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Jak Fazakerley and Danen Appasamy address the hazards to health and the environment caused by composting organic waste.
Defining bioaerosols
Bioaerosol, also referred to as biological aerosol or organic dust, is a term used to describe airborne microorganisms such as bacteria, actinomycetes, fungi, pollen, moulds, or viruses. It can also describe their products, including enzymes, endotoxins, mycotoxins, peptidoglycans, glucans and VOCs, which originate from organic matter, plants, soil, animals and humans.
Typically, bioaerosols consist of very fine particles measuring less than 20 microns in diameter and are therefore not visible to the naked human eye. Bioaerosols are ever-present and variable in time, space, size, shape, mass and composition. As such, some remain airborne for extended periods, while others fall back to the ground rapidly. In outdoor environments, local temperature, humidity and wind speed are all critical factors in determining the airborne concentration. A variety of new industrial activities has emerged in recent years in which exposure to biological agents can be prevalent.
Composting
A significant part of the government’s strategy for waste management is focused on diverting municipal waste, or resources, from landfill. Composting organic waste, that is, waste derived from plant and animal tissue, forms an increasingly important component of the waste management process. In simple terms, the process involves the controlled biological decomposition, stabilisation and maturation of organic substrates to produce a high-value product that can be utilised in land, horticulture and turf management.
In recent years there has been a surge in the number of composting operations taking place in the UK on the following scales:
• Small – households • Medium – community groups and farmers • Large – centralised sites including open-windrow, aerate static pile and in-vessel systems
The degree of process control employed varies depending upon the size and location of the site, the nature of the feedstock and the intended use of the end product.
Health effects of bioaerosols
Despite the benefits of organic waste composting, there are concerns that exposure to bioaerosols, created by the shredding, turning and screening operations, could be detrimental to health. There are two main routes of exposure to bioaerosols: ingestion, via hand to mouth contact, and inhalation. Bioaerosols that are less than 10 µm in size are considered repairable, meaning they are not filtered out by the hairs and specialised cells that line the nose, and can penetrate deeply into the lungs. This results in infectious diseases, respiratory symptoms such as asthma, coughs and bronchitis, gastrointestinal illness, eye irritation, dermatitis, acute toxic effects, allergies and even cancer.
Bacteria and fungi occur naturally in many different environments and are therefore commonly present in the air. Concentrations are highly variable, but background levels of gram-negative bacteria, total bacteria and Aspergillus fumigatus should not exceed the suggested threshold levels of 300, 1,000 and 500 colony forming units (CFU) per cubic metre of air (m 3 ) respectively (Afor, 2009).
Thermophilic actinomycetes are a group of spore forming bacteria naturally present in vegetation that are fundamental to the production of compost, as they break down celluloses and lignins. In actively composting material a succession of microorganisms grow with an increase in temperature until thermophilic actinomycetes flourish. As a consequence, they are likely to be present in numbers in excess of ten million spores per gram.
Aspergillus fumigatus is a species of fungus, again naturally present in vegetation and ubiquitous in the environment, but because it is a thermo-tolerant fungus it grows well at the raised temperatures experienced during the composting process.
Both thermophilic actinomycete species and the fungus Aspergillus fumigatus have been implicated in occupational allergic lung diseases such as farmers’ lung disease and mushroom workers’ lung disease, where gross exposure, typically greater than tens of millions of spores per cubic metre of air, have triggered the immune response. Repeated exposure to such allergens stimulates the immune system, causing responses including the release of histamines, constriction of airways and reduced lung capacity and resulting in chronic bronchitis, asthma or alveolitis. Individuals with severely compromised or suppressed immune systems, for example, following an organ transplant or if suffering from immune deficiency diseases, are at particular risk.
Research studies have been undertaken focused on environmental emissions and the potential risk to sensitive receptors, defined as those persons susceptible to ill health given the distance from the facility. It is known that bioaerosols from compost activities remain airborne and travel offsite. With this in mind, those potentially at risk, in order of scale of exposure, include:
• Employees closely associated with onsite activity, e.g. drivers of loading equipment • Employees intermittently associated with onsite activity, e.g. drivers of delivery vehicles and site maintenance workers • Persons working in close proximity to composting sites • Members of the public living in the vicinity of composting sites • Persons passing by the periphery of composting sites
There are no occupational exposure limits (OELs) for biological agents, and for most of them no dose-response relationship can be determined, due to the number of bioaerosols and the complexities associated with human responses to different microorganisms. Furthermore, cumulative exposure conditions may exist at workplaces. As such, workers’ exposure must be adequately controlled so far as is reasonably practicable. Depending on their work task and proximity to the bioaerosol source, measures can include personal protective equipment (PPE), engineering controls, such as air-conditioned vehicle cabs, and/or the establishment of risk zones for working practices.
A study conducted by the Health and Safety Executive (HSE, 2003) found that being protected by a vehicle cab while working with composting handling machinery reduced the chance of being exposed to more than 100,000 cfu/m 3 bacteria from 64% to 28% and reduced exposure to bacteria of more than one million cfu/m 3 from 28% to 5%, while reducing the chance of being exposed to more than 100,000 cfu/m 3 Aspergillus fumigatus spores drops from 24% down to 13%.
Natural variations
It is important to consider natural sources of variation when determining the concentration of bioaerosols present. These include time of day, geographic location, local meteorological conditions such as wind speed, wind direction, temperature and relative humidity, and seasonality. With regards to seasonality, it is anticipated that more green waste will be handled in summer and that microbial activity could be greater in the unprocessed green waste in warmer weather.
Cooler wetter conditions in winter may reduce bioaerosol generation, whereas windier conditions would aid dispersion and dilution, while lowering ambient temperature. Although it is not expected that overall exposure mitigation measures should be different between seasons, there may be the need for more frequent maintenance, such as replacement or cleaning of cab filters during drier conditions.
Site topography and layout is also a major factor. Sites should be designed so that the smallest surface area of composting activities is exposed to the prevailing wind and at the lowest elevations on site. Tree lines or bunds can be used to increase turbulence around the site, thus proliferating dispersion and reducing downwind concentrations.
Bioaerosol risk assessment
Bioaerosols can travel vast distances at concentrations likely to fluctuate significantly depending on location. Dilution and dispersion effects in open air will reduce the concentration of bioaerosols present. In England, it is generally accepted that at more than 200m from source bioaerosol levels return to background concentrations. There is great uncertainty, however, over the concentration to which bioaerosol exposure must be reduced to be considered ‘safe’.
In 2010 the Environment Agency (EA) for England adopted the position statement 031 (Version 1.0) on composting and the potential effects to human health of exposure to bioaerosols generated from composting. This risk-based approach places limitations on the location of composting facilities to prevent the siting of composting activity within 250 metres of a dwelling or workplace, unless justified by a site-specific risk assessment.
In order to gain an environmental permit, site operators must undertake monitoring and analysis to examine the presence of microbial fragments, allergens and microorganisms. This must be undertaken before starting new operations or making significant changes to existing ones as well as during normal operations to establish background bioaerosol emissions. If applicable, control measures should be adopted to reduce emissions. Such monitoring and analysis is specialist in nature and should be carried out by competent persons who are familiar with how to devise site-appropriate sampling plans and procedures.
A bioaerosol risk assessment should identify the hazard, assess exposure, and estimate and characterise the risk so as to establish the baseline and the environmental setting. The scope, level of detail and focus will be site-specific due to temporal and spatial scales, but the problem definition should be logical and transparent in accordance with industry methodology.
The problem definition should describe:
• Processing technology • Feedstock, tonnages processed and seasonal variations • Site layout • Location and description of sensitive receptors • Sources of bioaerosols from composting • Meteorological data such as wind speed, wind direction, temperature and relative humidity • Location of sensitive receptors • Brief summary of health risks associated with bioaerosols • Other sources of bioaerosols in the vicinity • Other emissions such as odour and dust
Bioaerosol monitoring
British Standard 13098:2001 ‘Workplace Guidelines for Measurement of Airborne Microorganisms and Endotoxins’ provides guidelines for the assessment of workplace exposure to airborne microorganisms, including the determination of total number and culturable number of microorganisms in the workplace atmosphere (BSI, 2001). In line with these guidelines the Association for Organics Recycling (AfOR), now part of the Renewable Energy Association (REA) and formerly the Composting Association, developed a standardised protocol for the sampling and enumeration of airborne microorganisms at composting facilities (AfOR, 2009).
Boundary monitoring
There are a number of strategies that may be used for measuring and estimating boundary bioaerosols, including liquid impinges, split samplers and those for active impact, such as Anderson samplers. It is the latter sampling technique that dominates. At least two (preferably four) single stage Andersen samplers are often used, fitted with a baffle at least 15cm high and placed on a tripod so that the top of the inlet cone is held at 1.5m to 1.8m above the ground. The samplers are connected to a vacuum pump calibrated to a constant airflow rate of 28.3 litres/min.
The air containing airborne organisms is drawn into the chamber at a measured quantity, due to the right angled design of airflow and the effects of inertia. The bioaerosols are impacted onto sterile agar plates that contain a special growth medium for specific types of airborne microorganisms. These agar plates are then incubated and the number of fungi or bacteria is enumerated.
The advantages of this method are that it requires minimal post-collection sample handling, pumps at a high rate, is capable of collecting bioaerosols at low concentrations, is sterilisable, relatively light weight and easy to use on site when compared to impinges made of glass parts. While the Andersen impaction method alleviates many concerns of subsequent contamination via analytical requirements, it is not a suitable method to use where bioaerosol and dust levels are high. Furthermore, the samplers are expensive, labour intensive and cannot be used for non-culturable microorganisms.
Once the bioaerosol concentration is recorded in CFU m -3 , the first step in interpreting the results is to compare with background concentrations specific to that site. In the absence of pre-operational bioaerosol concentrations, the sampling results should be compared to upwind samples. Downwind concentrations will be determined to assess the level of emission directly from the composting facility.
If the downwind measured concentrations exceed background concentrations or the suggested threshold limits, then three steps should be undertaken in the following order:
• The site and environmental conditions that existed during the sampling process should be examined for abnormal activities and/or meteorological conditions • The sampling process should be repeated • Mitigation measures should be explored by the site operator
The frequency of boundary sampling for bioaerosols should be determined by the level of risk from a particular site. With new sites, the level of bioaerosol emissions will initially be unknown, so more frequent sampling should be undertaken until the emissions are understood.
Typically, a three stage sampling regime will be proposed for bespoke permitted sites with a sensitive receptor within 250m of the site boundary:
1. Background sampling should be undertaken before a site begins operating to understand the level of bioaerosols before site activity.
2. New sites should be monitored quarterly for the first year of operations to provide some indication of the level of variance in emissions.
3. Under normal operating conditions, bioaerosol monitoring should be undertaken twice a year – to be discussed with the regulator on a site-specific basis.
It is anticipated that most sites will remain within stage three; however, monitoring may return to stage two and then progress through stage three if there is an increase in the amount of material being processed, a significant change in site operations, sensitive receptors identified close to the site such as within 250m, and/or high concentrations of bioaerosols are detected during routine sampling.
Personnel monitoring
A personal bioaerosol sampler is used to measure occupational exposure to airborne biological agents such as fungi and bacteria. The sampler is autonomous with a battery life of about 10 hours, covering a typical shift duration. It is light, easy to wear and does not impact on the worker’s activity, thus giving a true representation of the amount of bioaerosol present around the breathing zone of the worker. Moreover, it can sample much higher concentrations of bioaerosols compared to impactors such as Andersons and limits the mechanical stress on the microorganisms.
Biological particles are collected in 2ml of liquid inside a rotating cup fitted with radial vanes to maintain an air flow rate of 10L/min -1 at a rotational speed of approximately 7,000 rpm. The rotating cup is made of sterilisable material. The sampled particles follow a helicoidal trajectory as they are pushed to the surface of the liquid by centrifugal force, which creates a thin vertical liquid layer. Sterile water or another collecting liquid can be used.
Three particle size selectors allow health-related aerosol fractions to be sampled according to international conventions. Once they are collected, they are plated on various types of agar plates at different dilutions, incubated and counted. They can then be used for further analysis such as endotoxins or glucans.
Summary
Bioaerosol emissions from commercial waste composting activities will continue to be a health concern for workers on site and to near residential and commercial properties. Bioaerosol emissions rapidly decline as distance from the source increases, with site boundaries considered ‘typical’ background levels. The ‘risk zone’ approach provides a simple method that can be adopted for site operators and regulators to assess the potential for occupational exposure to bioaerosols originating from composting.
There is a need for robust and reliable information regarding the health risks associated with composting activities to ensure that adequate controls are in place to reduce occupational exposure. This should include an examination of factors affecting the dispersion of bioaerosol, with distance from source, such as immediate surroundings and beyond the site boundary, based on computational modelling.
Systematic monitoring and analysis of bioaerosols must be undertaken to validate such models. It is acknowledged, however, that there is a difficulty of interpreting health effect data as there is a variation in human health response. With this in mind there is a need to improve the knowledge of the biodiversity of bioaerosols so as to determine at what concentration individual constituents of bioaerosols are hazardous.
Published: 17th Dec 2014 in AWE International
