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STARTING UP A TERTIARY WASTEWATER MEMBRANE SYSTEM

FOR ENHANCED NUTRIENT REMOVAL

Michael S. Zeltner, HDR Engineering, Inc., 412 E Parkcenter Blvd., Suite 100,

Boise, ID 83706

michael.zeltner@hdrinc.com, Ph: 208-387-7089

Pierre Kwan, HDR Engineering, Inc., Bellevue, WA

Casey Fisher, Coeur d’Alene Wastewater Utility Department, Coeur d’Alene, ID

Abstract This manuscript describes the challenges faced and overcame during a start-up of a submerged GE ZeeWeed 500D system used for tertiary treatment at the City of Coeur d’Alene Advanced Wastewater Treatment Facility (AWTF). The AWTF discharges into the Spokane River which has extremely low effluent ammonia and phosphorus limits. The existing trickling filter-based treatment system is unable to meet the limitation in the most recent discharge permit. After several pilot studies, the City ultimately selected and installed tertiary membranes with chemical sludge recirculation to meet the lower limits. The system was designed such that an initial 1.0 million gallons per day (mgd) would be installed and operated as a full-scale pilot to confirm full-scale 6.0 mgd facility. The initial set of membranes was constructed in 2014 and began commissioning in early 2015. Once the process was seeded, the membranes began experiencing increase transmembrane pressures and reduced permeability. Due to an upstream process change in another part of the facility, the sludge became extremely difficult to filter and ultimately had to be drained from the system. Additionally, the membranes fouled to the point requiring a double-strength recovery clean to remedy. An alternative seeding process was utilized the second time with great results. Once the process became stable, the membrane system was unable to meet performance criteria. Ultimately additional membrane area was required to maintain stable operation of the membrane system.

Introduction The City of Coeur d’Alene, Idaho operates an Advanced Wastewater Treatment Facility (AWTF) to treat all of the wastewater flow within the city. The AWTF serves a population of approximately 46,000 people with an annual average discharge flow of approximately 3.3 million gallons per day (mgd). An aerial photo is shown in Figure 1.

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Figure 1. Aerial view of AWTF

Background

The AWTF was originally commissioned in 1939 providing a secondary level of treatment by utilizing a rock-media trickling filter before discharging effluent into the Spokane River. The original service population was approximately 10,000 people. In May 1981, the City Council imposed a moratorium on new sewer connections to avoid fines from the Environmental Protection Agency (EPA). Over the next year, facility planning and detailed design occurred which led to construction of the Phase 1 Improvements and thus lifting the moratorium. Between then and 1995, multiple phases of improvements occurred in response to population growth and regulatory changes. At that point, the liquid treatment process consisted of mechanical screening, grit removal, primary clarification, trickling filter / solids contact process, secondary clarification, and disinfection prior to discharge. Shortly thereafter in 1998, the Washington Department of Ecology started investigating dissolved oxygen levels in Lake Spokane. Initially, a phosphorus Total Maximum Daily Load (TMDL) was established which set a total phosphorus effluent discharge limit of 1.0 mg/L for the city. Over the next twelve years, multiple draft TMDLs were prepared before being finalized in 2010 at which point the City faced a total phosphorus effluent discharge limit of 0.036 mg/L. This initiated several rounds of pilot studies prior to selecting the current treatment process. Treatment Requirements

Facility treatment requirements are based on the effluent quality limits established in the National Pollution Discharge Elimination System (NPDES) permit. The City received a new NPDES permit in December 2014 which includes year-round effluent limits for carbonaceous

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biochemical oxygen demand (CBOD) and total suspended solids (TSS) limits, among other parameters, and seasonal effluent limits for total phosphorus and ammonia nitrogen. Table 1 summarizes the key effluent discharge limits.

Table 1. Key Effluent Discharge Limits

Parameter Units Average Monthly Limit

Average Weekly Limit

Maximum Daily Limit

CBOD (Nov – Jan)

mg/L 25 40 -

lb/day 1251 2002 -

% Removal 85 (min) - -

CBOD (Feb – Mar)

mg/L 25 40 -

lb/day Seasonal Average Limit: 226 lb/day

% Removal 85 (min) - -

CBOD (Apr – Oct)

mg/L 25 40 -

lb/day Seasonal Average Limit: 203 lb/day

% Removal 85 (min) - -

Total Suspended Solids

mg/L 30 45 -

lb/day 1501 2252 -

% Removal 85 (min) - -

Total Ammonia as N (Mar – Jun)

lb/day 649 - 1547

Total Ammonia as N (Jul – Sep)

mg/L 6.59 - 15.7

lb/day 330 - 786

Total Ammonia as N (Mar – Oct)

lb/day Seasonal Average Limit: 272 lb/day

Total Ammonia as N (Feb – Oct)

lb/day Seasonal Average Limit: 3.17 lb/day

Source: NPDES Permit No. ID0022853 Facility Planning and Pilot Studies

In anticipation of the more stringent effluent phosphorus and ammonia limits, the City completed substantial planning and process improvements over the past several years. Nitrification improvements were completed in 2008 and 2009 through the installation of integrated fixed-film activated sludge (IFAS) media in the solids contact basin. The nitrification improvements, and minor operational adjustments, enabled the City to meet the previous seasonal ammonia nitrogen limits for flows under 4.2 mgd. As influent flows and loads increase, the nitrification capacity of the existing treatment process is decreased resulting in the need for additional nitrification capacity. The 2009 Wastewater Facility Plan Amendment included three liquid stream alternatives to achieve low effluent phosphorus concentrations and provide increased treatment plant capacity. The results from the two-year long Low Phosphorus Demonstration Pilot Facility revealed that a tertiary membrane filtration (TMF) system with chemical sludge recirculation can reliably and consistently produce effluent phosphorus concentrations of less than 0.05 mg/L and simultaneous ammonia nitrogen removal. Based on these results and other factors, including the anticipated

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effluent phosphorus and ammonia nitrogen limits, site constraints, and life cycle cost, the recommended plan for implementation of liquid treatment process improvements included conversion of the existing wastewater treatment plant into an advanced water reclamation facility up through the original design capacity for the trickling filter/solids contact (TF/SC) facility. The original capacity of the TF/SC facility was 6 mgd prior to being de-rated to 4.2 mgd in 2004 to account for necessary ammonia nitrogen reduction capacity. The recommended approach consists of up to 6 mgd of tertiary membrane filtration of secondary effluent from the existing TF/SC facility. The 2013 Phase 5C Improvements Preliminary Engineering Report (PER) expanded on the recommendations presented in the 2012 Update to the 2009 Wastewater Facility Plan. The Phase 5C (Tertiary Treatment) improvements are being implemented in three incremental steps: 1 mgd, 5 mgd, and then 6 mgd of tertiary membrane capacity. Design Considerations

The two main types of submerged polymeric membranes utilized in wastewater treatment, flat plate and hollow fiber, were considered for the tertiary membrane filtration facility. Both were evaluated for their advantages and disadvantages as they relate to the Coeur d’Alene AWTF. Even when considering a double-deck or stacked cassette design of flat plate membranes, the required flat plate foot print was more than double that required by the hollow fiber design. Since the Coeur d’Alene AWTF site is very constrained, this was a very important consideration and flat plate membranes were not recommended. Three reinforced hollow fiber membranes were evaluated: GE ZeeWeed 500D, Koch Puron, and Mitsubishi Rayon Sterapore. Unreinforced hollow fibers membranes were not evaluated for this facility due to concerns regarding fiber breakage. The City considered several important factors in selecting a membrane system, including the unique nature of the pilot testing already conducted by the City to identify key design parameters, the need for a reinforced membrane product, and the lack of full scale operating experience at low effluent phosphorus levels for multiple membrane equipment suppliers. Changing to a different manufacturer for the first installment of full-scale facilities would forfeit the pilot testing data and operator experience, and would be disadvantageous to the City and the nearly $4M investment in previous pilot testing facilities. The City ultimately selected GE ZeeWeed 500D membranes as they are reinforced hollow fiber membranes and they are manufactured by the same manufacturer as was competitively bid for the Low Phosphorus Demonstration Pilot Facility. Table 2 summarizes the membrane design criteria for the Phase 5C Improvements.

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Table 2. Membrane Design Criteria Summary

Parameter Unit Phase 5C.1 Phase 5C.2 Phase 5C.3

Design Flow

Minimum mgd 0.40 2.00 2.40

Average mgd 1.00 5.00 6.00

Peak mgd 1.75 8.75 10.50

Membrane Tanks (Firm/Total) Qty 2/2 4/5 5/6

Membrane Tank TSS

Operational mg/L 6,000 – 8,000

Maximum mg/L 10,000

Membrane Flux at 20ºC

Average gfd 17.0

Peak gfd 30.0

Design Temperature ºC 20

Procurement

The membranes for the initial 1.0 mgd were procured by the City through a sole-source negotiation process with GE. In general, the equipment procured included strainers, permeate pumps, membranes, backwash pumps, chemical cleaning system, tank drain pumps, instruments, and a control system. The procured equipment was transferred to and installed by the construction general contractor. Construction

Construction of the TMF project started in July 2013 and the project was substantially completed in December 2014. The layout was orchestrated such that future expansion of the TMF system could be accomplished with modest facility improvements. Figure 2 shows a portion of the Pumping Gallery in the TMF Equipment Building where future permeate pumps can easily be added in the space provided. Figure 3 shows the space reserved for additional membrane tanks when the facility is expanded in future phases.

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Figure 2. TMF Equipment Building, Permeate Pumps in Pumping Gallery

Figure 3. TMF Equipment Building, Space for Future Membrane Tanks

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Commissioning The commissioning process is broken down into several phases. First is the pre-demonstration period when the individual pieces of equipment are checked-out by both the manufacturers and the engineer. During this time, the operations and maintenance staff is also trained. Next is the demonstration period when the systems are put into operation for a period of time to demonstrate complete functionality. For the project, most of the operating systems were demonstrated using clean water; however, some of the systems had to be demonstrated with wastewater after the process was seeded. Following successful demonstration periods, the systems can then be performance tested and operated for an extended period of time for acceptance testing. Seeding

Following the successful demonstration period for the membrane operating system, the TMF process was seeded on December 8 and 9, 2014 utilizing waste solids from the solids contact process within the existing plant. This initial seeding brought the mixed liquor concentration up to approximately half the design concentration in the Chemical Mixing Tank just upstream of the membrane tanks. The plan was to allow the biomass to acclimate and grow thus increasing the mixed liquor concentration up to the design concentration of between 4,000 and 6,000 mg/L prior to conducting the membrane performance testing scheduled for about five weeks after seeding. As the mixed liquor concentration began increasing, the wasting system was initiated to begin testing the control logic. Due to a minor programming glitch, the system over wasted and thinned the mixed liquor to a point where the decision was made to add additional seed to increase the mixed liquor concentration prior to the scheduled performance testing. Prior to the scheduled performance testing, a recovery clean was conducted on the first train. Due to the draining of the membrane tanks during the recovery clean, mixed liquor was lost and again thinned out. Additional seed sludge was added just prior to the performance testing. The first train performed well and the system was able to demonstrate peak hour flow throughput although the transmembrane pressure (TMP) was elevated due to the high flow rate. Similar to the first train, a recovery clean was conducted on the second train prior to the performance test. While this was occurring, a new scour air process was being implemented for the IFAS modules in the Solids Contact Tank to help reduce biomass accumulation on the media. After the second recovery clean, additional seed sludge was transferred from the solids contact process to the TMF process to increase the mixed liquor concentration. The performance test was conducted on the second train but it was unable to pass peak hour flow as high TMP alarms kept shutting down the system. It was noticed that the TMP was extremely high during normal operation but returned to expected levels during backpulsing. This indicated that the membrane fibers were being blinded with solids while permeating but the foulant was releasing from the surface of membrane fiber during backpulse.

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Fouling

After the failed performance test, discussions began with the manufacturer. They requested that the City conduct time-to-filter (TTF) tests on the mixed liquor within the TMF process and provided the recommended procedures. The results to filter 100 mL were significantly higher than anticipated. It is believed that the seeding event just prior to the second performance test transferred significant fines (colloidal material) that were present in the seed sludge due to the recently improved IFAS scour air system. The start of the increased air scour also correlated with a brief increase in turbidity of the secondary effluent thus further supporting the concept that significant fines were present. The membrane system continued to operate although the production cycle times were reduced from 12 minutes to 10 minutes. The wasting regime was slightly increased in hopes that it would slowly waste the fines from the system. After a period of about two weeks, approximately one-third of the mixed liquor had been wasted and while the operating TMP had decreased it was still not near the values observed during the first performance test. Over the next two weeks, the system experienced severe foaming events during the routine maintenance cleans due to the decreasing mixed liquor concentration. About four weeks after the seeding event, the decision was made to drain the system and start over with a modified seeding plan. On February 17, 2015, the process tanks were drained, flushed, and refilled with plant utility water. At this point, the system was returned to operation but without any seed or coagulant addition. After a brief period of time, the operations staff reported the water as having a light brown or yellow tint. This indicated there was still sludge, potentially containing fines, present in the system and it needed to be removed. The process tanks were drained again one week later and the operators also conducted a visual inspection of the membranes. During the visual inspection of the membranes, the operations staff observed significant fouling on the fibers located at the leading and trailing edges of the membrane cassettes (Figures 4 and 5) but very little fouling was observed within the interior of the cassettes (Figure 6).

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Figure 4. Train 1 Cassette 1 Showing Foulant at Perimeter of Cassette

Figure 5. Train 1 Cassette 1 Showing Foulant at Perimeter of Cassette

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Figure 6. Train 1 Cassette 1 Showing Interior of Cassette

Following observations of the heavy foulant, operations soaked the membranes in sodium hypochlorite overnight, with an initial concentration of 600 mg/L, degrading to 200 mg/L in the morning. The following morning, the membrane tanks were scoured at LEAP-high for one and a half hours before the tanks were drained for further inspection. Less fouling was observed on the membranes than the previous day however membrane fibers were observed as being “glued” together by the foulant (Figures 7 and 8).

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Figure 7. Train 2 Showing Foulant After Hypo Soak

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Figure 8. Train 2 Showing Foulant After Hypo Soak

Fiber Bundles A and H (the bundles on the outside of the cassettes, front and back) were found to be heavily fouled with a significant amount of material lodged between the base of the fibers. Fiber Bundles B and G were found to have minor fouling and than was observed on the fibers closer to Bundles A and H. Fiber Bundles C thru F (the bundles in the middle of the cassettes) were generally free of fouling and appeared clean. Fouling on Bundles A and H was observed approximately 75 percent of the way up the fibers as well as down approximately 2 IN from the top of the fibers. While the membrane tanks were drained, an investigation of the air scour was performed. The tank was filled with secondary effluent (TNMF influent flow) to approximately 1 foot over the

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base of the membrane cassettes. The membrane air scour system was turned on and the blower was observed running at 55 percent speed, which is the low air scour rate. A rolling boil with significant bubbles was observed adjacent to the cassettes (sides against wall). However, infrequent and discrete bubbles were observed at the front (Bundle A) and back (Bundle H) of the cassettes. The intensity of the air scour in these areas was significantly less than observed elsewhere. The tank was continued to be filled and the membrane air scour system was ramped up such that the blower was operating at full speed. While filling the tank, the membrane fibers in the middle of the cassette (front to back, approximately Bundles C thru F) were observed vibrating and shaking. The intensity of vibrating and shaking decreased significantly towards the front (Bundle A) and back (Bundle H) of the cassettes such that almost no movement was observed. Upon seeing that the foulant reacted positively to a sodium hypochlorite soak, some of the clay-like foulant was removed from the membranes and taken into the laboratory for testing. A piece of the foulant was soaked for a few hours in 12.5 percent sodium hypochlorite and after a period of just a few hours, it was completely dissolved with the exception of some minor white material that was both floating on the liquid surface as well as settled on the bottom of the beaker (Figure 9).

Figure 9. Foulant Soaking in 12.5% Hypo (Left) and Dissolved (Right)

At this point, the sodium hypochlorite maintenance cleans were increased to Monday, Wednesday, and Friday as compared to normal operating procedure which is just twice per week. The citric acid maintenance cleans were temporarily halted as no alum was being added. The membrane air scour was also increased to full speed all the time as compared to normal operating procedure for GE’s LEAP system which greatly reduces the air scour during the majority of the day. Beginning March 14, 2015, the operations staff cleaned the membranes with a double strength hypochlorite recovery clean soaking for at least 24 hours. After the recovery cleans, the fouling was significantly reduced. As a result, transmembrane pressure decreased and permeability increased. Re-seeding

While solutions were being generated to clean the membranes of the heavy foulant, alternative approaches were being generated for re-seeding the process. One approach was to slowly let the biomass accumulate overtime in the filtered secondary effluent as it had done in the Low Phosphorus Demonstration Pilot Facility but that could potentially take several months to build

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up the mixed liquor concentration to desired levels. The most favorable approach for re-seeding was to utilize seed sludge from the nearby Spokane County, WA membrane bioreactor (MBR). While the process at Spokane County is slightly different than the TMF process at Coeur d’Alene, the biomass in their MBR was much older (approximately 15 day solids retention time) than the biomass in the solids contact process (approximately 5 day solids retention time). Prior to introducing the new seed sludge to the Coeur d’Alene TMF, bench-scale testing was conducted to investigate the potential benefit or impacts with regards to times-to-filter. The testing found that adding even a small amount of Spokane County seed sludge to the existing very thin, low solids TMF mixed liquor greatly increased the filterability of the sludge. Beginning on March 18, 2015, the City began hauling seed sludge from Spokane County to Coeur d’Alene. The process immediately began to nitrify and the mixed liquor concentration slowly increased. Performance Testing

Once the mixed liquor concentration reached about 3,000 mg/L the process became very stable and efforts ceased to increase the concentration higher than that. The time-to-filter tests were generating favorable results (relatively low times) and treatment objectives were being met. Through routine operation, it quickly became apparent that the membrane system could not permeate peak flows without drastically increased TMP even though flux rates were within specification. Due to repeated high level alarms in the membrane tanks from the membrane system not keeping pace with the feed system, the operators began to decrease the feed flow set point. The design feed flow is 1 mgd and was gradually decreased to 0.8 mgd at which point the membrane system was able to keep pace and TMP were as expected. In early October, approximately 38 percent additional membranes were provided by GE and installed at Coeur d’Alene. After installation, the performance test was conducted and the system was able to meet performance criteria.

Treatment Performance Even though the initial operation was difficult, the TMF permeate and overall effluent water quality is quite good. Figures 10, 11, and 12 show the past six years of 30-day average AWTF effluent trends for BOD, ammonia and total phosphorus. BOD effluent performance has stabilized year round where previously is lagged during the colder months. Effluent ammonia has decreased substantially with winter highs around 10 mg/L and values as low as 3 mg/L during the summer. Effluent total phosphorus has also greatly improved during the winter due to operating the TMF facility with chemical removal year round. As the TMF facility is expanded to allow the majority of the plant flow to pass through the tertiary process, the performance trends will only further improve.

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Figure 10. 2010 – 2015 Annual Effluent BOD Trends (30-day Average)

Figure 11. 2010 – 2015 Annual Effluent Ammonia Trends (30-day Average)

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Figure 12. 2010 – 2015 Annual Effluent Total Phosphorus Trends (30-day Average)

Tertiary Membrane Filtration Facility

After stabilizing, the TMF facility has exceeded performance expectations. TMF nitrification performs as expected with the permeate ammonia near the detection limit (Figure 13). TMF effluent phosphorus has been between 0.02 mg/L and 0.03 mg/L with a tertiary alum dose of only 20 mg/L (Figure 14), which is only half the dose used during the pilot testing. The filterability of the TMF sludge has been of great concern and is routinely monitored by operations staff (Figure 15). Following the re-seeding effort, the filterability has steadily improved and as a result the TMP is between 1 and 2 psi (down from 7 to 8 psi during commissioning).

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/L]

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Figure 13. TMF Permeate Ammonia

Figure 14. TMF Permeate Total Phosphorus

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Figure 15. TMF Sludge Filterability

References Fredrickson, H. Sid (2014), “A Comprehensive History of The Wastewater Department for the

City of Coeur d’Alene, Idaho.” HDR Engineering, Inc. (2013), “Advanced Water Reclamation Facility Phase 5C Improvements

Preliminary Engineering Report.” HDR Engineering, Inc. (2015), “Tertiary Treatment Phase 2 Preliminary Engineering Report.”

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