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Long-Term Pilot Testing of Membrane Filtrationfor Ultra-Low Tertiary Phosphorus Removal at Spokane

Paul A. Mueller, CH2M, 1100 NE Circle Blvd, Suite 300, Corvallis OR 97333paul.mueller@ch2m.com, Ph: 541-768-3418Lars Hendron, City of Spokane, Washington

The City of Spokane, Washington is implementing tertiary membrane filtration at their RiversidePark Water Reclamation Facility, The primary objective of the tertiary membrane system is toproduce effluent with a seasonal average total phosphorus concentration of less than 18 partsper billion at a maximum month average daily flow capacity of 50 million gallons per day and a12-hour peak flow capacity of 75 mgd. To support membrane system selection, extensive side-by-side pilot testing of an immersed ultrafiltration system and a pressurized microfiltration systemwere performed to validate design and operating parameters, including membrane cleaningprocedures. Pilot test runs included fixed-flow simulation of maximum monthly flow andvariable-flow simulation of normal diurnal flow and storm flows based on real-time plant flowrates. This presentation will summarize the pilot plant configuration and present results of 16months of pilot testing. Particular focus will be placed on describing the response of themembrane systems to non-steady-state operations caused by storm flows and/or changes toupstream operations of the primary and secondary treatment processes at the plant.

Background

The City of Spokane, Washington is implementing a major tertiary upgrade to its Riverside ParkWater Reclamation Facility. The upgrade, known as the “Next Level of Treatment” project(NLT) will greatly improve the quality of effluent that is released to the Spokane River. TheNLT project will incorporate alum coagulation and direct membrane filtration to increase theoverall removal of phosphorus across the plant to more than 99 percent, from the current level of90 percent. The NLT system will also provide enhanced removal of heavy metals,polychlorinated biphenyls, and other pollutants. The NLT system will be designed to treat amaximum month average wastewater flow of 50 million gallons a day (mgd) and a 12-hour peakflow of 75 mgd.

The City is required to be in compliance with phosphorus standards for the plant's effluent byMarch 2021. The discharge permit for the plant requires the City to reduce the amount ofphosphorus going into the river during a 240-day “critical” period from March through October.

The NLT project is part of the City’s Integrated Clean Water Plan, a major initiative to improvethe health of the Spokane River. Integrated planning allowed the City to study all flows thatbring pollutants to the river, consider all viable technologies and options to manage those flows,and develop a comprehensive solution to deliver the best value for the investment. Essentially,the approach is designed to achieve enhanced results more quickly at a more affordable price.

A block flow diagram for the plant following the NLT implementation is shown in Figure 1. TheNLT project will consist of alum coagulation and direct membrane filtration at a firm capacity of50 mgd and a peak capacity of 75 mgd, along with improvements to increase primary treatmentcapacity to 125 mgd to match the existing secondary treatment capacity. Membrane filtration

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was selected to take advantage of the superior phosphorus removal afforded by the membranefiltration process in the context of the entire 240-day critical season, rather than on a day-to-daybasis. 50-mgd firm capacity and 75-mgd peak membrane capacities were selected based onstatistical evaluation of 10 years of plant flow data, as was the improvement of primary capacityto 125 mgd. Increasing primary capacity to 125 mgd reduces the expected number and durationof secondary treatment bypasses. Sizing the membrane system for 50 mgd firm capacity with a12-hour sustained peak capacity of 75 mgd allows for occasional tertiary treatment bypass bydesign, balancing the probability of bypass against the capital and operating cost of the tertiarytreatment system.

Membrane filtration was selected because it provides the best balance of critical seasonphosphorus removal and lifecycle costs. To achieve the critical season phosphorus removaltarget for the overall plant, the NLT treatment scheme consisting of alum addition, flocculation,and membrane filtration must be capable of averaging no more than 18 µg/L total phosphorus fora 30-day average daily flow of 50 mgd, with 12-hour sustained peak flows of 75 mgd.

Note that as shown in Figure 1, the treatment plant employs chemically-enhanced primarytreatment (CEPT), with a nominal alum dosage of 50 mg/L applied to the primary influent.CEPT also includes a nominal 0.3 mg/L dosage of a medium charge density anionicpolyacrylamide emulsion polymer (CLARIFLOC A-6350).

MethodsThe purpose of the pilot test was to confirm proposed design and operating criteria for the twovendors, and to confirm that membrane filtration effluent quality could meet or exceed thequality required for the selected pretreatment alternative from the Facilities planning. Designand operating parameters for the pilot system were presented in a previous paper (UsingCoagulant-Aided Tertiary Membrane Filtration to Achieve Ultra-low Phosphorus Limit for Cityof Spokane, AMTA/AWWA Membrane Technology Conference and Exposition, San Antonio,TX, February 2016). This paper presents the results of phosphorus removal during the pilottesting.

A schematic of the pilot system is presented in Figure 2. Characteristics of the pilot pretreatmentsystem are summarized in Table 1. Characteristics of the membrane systems tested in the studyare shown in Tables 2 and 3.

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FIGURE 1Spokane Riverside Park Water Reclamation Facility Block Flow Diagram

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FIGURE 2Membrane Filtration Pilot Testing Configuration

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TABLE 1Pretreatment Design Criteria

TABLE 2Vendor A Membrane Design and Operating Characteristics

Membrane PVDF, 0.04µm pore size

Module Tested (2) 440 SqFtFiltration Mode Outside - InProduction Interval 190 gallonsBackpulse At end of every

production intervalTank Deconcentration After every 5th backpulse

Backpulse Flux 40 gfdBackpulse Duration 15 seconds (30 seconds

during tankdeconcentration)

Intermittant Aeration During backpulse only

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TABLE 3Vendor B Membrane Design and Operating Characteristics

ResultsWater Quality – 2015 Critical Phosphorus SeasonAverage results for water quality are presented in Table 4. A probability plot of total phosphorusconcentration across the plant during the 2015 Critical Season is presented in Figure 3. Thesame data are presented in a box-and-whiskers plot in Figure 4, with separate filtrate data for thetwo types of membranes tested.

The average membrane filtrate total phosphorus concentration during the 2015 critical seasonwas 0.012 mg/L as P, better than the target value of 0.018 mg/L. The average filtrate totalphosphorus for membrane A, an ultrafilter with a rated pore size of 0.04 µm, was 0.0130 mg/L.The average for Membrane B, a microfilter with a rated pore size of 0.1 µm, was 0.0118 mg/L.

Membrane PVDF, 0.1µm pore sizeModule Tested (1) 6" module, 538 SqFtFiltration Mode Outside - InInstantaneous Filtrate Flux 25.7 gfd @ 50 mgd

simulationFeed Water Recovery 96%Backwash Air Scrub/Reverse Filtration

Frequency Every 400 gallons offiltrate production

Flow Rate 8 gpmAir Flow Rate 3.0 scfmDuration 60 seconds

Backwash Feed FlushFrequency Every 400 gallons of

filtrate productionFlow Rate 16 gpmDuration 23 seconds

Feed Water Recirculation 1.0 gpm fixed

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TABLE 4Pilot Plant Water Quality2015 Critical Season, March 1 – October 31

Parameter UnitsSecondary

EffluentPilot Plant

FeedMembrane

FeedMembrane

FiltrateAlkalinity mg/L as CaCO3 71.3 85.4Carbon, Dissolved Organic (0.45 µm) mg/L 5.03 4.64 4.28Coliform, Fecal #/100 mL 22.3Nitrogen, Ammonia mg/L as N 0.09Nitrogen, Nitrate mg/L as N 26.63Nitrogen, Nitrite mg/L as N 0.09Nitrogen, Total mg/L as N 27.82Oxygen Demand, Biochemical (5d) mg/L 7.09Oxygen Demand, Carbonaceous Biochemical (5d) mg/L 2.97Oxygen Demand, Chemical mg/L 20.7pH 6.65 6.62 6.81Phosphorus, Acid-Hydrolyzable (0.45µm) mg/L as P 0.054 0.008 0.007Phosphorus, Ortho (0.45µm) mg/L as P 0.063 0.068 0.008 0.006Phosphorus, Total mg/L as P 0.370 0.431 0.431 0.012Phosphorus, Total Reactive mg/L as P 0.237 0.278 0.007Solids, Total Suspended mg/L 8.03 10.83 17.4Solids, Volatile Suspended mg/L 6.27Temperature °C 18.49 18.62 18.62Turbidity ntu 2.82 4.39 0.037UV254 1/cm 0.104 0.094 0.106

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FIGURE 3Total Phosphorus Probability Plot2015 Critical Season, March 1 – October 31

FIGURE 4Total Phosphorus Box and Whiskers Plot2015 Critical Season, March 1 – October 31

0.001

0.01

0.1

1

10

PrimaryInfluent

SecondaryInfluent

SecondaryEffluent

Membrane AFiltrate

Membrane BFiltrate

mg/

L as

P

Total Phosphorus

Effluent Target

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Tertiary System Phosphorus Baseline and Storm Loading ProfileBecause it serves a combined sewer system, the Riverside Park facility is subject to significantpeak flows in response to storm events. To meet phosphorus removal objectives, the NLTsystem will need to produce low concentrations of phosphorus during storm events. Historicalwater quality data for the plant is based on analysis of flow-weighted composite samples. Therewas no data on the variability of phosphorus concentrations with time during normal diurnalflows or during storm events.

As a preliminary effort while mobilizing for pilot testing, an automatic sampling system wasdeployed on the secondary effluent flow stream and programmed to take one sample per hour for24 hours. The sampling system captured a storm event on June 29, 2014. Sampling wasrepeated on July 9, 2014 to profile phosphorus concentrations during the normal summer diurnalflow pattern. Results of these sampling events are presented in Figures 5-7.

Results of the phosphorus load study show that while the concentration of total phosphorus in thesecondary effluent does increase significantly during peak flow events, the concentration ofsoluble phosphorus requiring treatment through alum addition in the NLT system remains fairlyconstant. The storm profile showed a slight decrease in ortho-P concentration during the peakflow storm event, but not enough to warrant an adjustment to alum dosing during a storm event.

As the result of this study, the pilot testing and conceptual design for the NLT facility was basedon simple flow-paced alum addition at a fixed dosage.

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FIGURE 5Tertiary Phosphorus Load Profile: Flow

FIGURE 6Tertiary Phosphorus Load Profile: Phosphorus Concentration

FIGURE 7Tertiary Phosphorus Load Profile: Hourly Phosphorus Mass Load

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Fiber Cut StudySensitivity of filtrate total phosphorus concentrations to induced breaches of membrane integritywas investigated in this study. The protocol involved documenting baseline performance of eachmembrane pilot system filtrate for total phosphorus and turbidity, and then after cutting a singlefiber and after cutting a second fiber. A final sample was taken mid-way through the firstfiltration cycle following repair of the cut membrane fibers performed by representatives of therespective membrane suppliers. Testing was performed with the pretreatment system operatingat a fixed flow with two stages of flocculation and a tertiary alum dose of 25 mg/L. Watertemperature was 20.1°C at the time of the testing. Membrane A TMP was 9.8 psi during thefiltration cycles with the cut fibers. Membrane B TMP at the same time was 5.89 psi.

After each fiber cut, filtrate samples were taken at three points within the following filtrationcycle (production of filtrate between backwashes). The samples were taken 10 percent, 50percent, and 90 percent through the filtration cycle. For the immersed membrane system(Membrane A), backwashing consisted of a process tank blowdown after 5 cycles of reversefiltration backpulse. For that membrane system, the filtration cycle was defined as beingbetween the tank deconcentrations.

The automated air-pressure-hold membrane integrity test function was measured prior to thefiber cutting, after each fiber cut, and after the fibers were repaired. For Membrane A, thebaseline pressure decay prior to fiber cutting was 0.8 psi/min from an initial test pressure of10.67 psi. After cut of a single fiber, the measured pressure decay was 7.31 psi/min from aninitial pressure of 10.1 psi. After the second fiber was cut, measured pressure decay was 8.11psi/min from an initial pressure of 9.93 psi. For Membrane B, the baseline integrity test showeda loss of 0.6 psi in 5 minutes from an initial test pressure of 23.66 psi. The integrity test functionfailed to achieve a stable initial test pressure following cuts of one or two fibers. The integritytest following fiber repairs showed a 5-minute pressure decay of 0.51 psi from an initial testpressure of 25.46 psi.

Total phosphorus results for the fiber cut study are shown in Figure 8. Filtrate turbidity asmeasured by the on-line instrumentation on each pilot skid is summarized in Figure 9. ForMembrane A, filtrate total phosphorus generally did increase during each filtration cycle,especially with two cut fibers. The reported filtrate turbidity also increased during each filtrationcycle, and was higher with two cut fibers than with a single cut fiber. Measured total phosphorusdid not exceed the target value of 0.018 mg/L even with two cut fibers, despite significantincreases in filtrate turbidity and in the measured pressure decay rate.

For Membrane B, measured total phosphorus and reported turbidity for filtrate did not appear todegrade within filtration cycles when one or two fibers were cut. Filtrate quality was maintaineddespite failure of the membrane integrity test function with one or two cut fibers.

Soluble Aluminum ProfilePreliminary operation of the NLT pilot system utilized two flocculation stages each with 10minutes of detention time at maximum month average flow. Preliminary bench scale testing hadindicated that conversion of soluble reactive phosphorus to a filterable form was complete after asingle stage of flocculation. As a precursor to operating the pilot system on single-stage

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flocculation, profiling of soluble and colloidal aluminum through the pilot pretreatment systemwas executed.

Testing consisted of grab samples taken across 5 days of operation in October 21-27, 2015. Thepilot system was operated at fixed flow during this testing. Samples of pilot feed (prior totertiary alum addition), and outflow from first-stage and second-stage flocculated water weretaken and analyzed for aluminum. To assess the potential presence of colloidal aluminum,unfiltered samples were compared to filtered samples using standard laboratory filters (0.45 µm)and ultrafilters (250 kDa PVDF; Snyder Filtration YMBX475).

Average water quality during the testing is summarized in Table 5. Aluminum results from thissampling are presented in Figure 10. The results show the increase in total aluminum due totertiary alum dosing. Soluble (0.45 µm filtered) aluminum concentration decreased slightly instage 1 flocculation, and was essentially stable between stage 1 and stage 2. Colloidal (250 kDafiltered) aluminum increased slightly in stage 1 flocculation, and remained stable through stage 2flocculation.

Based on the results of this testing, subsequent operation of the NLT pilot system utilized single-stage flocculation.

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FIGURE 8Impact of Fiber Cuts on Filtrate Total Phosphorus

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FIGURE 9Impact of Fiber Cuts on Filtrate Turbidity (On-Line Measurement)

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TABLE 5Average Water Quality during Aluminum Profile Testing

FIGURE 10Aluminum Residual Profile55 mg/L alum dose upstream, 25 mg/L alum dose at tertiary

Parameter UnitsSecondary

Effluent

Pilot Feed(Strained

SecondaryEffluent)

MembraneFeed

Phosphorus, Total mg/L as P 0.19 0.22Phosphate, Ortho mg/L as P 0.022 0.006Phosphorus, Acid Hydrolyzable mg/L as P 0.020 0.006Phosphorus, Total Reactive mg/L as P 0.113Alkalinity mg/L as CaCO3 83.2pH 6.37 6.30Temperature °C 14.32 14.23Total Suspended Solids mg/L 5.3 6.0 16.8Turbidity ntu 2.0UV254 1/cm 0.092 0.081Dissolved Organic Carbon mg/L 4.99 4.31BOD5 mg/L 6.1

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Impact of Upstream Wastewater Operations on Membrane FoulingInitial conceptualization of the NLT System was based on limiting operation to the CriticalPhosphorus Season, March 1 to October 31 annually. As the project progressed, discussionswith stakeholders identified a desire to operate the system year-round to achieve maximumenvironmental benefit.

A decision was made to limit design flow through the NLT system to 50 mgd during the non-critical season, and pilot testing was extended to confirm operability of the membrane systemsduring the winter months. In addition to assessing the impacts of colder water, a possibleprocess adjustment during winter is reduction or elimination of alum and/or polymer addition aspart of the CEPT process.

To assess the impact of these process changes, stepwise reduction of CEPT alum and polymerdosing was performed in February, 2016. This occurred following a period of wet-weatheroperation, resulting in elevated plant flow rates during the months of December and January.Operation of the membrane pilot systems in the non-critical season beginning in November hadresulted in lower permeability and generally faster rates of fouling for both pilot membranesystems. To improve average permeability between the scheduled clean-in-place events, bothmembrane vendors investigated the use of acid solutions as part of the regime for mini-cleans(performed every 1-2 days).

Key primary and secondary process parameters are presented along with the permeability trendsfor each membrane in Figures 11-13. Each figure includes a group of three graphs:

A summary of the key primary and secondary process parameters, including primary andsecondary alum dose and primary polymer dose. Also included on those graphs is anestimate of the mean cell residence time in the secondary process.

The permeability profile for Membrane A, including permeability calculated before, duringand after each backpulse.

The permeability profile for Membrane B, including an indication of when hypochlorite andacid mini-clean procedures were used.

Figure 11 covers operation following the holiday beak until the next scheduled clean-in-place.Both membrane systems had been cleaned and placed in standby for a two-week period aroundChristmas and the New Year holiday. It provides a baseline for performance prior to any primaryor secondary process changes.

Figure 12 covers operation during the adjustment of the primary and secondary processparameters, as the CEPT alum and polymer doses were reduced and eliminated.

Figure 13 shows performance for the next cleaning cycle, as the primary and secondaryprocesses were returned to typical summer operation in time for the current summer permitseason beginning April 1.

All pilot system operation during this phase was at constant flow, simulating the maximummonth average daily flow of 50 mgd. The tertiary alum dose was maintained at 25 mg/L.

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FIGURE 11Baseline Winter Operation

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FIGURE 12Adjustment of Primary/Secondary Treatment Parameters

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FIGURE 13Adjustment to Typical Summer Operation

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Initial acid mini-cleans were performed using a 0.25 percent solution of citric acid. BeginningJanuary 29, 2016 a blend of 0.25 percent citric acid and 0.25 percent hydrochloric acid was used.Beginning February 18, 2016 sulfuric acid was substituted for hydrochloric acid at the same 0.25percent concentration. Beginning March 24, the blended acid solution strength was reduced to0.125 percent each of citric acid and sulfuric acid.

Data in Figures 11-13 show a clear stabilization in membrane permeability for both systemsduring this period. Permeability data for Membrane B shows a positive impact in the use of acidmini-cleans during, both in terms of increasing membrane permeability immediately after thecleaning, and in reducing the rate of permeability decline after the cleaning.

While the use of acid mini cleans was clearly beneficial for Membrane B, both membranesystems showed improved performance in response to changes in primary and secondarytreatment parameters. CEPT chemical dosing (alum and polymer) were adjusted during thisphase of testing specifically to observe impacts on membrane performance. At the same time,the mean cell residence time of the biological secondary treatment process was increasing tofacilitate nitrification in in advance of the initiation of the summer effluent ammonia permit limiton April 1.

Within this testing period, due to a temporary failure of the pilot control system, alum feed aheadof both membrane systems shut off for approximately 24 hours on March 17, 2016. This resultedin an immediate, pronounced drop in permeability for both membrane systems that was reversedupon re-initiation of the alum feed.

The net result of 12 months of operation of the pilot system, including optimization of mini-cleanprocedures, on performance of Membrane system B is shown in Figures 14 and 15. TheseFigures compare permeability profiles during the April-May timeframe between 2015 and 2016.The differences in pilot system design and operating parameters is summarized in Table 6.

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FIGURE 14Initial (Unoptimized) Critical Season Membrane Fouling ProfileOperation between April 2 and May 12, 2015

FIGURE 15Optimized Critical Season Membrane ProfileOperation between April 19 and May 20, 2016

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TABLE 6Membrane B Initial and Optimized Design and Operating Parameters

Parameter Units Initial Operation Optimized OperationPretreatment

Tertiary Alum Dose mg/L 30 25Rapid Mix 3 minutes @ G = 250 3 minutes @ G = 250

Flocculation2 Stages,

10 minutes @ G = 40each

1 Stage,10 minutes @ G = 40

Membrane FiltrationFlux (50 mgd) gfd 27.4 27.4Air Scrub/Reverse Filtration Interval gal/module 306 400Air Scrub/Reverse Filtration Water FlowRate gpm/module 8 8

Air Scrub/Reverse Filtration Air Flow Rate gpm/module 3 3Air Scrub/Reverse Filtration Duration seconds 60 60Feed Flush Interval gal/module 306 400Feed Flush Flow Rate gal/module 16 16Feed Flush Duration seconds 23 23XR Flow Rate gpm/module 1.5 1Maintenance Clean Type 1 Interval gal/module Daily 23,004Maintenance Clean Type 1 Chemical Sodium Hypochlorite Sodium HypochloriteMaintenance Clean Type 1 Concentration mg/L 600 500Maintenance Clean Type 2 Interval Not Used Once per Week

Maintenance Clean Type 2 Chemical Not Used Blended CitricAcid/Sulfuric Acid

Maintenance Clean Type 2 Concentration Not Used 0.125% EachMaintenance Clean Temperature °C 40 40Maintenance Clean Solution Volume gal 25 25Maintenance Clean Duration min 30 30Feed Water Recovery 95.2% 96.3%