|<< Back to OSHA Safety and Health Information Bulletins - Table of Contents|
OSHA Safety Hazard Information Bulletin (*) - Fire and Explosive Hazards
Associated with Biosolids Derived Fuel (BDF) and Waste Water Treatment Plants Shovel Attachments
April 8, 1996
FOOTNOTE(*) The Directorate of Technical Support issues Hazardous Information Bulletins (HIBs) in accordance with OSHA Instruction CPL 2.65 to provide relevant information regarding unrecognized or misunderstood health hazards, inadequacies of materials, devices, techniques, and safety engineering controls. HIBs are initiated based on information provided by the field staff, studies, reports and concerns expressed by safety and health professionals, employers, and the public. Information is compiled based on a thorough evaluation of available facts, literature and in coordination with appropriate parties. HIBs are used as an outreach tool for accident prevention.
The National Fire Protection Association (NFPA) forwarded to the Occupational Safety and Health Administration (OSHA) a research paper(1) which reported on potential hazards involved with processing, conveying and storing of dried biosolids derived fuel (BDF). BDF is used in a number of industrial applications such as The paper indicated that organic dust can be generated during the storage of BDF. It also asserted that another organic dust similar to biosolids dust was grain dust.
The paper outlined a number of design criteria which included both passive (venting) and active (inertial) systems to minimize and control explosions and fires connected with this organic dust. These criteria have been examined and tested and were found to be effective when implemented at the Los Angeles biosolids drying facility in its Hyperion Treatment Plant. The plant suffered no incidents where significant equipment damage or harm to personnel occurred. OSHA has not undertaken research nor made an evaluation on any of these matters.
The Office of Science and Technology Assessment (OSTA) has determined that the safety principles outlined in the research paper apply to most operations, including the handling or processing of biosolids derived fuel (BDF-powdered or pelletize organic material).
Dried BDF, which contains less than 10 percent moisture, is composed primarily of carbohydrates, proteins and fats. BDFs are the product of a procedure where waste water is subjected to a multiple evaporation process involving indirect steam dryers. Upon completion of the evaporation process, the prepared BDFs (in the form of "wet cakes") are stored for future use. During storage, BDFs may break down into a dust. Once created, BDF dust (depending upon its composition) is capable of self-heating and subsequent ignition. An organic dust suspended in air can rapidly ignite if exposed to an ignition source. Temperature, storage conditions and the presence of oxygen can be contributing factors.
According to the paper, the minimum explosible concentration of dried biosolids from Hyperion with 0% moisture is 320 gm/m(3). This is a high value, as most dusts, such as corn starch and coal dust, ignite at approximately 50 gm/m(3). The minimum ignition temperature for Hyperion dried biosolids is 355 degrees C and the minimum oxygen concentration for combustion of a dust cloud varies from 15 to 16% by volume.
Oxidation of organic materials is an exothermic reaction; any organic material can be oxidized in air. Information from Dust Explosions(2) indicates that any material that will burn in air, when in a solid form, may explode when it is a finely divided powder. According to the paper, dried biosolids must be processed and handled carefully to avoid both fire and explosion hazards. This is true whether the dried biosolids are powdery or pelletized. The paper calls for the following for the safe processing, handling, conveyance and storage of dried biosolids:
(1) All vessels which process, convey or store dried biosolids must be either equipped with explosion relief or be designed to withstand the maximum explosion pressure. "Passive" (venting) explosion vents are to be used even if an "active" (inertial) system, such as a nitrogen inertial system is also used. This criteria applies regardless of whether the dried biosolids are in a powder or a pelletized form.
(2) Use rupture type explosion vents on process and storage vessels.
(3) Explosion vents should be sized in order to vent the worst possible powder explosion in an air atmosphere. The venting should be determined based on laboratory testing of representative samples of the biosolids powder.
(4) The size of the explosion vent must be sufficient to vent the maximum expected gas flow rate without exceeding 90% of the yield strength of the storage vessel.
(5) Explosion vents must be designed to release discharged gases to a safe location, outside the buildings.
(6) Store and convey all dried and hot biosolids in a nitrogen inertial atmosphere. Maintain oxygen level below 5% by volume to reduce the potential for self heating and ignition of hot biosolids.
(7) All conductive elements of the system that contact the dried biosolids must be electrically bonded and grounded.
(8) Ducts, pipes and vessel made of non-conductive materials must not be used with dried biosolids.
In addition to the above, the following is a general guide for hazardous locations:
(1) Good housekeeping must be maintained.
(2) Maintenance workers must not enter any vessel unless all of the BDF powder has been removed prior to entry or the powder has been cooled to ambient temperature before safe entry clearance is given. It has been determined that a small dust layer will begin rapid self-heating and smoldering when exposed to temperatures between 145 and 195 degrees C.
(3) All facilities used to process, convey or store dried biosolids and all areas surrounding such facilities must be electrically classified as Class II, Division 1 or 2, as appropriate in accordance with NFPA 70.
(4) All potential heat sources within a classified area must be reviewed and eliminated or moved outside the classified area. All hot work (i.e., welding) must have a permit issued prior to any work commencing in a classified area.
This bulletin may be used for accident prevention and outreach purposes. Please distribute this bulletin to all area offices, State Plan States, Consultation Projects and appropriate local groups for their information.
1. Haug, R.T., Petino, G., Lewis, M.F., and Hartnett, W.J.,(1993) "Explosion Protection and Fire Prevention At A Biosolids Drying Facility," Proceedings of the 66th Annual Conference of the Water Environment Federation, Anaheim, CA.
2. Cross, J., and Farrer, D., (1982) Dust Explosions, Plenum Press, New York.