COMBINED NF AND RO CONCENTRATE DISPOSAL PIPING AND DEEP INJECTION WELL SCALING: A CASE STUDY. Background

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1 COMBINED NF AND RO CONCENTRATE DISPOSAL PIPING AND DEEP INJECTION WELL SCALING: A CASE STUDY Cristina Ortega-Castineiras, P.E., CH2M, 3150 SW 38 Avenue, Suite 700, Miami FL Cristina.Ortega@ch2m.com, Ph: Albert Jernej, City of Deerfield Beach, FL G. J. Schers, CH2M, Fort Lauderdale, FL Gerrit Bulman, P.G., CH2M, Fort Lauderdale, FL Mike Witwer, P.E., CH2M, Gainesville, FL Thomas Good, City of Deerfield Beach, FL Background The aeration of combined concentrate has resulted in unprecedented scaling in both the downstream disposal piping and deep injection well at the City of Deerfield Beach West Water Treatment Plant (WTP). The WTP, located in Southeast Florida, uses lime-softening, nanofiltration (NF), and reverse osmosis (RO) processes to treat water from Biscayne and Floridan aquifers. The NF system operates at 85% recovery and has a total production capacity of 12.7 mgd. A portion of the NF concentrate is blended with Floridan Aquifer groundwater to feed the RO trains. This practice increases the overall recovery of the treatment system. The RO system operates at 75% recovery and has a production capacity of 3.0 mgd. Typically, NF concentrate represents 30% of the RO feed. The remaining NF concentrate is combined with RO concentrate, and conveyed to a manhole. An air gap is created in this manhole as concentrate leaves the inlet pipe and drops down. The water then flows by gravity to the concentrate pump station wet well. Other WTP process waste streams drain to the concentrate pump station wet well, including raw water used for RO and NF system bypass start up and skid flush (flush waters), waste from the dual-zone monitor well, and analyzer waste from the RO building. The pump station discharge is disposed to either a deep injection well located at the WTP site primary disposal method, or to the wastewater collection system backup disposal method. The WTP injection well system consists of a Class I deep injection well (DIW) with a tubing and packer design and a dual-zone monitoring well (DZMW) located within 150 feet of the injection well. The injection zone is the Boulder Zone of the Oldsmar Formation between 3,020 and 3,520 feet below land surface. During routine plant maintenance in 2015 approximately two years after start-up of the RO systems, WTP staff observed a buildup of material in the concentrate pump station discharge piping, which coincided with independent observations that the pumps were not operating to their capacity and that the injection wellhead pressure was increasing. Figure 1 shows a photograph of the scale buildup in the discharge pipeline and around the submersible concentrate pump. In October 2015, dynamic losses in the DIW system reached levels at which pump capacity was greatly diminished and subsequently the City ceased operation of the DIW. Since that time, concentrate flows have been diverted to the wastewater collection system. Investigative work was performed on the DIW and in April 2016, through video logging, it was confirmed that the scale also was present on the inside of the injection well FRP tubing and in 1

2 the bottom of the open borehole. The scale buildup measures between 1 and 2 inches, has a clay-like consistence. The City has been actively investigating the scale source and is in the process of executing short term and long term measures to minimize future scaling, while returning to the use of the DIW as the main concentrate disposal method. Figure 1. Precipitated Scale in the Concentrate Pipeline and on the Concentrate Pump Treatment Process Characteristics The West WTP treats water from the Biscayne aquifer using lime softening and NF in parallel. Water from the Biscayne aquifer is fresh but high in hardness. Floridan aquifer water, which is brackish and high in hardness, is treated using RO, which both demineralizes and softens the well water. The lime softening facility includes the addition of lime and polymer, clarification, granular media filtration, and sludge handling facilities. The NF treatment process includes the addition of scale inhibitor, sulfuric acid (to minimize scaling in the NF system), cartridge filtration, and NF membranes. The recovery was increased to 85 percent from 78 percent after new low fouling, softening, NF membranes were loaded in early A portion of the NF concentrate is recycled and blended with water pumped from the Floridan aquifer. After blending, the water is pumped to the RO skids. The remainder of the NF concentrate is combined with the RO concentrate and discharged to the concentrate pump station. The portion of NF concentrate recycle is dependent on the number of operational NF and RO skids. If two NF skids and one RO skid are operational, which is a typical mode of operation, around 67 percent of the NF concentrate is recycled. Whereas, if one NF skid and one RO skid are operational, 100 percent of NF concentrate is recycled. Due to capacity limitations of the existing concentrate disposal system because of scaling, the West WTP has not operated with more than three membrane skids over the last year. The RO feed water, a blend of NF concentrate and Floridan water, is pretreated with scale inhibitor and sulfuric acid prior to cartridge filtration. Following this pre-filtration, the blended water pressure is boosted by the RO feed pump and treated by the RO system. Figure 2 presents the WTP process flow diagram. Table 1 shows NF and RO system design criteria and typical operating conditions. 2

3 Figure 2. Process Flow Diagram of the West Water Treatment Plant Biscayne Aquifer Wells Nanofiltration NF & RO Concentrate DIW Pump Station to DIW NF Concentrate RO Membranes Degasification Floridan Aquifer Well RO Concentrate to HSPS Biscayne Aquifer Wells Clearwell Ground Storage Tanks (2) Lime Softening Granular Media Filtration Table 1. NF and RO System Data and Typical Operating Conditions Parameter Nanofiltration Reverse Osmosis Number of skids 5 2 Number of pressure vessels in stage Number of pressure vessels in stage Number of membrane elements per 7 6 pressure vessel Membrane manufacturer and model Hydranautics ESNA1- Toray TMG20- LF-LD 430 Membrane area per element 400 ft ft 2 Membrane age <1 year 5 years Feed flow 2080 gpm per skid 1390 gpm per skid (420 gpm NF concentrate gpm groundwater) Permeate flow per skid 1760 gpm 1040 gpm Concentrate flow per skid 320 gpm 350 gpm Concentrate to DIW flow per skid 110 gpm 350 gpm Recovery rate 85 percent 75 percent Average flux Concentrate residual pressure 40 to 45 psi 160 to 170 psi Feed water ph Scale inhibitor dose 3 ppm 5 ppm 3

4 Scale Location and Composition As shown in Figure 1, the scale has precipitated on the outside of the DIW pump bowl, discharge column and in the pipelines downstream of the DIW pumps, and to a lesser extent inside the wet well walls. City staff inspected the concentrate piping upstream of the DIW pump station wet well and did not find scale on the inside of these pipes. Additionally, no membrane fouling/scaling was reported by the City. City staff indicated that the RO and NF membranes have never been chemically cleaned, and thus no CIP reports nor membrane autopsy reports are available. A thorough investigation was conducted to determine the origins and composition of the scale material and to develop a plan to prevent future scaling. Investigations included water quality and precipitant material analysis, process review, and water quality modeling. Two samples of the scale were collected and sent for inorganic analysis using x-ray fluorescence. Table 2 presents the results, which indicate that the major components of the scale are iron, phosphorus and calcium. Table 3 presents the most probable compounds present in the scale based on the x-ray fluorescence results as calculated by GE s Inorganic Deposit Analysis software (Version 3.0). The compounds, determined by stoichiometry and solubility, indicate that the potential major deposit components are iron oxide, calcium phosphate, iron phosphate and ferric sulfate. Table 2. X-ray Fluorescence Analysis Results from Samples of the Scale Deposited on the Interior of the Concentrate Piping a, b Parameter Sample #1 c Sample #2 c Iron, as Fe 2 O 3 56% 31% Phosphorus, as P 2 O 5 23% 18% Calcium, as CaO 12% 24% Sulfur, as SO 3 2% 11% Sodium, as Na 2 O 2% 6% Magnesium, as MgO 2% 2% Silica, as SiO 2 2% 1% Chloride, as Cl- 1% 7% a X-ray fluorescence detects elements between fluorine and uranium in atomic number. Any of these elements not reported are below detection limits. b This analysis was run on the non-extractables and refers to the mineral portion only. c Sampling date: 24-AUG-2015 Table 3. Potential Scaling Compounds Based on Calculations Using X-ray Fluorescence Analysis Results a Parameter Sample #1 Sample #2 Fe 2 O % 26.4% FePO % - Ca 3 (PO 4 ) % 39.2% Fe 2 (SO 4 ) 3 3% 11.6% Other (<7% of total composition) 7.4% 22.9% a Estimated Compounds and percentages of each were calculated using GE Water and Process Technologies Inorganic Deposit Analysis (Version 3) 4

5 Water Quality Analysis and Precipitation Model Results Table 4 presents raw, feed and concentrate water quality data. The concentrate quality data presented was used to run OLI s Stream Analyzer software (Version 9.2), to determine potential precipitates that would occur in the presence and absence of dissolved oxygen. OLI s Stream Analyzer is a computer software program used to simulate aqueous-based chemical systems, and utilizes a predictive thermodynamic framework for calculating the physical and chemical properties of multi-phase, aqueous-based systems. The model predicts reaction products, phase splits and complete speciation of all phases for a mixture of chemicals in water. NF and RO concentrate water quality data gathered on April 22, 2015 were used in the model. Concentration data from the August 7, 2015 sample was used for water quality parameters that were not available from the April 22, 2015 sample. Two scenarios were evaluated as follows: (1) Scenario A: RO concentrate only and (2) Scenario B: Combined NF and RO concentrate. Table 4 shows the parameters and concentrations used as model inputs as highlighted values. Aeration of the concentrate in the DIW pump station wet well was simulated by the addition of oxygen at various concentrations, ranging from 0.0 to 5 mg/l at intervals of 0.25 mg/l. Figures 3 and 4 present the Stream Analyzer outputs for Scenarios A and B, respectively. In both scenarios there were five dominant solids identified: iron hydroxide, calcium fluorophosphates, calcium carbonate, strontium sulfate and calcium fluoride. The precipitation rate for calcium fluorophosphates, strontium sulfate and calcium fluoride does not change with the addition of oxygen at the various concentrations. However, the model results indicate that when oxygen is added the iron oxidizes to form Fe(OH)3 and the CaCO3 levels drop slightly. The iron hydroxide precipitation increases between 0 to 1.0 mg/l of O2 addition. After that, the results show that all the iron has been oxidized. The results for both scenarios are similar, however the resulting precipitation for the combined NF and RO concentrate (Scenario B) is lower because the levels of the precipitating elements in the mixed concentrate are lower than the RO concentrate. These results were also consistent with some of the sparingly soluble salts identified as being controlled by the scale inhibitor in the scale inhibitor projections: calcium phosphate, barium sulfate and calcium carbonate for NF and barium sulfate, strontium sulfate, calcium carbonate and calcium fluoride for RO. Calcium carbonate was identified as the limiting salt. High iron concentration was also identified as a warning within the scale inhibitor projections. The Stream Analyzer software models equilibrium conditions and does not take into account or determine the precipitation reaction kinetics. The scale inhibitor added to the NF and RO system feed water minimizes scaling in the membrane system by reducing the rate of the precipitation reactions or by dispersing the elements taking part in the reaction. Therefore we would expect that these compounds would precipitate as the software indicates, however, it would occur over a longer time period as the effect of the scale inhibitor diminishes. It is possible that the cascading in the manhole and wet well (aeration), paired with the turbulence generated by the pumps, accelerate the loss of scale inhibition by the scale inhibitor and/or catalyzes the kinetics of the precipitation reaction of all the compounds identified. 5

6 Table 4. Water Quality and Flow Rate Data for RO and NF facilities Sample Location: NF Feed b NF Floridan RO Feed Concentrate Aquifer Well c RO Feed c RO RO RO Concentrate Concentrate Concentrate Flow d (gpm): Analyte a Mass Projection Date: 4/22/2015 4/22/2015 4/28/2015 4/22/2015 balance 4/22/2015 8/7/2015 Using calculation 4/22/15 data Combined Concentrate Mass balance calculation Calcium (Ca++) Magnesium (Mg++) Sodium (Na+) Potassium (K+) Barium (Ba++) Strontium (Sr++) Iron (Fe++) Manganese (Mn++) n/a 0.01 n/a 0.03 n/a n/a n/a Aluminum (Al+++) n/a 0.2 n/a 0.6 n/a n/a n/a Bicarbonate (HCO3-) n/a Sulfate (SO4--) , ,216 1,509 Chloride (Cl-) ,000 1,280 1,560 5, ,952 3,230 Fluoride (F-) 0.3 n/a n/a Nitrate (NO3-) 1 n/a <0.06 n/a n/a n/a n/a n/a n/a Phosphate (PO4---) 0.9 n/a n/a 2 n/a n/a Bromide (Br-) n/a n/a 11.5 n/a n/a n/a 24.2 n/a 4.9 Boron (B) n/a n/a n/a n/a n/a 0.7 n/a n/a Silica (SiO2) ph (units) n/a Alkalinity (mg/l as aco3) n/a Hardness (mg/l as CaCO3) ,580 3,539 2,540 2,538 Conductivity (us/cm) 539 1,890 6, ,584 17, n/a 11,414 TDS 315 1,263 3,880 2,946 3,199 12, ,539 7,838 a Units are in mg/l unless otherwise noted b Pretreated Biscayne Aquifer groundwater with sulfuric acid to reduce ph to 6.5 and scale inhibitor (3 ppm target dose). c RO feed is a combination of NF concentrate (~410 gpm) and Floridan Aquifer groundwater (~914 gpm), pretreated with sulfuric acid to reduce ph to 6.1 and scale inhibitor (5.0 ppm target dose). d Typical plant flow rates provided by the City 6

7 Figure 3. Scenario A RO Concentrate with Oxygen Addition Results Dominant Precipitate (mg/l) Calcium carbonate (calcite) Strontium sulfate (celestine) Calcium fluoride (fluorite) Calcium fluorophosphate (fluorapatite) Iron (III) hydroxide (bernalite) Oxygen Addition (mg/l) Figure 4. Scenario B Combined RO and NF Concentrate with Oxygen Addition Results Dominant Precipitate (mg/l) Calcium carbonate (calcite) Aluminum hydroxide bayerite) Calcium fluoride (fluorite) Calcium fluorophosphate (fluorapatite) Iron (III) hydroxide (bernalite) Oxygen Addition (mg/l) 7

8 Electron Microscopy and X-Ray Spectroscopy Analysis Scanning Electron Microscopy (SEM) analysis is used to determine the topography and morphology of a sample. The SEM shows very detailed 3-dimensional images at much higher magnification than an optical microscope. Energy Dispersive X-ray Spectroscopy (EDS) analysis is generally performed together with SEM to identify and quantify the elemental composition of deposits observed on the SEM. The targeted deposit is bombarded with electrons from the SEM, which produce X-rays. The X-rays are then measured by an X-ray dispersive spectrometer. Every chemical element has its own characteristic wavelength by which it can be identified. The SEM and EDS was done on 0.45 µm filters that had been used to filter water from four sample locations. Samples were collected from (1) RO concentrate, (2) NF concentrate, (3) combined concentrate prior to any air exposure and (4) wet well combined concentrate (after exposure to air). Samples were sent to and analyzed by AWC. Samples 1, 2 and 3 were not exposed to air during filtration. AWC analyzed the samples using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) with Superimposed Elemental Imaging (SEITM). Table 5 shows the material that was found on each filter from the four sample locations. Overall, the EDS results are consistent with x-ray fluorescence analysis of the scaling deposits and with the precipitation model results, with the main components of the scale including iron, phosphorous, calcium and sulfur. Table 5 Material Found on Filters from Four Sample Locations Organic Matter Phosphate Salts of Calcium Magnesium and Iron Ferric Hydroxide Iron Disulfide Iron Phosphate Ferric Silicate Calcium Sulfate Silica Scale Quartz Silts/Clays Calcium Carbonate NF Concentrate (Skid 2) RO Concentrate (Skid 6) Combined Concentrate Wet Well Water Elemental Sulfur ph, ORP, Temperature and Conductivity profile ph, ORP, temperature and conductivity were measured on the concentrate stream from the skids to the injection well pump station on July 14, Sample locations are shown in Figure 5. Field analysis results, provided in Table 6, indicate that as the concentrate passes through the air gap, the ph increases from approximately 6.75 to 6.94, likely due to the off-gassing of carbon dioxide. Data also indicates that the ORP increases from approximately -158 to -65 millivolts (mv) due to the introduction of oxygen. 8

9 Figure 5. ph and Oxidation Reduction Potential Sampling Location Schematic Table 6. ph and Oxidation Reduction Potential Field Sampling Results Sample Sample Location ID Temp ( C) ph (-) Conductivity (ms) 1 RO concentrate N/A a 2 NF concentrate Combined concentrate Analytical instrument discharge to air-gap wet well d Air-gap wet well DIW pump station wet well DIW pump station discharge Notes: a Unable to measure due to air within sample port. C = degrees Celsius d Composed mostly of NF and RO feed water ID = identification ms = millisiemens N/A = not applicable ORP (mv) 9

10 Interim Solutions Alternatives Comparison Scale Prevention Plan The following alternate solutions were considered for short term implementation to minimize scaling within the currently used concentrate disposal system, during planning and installation of permanent solutions, which could take up to two years to implement. Concentrate ph Reduction: Reduction of the concentrate ph downstream of the air gap may reduce scaling potential. OLI s Stream Analyzer software (Version 9.2), was used to determine the effects of ph reduction from 6.9 to around 6.0 of the concentrate on the precipitation potential (Scenario B2, Combined RO and NF Concentrate). Figure 6 presents the Stream Analyzer outputs for Scenario B2, which shows the same five dominant solids identified, including iron hydroxide, calcium fluorophosphates, calcium carbonate, strontium sulfate, and calcium fluoride. The precipitation rate of particular calcium carbonate is inhibited by the scale inhibitor and therefore the intent of the graph is to present trends of solids as a function of oxygen and ph levels and not the absolute solids values. In Scenario B2, ph reduction through sulfuric acid feed results in the decrease of calcium carbonate precipitation. However, it shows no significant change in the calcium phosphate precipitation potential. This is supported by the calcium phosphate stability index (SI) calculations proposed by Kubo et al., which indicate that the critical ph for calcium phosphate in the combined concentrate is 7.8. The actual ph is 6.9 and the SI is negative and non-scaling and therefore a further ph reduction with an acid will be have limited effect. Reduction of Nanofiltration Recovery: New NF membranes were recently installed on all skids. Prior to the membrane change, the NF system operated with a recovery rate of 78 percent. The City increased the recovery rate to 85 percent with installation of the new membranes. As a temporary measure, it is recommended that the recovery rate be reduced down to 80 to 82 percent, to decrease the concentration factor from 6.7 to approximately 5.0. The decrease in recovery rate also would achieve an increase in concentrate flow per element to a minimum value recommended by the membrane vendor and achieve a reduction on precipitation products of sparingly soluble salts. Table 7 summarizes the operating parameters and projected concentrate water quality for NF skid operation at 80, 82, and 85 percent recovery rates. The City is resource limited and therefore this interim solution needs to be balanced with raw water availability. Direct Discharge via Old Sewer Connection: Prior to the construction of the RO plant, the West WTP disposed of the NF concentrate through a pipe that provided a direct discharge to the Broward County wastewater collection system. With the concentrate pump station, a new pipe was installed to route the pump station discharge to the sewer force main and serve as the backup for concentrate disposal. A spool piece was removed from the old pipe and replaced with blind flanges to create an air gap. An interim solution could consist of switching back to using the old sewer connection, to eliminate the concentrate aeration that is currently occurring in the DIW pump station. In lieu of providing an air gap, the City should add a dual reduced pressure zone device (RPZD) to the old line to prevent any backflow from sewer, as required by regulatory agencies. 10

11 Figure 6. Scenario B2 Combined Reverse Osmosis and Nanofiltration Concentrate with ph Reduction Results Dominant Precipitate (mg/l) Calcium carbonate (calcite) Aluminum hydroxide bayerite) Calcium fluoride (fluorite) Calcium fluorophosphate (fluorapatite) Strontium sulfate (celestine) ph Table 7. Nanofiltration Projection Summary for 80, 82, and 85 Percent Recovery Recovery Rate 85 Percent Percent 80 Percent Parameter Current Interim Recommended Operating Operational by Hydranautics Condition Measure Permeate flow (gpm) Concentrate flow (gpm) Average flux rate (gfd) Concentrate Water Quality Calcium (mg/l) Magnesium (mg/l) Sulfate (mg/l) TDS (mg/l) ph (-) Langelier Saturation Index (-) CaSO 4 /ksp * 100, Saturation (%) Notes: CaSO 4 = calcium sulfate gfd = gallons per square foot of membrane per day ksp = solubility product TDS = total dissolved solids 11

12 Periodic Cleaning of Discharge Line to Sewer: Alternatively, if the City decides to continue using the current concentrate disposal method, through the pump station with discharge to sewer, the pipeline could be periodically cleaned to minimize scale accumulation. Full or partial cleaning of the scale may be achieved either through acidization or pigging of the line. Pigging the line would require the installation of a pig launching and a receiving station. Periodic acidization of the line would require installation of an acid feed system and a neutralization system to allow waste disposal. The feasibility of acidization and pigging for removing scale is unknown and should be tested prior to implementation of this temporary measure. From the interim options evaluated, two modifications can be considered for implementation in the short term: (1) reduction of NF recovery and (2) direct discharge via the old sewer connection, with the installation of a dual RPZD. The feasibility of the direct discharge depends on acceptance by the regulatory agencies. Further testing is recommended to verify effectiveness of acidification or pigging of the concentrate pipelines to dissolve/reduce scaling and restore capacity. The removal of scale may be necessary if and when the other interim options are not feasible or effective and until the permanent modification is operational. Long Term Solutions Alternatives Comparison Several permanent modification options to eliminate the air gap and minimize scaling within the currently used concentrate disposal system were evaluated. The considered alternatives are summarized below: In-line Booster Pump Station: The use of such pump station will eliminate the air gap and reduce consequent scale formation. The proposed in-line booster pump station would be integrated with the existing concentrate pump station, which would primarily be used for discharging to sewer and would receive drain flows from the RO Building and DZMW and instantaneous higher flows from the concentrate line due to startup activities. Other utilities in South Florida use this type of pump station to provide an additional pressure boost, if required, for DIW disposal. Discharge to Broward via old Sewer Connection: The direct discharge also would eliminate the air gap and would discharge continuous waste flows to sewer. The existing pump station would be maintained for the disposal of drain flows and high instantaneous flows. The existing DIW would be decommissioned and no additional backup disposal would be available. Because of this, a significant monthly disposal fee to Broward County wastewater collection system would apply. This option also requires Broward County Department of Health approval and would need, at a minimum, a dual RPZD. Split Disposal NF and RO: The RO concentrate has sufficient residual pressure and would be discharged directly into the DIW, while the NF concentrate would be discharged directly into the Broward County wastewater collection system and a disposal fee will apply. The existing pump station would be maintained for disposal of drain flows and high instantaneous flows. Both direct discharges would require some form of backflow prevention. Zero Liquid Discharge (ZLD) Treatment: The combined concentrate would undergo extensive treatment so that the only residual remaining is a solid waste product. The existing DIW would be decommissioned and the sewer connection would be used for the drain flows 12

13 only. Other utilities in arid areas of the U.S. (Texas and California) have tested this technology, but the number of actual applications is still limited. Concentrate Sedimentation Treatment: The combined concentrate would undergo intensive aeration to oxidize the reduced compounds (e.g., iron, sulfide) and then will be coagulated and settled in traditional lamella or alternative separation systems. The settled water would be discharged into the DIW and the coagulation sludge would be combined with lime sludge for further processing in the thickener and dewatering facility. The DIW would remain as the primary method of disposal and Broward County wastewater collection system would remain the backup method. Nitrogen Blanket in Wet Well: The combined concentrate still would flow to the existing manhole and concentrate pump station wet well; however, air in the underground wells would be replaced with a nitrogen blanket to limit the oxidation of reduced compounds with the oxygen in the air. This technology is used in the petrochemical industry; however, it is unknown in the water industry. The evaluation of permanent alternatives is summarized in Table 8. The Nitrogen Blanket in Wet Well alternative is considered to be a novel concept in the water industry and has not been proven for this specific application. The ZLD Treatment and Concentrate Sedimentation Treatment alternatives have been developed and tested by other utilities in the U.S. however, only a limited number of full-scale plants are currently operational. Further testing would be recommended for these applications to confirm feasibility and to develop design criteria for the treatment system. This would require additional implementation time. In addition, the cost to construct and operate the treatment facilities would be considerably more than the other alternatives considered. The Discharge to Broward via old Sewer Connection and Split Disposal NF and RO sewer disposal alternatives are straightforward and would require limited operation and maintenance involvement. However, there would be no backup for sewer disposal and the disposal fee would be high, around $40,000 per month. Therefore, the In-line Booster Pump Station alternative is considered the most viable of the alternatives (proven by other utilities in South Florida to be relatively cost-effective) and can be integrated with the other existing infrastructure at the WTP site. 13

14 Table 8. Scoring Table for Permanent Alternatives Considered Alternatives Criteria In-line Booster Pump Station Discharge to Broward via old Sewer Connect Split Disposal NF and RO t ZLD Treatmen Concentrate Sedimentati on Treatment Nitrogen Blanket in Wet Well Primary Disposal DIW Sewer DIW/Sewer Solids DIW DIW Backup Disposal Sewer - Sewer - Sewer Sewer State of Technology Further Testing Needed Implementation Time Capital Cost Operating Cost Notes: A + score is favorable compared to the score of other alternatives. A - score is less favorable compared to the score of other alternatives. A score of 0 is neutral when compared to the score of other alternatives. ZLD = zero liquid discharge DIW Rehabilitation Plan From the middle of 2013 through early 2014, DIW injectivity decreased from approximately 50 to 20 gallons per minute (gpm) per psi. The injectivity further decreased in A key component of the long term improvements to the concentrate disposal system is to rehabilitate the DIW. The West WTP DIW was constructed with an 18 inch outside diameter (OD), inch wall thickness, steel casing and a nominal inch fiberglass reinforced plastic (FRP) tubing, with a inch inside diameter (ID). The open hole injection interval extends from 3,020 to 3,520 feet (ft) below land surface (bls). Investigative work was performed on the DIW and in April 2016, through video logging, it was confirmed that the scale also was present on the inside of the injection well FRP tubing and in the bottom of the open borehole. The scale buildup measures between 1 and 2 inches, has a claylike consistence. DIW rehabilitation will entail further initial investigations of the downhole condition, followed by casing brushing, airlift development, re-drilling, and/or acidization or other chemical treatment, depending on actual conditions found during the rehabilitation process. Casing Brushing A casing brush will be used as an initial means of rehabilitation. A nylon fiber brush, with an inch diameter, will be used as an alternative to a steel brush to avoid damaging the FRP casing liner. The brush will be alternately raised and lowered within a 10-foot section of well casing or open hole for several minutes. This procedure will be repeated for each section of well casing or open hole with scale buildup. 14

15 Reverse airlift development will be used following casing brushing to remove loose material which has been scraped from the well casing. Discharge will be collected in temporary containers with settling tanks, allowing for the removal of solids. Discharge water will be transferred to the Broward County sewer collection system and collected solids will be transferred to an appropriate disposal location. Reverse Airlift Development Reverse airlift development will be used to attempt to dislodge and remove fill from the open borehole. Discharge will be collected in temporary containers with settling tanks, allowing for the removal of solids. Discharge water will be transferred to the Broward County collection system and collected solids will be transferred to an appropriate disposal location. In the event that reverse airlift development is unable to dislodge and remove the fill and open the borehole, reverse-air drilling method will be required with an appropriate sized drill bit. Rubber protectors (bumpers) will be required on all drill pipe connections to prevent FRP liner damage during re-drilling. Acidization of Open Borehole Formation Following brushing of the casing and reverse airlift development, acidization of the well may be completed to further develop and increase injectivity of the open borehole injection zone. One casing volume, 14,150 gallons, of 32% Hydrochloric (HCl) acid solution will be injected into the borehole formation by a drilling contractor supplied pump at approximately 75 gpm. Acidization will be completed using a 2 3/8 -inch pipe to inject the acid into the formation below the casing with simultaneous water injection from the wellhead at 250 gpm to help convey the acid solution into the formation. Alternatively, acidization may be performed by filling the casing with acid from the wellhead, followed by injecting water at a higher rate. The drilling contractor may obtain potable water from a hydrant on site approximately 400 feet from the wellhead. Following acidization, the well will be left static for 72 hrs before putting the well back in service. Upon approval from regulatory agencies, the acidization process may be repeated with other acids if sufficient capacity has not been restored with the procedure outlined above. Conclusions Investigation results indicated that iron, phosphorus, calcium, and sulfur make up the main components of the scale deposits found in the concentrate disposal system. These results were consistent with the water quality model, which identified iron (III) hydroxide, calcium fluorophosphate, calcium fluoride and strontium sulfate as the major potential precipitates. Filtered concentrate samples analyzed using SEM and ERS analysis also confirmed the presence of these precipitates. The water quality model results indicated that only iron (III) hydroxide and calcium carbonate precipitation are affected by the addition of oxygen. However, additional changes to the water chemistry, due to the release of carbon dioxide could also accelerate the chemical reaction kinetics and increase the precipitation potential of carbonate, phosphate and sulfate salts. These results were also consistent with some of the sparingly soluble salts identified 15

16 as being controlled by the scale inhibitor in the scale inhibitor projections: calcium phosphate, barium sulfate and calcium carbonate for NF and barium sulfate, strontium sulfate, calcium carbonate and calcium fluoride for RO. Calcium carbonate was identified as the limiting salt. High iron concentration was also identified as a warning within the scale inhibitor projections. Since no deposits were observed upstream of the point where the concentrate is exposed to air and carbon dioxide is off-gassed, and consistent with testing and model results, it is hypothesized that eliminating the aeration step will reduce the quantity of scale build up in the DIW piping and the DIW casing. Several permanent modifications were evaluated for implementation and the inline booster pump station is the recommended option. The existing concentrate pump station will remain operational for other drain flows and intermittent high volume waste streams from the membrane trains. The design and construction of the in-line booster station is planned to eliminate the aeration of the concentrate stream. This approach to concentrate disposal is successfully used at other South Florida WTPs that also use a combination of NF and RO treatment, similar to the West WTP. A DIW rehabilitation plan entailing casing brushing, reverse airlift development and potential acidization is currently underway. However, the level of success will only be known after rehabilitation work is completed, therewith dictating the design criteria for the new in-line booster station. In the meantime, short term operational changes will be implemented to minimize scaling potential. Several interim options were evaluated and two modifications are considered for implementation in the short term: (1) reduction of NF recovery and (2) direct discharge via the old sewer connection, with the installation of a dual RPZD. The feasibility of the direct discharge depends on acceptance by the regulatory agencies. The removal of scale may be necessary if and when the other interim options are not feasible or effective and until the permanent modification is operational. Further growth in potable water demands will drive the industry to increased membrane system recoveries. Systems with increased recovery have been operating successfully on groundwater in Florida; however, this case study from Deerfield Beach highlights significant scaling potential from a combined concentrate after aeration a precautionary example to be considered. 16