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This article reviews the basic process to help select an appropriate drying technology. Factors such as dryer operation, type of sludge, operation schedule, air emissions, fuel requirements, end-product quality and storage must all be considered.
Producing U.S. EPA 40 CFR, Part 503 Class A biosolids is the greatest benefit of heat-dried biosolids, allowing the dried material to be used for beneficial reuse. Heat drying also cuts sludge volume by a ratio of 4:1 and produces Class A biosolids with more than 90% dry solids content. By producing a Class A product, the amount of record keeping for state documentation is reduced or totally eliminated. But how do you select, with confidence, the dryer that will work best for your wastewater treatment plant (WWTP)?
Two Distinct Categories
Heat dryers typically fall into two distinct categories, convection and conduction. Convection dryers are commonly referred to as direct dryers and can include a variety of mechanical designs such as triple-pass drum dryers, single-pass drum dryers, belt dryers and fluidized bed systems. Some of the typical characteristics of a direct dryer are: its heat source comes into direct contact with the sludge being dried; it is equipment-intensive; the dried product must be back-mixed with the wet cake feed; a “pelletized” end-product is produced; large volumes of dryer air emissions must be handled and treated; and its operation requires increased levels of maintenance.
Conduction dryers are commonly referred to as indirect dryers and can include mechanical designs such as the rotating chamber, paddle-mixer style and batch dryers. Some of the typical characteristics of the indirect dryers are: its heat source does not come into contact with the sludge; it gives off low air emissions; less equipment is required; it involves one-step operation with optional back-mixing; and a “granular” end-product is produced.
Direct dryer. The required equipment and flow pattern of a typical direct dryer is shown in Figure 1 (see page 18). A mixture of wet sludge cake from the dewatering device (14% to 25% dry solids) and recycled dried product from the dryer (90% to 100% dry solids) are blended together to form a cake solid between 60% and 80% dry solids. The solids are then conveyed to the dryer’s inlet, where the drying process begins with a heated airflow (10,000 to 15,000 standard cu ft per minute [cfm]) coming in contact with the sludge. The airflow carries the product from the dryer into the separator/baghouse system, where the air and dried product are separated. The product is then conveyed to the screening system, which grades it into various sizes. The desired size product is conveyed to the storage area, and the rejected, dried product is reduced to a uniform size and conveyed to recycle storage.
The air from the separator/baghouse system is piped to one or more pieces of equipment that may include a heat exchanger for beneficial reuse of the waste-heat. It should also always include an odor-control device such as a thermal oxidizer.
Indirect dryer. The flow schematic in Figure 2 shows the required equipment and flow pattern of a typical indirect dryer. While the mechanics of the various indirect dryers may differ, the basic technology is the same. Most municipal indirect dryers use a synthetic heating fluid (hot oil) as the heat-transfer medium, but steam is also an option.
The sludge cake from the dewatering device (14% to 30% dry solids) is collected in the dryer’s feed hopper, from which it is metered into the dryer inlet port. The hot oil is heated in a heat exchanger to the preset temperature (typically 350°F to 500°F), pumped through the hollow flights/paddles of the internal mechanism and returned to the heat exchanger for reheating.
As Figure 2 shows, three individually controlled burners heat the drying chamber. However, in most indirect dryers, the hot oil is circulated around the drying chamber by the same pump systems that pump it through the internal mechanism. The sludge is dewatered inside the drying chamber by indirectly heating the sludge and releasing water as steam. The dried product is discharged from the dryer and is conveyed to the dried product storage. The steam from the drying chamber is drawn through a condenser that uses plant water to condense the steam back to water. The water from the condenser is returned to the plant’s headworks, and the vapors (200 to 1,500 cfm) from the condenser are carried to some type of odor-control device.
It should be noted that most all-combustible fuels could be used for both the direct and indirect dryers. Natural gas is the most commonly used and is one of the few fuels that can be piped to the dryer site. It is also possible to use two different types of fuel, such as natural gas and digester gas, for a dryer system.
Areas of Concern
Several areas of concern should be addressed when selecting a heat drying system. Just as WWTPs are different, the mechanics of each drying technology, direct and indirect, are also different. One important common area is the basic operational safety design of the dryer and its components. Regardless of the type of dryer being considered, at a minimum, the hot oil system, dehydration chamber, product discharge conveyors and end-product storage should be designed using established standards such as those of the National Fire Protection Association and the American Society of Mechanical Engineers.
The evaluation of a drying system should include, but not be limited to, the following areas:
The type of sludge to be dried. Various types of sludge have different handling characteristics and dry differently. Whether drying only one type of sludge or several different types mixed together, the consistency of the sludge being fed to the dryer is one of the most important aspects of the drying operation.
To ensure a smooth-running dryer that requires limited operator interface, it is very important to have a “managed” sludge feed to the dryer. Managed sludge means that the complete process is monitored and controlled—from the holding tank, dewatering device, dryer feed hopper and dried product—to ensure a consistent sludge. The municipality must understand that prior to installing a dryer system, managed sludge was not necessary because the sludge cake was sent to a landfill or for direct land application. For this reason, the cake’s consistency was not as important.
Operational schedule of the WWTP. The operation schedule of the WWTP must be considered when selecting a dryer. All dryers operate more efficiently when they run continuously. This means that a 24-hour operating schedule should be seriously considered because heating up a dryer and shutting it down each day wastes fuel and causes additional wear on the dryer’s components.
In the second half of this article, to be published in the March issue of Water & Wastes Digest, we will discuss: controlling abrasion and corrosion; avoiding combustion during operation, controlling air emissions; fuel requirements; building requirements; end-product quality and use; and end-product storage.