processing facilities (NSW Health Department 2001; The Staff of BioCycle 1991; Pillai 2002; Swan et al. 2002). Release of such pathogens into the air needs to be mitigated, because these pathogens are capable of causing severe infections in humans (Pillai 2002).
The principal types of particulate matter of concern to the community and the EPA are biological particulate matter, PM10 (size range < 10 µm), PM2.5 (size range < 2.5 µm) deposited matter and total suspended particulate matter (TSP) that are present at composting and related organics-processing facilities. The human health effects of different-sized airborne particulate matter differ. Larger particles PM10 are trapped in the nose and throat, whereas smaller particles (PM2.5) penetrate the lungs and are associated with a range of respiratory symptoms (Gilbert 1998; Swan et al. 2002). For example, workers at composting and related organics-processing facilities may be exposed to high levels of particulate matter (for short periods) if certain design features are not employed and key operational measures are not undertaken (Epstein et al. 2001). Appendix B provides some additional advice on controlling particulate emissions.
Decomposition of most organics in the absence of oxygen yields biogas - a mixture of approximately 65% methane and 35% carbon dioxide (Mata-Alverez 2003). Uncontrolled emission of biogas can pose a fire risk and other potential hazards to humans (see text later in this section).
Biogas generated from the decomposition of 'mixed residual waste containing putrescible organics' is likened to the biogas generated in landfills. Thus the principal key pollutants of concern arising from the decomposition of 'mixed residual waste containing putrescible organics' are methane, nitrogen oxides (NO2 and NO), sulfuric acid mist (H2SO4), sulfur oxides (SO3 and SO2), and non-methane volatile organic compounds (NMVOC) (Tchobanoglous et al. 1993). These pollutants are of concern because they can be toxic or highly odorous at quite low concentrations (a few parts per million by volume for odours).
Flaring of biogas from anaerobic processing minimises the release of odours generated from biogas and reduces the risk of explosions. Alternatively, energy recovery systems can be installed to recover energy from biogas.
Liquids condensed from biogas have the potential to cause pollution of waters and cause amenity impacts (such as odour), so they must be effectively managed. Where possible, the generation of condensed liquids from biogas should be avoided.
The emission of methane to the atmosphere is reported as the principal greenhouse impact of concern for composting and related organics-processing facilities, because methane has more than 20 times the greenhouse warming potential of carbon dioxide. In open windrow systems when an aerobic environment is maintained with proper moisture content to encourage aerobic decomposition of the organics, the composting process does not generate significant quantities of methane (USEPA 2002). Most experts agree that if isolated anaerobic pockets deep within a compost pile release methane then it is likely to be oxidised in the aerobic areas of the compost pile before any significant quantity is released to the atmosphere (e.g. Zeman & Rich 2001). Well designed and managed anaerobic systems are characterised by close monitoring and control of the generation and collection of methane (Mata-Alvarez 2003).
Zeman and Rich (2001) comment that only the carbon dioxide released by the use of fossil fuels (e.g. diesel and petrol) during transporting and processing contributes to global warming, because the carbon dioxide produced during the composting process would have been released in the longer term by the natural decay of the organic materials that are being turned into compost.
The well managed composting of organics will not produce methane, so this activity can contribute to a reduction of global warming by keeping organics out of landfill (Lechner et al. 2002). Landfills are usually oxygen poor, so methane is generally produced from the decomposition of organics in landfills. The capture of methane from landfills, even if it is included during the design phase, is never 100% efficient, so the landfilling of organics will always release methane, a powerful greenhouse gas, into the environment.
Putrescible organics have a tendency to generate leachates that need careful management (Haug 1993). For example, Category 3 organics such as food, meat, fish and fatty or oily sludges usually contain sufficient quantities of moisture to generate leachate without extra water being added. Category 1 organics such as garden materials, wood and fibrous materials generally form leachates only when additional water (including rainfall) is introduced.
Leachates can be acidic, especially when they are generated under anaerobic conditions. They can cause the dissolution of metals and metallic compounds that may be present in organics. Under aerobic conditions alkaline leachates can be formed from organics with low carbon/high nitrogen ratios, such as food and animal organics.
Leachates from composting and related organics-processing facilities have the potential to pollute groundwater and surface water bodies (such as rivers, creeks and dams). They can be high in nutrients; this makes them favourable host media for bacteria and other micro-organisms and gives them a high biological oxygen demand (BOD) (Tchobanoglous et al. 1993).
Stockpiles of raw organics and processed organics have the potential to pollute waters, because leachate may be generated when the stockpiled organics contain excessive moisture (for example, when too much rain falls on to the organics or if stockpiled organics are not sufficiently aerated or turned) (see Appendix B). Such stockpiles may also generate offensive odours because excessive moisture will tend to cause the stockpiled organics to become anaerobic if not managed competently (see discussion relating to odour earlier in this Section).
Surface water run-off from composting and related organics-processing facilities can cause unacceptable loads of sediment and suspended solids in receiving waters (Department of Housing 1998), while surface water run-on can lead to excessive generation of leachate (Tchobanoglous et al. 1993). Unvegetated exposed areas are a likely source of suspended sediment in surface water.
Note: A person who pollutes waters is guilty of an offence under the POEO Act.
Fire and other potential hazards
Methane emissions from poorly managed composting processes or uncontrolled biogas emissions represent a lost opportunity to recover energy or fuel from such facilities. Uncontrolled biogas emissions may create an explosion or fire risk and impose risks to humans (such as explosions and suffocation in confined spaces) (Rynk 2000; ASFCME 2001). So it is better to design facilities to avoid the release of these emissions, or at least to collect them.
Fire at composting and related organics processing facilities can attract public and industry concern about the perceived risks of composting activities, threaten damage and loss of equipment, and present potential dangers to workers and firefighters (Rynk 2000).
Possible causes of fires at composting and related organics processing facilities, include:
- spontaneous combustion (see below for further information)
- sparks from works activities such as welding
- lightning strikes
- build-up of particulate matter near engine manifolds and exhaust pipes of processing equipment
- bushfires and arson.
Cigarettes, sparks from welding activities and spontaneous combustion are reported as being the most common causes of fire at composting and related organics-processing facilities (Rynk 2000; Wilson 2002). Spontaneous combustion happens when decomposing organics self heat to a temperature high enough to ignite. The conditions for spontaneous combustion (such as large piles, limited air flow and time for temperature to build up) are usually more prevalent within large, undisturbed piles containing raw feedstock, curing compost or finished compost rather than in active composting systems (Rynk 2000).
If unauthorised access to the premises is not prevented, the risk of potential injury and damage to persons and equipment may be increased.
The Protection of the Environment Operations (Control of Burning) Regulation 2000:
- controls burning in the open or in incinerators in local government areas
- allows the EPA or local councils to grant approvals for burning in the open or in an incinerator in certain circumstances
- prohibits the burning of certain articles (including tyres, paint and solvent containers, and certain treated timbers)
- imposes a general duty on persons to prevent or minimise air pollution when burning in the open or in an incinerator.
Open burning in many council areas is prohibited unless approved by the EPA or conducted in an approved incinerator. For further information refer to the Protection of the Environment Operations (Control of Burning) Regulation 2000 or call the EPA's Environment Line on 131 555.
The potential negative impacts on local amenity from inappropriately managed composting and related organics-processing facilities include:
- odour pollution
- particulate matter (including deposited matter and biological matter) pollution
- noise from equipment or traffic.
These impacts may occur on and off the premises. The most common amenity issue is caused by the release of odour (Miller 1993). Text earlier in this section discussed air quality issues (such as odour and particulate matter).
Particulate matter leaving a poorly managed site as airborne dust may have a visual impact on the local amenity and may cause public health impacts.
Composting and related organics-processing facilities with exposed, rapidly biodegradable organics (see Glossary) may attract a large number of birds, particularly gulls and ibises; this can lead to noise problems and the spread of food scraps away from the site. Local amenity will also be reduced by the presence of large numbers of pests or vermin at composting and related organics-processing facilities and may pose environmental or health hazards.
Weed spread can have serious environmental and negative economic consequences. For example, noxious weeds can pose environmental harm and cause human health problems and loss of amenity in neighbouring areas. It is therefore important to prevent weeds from proliferating at the premises. It is also important that weeds, weed seeds and plant propagules are prevented from being transmitted to other locations via the products. The impact of pests on product quality is discussed later in this section.
Wind-blown litter emanating from composting and related organics-processing facilities can degrade the local amenity. The tracking of litter and mud on the wheels of vehicles leaving the premises may also have an impact on local amenity and on the quality of surface water run-off. Vehicles can also be a source of wind-blown litter (i.e. from uncovered loads).
Contamination of organics
Good design and management of organics-processing facilities are necessary to minimise the contamination of organics intended for beneficial use and thereby to avoid subsequent negative environmental and health impacts. It is important that facilities are designed and managed to keep contamination of final products to the lowest practicable levels.
The quality of the processed organics delivered to the market is an environmental issue that affects the viability and sustainability of a facility dedicated to producing organics for beneficial use. Contaminated organics used in the environment as composts, soil conditioners or mulches can potentially lead to the pollution of surface waters, soil and groundwater and the spreading of pathogens, pests and diseases; these in turn may pose health risks via the food chain.
The following paragraphs briefly describe the main classes of contaminants that need to be managed to prevent negative impacts on product acceptance and sustainability in the market.
Toxic organic chemicals and metal compounds present in composted organics can have the following properties and negative effects:
- organic chemicals may not degrade during processing and are, therefore, concentrated in the final products (e.g. persistent organochlorine pesticides such as DDT)
- metal compounds (such as those of cadmium, chromium, copper, mercury, lead, nickel and zinc) that tend to accumulate can have short-term and long-term toxic effects on organisms in the environment
- soil contamination by heavy metal compounds can necessitate costly remediation or even require storage of intractably contaminated soil
- significant health hazards can arise if contaminated composts are applied to agricultural and residential land and if these chemicals enter the food chain
- the presence of contaminants can endanger domestic animals, wildlife, plants and other living organisms and may have serious ecological consequences.
Excessive levels of contaminants, pathogens, pests or toxins in organic products will degrade the quality and value of organics. For example, contaminants may limit or prevent the usefulness of organic products and limit the acceptance of composted organics in the market (Ren 2003).
Historically, physical contaminants, such as shredded plastic and broken glass, have posed problems with the quality of final products processed from mixed residual waste (e.g. Rynk 2001; Lantz & Venters 2002). Although many techniques are available for removal of contaminants (either at the start or end of the composting process) complete elimination cannot be assured (e.g. Rynk 2001; Satkofsky 2001).
Pathogens, plant propagules and other pests
Pathogens, plant propagules and other pests in organic products are well documented as contaminants that have the potential to degrade the quality and value of organic products. For example, the NSW viticulture industry has been concerned that processed organics have the potential to carry and spread phylloxera (Daktulosphaira vitifoliae) (NSW Agriculture 2002) (see also Appendix B). However, Bishop et al. (2002) reported that the presence of phylloxera in the feedstock for composts is unlikely and that the temperatures reached during processing in well managed facilities readily destroy the insect. If composted organics are intended for transport to, and are used in, viticulture areas, it is essential to be aware of:
- the protocols for composting facilities to minimise the risk of phylloxera survival during processing and transport. This involves composting to Australian Standard AS4454 Composts, Soil Conditioners and Mulches and in particular maintaining the required temperature regime for the process (which is known to destroy the phylloxera) and managing the facility in a manner that avoids cross contamination
- NSW Agriculture compliance and licensing requirements for transport of compost into designated Phylloxera Free Zones within NSW. This involves organising a Compliance Agreement with NSW Agriculture.
Information on these phylloxera-related issues is available from NSW Agriculture.
There are also potential human health issues associated with organic products, including colonisation with spores of the fungus Aspergillus fumigatus (The Staff of BioCycle 1991) or with the bacterium Legionella longbeachae (NSW Health Department 2001).
Page last updated: 12 July 2012