BioCycle March 2006, Vol. 47, No. 3, p. 55
Dust can have a significant impact on working conditions, equipment maintenance, fire hazards – and odor. Here are some steps to minimize the impacts.
Robert Spencer and Charles M. Alix
WHEN considering composting facility emissions, odor is generally the first item that comes to mind. However, dust also can have a significant impact on working conditions, equipment maintenance, fire hazards – and odor. Dust consists of fine particles with sufficiently low density to remain suspended in the air for long periods of time. To achieve this low density, the particles usually must be dry. Water adds significantly to density and thus inhibits particles from becoming airborne and remaining in suspension.
Dust can be generated at all phases of a composting operation, from feedstock tipping to product shipping. Indoor and outdoor facilities both can have dust issues. Wherever fine particles can accumulate and become dry enough to remain suspended in the air, dust remains a constant factor in facility operations.
Although dust is defined as “particles that become airborne,” these same particles can create operational issues when they settle. For example, dust that settles on steel beams and becomes wet will hold the moisture in place, resulting in corrosion.
At most composting facilities, several aspects of the operation can generate dust, particularly waste receiving, loading, conveying, curing, screening, roadways, and hauling. At certain stages during composting, good process management practices will inherently lead to dust generation. For example, although moisture content during decomposition needs to be 60 to 65 percent, optimal screening requires the material to be drier (about 40 percent moisture). It is a challenge to hit the narrow moisture content window before the compost becomes too dry.
This article discusses aspects of compost facility management that are impacted by dust. It presents examples of dust issues that the authors have experienced in their years of working in the composting industry, as both operators and consultants. Dust management technologies and procedures will be discussed in greater detail in a future edition of Operator’s Forum.
Worker Health And Safety
Outside dust emissions are regulated by state and local air quality standards, while inside – or at least in worker-occupied areas – dust emissions are regulated by OSHA (Occupational Safety and Health Administration). OSHA limits worker exposure to 5 milligrams/cubic meter of dust less than five microns in size (known as respirable dust) and 15 milligrams/cubic meter of total dust (particles of any size). Workers subjected to dust concentrations equal to or higher than these limits must have protective dust masks and the workers must be trained in their proper use. Employees who do not wear dust masks or respirators, particularly in final screening areas, may experience irritation of mucous membranes and nose bleeds. Fortunately, occupational health studies around the world have shown that composting facilities can maintain a healthy work environment, but it requires good facility design, adherence to standard operating procedures, and use of personal protective equipment.
Besides the potential adverse health impacts of breathing dust, hazardous conditions can result for employees working in a dusty facility. Visibility may be impaired, particularly when operating loaders and other equipment. Material left in traffic areas is constantly agitated by wheel movement and thus is a persistent source of dust clouds. In indoor facilities, there is no wind to clear away the dust and the clouds continually agitated by traffic will obstruct vision for long periods. Keeping traffic areas clean is especially effective in reducing dust clouds. Where cleaning is not practical, such as dirt roadways, frequent passes with water spray trucks helps prevent dust. In most states, this is required practice.
Odors, Dust And Treatment Systems
The particular odor of a compost facility also is carried on dust particles generated by waste materials, bulking agents and compost. When dust from those operations is carried to other locations by wind, trucks and the compost product, odor goes with it. Screening dry compost can create a plume of dust, bearing the signature odor of the facility.
People who inhale dust particles will experience the odor associated with the dust at concentrations many time higher than the ambient odor concentration. Workers at compost plants may shower and change clothes, but they will continue to smell the compost odor they thought they left behind thanks to dust filtered out in nasal passages.
Design and operation of air collection and odor treatment systems should consider dust and its potential to clog scrubber ducts, packing, demisters, sumps, as well as the biofilter air distribution systems. The pressure loss across a scrubber, biofilter, or the combined system, as measured in inches of water column, is the best indicator of clogging. Some composting facility air permits require automated or regular monitoring of air system back pressure, with upper limits established and/or alarm set points.
The most common dust problem related to biofilters is dust accumulation in the air distribution system. The dust provides a surface for biofouling (slime) to grow and block holes, slots or other air passages. In biofilters employing geotextiles or filter fabrics, biofouling caused by dust accumulation may require the media be removed so the fabric or geotextiles can be cleaned.
Dust also can accumulate in ductwork. With exposure to moisture and buildup over time, a considerable load can be added to duct supports, causing ducts to collapse.
Air scrubbers often are installed between the ducts carrying process air from the processing area to the biofilter for humidification of the air. These scrubbers are prone to trapping dust on the surface of the internal components, providing food, warmth and moisture that is conducive to growth of microorganisms. (In fact, scrubbers are sometimes designed as biological treatment systems for odor, but the amount of scum can impede airflows if not properly controlled.) One consequence of dust accumulating in a scrubber is that the packing and demister materials provide extensive surface area for growth of a very nasty biological scum, similar to the rubbery scum that accumulates in the drain traps of bathroom sinks! A recent inspection of a scrubber at a biosolids composting plant revealed stalactites of scum, and clogged packing and demister material.
Once the dust and scum have accumulated to the point of impeding air flow, as indicated by back pressure reading across the scrubber, the operator is faced with cleaning the scrubber, which is usually a large fiberglass tank. Scrubber manufacturers often recommend that a solution of sodium hypochlorite be pumped through the scrubber to kill the biological scum. This can be a very difficult task since the concentrations required to effectively kill the organisms generate irritating chlorine concentrations in the facility air, a hazard to workers.
To remove and clean the packing is a daunting task when you have 10, 20 or 30 cubic yards of packing per scrubber. It has to be contained in something and then washed with a pressure washer or fire hose, sometimes handling each piece of the packing one at a time. Considering the large amount of surface in each sphere that harbors the scum, this can be an arduous task. Once the scum dries out, it is even more difficult to clean.
If it is not possible to clean the packing while in the scrubber, it is probably necessary to physically remove the packing, either by hand or with a vacuum. It can take several employees a couple of days to remove, clean, and reinstall scrubber packing. Employees must follow OSHA confined space entry procedures when a person enters a scrubber tank. At this point, facility managers sometimes make the decision to discard the old packing and purchase new, a considerable expense.
Where water is recirculated from a sump to the scrubber spray nozzles, particulates build up in the sump water, and may clog the spray nozzles. For this reason, sumps should be drained on a daily, or at least weekly, basis. Another factor is the percentage of scrubber water that is recirculated compared to use of fresh makeup water, so that excessive dust particulates are not recirculated to the spray nozzles. Similarly, the amount of water discharged from the scrubber as it is operating (known as blowdown water), can also impact the amount of dust particulates that build up in the sump recirculation water. Usually, higher rates of blowdown minimize the accumulation of dust, and nozzle clogging.
Demisters are sometimes installed at the discharge end of a scrubber to trap water particles so they do not carry over into the downstream air ducts where they can clog pipes, and contribute to corrosion. At one facility, so much water was carried over into a duct that conveyed compost building air to the adjacent biofilter that it leaked out of the duct seams and saturated the surrounding insulation, creating a noticeable odor outside of the building.
Leachate And Dust Issues
The water that drains out of scrubbers, biofilters, tip floors, and aeration floors contains dust particles that are a source of odor and BOD (biological oxygen demand). As soon as dust is trapped in the aqueous discharge from the scrubber, it becomes wastewater, or leachate.
Leachate can be collected and used for irrigation of biofilters, and perhaps compost piles. However, the amount of dust in the leachate as measured in total suspended solids, and BOD, may trigger other problems. One facility that constructed detention basins to collect scrubber and biofilter leachate for biofilter irrigation water experienced significant odor issues from the outside aerated basins. This is particularly true if the water remains stagnant and unaerated for long periods of time.
Leachate also may be regulated as an industrial wastewater, and testing of the leachate may be required to determine if it is suitable for discharge into a municipal wastewater treatment system. Typically, such leachates are comparable to septage in terms of BOD and TSS (total suspended solids), and can be handled by WWTPs. If a public WWTP is not available, the leachate may have to be collected and hauled to off-site treatment plants, or an on-site septic system must be installed.
Obviously, design of compost facilities should incorporate estimates of the dust loading, and its impact on leachate quality. At operating facilities, dust concentrations can be measured by setting out canisters that collect dust over a certain time period, which helps to estimate the dust load on scrubbers and biofilters.
Fire Protection, Equipment And Buildings
Fire protection requires that dust accumulations be kept to a minimum, a challenge in most compost plants. A layer of dust on screening equipment can foster the right conditions for a welder’s spark to smolder for hours, then igniting into flames later that night or weekend when the facility is closed. When welding, OSHA “hot work” procedures must be followed, including a fire watch. Electrical equipment can also generate sparks that will ignite dust. Dust also can accumulate to concentrations that become explosive in the presence of an energy source.
Dust is a major enemy of equipment and buildings since it clogs air intakes on loader radiators, gets into the lubrication systems of machines, and accumulates to depths where its insulating properties can cause machinery to overheat.
Dust settling on steel structural beams and purlins will trap and hold moisture. This leads to corrosion. For this reason, housekeeping procedures should include industrial scale dusting and cleaning.
Dust Control Measures
The US Composting Council’s “Compost Facility Operating Guide” presents the following dust control measures:
Dampen loads on the tipping floor, either manually or with an overhead misting system.
Enclose and ventilate potentially dusty process areas such as tipping floor, picking lines, and storage and packaging areas.
Provide filter mask for employees
Install dust hoods around conveyor transfer points or other localized dust sources. Route air and dust collected from the dust hoods to a particulate removal system and central odor control system.
Pave on-site roads and keep them free of debris. Compost spilled on roads tends to dry out and cause dust problems.
Regularly sweep spilled compost and other materials from loading or processing areas.
Periodically clean beams, light fixtures, equipment, and other surfaces as necessary even if proper dust control measures are in place.
Although some facilities install spray misters around trommel screens to knock out dust, it is important to note that the use of a water spray to control dust involves detailed consideration of the amount of water and the conditions provided for water:dust impact. Point source capture of dust with hoods is also a good approach to reducing spread of dust throughout the facility.
Standard operating procedures for housekeeping to control dust are advisable. This usually requires sufficient numbers of employees assigned to cleaning. Some facilities subcontract for periodic industrial cleaning services rather than using their own employees.
Composting operations are industrial facilities, like factories. As such, the same reasoning and concepts of industrial hygiene that apply to any other type of factory also apply to compost factories.
Robert Spencer is an Environmental Planning Consultant based in Sterling, Massachusetts. Charles M. Alix is a Senior Engineer with MWH Americas, Inc. in Boston, Massachusetts.
March 27, 2006 | General
Dust Management, Mitigation At Composting Facilitites
BioCycle March 2006, Vol. 47, No. 3, p. 55