Sewage sludge pellets: fire and explosion risks
The article below was published by the Ontario Fire Marshall's Office in the Publication: Ontario Fire Service Messenger November/December 2002
Sewage sludge pellets: fire and explosion risks
There are rabbit food pellets, pellets for guns, wood pellets for burning in stoves, and there are even sewage sludge pellets! What will they think of next? Introduction Sewage sludge is the nutrient-rich organic byproduct of the wastewater treatment process. It contains most of the nutrients required for crop growth, and organic matter, which can enrich soil, and may also be called “biosolids”.
For some people, spreading biosolids on farmlands is considered to be perfectly safe way of returning nutrients to the ground if appropriate procedures are in place. Recycling this nutrient source is viewed to be better than putting it in a landfill site or incinerating it. Other people believe that cities are simply transferring urban pollution to the countryside, and site concerns about the fumes, respiratory infections and other negative health effects that may stem from exposure to biosolids, and the potential for contaminated well water and water courses.
Sewage sludge can exist in liquid forms and can also be converted into granules or pellets by removing the moisture. In this solid form, pellets are easier to handle and store, and transportation costs are reduced, as compared to liquid sludge. Although people may be aware of the environment aspects associated with the disposal of sewage sludge, they may not be aware that sewage sludge pellets have been associated with numerous fires and explosions.
For instance, Sludge pellets stockpiled at a farm in Windsor caused a smoldering fire. There were a series of explosions in a Windsor pelletization plant, most recently in October 2002. The City of Toronto had problems with the self-heating of sewage pellets in a storage silo. An explosion at the Miloganite plant in Milwaukee in 1996 caused serious injuries to a worker and $ 4.5 million worth of damage to the plant and property.
This article has been prepared to provide information on potential fire hazards associated with sludge pellets, safe storage and effective fire suppression. Self-heating properties of the product: Sewage sludge is mainly derived from human waste, but may also contain animal products, paper, high fat content from processing plants, heavy metals, organic contaminants and petroleum products from petroleum and diesel spills. When formed into pellets, the finished product has less than 5% moisture content. Should the moisture content of this material reach between 5- 10% by weight of the product, aerobic biological decomposition occurs, causing self-heating.
Water generated by this process is absorbed by the surrounding sludge, which intensifies the self-heating process. A smoldering fire may occur if the heat generated by this self-heating process is not dissipated to the surroundings. Processing and Handling In the initial stages of sewage treatment, the digestion process produces methane and carbon dioxide. If raw sludge is stored it will decompose and produce hydrogen sulphide and other volatile sulpher compounds. With the addition of chemicals to dewater the sludge, hydrogen sulphide and ammonia may be released. Conversion of sewage sludge into granules or pellets, by removing the moisture, is the final stage of the sewage treatment process.
The amount of dust produced in the drying process and later processing is affected by the method of drying and type of final product. Sewage sludge dust is about the same size and similar hazard as wood dust. Depending on the design of the plant, there is the potential for a dust explosion to occur at the main dryer, dust collector and handling plant, pelletizer and final product discharge plant.
In pellet form the product is sufficiently hard to withstand the normal conditions of mixing, handling and transportation without producing excessive levels of dust. These pellets have a relatively low auto-ignition temperature, as low as 2650C, and may be easily ignited without process precautions. A risk assessment followed by implementation of suitable prevention and protection measures is required for all parts of the process.
Special attention should be given to the specific hazards associated with the generation of methane, hydrogen suphide, and dusts. Appropriate ventilation, relief venting, suppression systems, containment features, avoidance of ignition sources, and safe handling and storage practices also need to be considered. Storage Once dried, pellets may self-heat to the point of ignition and slow burn.
To minimize the potential for self-heating, sewage sludge pellets should be kept cool and dry and should not be stored in large piles. Storage silos should be designed to aid cooling and be sized to allow thermal dissipation of heat. For this reason, tall narrow silos are preferable to wide silos. Where significant levels of dust are likely to be produced in the storage silos, they should be designed to mitigate the effect of any explosion. The simplest protection is the provision of explosion relief panels venting to a safe location. Silos should be designed to identify and contain a fire. A slow burning silo fire is likely to be starved of oxygen and therefore produce carbon monoxide. A carbon monoxide detector in the silo will indicate an incipient fire.
As well, multi-point temperature probes may be installed to monitor the temperature of the product. As an alternative to indoor storage, pellets should be transported to a site location and be off-loaded and turned into the soil as soon as possible. If this is not possible, the material should be spread on the ground evenly in the form of a very thin layer. This configuration will dissipate any heat generated into the ground and atmosphere.
Fire suppression
Inside a silo, an inert gas can be used to contain, but not necessarily extinguish, a fire. The injection of an inert gas will cause a drop in temperature, but may only have a limited effect. The temperatures should be monitored for several hours before deciding if the fire has been controlled. Procedures to deal with a silo fire may include the gradual emptying of the silo to a safe location. Outside, a sewage sludge pellet fire typically smolders at the surface with a relatively low burning temperature and emits dense white smoke and products of incomplete combustion. The smoke may contain organic acids and other compounds that are irritating agents.
The simplest way to deal with such a fire is to dissipate the heat by spreading out the pellets. It may also be extinguished by confining and smothering. Alternately, the pellets may be mixed into the soil or stamped with heavy earth moving equipment. In some cases, the use of Class A foam may be considered for fire suppression. Class A foam is a special formulation of hydrocarbon surfactants, that reduces the surface tension of water and provides for better water penetration and increased effectiveness. Class A foam acts as a surface barrier to stop or prevent further combustion. The use of water to suppress this type of fire is controversial.
The application of water may actually support a fire by contributing to the process of aerobic decomposition. Further, adding water may return the dried sewage into liquid sewage and create additional leachate and runoff. In turn, this may contaminate ground and surface waters surrounding the site and could cause significant environmental and health risks. Overall, fire fighting tactics need to consider a range of circumstances, not the least of which include the size of fire, location, wind/weather conditions, water supply, personnel safety, access to heavy equipment, and environmental impact.
Conclusions
Although fires involving sludge pellets are not common occurrences, they do tend to attract a great deal of public attention and challenge the fire service. By working with the public, pellet factory owners, and owners of sites used to spread sewage sludge pellets, the fire service can ensure that safe practices are employed, thereby protecting the public, environment and emergency responders.
Article prepared by OFM Fire Protection Engineers Beth Tate and M. Mailvaganam.
The article below was published by the Ontario Fire Marshall's Office in the Publication: Ontario Fire Service Messenger November/December 2002
Sewage sludge pellets: fire and explosion risks
There are rabbit food pellets, pellets for guns, wood pellets for burning in stoves, and there are even sewage sludge pellets! What will they think of next? Introduction Sewage sludge is the nutrient-rich organic byproduct of the wastewater treatment process. It contains most of the nutrients required for crop growth, and organic matter, which can enrich soil, and may also be called “biosolids”.
For some people, spreading biosolids on farmlands is considered to be perfectly safe way of returning nutrients to the ground if appropriate procedures are in place. Recycling this nutrient source is viewed to be better than putting it in a landfill site or incinerating it. Other people believe that cities are simply transferring urban pollution to the countryside, and site concerns about the fumes, respiratory infections and other negative health effects that may stem from exposure to biosolids, and the potential for contaminated well water and water courses.
Sewage sludge can exist in liquid forms and can also be converted into granules or pellets by removing the moisture. In this solid form, pellets are easier to handle and store, and transportation costs are reduced, as compared to liquid sludge. Although people may be aware of the environment aspects associated with the disposal of sewage sludge, they may not be aware that sewage sludge pellets have been associated with numerous fires and explosions.
For instance, Sludge pellets stockpiled at a farm in Windsor caused a smoldering fire. There were a series of explosions in a Windsor pelletization plant, most recently in October 2002. The City of Toronto had problems with the self-heating of sewage pellets in a storage silo. An explosion at the Miloganite plant in Milwaukee in 1996 caused serious injuries to a worker and $ 4.5 million worth of damage to the plant and property.
This article has been prepared to provide information on potential fire hazards associated with sludge pellets, safe storage and effective fire suppression. Self-heating properties of the product: Sewage sludge is mainly derived from human waste, but may also contain animal products, paper, high fat content from processing plants, heavy metals, organic contaminants and petroleum products from petroleum and diesel spills. When formed into pellets, the finished product has less than 5% moisture content. Should the moisture content of this material reach between 5- 10% by weight of the product, aerobic biological decomposition occurs, causing self-heating.
Water generated by this process is absorbed by the surrounding sludge, which intensifies the self-heating process. A smoldering fire may occur if the heat generated by this self-heating process is not dissipated to the surroundings. Processing and Handling In the initial stages of sewage treatment, the digestion process produces methane and carbon dioxide. If raw sludge is stored it will decompose and produce hydrogen sulphide and other volatile sulpher compounds. With the addition of chemicals to dewater the sludge, hydrogen sulphide and ammonia may be released. Conversion of sewage sludge into granules or pellets, by removing the moisture, is the final stage of the sewage treatment process.
The amount of dust produced in the drying process and later processing is affected by the method of drying and type of final product. Sewage sludge dust is about the same size and similar hazard as wood dust. Depending on the design of the plant, there is the potential for a dust explosion to occur at the main dryer, dust collector and handling plant, pelletizer and final product discharge plant.
In pellet form the product is sufficiently hard to withstand the normal conditions of mixing, handling and transportation without producing excessive levels of dust. These pellets have a relatively low auto-ignition temperature, as low as 2650C, and may be easily ignited without process precautions. A risk assessment followed by implementation of suitable prevention and protection measures is required for all parts of the process.
Special attention should be given to the specific hazards associated with the generation of methane, hydrogen suphide, and dusts. Appropriate ventilation, relief venting, suppression systems, containment features, avoidance of ignition sources, and safe handling and storage practices also need to be considered. Storage Once dried, pellets may self-heat to the point of ignition and slow burn.
To minimize the potential for self-heating, sewage sludge pellets should be kept cool and dry and should not be stored in large piles. Storage silos should be designed to aid cooling and be sized to allow thermal dissipation of heat. For this reason, tall narrow silos are preferable to wide silos. Where significant levels of dust are likely to be produced in the storage silos, they should be designed to mitigate the effect of any explosion. The simplest protection is the provision of explosion relief panels venting to a safe location. Silos should be designed to identify and contain a fire. A slow burning silo fire is likely to be starved of oxygen and therefore produce carbon monoxide. A carbon monoxide detector in the silo will indicate an incipient fire.
As well, multi-point temperature probes may be installed to monitor the temperature of the product. As an alternative to indoor storage, pellets should be transported to a site location and be off-loaded and turned into the soil as soon as possible. If this is not possible, the material should be spread on the ground evenly in the form of a very thin layer. This configuration will dissipate any heat generated into the ground and atmosphere.
Fire suppression
Inside a silo, an inert gas can be used to contain, but not necessarily extinguish, a fire. The injection of an inert gas will cause a drop in temperature, but may only have a limited effect. The temperatures should be monitored for several hours before deciding if the fire has been controlled. Procedures to deal with a silo fire may include the gradual emptying of the silo to a safe location. Outside, a sewage sludge pellet fire typically smolders at the surface with a relatively low burning temperature and emits dense white smoke and products of incomplete combustion. The smoke may contain organic acids and other compounds that are irritating agents.
The simplest way to deal with such a fire is to dissipate the heat by spreading out the pellets. It may also be extinguished by confining and smothering. Alternately, the pellets may be mixed into the soil or stamped with heavy earth moving equipment. In some cases, the use of Class A foam may be considered for fire suppression. Class A foam is a special formulation of hydrocarbon surfactants, that reduces the surface tension of water and provides for better water penetration and increased effectiveness. Class A foam acts as a surface barrier to stop or prevent further combustion. The use of water to suppress this type of fire is controversial.
The application of water may actually support a fire by contributing to the process of aerobic decomposition. Further, adding water may return the dried sewage into liquid sewage and create additional leachate and runoff. In turn, this may contaminate ground and surface waters surrounding the site and could cause significant environmental and health risks. Overall, fire fighting tactics need to consider a range of circumstances, not the least of which include the size of fire, location, wind/weather conditions, water supply, personnel safety, access to heavy equipment, and environmental impact.
Conclusions
Although fires involving sludge pellets are not common occurrences, they do tend to attract a great deal of public attention and challenge the fire service. By working with the public, pellet factory owners, and owners of sites used to spread sewage sludge pellets, the fire service can ensure that safe practices are employed, thereby protecting the public, environment and emergency responders.
Article prepared by OFM Fire Protection Engineers Beth Tate and M. Mailvaganam.
posted by Sludge at 7:04 PM
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