IMMERSED ULTRAFILTRATION MEMBRANES IN A SIPHON DESIGN: A CASE STUDY OF THE PENDLETON OREGON WTP

S. Rambi, B. Sc. (Eng); G. Best, P. Eng.; P. Schuler,

ZENON Environmental Inc., Oakville, Ontario; Bob Patterson, City of Pendleton, Pendleton, Oregon

ABSTRACT Municipalities constructing new drinking water treatment plants face many challenges including: availability of suitable raw water, varying demand, color and organics and THM formation, wastewater handling, as well as the capital and operating costs of the facilities. The design team for the City of Pendleton Oregon, consisting of city officials, as well as engineers from CH2M Hill and ZENON Environmental, was faced with these usual difficulties including a limit on feed water availability from the Umatilla River, as well as a project site with an elevation differential approaching thirty feet. The team devised an innovative solution to meet the unique requirement of the Pendleton project. In order to meet the increasingly stringent EPA requirements while removing color and organics to minimize THM formation, the City selected Immersed Ultrafiltration Membranes in combination with coagulation using aluminum chlorohydrate (ACH) based on a successful pilot test. The membrane plant is designed to operate at a lower flow during the Summer when less river water is available. The City makes up the total demand to the distribution system by supplementing the membrane permeate with well water. In the Winter, when river water is more readily available, the membrane plant operates at a higher flow rate and the excess permeate is used to recharge the aquifer. In order to increase system recovery, the reject from the 95% recovery membrane plant is sent to lagoons from which supernatant is recycled to the head of the membrane plant. The most interesting design aspect of the plant however is the use of the 30 foot differential on the site to achieve permeation from the membranes under siphon. This improvement eliminated the requirement for permeate pumps and led to significant energy savings. This paper will present membrane ultrafiltration and its use in a coagulation-filtration process, the City of Pendleton water treatment plant including process flow and design parameters, operating data, capital and operating costs, and will focus on the design of the siphon system for permeation and its advantages. INTRODUCTION AND BACKGROUND The underlying driving factors for the construction of drinking water treatment plants can range from increased water demand related to population growth, to aging infrastructure, or to increasingly stringent regulations. In all cases, municipalities requiring additional water treatment capacity, existing plant upgrades, or new plant construction face similar challenges. These include the availability of suitable raw water, varying demand, treatment method

suitability, treated water quality, residuals handling, as well as the capital and operating costs of the facilities. The City of Pendleton, located in northeast Oregon, faced many of these challenges in providing a safe and reliable drinking water supply for their population of just under 20,000 people. The City was being supplied with drinking water in the form of groundwater from the Thornhollow Springs. Since the amount of water available from the Springs was directly proportional to the flow of the adjacent Umatilla River, additional water was required during the Summer months when the River level was low. This was supplied from eight (8) wells, but the level of the aquifer was severely decreased whenever the wells were used. In 1979, the USEPA informed the City of Pendleton that filtering of the Spring water supply would likely be required. It wasnt until 1999 that the City received official notice from the State of Oregons Department of Human Services (DHS) Drinking Water Program that changes would be required to the treatment of the surface water influenced groundwater source. These changes were based on compliance with the USEPAs Surface Water Treatment Rule and were required in order to protect the residents from waterborne bacteria such as Giardia and Cryptosporidium. Of the options put forward by the DHS, the City of Pendleton selected filtration. Due to the limited water available from the Springs and wells and water rights associated with these and the nearby Umatilla River, the City researched alternatives for a water source to the new proposed filtration plant. Since the well supply was insufficient and it was desirable to abandon the Springs as a source of drinking water due to aging infrastructure, the City put in an application with the state to transfer their water rights for the Springs to the Umatilla River. With the abandonment of the Springs as a source of drinking water, the water that would have been removed from the Springs would instead move towards the Umatilla River, augmenting its flow and cooling it. This would have benefits from a fisheries perspective. According to the transfer application, the City could then draw from the Umatilla River instead of the Springs at a slightly reduced flow. The transfer right would be dependent on a minimum flowrate in the River of 250 cfs. This flowrate is achieved only in the Winter months. In order to make the best use of the available water sources, the City put forth an innovative plan. As much water as possible would be drawn from the Umatilla River in the Winter when the flow was above 250 cfs. The water would be treated in a new filtration drinking water plant and the portion of treated drinking water that was not required to meet the demand of the City residents would be transferred to the basalt aquifer through the existing wells in an aquifer storage and recovery (ASR) program. In the Summer, when the right to draw water from the River would be decreased, the minimum amount of water would be drawn from the River for treatment and the wells would be used as backup as in the past. Due to the requirement for high water quality in the ASR water being delivered to the aquifer, membrane filtration was selected for the new drinking water treatment plant. Based on the results of a pilot study, ZENON was selected to supply immersed hollow-fiber membrane ultrafiltration for the new plant. Key features of the plant included the coagulation pre-treatment

with aluminum chlorohydrate (ACH), and the innovative siphon design that eliminated the need for permeate pumps. This paper will describe the new membrane ultrafiltration plant, its use in an enhanced coagulation mode, operating data, capital and operating costs, and will focus particularly on the design of the siphon system that is used for permeation and its advantages. IMMERSED MEMBRANE ULTRAFILTRATION The flow sheet for an immersed ultrafiltration membrane plant with enhanced coagulation is shown in Figure 1. It consists of pre-screening, rapid mix coagulation, flocculation and ultrafiltration with no requirement for clarification. The flocculation tank may be a separate tank, or a separate compartment within the process tank. Mixing is provided either with air (as shown in the figure), or with mechanical mixers. The flocculated water flows from the flocculation tank to the process tank compartment where shell-less hollow-fiber ultrafiltration membranes are immersed directly in the flocculated water. Most often, permeate or process pumps are used to draw clean water (permeate) from the inside of the membrane fibers under a slight vacuum. When site elevations permit however, it is possible to obtain permeation by siphoning. The outer surface of the membrane fibers is intermittently scoured using air provided by blowers to minimize fouling.

Figure 1: Immersed Ultrafiltration with Coagulation Process Flow Diagram (adapted from Best et al, 1999)

ZENONs ZeeWeed 500 series membranes, used in most enhanced coagulation applications including the Pendleton plant, have a nominal pore size of 0.04 m and an absolute pore size of 0.1 m. As such, they act as a barrier to turbidity, bacteria and viruses, including Giardia cysts (6-16 m) and Cryptosporidium oocysts (4-7 m). Without coagulation, dissolved natural organic matter would pass through the membrane pores; but with proper coagulation, color

Air

Permeate Pump

Feed Water

Concentrate

CCoonnttaacctt // FFllooccccuullaattiioonn TTaannkk

Coagulant

Flash Mixer Treated Water

Immersed Outside-in Hollow Fibre UF

Membrane

Flocculation Tank

Membrane Process Tank

removals approaching 95% and TOC removals up to 75% can be achieved depending on the feed water characteristics, coagulant dosing and pH adjustment. This is beneficial for minimizing the formation of disinfection byproducts including trihalomethanes (THMs) and haloacetic acids (HAA). The membrane fibers are supported with an inner braid which provides durability. The relatively large outside diameter of the fibers of 0.075 inches (1.9 mm) affords them a high resistance to breakage. These factors ensure a long membrane life, even with high solids concentrations in the raw water. In addition, the membranes are resistant to oxidants and to a wide pH range, allowing them to be easily cleaned with acids, bases or oxidants. The individual membrane fibers are assembled into building blocks known as modules or elements. In turn, these membrane elements are assembled into larger units known as cassettes. Several cassettes may be installed together in parallel into a single process train (refer to Figure 2), and several process trains may be used to meet a particular plants production capacity. The modular nature of the cassettes allows them to be used to retrofit existing tankage in conventional drinking water treatment plants, as well as in new plants.

Figure 2: ZeeWeed Ultrafiltration Process Train (4 cassettes)

The membranes are cleaned in several different ways. During operation, intermittent aeration is provided by blowers to scour the outer surface of the membranes to minimize fouling. Typically, in each 15 minute production cycle, the membranes are backpulsed with permeate for approximately 30 seconds using treated water which is delivered to the inside of the membrane fibers. There is no waste associated with the backpulsing as the backpulse volume is held within the membrane tank and re-permeated. The downtime and water requirements associated with the backpulsing are taken into account in the design of the plant such that there is no loss in production capacity associated with the backpulsing.

Permeate

Reject

Cassette 1

Cassette 2

Cassette 3

Cassette 4

Chemicals can be used to better clean the membranes. Maintenance cleans can be performed automatically as often as daily using a low concentration of sodium hypochlorite. Typically, permeate with 10 mg/L of sodium hypochlorite is backpulsed through the membranes while the membrane process tank is full. This means of cleaning is meant to be used to help maintain the membrane permeability. Over time however, the membranes may become fouled due to accumulation of organic matter or crystallization of salts within the membrane pores. More intensive cleaning is required to restore the membrane permeability at this point. Higher concentrations of sodium hypochlorite or citric acid are backpulsed through the membranes into the empty process tank. The tank is then filled to allow the membranes to soak in the cleaning solution for four (4) hours or more. THE PENDLETON WTP The design of the Pendleton Drinking Water Treatment Plant was based on the raw water characteristics of the Umatilla River as described in Table 1 below. With headwaters in the Blue Mountains of Northeast Oregon, the River contains low levels of organics and during Spring runoff conditions, can have turbidity levels as high as 100 NTU.

Table 1: Umatilla River Characteristics Parameter Units

Temperature 43-75 F Turbidity 3-6 (90% of the time)

6-100 (10% of the time) NTU NTU

Total Organic Carbon (TOC) 1.5-2.5 mg/L The treated water requirements for the plant were based on a combination of drinking water regulations, and the high quality required for the ASR program. These are outlined in Table 2.

Table 2: Treated Water Requirements Parameter Units

Turbidity 0.05 (95% of the time) 0.1 (100% of the time)

NTU NTU

Total Organic Carbon (TOC) 2.0 Note 3 mg/L Particle Counts (> 2 micron) < 5 (average)

< 10 (99% of the time)

Giardia 6 Note 1 Log removal Cryptosporidium 6 Note 1 Log removal Viruses 2 Note 2 Log removal

Note 1: The ZeeWeed Membrane is guaranteed to achieve 6 log removal of Giardia and Cryptosporidium to the limits of detection, however it must be realized that 6 log removal can only be achieved if > 106 cysts/oocysts are present in the raw water.

Note 2: Viruses are usually less than 0.1 microns, however they are typically associated with host bacteria or attached to particulates larger than 0.1 microns and can therefore be removed by the ZeeWeed Membrane. ZENON has received a minimum of 2.0 log virus rejection certification by the DHS based on the results of the California DHS Certification Testing which showed a minimum virus rejection of 2.5 log for the ZeeWeed Immersed Ultrafiltration Membrane.

Note 3: TOC removal is a function of three variables as follows:

1. The actual TOC concentration levels in the water 2. The type of coagulant used 3. The pH of the water. ZENONs pilot testing was based on the use of Aluminum Chlorohydrate. TOC removal efficiency decreases as the TOC concentration in the raw water decreases.

Based on the results of a ten week pilot study performed in the Spring of 2000 on the Umatilla River, ACH was recommended in dosages of up to 15 mg/L. Pilot results indicated that without coagulant, TOC removal was about 24% and color removal was up to 80%. The color and TOC removal rates were increased to 40% and 100% respectively with the addition of coagulant. pH adjustment was not tested since TOC and color removal were adequate without it. In general, the optimum pH for TOC removal with ACH is 6.8 to 7.0. Since ACH is not very acidic, the pH does not drop much following coagulant addition such that the pH is more likely to be in the optimum range for organics removal without adjustment. The design of the full scale plant therefore includes coagulant addition without pH adjustment. At the current design flowrate, a retention time of approximately nine (9) minutes is provided in the flocculation tanks. With future expansion of the existing trains, the detention time will be a minimum of five (5) minutes. This pre-treatment minimizes the potential for formation of THMs and HAAs when the filtered water is chlorinated. The Pendleton WTP is designed for a total initial capacity of 6 MGD in four (4) trains of eight (8) cassettes of ZeeWeed 500c membranes. The corresponding net flux is 32.8 gfd for each trains operating capacity of 1.5 MGD. In order to account for the downtime and permeate requirements associated with backpulsing, as well as membrane integrity testing and maintenance cleaning, the plant actually operates at a slightly higher flux to ensure that the net production capacity is met. In addition, when one train is taken offline for recovery cleaning or maintenance, the remaining three trains can produce a total of 5 MGD for up to 24 hours to minimize the impact of the offline train. This brings each trains peak capacity to 1.67 MGD with a corresponding net flux of 36.4 gfd. The plant is hydraulically sized for future expansion to 9 MGD with the addition of four (4) cassettes into each of the existing trains. With this configuration, the net flux remains at 32.8 gfd. The City of Pendleton may also choose to add two (2) additional trains of twelve (12) cassettes to bring the total capacity to 13.5 MGD. These expansion figures are based on expanding with ZW500c membranes identical to the membranes that are currently installed. However, piping has been hydraulically sized for an additional 23% capacity based on anticipated membrane improvements whereby additional membrane area could be installed in the same volume for additional production capacity. With this additional 23%, the four (4) existing trains could be expanded to 11 MGD, and with the addition of two (2) more trains, the capacity could be increased to 16.5 MGD. The current demand for the City of Pendleton is only 2.5 to 2.8 MGD. During the Winter, when raw water is available from the Umatilla River, the plant operates at 6 MGD based on a permeate flow set point. The excess treated water is pumped into the basalt aquifer for storage as part of the ASR program. In the Summer, the WTP operates based on a fixed feed flowrate of 2.2 MGD based on the water rights for feed water from the river. In order to minimize operating costs, the number of simultaneously operating trains is decreased in the Summer and the system rotates

through the trains. The Citys demand is met by the permeate produced at this flowrate, supplemented by groundwater from the wells. A simple Process Flow Diagram (PFD) for the plant is shown in Figure 3. Raw water from the Umatilla River is passed through a 0.069 inch (1.75 mm) vertical bar screen and pumped to the WTP. The river water is pre-chlorinated prior to the addition of ACH in an inline mechanical mixer. The flow is divided at a splitter box to two (2) flocculation tanks that overflow to a distribution channel. From the distribution channel, the flocculated raw water is delivered to the membrane tanks.

Figure 3: Pendleton WTP Process Flow Diagram

The membranes are immersed in the flocculated raw water in the membrane tanks. A siphon is used to draw clean water to the insides of the membrane fibers and to the clearwell located several feet below the membrane tanks. Additional details on this aspect of the design will be discussed further in this paper. The membrane plant operates at a recovery of 90 to 95%. Reject flows continuously from the membrane tanks by gravity, controlled by a magnetic flow transmitter and flow control valve based on a recovery set point. Periodic tank deconcentrations are also used every 3 to 4 days, whereby the entire contents of the tank are drained to waste. The reject water is delivered to onsite settling ponds in which the solids are allowed to settle. The clarified water is pumped from the ponds and back to the head works of the plant. This maintains the overall recovery of the plant at almost 100% with minor losses due to evaporation from the settling ponds. This high recovery allows the City of Pendleton to maximize the amount of water available to them.

feed pump

Umatilla River water

screen rapid mix

splitterbox

flocculation

distribution channel

ACH

to clearwell

membrane tanks

Backpulse water is provided by dedicated pumps which draw permeate from the clearwell. A dedicated pump is also supplied for maintenance and recovery cleaning. This pump draws from a CIP tank. Chemicals used to clean the membranes are citric acid which is purchased in liquid form, and sodium hypochlorite. The hypochlorite is produced on site. OPERATING DATA While the new membrane ultrafiltration WTP first began sending treated water to the City of Pendletons distribution system on June 13, 2003, it did not actually begin treating raw water from the Umatilla River until December 5, 2003 since the river water level was too low and the transfer of water rights from the Springs had not yet been approved. Following an initial commissioning period on the new water source, the plant has been operating at full capacity on River water feed. As demonstrated during a fourteen (14) day performance test in March 2004, the membrane plant has been providing the guaranteed volume and quality of treated water. During the performance test, the production capacity was monitored and it was shown that the capacity of 6 MGD was exceeded for every day of the test with the exception of one day when one of the trains was down for repairs due to a faulty valve. The capacity was achieved despite the very low raw water temperatures that were experienced from January to March. The plant was designed for a feed water temperature of 43 to 75 F (6 to 24 C), but as shown in Figure 4, the water temperature varied between 0.5 and 13.3 C (33 and 56 F) between December 5, 2003 and the end of April 2004. Lower water temperatures increase the viscosity of the water such that a higher vacuum is required for permeation.

Figure 4: Water Temperature

The turbidity of the feed water is shown in Figure 5 and has ranged from 1 to > 100 NTU since the plant began receiving feed water from the river. The high turbidity events represent heavy rainfall events and those in March represent the effect of Spring runoff.

Figure 5: Raw Water Turbidity

As shown in Figure 6, the treated water turbidity remained consistently low at less than 0.07 NTU regardless of the feed water turbidity. This is typical of performance that is observed in other ZeeWeed immersed ultrafiltration membrane plants.

Figure 6: Permeate Turbidity

Treated water particle counts are also monitored, with both turbidity and particle counts used as continuous means of verifying the integrity of the membranes. The permeate particle counts greater than 2 microns are plotted in Figure 7. As shown, the total particle counts have remained well below the guaranteed maximum of 10 counts 99% of the time and the average of 5 counts. In fact, the average has generally been closer to 2 counts.

Figure 7: Permeate Particle Counts

The other means of membrane integrity monitoring consists of pressurizing the insides of the membranes with air to a pressure of 5 to 6 psig, then isolating the membranes and measuring the rate of pressure decay over ten (10) minutes. This is called a membrane integrity test (MIT) or pressure decay test (PDT). The rate of decay during the test is used as an indication of whether there are any leaks in the membranes or piping. The first criterion for passing the MIT is that the rate of pressure decay shall not exceed 1 psi. The second criterion for passing is that the pressure decay during the current MIT shall not exceed the pressure decay from the previous MIT by more than 0.3 psi. The test is completely automated and can be programmed through the PLC to take place on a user adjustable frequency. The Pendleton WTP is testing each train once per week. The eight (8) membrane cassettes in each process train are separated into two (2) groups of four (4) cassettes for the purposes of the test, such that the cassettes in group A and group C are tested separately. If a leak is identified by an MIT, the affected cassette is isolated and removed from the tank. The broken or leaking fibers can be cut and the ends filled with silicone prior to returning the cassette to service. The loss of a few fibers is insignificant since each membrane cassette contains over 50,000 fibers. The MIT results during the performance test confirmed the absence of leaks.

With the use of 2 to 5 mg/L of ACH coagulant, the TOC removal ranged from 34.6 to 80% and the color removal ranged from 78.9 to 100%. The TOC and color data from the performance test are outlined in Tables 3 and 4 below.

Table 3: TOC Data

Date

Coagulant Dose

Raw Water TOC

Permeate TOC

TOC Removal

M/D/Y mg/L mg/L mg/L % 3/16/2004 5 2.6 1.7 34.6 3/17/2004 5 2.4 1.3 45.8 3/18/2004 5 3.9 2.1 46.2 3/19/2004 5 3.4 2.4 29.4 3/20/2004 5 5.1 2.1 58.8 3/21/2004 3 3.7 1.9 48.6 3/22/2004 3 2.8 1.6 42.9 3/23/2004 5 2.8 1.5 46.4 3/24/2004 5 3.5 1.5 57.1 3/25/2004 5 3.9 0.9 76.9 3/26/2004 2 2.0 0.4 80.0 3/27/2004 2 1.2 0.7 41.7 3/28/2004 2 1.7 0.8 52.9 3/29/2004 2 2.1 0.7 66.7

Table 4: Color Data

Date

Coagulant Dose

Raw Water True Color

Permeate True Color

Color Removal

M/D/Y mg/L PCU PCU % 3/16/2004 5 15 3 80.0 3/17/2004 5 12 1 91.7 3/18/2004 5 15 4 73.3 3/19/2004 5 16 0 100.0 3/20/2004 5 14 2 85.7 3/21/2004 3 16 0 100.0 3/22/2004 3 18 0 100.0 3/23/2004 5 19 0 100.0 3/24/2004 5 23 3 87.0 3/25/2004 5 15 3 80.0 3/26/2004 2 19 2 89.5 3/27/2004 2 19 4 78.9 3/28/2004 2 17 2 88.2 3/29/2004 2 15 1 93.3

Membrane performance is generally discussed in terms of the interval between recovery (soak) cleanings. This interval is representative of the fouling rate of the membranes and is determined

by considering the transmembrane pressure (TMP) across the membranes, as well as the membrane permeability which is the ratio of flux to TMP and is measured in gfd/psi. The 500c membranes installed at the Pendleton plant can operate at a TMP as high as 10 psi; and in general, permeability should be maintained above 3.6 gfd/psi (36.4 gfd/10 psi) to ensure that the production capacity can be met. The membranes at the Pendleton plant had been cleaned in early March in preparation for the performance test. Since then, the membranes have not required cleaning and although they are operating in a slightly high TMP range between 6.5 and 8.5 psig, the operation is stable. As can be expected since the permeate flow has not changed, the membrane permeability corrected to 68 F (20 C) has also been stable. The data for Train 1 which is typical of all four trains is shown in Figure 8. Prior to April 14, 2004, the temperature correction equation was using a single temperature point for the day. Since the temperature of the water varied significantly over the course of a day, this error led to the temperature corrected permeability showing diurnal variations due to temperature when in fact the temperature corrected permeability should have been relatively constant. This becomes clear in the data after April 14, 2004 when the temperature correction calculation was rectified.

Figure 8: Temperature Corrected Permeability

The stable membrane permeability is indicative of a cleaning interval of approximately three (3) months for this raw water quality.

SIPHON DESIGN As presented earlier, most plants using the ZeeWeed immersed ultrafiltration membranes use process pumps to provide the required suction inside the membrane fibers to draw clean water through from the process tank. A variable frequency drive (VFD) on the process pump is used to control the flow of treated water. This is depicted in Figure 9. Depending on the temperature of the water and the design flux, which is a measure of the volume of treated water required divided by the membrane area available, a vacuum of 2 to 10 psi may be required to be drawn by the process pump at the membranes. Additional vacuum would be required to overcome losses and changes in elevation if applicable.

Figure 9: Membrane Flow Diagram With Process Pump

Since the aeration provided to the membranes during the filtration process to scour the outer surface of the membranes saturates the water with air, and the low pressure operation of the system allows air to come out of solution due to the applied vacuum, the treated water may contain up to ten (10) percent entrained air at the higher applied vacuum. In order to prevent problems associated with cavitation in the pumps, or air locks in the piping and pump system which could cause the process pump to lose its prime, an air removal system is provided with each plant. The system includes a vertical air separation column for each process train which is located on the common permeate header. Air released from the treated water accumulates at the high point in this column and is automatically vented from the system by an air release valve at the top of the column. The air release valve is connected to a vacuum pump that operates continuously. The valve closes to prevent water from reaching the vacuum pumps in the event that all of the air has been removed from the air separator. In some cases, the process pump can be eliminated and a hydraulic gradient (h) utilized to operate the system in a siphon mode. A process flow diagram of this operating mode is shown in Figure 10 for a case where the difference in elevation between the membrane tank and the clearwell produces the required hydraulic gradient (figure is not to scale). The hydraulic gradient must be sufficient to overcome the maximum TMP across the membranes, as well as all of the line losses. The air separation system is still present, as an air lock in the piping would break the siphon although the system is less sensitive to air that a pumped system. In lieu of the VFD on the process pump, different methods may be used to control the vacuum applied and the resulting permeate flow. In one option, a flow control valve can be used to control the permeate flow to the desired level. When the membranes are clean and the water is warmer, the TMP is

Membrane Process Tank

FIT

Process Pump

Air Separator Vacuum

Pump

air removed to atmosphere

VFD

Clearwell

lower and the flow control valve is used to generate artificial backpressure to maintain a constant permeate flow. This is the approach in use at Pendleton, but the clearwell level can be changed if required. Alternatively, the vacuum applied can be controlled entirely by varying the level in the clearwell if a wide enough operating range is available. In this case, the flow control valves would be used only to throttle the flow to maintain it below the maximum design flow for the train. A combination of both approaches is also possible.

Figure 10: Membrane Flow Diagram in a Siphon Mode

Another option for the elimination of the process pump when site elevation does not allow for a clearwell elevation below that of the membrane tanks is to excavate a wet well to create the required elevation difference from the membrane tank. This wet well can be smaller than a typical clearwell with a retention time of approximately fifteen (15) minutes, thus reducing the excavation costs. While pumps would be required to transfer the contents of the wet well to the clearwell, these could be more efficient vertical turbine pumps as compared to the end suction centrifugal pumps that would be used as process pumps. In addition, these transfer pumps may not require VFDs as would the process pumps. The difference in elevation required to utilize the siphon mode of operation is determined as follows:

TMPmax lossesfriction +=h For the Pendleton WTP, the data is as follows:

Membrane process tank operating level elevation 1359.5 ft Clearwell maximum level elevation 1339.5 ft Clearwell minimum level elevation 1328 ft Line losses 5 ft Maximum membrane TMP 23 ft

Using this data, the maximum required elevation difference between the membrane tank and clearwell is 28 ft (23 ft + 5 ft). The actual difference in elevation is 20 to 31.5 ft. At membrane TMPs less than 15 ft (6.5 psi), siphon is possible at the maximum clearwell elevation while at the maximum TMP of 23 ft (10 psi), the clearwell level must be 1331.5 ft or less. The range of operating levels on the clearwell is sufficient to allow siphon operation over the entire range of membrane TMPs.

FIT

Flow Control Valve

Air Separator Vacuum

Pump

air removed to atmosphere

Membrane Process Tank

Clearwell

h

The system design allows a siphon to be generated in approximately fifteen (15) to twenty (20) minutes. When starting up the plant, the flow control valves located at the base of the air separation vessels on each train are closed. The vacuum pump is turned on and draws air from the upstream side of the flow control valves from air release valves at the top of each trains air separation vessel. As the air is removed at the air separation vessels, the vacuum pump eventually draws water through the membranes to fill the vessels. The vacuum pump also draws from the downstream side of the flow control valves through an air release valve at the top of a standpipe that is used for chlorine injection to the treated water. In order to speed up the air removal, a second air removal line was installed parallel to the air release valve line through which it is connected to the vacuum pump. This line is isolated with a manual valve. The manual valve is opened when it is necessary to create the siphon for the plant. Once the plant is in operation, the manual valve is closed. As air is drawn through these parallel lines on the standpipe, water is eventually pulled up into the piping from the clearwell since the inlet to the clearwell is submerged. When the water level gauges in the standpipe and air separation vessels indicate that air has been sufficiently removed from these zones, the plant is started. The flow control valves open on each train based on a ninety second timer and the siphon is induced. The siphon can also be induced with the manual valve at the clearwell inlet closed and the flow control valves open. In this method, air is removed from all of the piping and the piping is filled solely with water drawn through the membranes. This method has been observed to induce siphoning faster, but the opening and closing of the manual valve at the clearwell is a long process due to the size of the valve and its location with respect to the membrane plant. For this reason, this method is infrequently used. With the two air removal locations, it would also be possible to induce siphon with the flow control valves closed and the clearwell inlet valve closed. During normal operation, the vacuum pump draws air from the top of the air separation vessels as well as from the standpipe downstream of the flow control valves to keep air out of the piping. The siphon is not broken by shutting down trains such that trains can be restarted quickly. Other ZENON plants currently operating under siphon include the Chestnut Singapore WTP, Sudbury Ontario WTP, Lowestoft UK WWTP and the ISP Chemicals (Envirogen) WWTP in Ohio. Other plants currently in design with siphon operation include the Buxton UK WWTP and Linwood Georgia WWTP. CAPITAL AND OPERATING COSTS For the City of Pendleton, the selection of a site that allowed the membranes to operate under siphon had several implications. Having the membrane tanks positioned higher than the clearwell to achieve a siphon design meant the replacement of four (4) process pumps with flow control valves, as well as the elimination of four (4) reject pumps since reject could now flow from the membrane tanks to onsite settling ponds by gravity. This benefit was partially outweighed however by the requirement for high service pumps to transfer the finished water from the clearwell to the gravity system reservoirs located throughout the distribution system. These pumps were required since in placing the clearwell at the required elevation for siphoning from the membrane tanks, there was no longer sufficient pressure to deliver water to the

reservoirs based on their elevation. The impact of having the membrane tanks located higher than the clearwell was insignificant with respect to the raw water pumping station. The interrelationships between all of these factors and the number of design modifications that were associated with changing the design to a siphon complicate the cost analysis. On a level site, the process pumps and reject pumps would likely have required motors of approximately 50 hp and 5 hp, respectively. The 50 hp motors would have required VFDs. This would represent eight (8) pumps and four (4) VFDs requiring maintenance and repairs over the years. The elimination of these pumps required the addition of high lift pumps. Currently, two (2) pumps with VFDs have been installed, one with a 50 hp motor and another with a 20 hp motor. For the future buildout, a third 50 hp pump will be required. These represent only three (3) pumps and three (3) VFDs, thus reducing the maintenance and repair requirements as compared to the process and reject pumps. The high lift pumps also have lower power requirements. For the process and reject pumps, the power costs had been estimated at $12,000 US per year by an independent engineer who performed a power study for the site. Comparing the total connected horsepower of the high service pumps to the estimated connected horsepower for the process and reject pumps, the cost of operating the high service pumps at the future buildout can be roughly estimated at $6,500 US per year. The net savings would therefore be $5,500 US per year in power costs alone. The maintenance and repair savings are over and above this. The installation cost was decreased by the replacement of the process pumps with flow control valves as the costs associated with installation, testing, aligning and vibration testing of these pumps as well as the reject pumps were eliminated. The savings were not quantified as installation costs were not determined until after the pumps were removed from the scope of the project. The total capital cost of the 6 MGD membrane plant equipment, hydraulically sized for 11 MGD, was $2.8 million US for a siphon design. The total installed price of the plant came to $10.4 million US. This includes the supply of the membrane equipment, engineering and construction for the plant and required ancillaries such at the intake structure and settling ponds. The actual operating costs for the plant for the first year of operation came to $260,000 US based on the production of 1,200 million gallons of finished water. Figure 11 below outlines the operating costs on the basis of US dollars per million gallons of water produced.

Figure 11: Pendleton Operating Costs

Pendleton Siphon WTP Annual Operating Costs$ US / million gallons filtered water

Electricity and Gas, $125.00

Personnel - Operating, $20.00

Personnel - Cleaning/Maintenance,

$11.25

Chemical Analysis, $4.17

Repair & Maintenance, $31.25

Chemical Supplied, $25.00

Considering the power costs of $150,000 US per year, the savings of $5,500 US per year represented by the elimination of the process and reject pumps and addition of the high service pumps represents 3.7% savings on power alone. In general, ZENONs studies on siphon design have demonstrated that on a lifecycle basis, savings of 6 to 8% are possible by opting for a siphon design as opposed to a dedicated process pump design. This was determined using a siphon design with transfer pumps as in Pendletons case. If a wet well is required to achieve the siphon, the savings are decreased. REFERENCES Best, G, et al. (1999) Application of Immersed Ultrafiltration Membranes for Colour and TOC Removal. Paper presented at the AWWA Conference, Chicago, June 1999.