Biocycle - Advancing Composting, Organics Recycling & Renewable Energy

Controlling Dust and Bioaerosols at a Biosolids Composting Faclity


Courtesy of Biocycle - Advancing Composting, Organics Recycling & Renewable Energy

Studies at enclosed Longmont, Colorado project evaluate exposure level of employees. Operational changes reduce dust, endotoxin and A. Fumigatus by 90 percent.

WHEN dealing with public health issues and composting, attention has been predominantly focused on potential exposure from the release of Aspergillus fumigatus (A. fumigatus) from composting facilities. Numerous studies were conducted to assess the level of A. fumigatus spores concentration in the areas surrounding composting facilities and to compare this to the levels of A. fumigatus found in remote areas (areas unrelated to composting facilities). Two early studies — in 1984 and 1987 — focused on worker exposure, but neither study showed any significant worker health problems.

Recently, the increase in composting activities, particularly when composting involves the handling of yard trimmings, and the trend towards controlling odors by enclosing facilities, have revived concern for worker exposure to A. fumigatus and other bioaerosols. Although there are no reported cases in the literature of occupational impacts at biosolids composting facilities in the United States, E&A Environmental Consultants, Inc. (E&A) has encountered two cases of employees developing symptoms related to dust. One individual — an employee at a biosolids/municipal solid waste facility (and, incidentally, a heavy smoker) — reported respiratory discomfort at the end of the work week that would improve over the weekend. Symptoms disappeared during his vacations. A second employee at a biosolids/wood chip composting facility developed a rash on the face and scalp. The study reported here was designed for the city of Longmont, Colorado, which wanted to evaluate the level of exposure of employees to dust, endotoxin, and A. fumigatus, and to determine if operational and design changes to the facility could reduce the airborne concentration of these constituents. Longmont has operated a 7.7 dry metric tons/day aerated static pile biosolids composting facility since 1991. The facility consists of a totally enclosed mixing building and a separate, totally enclosed composting/curing and screening building. Dust is a major problem in the facility due to the extremely dry climatic conditions (see sidebar).

In this paper, E&A presents data on dust, endotoxin, and A. fumigatus concentrations as related to various activities within the composting facility. Based on these results, operational mitigation measures were recommended, and the impacts of those modifications are reported. Sampling Air Quality Inside Facilities Two air sampling events were conducted to determine facility conditions in winter and summer climates. Air quality parameters included total dust, respirable dust, endotoxin, and A. fumigatus. In addition, eight-stage Marple Multiple Cascade Impactors — which collect particulates in separate size ranges between 0.52 and 21.30 µ — were worn by employees for the entire work shift during the winter monitoring period. Ambient temperatures during the winter ranged from -18°C to -7°C; summer temperatures ranged from 13°C to 18°C. All air samples were taken at a height of 1.5 meters. Airflows were calibrated daily using a bubble calibrator. Air monitoring was conducted during the following activities: feedstock mixing, pile construction, pile covering (winter monitoring only), pile teardown, and screening. Sampling was also conducted during a period with no activity.

Total airborne dust was determined using NIOSH Method 0500, and respirable dust concentrations were determined by collecting samplings through a nylon cyclone and using NIOSH Method 0600 (NIOSH is the National Institute of Occupational Safety and Health.) Airborne endotoxin concentrations were determined using the Limulus amebocyte lysate test on samples collected using NIOSH Method 0500. Samples for A. fumigatus evaluation were collected using an Andersen Two-Stage Impactor, according to ASTM Method E 884-82. Samples were collected on oxgall-gentamicin petri plates at a rate of 28.3 liters per minute. Dust and Endotoxin Table 1 shows the total and respirable dust concentrations measured during sampling periods in the winter of 1996 and the summer of 1997 as related to the various composting activities in the enclosed facility. The winter sampling event found very low dust levels that never exceeded OSHA PEL standards (Occupational Safety and Health Agency — particulate emission limit) of 15 mg/m3 for total dust and 5 mg/m3 for respirable dust. Much higher total dust concentrations were found during the summer sampling period. During pile breakdown, which is performed with a front-end loader, and during screening, high total dust levels during the single time measurements did exceed the OSHA PEL standards. Table 2 shows the data obtained using the Marple Multiple Cascade Impactor personal dust samplers. Two employees wore the samplers; one employee (Operator #2) participated on two different days so that data from three different workdays were obtained. Operator #1 conducted most of his activities from the cab of an enclosed front-end loader. Operator #2 spent his time both in the loader. cab and on the open floor where most of the activities take place. On Day 1, Operator #2 was in the mixing building where biosolids and sawdust were being mixed. This activity required Operator #2 to frequently exit the loader cab and monitor the mixing operations from ground level. On the second day, Operator #2 was involved in pile teardown and screening using a loader. As shown in Table 2, the operators in the mixing area are exposed to a greater overall dust concentration than operators involved in the other two activities. The percent of dust that is respirable is lower in the mixing area than in the other two areas; however, exposure to respirable dust appears to be higher in the mixing area. Table 3 shows the endotoxin and A. fumigatus concentrations. There are no regulatory standards for endotoxin although some thresholds have been proposed. The International Committee on Occupational Health (ICOH) has proposed the following threshold levels: 20 to 50 ng/m3 = mucous membrane irritation; 100 to 200 ng/m3 = acute bronchial constriction; 100 to 2,000 ng/m3 = organic dust toxic syndrome (ODTS). The Dutch Expert Committee on Occupational Standards of the National Health Council proposed a health-based recommended limit value of 4.5 ng/m3 over an eight-hour exposure period. In most cases, the endotoxin levels were sufficiently high to elicit respiratory response. None of the results suggest that workers may be susceptible to ODTS. Employees at the facility predominantly work in enclosed cabs of loaders, which have filters. Furthermore, the employees spend relatively few hours at any one location within the three buildings on the site. During part of the day, they are in areas in which dust and bioaerosol levels are considerably lower. Therefore, over an eight-hour working period, the average exposure may be lower than the reported values.

Aspergillus fumigatus The winter data for A. fumigatus were questionable since the air temperature at the time of sampling was very low, and it is possible that the spores did not adhere to the agar in the plates. The summer data indicated that A. fumigatus was greatest during feedstock mixing. Low levels of A. fumigatus were found during the screening operation, in contrast to other reported studies. It is possible that the extremely dry conditions reduced the viability of existing spores; the Anderson sampler methodology only determines the concentration of viable spores. Although the nonviable spores do not represent a pathological health hazard (i.e., Aspergillosis), they can elicit an allergic response in humans. Future sampling should include methods of A. fumigatus determination that measure both viable and nonviable spores. Recommendations For Dust Mitigation And Reducing Worker Exposure As a result of these data, E&A recommended several dust mitigation measures. The primary recommendations included: Increase moisture content of the feed mix; Adjust air flow to reduce drying prior to screening; Suppress dust on the composting area floor using a water truck; Place a dust hood and a baghouse dust collection system over the screen; and Place a water misting system over screen conveyors. The first four recommendations have been implemented. Significant reductions (90 percent) in total dust, respirable dust, endotoxin, and A. fumigatus were found in the composting hall following the implementation of the mitigation measures (see Figures 1 and 2). Workers at enclosed biosolids composting sites may be subject to frequent exposure to high levels of dust, endotoxin, and A. fumigatus for short periods of time. There are several very effective design and operational measures that can reduce worker exposure. These include: Moisture control of feedstock and composting; Isolation of the screening operation from the composting operations; Dust control and collection systems in dry climates; Sweepers and water vehicles to control dust in roadways; Air filters in loaders and other mobile equipment; Adequate ventilation in buildings; and Personal hygiene equipment. Eliot Epstein is with E&A Environmental Consultants in Canton, Massachusetts. Nerissa Wu was with E&A when this work was done. Calvin Youngberg is with the City of Longmont, Colorado. Gerald Croteau is in the Bothell, Washington office of E&A.

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