Based on the Composting Council of Canada’s 1998 survey, there are 344 composting facilities in the country. Many of these are small leaf and yard waste operations, but the level of composting sophistication is on the increase and many can now accept a wide range of organics.
Because of their potential to negatively affect health, bioaerosols must be considered when designing and operating a composting facility.
Bioaerosols are aerosolized microorganisms. Given that composting is a microbial process, bioaerosolization occurs when composting material is disturbed. Bioaerosolization activities result from the turning of a compost windrow or from air that’s mechanically forced through a composting mass. Small particles of dust can adhere to very small water droplets. There may be a greater concentration of microorganisms in the air surrounding the direct area of activity compared to background concentrations.
Two main bioaerosols are of concern: the thermo-tolerant fungus Aspergillus Fumigatus (AF) and a constituent of the cell walls of gram-negative bacteria called endotoxin. Both are ubiquitous. AF is found in many diverse environments, from Antarctica to the tropics and is able to survive or grow in a wide temperature range (12-50C), relative humidities and substrates. Bacterial endotoxin consists of a portion of the outer layer of the cell walls of gram-negative bacteria and is often found in organic dusts.
Human responses to bioaerosols are host and dose dependent; that is, some individuals may respond to a dose that does not affect others. Healthy individuals are typically not at great risk.
Potential responses to bioaerosols can include inflammation responses (such as mucous membrane irritation), allergic responses (such as hypersensitivity pneumonitis) and infection. Most reported cases of aspergillosis (i.e., the condition caused by AF) have occurred in immuno-compromised individuals. Instances of aspergillosis in healthy individuals are rare, even for people with occupational exposure to high levels of airborne AF.
Of course, active composting areas have high bioaerosol concentrations relative to the normal background environment. Typically, concentrations fall to background levels shortly after the activity ceases and at a short distance (i.e., hundreds of feet).
It’s important to note that not many studies have been conducted in Canada. The most recent was conducted in 1995.
In response to concerns from Ontario’s labour ministry over bioaerosol concentrations at an outdoor leaf and yard waste composting facility in Sarnia a comprehensive study was conducted by Green Lane Environmental Group Ltd. A work order was issued to “reduce bioaerosol concentration to the lowest practical limit.”
Over a two-day period, concentrations were assessed during various activities that included debagging, turning active compost, turning curing compost and loading finished compost into a truck. Although the focus of the study was on personal samples (i.e., to assess respirable or inhaleable bioaerosols), upwind and downwind samples were also collected.
The study found that: higher concentrations of bioaerosols existed during turning of the active and curing compost; debagging of yard waste (mostly grass) resulted in the highest concentration of endotoxin; and, shipping finished compost resulted in relatively low concentrations of bioaerosols.
For the most part, the measurement of microorganisms in the study fell below levels that pose any significant risk. Endotoxin concentrations during debagging and the turning of active compost were in the range associated with potential mucous membrane irritation. As in other studies, it was concluded that proper site design, operations and housekeeping reduce dust and exposure to secondary pathogens and endotoxin.
A multi-disciplinary team including occupational health and safety experts and a medical doctor indicated that risk for the average worker at the site was minimal.
As part of the planning process, compost facility operators should develop a bioaerosol control plan. A set moisture content of 50 to 60 per cent is ideal for reducing the quantity of dust to which microorganisms may adhere. Similarly, maintenance of a clean site reduces dust.
Protective controls are very important and can be simple. Rotating workers in activities that generate greater bioaerosolization reduces their exposure. Operators that mechanically turn active or curing compost should use some kind of respiratory control. For instance, an N-95 dust-mist mask is effective.
Worker hygiene is also important. One of the exposure pathways for microorganisms is hand-to-mouth, so measures such as a ban against eating, drinking and smoking at the site and hand-washing protocols are effective.
Paul van der Werf, M.Sc. is manager of composting operations for Green Lane Environmental Group Ltd. and principal of 2CG Consulting, in London, Ontario.