As European Regulations (Landfill Directive and Council Decision 2003/33/EC) and its transposition into UK law prohibit the disposal of organic matter in landfills, and agricultural applications require high-quality solids, heat drying remains an attractive treatment step. It transforms biosolids into an aesthetically pleasing and disinfected product with multiple use opportunities.
Also, a significant reduction in both volume and weight of biosolids for off-site hauling is achieved. As the number of heat drying installations increases, so too do reports of safety incidents.
Dried biosolids are a combustible material, which will burn if present with air and an ignition source. As a result, safety measures commonly applied to combustible material should also be applied to the design and operation of dryers. Heat drying of biosolids can also 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 biomass, 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 smouldering fire and, if left unattended, an uncontrolled fire. This fire in turn can provide an ignition source for explosion of nearby combustible dust.
Some influent constituents end up in the solids processed for ultimate disposal, such as excessive fiber or grease, which create problems when the solids are heat dried.
High levels of fiber in the feed can produce build up of fiber within air filters, screens and product coolers. This results in blockages, and can also result in poor granule formation, leading to high dust production.
Large amounts of grease can volatise during the drying process, producing combustible vapours. Grease in excessive amounts can also deposit on the inside of equipment and create blockages and a source of combustible material.
Design of safe drying systems should include provisions for adequate fire and explosion protection systems, as detailed in industry standards and system supplier experience. In the UK, a specific document provides background information on the biosolids drying process and guidance for risk assessments: British Health and Safety Executive HSE 847/9, Control of Health and Safety Risks at Sewage Sludge Drying Plants. This standard provides much information relating to specific risks and design considerations associated with biosolids drying facilities.
In addition to adhering to applicable standards, it is important to select a drying system supplier that has adequate experience with drying municipal biosolids, and provides sufficient relevant protection systems, incorporated into equipment in response to a particular problem that previously occurred.
Prevention is the first specific means of protection for drying systems. Preventive measures include:
- The drying area is typically inerted with evaporative steam during operation to ensure that the oxygen level stays below the lower explosive limit – the level of oxygen concentration that would sustain a dust deflagration. Various means can be used to inert the drying area during start-up and shutdown: steam created by water injection sprays; exhaust gases created by the combustion process in direct fired dryers; or nitrogen gas supplied from a purge system
- Most drying systems have deluge water systems to drench material in the drying area if monitoring indicates that incipient fire conditions, such as elevated temperature, are present
- All drying systems have some means of temperature control for the drying area, to ensure that temperatures do not rise to unsafe levels and material does not become too dry – which would create excessive dust, so that preventive systems are activated in case of elevated temperatures
- Product storage and handling areas are typically monitored with respect to both temperature and carbon monoxide (combustion by-product gas) to raise the alarm if conditions become unsafe and activate inerting systems. These areas are typically blanketed with inert nitrogen gas, either on a continuous basis, or in response to indications of incipient fire conditions
- Most recently manufactured drying systems cool the final product before storage to reduce the potential for auto-oxidation of dried material with lower temperatures throughout the dried material handling systems
- Ventilation of dried material handling equipment is often provided to keep the dust concentration below the lower explosive limit and to remove moisture that could lead to build up of material on the inside of equipment
- Some drying systems are provided with spark detection devices, which can detect glowing or burning material at key transition areas such as the discharge of the dryer or gas/product separator, and activate deluge systems as required
Mitigation for biosolids drying includes some of the more important systems:
- Explosion venting is provided for the product storage areas to minimise hazards from a deflagration, and for higher risk handling equipment, such as bucket elevators
- Isolation is provided between system components to ensure that a deflagration will not propagate to other areas, such as recycle bins or storage silos, or to areas of high dust concentrations, such as baghouses
Adequate instrumentation is important in providing operators with sufficient information regarding the state of dryer operation and alerting them to upset conditions. Some of the more important components of the instrumentation systems include:
- Temperature monitoring for controlling the drying process and ensuring that safe conditions and operation within acceptable limits are maintained. These cover: product or process gas discharge; cooler discharge; storage areas to monitor for reheating of product; furnace or heat exchanger outlets; water into and out of the cooler to monitor heat exchanger performance; process gas to monitor drying conditions; and scrubber water discharge to monitor performance
- Gas monitoring to ensure oxygen levels in drying areas are below levels that can sustain a deflagration, and for detecting elevated carbon monoxide levels which, as a by-product of biosolids combustion, can provide indication of a fire
- Pressure monitoring throughout gas systems can indicate blockages in equipment, including ductwork and mist eliminators, and in water supplies ensures proper operation of equipment or indicates partial blockages in water sprays
Drying systems should be designed to provide flexibility in handling material under non-standard conditions, such as the removal of dried product from the dryer or recycle bin, to address off-spec product or empty storage systems for prolonged shutdown periods. For drying systems operated with centrifuges, excessively wet material should be diverted from the drying system, as sloppy material can lead to clumping of material, build-up of material inside equipment, and moisture to initiate auto-oxidation of material.
Not all unique hazards identified with biosolids drying – combustible dust accumulation, auto-oxidation of material, and poor feed material – can be prevented by the design of protection systems, underscoring the importance of proper operating practices to avoid unsafe conditions.
Safe Drying Operation entails maintaining steady-state conditions within optimum limits, for feed moisture content, mixed wet feed, recycle (if provided), and the dried material. When a drying system includes back-mixing and product recycle, operation within optimum moisture limits produces a cohesive granulate. Almost all of the material is encapsulated in the pellet, and build up of material–whether as dust or sticky cake – inside the equipment is minimised. The final product is dry enough to retard auto-oxidation, and cohesive enough to prevent the granules from breaking apart and producing dust.
Certain periods and situations of dryer operation pose higher than normal risks, including start-up and shutdown and opening of baghouses or other equipment when smouldering conditions are present.
Start-up of dryers can be especially risky if the unit was shut down with too-wet material left in it, as it can smoulder during shutdown, producing small fires. And restart of the unit with its fans can introduce large volumes of oxygen, which can increase the intensity of the combustion.
The start-up of fans can also suspend accumulated dust to provide an explosive mixture for which the smouldering material can provide an ignition source. In addition, the start-up of material handling equipment can cause smouldering material to be transferred to other areas, including areas that may have accumulated dust, and spread the areas of fire. Clean out of baghouses to address smouldering material should be addressed carefully, as opening the unit to atmosphere will provide oxygen for the fire, which may have been significantly retarded by an oxygen-reduced atmosphere.
Optimisation procedures can be effective for determining acceptable moisture levels and monitoring to ensure acceptable levels are maintained, and are especially useful during the initial operating period. That is when operators have little experience and have not yet developed an intuitive understanding of what feed and final product material characteristics and dryer operating parameters will avoid unsafe conditions.
The optimisation process should include a systematic set of procedures to monitor moisture concentration of the feed solids and at several internal dryer locations.
Statistical analysis should be performed to correlate the moisture concentrations data with process malfunctions and then establish acceptable operating ranges for each of the locations monitored.
Finally, moisture content should be routinely monitored to quickly identify when material moisture content is approaching limits which increase the risk of dryer malfunction and operating procedures should be established to direct material moisture content back to within acceptable levels.