Frequently Asked Questions
by Topic
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Technology
Q: What type of drying technology does the indirectly heated dryer incorporate?
A: Biosolids in the dryer system is heated by an indirect process. The dehydration chamber and the hollow flight auger which carries the sludge, rotate in opposite directions, never allowing the sludge to come in direct contact with the heat source.
Operating Parameters
Q: What is the minimum percent solids that the indirectly heated dryer will accept?
A: Solids as low as 12% can be fed into the dryer.
Q: What is the maximum percent solids that the indirectly heated dryer will accept?
A: Solids as high as 28% can be fed into the dryer. For solids higher than 28%, please contact the indirectly heated dryer product manager, e-mail john.igoe@siemens.com, or call 281.360.4971.
Q: Does it matter what type of sludge is fed into the dryer, such as digested, primary, secondary, WAS, etc.?
A: No. The dryer can accept all types of municipal solids.
Q: Can the indirectly heated dryer operate to match the feed rate of a belt press or centrifuge?
A: Typically the dewatering equipment's operational feed is set to match the indirectly heated dryer, but adjustments to the drying process of the indirectly heated dryer can be made to match a particular dewatering scenario. For example, by speeding up the rotation of the dehydration chamber and the hollow flight auger, and making adjustments to the burners, the process capacity can be increased. By reversing this logic we can decrease the process capacity of the dryer. It is also possible and advisable to install an oversized feed hopper on the feed side of the dryer to allow an accumulation of sludge upstream of the dryer and down stream of the dewatering equipment. With a standard indirectly heated dryer installation, a 5 to 20 cubic yard feed hopper is included.
Q: Can Siemens test dry my sludge?
A: Yes. Siemens has a full scale demo/prototype unit, designed to dry 4 wet tons of sludge cake in a 24 hour period. Please contact your regional representative to arrange the testing.
Q: What happens to the dryer if the percent of solids from dewatering changes?
A: If the change in sludge quality is large enough to affect the final products moisture to an unacceptable level, adjustments can be made by:
1. Adjusting the retention time with the thermal auger speed, increasing speed to decrease dryness or decrease the speed to increase the dryness, or
2. The temperature set points for burners #1, #2, or #3 can be adjusted to compensate for the change in solids. Increasing the temperature setting of one or more of the burners will increase the final product dryness and visa versa.
Q: Some of our sludge may contain small plastic strips and other plastic debris that entered the plant. Will this affect the dryer?
A: The small plastic items in the sludge should have no affect on the dryer. At existing locations with operating dryers, small pieces of plastic have been seen entering the dryer. Upon investigation of the dried product, no evidence of the plastic can be detected. Nor has any problem with the dryer been experienced due to the presence of these foreign objects.
Standard Operating Procedures
Q: Is a warm up or cool down period required before or after using the dryer? If so, how long?
A: Yes, from a cold start- a warm up period of approximately 2 to 2-1/2 hours is required before using the dryer. A cool down period of approximately 3 hours is required after the dryer's operation.
Operating Conditions
Q: What is the evaporation rate?
A: Maximum design evaporation rate for
Series 10020: gallons of water = 7,220 lb./hour
Series 6012: gallons of water = 4,333 lb./hour
Series 4008: gallons of water = 2,889 lb./hour
Series 2004: gallons of water = 1,446 lb./hour
Series 1002: gallons of water = 722 lb./hour
Q: How does the dryer stay sealed during operation?
A: There are mechanical seals at each end of the hollow flight auger and the rotating drum. The design of the sludge hopper feed auger provides a seal on the wet end. The air lock at the product discharge provides an air seal at the dry end.
Q: Is grease a problem?
A: Not in the quantities normally found in municipal sludge.
Q: Does the indirectly heated dryer recycle dried material?
A: No, the indirectly heated dryer does not rely on recycling previously dried solids as a part of the drying process. Recycling is not necessary. Recycling unnecessarily complicates the process, requires additional costly screening, conveyence, and blending equipment to return the already dry product back into the wet sludge being feed into the dryer.
Operating Costs
Q: What is the utility cost to dry sludge using your equipment?
A: The actual cost will depend on the initial dryness and the desired final dryness of the sludge and the cost per unit of natural gas and electricity. We can provide an accurate estimate of the utility cost per day and wet ton for a given scenario.
Q: What is the evaporation efficiency of the indirectly heated dryer?
A: Depending on the sludge type, the indirectly heated dryer has an evaporation efficiency of 1,300 - 1,500 BTU/lb. of water.
Q: Can dewatered sludge from the belt filter press be stored in a supply hopper so the dryer can be fed around the clock even though the press operations are ongoing only one shift per day?
A: Yes, dewatered sludge can be stored in the feed hopper ahead of the dryer.
Q: Would Siemens be willing to guarantee the performance and operating cost?
A: Siemens has and will guarantee a minimum performance level capability for any dryer installation. Total operating costs are a function of performance and therefore will also be guaranteed. Siemens will work with the engineer/client to provide a project specific performance bond.
Mechanical
Q: Where is the indirectly heated dryer engineered and fabricated?
A: The indirectly heated dryer is engineered and fabricated in Siemens ASME coded fabrication facility in Thomasville, Georgia. All technical and field services are supported from our Cranberry, Pennsylvania; Thomasville, Georgia; Canton, Georgia and Sarasota, Florida offices.
Q: What is the material of construction of the indirectly heated dryer?
A: All of the components of the indirectly heated dryer are of a corrosion resistant design. The corrosion resistance of the components has been successfully demonstrated and proven by application in handling municipal biosolids. The following is a break down of the equipment and the materials of construction.
| EQUIPMENT |
MATERIAL OF CONSTRUCTION |
| Feed Hopper |
Stainless Steel, SS Lined or Carbon Steel, epoxy coated |
| Condenser/ Scrubber |
Type 304 and/or 316 Stainless Steel |
| Condenser Piping |
Type 304 and/or 316 Stainless Steel |
| Condenser Spray Nozzles |
Type 304 Stainless Steel |
| End Housings |
Type 304 and/or 316 Stainless Steel |
| Dryer Auger |
Carbon Steel - 516 Grade 70 |
| Dryer Dehydration Tube |
Carbon Steel - 516 Grade 70 |
| Electrical Panels |
Stainless Steel |
| External Fasteners |
Stainless Steel |
Q: Why isn’t the dryer tube and auger constructed from grade 304 or 316 stainless steel?
A: All associated moisture from the sludge, which could cause corrosion, is evaporated during the drying process. The absence of moisture and the superior thermal heat transfer properties of the carbon steel dictate the use of carbon steel as the preferred material of construction. The carbon steel components of the indirectly heated dryer have proven themselves superior in resistance to, corrosion, thermal cycling, stress cracking and strength loss at elevated temperatures.
Q: If requested by the customer/engineer, will Siemens build the complete dryer from stainless steel?
A: Yes. Siemens can build the complete dryer from stainless steel, but for superior performance when drying municipal sludge, we prefer to use carbon steel for the dehydration chamber and the hollow flight auger.
Q: What would happen if the auger wore through?
A: Due to the extremely slow rotation (less than 1/3 rpm) of the auger and the ability to inspect the wear strip at each end of the auger, this should never occur. If after years of operation it is determined that wear is occurring the auger can be rolled out and be repaired or replaced.
Q: Is corrosion a problem?
A: At the end of each run essentially all material is removed from the dryer, leaving essentially no wet material to corrode the dryer. The wet and dry end housings are the only parts of the dryer subject to corrosive conditions and are constructed of Grade 304 and/or 316 stainless steel to prevent any corrosion.
Q: Is abrasion a problem?
A: The abrasiveness of dried sludge depends on the various components and varies greatly from location to location. The speeds of the major rotating components are extremely slow. The dehydration chamber rotates at 1 to 2 RPM’s and the dehydration auger rotates at 0.125 to 0.333 RPM. At these speeds we do not foresee abrasion creating the failure of any major component.
Hot Oil Heating Unit
Q: What type of hot oil heater/unit is used?
A: An ASME designed, built, tested and code stamped triple pass high efficient hot oil unit is used.
Q: What is the efficiency of the hot oil unit?
A: The operating efficiency is around 90%.
Q: Is the hot oil system ASME (pressure vessel code) code stamped?
A: Yes, the hot oil heater coils, expansion tank and the hollow flight auger are ASME code stamped and the remainder of the unit is built to code but does not require a stamp.
Q: Why does it need to be ASME code stamped?
A: The hot oil system operates, across the heating coils and hollow flight auger, at a pressure above 15 psi. Operating at pressure about 15 psi requires an ASME code stamp to comply with state laws governing pressure vessels.
Q: Does this law effect the hollow flight auger?
A: Yes, the operating pressure across the hollow flight auger can exceed 15 psi.
Q: What type of oil is used in the hot oil unit?
A: The system uses an environmentally friendly, non-toxic and non-hazardous heat transfer fluid. Used or contaminated fluid can be safely combined with spent lube oils and sent to the local oil recycler or burned for BTU value.
Permitting
Q: Are any air emission permits required with your dryer?
A: Permit requirements vary and depend on your state’s particular requirements. In the municipal sludge market it is not anticipated that any air quality permits will be required for the indirectly heated dryer.
Alternate Fuels
Q: Can steam be used to heat the dryer?
A: The dehydration chamber cannot be heated with steam or any liquid medium because it is an outer rotating chamber. The thermal oil for the hollow flight auger could be heated with steam, but we have determined that a thermal transfer fluid is the safer medium, and therefore, we promote "hot oil."
Q: Can digester gas be used to supplement the natural gas demand?
A: Yes, digester gas can be used to supplement the natural gas demand for the dryer.
Q: If so, would any cleaning of the gas be required?
A: General speaking burners are available to burn digester gas directly off the digester. The indirectly heated dryer employs four burners (one for the hot oil unit and three for the dehydration chamber) and if only digester gas is being used the answer would be yes, the gas must be cleaned, condensed and compressed prior to use. If one burner is isolated and setup to burn the methane gas then the gas directly off the digester could be used. It is also possible to burn a mixture of natural gas and methane.
Q: What type of cleaning, for digester gas, would be recommended and what costs would be associated with this cleaning?
A: The main constituents of concern to be eliminated are the hydrogen sulfide compounds and the water. You would need to analyze your digester gas and consult with the supplier of gas conditioning equipment in order to determine the required cleaning and associated equipment.
Q: Have any of your installations relied on 100% sewer gas?
A: No.
Manpower for Operation
Q: How much manpower is recommended for operation?
A: At existing locations with operating dryers, the personnel operating the dewatering equipment also monitor the dryer.
Q: Should the dryer be attended at all times when in operation?
A: The dryer requires monitoring of the general operating status and the output temperature of the product. It is recommended that the operator inspect the dryer's operation each hour and record the wet and dry product % solids as well as the dryer's operating temperatures.
Q: Are any periodic or routine operational adjustments needed?
A: The output temperature is used as an indicator of the dryness of the finished product. If the output temperature changes, then minor adjustments are made to the set point temperatures of the dehydration tube burners.
Q: Can the indirectly heated dryer operate safely unattended?
A: Yes, but not for extended periods of time. Someone should be on the plant site that could respond in the event of an alarm that would shut the dryer down. It is also recommended that a check of the operational readings be recorded every hour as well as performing a walk around inspection of the dryer.
Q: Is a building recommended?
A: This would be dependent upon the geographical location. In colder climates a building with a HVAC system is recommended and at a minimum a covered area is recommended for the installation of the dryer equipment.
Q: What would be the ventilation requirements?
A: The ventilation required depends on the owner’s needs. As a minimum, roof and wall vents with fans should be provided to provide natural and/or forced convection ventilation.
Q: How large does the building need to be (i.e. how much clearance around the dryer and other equipment)?
A: We can provide a plan layout drawing for each application showing the building size and typical layout of the equipment. Please contact the indirectly heated dryer Product Manager.
Maintenance Requirements
Q: Approximately how often will the dryer need repair?
A: The indirectly heated dryer under normal operating conditions will only require scheduled maintenance inspections and repairs.
Q: What major components would typically be expected to fail or require routine replacement?
A: The speeds of the major rotating components are extremely slow. The dehydration tube rotates at 1 to 2 RPM’s and the dehydration auger rotates at 0.125 to 0.333 RPM. At these speeds, we have not experienced nor do not foresee the failure of any major component. Other components such as IR sensors and mechanical seals could fail and require replacement. A good preventative maintenance program will help minimize any unexpected downtime due to component failure.
Q: What are the maintenance requirements of the indirectly heated dryer?
A:
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Perform daily visual inspections of all operating parts to assure correct operation.
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Assure proper lubrication of the mechanical seals.
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Assure lubrication and inspection of drive chains.
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Perform monthly inspection and testing of burner air/gas ratio settings.
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Maintain good housekeeping.
Q: What is the expected life of the dryer?
A: With proper operation and a good preventative maintenance program, the expected useful life of the dryer is 20 years.
CFR 503 Compliance
Q: How does the dryer meet 503PFRP?
A: With a retention time of over 150 minutes, and with the material reaching and exceeding the required 80 degrees C. the time/temperature requirements for pathogen kill are easily met.
Q: How does the dryer meet 503VAR?
A: By drying stabilized biosolids to at least 75% solids content or by drying un-stabilized biosolids to at least 90% solids content.
Process Control
Q: In order to consistently maintain an output of 75% or 90% solids, what set-point must be used (for design and for operation) to ensure that the solids do not drop below the EPA allowable minimum?
A: The set point used to estimate percent dryness is the final output temperature of the dried product. Temperatures of the various zones of the dryer and the auger rotation speed are adjusted to maintain the output temperature that correlates to the dryness required.
Q: Is this considered in your estimate of operating cost?
A: The energy cost estimates we provide are based on the proven efficiency of the dryer and the quantity of water to be driven from the product. The energy required to remove the water is independent of the actual set points of the dryer.
Q: How do you control the product moisture?
A: By varying the drying chamber and auger rotation speed and the set point temperatures of burners #1, #2, & #3. The auger rotation speed controls the product retention time inside the drying chamber. Normal product retention time ranges from 1.5 to 5 hours. The desired product retention time depends on the feed sludge solids content and the sludge characteristics.
Odor and Air Pollution Control
Q: Are odor control and air pollution control components desired or required?
A: The odors driven from the biosolids during the drying process are collected and must eventually be dealt with.
Q: If so, what type of odor control is used?
A: We recommend these odors be blown into an aeration basin with an appropriate amount of submergence at the outlet. We have found aeration basins very effective in removing the odors from this relatively small stream of air. The approximate volume is 300 CFM per dryer. The operating cost of the blower is dependent on the particular size of blower required as well as the planned duration of operating the dryer. The size of the blower is dependent on the size and length of the piping used to transport the air as well as the depth of submergence of the diffuser and the type of diffuser to be employed.
Q: Have any trends been noted regarding the factors that may effect odor (e.g. type of sludge digestion or temperature of drying)? If the Waste Activated Sludge were to be digested, would this impact the odor potential?
A: Odors are a function of the nature and composition of your sludge. Primary sludge will have a higher odor potential. Digested sludge should have a lower odor potential. However, the ability of the indirectly heated dryer to keep the odor stream separate from the combustion exhaust stream constrains the gas containing odors to a relatively small and easy to handle volume.
Recycle Streams
Q: Do you have any data on the loading (flow, BOD, TSS, ammonia, etc.) of recycle streams (if any) returned to the WWTP from condensers or scrubbers, etc.?
A: Yes, the flow of condensate from the dryer will be the volume of water driven from the sludge plus approximately 50 - 150 GPM of cooling water. The flow of gas from the dryer is approximately 300 CFM. As for loading, the quantity of BOD, TSS, ammonia and nutrients returned to the plant will be site specific. But to date all installations indicate this loading to be negligible on the performance of the WWTP where dryers are operating.
Q: What are the approximate solids loading of the return condensate?
A: Approximate 0.23% by weight of the returning condensate flow is composed of solids.
Q: Where does the condensate go?
A: The condensate is pumped back to the head works of the plant for re-treatment.
Q: What is the O2 content of the vapor from the dryer, which is going to the aeration basin?
A: The O2 content of the vapor from the dryer returned to the aeration basin is above 20% by content.
Dried Product Quality
Q: Is the product dusty?
A: The amount of dust in the final product will depend on the percent solids content of the product. If the product is taken to 90-95% it is dustier than 80-85%. The amount of lint and other fines in the sludge can also effect the size and distribution of the dried sludge particles.
Q: Can fines (dust) escape into the room?
A: No, the indirectly heated dryer and its conveyors are totally enclosed and the dryer itself is operated under a vacuum.
Q: What is the approximate bulk density (pounds/cubic yard) of the dried sludge?
A: The density of the dried product will be site specific and dependent on the level of dryness. The measured density of the dried sludge ranges from 35 to 45 lb/ft3.
Q: What will be the range of particle sizes of the dried material?
A: The particles of the dried product are angular from fines to 4 mm in length and width.
Q: Is it possible to use a binding agent (dust control) on the end product?
A: Yes. It is recommended that a binding agent be used. Siemens can supply a binding agent system as part of the dryer supply.
Disposal of Dried Solids
Q: Have any problems arisen at any of your installations regarding getting rid of the biosolids? If the biosolids are applied to farmland, are there any problems? Is dusting a concern or a complaint? If a sod farm is near, do you foresee any problems with using the dried biosolids to grow sod?
A: No major objections or problems have been reported of the use of the dried biosolids product produced by the dryer. Farmers, sod farms and fruit orchards are the primary consumers of the dried product. Dust can become a minor problem if large quantities of the dried product are applied at extreme rates under dry weather conditions. The granular size of the dried product is the preferred size for application to grasses. The dried product granules readily penetrate the thatch of sods and grasses in order to become an effective nutrient source and soil amendment.
Q: What are dried biosolids used for?
A: As long as it is dried to a Class A product there are no restrictions on its use as a soil amendment. It may be used on lawns, gardens, athletic fields, golf courses, pastures, farmland, etc.
Q: Can the Class A product produced by the indirectly heated dryer be sold?
A: Yes, it can be sold and is being sold. Prices vary for the dried product from a few dollars to $30 per dry ton with the buyer supplying transportation. It has been our experience that dried sludge is a superior organic fertilizer product and an analysis of the sludge to determine its nitrogen content will set it’s value as a fertilizer.
Fire Concerns
Q: Is there any concern with fire either during drying or storage? What other safety issues are of concern?
A: Basically the dryer is a low temperature slow rotating furnace. The internal temperatures are to be maintained below the flash point of the biosolids. Water spray nozzles are installed on the discharge end of the dryer. If the temperature of the product increases above the set point, the sprinklers are activated. Once the sprinklers are activated the temperature of the product is quickly reduced. The all-steel construction of the contact surfaces of the dryer provides a fireproof enclosure. Therefore, the dryer can easily contain and withstand an internal fire if one was to occur.
Safety
Q: Does the indirectly heated dryer comply with any nationally known and industry accepted safety guidelines?
A: Yes. The indirectly heated dryer is designed and built in compliance with the National Fire Protection Association (NFPA) guidelines and the American Society of Mechanical Engineers (ASME) codes.
Q: Does the indirectly heated dryer employ a pressure relief management system to protect the dryer's equipment and the operators?
A: Yes. The indirectly heated dryer pressure relief management system is designed using the NFPA 68 guidelines. An increase in pressure will be deverted to the atmosphere and away from the operational personnel.
Other Installations
Q: How many plants do you have the indirectly heated dryer in operation?
A: Please contact our offices for a up-to-date list of projects and contacts.
Q: What is the longest running?
A: The longest running on municipal sludge is Forest City, NC. It went into full production in April 1997.