Closing the Bottom Ash Loop Pilot Testing Treatment and Reuse for FGD Makeup

Transcription

1 Closing the Bottom Ash Loop Pilot Testing Treatment and Reuse for FGD Makeup CHAD ROBY, P.E., BCEE CH2M Columbus, OH ROBERT MUEHLENKAMP, P.E. We Energies Milwaukee, WI THOMAS E. HIGGINS, Ph.D., P.E. CH2M St. Augustine, FL

2 KEY WORDS: Bottom Ash Transport Water, FGD Makeup, Closed-loop Bottom Ash System, Pilot Testing ABSTRACT The 2015 Steam Electric Effluent Limitation Guidelines (ELGs) ban discharge of ash transport water. Many power stations will continue to use their wet-sluicing bottom ash systems in a closed-loop operation. A purge from the system will likely be necessary. The ELGs allow use of transport water for FGD makeup water. The industry has not yet defined what impact this would have. This study included conducting bench-scale testing followed by pilot testing to determine what impact ash transport water would have on FGD operations.

3 INTRODUCTION The U.S. Environmental Protection Agency (EPA) issued Effluent Limitations Guidelines and Standards for the Steam Electric Power Generating Point Source Category (ELGs) in November The ELG update bans the discharge of ash transport water. The ELGs define ash transport water as [w]astewater that is used to convey fly ash, bottom ash, or economizer ash from the ash collection or storage equipment, or boiler, and has direct contact with the ash. The ELGs included an anti-circumvention provision that prohibits sending bottom ash transport water to a process that eventually discharges to a permitted outfall, with the exception of sending to a flue gas desulfurization (FGD) system. Compliance with the no-discharge of bottom ash transport water requirement can be met through either installing dry ash handling systems or operating closed-loop systems. This paper focuses on managing a closed-loop system. There are generally two aspects to developing a closed-loop bottom ash management system beyond the actual selection of the equipment in which the bottom ash will be settled out of the transport water. These aspects are: Closing the water balance (balancing flows into and out of the loop) Maintaining the closed-loop water quality to avoid operational issues Closing the water balance consists of determining what waters would be included in the closedloop system. Water streams flowing into the ash transport loop include process waters (transport and non-transport), makeup water, and in some cases, stormwater. Water streams exiting the system are primarily from water entrained in the dewatered solids, but also include evaporation losses and, if used, purge streams. Plants with existing dewatering systems may need to make modifications to close the water balance. These may include limiting fresh waters into the closed loop, accounting for water loss from dewatered ash (prior to landfilling), changing equipment operations, and building in storage capacity for excess water. Operating a closed-loop water balance creates numerous water quality concerns, such as build-up of solids in the system and abrasion, scaling, and corrosion. Some solids are not removed in ash settling systems, carrying over into their effluent and subsequently into the recirculated water supply. These are the small and/or less-dense particles that are more difficult to remove by settling during the dewatering process. Power plants that have gone to closed-loop bottom ash management have reported problems, such as plugging of pipes, nozzles, and other equipment, related to solids carryover during some point of operation. These solids also accumulate in sumps, tanks, and other wide spots in the recirculated water loop. Another potential problem with solids carryover is abrasion of pipes, valves, pumps, and in-line instruments, which could lead to extra maintenance. Scaling refers to the formation of precipitates on the wetted surfaces of pipes, pumps, tanks, and other equipment. It occurs due to the changes in the concentrations of ions that, combined with the recirculated water conditions such as temperature and ph, result in a compound s solubility being exceeded resulting in solids formation. Closing the water balance and management of closed-loop water quality drives the need to operate a purge from a closed-loop bottom ash system; however, the purge cannot be discharged due to the ELG ban, necessitating an alternative disposal method. Our experience has shown that

4 the prime disposal alternative for ash transport water is for use as FGD makeup. Other alternatives are to use the ash transport water for fly ash conditioning or to send to an available deep well injection facility. Little experience exists to determine the impact of using bottom ash transport water in an FGD system has on FGD operations and gypsum quality. GOALS AND OBJECTIVES The primary objective of this study was to evaluate the impact to FGD operations and gypsum quality if a purge from the plant s bottom ash transport water (BATW) was sent to the FGD system. A secondary objective was to evaluate treatment (ph adjustment and filtration) requirements needed to send a purge to the FGD system. The concern of sending BATW to the FGD system is the potential impact of ash solids and aluminum, which, at elevated concentrations present in the BATW closed loop, could cause excessive foaming or interfere with gypsum formation and quality. The evaluation began by conducting bench-scale testing to understand removal of aluminum by adjusting ph and determining the amount of acid needed for ph adjustment. This was followed by a pilot test of bottom ash purge water treatment and transfer to the FGD system for use as makeup. The goals of this pilot test were to evaluate the following: Impacts to FGD operations Impacts to gypsum quality ph adjustment of the closed-loop BATW for aluminum precipitation/removal. o Aluminum is a parameter of concern because it may negatively impact gypsum formation in the scrubber. o The closed-loop system currently uses sulfuric acid to target a ph of 8.0 standard units (s.u.). During pilot testing, the ph target was 6.5 to 7.0 to promote aluminum precipitation. Filtration requirements (optimal filter pore size and solids loading) to pretreat BATW for ash and aluminum removal prior to the FGD scrubber. CURRENT BOTTOM ASH OPERATIONS We Energies operates a two-unit coal-fired power plant in Pleasant Prairie, Wisconsin. Each unit is fueled with Powder River Basin (PRB) sub-bituminous coal and generates megawatts (gross) and 595 megawatts (net) of electricity. The Pleasant Prairie Power Plant (P4) does not discharge fly ash transport water. It currently does discharge BATW, so changes will be required to comply with the ELG. Bottom ash is collected in ash hoppers and is sluiced to a remote location. The BATW is treated in two hydrobins followed by a settling tank and then a surge tank, as shown in Figures 1 and 2. The treated ash transport water is recirculated back to the bottom ash transport system for reuse. Reusing water for ash transport will increase the concentration of solids (suspended and dissolved) in the water, which could lead to abrasion, scaling, and/or corrosion. As such, a purge from the system is needed. The P4 system purges from the closed-loop system to maintain a conductivity of 1,500 micromhos per centimeter.

5 Purge flow depends on the makeup source used for the BATW system. We Energies determined that 150 gpm is needed when using lake water as makeup. If using cooling tower blowdown, additional purge flow may occasionally be needed. The system is purged by opening a valve on the low-pressure ash system (return to ash hoppers) to direct water to an existing drain. The purge is then routed to the Low Volume Waste Basin for treatment. Hydrobins (2) Surge Tank (1) (Not visible) Settling Tank (1) Figure 1: Photograph of Existing BATW Treatment System Figure 2: Block Process Flow Diagram for Existing BATW Treatment System

6 Log C IWC WATER QUALITY DRIVERS The primary objective of this study is to determine what impacts BATW has on FGD operations, with a secondary goal of understanding treatment requirements. P4 gypsum is beneficially reused and therefore must meet specifications, which require moisture to be less than 10 percent and gypsum purity to be greater than 95 percent. It is also important to prevent discoloration of the gypsum. Gypsum crystallization and FGD operational setpoints directly affect how well gypsum will dewater. Kruger, 2001 and others have studied the effect of ionic contaminants, including aluminum. Aluminum has been shown to retard the crystallization process and change the shape of the crystals. This is partially due to aluminum-fluoride blinding, which is the aluminumfluoride complex blocking limestone from dissolving. This condition has previously been attributed to high concentrations of fly ash entering the scrubber. Limestone impurities can also contribute to ionic contaminants, including aluminum. Since aluminum can impact gypsum crystallization, it is important to understand the chemistry and how aluminum can be removed. Aluminum solubility under various conditions is well documented in many studies. Actual solubility will depend on the quality of a specific water, but it generally follows the solubility curves presented in Figure 3. The dominant aluminum species present will depend on the total aluminum concentration and ph of the water. Aluminum will primarily come out of solution and precipitate in a ph range of about 6 to 8. Figure 3. Aluminum Solubility Curves (Holt, 2002) An important consideration is what levels of aluminum are present in the makeup to the FGD, in the FGD absorber, and in the BATW. The average aluminum (total) in the makeup (Lake Michigan water) varies, but available data suggest ranges from milligrams per liter (mg/l) to mg/l. The average aluminum concentration in the BATW is 8.5 mg/l and ranges from 6.5 to 11.0 mg/l. Elevated aluminum data in the BATW made it a potential water quality concern. Operating levels of aluminum within the absorbers was measured for comparison purposes. FGD absorber aluminum levels are shown in Table 1 and represent aluminum concentration in the absorber during pilot testing.

7 Table 1: Absorber Liquor Aluminum Levels During Pilot Testing FGD Receiving Absorber Unit Sample Date Aluminum, Total (mg/l) Aluminum, Total (lb) a Aluminum, Dissolved (mg/l) Aluminum, Dissolved (lb) a Treated Bottom Ash Transport Water? 1 1/6/ Yes 1 1/9/ Yes 1 2/23/ No 2 1/3/ No 2 1/9/ No a Based on a volume of 572,000 gallons. BENCH TESTING The purpose of the bench-scale testing was to determine the ph adjustment requirements necessary to reduce aluminum concentrations in the BATW at P4. Information gathered from bench-scale testing was used to assess the impacts of ph adjustment on aluminum concentrations, the filtration requirements (size range), the sulfuric acid dose requirements to achieve target ph, and to assist with planning the pilot test. Three 2-liter jars of bottom ash water were tested in parallel (Table 2). The jars were mixed at 100 revolutions per minute and 70 degrees Fahrenheit while the ph was adjusted to the target range and recorded. Mixing was continued for 5 minutes to ensure the ph adjustment was stable. The volume and strength of sulfuric acid added to achieve the target ph was recorded and used to calculate the dose of sulfuric acid required. Once the ph was adjusted, aliquots of the water were tested for total suspended solids (TSS). Aliquots were then filtered through a 5-, 0.45-, or 0.10-micrometer (µm) filter. The filtrate was collected and analyzed for aluminum. The native unfiltered samples were also tested for ph and total aluminum. The purpose of conducting ph titrations was to determine the amount of acid needed to reach the target ph range of 6.5 to 7.5 for aluminum precipitation and prepare the samples for measurement of total and filtered aluminum at the native ph, ph 7.5, and ph 6.5, as shown in Table 2. The results of the titrations are shown in Figure 4. Test No. Table 2. Testing Matrix for P4 BATW Bench Testing Al, 5.00-µm Filtered Al, 0.45-µm Filtered Al, 0.10-µm Filtered Target ph (s.u.) TSS, Unfiltered Total Al, Unfiltered 1 Native X X X X X X X X X X X X X

8 Aluminum Concentration µg/l 12,900 Resultant ph - s.u. IWC Pleasant Prairie Power Plant ph Titration Screening Test Sulfuric Acid Dose Applied - mg/l Figure 4: P4 ph Acid Titration (Left) and Photograph of Sample at ph 6.5 (Right) The purpose of measuring aluminum and TSS was to determine how much aluminum could be removed through ph adjustment paired with effective settling or sand filtration (both of which can remove particles down to roughly 5 µm) and ultrafiltration (0.45 µm). A 0.10-µm filter was also used to determine if any particulate aluminum was present but too fine to be removed by a 0.45-µm filter. This possibility was investigated because CH2M identified a common treatment issue of precipitation being able to produce solids, but those solids being sheared into smaller particles by mixers, some of which are not removed by the settling or filtration step. Aluminum and TSS results are shown in Figure 5. Much of the aluminum was insoluble at the initial ( native ) sample ph of 8.0, as evidenced by the aluminum reduction from the total results to the filtered samples at this ph. An increase in insoluble aluminum was found at a ph of 7.5 and a ph of 6.5, as expected. Maximum formation of insoluble aluminum was at a ph of ,000 Aluminum Pleasant Prairie Power Plant Total Aluminum 5 µm Filtered 0.45 µm Filtered 0.10 µm Filtered 14,000 12,000 10,000 8,000 6,000 4,000 2, ph 7.5s.u. 6.5 Figure 5: P4 Bench-Scale Testing Aluminum Results

9 PILOT SYSTEM DESCRIPTION AND OPERATION The pilot test consisted of treating a purge from the BATW loop by ph adjustment (using the existing sulfuric acid feed and controller in the BATW loop) and filtering a purge stream from the loop, followed by transferring treated water to the FGD scrubber. The pilot treatment system consisted of skid-mounted pumps, bag filters, and cartridge filters. A process flow diagram is provided in Figure 6. Bottom ash purge water was routed through a 2-inch line off the lowpressure recirculated water supply system, with a portion being sent through the filters and excess water returned to the bottom ash settling tank. The ph was adjusted at the settling tank with the existing sulfuric acid system to a target of 6.5 to 7.5 s.u. The pilot study team decided early in planning to use bag and cartridge filters despite the potential for solids overloading. Use of a pilot pressure leaf filter was investigated, but logistics and training on equipment was considered to make the project schedule too long, so the team elected to go with a system that could be implemented more quickly. Pressure leaf filter technology is described in the Proposed BATW Treatment System section. Planning for the pilot began in early November 2016 with a goal of getting the pilot started quickly. We Energies needed quick initial feedback to determine whether sending BATW to the FGD would negatively impact scrubber operations and gypsum quality. The pilot test commenced operation on December 30, 2016, and concluded operation on February 21, The pilot plan called for 5-µm filters for the bag filters and 1-µm filters for the cartridge filters. Issues with solids loading required operators to change filters every 2 to 3 hours and also quickly built up pressure across the system. A weir tank (a frac tank equipped with baffles) was added to the process to reduce solids loading to the filters, with some improvement on January 18, To improve settling, polymer was used to increase settling in the weir tank. Solids loading continued to be an issue, and the filter sizes were changed to 10-µm and 5-µm for the bag and cartridge filters, respectively. Solids loading reduced the amount of flow that could be routed to the FGD due to plugging of the filters. The plan was to send up to 216,000 gallons per day (gpd), but the maximum during the pilot was 43,200 gpd.

10 Figure 6: P4 BATW Purge Pilot Process Flow Diagram IWC 17-59

11 Data for the pilot was captured by the plant distributed control system (DCS). Ten grab samples were collected throughout the pilot to measure aluminum (total and dissolved), TSS, and total dissolved solids (TDS). ph in the closed-loop bottom ash system was measured continuously by existing ph probes. The plant also continued measuring standard monitoring parameters, including gypsum moisture and purity. Operators routinely monitored the FGD absorber vessels for foaming and increased their daily checks. Plant personnel routinely took grab samples from the filter belts to confirm there were no visual impacts to the gypsum. PILOT TEST RESULTS The most important objective of this pilot study was to determine the impacts of sending BATW to the FGD as a portion of the makeup water. Plant personnel monitored foaming and gypsum color closely during the pilot. Plant personnel did not observe any increase in foaming or impacts to gypsum color. Plant personnel continued to monitor gypsum moisture and purity. The specification for gypsum moisture is less than 10 percent and for gypsum purity greater than 90 percent. The gypsum was within specification during the pilot with an average moisture of 3.9 percent and purity of 96.6 percent. The averages during the pilot study correlate closely with the averages in the 2 months preceding the pilot. The main purpose of measuring water quality was to characterize the water quality of BATW going to the FGD as makeup water. Water quality results are shown in Table 3 and Figure 7. It also provided insights on how well the pilot treatment system removed TSS and aluminum. The ph of the closed loop was controlled at an average of 7.1, enabling aluminum to be removed by the existing hydrobin, settling, and surge tank system. Removal of aluminum in the existing hydrobin, settling, and surge tank system was evidenced by the below average aluminum levels to the pilot system and the increase in aluminum concentration to the pilot system during a ph upset event. Aluminum (total) average in the closed-loop BATW system was 7.1 mg/l with an average influent concentration to the filtration system of 4.3 mg/l, indicating removal by the existing closed-loop system using the existing hydrobin, settling, and surge tank system. Aluminum (total) was reduced to an average of 3.6 mg/l after the bag filter and reduced to an average of 1.15 mg/l after the cartridge filters. The average TSS was 36 mg/l into the system and 7 mg/l out of the pilot system. There was a high-ph upset on January 31, 2017, that resulted in increased aluminum into the pilot system and ultimately to the FGD scrubber. Aluminum (total) into the pilot system was 7.1 mg/l and was reduced to 4.4 mg/l across the pilot system during the high-ph event. Finally, there was an event in which a filter vessel was inadvertently left without a filter resulting in some unfiltered water making it to the FGD. No measurements were taken during this period and the exact time frame is not known. Visual observations after discovering the missing filtered showed no impacts.

12 µg/l µg/l IWC Table 3: BATW Water Quality Across Pilot Closed-Loop System Influent to System After Bag Filter After Cartridge Filter (To FGD) Parameter Units Min Avg 95th Max Min Avg 95th Max Min Avg 95th Max Min Avg 95th Max Aluminum, Total µg/l 2,600 4,330 6,640 7,900 2,200 3,600 5,820 6, ,149 3,500 4,400 Aluminum, Dissolved µg/l ,385 1, ,271 1, ,115 1,700 Total Suspended Solids mg/l Total Dissolved Solids mg/l 1,180 1,376 1,576 1,630 1,150 1,372 1,585 1,630 1,140 1,368 1,583 1,610 ph s.u Note: Ten samples were collected from 12/30/2016 to 02/16/2017. BATW was sent to the FGD from 12/30/2016 to 02/21/2017. Aluminum, Total Influent to System After Bag Filter After Cartridge Filter (Effluent to FGD) Aluminum, Dissolved Influent to System After Bag Filter After Cartridge Filter (Effluent to FGD) ph Upset (8.6) Sample Event ph Upset (8.6) Sample Event Figure 7: BATW Aluminum (Total and Dissolved) Results Across Pilot

13 The flow rate to the absorber varied but reached a maximum of 43,200 gpd (30 gpm). The contribution of pounds of aluminum from the BATW to the absorber ranged from 0.05 lb/day to 1.59 lb/day. As shown in Table 1, the operating load in the absorbers ranged from 468 to 716 lbs and no significant difference between the absorber receiving BATW and the absorber not receiving BATW. This indicates that the loading from the BATW during this pilot was relatively small compared to other sources of aluminum. PILOT TEST EVALUATION The main objective of the pilot test (to determine the impacts of sending BATW to the FGD on FGD operations and gypsum quality) was partially met. Although no impacts were observed, the desired flow rate was not obtained, so the potential impacts could not be fully assessed. However, there was no increased foaming and no impact to gypsum quality, even during an excursion when a cartridge filter was inadvertently left out. The pilot proved that some BATW can go to the FGD as makeup water with no discernible impacts. The secondary objective was to understand removal of TSS and aluminum. Operating at a ph lower than the original ph of 8.0 within the closed-loop BATW system showed good removal of aluminum by the bottom ash bin/tank settling system. TSS and aluminum removal across the filtration system was also good. The pilot correlated with the bench-testing results, where a lower ph produced insoluble aluminum but a 5-µm filter did not remove much aluminum. Good removal of aluminum was observed with a 1-µm filter. This information should be used to select a permanent system (e.g., a purge stream treatment system with either settling with flocculation or a filter able to remove down to 1-µm particles such as a pressure leaf system filter). Another important consideration is the aluminum concentration of the current makeup source to the FGD along with the current operating concentration within the absorber vessels. Current makeup to the FGD is settled water from Lake Michigan. The average aluminum (total) in Lake Michigan varies, but available data suggest the average level ranges from mg/l to mg/l. This suggests that aluminum concentrations from an FGD makeup source (such as BATW purge) should be at or below these levels. The average makeup flow to the FGD from Lake Michigan is 1,388 gpm, meaning aluminum loading from this source water to the absorber ranges from 0.6 to 4.3 lb/day. The pilot consisted of flows up to 43,200 gpd, but purge flow to the absorber could be up to 108,000 gpd per absorber. BATW aluminum loading would decrease as treatment is increased from simple ph adjustment to use of 1-µm filters. The concentration of aluminum in BATW is considerably higher than in water from Lake Michigan. However, aluminum concentrations in the absorber (typically 98 to 150) are well above any cycling effect the aluminum concentration in the makeup source (0.037 to mg/l) has, suggesting that other factors could be contributing to the aluminum levels (e.g., limestone contaminants). The following conclusions can be made from the pilot test: Bench-scale testing provided a good basis for defining the operating conditions of the pilot test. There were a number of issues in operating the pilot system related to solids loading and other operational constraints. A follow-up pilot could mitigate these issues using a

14 technology more suitable for elevated solids loading. Solids loading would also be taken into account for a full-scale system. The maximum flow to the FGD during the pilot was 43,200 gpd (30 gpm). The target flow was 216,000 gpd (150 gpm) (the assumed requisite purge from the BATW loop). Aluminum loading from BATW varies depending on flow and treatment but is small compared to absorber aluminum operating levels. No impact to FGD operations was observed. No impact to gypsum quality was observed. No impacts were observed even when BATW was sent to the FGD untreated or when a larger pore filter was used for a short time period; however, the flow was less than the target flow and for a short period, so potential impacts from higher flows and longer duration could not be observed. Adjusting ph in the closed-loop ash system resulted in lower aluminum concentrations in the loop than previously observed. The pilot filtration system showed good removal of solids and aluminum, reaching levels close to but not below the current source water. Current operating aluminum levels within the absorbers (taking into account cycling in the absorber) are higher than in the makeup water, indicating there are contributors of aluminum other than the makeup water. Therefore, contributions of aluminum from a well-operated purge treatment system would have negligible impact even at higher flows than those tested during this pilot. PROPOSED BATW TREATMENT SYSTEM The use of untreated BATW could cause build-up of ash in the absorber and other unforeseen impacts. The pilot work showed no observable impact to FGD operations or gypsum quality, though the desired flow rate was not obtained. It is recommended to continue to control ph in the closed-loop ash system to maximize aluminum removal in the hydrobin, settling tank, and surge tank system. To further reduce the solids and aluminum loading on the FGD from a bottom ash loop purge, it would be prudent to plan to use a BATW purge treatment system. A purge stream could be directed to a pressure leaf filter sized for a flow of 150 gpm. A pressure leaf filter is well suited for this application, given that the solids are too low for a filter press, and using a conventional filter would result in a backwash stream that would have to be managed. Other sites have successfully employed pressure leaf filters in similar applications but a pilot would be necessary to confirm suitability and design parameters. Pressure leaf filters are pressure vessels containing permanent filter leaves. These leaves can be the final filtration media but typically use diatomaceous earth or other filter aids as a pre-coat and/or body feed to improve filtration rate and product clarity. The diatomaceous earth and the filtered materials form a cake on the leaves during the filtration cycle. As the cycle progresses, the pressure drop across the leaves increases, while volumetric flow rate remains constant. High flow rates are easily accommodated as flux rates (flow per unit area) may be as high as 2 gpm per square foot filter area. Liquid inlet feed should have concentrations of suspended solids less than 2.0 percent by weight or 20,000 mg/l (which is adequate for bottom ash water purge, as the bottom ash water TSS is roughly 100 mg/l).

15 Cake discharges from the filter in a similar manner as from plate and frame filter presses. Systems come either in a dry cake discharge or wet cake discharge configuration. Dry cake discharge uses vibratory equipment as needed to facilitate cake removal, although particularly dry material may not need assistance. A wet cake discharge uses the filtered water to spray filtered solid material off of the filter leaves. A dry cake configuration is recommended for this application. A proposed process flow diagram using a pressure leaf filter for treatment of BATW is shown in Figure 8. Existing tanks will periodically need to be drained for maintenance and cannot be discharged as currently done. A secondary tank is needed to hold BATW from the other tanks during maintenance activities since that water could not be discharged. The estimated project cost (installed) to implement a purge BATW treatment system sized for 150 gpm is approximately $2.8 million. A significant portion of the project cost consists of the secondary storage tank included for draining other tanks and not directly related to the proposed treatment system. Considerations and major assumptions in the cost estimate are as follows: Equipment includes: - One 8,000-gallon sulfuric acid storage tank - Two transfer pumps (to pump decant from sump back into system) - One 200,000-gallon secondary surge tank (for maintenance purposes) - Two 150-gpm feed pumps - One pressure leaf filter package A building is included to house the pressure leaf filter system. Costs are for 2017 and do not include escalation. Owner administration and overhead costs are included. Estimate includes 30 percent allowance for undefined scope of work. Estimates by CH2M were developed using the methodology for a Class 4 estimate as defined by AACE including equipment factored or parametric models.

16 Figure 8: Proposed Conceptual BATW Treatment System. SUMMARY AND NEXT STEPS The pilot study showed no impacts to FGD operations and gypsum quality at a flow up to 30 gpm as it relates to scrubber operations (e.g. foaming) or meeting gypsum specifications such as purity and moisture. A purge flow rate from the bottom ash loop of 30 gpm may be sufficient if treated lake water were used but the ability to purge up 150 gpm is desired. However, the current use of cooling tower blowdown as the source water to the BATW loop requires a purge flow well beyond what was tested during this pilot. As previously discussed, the flow to the FGD was limited due to treatment capacity. A second pilot could be operated using a pilot pressure leaf filter. This would be the most valuable demonstration that flow above 30 gpm would have no impact to FGD operations and gypsum quality. The data from this study are suitable for design of a full-scale treatment system at Pleasant Prairie Power Plant. Other sites would need to take into account site specific chemistry and operational parameters along with a site specific pilot study.

17 REFERENCES Kruger, A., W.W. Focke, Z. Kwela, and R. Fowles Effect of Ionic Impurities on the Crystallization of Gypsum in Wet-Process Phosphoric Acid. Industrial & Engineering Chemistry Research 2001, 40 (5), pp P. Holt, 2002, Electrolytic Treatment of Wastewater in the Oil Industry. Ph. D. Thesis, University of Sydney, Australia.