PLANT OPTIMIZATION FOR AN INLAND INDUSTRIAL RO REUSE FACILITY. Abstract

Transcription

1 PLANT OPTIMIZATION FOR AN INLAND INDUSTRIAL RO REUSE FACILITY William Lovins, PhD, PE, AECOM Water; 150 N. Orange Ave, Orlando, FL Phone: Brit Johnston, PE, Tampa Electric Company; 702 N. Franklin Street, Tampa, FL Phone: x Jaya Tharamapalan, PhD, PE, AECOM Water; Orlando FL C. Scott Lee, PE, AECOM Water; Orlando, FL Catherine Magliocco, Tampa Electric; Polk County, FL Bret Nicholas, Tampa Electric; Polk County, FL Abstract The Tampa Electric Company (TEC) recently commissioned a $100 million industrial water reuse facility at their Polk Power Station located approximately 40 miles southeast of Tampa, Florida. The project includes a wetlands water intake and pumping station, 15-mile transmission pipeline, advanced reverse osmosis (RO) desalination water treatment system and deep underground injection system for RO concentrate disposal. As an alternative to fresh ground water, the TEC facility uses RO to lower total dissolved solids from reclaimed water stored in a wetlands tertiary treatment system and formerly discharged to the Alafia River. RO pretreatment consists of high rate clarification and multimedia filtration with added chemical feeds for disinfection and RO feed water conditioning. With a low quality surface water source, high level and reliable pretreatment is paramount for sustainable RO operation. Initial RO performance showed a high rate of fouling with a very short 1-week run time before offline cleaning was required. Through a series of methodical investigations a good working understanding was established for the high RO fouling nature and pretreatment requirements for this challenging water supply. Over the course of multiple runs, RO run times were incrementally improved to between 1 to 2 months duration. This paper presents treatment optimization results from plant start-up and commissioning and ongoing efforts to further enhance RO performance at this new industrial water reuse facility. Introduction The Tampa Electric Polk Power Station (PPS) is an integrated gasification combined cycle (IGCC) power plant built on reclaimed land previously used for phosphate mining. The plant uses a closed loop cooling water reservoir to dissipate waste heat generated by the PPS power generation process. To meet future electric demands, TEC expanded the PPS, which added capacity with the completion of a 6 th generating unit. The increased capacity results in additional waste heat loading to the cooling reservoir, and increased evaporation losses. Evaporative losses from the 3.5 billion gallon cooling reservoir were previously replaced by groundwater pumped 1

2 from the Floridian aquifer, historically requiring approximately 3.1 MGD of groundwater pumping (Magliocco et al. 2016). As a means of eliminating reservoir replenishment using valuable, potable water quality groundwater supplies, Tampa Electric, the Southwest Florida Water Management District (SWFWMD), the City of Lakeland, Polk County, and the City of Mulberry collaboratively developed a concept of utilizing reclaimed water from Lakeland s wetland wastewater treatment system. The wetland system currently acts as a final treatment stage for the treated wastewater effluent from the Glendale and Lakeland Wastewater Treatment Plants, prior to NPDES-regulated discharge into the Alafia River. A pump station at the wetland transfers water to the Reverse Osmosis Treatment Plant (RTP) at PPS via approximately 15 miles of 30-inch diameter single transmission pipeline. The initial design raw water pretreatment capacity at the RTP is 8.5 millon gallons per day (MGD) and 5.7 MGD RO feed water capacity. The facility was constructed for expansion in stages up to 17.2 MGD raw water flow. Pilot Testing AECOM mobilized a pilot test site to operate an integrated membrane process over a 12- week period in The temporary pilot facility was located at the City of Lakeland s Wetland treatment system in Mulberry, Florida. The purpose of the pilot study was to prove the feasibility of treatment concepts and provide design criteria for the proposed treatment plant. Integral to the project was process selection through pilot testing under anticipated fullscale conditions. Figure 1 shows schematically the treatment processes configuration during the pilot study that consisted of high rate clarification, rapid sand filters, and RO. The testing protocols for the pilot study were structured to identify the pre-treatment options and membrane configurations that yielded the highest recovery rate while limiting the fouling potential on the RO processes. A high recovery rate was mandated by anticipated deep injection well capacity constraints. Key findings from the pilot study relative to pretreatment and RO process optimization are briefly outlined below: Enhanced coagulation was necessary for organics and suspended solids removal prior to filtration. A non-oxidizing biocide was required for biological RO fouling control. Other fouling control measures included: ph for residual metal control (dissolved iron from ferric coagulation), and solids loading control with reduced water flux rate. The RO foulant matrix was predominantly composed of an organic / biological matrix with small amounts of inorganics in localized areas. Chemical cleaning was effective with a broad spectrum high ph detergents followed by a low ph cleaner. 2

3 FIGURE 1: Pilot Study Process Configuration 3

4 The RO process provided a consistent highly quality product water. RO conversion efficiency was successful up to 85-percent recovery. Algae from the wetlands proved to be the rate limiting constituent during the pilot study (Lovins et al. 2012). The wetland is a man-made 1,600 acre system that consists of 7 holding cells in series. The treated wastewater effluents from the McIntosh Power Plant, and Glendale and Lakeland Wastewater Treatment Plants meander through the cells where it is treated naturally while flowing through a diverse habitat with uplands, swamps, marshes, and open water lakes. With high nutrient levels, coupled with shallow water bodies exposed to Central Florida heat and sunshine, year round algae activity was anticipated. Algae control on the pilot required the use of a filter aid (ferric coagulant) and pre-oxidation with sodium hypochlorite dosing to the raw water. Based on the pilot test results, the following treatment process train was recommended: RO Pretreatment: - Ferric sulfate coagulation, flocculation and sedimentation using high rate upflow solids contact clarification. Ferric sulfate dosage of 135 mg/l with supplemental sulfuric acid for ph control for optimal settling and iron carryover control. - Filtration of coagulated and settled water using granular media filters. - Anti-scalant addition plus biocide followed by 5-micron cartridge filtration. Pretreatment dosages are 2 mg/l anti-scalant and 10 mg/l biocide. RO Desalting: - Single pass 3 Stage RO trains, 7 elements per pressure vessel, employing thinfilm composite polyamide RO membranes. - Operating recovery of 85-percent and 10.2 gsfd water flux. - Hybrid RO array for flux balancing using a hybrid membrane array. Plant Overview The construction works for the Tampa Electric PPS Reclaimed Treatment Plant (RTP) was started in March 2014 and completed in April Figure 2 shows the schematic of the processes of the RTP. 4

5 FIGURE 2: Process Configuration at Reclaimed Treatment Plant at Polk Power Station 5

6 Pre-treatment The TEC RTP s pre-treatment configuration includes the clarifier and mixed media filtration systems. The clarifier and filter systems are proprietary systems, Densadeg TM and Greenleaf TM, manufactured by Infilco Degremont. The clarifier system is a high-rate upflow solids contact clarifier which combines coagulation, flocculation, internal and external sludge recirculation, and plate settling in two conjoined vessels. The feedwater to the clarifier includes a mix of the raw water from the wetlands located 15 miles away, and the recycled process streams from the RTP. Recycled water is comprised primarily of spent wash water (SWW) from the mixed media filters that is recycled via the onsite SWW storage tank. The recirculated SWW was planned at about 10% of clarified feed water. The pilot studies in 2011 tested two coagulants ferric chloride and ferric sulfate. Ferric sulfate was selected for plant operations based on cost, availability and less corrosion impacts compared to ferric chloride. An anionic polymer was also implemented as coagulant aid. Sodium hypochlorite was also added in the rapid mix tank for pre-oxidation of algae. Factors affecting clarifier performance generally include: Amount of coagulant, polymer and sodium hypochlorite used; ph of the flocculated and clarified water; Amount of sludge recycle from the clarifier sludge blanket; Raw water quality, turbidity, algae level, and alkalinity; and Total suspended solids level in the flocculation / reactor vessel. The mixed media filtration system consists of 4 filter cells. The mixed media in the system consists of anthracite (23 inches), silica sand (7 inches), fine garnet (3 inches) and coarse garnet (3 inches) a total of 36 inches of mixed media. Ferric sulfate is used as filter aid to the incoming flume to the filter media, to strengthen and enhance filterability of the floc carry over. Factors affecting mixed media filtration generally include: Influent turbidity; Amount of filter aid used; Amount of sodium hypochlorite added to the raw water and/or the filter influent plume; and Filter backwash regime. 6

7 Reverse Osmosis Process The RO system consists of a 3-stage, single pass spiral-wound RO system. Treatment capacity is 5.7 MGD for 2 skids (i.e. ~1960 GPM feed flow rate in each skid). The RO process is designed for expansion with another 4 skids, bringing the treatment capacity to 17 MGD raw water flow. The first, second, and third stages of each train contain arrays of 48, 24, and 12 pressure vessels, respectively, with seven membrane elements. Hydranautics CPA5-LD, ESPA2-LD, and ESNA1-LF2 membranes have been placed in the first, second, and third arrays, respectively. The RO system is designed to operate at a recovery of 85%. The schematic of each of RO skid s recovery rate per stage; the permeate production per stage and total permeate and concentrate production are as shown in Figure 3. FIGURE 3: Schematic of Permeate and Concentrate flow rates at 85% RO recovery The filtrate from the filters is pretreated and filtered through cartridge filters before feeding to the RO process. RO pretreatment chemicals include anti-scalant (i.e. scale inhibitor), biocide, and sulfuric acid. Anti-scalant acts to inhibit the precipitation of certain metals and biocide will prevent biological fouling. Acid is not typically required due to the impact that ferric sulfate has on depressing the ph. However, an acid injection point is available to provide ph control, if needed. Sodium bisulfite is added as needed for de-chlorination as free chlorine carry over will oxidize and damage the RO membranes. Unknown Factors Going into Initial Full-scale Plant Operation TEC commenced performance testing of the full facility on April 27, The goals of the performance testing were to ensure that each of the pre-treatment processes and the RO process performed to their performance specifications and that the integrated treatment system performed as planned. System success hinged on RO permeate water quantity and quality, system reliability and operability, and ability of the pre-treatment processes to control the fouling rate on the RO membranes, thereby prolonging the run times and reducing 7

8 downtime for chemical cleanings. Tables 1 and 2 compare the start-up and pilot water quality in both raw and filtered and the notable differences in water quality are summarized below: Raw Water Increased total organic carbon (TOC) Increased ph Increased suspended solids Increased iron Decreased silica Decreased ammonia Decreased phosphorous Increased Heterotrophic Plate Count Filtered RO Feed Water Decreased suspended solids Decreased silica Increased sulfate Decreased ammonia Decreased Heterotrophic Plate Count Though the RTP was designed taking into consideration the findings of the pilot study, multiple unknown factors remained going into the full plant start-up operation. The following summarize areas of opportunity for building upon the accelerated and relatively brief 12- week pilot test program. Understanding of seasonal changes in wetland water quality and impacts on treatment process performance. Understanding of TOC loads and removal efficiencies through individual pretreatment unit operations (Johnston et al. 2016). Understanding of impact and effectiveness of direct application of biocides to individual RO stages on overall RO performance. Impacts of treated effluent water discharges from the Glendale and Lakeland Wastewater Treatment facilities and the McIntosh Power Plant which uses Lakeland treated secondary effluent for cooling water make-up. 8

9 TABLE 1: Comparison of Start-up & Pilot Water Quality - Raw Water Parameter Units Start-up Pilot Min Ave Max Min Ave Max ph SU Total Suspended Solids ppm Total Dissolved Solids ppm Organic Carbon, Total (TOC) ppm UV Absorbance cm Specific UV Abs., calculated L/mg-m Total Alkalinity, as CaCO 3 ppm Calcium, as CaCO 3 ppm Magnesium, as CaCO 3 ppm Total Hardness, as CaCO 3 ppm Calcium ppm Iron ppm Magnesium ppm Manganese ppm Potassium ppm Sodium ppm Strontium ppm Chloride ppm Fluoride ppm Silica ppb Sulfate ppm Ammonia, as N ppm Nitrogen, Total as N ppm Phosphorous, Total as P ppm Chlorophyll A mg/m Quaternary Amine Compounds ppm Heterotrophic Plate Count cfu/ml 28,000 89K 150K 1,500 6,633 18,000 Notes: Start-up sampling events correspond to sampling on June 10 and June 18, Quaternary amine compounds are shown from sampling between July 30 and October 21, Pilot data corresponds wetland treatment system sampling between Dec. 23, 2010 and May 8,

10 TABLE 2: Comparison of Start-up & Pilot Water Quality - Filtered RO Feed Water Parameter Units Start-up Pilot Min Ave. Max Min Ave Max ph SU Total Suspended Solids ppm Total Dissolved Solids ppm Organic Carbon, Total (TOC) ppm UV Absorbance cm Specific UV Abs., calculated L/mg-m Total Alkalinity, as CaCO 3 ppm Calcium, as CaCO 3 Ppm Magnesium, as CaCO 3 ppm Total Hardness, as CaCO 3 ppm Calcium ppm Iron ppm Magnesium ppm Manganese ppm Potassium ppm Sodium ppm Strontium ppm Chloride ppm Fluoride ppm Silica ppb Sulfate ppm Ammonia, as N ppm Nitrogen, Total as N ppm Phosphorous, Total as P ppm Chlorophyll A mg/m Quaternary Amine Compounds ppm Heterotrophic Plate Count cfu/ml , ,816 33,000 Notes: Start-up sampling events correspond to sampling on June 10 and June 18, Quaternary amine compounds are shown from sampling between July 30 and October 21, Pilot data corresponds wetland treatment system sampling between Dec. 23, 2010 and May 8, Demonstration of long-term fouling rates of the RO membranes. Impacts of bio-growth in the raw water pipeline en route to the RTP from the wetland treatment system. Residual waste stream recycling impacts on pre-treatment and RO performance from the filter backwash, thickening, and dewatering operations. Better identification of bio-fouling mechanisms including testing for slime forming bacteria in raw water sources. 10

11 The limitations must be viewed with the understanding the near impossible task of piloting each and every aspect of the treatment process. Therefore, an intensive optimization effort was recognized as a vital aspect of the first one to two years of plant operation, especially in light of full pretreatment and sludge waste liquid stream recycling to maximize water utilization and to minimize water volume disposed to deep injection. Initial RO Process Performance Full plant start-up commenced with two RO Trains in service. The RO process operated at the design 85% recovery rate, producing 2.4 MGD (1,670 gpm) of permeate per train, from an average feed rate of 2.82 MGD (1,960 gpm). The predetermined loss of membrane productivity, in terms of water and salt transport coefficients, at the start of Run #1 was 15%. As there was rapid decline in productivity on Stage 3, Run # 1 was stopped after 6-days of operation. Figures 4, 5 and 6 show the trending of the RO performance in terms of water transport and salt mass transport coefficients and differential pressure respectively, during start-up (Run # 1) in Train A. As shown, both RO Trains, A and B experienced significant fouling. During the slightly more than 6 days of operations, there was sufficient loss of membrane performance, in terms of water transport and salt transport coefficient, across all 3-Stages, to trigger cleaning. In particular, water transport coefficients dropped 9%, 12% and 21% across Stages 1, 2 and 3 respectively. While the salt transport coefficients dropped 19%, 12% and 47% across Stages 1, 2 and 3 respectively. On the other hand, the differential pressure drop (i.e. feed channel pressure drop) across all 3 stages remained relatively stable. Incremental RO Performance Improvements Following the observed rapid RO performance decline during start-up and commissioning, TEC lead a collaborated approach with AECOM to systematically identify potential membrane fouling mechanisms and make incremental adjustments to the pre-treatment and RO operations with the goal of maximizing RO run times between chemical cleaning. Quaternary amine compounds (QACs) that have been known to foul membranes were identified in the treated effluent streams feeding into the wetlands and also in the raw water to the RTP (0.9 to 2.5 ppm); Adsorption of residual natural organic matter onto the membrane surface; Excessive chlorination and dechlorination in pretreatment processes resulting in the creation of food for bio-sliming organisms; 11

12 FIGURE 4: Water Transport Coefficient during Start-Up at 85% recovery in Train A FIGURE 5: Salt-Transport Coefficient during Start-Up at 85% recovery in Train A 12

13 FIGURE 6: Differential Pressure across Train A during Start-Up at 85% recovery Another 5 RO Runs (total 6 RO Runs including startup) were completed to systematically identify the fouling mechanisms and determine optimum conditions to operate the RTP. Potential fouling mechanisms and process improvements identified and systematically addressed include: RO processes and pretreatment processes were started almost simultaneous before the pretreatment processes were stable, possibly resulting in poorer quality water being discharged to each subsequent unit process and carrying into the RO trains; Particulate fouling was more likely to happen when the pretreatment operations were not stable or an individual media filter cell and/or backwash performance was inconsistent; Direct measurement of Total Organic Carbon across each operating unit of the pretreatment system (coagulation process, multi-media filters, and SWW recycle to the coagulation process) Polymer carryover due to overdosing, resulting from incomplete chemical dosing control during the coagulation process and during the solids dewatering process; 13

14 Implementation of direct application of biocide (DBNPA) to all RO stages in order to control biofouling; Lack of disinfection and excessive bio-growth downstream of the media filters in the RO transfer wet well, cartridge filter media, and RO feed water pipeline; and Even after cleaning during the cleaning studies, fibrous (filamentous) material remained without slime, on the concentrate end of elements and the last 2-3 inches of the feed channel spacer. A number of operational changes were carried out during each of the runs. The key changes prior to a given Run, expectations following the changes, and the resulting outcomes, are tabulated in Table 3. Membrane autopsies, cleaning studies and RO membrane performance across each of the 3-stages were used in carrying out the assessments and operational changes that were deemed necessary. RO performance trends in terms of water transport, salt transport, and differential pressures during these runs are as shown in Figures 7, 8 and 9 respectively. Chemical Fouling RO Fouling Control Highlights In order to minimize the risk of fouling due to polymer carry over, jar testing identified a non-ionic polymer to replace an anionic polymer used for pilot testing and start-up. Nonionic polymer was implemented starting in Run #3. Quaternary amine compounds (QAC) are known to cause a rapid reduction in RO water flux and solute rejection from ionic interactions with the surface, and it is often irreversible. Though QACs are present in the raw water (0.9 to 2.5 ppm, they were not positively identified in the membrane autopsies. However, the lack of a positive identification may be due to limited analytical techniques and interference from the composite membrane material. Nonetheless regular water sampling and analysis was recommended to check for spikes in concentrations and possible correlation with irreversible RO fouling. For reference, QACs are organic molecules found in surfactants, disinfectants, pesticides, and corrosion inhibitors. Common discharge sources include wastewater effluents, laundry and hospital facilities as well as aquatic sediments. The QAC molecule consists of hydrophobic alkyl groups with a hydrophilic positive central nitrogen atom. The QACs can act as a surfactant and affect the performance of polyamide membranes by bonding to the membrane through hydrophobic interactions. 14

15 Run No. #1 (Startup) TABLE 3: Changes in Operating Conditions During Runs #1 to Run #6 and Resulting RO Performance Changes RO Run Time Pretreatment Flow Rate (gpm) Targeted Foulant 6 4,000 Initial Run RO Start-up Conditions: 85% Recovery; Densadeg Fe dose 295ppm; Densadeg Cl dose up to 15 ppm; Mixed media bed Cl dose up to 4.4 ppm #2 22 4,000 Chemical; Biological #3 10 4,000 Particulates; Biological #4 20 4,000 Particulates; Organic Matter; QAC; Biological #5 34 2,000 Biological; Organic #6 34 2,000 Biological; Organic Key Operations Changes Expectations Outcome/ Performance Lowered raw water Cl dose from 15ppm to 5ppm; Densadeg Fe dose 260ppm;Densadeg polymer switched from anionic to non-ionic, AS1200; Mixed media bed Cl dose introduced at 0 to 13 ppm as needed; Lowered cartridge filter from 5 µm to 1 µm; Densadeg Fe dose 216ppm; Mixed media bed Cl dose increased to 25 ppm; Increased biocide isothazoline dose to RO feed stream from 17.5 ppm to 20 ppm Densadeg Fe dose 278ppm. Lowered cartridge filter from 1 µm to 0.5 µm; Reduced RO Recovery to 75% Increased RO Recovery to 80%. Weekly Cl shock to mixed media; wet well; cartridge filter; Daily DBNPA shock 20ppm; Weekly DBNPA CIP 20ppm; ph 10 mini-cip in week 3 Densadeg Fe dose 345ppm; Continue: Daily DBNPA shock 20ppm; Weekly DBNPA CIP 20ppm; ph 10 mini-cip in week 3 Eliminate chemical fouling Eliminate inorganic and organic fouling Reduce biological and organic fouling Reduce biological and organic fouling Reduce biological and organic fouling Root Cause - Chemical & Polymer Fouling; Some inorganic/organic fouling Non-ionic switch resulted in less polymer carryover to RO; No significant Fe carryover to RO; No Ca scaling; Root cause - Industrial adhesive. Post- Run Membrane Inspection - slime/gel on ATDs, no odor. Stg 1 Foulant: Organic & SS; Stg 3 Foulant: Hydrophilic surfactant (maybe QAC); Main foulant was organic matter and SS. Shorter than expected run. 3 rd Stage Membrane Autopsy organic and some biological fouling. Post- Run Membrane Inspection - slime/gel on ATDs, foulant material exiting element feed channel, no odor. Organic removal was optimized with ferric dose; Biofouling present in membrane spacers. Post- Run Membrane Inspection - slime/gel on ATDs, foulant material exiting element feed channel, no odor. Post- Caustic ph 10 Clean Inspection Slime observed on membrane ATDs. Organic removal was optimized with ferric dose. Biofouling present in membrane spacers. Post- Run Membrane Inspection - slime/gel on ATDs, foulant material exiting element feed channel, no odor. Caustic ph 10 Clean Inspection - Slime observed on membrane ATDs. Organic removal was optimized with ferric dose. Biofouling present in membrane spacers. Post- Run Membrane Inspection - slime/gel on ATDs, foulant material exiting element feed channel, no odor. Caustic ph 10 Clean Inspection - Slime observed on membrane ATDs. 15

16 FIGURE 7: Water Transport Coefficient across Train A during Runs #1 through #6 FIGURE 8: Salt-Transport Coefficient across Train A during Runs #1 through #6 16

17 FIGURE 9: Differential Pressure across Train A during Runs #1 through #6 Particulate Fouling Control The cartridge filter pore size was reduced from 5 micron (startup) to 1 micro (Run #3) pore size, and then to 0.5 micron pore size (Run #4 onwards) for enhanced particulate fouling control. Particulates were concluded to be a minor contributor to overall RO fouling at this plant. Biological Fouling Control Chlorination is an integral part of the RTP biofouling control program. The clarifier was chlorinated at 5 ppm (chlorine demand in raw water is about 30 ppm) following Run #1, and at start-up, Run #1, the chlorination dose was 15 ppm. Partial chlorination of feedwater to carry a 0.5 to 1 ppm residual through the media filters is necessary to pre-oxidize raw water solids, provide disinfection, and help prevent the sludge in the clarifier from going septic. RO inter-stage injection of DBNPA as a shock biocide treatment is critical to controlling RO biofouling at this facility. During Run #5 daily DBNPA shock treatment of the RO feed water was implemented. The protocol involved temporarily discontinuing isothiazoline injection 1-hour before DBNPA injection. The DBNPA injection was carried out for 1-hour in an effort to overcome any potential development of resistance of the biofoulants and biofilm. During Run #6 the DBNPA injection protocol was further refined with injection in all 3-stages, one at a time, for 20 mins in each stage. Run # 6, with interstage DBNPA 17

18 injection was the first long-term run where the differential pressure across all 3-stages remained stable (see composite Figure 9). Natural Organic Fouling Control Increased coagulation rates are necessary to maximize the removal of natural organic compounds in the pre-treatment process. TOC levels in Run #1 showed a 10.6 ppm C feeding into the RO process. Subsequent improvements in RO run time coincided with higher TOC removal efficiency with RO feed water levels consistently below 6 ppm C. Efforts continue in quantifying and lessing the impact of recycle waste stream and filter backwashing regime on organic loading to the RO membranes. In addition, it is critically important that TOC measurements are systematically taken on a regular basis to understand the effective removal across each unit operation. While continuous online TOC readings would be most effective, at a minimum daily measurement is suggested. Comparison of system changes (i.e. coagulant dose changes, equipment operation modifications) can reveal real-time information about the effectiveness of the change on the removal of organics in the pre-treatment system. TOC measurement techniques can range from simple (i.e. use of portable UV254 meters) to more advanced instruments (i.e. use of laboratory style TOC Analyzers to direct online process measurements). Membrane Cleaning Following Run #1 a bench-scale cleaning study showed fouling was reversible with a high ph peroxide solution. Full-scale cleaning results showed good performance with only small amounts of irreversible fouling. A follow-on cleaning study at the end of Run #6 confirmed that high ph peroxide solution was the best overall chemical for dissolving the foulant when compared with soak tests with a wide range of 26 different cleaning chemicals. Caustic cleaning at ph 10 and 95 F was also identified as an alternative low-cost cleaning regime. Table 4 summarizes the results of the cleaning study with a full element using three separate chemicals. Water flux returned to normal rate after cleaning. Salt rejection was reduced but expected to return to normal after hours of operation or if the high ph clean is followed with low ph solution cleaning. Intermittent caustic soak at ph 10 and rinse by stage helped control organic and biofouling potential and was used to extend RO run time between cleaning. During Runs #5 and #6 caustic rinses at ph 10 was carried out in all 3 Stages. This mini-cip regime, carried out for 1 hour per stage, extended the membrane operations by an estimated one week. See Figures 7, and 8. 18

19 Condition / Cleaning Step Table 4: Cleaning Study Results - Run #6 2 nd Stage ESPA Membrane Cleaning Regime Agent ph Condition Temp. ( F) Soak Time (Hrs) Air Flow (gpm) Comment 1 C No Foulant mostly removed with some fibrous material remaining in feed spacer and ATD 2 NaOH No Not as much foulant on ATD Foulant became fibrous and 3 MT Yes visible on concentrate end and ATD 4 NaOH No Not recommended at facility, no complete removal of foulant 5 C Yes No significant improvement membrane productivity, in terms of water and salt transport coefficients, that was set at 15%. At the end of Run #4 cleaning triggers were refined for Run #5 and then again for Run #6, as outlined in Table 5. These thresholds were referenced to the start-up flux rate or differential pressure between 12 to 24-hours of operation at the start of each run. Table 5: Predetermined Shutdown Thresholds on Train A during Runs #5 and #6 Parameter Units RO Stage % Δ for CIP Trigger Run #5 Run # Water Transport Coeff. m/s-kpa Salt Transport Coeff. m/s Differential Pressure psi

20 Summary and Conclusions Through the public-private partnership between Tampa Electric Company, City of Lakeland, and Southwest Florida Water Management District, initial plant operations have proven successful in reclaiming a challenging and low quality source for beneficial reuse. However, the experience reinforces the dependence of RO performance on pretreatment effectiveness for membrane fouling control. Optimization efforts as of the fall of 2016 have yielded RO run times of between 5 to 7 weeks. In addition, membrane cleaning regimes are effectively recovering performance between runs. While the membrane life expectancy is unknown at this point, an estimated 3 to 5 years life is the best likely outcome for this challenging source water. While optimization will remain an ongoing effort for the first few years of this plant s operation, the steps to achieve robust pre-treatment for shielding the RO from numerous raw water foulants are as follows: Pilot testing was critical for establishing the basis of design and initial process operating targets. With challenging water sources, in order reduce the potential for protracted start-up and commissioning measures, pilot testing periods should be increased to 4 to 6 month periods at a minimum in order to observe seasonal changes in water quality and operation with all recycle waste streams. However, a protracted start-up and commissioning process should be expected for challenging water sources where the effects of a long raw water transmission line that is subject to biogrowth and solids accumulation cannot be understood until the plant is in full operation. Enhanced coagulation was necessary for organics and suspended solids removal prior to filtration. Optimal operating conditions correspond to 250 mg/l ferric sulfate (as 72% hydrated product), 1.5 ppm neat non-ionic polymer feed, 1.5 to 2 percent sludge blanket suspended solids concentration, 2.5-percent sludge recycle rate, and less than 5-percent spent wash water or filter backwash water recycle rate. The high rate clarifier typically produced settled turbidity below 1 NTU and TOC of about 6 mg/l. With challenging waters where organic fouling is determined to be a primary foulant of RO membranes it is paramount to establish TOC removal efficiencies across each pre-treatment process with frequent and regular direct measurement. Algae is a primary constituent of concern for this supply. Successful control requires the use of a filter aid (ferric sulfate) and pre-oxidation with sodium hypochlorite dosing to the raw water. Sodium hypochlorite dosing to achieve 0.5 to 1 ppm free chlorine residual through the media filters has proved beneficial in controlling algae, biogrowth, and preventing septic sludge conditions in the clarifier blanket and sludge thickening and storage processes. Significant RO biofouling was indicated by rapid increases in feed channel pressure drop trends. Intermittent shock biocide dosing (DBNPA) proved to be a critical 20

21 counter measure using a daily 20 ppm continuous feed for 20 minutes to each stage of membranes. A non-oxidizing biocide, isothazoline, is dosed on a continuous basis to RO feed stream at 15 ppm for added biological fouling control. However, RO performance has shown isothazoline is not an effective stand-alone biogrowth control measure for this particular application. Other fouling control measures include flocculating at ph 4.8 to 5.2 for residual metal control with dissolved iron from ferric coagulation typically <0.1 ppm Fe. The RO foulant matrix was predominantly composed of an organic / biological matrix with small amounts of inorganics in localized areas. Chemical cleaning was effective with broad spectrum high ph detergents followed by a low ph cleaner. The RO process provides a consistent highly quality product water. RO conversion efficiency is successful at the design 85-percent recovery rate. As of this writing, RO run time of 55 days was achieved with 5.7 MGD raw water flow and pretreatment operations were proceeding successfully at full system capacity with 8.64 MGD raw water flow. The longer-term outlook for this facility is operation at full capacity with an additional two weeks RO run time between cleaning to improve to 60-day RO run cycles. Acknowledgements The authors acknowledge the Tampa Electric Company for their commitment and efforts in support of this project. The project was made possible with valuable support and oversight provided by TEC engineering, operations and management team, and ASI contract operations staff. We also thank the vendors who supplied valuable technical assistance; Alkema (formely American Water Chemicals), Avista Technologies, David H. Paul, Inc, Doosan Hydro, Hydranautics, Infilco Degrement, Nalco, and Mr. Bill Anderson with the City of Lakeland who was instrumental in coordinating sampling activities at the wetland treatment test site. The Southwest Florida Water Management District is acknowledged for funding this innovative public-private partnership with both TEC and City of Lakeland. References Lovins, W.A. et al. (June 2012), Getting the Algae Out: Fouling Control for Sustainable RO Treatment at an Inland Facility. AWWA Annual Conference Proceedings, Dallas, TX. Magliocco, C. et al. (April 2016), Reclaimed Water Use in the Power Industry. Florida Water Resources Conference Proceedings, Orlando, FL. Johnston, B. et al. (September 2016), Reclaimed Water Addition for Polk Power Station A Case Study. 31st Annual WateReuse Symposium Proceedings, Tampa, FL. 21