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T ransforming sludge into a Class A biosolids productper 40 CFR 503 standards leads to a dramatic increasein safety relative to sludge handling and dispos-al because it dramatically reduces disease-bear-ing pathogens, vector attraction, and when drying is partof the process, the volume of material that must be trans-ported offsite. However, if appropriate electrical, mechan-ical, and process safeguards are not used when dryingbiosolids, the inadvertent potential safety hazards couldoutweigh the benefits of producing a Class A product.

All processes for transforming sludge into Class Abiosolids heat-drying, thermal dehydration, pasteur-ization, and thermophilic digestion involve the use ofheat and so require special attention to safety to avoid fires,explosions, and other adverse events. So, although this arti-

cle focuses on heat-drying systems, it actually relates to allClass A processes.

Heat-drying can be divided into two broad categories:convection (direct heat application) and conduction (indi-rect heat application). Some literature suggests that indi-rect dryers are inherently safer than direct dryers, butboth types can create operating hazards if certain common-sense guidelines or standards are not followed. The guide-lines presented in this article include both preventionmeasures and safety response systems.

Drying Systems

During the last several years, drying systems haveevolved to incorporate more mechanical and process safe-guards to make them inherently safer. Such changes include

Key safety considerations when drying biosolids to Class A specificationsRay Barrett and Joey Herndon

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installation of appropriate inert purge systems, securitysystems upgrades, implementation of proper mainte-nance practices, and stricter manufacturing of heaters,boilers, and gas burners and trains.

Inert purge systems reduce combustion potential.Explosion prevention is important when designing dryingsystems. It has been demonstrated that using inert gas topurge available oxygen is a most effective option, regard-less of whether the systems thermal heat source is director indirect.

Inert purge systems lower the available oxygen belowthe limiting oxygen concentration required for combus-tion to occur. The guidelines for inert-gas systems forcombustion prevention are provided in the National FireProtection Association (NFPA; Quincy, Mass.) Standard onExplosion Prevention Systems (NFPA 69).

With indirect drying, an inert purge can be created withsteam, but other inert gases, such as nitrogen, also can beused. Many direct dryers use recycled combustion gasesfor the inert purge, along with supplemental nitrogen gas.(The combination of combustion gas and nitrogen gas

requires more safety design considerations to avoidexposing personnel to inert gases.)

Maintaining a minimal oxygen level in the dehydrationchamber is essential during active biosolids drying. But itis even more critical during startup and shutdown of thebiosolids dryer when wet biosolids are not present because, historically, this is when most adverse dryerevents have occurred. With indirect dryers, an inert-gaspurge during startup and shutdown can be easily accom-plished by spraying a calculated volume of water onto a sur-face heated above 100C (212F) in the dehydration cham-ber. This will produce an inert purge of steam. The waterrequirements are detailed in NFPA 69, Appendix A.

During normal operation of indirect dryers, more inertgases are not required to create an inert blanket. However,direct dryer designs typically require the addition of inertgases during normal operation.

Eliminate ignition sources to prevent fires. As withmost dry substances, dried biosolids can ignite under cer-tain conditions. Thats why dryer systems should bedesigned to minimize the possibility of introducing an igni-tion source due to maintenance activities or foreign objectsmoving through the dryer, which can cause impact sparks.This can be accomplished by using appropriate detectiondevices and implementing prevention strategies.

For example, after dried biosolids leave the dryer, thematerial can be passed over a magnet to remove anymetal present before the biosolids are conveyed to storage.Also, all drying equipment should be bonded and ground-ed to avoid static electricity buildup and discharge. Likewise,all field electrical devices and enclosures should have aminimum rating by the National Electrical ManufacturersAssociation (NEMA; Rosslyn, Va.) of NEMA 4.

Proper gas trains and burners ensure safety. Becausehighly combustible fuels must be handled properly, theonly burner gas trains that should be considered for usein drying systems are those that meet Industrial RiskInsurers (Hartford, Conn.) and NFPA specifications, as

For More InformationHaug, Roger T, et al., Explosion Protection and FirePrevention at a Biosolids Drying Facility, ProceedingsWEF.

Pascucci, J. and Koch, C.M., Fire and ExplosionPrevention and Protection and the Design of DriedSludge Storage Vessels, Proceedings WEF, 1992.

Packer Engineering, Safety Audit, Report of Findings,2004.

USFilter, Dragon Dryer, Company Brochure, 2005.

National Fire Protection Association (NFPA; Quincy,Mass.) 68, 69, 654, 820

Rupture disks upstream and downstream of thedrying chamber on this Tallahassee, Fla., dryingsystem help protect it from explosive pressures.

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well as all applicable local codes and regulations. Also, allgas train components should meet at least a NEMA 4 rat-ing. If burner management controls are not housed in thedryers main control panel, they should be in a NEMA 4Xstainless steel enclosure certified by UnderwritersLaboratories Inc. (UL; Northbrook, Ill.)

All burners used in dryers should be manufactured bya reputable, nationally recognized firm and meet or exceedthe requirements of Factory Mutual (Johnston, R.I.). Inaddition, burner controls should have a sufficient turndownto operate safely at low feed conditions. In all cases, how-ever, a facility gas emergency shutoff switch must beinstalled outside the dryer building.

Oil heaters and steam boilers. Oil heaters and steamboilers are commonly used as a heat source for indirect dry-ing systems. Because of the stresses to which they are typ-ically exposed, heaters and boilers must be built to the high-est safety standards. Thats why the American Society ofMechanical Engineers (ASME; New York) code, common-ly known as the pressure vessel code, has become thestandard for these critical systems.

To ensure continuous oversight of heaters and boilers,they should be operated and monitored by the dryer systemsprogrammable logic controller and linked to its safety net-work. Likewise, ASME-approved pressure-relief valves shouldbe used on the systems feed and return lines, and thesevalves must be hard-piped to a safe discharge area to elimi-nate danger to personnel if over-pressurization occurs.

For the safety of personnel, the heater or boiler shouldbe isolated in a separate room away from other drying

equipment. This room should comply with NFPA guidelinesand local fire codes.

The operating pressures for thermal transfer fluids (hotoil) or steam must be kept to a minimum to reduce theseverity of a leak if any piping, joints, or valves are faulty orbecome damaged. Typically, thermal oil systems operate atsignificantly lower pressures than steam-based systems.Some dryer systems that use hot oil recommend using a heatexchanger to cool the oil during shutdown in order to pre-vent overheating the dried product.

Level sensors are necessary. Another important factorfor all heat dryers is a system that ensures the dryer doesnot unexpectedly run out of biosolids while operating.The biosolids provide a heat sink to absorb applied heat,and operating without sufficient biosolids could cause thedryer to overheat and combust dried biosolids.

To avoid this, the control system should include instru-mentation and simple interlocks designed to respondappropriately if the biosolids feed runs out. Typically, thiswould involve mechanical flow switches or a meter of feedhopper level.

Control systems must be safe and secure. Control systemstypically record such critical information as temperaturedata from all sensing points, level indication systems, and allsystem commands and alarms, as well as their date andtime. So, a secure control system is critical to keeping theprocess functioning appropriately and providing the infor-mation needed to identify potential operating hazards.

Several control system practices can ensure better per-sonnel safety and security. One particularly important

2005 Dryer Safety Standards Check List An inert-gas purge system per the National Fire Protection Association (NFPA; Quincy, Mass.) Standard on

Explosion Prevention Systems (NFPA 69). Level sensors and flow sensors located on the in-feed hopper and communicating with the dryers control

system to confirm sludge feed to the dryer. An independent spark-detection system designed to communicate with the dryers programmable logic

controller in place at the dryers out-feed. Careful consideration of the conveyance system used to transport the dried product. Control hardware designed and built to meet the requirements of Underwriters Laboratories Inc. (Northbrook,

Ill.) and, at a minimum, a National Electrical Manufacturers Association (NEMA; Rosslyn, Va.) NEMA 4 rating. Redundant safety reporting systems and instrumentation. A humanmachine interface protected by multilevel pass-code protocols. Historic trending information recorded and pass-code protected in the control system. Building(s) for drying systems that meet all codes and requirements of the local jurisdiction and NFPA 820,

as appropriate. Hot-oil systems and steam boilers isolated behind fire resistance walls. Hot-oil systems designed, built, and code-stamped per American Society of Mechanical Engineers (ASME; New

York) code. (This applies to the entire system, not simply the hot-oil heater.) Storage systems (confined) equipped with rupture disks or panels and designed per NFPA codes. Storage systems (confined) equipped with temperature and carbon monoxide sensors. Rupture disks incorporated into dryer design per guidelines detailed in the NFPA codes. Safety systems for heat dryers designed around specific maximum deflagration pressure (Pmax) and defla-

gration index (Kst) values.

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practice is the use of enclosures and components thatmeet industry standards, such as UL and NEMA 4 ratings,to ensure that the electronic equipment in control systemsis the highest quality.

Another operational safety net is to control systemredundancy to ensure that the supply of essential infor-mation is not compromised. Redundant systems are sug-gested for alarms, motor controls, temperature-sensingdevices, pressure-sensing devices, and all other safety-crit-ical components.

The humanmachine interface (usually in the form ofcommunicating with software via touch screens) can beimportant to both safety and security, so all relevantequipment should be pass-code protected to preventinappropriate or unauthorized access. For example, accessto trending records should be limited, as should accessto safety-critical settings and alarms to the manufactur-er. Also, for internal security, operator login and logoutprocedures should be well defined and protected fromchange, except by specified passcode-enabled supervisors.Having a protected history of changes (who, what, andwhen) is also important for long-term safety appraisals.

Safety Response Systems

Sometimes overpressurization events occur even whenall prevention techniques have been used, so its impor-tant to incorporate pressure-relief management into dryersystem design to manage such an event safely and directit away from personnel. Typically, rupture disks or pan-els are used for this purpose. These disks and panelsshould be properly sized and placed where they will bestlower the pressure from the drying chamber.

To size relief vents properly, designers must know thedeflagration characteristics of the specific biosolids beingdried. The two most significant dust properties for sizingvents are maximum deflagration pressure (Pmax) and thedeflagration index (Kst). These characteristics are typicallyprovided by the consulting engineer or municipal officialsto the drying system manufacturer. Pmax and Kst are often

derived from averages or projected biosolidscontent for new treatment facilities or fromanalysis of sludge at existing facilities. NFPA68, Guide for Venting of Deflagrations, providesguidelines for applying dust characteristics andmechanical geometry to properly size defla-gration relief systems.

Vent size also is affected by the confinedvolume of the dehydration chamber. Directdryers typically have a larger dehydration cham-ber than indirect dryers, so they typicallyrequire larger relief vents to protect the vesselfrom overpressurization.

Dryer systems also should include fire-sup-pression technologies, such as water or inert-gas purge systems.

Appropriate Dryer Housing

Not only must the dryer system be engineered and builtfor safety, but so must the building in which it is housed.At a minimum, this building should meet all the codes andrequirements of the authority with jurisdiction (typical-ly the local fire authority), as well as NFPA 820 guide-lines, as appropriate.

It is also important that dryer equipment be arranged inthe building with personnel access in mind. Walkwaysaround the equipment must meet U.S. Occupational Safetyand Health Administration (OSHA) standards and, for emer-gency purposes, at least two exits from the dryer area and

As The Saying Goes: Safety Is No AccidentDrying sludge to Class A requirements is a major contribution to the

long-term safety of our communities. However, it is important that thecommon-sense, day-to-day, safety considerations associated with heat-drying sludge be emphasized in the design, construction, installation,and operation of drying systems regardless of whether they arebased on direct or indirect drying technologies. It is also important thatthe storage facilities associated with these drying technologies meet thesame rigorous requirements for safety.

Heat-drying is an excellent solution to the growing problem ofsludge disposal, but a dryer system cannot be purchased at theexpense of safety and more importantly, it doesnt have to be. Safe,reliable, and economical drying systems exist and are currently inoperation, so there is no need to settle for anything less. Safety mustbe the first priority of any drying-system design.

This Ocala, Fla., drying system has beenequipped with the industrys most advancedsafety systems.

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building are recommended. Also, the dryers ancillary equip-ment, including platforms, catwalks, ladders, handrails, andstairways, must conform to OSHA, International BuildingCode, and other applicable codes and standards.

Hot-oil systems, steam boilers, and similar devicesshould be housed behind fire-safe walls and otherwiseisolated from both the drying system and personnel inthe area, as per local codes and NFPA guidelines.

Dry Product Conveyance and Storage

Storage. The possibility of a fire or explosion in a stor-age silo is actually greater than in a properly engineereddryer system. An independent spark-detection system canhelp prevent combusted biosolids from entering the dried-product conveyance and storage system. This detectionsystem should operate independently of all other moni-toring systems and be able to extinguish and divert anydetected embers from the conveyance or storage facility.(Typically, the spark detection system is best locatedbefore the dry product conveying system.)

At a minimum, silos should have rupture panels forpressure relief, as detailed in NFPA 68 guidelines. They alsoshould have redundant temperature-sensing devices andcarbon monoxide sensors. The safety sensors should bepart of an automated control system to announce dan-gerous conditions and react, as appropriate, to prevent asilo explosion. All silos and silo equipment also should bebonded and grounded.

The only reliable way to extinguish a fire in a silo is byusing a water deluge system that completely floods the silo.But this means the silo must be designed to be flooded,which typically means the silo must be built at groundlevel, rather than elevated on legs, and use a mechanicaldischarge device, rather than a gravity unloading system(see figure, above).

An inert-gas system can be used to extinguish silo fires,but the oxygen level will rise immediately on discharge to theenvironment, so combustion could recur. If inert gas is usedto extinguish silo fires, a water spray or carbon dioxide sys-tem will be needed at the truck loading station. Also, all

trucks or containers loaded with dried biosolids shouldhave a removable grounding and bonding system connect-ed to the silo grounding and bonding system to prevent sta-tic electricity from becoming an ignition source.

Conveyance. Once the Class A product is safely dis-charged, its transportation to storage must be designed to min-imize dust production. When logistically possible, non-static-producing tube, drag-type conveyor systems are preferred.

Using pneumatic transport systems (dense- and dilute-phase) is a safety concern because they introduce more airinto the storage silo. They also require a fugitive dust sys-tem (FDS), which can create more safety hazards due todust management.

Fugitive dust system. An FDS may be required for the siloand dry-product conveyor system to remove dust from thestorage silo, loading spouts, and any dry-product screw con-veyors. It generally consists of a fabric filter system (bag-house), a high-speed, spark-free fan, and associated pipingand controls. This equipment typically is designed for out-

door operation and supported by a steel structure thatelevates the FDS so a dumpster can be placed under itto collect the dust.

NFPA recommends that the FDS be designed tomaintain a combustible concentration of less than60% of the lower flammable limit. The FDS baghousealso must have rupture panels for pressure relief anda suppression system that meets NFPA 68.

Ray Barrett is an engineer of residual management sys-tems for USFilter Davco (Thomasville, Ga.) and has morethe 24 years experience in composting, drying systems, andresiduals handling systems. Joey Herndon is the DragonDryer product manager at USFilter (Warrendale, Pa.) andhas more than 30 years experience in the wastewatertreatment and metal fabrication industry.

Explosion PotentialAny material that will burn in air when it is in solid form may

explode when in the form of a finely divided powder. Biosolidsare composed primarily of carbohydrates, proteins, and fats, andwill burn readily in solid form. In order for an explosion to occur,there must be dust present in a sufficient explosive concentra-tion, oxygen in a sufficient concentration, and a source of igni-tion. Also, for a dust explosion to occur, particles must be suffi-ciently close together so the heat released from one particle willheat the surrounding particles. According to published studies,an explosive concentration of dust (320 g/m3) in the presence ofoxygen at a concentration greater than 15% will explode whenexposed to a spark at a temperature exceeding 355C (671F).

Using a water deluge system willrequire the silo to be designedfor a flooded condition.

Reprinted with permission from Water Environment & Technology, April 2005, by The Reprint Dept., 800 259-0470, 9574-1204; #9830-0705