Water Environment Federation (WEF)

Biosolids Heat Drying: Safety in Design and Owner Operation

Interest continues to grow in heat drying as a means of processing biosolids into an aesthetically pleasing product form with multiple use opportunities, while also achieving a significant reduction in both volume and weight of biosolids for off-site hauling. As the number of heat drying installations increase, so too do reports of safety incidents—an indication that Owners and facility designers must give greater attention to safety in both the design and operation of new facilities. This paper reviews hazards associated with drying and presents means of reducing the risks of safety incidents in design and operation of drying systems.

Dried biosolids are a combustible material which, if present with air and an ignition source, will burn. As a result, safety measures commonly applied to combustible material should also be applied to the design and operation of biosolids drying facilities. In addition to the typical hazards associated with combustible material, heat drying of biosolids can create some unique hazards, including overheating due to auto-oxidation of the dried material, and unstable operating conditions resulting from fluctuations in the moisture content of the feed solids from dewatering.

Combustible dust is produced with material handling of the dried product. Dust accumulation can occur if there is excessive dust production because of solids that are too dry or if there is inadequate removal of dust from equipment. Combustible dust can be an explosive hazard if it is suspended in air in sufficient concentrations when an ignition
source is present.

Dried biosolids contain biological material which can undergo auto-oxidation if it is rewetted from condensation in storage bins or if too much moisture remains in the product after drying. The auto-oxidation process generates heat which, if not dissipated, could result in a smoldering fire and, if left unattended, an uncontrolled fire. In addition to the hazards associated with the fire itself, the fire can provide an ignition source for explosion of nearby combustible dust. Smoldering material can produce carbon monoxide, which is a combustible gas--although opinions are divided as to whether explosive levels would ever be produced in a drying system.

Publicly Owned Treatment Works (POTWs) have only limited control over the constituents contained in the influent wastewater and in the solids processed for ultimate disposal. Excessive fiber or grease in the influent can end up in the solids and create problems when the solids are heat dried. High levels of fiber in the feed can produce buildup of fiber within air filters, screens, and product coolers, resulting in blockages, and can also result in poor granule formation, leading to high dust production. Large amounts of grease can volatize during the drying process, producing combustible vapors. Grease in excessive amounts can also deposit on the inside of equipment and create blockages and a source of combustible material.

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