SELECTING THE BEST DEWATERING TECHNOLOGY FOR A CHALLENGING ASH SLURRY

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1 SELECTING THE BEST DEWATERING TECHNOLOGY FOR A CHALLENGING ASH SLURRY Elijah Williams* and Don Howard City of Greensboro, Water Resources Department Christopher Crotwell, Mary Knosby, and Will Shull HDR *Elijah Williams, PE, City of Greensboro, Operations Engineer, Water Resources Dept, 2602 S. Elm-Eugene Street, PO Box 3136, Greensboro, NC 27402, , Elijah.Williams@greensboro-nc.gov Abstract The City of Greensboro s TZ Osborne (TZO) Water Reclamation Facility (WRF) is rated to treat 40 mgd and has been in service since Dewatered WAS and primary sludge are sent to a fluidized bed incinerator. The exhaust from the incinerator is quenched with plant water and an ash slurry is produced. The ash slurry is currently sent to a thickening clarifier and then to a belt filter press for dewatering before hauling to a landfill for ultimate disposal. The existing belt filter press is reaching the end of its useful life and requires frequent maintenance. The City wanted to evaluate alternatives to dewater the ash slurry which would produce equal or higher cake percent solids and also require less maintenance. Several technologies were evaluated, including belt filter press, plate and frame press, dewatering screw, screw press, centrifuge, rotary fan press, and dewatering bin. The fine and abrasive characteristics of TZO s ash presented challenges for several technologies based on the sieve analysis and bench top testing. Ultimately the rotary fan press and the screw press were evaluated further and pilot tests were performed on-site. This presentation will walk through the challenges of dewatering ash slurry, review current dewatering technologies, present both bench top and pilot results from various manufacturers, as well as present the final design criteria. This project is anticipated to be in construction by spring Introduction The City of Greensboro s (City) T. Z. Osborne (TZO) Water Reclamation Facility (WRF) is rated for 40 mgd and has been in-service since TZO utilizes a fluidized bed incinerator to dispose of municipal dewatered sludge. An overview of the existing ash dewatering process, summarized in Figure 1, consists of pumping ash slurry to an ash clarifier for thickening followed by pumping the thickened ash to a belt filter press for dewatering. Figure 2 emphasizes the influent and effluent to the existing belt filter press and includes a mass balance of the process. Flow to the ash dewatering equipment would be a maximum of 47 gpm (gravity flow) with a maximum solids loading rate of 1,900 lb/hr. Currently, the raw feed to ash clarifier is approximately 0.38% solids, the percent solids from the ash clarifier to the belt press is approximately 8%, and the cake solids from the belt press is approximately 47%. Table 1, below, shows a 7 day range of data which includes the flow rate, percent solids, wet lbs/hr, and dry lbs/hr from the ash clarifier to the belt filter press. Flow rate data was recorded at the same time as a sample was taken. The data in Table 1 shows that on average the influent solids to the belt filter press are 4.2% but that it can fluctuate between 2.0% to 9.6%. Table 1. Ash Slurry Characteristics Date % Solids Flow (gpm) Wet lbs/hr Dry lbs/hr 3/25/ % 46 23, % 46 23,

2 Table 1. Ash Slurry Characteristics Date % Solids Flow (gpm) Wet lbs/hr Dry lbs/hr 3/26/2013 3/27/ % 40 20,016 1, % 47 23, % 38 19, % 37 18,515 1,448 3/28/ % 49 24,520 1,165 3/29/ % 47 23,519 1,077 3/30/ % 46 23, /1/ % 44 22, AVERAGE 4.23% 44 20, The dry ash is described as a talcum powder-like material. A sieve analysis was performed to obtain a characterization of the ash. The ash slurry itself is more challenging to dewater than most fluidized bed municipal incinerator ash, which might be a result of the finer grade sand, which was put into service in recent years. This sand is smaller in size than typically used in incinerators and has resulted in less being required. Plant staff has reported that 200 lbs of sand are fed into the incinerator per month. Additionally, the sieve analysis shows 98% of solids were below 2mm in diameter. The City also built a pilot scale dewatering inclined screw that produced decent results for short periods of time. However, over time the ash slurry would tend to form a dam and not allow water to drain back down the screw. This would lead to wetter cake being discharged. Review of Dewatering Technologies An initial screening of the available dewatering technologies was performed to reduce the number of alternatives selected for detailed evaluation. Table 2, at the end of this section, summarizes the advantages and disadvantages associated with each alternative, followed by a conclusion regarding further evaluation. Manufacturers were contacted to obtain information regarding each technology. Samples of the ash slurry were also sent to each manufacturer for bench top analysis to determine the expected cake percent solids and the amount and type of polymer required. Each type of dewatering equipment was evaluated based on the manufacturer s information/analysis, engineer s experience, and Owner s input to arrive at a recommendation for solids dewatering. All of the alternatives except the plate and frame press require polymer addition to adequately dewater the solids. A description of the various types of dewatering equipment and manufacturer s comments are summarized below. A summary of the sample analysis results for each technology can be found in Table 2. Screw Press Screw presses have been used for dewatering for over 20 years. A screw press consists of a helical screw mounted on a shaft inside an enclosed tube made of perforated screen. The screw rotates to move sludge from the inlet end of the screw to the outlet. The screw volume between flights decreases towards the outlet of the screw. This forces greater and greater pressure on the sludge as it approaches the outlet. As the pressure builds, water is forced out of the sludge. There are several variations on this basic design. Some manufacturers rotate the perforated tube while the screw remains still. Some screws are mounted horizontal while others are at an angle. FKC has an option to allow steam into the screw press. This increases the cake dryness but is typically used if there is 2

3 already steam available. Screw presses are sturdy, slow rotating machines that generate solids nearly as high as centrifuges. Typically units have a long and narrow footprint. Benefits of the screw press are: Relatively high solids Fairly rugged equipment Low water usage Centrifuge Centrifuges are a proven dewatering technology in the wastewater industry. The sludge is pumped into the unit which uses centrifugal forces from the high speed machine to separate the solids. Like screw presses, centrifuges typically have a long and narrow footprint. Centrifuges have the following benefits: Higher solids. Limited water usage. Reduced installation costs. Centrifuges are high speed machines and can require specialized maintenance which on-site staff may not be qualified to perform. Grit in sludge can result in premature wear to centrifuges. Belt Filter Press Belt filter presses are also a well know dewatering technology and the City currently uses a belt filter press for ash dewatering. Belt filter presses have three zones; gravity zone, wedge zone, and pressure zone. The sludge is pumped on to the top of the gravity section of the press which drains all free water away and thickens the sludge. Then the sludge travels around the end roller into the wedge zone where it is squeezed between two rollers, and then into the pressure zone where it gets squeezed a second time between two belts. Belt filter presses have the following benefits: Performance of presses are simple to monitor Operator friendly, since the top deck and discharge of the unit are visible Recessed Chamber Filter Press The recessed chamber filter press (filter press) is also known as a plate and frame press. The filter press is a well proven dewatering technology, which unlike other technologies, is a batch process. The filter press equipment consists of several plate pairs which are secured by two shafts on either side of the plate. A hydraulic piston is used to apply force to the plates to keep sludge from leaking out between the plates. Each pair fits tightly together with a gasket. There is a recessed cavity or chamber between the two plates in which sludge is pumped at the beginning of the batch process. Each rectangular plate is fitted with a filter cloth or a diaphragm that allows for water to pass through while trapping solids. Dewatering occurs by applying pressure to the sludge from the sludge feed pumps. This pressure is applied and maintained for a specific time period (typically around 2 hours) during which water is forced out of the sludge inside each cavity. At the end of the cycle, the sludge feed pump is stopped, the plates are opened and the dewatered cake falls out below the filter press. A conveyor can be used to transfer the cake or a roll off storage bin can be located directly under the press. Filter presses also have a long and narrow footprint. Advantages to the filter process include a very dry cake solids product and the ability to handle grit in the sludge. Rotary Fan Press The rotary fan press is a relatively new dewatering technology. The equipment consists of rectangular channels around the outside of a wheel. The sidewalls of the channel rotate. The sidewalls are made of 3

4 fine screen elements that allow liquid through while retaining the solids. As it moves, the wheel compresses and transports the sludge. Similar to the filter press, the sludge feed pump applies pressure to the sludge; however, at a much lower pressure (2-7 psi). A restricted outlet at the end of the channel applies back pressure to the sludge with a pneumatic piston. The force on the restricted outlet controls the dryness of the cake. As the channel rotates, sludge is forced towards the outlet by way of friction from the sides of the channel and, thereby, force water out through the screen as the volume decreases. A rotary fan press requires minimal wash water, has a small footprint, low rpm, lower power cost, and some redundancy within the unit. The redundancy is due to there being multiple channels on each unit. Each channel can be taken out of service if required while the other channels are still in service. Dewatering Bin The dewatering bin is a dewatering technology that has been used for dewatering ash slurry from coal plants and other industries. The dewatering bin is a batch process that consists of filling the bin and allowing for the sludge to settle at the bottom. There is an outlet at the center of the bin and also on the outside of the tank at the bottom. These outlets are perforated to allow for water to be removed. After the bin is filled, slurry is continuously pumped into the bin. Once the blanket of slurry reaches a certain height in the bin, the pump stops providing flow to the bin to prevent solids carry over. At this point a second bin could be provided to accept additional flow. The first bin would allow for water to filter out the outlets for a period of time (around 8 hours) while the slurry drains. After the cake has reached its desired level of dryness, a bottom gate valve is opened allowing the cake to fall into a truck bed below for disposal at a landfill. Alternative Table 2. Summary of Dewatering Technologies Development Advantages Disadvantages Stage Centrifuge Well-proven Becoming more common for medium to large WWTPs. Drier cake solids than most options. Small footprint. Minimal wash water required and therefore smaller recycle stream. Belt-filter Press Filter Press (Plate-and- Frame) Rotary Fan Press Screw Press Well-proven Well-proven Somewhat Proven Somewhat Proven Most common option in existing plants. Easy to view solids during dewatering process. Can handle grit in sludge. Lower power cost. Drier cake solids than most options. Can handle grit in sludge. Minimal wash water required (intermittent) Slow speed Small footprint Cake solids nearly as dry as centrifuge. Minimal wash water. Sturdy, reliable equipment. Can not handle high grit sludge. Slightly higher equipment costs. Higher power costs. Can be more difficult to maintain for local maintenance staff Continuous wash water is required which generates recycle stream Dewatered solids not as dry as centrifuge Maintenance intensive High equipment cost. Labor intensive process with batch operation High O&M cost Cake solids generally not as high as centrifuge, but drier than belt filter press. High sand content in sludge can lead to premature wear of the screens inside channels. Long narrow footprint 4

5 Alternative Dewatering Bin Notes: 1. EPA Report on Emerging Management Technologies (2006) 2. Wastewater Treatment & Reuse (2004); WEFTEC 08 proceedings 3. Riedel, D: An Investigation into the Mechanisms of Sludge Reduction Technologies (2009) Sampling Results Several different manufacturers of the technologies listed in Table 2 requested a sample of the ash sludge for bench scale testing. The ash sludge samples were collected according to each manufacturer s instructions and then shipped for testing. Table 3 summarizes the results of the ash samples that were tested. The results of the bench scale tests show that the percent solids vary. Based on the lab tests all technologies could achieve a capture rate of 90% or higher, and four of the technologies were able to obtain greater than 50% cake dryness. The polymer required for each technology varied from none to 7 dry lbs/ton. Belt Filter Press 1 BDP Industries Recommend -ed Model Table 3. Sample Results Total Solids (% by weight) ph 1.5m 3DP Polymer Used Polymer name not provided Polymer Dose (lb/ton) 4-7 Cake Solids (%SBW ) 45% - 50% Capture (%) 2 Andritz Power Press FS % 95% 3 Phoenix WXG Centri fuge 4 Centrisys CS Recessed Chamber Filter Press (Plate and Frame Type) 5 Andritz 6 M.W. Watermark Screw Press 7 FKC Model 1000/LP Polydyne C-6257 None - polymer 95% % 90% % - 70% 98% % - - None n/a 48% 98% 1500MM None n/a 71% 99% Table 2. Summary of Dewatering Technologies Development Advantages Disadvantages Stage Well-proven Simple operation Batch process Low maintenance Finer particles are not collected Experience with coal ash; limited experience with municipal sludge incinerated ash Technology Manufacturer SHX- 800x4500L Polydyne C % 92% 5

6 Table 3. Sample Results Technology Manufacturer Recommend -ed Model Total Solids (% by weight) ph Polymer Used Polymer Dose (lb/ton) Cake Solids (%SBW ) Capture (%) Rotary Fan Press 8 Fournier 6-900/6000CVP Ashland K274FLX % 95% Evaluation of Design of Dewatering Alternatives A proposal submitted by each manufacturer listed in Table 3 was reviewed based on the sampling results, the required space at the WRF, the City s comfort level with the technologies, and equipment capital cost. The plate and frame technology was found to be heavily maintenance intensive, not fully automated, and would not fit into the proposed equipment location. The plate and frame press is also a batch process, which could potentially complicate the incinerator process because it operates continuously. The centrifuge was able to obtain dry cake and had a high capture rate; however it was eliminated due to the high speed at which the bowl turns. The mildly abrasive characteristics of the ash slurry would not be a good application for a centrifuge. The capital cost of the equipment was also the highest. The City currently operates a belt filter press (BFP) and is familiar with the frequent and costly maintenance required. Additionally, the large footprint of the BFP makes it difficult to fit in the desired location while still have room to maneuver and perform maintenance. A sample was sent to the dewatering bin manufacturer. Due to the small size of some of the finer particles, this technology would not work for Greensboro s application. The finer particles would not be captured in the bin and would end up at the head of the WWTP. After the preliminary evaluation it was decided to eliminate the BFP, plate and frame press, and centrifuge technologies because of the reasons listed above leaving the remaining two technologies the be further evaluated: Alternative No. 1 - Screw Press (FKC) Alternative No. 2 Rotary Fan Press (Fournier) To further evaluate these technologies a site visit to witness both technologies in operation and speak to appropriate O&M personnel was completed. Following the site visits, both technologies were further evaluated in respect to on-site pilot results, life cycle cost, and how the equipment would be integrated into the existing incinerator building. Pilot Testing and Results FKC and Fournier were invited to run a pilot test at the TZ Osborne WRF. The pilot would test the effectiveness of each technology on dewatering the incinerator ash sludge and utilizing the City s existing polymer. FKC Screw Press The FKC pilot trailer arrived on site February 4, 2013 and stayed until February 7, Initial testing revealed problems conditioning the sludge, which lead to a poor capture rate. One issue was that the 6

7 flocculation chamber cascaded into the screw press and this turbulence appeared to break apart the floc. Several polymers, including the WRF s current polymer, were used to optimize conditioning. A pilot result summary report of the first site visit, including cake dryness and capture rate, was not provided by FKC. The pilot operator decided to take back a sample of the sludge to do more testing at FKC s lab, where more polymers were available, to develop a new strategy. FKC returned the week of March 11th through 15th. They were then able to produce a cake with 50%+ solids. However, to achieve these results required using two different polymers in combination. The amount of new anionic polymer and current cationic polymer needed to achieve these results were 1.3 lbs/dry ton and 6.48 lbs/dry ton respectively. One test was performed with the City s polymer. During this test the screw press was able to produce a cake with 48% solids and a capture rate of 85% by using 6.48 lbs/dry ton of polymer. Rotary Fan Press The Fournier pilot trailer arrived at the WRF on March 1, 2013 and stayed until March 6, The unit was able to produce visibly dry cake within 30 minutes of setting up. The pilot operator used a different polymer the first day and half of the second day. The remaining trials were conducted using the WRF s current polymer. The most favorable results were achieved using the WRF s current polymer. The results include a capture rate of 86% and a cake dryness of 47%+. The amount of polymer needed to attain these results on average was 3 lbs/dry ton. Pilot Testing Summary Both pilot tests showed significantly lower capture rate and cake dryness than in the bench top lab tests although the same amount of polymer was used. Both manufacturers reinforced that the unique consistency of the ash led to the decrease in performance. Economic Analysis Table 4 lists the economic data and assumptions used to perform the economic analysis. As shown in Table 5, the Fournier Press has a lower energy usage and polymer usage than FKC, but it also has more maintenance cost, slightly higher capital cost, and the cake dryness is 3% less than FKC. A 20-year net present value (NPV) cost showed there Fournier to be approximately $276,622 higher than the FKC screw press. Table 4. Economic Assumptions Parameter Value Hauling Cost (tipping fee) $38/ton Power Cost $0.066/kWhr Neat Polymer Cost (Polydyne) $0.90/lb Discount Rate 4.5% Operation 24/7 Table 5. Equipment Data Assumption Screw Press Rotary Fan Press Total Hp 17.5 hp 4 hp Polymer Usage 8 3 Capture Rate 86% 86% Cake %Total solids (TS) Annualized O&M Costs $3,000 (1) $30,000 (2) 7

8 Table 5. Equipment Data Assumption Screw Press Rotary Fan Press Equipment Cost $425,000 $472, year NPV $8.3 million (3) $8.6 million Notes: (1) O&M costs include replacement of wear plates every 5 years, screens every 5 years and labor for these replacements. (2) O&M costs include replacement of blades and deflectors every 9 months and replacement of screens every 3 years. (3) Does not include costs associated with new polymer/polymer system (pumps, pipe, mixing equipment, polymer trials, cost of new polymer) Conclusions and Recommendations The City decided to select the Fournier rotary fan press technology to replace the belt filter press for incinerator ash dewatering based on the advantages listed above. The FKC screw press did not produce consistent results during the pilot testing, did not provide multiple results with the City s existing polymer and, due to the screw press s size, does not allow for future installation and does not have any redundancy. Additionally, due to the higher height of the screw press, it presented a challenging installation in the existing room. 8

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