Biological media air filters (biofilters) are currently being used for the treatment of inorganic and organic gases from sewage treatment plants, industrial processes, and remediation systems. The media may be organic material such as compost, wood chips, or synthetic plastic media, each with a large surface area for microorganism growth and activity. An occupational health and safety graduate student team (OHS team) evaluated potential particulate and bioaerosol exposure from a biofilter unit process used to treat hydrogen sulfide (H2S) gas generated from a primary sludge settling unit process. The OHS team included an industrial hygiene/environmental health engineering specialist, an occupational safety specialist, an occupational health physician, and an occupational health nurse. Concerns were raised regarding the possibility of adverse health effects to maintenance workers during "conditioning" of the biofilter compost-like media beds. Conditioning activities may include in-situ rearrangement of the existing media, removal from the tank/surface, drying/reinsertion of the existing media, or complete removal of the media, and replacement with new. Neither the design engineering firm nor the manufacturer had specific written recommendations or precautions regarding exposure during the conditioning of the compost beds. No personal protection equipment has been used for this activity. The expected agents for adverse health effects associated with this unit process are respirable particulate dust and bioaerosols, which may contain viable bacteria and fungi, as well as endotoxin. Safety procedures are already in place for H2S. Mixed dust from the compost media bed may cause irritation of pre-existing health conditions such as asthma, chronic lung disease, and some skin conditions, and may also lead to new health problems such as inhalation fever, occupational asthma, hypersensitivity pneumonitis, skin rashes and/or skin infections, and upper or lower respiratory infections. Air samples were taken immediately before (background samples) and during a simulation of the conditioning procedure of one of the two compost beds. Airborne samples collected during the simulated procedure yielded more bacteria per volume of collected air than the background samples. These findings suggest an increased risk for bacterial infection during the conditioning procedure. The airborne samples cultured for fungi during the simulation were less than the background sample. Stachybotrys chartarum was not detected in a sample of the filter media. As with the fungi data, endotoxin air sampling yielded less endotoxin during the simulated conditioning procedure than in the background. Two inches of rain had fallen in the 24 hours preceding the sampling, soaking the upper layers of the compost media. Sampling during an actual conditioning procedure on a warm, dry day may yield additional evidence relevant to potential health risks from dust and microorganisms. Although the data is not representative of an actual conditioning procedure, the following common sense precautions are recommended: (1) Dust minimization during dry conditions can be achieved by thoroughly wetting the compost bed prior to removing the compost, and wetting the compost again before returning it to the bed. (2) Based on scientific literature rather than this project's sampling results, the use of an appropriate respirator should be proactively considered. In addition, consideration must be given to the worker's cardiopulmonary effects associated with respirator use, decreased work performance effects, and OSHA administrative requirements of a respirator protection program. (3) Workers with immunocompromised status, asthma, allergic rhinitis/conjunctivitis, or allergic conditions of the skin, eyes, or lungs should be excluded from this particular job, unless it can be shown that their personal protective equipment is effective. (4) Protective outer wear, goggles, and gloves should be worn to minimize health risks associated with clothing garments contaminated by microorganisms.
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May 14, 2003
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