app h--evaluation of sludge dryer and dewatering facilities

TECHNICAL MEMORANDUMTo: Copies:

Western Wake Project PartnersFrom:

Western Wake Design Team FileDate:

ARCADIS/CH2M HILLSubject:

March 10, 2008

Evaluation of Sludge Dryer and Dewatering Facilities at the Western Wake Regional Water Reclamation Facility

11.1

IntroductionPurpose of Evaluation

This evaluation is a continuation of the process of determining the most appropriate solids handling process for the Western Wake regional Water Reclamation Facility (WWRWRF). ARCADIS/CH2M HILL prepared a Technical Memorandum (TM), in September 2007, along with an associated Addendum in November 2007, which compared the current design to an alternative that included primary clarifiers and anaerobic digestion. Having reviewed that TM and revisited the operational cost of their existing dryer, the Project Partners asked that biosolids dryer technology be considered as an alternative solids handling process. The Project Partners also asked that available sludge dewatering technologies be assessed to determine whether solid-bowl centrifuges as currently proposed are the most suitable dewatering equipment for the WWRWRF. The purpose of this TM is to provide design development discussion and recommendations on dewatering and drying technologies, and to provide more detailed criteria and a design data summary for the modified aerated sludge holding facility. 1.2 Background to Dryer Alternative

Several factors went into the development of the dryer alternative as an option for evaluation. It had previously been considered that the operation of a dryer was more costly than contract composting at a nearby merchant composting facility. Recent evaluations by the Project Partners demonstrated that a biosolids heat-drying facility as currently conceived provides lower operational costs than contract composting, although there is a significant capital investment to consider. The Alternative WWRWRF Design, with primary clarifiers and anaerobic digestion, had introduced the need to add metal salt and methanol to achieve the same level of biological nutrient removal as the Current WWRWRF Design. Town of Cary staff considered that the original process configuration, without primary clarifiers or anaerobic digestion, would yield the best biological nutrient removal performance for the WWRWRF.Western Wake RWRF Sludge Dryer and Dewatering Evaluation TM

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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityWith anaerobic digestion there would be a release of phosphorus and ammonia, which would be removed from the digested biosolids during dewatering and returned back to the BNR basins in the dewatering recycle stream. With the dryer alternative and no digestion, the anaerobic digesters are not required and there would be minimal to no release of phosphorus and ammonia during dewatering. The nutrients would mostly be retained in the dewatered cake and the dried product, and would not require treatment as a side stream or in the main plant. One of the principal aims of including a sludge dryer at the WWRWRF is to produce Class A biosolids per EPA requirements with the flexibility for beneficial use or disposal that brings. A primary advantage of a Class A product is that it can be land applied with very few restrictions. Under the Current and Alternative WWRWRF Designs, unclassified or Class B sludge would be composted off site at a commercial facility. In the emergency situation where it were not possible for the contractor to accept the product for composting, then the options open to the Project Partners would be land application (if Class B standards could be attained) or hauling to landfill, neither of which the Project Partners are currently prepared or equipped to handle. The dryer option can provide the Project Partners proven technology, more flexibility given the Class A product, and reduced volume of product for beneficial use or disposal. Among the factors that make the dryer alternative attractive for the Project Partners are: The Town of Cary and the Project Partners have a preference for controlling the ultimate use or disposal of biosolids, to the extent that it is practical and affordable. Dependence on outside contractors for biosolids processing places the Project Partners in a vulnerable position in the event of an emergency or change in the outside contractors business. Class A certification provides a positive public perception as compared to an unclassified or even a Class B product. The emergence of phosphorus loading as a controlling factor on potential land application sites will impact land application (whether Class A or Class B) as a potential backup plan for composting. Compost and dried biosolids are impacted to a much lesser extent by nutrient limits because those products have a considerably larger marketplace that will distribute nutrients over a much larger area. The utilization of outside contractors for biosolids disposal places an added obligation on the Project Partners to overbuild some facilities, such as aerobic digestion and dewatered cake storage processes, to accommodate unavailability on the part of an outside contractor. Reduced volume for onsite biosolids storage makes containment more practical and helps limit odor concerns. Reduced volume also minimizes hauling and disposal costs. Class A dried biosolids may be applied in locations such as forested land, parks and golf courses where dewatered cake, or liquid, digested biosolids, would not be acceptable, regardless of whether it is a Class A product. Among potential biosolids products, only heat-dried biosolids and compost are typically accepted for use on areas other than farmland. Several days inventory of Class A dried biosolids can be stored onsite in cases where they cannot be removed on a regular schedule. Safety measures to prevent re-heating of stored product are still required.

On the basis of these considerations, the dryer alternative was developed, assuming no sludge digestion other than the minimal aerobic digestion that would occur during several days of aerated sludge holding prior to dewatering (see below).

Western Wake RWRF Sludge Dryer and Dewatering Evaluation TM

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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation Facility

22.1

DefinitionsDefinition of Treatment Alternatives

In the September 2007 Evaluation TM the current and alternative WWRWRF project are defined and the components of each are briefly summarized as follows: Current WWRWRF Design - Preliminary Treatment with Odor Control, Biological Reactor Basins, Secondary Clarifiers, Effluent Filters, Ultraviolet (UV) Disinfection, Post Aeration, WAS Thickening, Aerated Sludge Holding with Partial-Stabilization, Solids Dewatering. Alternative WWRWRF Design Preliminary Treatment with Odor Control, Primary Treatment with Odor Control, Biological Reactor Basins, Secondary Clarifiers, Effluent Filters, UV Disinfection, Post Aeration, Waste Activated Sludge (WAS) Thickening and Aerated TWAS Holding, Blending Tank, Anaerobic Digestion, Digested Solids Dewatering with Odor Control. The Dryer Alternative essentially preserves the features of the Current WWRWRF Design but adds a biosolids dryer to reduce the volume for disposal and to provide a Class A product. An additional change is in the Aerated Sludge Holding tank size. For the Current WWRWRF Design, the aerated sludge holding tank provides 20-days of hydraulic residence time (HRT) to give partial stabilization. That HRT is reduced to 5 days HRT for the Dryer Alternative evaluation. For the purposes of this screening level evaluation, the dryer alternative design option for the WWRWRF is defined as follows: Primary clarifiers will not be provided. The current design for liquid treatment facilities, as described in the most recent (December 2007) Final Design deliverables, is maintained. Aerated sludge holding tank facilities will provide for a 5-day HRT at design conditions. The ultimate HRT would be determined during detailed design, but is assumed to be 5 days for the purpose of this analysis. Two dewatering alternatives will be compared: centrifuges and rotary presses. Two biosolids management alternatives will be compared: biosolids heat-drying and hauling dewatered biosolids cake to an off-site contracted composting operation. Three alternative biosolids dryer options will be considered in the analysis. The biosolids dryer would be fueled by natural gas. Truck loading and on-site dried biosolids silo storage will be provided. A design alternate will be investigated which includes additional equipment to accommodate use of dried biosolids pellets as a supplemental fuel source. Appropriate odor control and dryer emissions control facilities will be included in the alternative to handle the solids handling facilities and dryers. The Current and Alternative WWRWRF Designs will include composting via an outside contractor as the ultimate biosolids disposal process.


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation Facility2.2 Design Criteria/Facility Sizing

The design criteria for the sludge dryer options are presented in Table 1. These values were used by equipment manufacturers to base their proposals.TABLE 1 DESIGN CRITERIA PROVIDED TO VENDORS Design Criteria Dry Sludge Load (dry ton/year) Dry Solids in Sludge Cake (%) Sludge Cake Load (ton/year) Solids Mass Load (lb/day) Water Mass Load (ld/day) Operation Time (hr/year)1

Phase I 5,500 18% 30,600 30,100 137,300 6,240 90% 90

Phase II 10,530 17% 61,900 57,700 281,700 6,240 90% 90

Minimum Dry Solids in dried sludge (%) Discharge Product Temperature after product cooling (degrees F)1 Based on 24 hr/day, 5 day/wk

The above sludge quantities were based on a review of the sludge production at the South Cary WRF, which averaged 1,650 lbs/MG, and the design flows for each phase. Sludge quantities were also estimated using the process models, as shown in Table 2. These differ from the values above; the values in Table 1 are based on what the Town of Cary has found in terms of sludge loading at their existing water reclamation facilities (WRFs). The values in Table 2 are derived from the process models for the WWRWRF and predict a 10-15 percent reduction in solids generation per unit of wastewater treated, compared to Carys existing treatment facilities. This is due to predicted higher efficiencies of treatment and lower solids generation for the WWRWRF as compared to Carys two existing treatment facilities. Nevertheless, the drying facilities have been sized based on the Table 1 values because they are more conservative than Table 2. As the dryer equipment included in the evaluation was sized based on the slightly more conservative values in Table 1, there is a certain amount of contingency capacity built into the equipment sizing.


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityTABLE 2 ESTIMATED SLUDGE QUANTITIES FROM PROCESS MODELING Start Up Description Annual Average 8.3 Maximum Month 9.9 Phase I Annual Average 15.3 Maximum Month 18.0 Phase II Annual Average 25.4 Maximum Month 30.0

Flow (MGD) Thickened WAS Flow. gpd Solids Mass Load, lb/day Concentration, % Dewatered Sludge Flow. gpd Solids Mass Load, lb/day Water Mass Load, lb/day Concentration, % Dry Sludge Load (dry ton/year)

57,900 14,500 3

68,100 17,000 3

75,200 25,100 4

88,500 29,500 4

133,000 44,300 4

155,000 51,800 4

6,300 11,100 50,400 18 2,020

7,400 13,200 60,200 18 2,410

13,500 20,200 92,100 18 3,690

15,800 23,800 108,400 18 4,340

23,000 34,600 157,600 18 6,310

27,200 40,800 185,900 18 7,450

33.1

Sludge Holding and Thickening FacilityAerated Sludge Holding Tank and Aeration Equipment

The role of the Aerated Sludge Holding Tank is slightly different for each of the design scenarios. For the current design, the Aerated Sludge Holding tank provides 20 days of detention time and allows for substantial aerobic digestion of waste activated sludge. For the alternative design, the aerated holding tank primarily serves to buffer the feed rates and times to the anaerobic digester facility. For the dryer alternative, the aerated holding tank provides some minimal VSS reduction prior to drying, allows continuous wasting from the activated sludge process, and provides a storage area during times the dewatering and drying operations are inactive. For the dryer alternative, the aerated sludge holding tanks have been designed to provide 5 days detention time with one tank out of service at the design flow of 18 MGD and a solids concentration of 3.5 percent. Waste activated sludge is pumped directly from the clarifiers to the aerated sludge holding tanks, thus sludge wasting can occur continuously. The sludge will be concentrated using Rotary Drum Thickeners, described later in this TM. The contents of the aerated sludge holding tanks are mixed and aerated using jet aeration equipment, which is also designed for a solids concentration of 3.5 percent. Jet aeration equipment is preferred rather than conventional fine or course bubble diffusers for the following reasons: Jet aeration utilizes a recirculation pump for mixing of the tank contents. This provides independent control of mixing and oxygen transfer Better oxygen transfer efficiency in concentrated sludges Limited need to access the equipment or drain the basins for maintenance

The specifics of the tanks and the jet aeration equipment are shown in Table 3.


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityTABLE 3 AERATED SLUDGE HOLDING TANKS SUMMARY Description Type of Tank Number of Tanks Volume, Each Tank Dimensions, Each Tank Aeration Equipment Blower Type Number of Units Design Airflow Rate, each Recirculation Pump Type Number of Pumps Pump Capacity, each 3 11,000 gpm 3 2300 scfm Screw Centrifugal 5 3 365,000 Gallons 74 x 30 x 25 (3 freeboard) Jet Header Positive Displacement 5 Phase I (18 MGD) Phase II (30 MGD)

Aerated, Covered 5

In order to reduce odor potential, the tanks are covered and the exhaust air is sent to odor control facilities. The tanks are rectangular in shape for several reasons. Using rectangular tanks allows common wall construction, allows one wall to be common with the sludge processing building, and easily lends itself to the installation of Rotating Drum Thickeners (RDTs) on the top. The rectangular shape is also best suited to the header type jet aeration system which is proposed. The width of the tank (30 feet) matches up with the suggested optimum throw of the jet system in thickened sludges of 15 feet (each side of the header). Concrete is the preferred material for the tanks, both in terms of durability, corrosion resistance, and ease of providing wall penetrations. The oxygen demand is based on typical aerobic digestion requirements for solids concentrations of 3.5 to 4 percent solids, with 80 percent influent VSS concentration. The resulting ratio of air to tank volume is approximately 47 cfm/1000 cubic feet, compared to recommended ratios of 20 to 40 cfm/1000 cubic feet (M&E). Positive displacement blowers are recommended for this application, as the reduced size of the tank and the resulting reduced oxygen demand does not justify the single stage centrifugal blowers. The blowers are to be located on top of the tanks, under a canopy, with sound attenuating enclosures. Given the grading of the site in this area, the top of the AHT is close to ground level. This would allow for easy access for maintenance. In a typical aerobic digestion facility, the goal is to reduce the volatile suspended solids (VSS) by 38 percent in order to meet vector attraction reduction requirements. The VSS concentration can be an indicator of the odor potential in the aerobic digesters. For the dryer evaluation, however, it is not considered necessary to meet typical values for VSS reduction. The goal is only to reduce VSS and odor potential to a degree such that nuisance odors are prevented and the feed concentrations to the dryer are consistent. Figure 1 shows theoretical VSS reduction versus time assuming a 20oC digester liquid temperature, and Figure 2 shows theoretical VSS residue versus time, for conventional aerobic digestion. Note that the most rapid VSS reduction comes in the first 5 to 7 days, then the rate of reduction then decreases. Note that neither of these graphs takes into account any prior VSS reduction that may have occurred in the activated sludge process.Western Wake RWRF Sludge Dryer and Dewatering Evaluation TM

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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityFIGURE 1 - VSS REDUCTION

VSS Reduction v.s. Sludge Age in Aerobic Digester60 50 40

Volatile Solids Reduction, %

30 20 10 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Sludge Age, days


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityFIGURE 2 - VSS RESIDUE

VSS Residue v.s. Sludge Age in Aerobic Digester110 100 90 VSS Residue, % 80 70 60 50 40 30 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Sludge Age, days

The oxygen demand over time follows a similar curve. As the total VSS concentration is reduced, the overall oxygen demand is reduced (See Figure 3).


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityFIGURE 3 - PLOT OF OXYGEN DEMAND VERSUS TIME

Oxygen Requirement v.s. Sludge Age in Aerobic Digester90,000 80,000

Oxygen Requirement, lb/day

70,000 60,000 50,000

40,000 30,000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Sludge Age, days

Based on the theoretical VSS reduction, oxygen demand, and past experience, we consider 5 days storage to be adequate preceding thermal drying, provided the market for dried biosolids remains restricted to agricultural or forested application sites. If the product were to be marketed to the general public, then a longer retention time would be recommended, in order to reduce VSS and potential for odor in the product. We recommend that the tanks be operated in series, such that fresh waste activated sludge is continuously fed into a full or almost full tank. Feed to the dewatering operations is to be from the second or third tank, depending upon plant flow. Piping and valves will be provided to allow parallel operation, which may be required for maintenance reasons. 3.23.2.1

Waste Activated Sludge ThickeningWAS Thickening

Both the current and alternative designs used gravity belt thickeners (GBTs) for thickening of waste activated sludge. The GBTs were located in the solids processing building, and were sized for shift operations with a 5-day work week. For the dryer alternative, the GBTs have been replaced using Rotary Drum Thickeners (RDTs), which are to be located directly on top of the aerated sludge holding tanks. The RDT unit is fed from the aerated sludge holding tank, and acts more as a tank concentrator than a thickener. Several advantages of this strategy include: The RDTs require less operator attention, and can run on a continuous basis. This allows units of smaller hydraulic capacity to be utilized The RDTs are enclosed by stainless steel and therefore not required to be located inside a building.9/77


Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation Facility Locating the RDTs on top of the aerated sludge holding tank allows the thickened sludge to be dropped directly into the tank

The aerated sludge holding tanks are fed directly from the activated sludge process; therefore the feed sludge to the holding tank is at a lower solids concentration (thus reducing the initial oxygen demand). By concentrating a blend of fresh sludge with the existing tank contents, the overall oxygen demand is decreased. When the aerated sludge holding tanks are operated in series, the solids concentrations can be increased in succeeding tanks, since oxygen demands are decreasing. A schematic of the proposed aerated holding tanks with RDTs is shown in Figure 4.FIGURE 4 PROPOSED AHT SCHEMATIC

P

P

P

RDT

RDT

RDT

WAS

AEROBIC HOLDING

AEROBIC HOLDING

AEROBIC HOLDING TRUCK

CENTRIFUGE

A photograph of a RDT is shown in Figure 5.


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityFIGURE 5 ROTARY DRUM THICKENER PHOTOGRAPH

The specifics of the RDT application are shown in Table 4.TABLE 4 ROTARY DRUM THICKENER SUMMARY OF WWRWRF APPLICATION Description Type of Thickener Number of Units Capacity, Each Unit Feed pump type 3 300 gpm Screw Centrifugal Phase I (18 MGD) Rotary Drum 5 Phase II (30 MGD)

Ancillary equipment for the RDT installation includes feed pumps, polymer storage and feed systems, and if desired, a distribution conveyor.

44.1

Technology EvaluationsDewatering Technologies

Two different dewatering technologies were considered for the Dryer alternative rotary presses and centrifuges. The rotary press technology was introduced as a possible lower cost alternative to the centrifuge technology during follow up activities after the Value Engineering Review. Descriptions of the technologies follow. Centrifugation is widely used in the wastewater treatment industry for dewatering of waste biosolids. This is the dewatering technology currently used at the South Cary WRF. Waste sludge is conditioned using polymer or other chemicals, and is then fed at a constant rate into the rotating bowl of the centrifuge. The high speed rotation of the bowl causes the solids to settle on the bowl inner wall. A scroll or conveyor,Western Wake RWRF Sludge Dryer and Dewatering Evaluation TM

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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation Facilityrotates in the same direction as the bowl, but at a different speed. This differential speed creates torque in the solids discharge section to provide additional water removal. The differential speed also provides the conveying capability to discharge the cake. Typical values for solids concentrations of centrifuged sludge range from 15 to 30 percent. The feed solids concentrations for a thermal dryer installation range from 10 to 30 percent. The lower the percentage of solids, the more thermal energy that must be supplied by the dryer to evaporate water so the desired feed is at least greater than 15%, and preferably above 18%. A photograph of a dewatering centrifuge installation is shown in Figure 6.FIGURE 6 DEWATERING CENTRIFUGE

The rotary press is a relatively new entrant into the wastewater treatment industry. However, this technology has been widely used in the mining and pulp and paper industries. Several utilities have installed rotary presses for dewatering of municipal sludges, including the Orange Water and Sewer Authority and the Charleston (SC) Water Utility. The rotary press operates primarily by using the forces of friction to dewater sludge. Sludge is conditioned using polymer, then fed into a rectangular channel and rotated between two parallel revolving perforated screens. The filtrate passes through the screens as the flocculated sludge advances within the channel. The sludge continues to dewater as it travels around the channel, eventually forming a cake near the outlet side of the press. The frictional force of the slow moving screens, coupled with an outlet restriction, results in the extrusion of a cake. A photograph of a rotary press is shown in Figure 7.Western Wake RWRF Sludge Dryer and Dewatering Evaluation TM

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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityFIGURE 7 - ROTARY PRESS

4.2

Pilot Testing

As part of the evaluation, on site pilot tests of the rotary press technologies were performed at the North Cary WRF. Fournier Industries pilot tested their RDT unit at the North Cary WRF from December 4 until December 7, 2007. Various feed sludge ages were tested, including fully digested, partially digested, and undigested. Results of this testing are shown in Table 5.TABLE 5 FOURNIER ROTARY PRESS PILOT RESULTS Date Sludge Age, days 2-9 20-30 2-9 From Clarifier Feed Concentration, % 0.1 0.1 0.1 0.1 Capture Rate, % TSS 87.6 84.2 89.9 91.6 Filtrate TSS Concentration, % 0.25 0.33 0.23 0.05 Cake Solids Concentration, % 12.02 12.06 11.93 12.68

12/4,5 12/5,6 12/6,7 12/7

Prime Solutions, Inc. tested their rotary press at the North Cary WRF on December 11 and 12, 2007. Although less data from the test is available, the range of cake solids for these tests was from 11.8 to 14.6 percent solids.Western Wake RWRF Sludge Dryer and Dewatering Evaluation TM

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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityWaste sludge samples were also shipped to two potential centrifuge vendors for bench testing. Results of these tests are presented below. A 5-gallon sludge sample from the North Cary WRF was shipped to Andritz and received on January 3, 2008. Laboratory centrifuge testing indicates that a solids concentration of 18 percent (+/- 2 percent) can be achieved with a polymer dosage of 12 lbs/dry ton. A second sample from the North Cary WRF was shipped to Westfalia Separator. Test results from Westfalia indicate solids concentrations of 15.2 to 16.5 percent, using various polymers at reduced dosages. Onsite pilot testing of both manufacturers equipment is currently being arranged. Since the preferred feed solids concentrations for all the drying technologies being evaluated is 18 to 30 percent (although 30% may be considered by some operators to be too dry), the rotary press is not a viable technology for the WWRWRF. Therefore, it is our recommendation to proceed with the original concept of using centrifuges for waste sludge dewatering. 4.3 Sludge Drying Technologies

Heat drying technology is based on removal of water from sludge generated during wastewater treatment. Typically, dewatered sludge of 15 to 30 percent dry solids concentration (i.e. 70-85 percent water content) is delivered to the heat drying system. In the heat drying system the temperature of the sludge is raised sufficiently to evaporate and remove most of the entrained water. By removing most of the water from the sludge, heat drying results in a significant reduction in both sludge volume and mass. Material produced in the heat drying process typically has a dry solids content of 90% to 96%, with an optimum range of 92% to 95% dry solids. A variety of heat drying systems are currently offered by equipment manufacturers. Heat drying systems are generally divided into two main categories: direct and indirect. This classification is based on how the thermal energy is delivered to the sludge in the drying process. In direct heat dryers, wet sludge comes into direct contact with hot air. Typically, hot air flows through a process vessel and comes into contact with the wet sludge. The contact between the hot air and the cold sludge allows the transfer of thermal energy, which causes an increase in sludge temperature that is needed for evaporation of water. The predominant method of heat transfer in direct drying systems is convection. Examples of direct drying equipment are rotary drum dryers, flash dryers, fluidized-bed dryers, and belt dryers. In indirect heat dryers, materials with high heat-conducting capacity such as steel separate the wet sludge from the heat transfer medium (steam, hot water, or hot oil). Thermal energy is transferred from the hot transfer medium into the metal wall and then from the metal wall into the cold sludge. The sludge is heated by contact with the hot metal surfaces and never comes into contact with the heating medium. The predominant method of heat transfer is by conduction. Indirect heat drying equipment includes multiplehearth dryers, paddle dryers, and disk dryers. This dryer technology evaluation reviews alternative technologies, and compares performance and net present worth for rotary drum dryers, belt dryers, and paddle dryers. The rotary drum and belt dryers are direct-type dryers, while the paddle dryer is an indirect type.4.3.1 Dryer Evaluation Criteria

The evaluation of the biosolids dryer option for WWRWRF is based upon both cost and non-cost criteria. A comparison of estimated construction costs for several different dryer alternatives, and estimated 20year and 30-year net present value (NPV) cost incorporating operation costs for the dryer option comparedWestern Wake RWRF Sludge Dryer and Dewatering Evaluation TM

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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation Facilitywith the current design and the previous primary clarifier/anaerobic digester alternative, is provided in Section 5. In addition to construction cost and operating cost, non-cost criteria are also considered in evaluating dryer equipment. Discussion on these factors is provided in Section 4.5.4.3.2 (a) Dryer Options Rotary Drum Dryer

Direct type, rotary drum drying technology is based on evaporation of water by direct contact of material with a stream of hot air. The major components of the rotary drum dryer system are the wet cake bin, recycle bin, mixer, furnace, drying drum, air/solids separator, screen, crusher, cooler, main fan, saturator, and storage silos. A schematic for a direct type, rotary drum system is shown in Figure 8:FIGURE 8 - DIRECT TYPE, ROTARY DRUM DRYER PROCESS FLOW DIAGRAM

The dewatered cake entering the drying system passes through a wet cake bin and is then mixed with recycled dry product in the mixer to create an approximately 70 percent dry mixture. The hot air required for the heat drying process is produced in a furnace that is usually gas-fired (other types of fuel are sometimes used). The furnace produces a stream of hot air/exhaust at temperatures between 800 and 1,000 F. The evaporation process takes place in a horizontally mounted, slowly rotating drying drum. The driedWestern Wake RWRF Sludge Dryer and Dewatering Evaluation TM

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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation Facilitymaterial is conveyed through the drum where the hot air stream comes in direct contact with wet biosolids, heats the material, and evaporates the water contained in the biosolids. Dried pellets and the moistureladen air stream leave the drying drum at temperatures between 190 and 220 F. Both dried solids and hot air pass together through the air/solids separator, where solid particles fall out and separate from the air stream. Dry solids are separated into oversized, product, and undersized (fine) fractions on vibrating screens. Product size pellets (1 to 4 mm diameter pellets are typically most desirable) are cooled and then pneumatically conveyed to storage silos for storage and loading onto trucks or railroad cars. Large size pellets are crushed in the crusher, combined with fine size pellets and passed through the recycle bin into the mixer for mixing with dewatered cake. The hot, moisture-laden air is forced by main fan into the saturator, where the air is cooled and water vapor condensed by a counter-current flow of hot process air and cold cooling water. Plant re-use water can be used as the cooling water, and the condensate is returned to treatment. In the case of Western Wake, condensate would pass to the filtrate/centrate pump station. Most of the cooled air is recycled back to the drying drum, while approximately 10 to 30 percent of the flow is treated in the air emissions control system and discharged to the atmosphere. The direct type, rotary drum drying system is generally capable of producing well-sorted, semi-round pellets with approximate size of 1 to 4 mm. Systems with full product recycle and mixing produce pellets with hardness and dust characteristics that are typically better than those produced by other heat drying technologies. Direct dried product rarely requires additional processing to make it marketable. Direct type, rotary drum dryers are the most prevalent type of dryer found in United States, with more facilities in operation drying biosolids than any other type of dryer. They are available from Andritz, Berlie, NEFCO, and Siemens (formerly Sernagiotto). A photograph of the rotary drum installation at the South Cary WRF (Andritz Model DDS 40) is shown in Figure 9.FIGURE 9 ROTARY DRUM DRYER


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation Facility(b) Belt Dryer

Belt dryers use direct contact of circulating hot air with wet sludge that is pumped onto and conveyed by a slowly moving horizontal belt enclosed in a metal enclosure. The wet material moves through several drying chambers, where the moisture is released into the circulating air. After passing through the drying chambers, the dried cake falls off from the belt onto a hopper and is conveyed to a loading or storage facility. Each drying zone has its own circulating fans and air temperature control. Excess moisture is removed from the air stream in a saturator. Heat for the air circulation loop in each zone is provided in a heat exchanger by indirect contact with steam, hot water, thermal oil, or hot air serving as the heat source. The drying temperatures are controlled at approximately 300F at the belt entry and at 210F at the belt discharge. The sludge is heated to approximately 170F in its dried state. The lower drying temperature is claimed to produce a less odorous exhaust stream, and the drying process is less prone to accidental combustion than rotary drum dryers, which operate at much higher temperatures. Dried material produced by the dryer is composed of larger fragments, non-uniform in shape, with sizes between 1 and 10 mm. A pelletizer must be added if smaller pellets of uniform size are desired. Since the sludge is not excessively moved in this system, dust formation is reduced in the dryer itself, although dust may form in subsequent handling of the dried product. Belt dryers are available from Andritz, Kruger, and Huber. Kruger belt dryers were first installed in Europe in 1995, and Andritzs first was installed in 2002. At present there are 15 Kruger and 20 Andritz belt dryers either in operation or development worldwide. Of these Kruger has 4 US installations, and Andritz has no US belt dryer installation. A schematic of the belt dryer technology is shown in Figure 10.FIGURE 10 - DIRECT TYPE, BELT DRYER PROCESS FLOW DIAGRAM Dewatered Biosolids Cake Feed Hopper Recycled Dried Solids

Biosolids Mixer / Conveyor Dried Biosolids

Biosolids Belt Dryer Hot Air

Product Screen

Recycle Air Natural Gas Air Heat Exchanger Air Exhaust to Air Pollution Control Condenser Cooling Water Condensate

Product Cooler

Storage Silo

Dried Biosolids


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityA photograph of a belt dryer (Andritz BDS 0.75) is shown in Figure 11.FIGURE 11 BELT DRYER

(c)

Paddle Dryer

Paddle-type heat dryers are based on raising the temperature of dried biosolids above 220F by contact with hot metal surfaces. The evaporation process takes place within an enclosed, insulated vessel that is usually mounted in a horizontal position. The vessel is fitted with two rotating shafts fitted with selfcleaning paddles that rotate in the opposite direction, much like in a pug mill mixer. Shaft and paddles are internally heated by thermal oil or steam circulating through their hollow interior and allow transfer of heat energy from steam to sludge by conduction. Rotation of shafts provides for good mixing and thorough contact of sludge with heated metal surfaces. Wet sludge enters the dryer at one end and dried material exits at the other end. Evaporated moisture is exhausted from the dryer and there is headspace above the drying screws is to provide a plenum to allow the evaporated moisture to be absorbed by the exhaust air and removed. Paddle dryers have high thermal efficiency and produce less odorous air than direct dryers. Due to continuous break up of the dried material by the paddles in the vessel, paddle-dryers produce pellets of a smaller size, and a higher potential for dust formation in the product. Nara Machinery of Japan was the original patent holder and developer of the paddle dryer. KomlineSanderson acquired the rights to market the paddle dryer in North America and has about 25 industrial paddle dryers and 12 municipal paddle dryers in operation at present. The same paddle dryer technology is being applied by GMF-Gouda of the Netherlands, who have 30 installations worldwide, but none in North America at present. Both Komline-Sanderson and GMF-Gouda have continued to refine their respective versions of the paddle dryer, so there are various differences between the two even though they both operate using the same basic technology. GMF-Gouda is beginning to market its paddle dryer as a competitor to Komline-Sanderson in the US. A photograph of a paddle dryer (Komline-Sanderson Model 13W 2000) is shown in Figure 12.Western Wake RWRF Sludge Dryer and Dewatering Evaluation TM

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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityFIGURE 12 PADDLE DRYER

4.3.3

Vendor Information

Three representative manufacturers of each type of dryer (Kruger, Andritz, and Komline-Sanderson) have provided information on their dryer equipment and this is summarized in the following table and sections. Table 6 presents the vendor provided sizing and equipment summary for Phase I requirements. Where information for Phase II has been provided that will be summarized under the discussion of each vendors proposal.


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityTABLE 6 VENDOR INFORMATION SUMMARY (FOR PHASE I) Description Design Data Sludge Load (dry ton/year) Dry Solids is sludge cake (%) Sludge cake load (wet ton/year) Vendor Data Number of Trains Model Drying System, Design Evaporative Rate (lb water/hr) Drying System, Evaporative Capacity 4 (lb water/hr) Required Evaporative Energy (BTU/lb 5 water evaporated) Drying System, Sludge Cake Load (lb/hr) Operating Time (hr/yr)2 Dry Solids in Dry Sludge (%) Dried Sludge (lb/hr) Fuel Consumption (MMbtu/hr)1 Natural Gas Utilization (million cf/yr) Electrical Load (kW)7 Vendor Quoted Cost ($m)31 2 3 4 5 1 MMbtu/hr = 1,000,000 btu/hr Operating time based on 5 days per week at 24 hours per day. Kruger based on 52 weeks, others on 51 weeks. The difference in total hours does not affect the economic analysis. Andritz Belt cost provided excludes concrete tank (included in capital cost estimates). Komline evaporative load capacity based on 15% safety factor These values were provided by the vendors as being representative of values they would be expected to provide as a guarantee with their bids. In the economic analyses all have been assumed to be 1500 BTU/lb water evaporated. A minimum of 92% solids would be required with vendor bids and guarantees. Electrical load values were provided by the vendors and would need to be evaluated more closely when provided with bids. When corrected for 1,500 BTU/lb water evaporated, 1,000 BTU/cf for NG, and 92% dry solids in product, these values all become 73.8 million cf/yr8

Kruger Belt

Andritz Belt

Andritz Drum

KomlineSanderson Paddle

5,500 18 30,556

5,500 18 30,556

5,500 18 30,556

5,500 18 30,556

2 DR1500SAZN 7,835 8,260 1,530 9,793 6,240 906

1 BDS 4.0 8,034 8,800 1,400 9,988 6,120 92 1,954 11.25 68.8 330 $5.7 m

1 DDS 40 8,034 8,800 1,500 9,988 6,120 92 1,954 12.05 73.8 328 $5.7 m

1 13W-2200 7,900 9,080 1,440 9,815 6,120 92 1,920 11.3 69.4 200 $4.1 m

1,959 12 74.8 184 $5.0m

6 7 8


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Technical Memorandum Evaluation of Sludge Drying and Dewatering Facilities at the Western Wake Water Reclamation FacilityTable 7 summarizes the main features of the different dryers, based on the information provided by the vendors.TABLE 7 DRYER FEATURE COMPARISON Feature Installations Kruger Belt Approx. 15 worldwide including 11 in Europe and 4 in U.S. (Operational or in development)st 1 in world 1995

Andritz Belt Dryer 20 in Europe. None in US. (Operational or in development)st 1 in world 2002

Andritz Rotary Drum 70 worldwide including 21 in US (operational or in development) 1 in world 1974 (Switzerland) 1st in US 1995st

Komline-Sanderson Paddle Approx 78 industrial and municipal worldwide 1 in world early 1970s (Japan, by Nara Machinery) 1 US municipal 1992 8 hour, 50 cu yd Live bottom bin Progressive Cavity Pump for even feedst st

1st in US 2006 Cake Storage 24 hour cake bin 4 hours. Live bottom bin Screw and distribution coil

4 hours. Live bottom bin Mix of wet cake and dried product. Dosing screw with variable speed drive Natural Gas Direct Heated Air

Cake Feed

Progressive cavity pump to oscillating nozzle depositors Natural Gas Direct Heated Air

Energy Source Heat Source

Natural Gas Direct Heated Air

Natural Gas Indirect - Thermal Oil (High pressure steam also an option)

Operating Temperature

350 F zone and 210 F zone over belt

Feed temp. 250 300 F 265 F over belt

Feed temp 800 o 900 F Exit air at 195-205 F Vibrating screen Under-sized and over-sized particle (crushed) as all/part of recycle to obtain non-sticky feed Parallel plate, noncontact heat exchanger using plant effluent. Cools product to 90 100 F

Oil at 350 400 F Product 250 260 F at discharge Vibrating screen Particles > to rolloff container; Particles < 0.5mm returned to dryer Two coolers, one before and one after product handling. Cools product to 120 F Granular. 0.5 mm to

Product Classification Product Recycle

None No

None Yes, for granulation Approx 50% recycled

Product Cooling

Dried product cools within dilute phase transport system

Cooled using ambient air. Cools product to o