Design Considerations

The most important aspects of the design of a thermal drying system are discussed in the following sections.

Moisture Content of Feed Sludge Thermal evaporation of water from sludge requires considerable energy. The amount of fuel required to dry sludge depends on the amount of water evaporated. As mechanical dewatering methods are more efficient than thermal drying methods per volume of water removed, it is imperative that a dewatering step precede thermal drying so that overall energy requirements can be minimized.

Often, drying requires mixing of feed sludge with a portion of the dried sludge to achieve a solids content exceeding the plastic stage to prevent agglomeration and the fouling of internal dryer surfaces. Paddle mixers, pug mills, and hammer mills are devices for the thorough mixing of sludge cake and recycled product.

Storage Storage requirements for both the dewatered sludge feed and the dried product should be considered in the design of a drying system. Sufficient dewatered sludge storage should be provided to allow orderly shutdown of the drying system and to attenuate variations in production. A storage capacity of a minimum of three days of dewatered sludge production is recommended. Storage for the dried product depends on the final disposal arrangement. Sales of the product are likely to be seasonal, and storage for 90 days may be necessary unless bulk buyers provide off-site storage. If the dried product is burned as a fuel or it undergoes further processing, storage requirements are dictated by subsequent steps in the sludge processing system.

Dust can become a problem if the dried product is stored in bulk and is not pelletized. Stored product with a moisture content below 10% may combust spontaneously, especially when it contains large quantities of dust. To minimize dust generation, long conveyors that create an abrasive action, such as screw conveyors and drag conveyors, should be avoided. Even the use of pneumatic conveyors may be too abrasive. Open or folded belt conveyors are the preferred choices. Dried product with moisture content below 20% may absorb water from the environment.

Fire and Explosion Hazards Drying systems exposed to heavy dusting have had problems with fires. The combination of combustible particles, warm temperatures, sufficient oxygen, and high gas velocities make these systems susceptible to fire and explosion. Any material that will burn in air when it is in solid form may explode when in the form of finely divided powder. Biosolids are composed primarily of carbohydrates, proteins, and fats and will burn readily in solid form. For an explosion to occur, dust must be present in sufficient explosive concentration, and there must be a source of ignition. In addition, for an explosion to occur, particles must be sufficiently close together so that the heat released from one particle will heat the surrounding particles. An explosive concentration of dust (320 g/m3) in the presence of oxygen at a concentration greater than 15% will explode when exposed to a spark at a temperature exceeding 355°C (671°F) (Barrett and Herndon, 2005). Cyclone separators, wet scrubbers, baghouses, or a combination of these can be provided to remove dust and fine particles from the process airstream.

Maintaining a minimum oxygen level in the dehydration chamber is essential during active sludge drying. It is even more critical during startup and shutdown of the dryer (when wet sludge is not present) because, historically, this is when most adverse dryer events have occurred. It has been demonstrated that using an inert gas such as nitrogen to purge available oxygen is the most effective option. Well-trained operators, controls with interlocks, and monitoring equipment are important considerations in reducing fire hazards.

When designing a sludge drying system, the following dryer safety standard checklist should be followed to avoid fire and explosion hazards (adapted from Barrett and Herndon, 2005). The items of importance are:

• An inert-gas purge system following the National Fire Protection Association (NFPA) Standard on Explosion Prevention Systems (NFPA 69)

• Level sensors and flow sensors located on the in-feed hopper and communicating with the dryer's control system to confirm sludge feed to the dryer

• An independent spark-detection system in place at the dryer's out-feed that communicates with the dryer's control system

• Careful consideration of the conveyance system used to transport dried product

• Control hardware designed and built to meet the requirements of Underwriters' Laboratories and, at a minimum, the National Electrical Manufacturers Association (NEMA) NEMA 4 rating

• Redundant safety reporting systems and instrumentation

• A human-machine interface protected by a multilevel pass-code protocol

• 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-resistant walls

• Hot oil systems designed, built, and code-stamped according to the American Society of Mechanical Engineers (ASME) code (this applies to the entire system, not simply the hot oil heater)

• Storage systems (confined) equipped with temperature and carbon monoxide sensors

• Rupture disks incorporated into the dryer design following the guidelines in the NFPA codes

• Safety systems for heat dryers designed around specific maximum deflagration pressure (Pmax) and deflagration index (Kst) values

Emissions and Odor Control Dryer equipment and handling and storage areas should be contained and vented to air pollution control equipment. Cyclone separators, wet scrubbers, baghouses, or a combination of these remove particulates from airstreams. Afterburners (incinerators) and chemical scrubbers typically control odors. Thermal oxidizers have proved to be the most effective in removing aldehydes and various species of sulfides and disulfides (methyl, dimethyl, and carbonyl).

Sidestreams Liquid sidestreams are produced by certain ancillary equipment such as wet scrubbers. Odorous liquid sidestreams produced by condensation of water vapor from dryers contain both organic oils and ammonia. These sidestreams can frequently be recycled to the treatment plant influent but may sometimes require separate treatment.

Heat Source and Heat Recovery The large amounts of energy required for thermal drying of sludge dictate that close attention be given to the source used to heat the drying medium. Natural gas and fuel oil are most frequently used but are becoming more expensive. Dryers should be designed for heat recovery and reuse to reduce energy use. Recovered heat from dryer or furnace exhaust may be reused to preheat combustion air, preheat feed sludge, or supplement plant heating requirements. The dried sludge itself has a fuel value and may be used as a heat source for the drying medium.

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