Biosolids Technology Fact Sheet Heat Drying
DESCRIPTION users for many years. They can be directly ap- plied to agricultural fields, lawns, etc. or mixed Heat drying, in which heat from direct or indirect with other ingredients prior to application. dryers is used to evaporate water from wastewa- ter solids, is one of several methods that can be used to reduce the volume and improve the qual- APPLICABILITY ity of wastewater biosolids. A major advantage of Heat drying is an effective biosolids management heat drying versus other biosolids improvement option for many facilities that desire to reduce methods, however, is that heat drying is ideal for biosolids volume while also producing an end- producing Class A biosolids. product that can be beneficially reused. For ex- Class A biosolids, as defined in 40 CFR Part 503, ample, the Milwaukee Metropolitan Sewage are biosolids that have met “the highest quality” District (MMSD) has been heat-drying wastewa- pathogen reduction requirements confirmed by ter solids and marketing the end-product as a analytical testing and/or the use of a Process to fertilizer since the 1920s (USEPA 1979). The Further Reduce Pathogens (PFRP) as defined in technology has gained popularity since the mid- 40 CFR Part 257. One advantage of Class A bio- 1980s, as many large urban wastewater solids solids is that they are approved for unrestricted generators, especially on the east coast, have use. For example, Class A biosolids that also shifted from ocean disposal to land-based, bene- meet appropriate metals limits and vector attrac- ficial use of biosolids. Most of the new tion reduction requirements can be sold or given wastewater solids processing facilities use direct away for residential use, such as for use on lawns rotary dryers. Table 1 presents a representative and home gardens. They can also be land-applied list of facilities that heat-dry wastewater solids. in public areas without restriction in addition to Table 1. use as an agricultural amendment. The pellets Representative Wastewater Solids formed from the heat-drying process have been Dryers in the United States successfully marketed to a wide range of Type of Type of Biosolids Location Dryer Dried Milwaukee, WI Direct, rotary Blend of raw secondary with digested primary Baltimore, MD Direct, rotary Blend of raw primary (Patapsco) with secondary North Andover, Direct, rotary Anaerobically digested MA Newport, TN Indirect, rotary Anaerobically digested chamber Sacramento, Direct, rotary Anaerobically digested CA Ocean County, Direct, rotary Anaerobically digested NJ Waco, TX Direct, rotary Anaerobically digested New York City, Direct, rotary Anaerobically digested NY Used by permission of CH2M Hill, Inc. Amsterdam, NY Indirect, disc Anaerobically digested Figure 1. Biosolids Dried Product Distribution Sources: Shimp et al. 2000; Pepperman 2005. Center.
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Heat drying is applicable in both urban and sub- ADVANTAGES AND DISADVANTAGES urban settings because it requires a relatively There are both advantages and disadvantages to small amount of land and facility design allows using heat drying to stabilize wastewater solids. process air to be captured for treatment. Markets Several of these advantages and disadvantages for dried products are generally more prevalent in are summarized below. suburban and rural areas than in urban settings. However, because heat drying reduces the vol- Advantages ume of the solids to such a great extent, transport of the end-product from urban areas to rural mar- • Requires a relatively small footprint com- kets is usually economical. Heat drying is also pared with other stabilization processes, such becoming more cost-effective even for small sys- as composting, alkaline stabilization, and air tems (< 20 dry tons/day), particularly with indirect drying/long term storage. drying systems. For example, recent changes in • Can be designed to accept a variety of feed the regulations in Texas over the past several years material characteristics. have made it harder to find areas on which to land- apply Class B biosolids. As urbanization spreads • Greatly reduces the volume of material that outward from larger communities, close-in farms needs to be transported. The typical heat- where Class B biosolids can be land-applied are dried product is at least 90 percent solids, being developed, leaving only the farms farther compared to 15 to 30 percent solids com- out. With the rising costs of fuel, communities monly produced by mechanical dewatering are turning to heat dryers to produce a Class A operations. This feature is particularly impor- biosolids product to facilitate transport and tant for major urban areas, where the end- enhance its value. product might need to be transported for con- siderable distances for use or marketing. The physical characteristics of most wastewater • Reduces traffic into and out of a facility. The solids allow for successful drying. But the facili- number of trucks required to remove material ties most likely to find heat drying feasible include is reduced because of the smaller volume of those that have the following characteristics: the final biosolids product. In addition, no • Produce 10 or more dry tons of solids per day. additives or amendments need to be trans- ported into the facility. • Dewater up to 25 percent solids or greater. • Generates a readily marketable product. • Produce digested solids (heat drying of raw wastewater solids tends to produce a more odorous product, thus reducing its market- Disadvantages ability). • Requires a substantial capital investment. Capital costs often are weighed against the • Produce high-quality solids with respect to long-term financial return that can be realized metals content. by the sale of the heat-dried pellets. • Are located in an area where landfilling, incin- • Requires a large amount of energy. Heat- eration, and land application of Class B drying systems can require 1,400–1,700 Brit- biosolids are expensive or not feasible. ish thermal units per pound of water Although these characteristics might make spe- evaporated. This makes heat drying less en- cific facilities better candidates for heat drying, ergy-efficient per pound of final material than some of these characteristics also affect design other beneficial reuse methods, such as com- decisions for construction of the heat-drying posting and land application. (Sapienza and operations. These factors are discussed in the Bauer 2005). In some cases, this can be at least “Design Criteria” section addressed later. partially offset through the use of on-site en- ergy sources. For example, some facilities use gas from their anaerobic digesters to fuel the heat-drying units. Wood chips have also been
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used as a fuel source to produce the hot gases used to fuel the heat dryers, thereby reducing used in direct dryers. Recycling of these gases plant operating costs. also reduces fuel costs. • Results in an end-product that might have • Generates dust that can affect plant workers properties (such as offensive odor) that affect and neighbors in the local community and its value and marketability. Sapienza and must be controlled to avoid problems during Bauer (2005), however, note that the most storage and transport of the product. The current designs for heat-drying operations in- health effects of the dust are similar to those corporate recirculation of dryer exhaust gas caused by exposure to other sources of dust and the use of regenerative thermal oxidizers and primarily affect lung function. Controls and other techniques to reduce the odor of the are available to address dust concerns. Dust final exhaust gas. Therefore, the authors con- control is further discussed in the “System De- clude that odorous emissions are no longer a sign Considerations” section below. significant problem for heat drying facilities. (See discussion on “End-Product Characteris- • Creates an explosive hazard from dust gener- tics” below). ated in the drying process. (Sieger and Burrowes (2006)) Dryer installations have experienced fires, deflagrations, and explo- DESIGN CRITERIA sions. Much of the recent work in thermal Operators and planners should consider three drying systems has been focused on enhanc- basic questions when selecting or designing a ing their safety. (See discussions of thermal heat-drying system: drying safety in the “Design Criteria” and “Performance” sections below.) 1. What characteristics are desirable in my end- product? • Requires systems that are relatively complex in comparison with other solids-processing 2. How could the heat-drying system be config- systems and need skilled labor for operation ured to achieve my desired end-product, and maintenance. ensure efficient operation, and meet safety standards? • Can produce nuisance odors that could nega- tively affect community acceptance of the 3. What type of dryer is best suited for my process. Sapienza and Bauer (2005) note that specific system? odor was “probably the single most detrimen- tal impact from thermal drying plants.” For The following discussions provide background example, the Morris Forman Wastewater information that should enable treatment plant Treatment Plant in Louisville, Kentucky, operators and planners to answer these questions struggled with odor control in its heat-drying and identify an appropriate heat-drying system process for a decade. However, in 2003 the for their needs. plant completed an upgrade to its solids- handling process that replaced an odor- End-Product Characteristics causing low-pressure oxidation system with a Heat-drying systems are typically designed to pro- system that includes anaerobic digestion and duce Class A biosolids. Although Class B blending of biosolids with secondary solids biosolids can be produced using a heat-drying prior to dewatering and drying. The new system, the lower market value of a Class B prod- design not only significantly reduced odors uct typically does not justify the energy and cost emitted to the atmosphere from the heat- required to run the system. The regulatory re- drying process, but it also reduced the volume quirements for a heat-drying process to be of solid waste produced at the plant and the considered a Process to Significantly Reduce subsequent landfill charges that go along with Pathogens for the production of Class A biosolids solid waste disposal. In addition, methane are discussed later in the “System Design Con- produced in the anaerobic digesters can be siderations” section.
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Although federal regulations allow for Class A be present in the biosolids to warrant the costs biosolids that also meet the metal limits and vec- associated with transporting and applying tor attraction reduction requirements to be them as fertilizer. A reliable sampling pro- distributed to the public for unrestricted use, not gram must be established to determine the all Class A biosolids have the same market value nutrient content, and this information should to consumers. The following list describes sev- be provided to potential users (NBP 2005). eral biosolids end-product characteristics that can • Mechanical durability. It is important to be controlled to improve product marketability. ensure that the product will maintain its form • Odors. It is preferable that the pellets be free through bagging, conveyance, handling, and of offensive odors. Undigested solids tend to storage. Pellets that are not within the stan- create more odorous pellets than those made dard range for mechanical durability may from digested or waste-activated solids crumble during handling; therefore, they may (Dolak et al. 2001). Odors can increase if the not be acceptable even if they have sufficient pellets become wet, which can happen from nutrient content. condensation during cooling or through other • Particle size distribution. Pellets produced mechanisms. The best way to reduce odors in by heat-drying wastewater solids range in size the finished product is to continue to digest from 1 to 4 millimeters and are angular in prior to dewatering and drying (NBP 2005). shape. Screening and sizing abrade the pellets In addition, the end-product must be properly into a more spherical shape. Irregular particle stored to ensure that it is not exposed to mois- sizes can result in larger particles settling ture before use. Exposure to significant faster than smaller ones. Some users (such as moisture presents a potential for anaerobic fertilizer blenders) must ensure that products decomposition (leading to odors). remain well mixed throughout shipment to Undigested biosolids led to odor problems at their customers. End users may associate ir- the Hagerstown, Maryland, pelletizing plant. regular pellet sizes with an inferior product. The plant mixed an undigested primary • Moisture content. Too much moisture in the sludge (typically high in odor) with waste ac- pellets can cause odor problems and might tivated secondary sludge prior to drying the also cause the pellets to smolder. Adequate material. Influent to the plant also contained cooling before the pellets are stored or trans- waste from local dairy processors, which ported will reduce the potential for odor and added a pungent odor to the primary sludge. smoldering, and therefore this step should be When the product was first dried, there was included as part of the facility’s biosolids no odor to the pellets. However, after the pel- process (NBP 2005). lets cooled, they released a strong offensive odor (R. Pepperman, personal communica- • Dust content. Dust from pellets can be prob- tions, 2005). The facility eventually added an lematic for several reasons. First, dust can be odor-masking compound to make the pellets an explosion hazard. Second, dust might more marketable to the agricultural commu- cause human health problems. And third, nity. Further information on the control of some potential end-users may not accept odors in biosolids (related to more than heat dusty pellets; since many potential users of drying) can be obtained from the fact sheet biosolids pellets find excessive dust unac- Odor Control in Biosolids Management ceptable or at least characteristic of an (USEPA 2002). inferior product (NBP 2005). Dust can be generated because the pellets were not suffi- • Nutrient content. One of the main reasons ciently dried and hardened during heat-drying that heat-dried biosolids can be sold and used or because the pellets were not otherwise as fertilizer is their nutrient content. Heat- processed to minimize their potential to cause dried biosolids pellets contain up to 6 percent dust. Sapienza and Bauer (2005) note that, nitrogen, up to 5 percent phosphorus, and a typically, the harder the heat-dried material, trace of potassium. Sufficient nutrients must the less potential there is to generate dust. 4
Repeated handling of some pellets during the dryer air, which keeps the solids from storage and/or transport, however, can result sticking and overdrying at the entrance to the in dust generation, which may be a concern process. The dryer uses the VADEB multi- for fertilizer blenders who must comply with pass system, which keeps the material from air emission requirements. Coating pellets being over-dried in the dryer. Finally, the dried with vegetable oil or paraffin minimizes dust particles are entrained in exhaust air, from production. which they are separated by size. The under- sized particles go back into the process to be System Design Considerations mixed with incoming solids, the oversized par- Once the planners determine the desired charac- ticles are correctly resized in a crusher, and the teristics of the heat-dried biosolids end-product, correctly sized particles go to storage. Most of they must design a system that can produce that the exhaust air is then recycled, while some is end-product. The following items must be ac- vented to the environment through an in-line counted for in the design process. afterburner. • Characteristics of feed solids. The moisture • Storage for feed solids and the finished content of the feed solids partially dictates the product. Control of dust and odor is neces- required dryer capacity and affects decisions sary when storing both feed and dried on appropriate conveyance technologies and biosolids. Feed solids can be stored in day the amount of previously dried material to be bins, which are common in solids-processing mixed with the feed solids. Many experts rec- facilities. However, special considerations ommend that biosolids be digested prior to must be made for storing the dried biosolids. heat drying to minimize odors produced at the High solids content can make the potential for processing facility and in the final product. dust formation high. Nitrogen or some other (See the discussion on odors under “End- inert agent is usually injected into storage si- Product Characteristics” above.) Mixing pre- los to reduce the fire hazard. Care also must viously dried product into the feed solids will be taken to ensure that the dried biosolids are reduce the moisture content of the mixture stable, reducing the potential for odors. (See and help to prevent the solids from sticking in “Odors” discussion above.) the dryer. There are several options • Compliance monitoring. If Class A biosol- for mixing, including pug mills and paddle ids are to be produced, a system to monitor mixers. the heat-drying process must be incorporated • Process dust control. Dust control during the to ensure (1) that the moisture content is 10 actual heat-drying process is important to pro- percent or lower and (2) that the temperature tect worker health and safety, as well as to of the biosolids particles or the wet bulb tem- minimize the potential for fire and explosion. perature of the gas in contact with the (See “Safety Considerations” later in this sec- biosolids exceeds 176 °F (80 °C). In addition, tion.) Dust can be controlled by enclosing the heat-dried biosolids must be tested for fecal drying system and using cyclone separators, coliform bacteria or Salmonella sp. at the last wet scrubbers, or bag houses. Site-specific air point before being used or disposed of modeling is recommended during the concep- (USEPA 1999). tual design of heat-drying facilities to • Location of dewatering and drying sys- determine the potential for dust migration tems. The heat-drying system should be off-site. located near the dewatering system to cut A process patented by Dutch company Gront- down on biosolids handling and transport mij Vandenbroek International has several within the facility. innovations to reduce the potential for dust to • System capacity. The heat-drying system must become an explosive hazard. The process feed be sized to allow for required equipment main- does not enter the dryer at the same location as tenance. If a single system is implemented,
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use of standby grinders, fuel pumps, an air setts Sanitary District Biosolids Drying compressor (if applicable), and dual sludge Facility, a direct rotary kiln dryer that uses pumps should be provided, and this equip- digester gas as a fuel source. The system, ment should be in good working condition. A which came online in 2002 at a cost of reasonable downtime for maintenance and re- $13 million, has a capacity of 38 dry tons of pair based on data from comparable facilities Class A biosolids/day and is estimated to save is typically included in the design. A good the District an estimated $600,000 in opera- rule of thumb is to provide storage or alterna- tions costs annually relative to other drying tive handling for at least 3 days of peak solids options because of the alternative fuel source production. Maximizing storage capacity (NEFCO 2006). A second NEFCO installa- (based on available land area and economics) tion in Palm Beach County, Florida, that can increases program flexibility. Additional stor- accommodate 600 wet tons/day will use age also enables a facility to store its finished 2,000 scfm of landfill gas as its fuel source product if market demand fluctuates or if and will use only natural gas as a backup. weather conditions make transporting pellets Hillsborough County, Florida, uses the biogas off-site more hazardous. generated from a local landfill to operate the dryers. • Adequate space for screening equipment. Depending on the type of dryer and intended • Safety considerations. Because of their high end use of the product, additional processing, organic content, both the heat-drying end- such as sizing, screening, coating, or pelleti- product and the dust generated during produc- zation, might be necessary. Sizing and tion of the end-product are flammable, and screening equipment is used to sort out parti- precautions must be taken to design the heat- cles that do not meet an end user’s drying process, equipment, and storage to specifications or to recycle unacceptable ma- minimize the potential for explosion or fire. terial back to the infeed—directly with small Various design modifications can be made to particles or after further processing (such as minimize the potential for fire or explosion, milling) for large particles. Adequate space including minimizing dust through the use of for this type of equipment should be factored cyclone separators, wet scrubbers, or bag into any construction design. houses; minimizing oxidation potential by using an inert gas; and minimizing combus- • Energy considerations. As discussed above, tion by cooling the end-product and ensuring heat dryers require a large amount of energy, that the end-product is not produced or stored and they are less energy-efficient per pound near heat sources, such as dewatering proc- of final material than other beneficial reuse esses. Sieger and Burrowes (2006) also methods. Innovative designs, however, allow indicate that in addition to inertization, other newer dryers to operate at lower temperatures safety considerations include isolation, explo- than older dryers, and thus they require less sion suppression, explosion relief, and energy. This has allowed some dryers to use venting and extinguishing. Designers should low-energy waste streams as power sources. work with the vendors to ensure that the vari- Moss and Sapienza (2005) indicate that direct ous safety considerations in designing and dryers can use biogas, landfill gas, gas turbine implementing the system are well understood. exhausts, and wood-fired gasifiers as energy sources, while indirect systems can use these sources as well as steam or hot water genera- Types of Dryers tor exhaust, or waste heat from water circuits. The most important feature of a heat-drying sys- For example, MMSD uses waste heat from tem is the dryer. Typically, the rest of the facility turbine generators to power its sludge dryers is designed around this integral piece of equip- (MMSD 2005). New England Fertilizer ment. Dryers can be classified as direct, indirect, Company (NEFCO) designed, built, and is or other. Direct and indirect dryers typically have operating the Greater Lawrence, Massachu- been most successful for drying wastewater solids.
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Direct Dryers. In direct dryers, the wastewater occurs in mixtures with between 40 and 60 per- solids come into contact with hot gases, which cent solids, and it renders the material difficult to cause evaporation of moisture. mix and move inside the dryer. Direct dryers, which include rotary dryers (the Indirect Dryers. In indirect dryers, the solids most common dryers in use today, shown in Fig- remain separated from the heating medium (usu- ure 2), flash dryers, spray dryers, the SWISS ally thermal oil or steam) by metal walls, and the COMBI ecoDry process, and toroidal dryers, are solids never come into direct contact with the most often the technology of choice when the heating medium. Moisture evaporates when the product is intended to be marketed as an agricul- wastewater solids contact the metal surface tural product. heated by the hot medium. The heat transfer sur- face is composed of a series of hollow metal Pellets from direct dryers are usually uniform in discs or paddles mounted on a rotating shaft, texture, size, and durability, and therefore they through which the heating medium flows. The rarely require additional processing to make them rotating action of the shaft agitates the solids, marketable. Generally, the plant must mix proc- improving heat transfer and facilitating the sol- essed solids (usually undersized fine particles) ids’ movement through the dryer. Mixing of into the feed solids to raise the solids content of previously dried material with feed solids is re- the feed mixture and avoid a condition referred to quired in some indirect drying systems. as the “sticky” or “plastic” phase. This phase
Source: WEF, 1992. Figure 2. Rotary Dryer: (a) Isometric View and (b) Alternative Flight Arrangements.
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Indirect dryers, which include steam dryers, hol- consistency, and durability of the product. Such low-flight dryers (Figure 3), and tray dryers, processing can improve the marketability of the produce smaller quantities of noncondensable gas pellets from indirect drying facilities, but it also than direct dryers, which means that the process increases costs. produces less odor and requires less odor control equipment. Indirect dryers usually have a higher A comparison of direct and indirect dryers is pre- thermal efficiency and are more suitable when sented in Table 2. pellets are to be used in energy production or Table 2. combusted. Indirect dryers also produce less dust Comparison of Direct Versus during the drying process and have a lower risk Indirect Drying of explosion than direct dryers. However, the Direct Indirect end-product of indirect dryers (the pelletized ma- Dried solids recycling Dried solids recycling terial) tends to be dustier than a dried product required. sometimes required. from a direct dryer, and therefore it is not as mar- Many operating facilities in Limited number of operating ketable to some users. Finally, indirect dryers the United States. facilities in the United often produce oversized pellets, which are not as States; several successful desirable in the agricultural market (R. Pepper- operations in Europe. man, personal communication, 2005). Additional Source: Summarized by Parsons 2005. processing (such as granulation or compaction) might be required to increase the uniformity,
Source: WEF, 1992. Figure 3. Flow Diagram of Hollow-Flight Dryer System.
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Other Types of Dryers. Other types of dryers The volatile solids content and temperature of the include those that use a combination of direct and pellets also affect their explosion potential. indirect drying or use special carrier fluids. For Therefore, pellets must be cooled to avoid com- example, the Jones Island Wastewater Treatment bustion in storage facilities. Most facilities Plant in Milwaukee, Wisconsin, which has blanket the pellets stored in storage silos with been in operation longer than any other facility inert material (such as nitrogen) to lessen the using heat drying in the United States, uses a explosion potential. Facilities can also monitor combination direct-indirect rotary system. the silos using thermal sensors (to detect in- Carver-Greenefield has patented a dryer that uses creases in temperature) or carbon monoxide carrier oil. In this system, wastewater solids are monitors (to detect increases in carbon monox- mixed with the oil, and the mixture flows through ide), both of which could indicate potential fire a multi-effect evaporator, where moisture is re- hazards (Sapienza and Bauer 2005). moved. Although a number of Carver-Greenefield biosolids dryer facilities were constructed (in- The Occupational Safety and Health Administra- cluding facilities for the Los Angeles County tion (OSHA) issued a Hazard Information Bulletin Sanitation District, the Ocean County [New Jer- in December 1995 that described required safety sey] Utility Authority, and the Mercer County precautions for facilities that process, convey, or [New Jersey] Improvement Authority), none are store dried biosolids. OSHA has outlined design currently operated. This system required consid- criteria that help minimize and control explosion erable maintenance to operate reliably, and its and fires connected with the organic dust from working capacity was smaller than that indicated heat-dried biosolids. These criteria include vent- by the designer. ing systems to release any buildup of pressure within the drying vessels or storage areas, safely Microwave Dryers. Burch Biowave has devel- releasing gas from drying facilities, using non- oped a system that uses a high-efficiency, multi- conductive materials in areas of drying or product mode microwave specifically designed to remove storage, reviewing all heat sources in and around moisture and destroy pathogens. The process heat-drying processes and storage areas, and en- does not affect the nutrient content of the end- suring that workers in these areas employ good product and can produce Class A biosolids. A housekeeping practices (OSHA 1995). Sapienza Burch Biowave system in Fredericktown, Ohio, and Bauer (2005) also note that maintaining an began operations in 2004, and another is planned oxygen-deficient atmosphere in the process com- for Zanesville, Ohio. ponents (dryer, solids separator, recirculation duct) can help to minimize this potential problem. Annual buyer’s guides published by trade organi- zations such as the Water Environment Federation and the Solid Waste Association of North America are good sources of additional information on heat dryer manufacturers.
PERFORMANCE Heat-drying technology is generally very reliable, and few facilities experience significant periods of unscheduled downtime. Nevertheless, some instal- lations have experienced performance problems. Spontaneous heating in storage areas is a concern because of the organic matter content of pellets derived from wastewater solids, and improper Used by permission of CH2M Hill, Inc. product storage procedures and dust accumulation Figure 4. Rotary Dryers, the Most Common have caused fires in some locations. Type Used for Drying Wastewater Solids.
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Sieger and Burrowes (2006) also presented in- Sapienza and Bauer (2005) report that historical formation on the safety and design of heat-drying costs for heat-drying equipment typically ranged systems. between $110,000 and $180,000 per dry ton/day of solids processing capacity for facilities proc- essing between 20 and 100 dry tons/day. Capital OPERATION AND MAINTENANCE costs for the entire heat-drying operation, includ- Heat-drying systems are sometimes highly ing buildings, site work, utilities, dewatered cake mechanized to maintain proper temperatures and conveyance, product storage, performance test- inflow/outflow. Therefore, operation of such ing, and so forth can be in the $220,000– systems require skilled operators. Preventive $300,000 per ton per day range (Sapienza and maintenance is a necessary part of day-to-day Bauer, 2005). The city of Leesburg, Virginia, operations. Routine cleaning helps to avoid cor- installed a direct rotary dryer system with an rosion caused by the properties of the solids. evaporative capacity of 2,000 kg/hr in 2001 as Multiple units are often used to avoid disruption part of a biosolids management upgrade project. to treatment works operation when units are not The project, which cost $11.5 million, also in- in service. All units should be in proper working cluded a screening building and a 350,000-gal order so they can be used if needed. sludge storage tank. The city chose an Andritz Several operating heat-drying systems report system in which hot gases are routed directly into common problems, including pitting of convey- the dryer instead of an alternative system with a ance equipment and dryer drums due to the heat exchanger because the Andritz system could abrasive nature of the wastewater solids, and start up and shut down more quickly. This feature scale formation on dryers and piping. Scale can was important because the city does not run the be removed by washing with acid or high- system constantly (S. Cawthron, City of Lees- pressure water jets. Mixing oil with the solids burg, personal communication, 2006). also helps to prevent scale formation. Items that must be considered when estimating capital costs include COST • Dewatering feed solids Capital and O&M costs for heat-drying facilities • Feed solids mixing are typically high relative to other solids alterna- tives, such as land application and alkaline • Dryer stabilization (Sapienza and Bauer 2005). It is • Conveyance to and from dryer difficult, however, to estimate the exact costs of heat-drying wastewater solids without design • Air emission (including odor and dust) control details such as the specific type of dryer, fuel • Product classification, screening, and/or pel- source, and moisture content of the feed solids. letizing Santa Barbara County, California, (2004) esti- • Product cooling prior to storage mated that heat drying would cost from $51 to $58 per wet ton, depending on the availability of • Product storage, including provisions for ni- biogas or waste heat from co-generation facili- trogen blanketing ties. These costs are based on an average biosolids solids content of 18 percent. Grace et al. Sapienza and Bauer (2005) indicate that O&M (1994) compared the cost of direct versus indirect costs for heat-drying facilities typically range drying of approximately 35 dry metric tons of from $180 to $300 per dry ton of material proc- wastewater solids per day and estimated $323 per essed. These costs include costs for fuel, power, dry ton for indirect drying and $441 per dry ton O&M labor, and maintenance materials and sup- for direct drying. These figures included capital plies. Costs for fuel can be a significant part of costs of $26.8 million for the indirect dryer ver- these costs and can range from 25 percent to 55 sus $37 million for the direct drying system. percent of the total O&M costs.
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Typical O&M costs include REFERENCES • Labor Other related fact sheets: • Auxiliary fuel Odor Control in Biosolids Management • Air emission control chemicals and mainte- EPA 832-F-00-067 nance September 2002 • Equipment maintenance Centrifugal Thickening and Dewatering EPA 832-F-00-053 • Product transport September 2002 • Product marketing Belt Filter Press Another facet of costs related to drying is the sale EPA 832-F-00-057 of the resulting product. Biosolid pellets from September 2002 dryers are historically very marketable products. Other EPA fact sheets are available at the The factors that influence the price received for following Web address: the pellets are nutrient content, particle size dis- http://www.epa.gov/owmitnet/mtbfact.htm tribution, dust potential and mechanical durability (which are closely related), bulk density, mois- Cawthron, S., City of Leesburg Wastewater ture content, and odor. Treatment Plant. 2006. Personal communica- tion. Nutrient content usually has the greatest impact on the price because most buyers base their pur- Dolak, I., S. Murthy, and T. Bauer. 2001. Impact chase on the amount of nitrogen in the pellets. of Upstream Processes on Heat-drying Tech- Many facilities sell dried biosolids to users with nology. In Proceedings of the Water the price based on the nitrogen content of the Environment Federation, American Water product. Current prices are typically around $9 Works Association and California Water Envi- per metric ton ($10 per ton) of material per per- ronment Association Specialty Conference, cent nitrogen. Sapienza and Bauer (2005) report a Biosolids 2001: Building Public Support. Ar- range in value from $0 to $36 per metric ton ($0 lington, VA: Water Environment Federation. to $40 per ton). As with many types of products, Feindler, K.S., and C.A. Holley, 1994. Method however, prices can fluctuate with the seasonal for Upgrading Thermally Dried Wastewater demands of users and in response to supply. The Solids into a Competitive Organic Fertilizer. operation of several large dryers has recently In Proceedings for the Management of Water increased supply and led to falling prices. Being and Wastewater Solids for the 21st Century: A able to store the products until supply is low Global Perspective. Alexandria, VA: Water might also help the bottom line. Producers that Environment Federation. can hold the product until users are ready might net a higher price than those who move the prod- Foess, G.W., D. Fredericks, and F. Coulter. 1993. uct from the site every day regardless of price. Evaluation of Class A Residuals Stabilization Technologies for South Broward County, Although the sale of dried biosolids provides a Florida. In Proceedings of the Water Envi- welcome source of revenue to wastewater treat- ronment Federation 66th Annual Conference ment plants to help offset O&M costs, it should & Exposition, Sludge Management. Arlington, be noted that selling the end-product typically VA: Water Environment Federation. does not completely offset heat-drying processing costs.
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Grace, N., G. Carr, and J. Finley. 1994. Direct Pentecost, D.J. 2004. Biosolids 101: Understand- Versus Indirect Thermal Drying of Biosolids: ing the Pathogen Classes. Pollution A Comparative Evaluation. In Proceedings of Engineering 36(8): 20–23 the Water Environment Federation Specialty Conference, The Management of Water and Pepperman, R. 2005. Personal communication. Wastewater Solids for the 21st Century: A Pepperman, R. 2006. Personal communication. Global Perspective. Arlington, VA: Water Environment Federation. Santa Barbara County, California. 2004. Strategic County-Wide Biosolids Master Plan. Prepared Grontmij Vandenbroek. 2006. VADEB Thermal by CH2MHill. Kinetic Drying Technology.
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WEF (Water Environment Federation). 1992. The mention of trade names or commercial Design of Municipal Wastewater Treatment products does not constitute endorsement or Plants. WEF Manual of Practice No. 8. Alex- recommendation for use by the U.S. andria, VA: Water Environment Federation. Environmental Protection Agency. Office of Water ADDITIONAL INFORMATION EPA 832-F-06-029 Synagro Corporation September 2006 Karl von Lindenberg P.O. Box 9974 For more information contact: Baltimore, MD 21224 Municipal Technology Branch U.S. Environmental Protection Agency Milwaukee Metropolitan Sewerage Mail Code 4204 Paul Schlect 1200 Pennsylvania Avenue, NW 260 West Seeboth Street Washington, DC 20460 Milwaukee, WI 53204-1446
New England Fertilizer Company Virginia Grace 500 Victory Road North Quincy, MA 02171 New York Organic Fertilizers Company Peter Scorziello 1169 Oakpoint Avenue The Bronx, NY 10474 New York Department of Environmental Protection Tom Murphy 96-05 Horace Harding Expressway Corona, NY 11368
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