DRYING OF WASTEWATER SOLIDS
WEF Residuals and Biosolids Committee,
Bioenergy Technology Subcommittee
For additional Biosolids information, please see biosolids.org.
Water Environment Federation
601 Wythe Street Alexandria, VA 22314
Copyright © 2014 Water Environment Federation. All rights reserved.
This Fact Sheet is an update of the “Drying of Wastewater Solids” White Paper (January 2004)
Archis Ambulkar, Brinjac Engineering
Amber Barritt, CDM Smith
Robert Bates, Louisville MSD
Jason Boyd, Kruger
Scott Carr, CDM Smith
Bill Holloway, Kurita America
Christopher Komline, Komline Sanderson
Hari Santha, Black & Veatch
Frank Sapienza, CDM Smith
Dieter Weinert, Huber Technology
Ron Sieger, CH2M HILL, Chair
Peter Brady, Alpine Technology, Vice Chair
John Donovan, CDM, Primary Reviewer
Tim Shea, CH2M HILL, Primary Reviewer
Frank Sapienza, CDM, Chair (Principal Author)
Tim Bauer, CH2M HILL, Vice Chair (Principal Author)
Saeed Assef, Louisville MSD
Andrew Bosinger, Synagro
Mark Hoey, Tighe & Bond
Bill Holloway, Andritz
Terry Logan, N-Viro
Dirk Eeraerts, Seghers
Steve McDonough, Hankin
Jeff Berk, USFilter
Jon Orr, Virginia Tech
Wilfried Schnabel, Bird Machine
Virginia Grace, NEFCO
Tom McGowan, TMTS
Mike Sweeney, Louisville MSD
Perry Schnuck, Synagro
Bruce Miller, USFilter
Rich Chaney, Dupps Company
Robert Pepperman, United Water
David Dahlstrom, Alstom Power
In January 2004, the Water Environment Federation’s (WEF) Bioenergy Technology Subcommittee developed a Whitepaper on “Thermal Drying of Wastewater Biosolids”. The paper provided an overview of the status of biosolids drying systems, including various technology descriptions, use of dried biosolids products, sustainability of the drying process, and associated safety issues. A Subcommittee taskforce was formed in 2011 to update the Whitepaper to include information from operating facilities as well as to update the state of the technology in the U.S. municipal market. The update team also expanded the thermal drying aspect to include solar drying, hence the name change to “Drying of Wastewater Solids.”
Drying is one of the alternative technologies available for processing of biosolids produced at municipal wastewater treatment plants (WWTPs). This technology has been successfully implemented at WWTPs since the 1920s’ and produces a marketable dry solids product that can be used as a fertilizer or biofuel. As of 2012, there are more than 60 drying systems operating in the U.S., and more than 100 in Europe.
Drying is based on the removal of water from dewatered solids, which accomplishes both volume and weight reduction. At municipal WWTPs, dewatered biosolids are conveyed to the drying system where the temperature of the wet solids mass is raised and most of the water is removed via evaporation, resulting in a product with approximately 90% or higher total solids. This drying process does not significantly alter the nutrient content of the biosolids.
During drying, a significant amount of thermal energy needs to be transferred to the wet solids (cake) to evaporate the water. Energy is required not only to evaporate water, but also to heat the solids and remaining water. This energy can be provided by combustion of various fuels (such as natural gas, digester gas, heating oil, wood, etc.), by reusing waste heat, via solar radiation, or by conversion of electrical power into thermal energy.
For most systems, the high temperatures used in drying assure that the US EPA time and temperature requirements for pathogen kill are met. Drying also meets the EPA vector attraction reduction standards by desiccating the wastewater solids to greater than 90% solids (or to greater than 75% solids if the solids have been previously stabilized). Although high temperatures are used in many drying systems, the temperatures are generally low enough to prevent oxidation (burning) of the organic matter. Thus, most of the organic matter is preserved in the dried material.
Material produced in the drying process generally has a dry solids content ranging from approximately 75% for solar drying systems to greater than 90% for systems using fossil fuels or other heat sources. Drying systems may produce a variety of forms of dry material, including fine dust, flakes, small pellets, or larger fragments, depending on the type of drying system used, the characteristics of the solids processed, and the intended use of the final product.
Thermal drying typically must be preceded by, or done in conjunction with a dewatering process, and drying is usually the last stage in processing of solids at municipal WWTPs. After the drying process, dried material can be used for a variety of purposes.
In the most general terms, drying is the use of heat to evaporate water from wastewater residual solids. The drying system, in addition to the dryer itself, generally consists of materials handling and storage equipment, heat generation and transfer equipment, air movement and distribution equipment, emissions control equipment, and ancillary systems. These equipment systems can take many forms, the details of which are beyond the scope of this paper. However, drying systems use different methods for heat transfer, including convection, conduction, and radiation heating. To some extent, multiple methods of heat transfer are used by individual systems, but they are generally categorized by their primary method of heat transfer.
Systems that primarily use convection for heat transfer are often referred to as “direct” dryers. In direct heat dryers, hot air/gas flows through a process vessel and comes into direct contact with particles of wet solids. The contact between the hot air and cold wet cake allows the transfer of thermal energy, which causes an increase in wet cake temperature and evaporation of water. The hot air/gas can be produced by almost any source of heat, but most often is produced by a gas or oil-fired furnace. Examples of direct drying equipment are rotary drum dryers and belt dryers. A schematic diagram of a typical rotary drum drying system is shown in Figure 1.
Figure 1: Direct Type Rotary Drum Drying System
In this type of system, the heat supply is via a fuel-burning furnace that exhausts directly to the dryer drum. The dried material is separated from the warm exhaust gas and is then screened and processed for either recycling back to the dryer or routed to storage silos. The exhaust air/gas is cooled and part of it is recycled back to the dryer. The remainder of the air/gas is treated in air pollution control equipment and then vented to the atmosphere. Recirculation of the dryer exhaust accomplishes three important functions. First, it increases the overall thermal efficiency of the dryer system, second, it minimizes the volume of exhaust gas requiring air pollution control (APC), and third, it provides a safety feature by limiting the oxygen concentration in the system, which reduces the risk for explosions. APC systems for drum dryers typically consist of additional particulate removal followed by regenerative thermal oxidation to destroy odors and volatile organic compounds (VOCs). Other methods of APC, such as biofilters, are often used with different drying systems. Present day direct drying systems typically recirculate 70% to 90% of the dryer exhaust, thereby greatly reducing the size of the APC equipment. Direct drying systems vary considerably depending upon the type of equipment used to process the wet and the dried biosolids. Even rotary drum systems as shown in this figure vary considerably in general layout and the equipment used.
Another type of direct dryer that is seeing increased use in the U.S. and Europe is the belt dryer. This is typically a lower temperature system compared to a rotary drum system. The heat supply is usually a fuel-burning furnace, but in contrast to the rotary drum system, the system exchanges its heat to a thermal fluid, hot water or flue gas to air heat exchanger instead of the furnace exhausting directly into the dryer cabinet. The belt drying system distributes dewatered cake onto a slow moving belt, allowing for high surface area exposure to the hot air. Belt drying systems can utilize multiple belts to help minimize the size of the dryer cabinet. High dryer air recirculation (>90%) and low vent rates are common. Due to the gentle handling on the slow moving belts, dust generation within the dryer cabinet is low and the quantity of fines in the dried product should be low. Some belt drying technologies require dried product recycling to elevate the inlet solids composition to above the sticking point, while others inject dewatered sludge cake without additional recirculation equipment. The lower temperature belt drying system can more adequately utilize lower grade waste heat (in addition to high temperature waste heat).
Systems that primarily use conduction for heat transfer are referred to as “indirect” dryers. With indirect dryers, solid metal walls separate the wet cake from the heat transfer medium (such as steam, hot water, or oil). Thermal energy is transferred from the heat transfer medium into the metal wall and then from the metal wall into the cold cake. The solids temperature is elevated by contact with hot metal surfaces and the solids never come in direct contact with the primary heating medium. Some types of indirect dryers do not require recycle of dried material, simplifying the system. Indirect thermal drying equipment includes paddle dryers with varying configurations, vertical tray dryers, and an indirect-type of fluidized bed dryer. A schematic diagram of a typical paddle drying system is shown in Figure 2.
Figure 2: Indirect Type Paddle Drying System
In this type of system, the heat supply is via a fuel-burning furnace that exhausts to a heat exchanger to heat oil, which is recirculated through the dryer. Steam, air, water, or other heat transfer fluids are other media that can be used. The solids are mechanically moved through the dryer and pick up heat from direct contact with the hot surfaces. Following the dryer, the material handling equipment is similar to that used in the direct system. In this system, the dryer exhaust primarily consists of water vapor and a small quantity of air which inadvertently enters the dryer with the wet feed. The exhaust from the dryer is sent to a condenser where the water vapor is condensed and sent back to the WWTP and the small air flow (containing some non-condensable organics) is then treated using various APC methods, depending on the system and supplier. For example, some systems send the exhaust to the furnace for use as combustion air, while others use a wet scrubbing system. Similar to direct drying systems, indirect systems also vary considerably in the type of equipment used to process the wet and dried material through the system (WEF MOP No. 8).
Solar drying systems rely on radiant energy from the sun. Dewatered solids are distributed into the greenhouse uniformly, either by automated mechanical means or by a manually operated tractor or truck. The sun’s radiant energy passes through the greenhouse enclosure (walls and roof) to heat and evaporate moisture from the sludge. The greenhouse enclosure prevents rain from adding water to the sludge and allows for a semi-controlled greenhouse environment, including air convection to help accelerate evaporation and enclosure of odors that can be processed through an odor control system. Solar drying systems are sensitive to local weather conditions, including solar radiation (considering typical cloud cover), relative humidity, temperature and wind speed and require thorough analysis during the design process. The solar drying system design will need to consider seasonality and historical weather data to ensure adequate design, and for some deviation from “average” weather years, some overdesign may be necessary. Solar drying processes are typically designed to provide solids compositions to exceed 75% dry solids, but can be difficult to sustain above 80-85%, depending on how loaded the system is. Solar drying systems typically provide a mechanical means for mixing and aerating the sludge periodically to promote the drying process. Solar drying is most viable in the southerly latitudes, for municipalities who have land available, and where snow cover is minimal. Radiant heat flooring can also be incorporated into the concrete greenhouse floor to reduce required greenhouse space and reduce the effects of weather variations to process performance. There are multiple solar drying systems available and vary in their equipment supply and how they operate.
Drying systems are a net energy consumer. By virtue of the inherent need to evaporate water (which theoretically requires 970 Btu/pound of water), an external source of heat is required. The most common energy sources are natural gas, digester gas, and fuel oil. However, in some instances waste heat from a nearby combustion source or thermal process can be used, including the combustion of dried biosolids. Solar systems use radiant energy primarily, but the fans and other equipment consume electric power.
Energy consumed in a thermal drying system typically includes fuel/thermal energy, and electrical power to operate equipment. The thermal energy consump¬tion is based on the amount of water to be evaporated and the thermal efficiency of the drying system. The thermal efficiency of drying systems typically ranges from approximately 1,400 Btu per pound of water evaporated to 1,700 Btu per pound of water evaporated. Use of energy recovery systems may be able to further reduce these values. Cost of fuel is one of the largest operating costs for most drying systems, except for solar systems. If digester gas or another waste heat source is available, considerable savings in fuel costs can be realized. For example, in 2011, one utility surveyed indicated they were able to reduce operating costs by approximately $40 per wet ton of cake solids processed by using landfill gas as opposed to natural gas. Lower temperature dryers, such as belt dryers, are more able to utilizing lower temperature waste heat from existing plant operations.
Electricity is used to power various moving components of any drying system. Large consumers of power are fans/blowers, mixers, conveyors, elevators, and screens. The amount of power consumed by drying systems with identical capacity may vary depending on the type of drying system.
Sustainability is the ability of a process to endure and remain an economically and environmentally sound means of wastewater solids management. In general, the demands on solids processing alternatives are always changing and becoming more difficult to fulfill. The demands and pressures on drying can come from state and federal regulatory agencies, the general public, or from economic conditions. Regulatory agencies are continually scrutinizing pollutant and odorous air emissions from drying plants and imposing tighter emissions criteria on new facilities. Recent drying plants have shown that they can meet the strictest odor and pollutant emissions criteria. Federal and state statutes also regulate the quality of the product. Specifically, for a heat-dried product to be applied to land as an Exceptional Quality (EQ) product, it must meet stringent quality parameters including pathogen density reduction (Class A), vector attraction reduction, and low metals concentrations. These parameters are fully specified in the US EPA Part 503 regulations (40 CFR Part 503). Operating experience at drying facilities has shown that these criteria can be confidently and consistently met.
In the past odors were probably the single most detrimental impact from drying plants. However, present day design of drying plants has incorporated recirculation of dryer exhaust gas and the use of regenerative thermal oxidizers (and other techniques) to deodorize the final exhaust gas such that odorous emissions are no longer a significant impact. Dryers with high air recirculation rates or indirect dryers with low off-gas volumes can tie their low vent flow directly to the plant’s existing odor control system. In general, the public now perceives drying as an environmentally acceptable technology for solids processing.
Presently, one of the major pressures on drying systems is the energy demand of the process, particularly the high fuel usage. Drying does use a considerable amount of fuel in comparison with other beneficial reuse technologies, i.e. composting, alkaline stabilization, and land application. However, the value and acceptability of the final product is much higher for a heat-dried product than a product from these alternate technologies. Municipalities producing a heat-dried product typically have had little difficulty marketing the product, whereas products from alternative technologies have required more effort to find suitable uses or markets for these products. Thus, the energy demands and associated costs of drying have been acceptable because the municipality is assured that the final product can be safely used and in many cases will generate income. Furthermore, innovations have been developed in the last decade to improve the ability of some systems to use waste heat to reduce or eliminate energy consumption. Thus, drying meets all of the present demands placed on it and should continue to be a highly sustainable solids processing technology in the future.
Because of the large variety in types of drying systems, levels of processing, procurement methods, and general equipment differences, the capital cost of thermal drying systems varies a great deal. Factors that affect capital cost include the type of system selected, existing infrastructure, such as buildings and utilities, conveyance needs for moving dewatered solids to the process, and finished product storage requirements.
The operating and maintenance (O&M) costs of thermal drying systems are also dependent on the type of system selected and the energy source. Systems that rely on combustion of fossil fuels will have significantly higher O&M costs than systems that recover heat from other processes or rely on solar radiation. Energy recovery can make O&M costs very competitive with O&M costs for other methods of solids processing, especially other systems that create Class A stabilized products. The level of mechanization and automation used in the system will also have a significant effect on labor and maintenance costs.
Drying systems should be evaluated on their life-cycle costs as opposed to any single economic parameter. Although drying systems may have higher capital costs than other processes, the substantial reduction in volume of material to be transported off-site, the flexibility of outlets available, and the value of the product can help make these systems cost-competitive with other solids processing systems.
Heat dried municipal biosolids product has qualities and characteristics that make it suitable for application on land or for use as a biofuel. Most of the heat-dried material produced in the U.S. is applied to land as soil conditioner, fertilizer, or fertilizer supplement. The material is used by agricultural users (farmers), golf courses, nurseries, parks, and is also marketed through retail outlets for home use.
Due to its characteristics, the dried product typically has a positive commercial value at the drying facility. Most of the municipalities or private parties in the U.S. that operate drying systems are able to sell the dried product and generate revenue that offsets a small portion of the costs for operating the drying process.
The market value of dried biosolids as a fertilizer depends on local market conditions, nutrient content, physical characteristics of the product, and other factors. The commercial value of dried biosolids in the U.S. is typically between $0 and $40 per ton of dry material, and varies based on agricultural practices in the region, product characteristics, and marketing strategies of the producer. Obtaining the high range of value for the product is less common and usually requires a significant marketing effort from the producer or a third party.
Some of the specific factors that impact commercial value of dried biosolids as a fertilizer include the following:
In recent years there has been a growing interest in the use of biosolids as a biofuel. European facilities have shown this to be a viable practice, burning product in coal fired power plants to a limited extent, and more commonly in cement kilns and waste-to-energy facilities. In the U.S., use of the dried product as a biofuel is still in a developmental stage with the product primarily used in cement kilns. The heat value of the product will vary depending on the feed solids characteristics, with digested solids typically in the 7,000 Btu/dry pound range (3,900 kCal/kg, 3,400 kJ/kg) and raw solids as high as 9,000 Btu/dry pound (5,000 kCal/kg, 4,300 kJ/kg).
Drying technologies offer relative advantages and disadvantages as compared to other solids processing technologies. Some of these are listed below:
The National Biosolids Partnership (NBP) is a not-for-profit alliance formed in 1997 with the National Association of Clean Water Agencies (NACWA), Water Environment Federation (WEF), and U.S. Environmental Protection Agency. Its goal is to provide technically accurate solids management information and to promote environmentally sound and accepted solids management practices. In addition, the NBP acts as a clearinghouse for gathering and disseminating information on technical issues related to biosolids. For example, operational controls for thermal dryers are discussed in the NBP guidance document, Manual of Good Practice for Biosolids. Producers, service contractors, and users - together with stakeholders from regulatory agencies, universities, the farming community, and environmental organizations all have input into shaping NBP priorities through scientific and technical support and communications. Furthermore, one of the roles of the WEF Bioenergy Technology Subcommittee is to be an experienced and knowledgeable source of technical expertise to the NBP.
Another goal of the NBP is to promote certification and recognition of municipal wastewater treatment organizations. To become certified or recognized, an organization must comply with the NBP’s Code of Good Practice which requires having an acceptable Environment Management System for biosolids. Since drying produces a valuable biosolids product which is not offensively odorous, is easily transported and blended with commercial fertilizers, has a high content of slow release nitrogen, and meets the EPA Part 503 requirements of a Class A product, drying can be a key component in an environmentally sound solids management system. Thus, municipal agencies with drying systems should be encouraged to join the NBP. In addition, producers, distributors and end users of dried products should be informed of the NBP so they can relate their experience, which would definitely enhance the growth of beneficial use of biosolids.
Public acceptance of biosolids is dependent on several key safety and environmental issues, which the public must be reassured of. These issues include:
While numerous state and federal regulations have been developed to address these issues and protect the public, the public can be quite skeptical of “official “statements and policies. In many cases, it is the one odorous incident or mismanagement practice which catches the attention of the media and the public. Therefore, it is important that the whole picture regarding the use of biosolids be accurately portrayed to the public. It is important that the public understand that with proper management of biosolids these issues are satisfactorily addressed and that biosolids, instead of being an environmental detriment, is an environmental asset. In comparison with other biosolids processes (i.e. composting, alkaline stabilization, land application), drying should be promoted as a beneficial biosolids process since it produces a product that has significant nutrient value as a fertilizer and as a biofuel, meets the pathogen and vector reduction requirements, and is not offensive to the general public.
In the past there were serious concerns with the safety aspects of wastewater solids thermal drying plants. These safety concerns included the following:
As the design of drying plants has evolved, engineers and system suppliers have learned how to design safer drying plants. The potential for fires in direct dryers has been greatly reduced by maintaining an oxygen-deficient atmosphere in the dryer. This is done by recirculating the dryer exhaust gas and limiting the amount of infiltration air such that the oxygen level in the dryer is held at 3% to 9%. Typically, the oxygen level must be greater than 10% to support combustion. Note that one study indicated a maximum permissible oxygen concentration of 6% is necessary to prevent combustion depending on the type of solids (HSE 847/9). In addition, dryers are equipped with quench sprays to extinguish a fire or burning embers in the dryer. Quench sprays are usually automatically activated based on a rise in the dryer exhaust gas temperature indicating that combustion is occurring in the dryer.
The potential for a dust explosion in many of the system components (dryer, solids separator vessel, recirculation duct) has been eliminated by maintaining an oxygen deficient atmosphere in these components. In some plants select equipment is provided with nitrogen blanketing to prevent explosions.
Similarly, the potential for fires in the product storage silo has been addressed by using inert gas (nitrogen) blanketing systems to maintain an oxygen deficient atmosphere in the product silo. In addition, cooling of the product prior to storage has proven to be an effective means of retarding auto-oxidation of the material and preventing fires. Storage silos are typically monitored by thermal sensors hung within the silos to detect any rise in temperature. Another monitoring technique is to use carbon monoxide monitors, which can detect the initiation of any combustion reactions. Thus, through experience and careful engineering the potential safety concerns with thermal drying systems have been satisfactorily addressed.
As part of the 2012 update, the Taskforce solicited feedback from operators of thermal drying systems related to potential design improvements to assist operators and improve safety. The purpose of the survey was to collect information for use by others considering new drying facilities, or for those that have existing facilities and are interested in what others have done to correct certain issues. This section summarizes the feedback received.
Equipment access was one of the key issues identified by the respondents. One respondent noted that the equipment design didn’t consider OSHA regulations regarding fall protection. Due to the height of the equipment, which can be greater than 50 feet on some systems, some platforms had to be extended to allow staff to read monitoring equipment.
Multiple respondents indicated that consideration should be given for accessibility to all components during design, with thought to regular inspection and servicing of the equipments. One utility used access to the storage silo as an example. Some of the air hoses and slide gates need to be inspected weekly. Therefore, they recommended consideration be given to making these types of components more accessible to plant staff. In addition to being able to access the equipment, confined space entry requirements should especially be considered during the design of the systems, as well as during development of the operation and maintenance manual and training programs. Examples cited where this is necessary include the drying drum or shell, cyclone separators87978, fugitive dust collector, condenser, venturi scrubber, cake wet-feed bin, furnace, and regenerative thermal oxidizer.
One respondent recommended having access doors installed in the chutes between the bottom of the product silo and the motorized part of the chute that lowers into the truck bed. This will provide access at the slide gate to clear the silo if it becomes plugged due to clumps of pellets that can form due to any water leaking into the silo. Similarly, another respondent recommended that all bins, especially recycle bins, should have some means of emptying to a truck or other collection point to allow for cleaning and inspection of the bin.
Another responder indicated the lower condenser drain line will sometimes clog due to solids in the discharge water. Their drain line had a butterfly cut-off valve in it. The problem was that the drain line had no access for staff to unclog the line if it was clogged. Someone had to go inside the condenser to flush the drain. The butterfly valve would not let a normal size garden hose pass through it. The respondent removed the butterfly valve and installed a ball valve, which allowed a garden hose to pass through the pipe to help remove any clogs. They also installed a "TEE" with a blind flange that allows cleaning the drain line from the outside of the condenser. Some other access suggestions include:
Product safety in the storage silo has proven to be a critical issue. There have been many different approaches for the implementation of safety features. One respondent indicated that during design, consideration was given to installing a sprinkler system in the final product silos. However, since the design already included a nitrogen purge system installed to deplete the oxygen in the silos in the event of high silo temperatures, it was agreed to by all parties, including the engineer and fire marshal, that a sprinkler system was unnecessary.
One utility reported concerns with bagging systems. They use a semi-automated bagging system for bagging and palletization, and had a few back injuries as a result of moving the filled and sealed bags from the conveyor to the palletizer.
The following general comments related to operations and maintenance were provided.