SKI RESORT MBR WWTP: CHALLENGES OF YEAR-ROUND OPERATION WITH VERY HIGH FLOW AND LOAD VARIATIONS BETWEEN SEASONS

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1 SKI RESORT MBR WWTP: CHALLENGES OF YEAR-ROUND OPERATION WITH VERY HIGH FLOW AND LOAD VARIATIONS BETWEEN SEASONS Marie-Laure Pellegrin, Ph.D.,* Lawrence Riegert, P.E.,** Steve Brewer, Class III Operator,*** *** *HDR, Inc 4435 Main Street, Suite 1000 Kansas City KS ABSTRACT ** Hammond Collier Wade Livingstone, Wenatchee, WA *** Stevens Pass Sewer District, Stevens Pass, WA The information presented in this paper is developed based on the Stevens Pass Wastewater District Membrane BioReactor Treatment Plant. The main objective is to discuss the unique process issues of ski area facilities which are faced with seasonal flow and load variations: (i) how to handle these variations within the system, (ii) how to grow enough biomass before the ski season starts and the flow and load go by a ratio of 1 to 800 (lowest to highest) in literally one day. Other unique design considerations include high flow at low water temperatures (worst conditions for membrane operation), difficult plant access, and operating in a pristine mountain environment. Process issues include an elevated TKN in the influent, approach to minimize alkalinity feed requirement and a question of adequately reduced carbon considerations in the influent to achieve denitrification and associated alkalinity recovery. KEYWORDS Membranes, MBR, Ski Resort, Startup, Denitrification, COD/N ratio, BOD/N ratio INTRODUCTION The Stevens Pass Ski Area is located on the crest of the Cascade Mountain Range and receives abundant snow over the winter ski season. In order to meet the more stringent effluent ammonia requirements imposed in the effluent permit, a Membrane BioReactor process technology was installed in the existing process tanks. Reliability and equipment durability was a primary concern as the plant is located in a remote area and difficult to access. The plant was designed for a wide range of flows depending on the design year and the season (see table 1). The ultimate peak day flow is mgd (winter 2025) but the summer conditions for year 2007 have a peak day flow of only mgd. The Stevens Pass sewer treatment plant is a hollow fiber membrane project with equipment supplied by USFilter. The process is the MBR version of the modified Ludzack-Ettinger in which there is an internal Mixed Liquor Suspended Solids (MLSS) recycle from the aerobic stage to the anoxic stage to return nitrified MLSS at a regulated rate. The process is designed 7070

2 with two completely independent process trains operating in parallel at the design flow in year The treatment trains are independent, so that if a single train fails, there is still 50% of the process capacity remaining in operation. Current flows are about 50% of the design capacity of the plant. Therefore, a single treatment train meets the needs of the existing facility. Only half of the membrane complement is provided at this time. Installation of the additional membranes is anticipated at a point in time when a substantial increase in inflow is observed. However, the only scenario under which additional flows are likely to be observed, would be a substantial increase in the existing ski area. Since a major increase in the ski area is not expected to occur for many years, a single train operation will continue for the foreseeable future. In this paper, single train operation is associated with design flows for year The following table provides an overview of the anticipated flow scheme in design year 2025 as well as single train operation in year Table 1 - Stevens Pass Flow Scheme Design Year Design Flow (Peak Day) Process Trains Operational Membrane Racks in Service 2025 (winter) mgd (winter) 0.14 mgd 1 4 Summer Conditions mgd Non Standard (see below) 4 The overall design criteria for the membrane process (including the full complement of eight membrane racks) is provided in table 2 and 3. Note that the design criteria provided will be achieved following the installation of four additional membrane racks in addition to the four racks currently installed. Table 2 - Effluent Design Criteria Parameter Discharge Limit Design Year Winter 2007 Winter 2025 BOD5 10 mg/l Same TSS 10 mg/l Same NH4-N 3.0 mg/l Same 7071

3 Table 3 - Design Criteria for Stevens Pass Wastewater Treatment Plant (Criteria are for the full complement of eight membrane racks) Parameter Design Value Design Year Winter 2025 Average Day in Maximum Month mgd Peak Daily Flow mgd Peak Hourly Flow mgd (210 gpm) CBOD5 261 mg/l Max Month CBOD5 244 lbs/d Peak Day CBOD5 464 lbs/d TSS 647 mg/l Max Month TSS 602 lbs/d Peak Day TSS 1,150 lbs/d NH4-N 93 mg/l Max Month NH4-N 166 lbs/d TKN 140 mg/l Max Month TKN 131 lbs/d Peak Day TKN 249 lbs/d COD 1,316 mg/l scod 591 mg/l Max Month COD 1,230 lbs/d Peak Day COD 2,338 lbs/d Peak sustained COD (no more than 7 consecutive days) 1,974 lbs/d VSS 80% TDS 500 mg/l Winter Temperature 50oF (10oC) Summer Temperature 77oF (25oC) METHODOLOGY Modes of Operation Because of the complexity of the Stevens Pass wastewater treatment plant project regarding seasonal flow and load variations, the predesign engineering report called for 10 different modes of operation. When the plant started in 2004, 6 of these modes were considered unrealistic and consequently only 4 were implemented. These operational scenarios have been designed for winter vs. summer operation for 2007 and 2025 forecasted conditions. For example, the winter 2005 flow and load conditions determined the full plant volume and capacity (mode 2). Similar calculations were run for winter 2007 flow and load conditions (mode 7 or mode 8) which determined the size of each train. Finally summer 2007 flow and load conditions (mode 12) were used to determine the volume needed and the control strategy implemented. Flow diagrams are shown on figure

4 Figure 1 - Process Flow Diagram for different modes of operation Mode 2 Description: Mode 2 Description: Mode 2 involves splitting the screened influent flow at the headworks and routing one half of the influent to AX1 (anoxic tank train1) while the other half of the influent is routed to AX2 (anoxic tank train 2). In this mode both process trains and membrane tanks are operating in parallel, as will be the case in the final upgraded plant. The anoxic tanks and associated mixing equipment as well as the process recycle pumps are functional in Mode 2. Mode 7 Description: Mode 7 is one of the normal operational modes used in winter ski season operation (along with Mode 8) as long as there are only two racks per membrane tank installed. Mode 7 includes process basins AX1 and AB1 (aeration basin train 1) supplementing the membrane operating system. Screened influent flow is routed to AX1. The process basins to train 2 (AX2 & AB2, aeration basin train 2) are dewatered and not functional under Mode 7. Mode 7 combines both membrane operating systems with a single process tank configuration consisting of AB1 and AX

5 Mode 8 Description: Mode 8 is the mirror image of Mode 7 with train 2 in operation. In Mode 8 operation process tanks AB2 (aeration basin train 2) and AX2, along with the associated process recirculation pump are operational while train 1 tankage is dewatered (AX1 and AB1). As is the case with Mode 7, both membrane operating systems are functional. In Mode 8 all membrane recirculation flow is drawn from and returned to AB2. Mode 12 Description: Mode 12 is a batch process consisting of a fill stage, in which the incoming wastewater is stored and aerated in the aeration basins as it enters the plant. When the aeration basins are filled, the filtrate pumps are started and the aeration basins are pumped down to a low level during the discharge stage. The process is then repeated. The discharge rate of the filtrate pumps is an operator adjustable setpoint. The discharge stage can also be initiated by the operator before the aeration basins are completely filled. The anoxic basins are not used in Mode 12 operation except as potential emergency overflow holding basins. Mode 12 operation involves evenly splitting the influent flow and routing the flow into the two aeration basins (AB1 and AB2). The anoxic tanks are dewatered and the process recirculation pumps and mixers are shut down. During the batch fill phase the filtrate pumps will be shut down and the influent wastewater fills the aeration basins. After filling to the operator adjustable setpoint aeration basin water level, the filtrate pumps are automatically activated and pump down to an operator adjustable specified shut down level. Membrane Performance Measurement Several typical parameters are used to evaluate the performance of the MemJet membrane systems. Equations for calculating each of these parameters are detailed below. Filtrate Flux: The average filtrate flux is the flow of product water divided by the surface area of the membrane. Filtrate flux is calculated according to the following formula: J t = Q S p where J t = filtrate flux at time t (gfd or gal/d/ft 2, LMH or L/h/m 2 ) Q p = filtrate flow (gpd, L/h) S = membrane surface area (ft 2, m 2 ) 7074

6 Trans-Membrane Pressure (TMP): The average trans-membrane pressure for dead-end filtration is calculated as: P tm = P P f p where P tm = transmembrane pressure (psi, bar) P f = pressure at the outlet of the membrane module (psi, bar) P p = filtrate pressure (psi, bar) Temperature Adjustment for Flux Calculation: Temperature corrections to 20 C for filtrate flux can be made to account for the variation of water viscosity with temperature. The following equation may be employed: J tm o ( ) ( T 20 at 20 C = J e ) t ( ) where J tm = instantaneous 20 C (gfd, LMH) J t = filtrate flux at time t (gfd, LMH) T = temperature, ( C) Permeability (Normalized Specific Flux): Permeability may be calculated as follows: J P = P tm tm where J tm = instantaneous 20 C (gfd, LMH) P tm = transmembrane pressure (psi, bar) P = permeability (gfd/psi, LMH/bar) Alkalinity requirement The aeration basins (aerobic zone) will biologically oxidize the ammonia nitrogen in the wastewater to nitrate, a process known as nitrification. Ammonia nitrogen is found in two forms: ammonia (NH 3 ) and ammonium ion (NH 4 + ). The relative concentration of these two forms is dependent upon the ph of the wastewater. At the ph typically found in wastewater, the ammonium ion predominates. Confusion can arise when the ammonia nitrogen is referred to simply as ammonia, as it actually includes both forms described above. Chemical ammonium ion (NH 4 + ) is the ammonia species being oxidized. The bacteria involved in the oxidation of ammonia nitrogen include Nitrosomonas and Nitrobacter. Nitrosomonas converts ammonia to nitrite, while Nitrobacter oxidizes nitrite to nitrate. Nitrite is relatively unimportant in the above steps as it is easily oxidized to nitrate. Both of these bacteria use carbon dioxide as their carbon source (instead of organic matter) and inorganic oxidation/reduction reactions as their energy source. This type of metabolism classifies these bacteria as autotrophic and chemosynthetic. 7075

7 The overall reaction for the conversion of ammonia nitrogen to nitrate is shown below. NH O 2 NO H + + H 2 O or Bacteria + NH O HCO - 3 NO H 2 CO 3 + H 2 O + Additional Bacteria or NH O HCO C5H 7NO H 2O NO H 2CO3 These equations show that alkalinity (HCO 3 - ) is destroyed during the nitrification process at a rate of 8.6 mg of alkalinity as CaCO 3 is used per 1.0 mg of ammonia nitrogen oxidized. The effect of cell synthesis on ammonia removal is relatively small. For example, if a wastewater has 25 mg/l of ammonia nitrogen but does not have sufficient alkalinity, it would not be buffered during the nitrification process. This condition is significant as nitrification during the activated sludge process may drop the ph below 7.0, resulting in a possible violation of the permit limit for effluent ph. Low ph has also been shown to inhibit nitrification. Biological nitrogen removal (BNR) can be achieved in the activated sludge process by combining nitrification with the biological process known as denitrification. BNR requires separate zones of high (nitrification) and low (denitrification) dissolved oxygen concentrations (DO). Therefore, part of the process must be able to provide mixing without producing a high DO. This is achieved by providing the anoxic tanks with internal mixers to provide mixing and prevent settling of solids in the anoxic tanks. The process of biological denitrification is nitrate reduction and involves the conversion of nitrate nitrogen to gaseous nitrogen under anoxic (oxygen-free) conditions. The gaseous product is primarily nitrogen gas, but may also be nitrous oxide or nitric acid. These gases are then released to the atmosphere where they have no discernable effect on the environment. A large group of bacteria are able to accomplish denitrification including Archromobacter, Pseudomonas and Bacillus. These bacteria, known as facultative heterotrophs, accomplish nitrate reduction by replacing oxygen with nitrite or nitrate as the terminal electron acceptor in the oxidation of organic matter. Thus, this process is said to take place under anoxic conditions, not anaerobic. Because of the similarity of these biochemical pathways with those used for the consumption of carbonaceous material under aerobic conditions, denitrifiers can switch between using nitrate or oxygen as the terminal electron acceptor quite easily. However, since the use of oxygen as the terminal electron acceptor results in a greater release of free energy than nitrate, oxygen is favored whenever it is available. The chemical equation showing the overall denitrification process is given below. Bacteria + 10 NO C 10 H 19 O 3 N 10 CO N H 2 O + NH OH - + Additional Bacteria 7076

8 Actually, nitrate reduction is a three-step process. First nitrate is converted to nitrite, nitrite to nitrous oxide, and then nitrous oxide to nitrogen gas. In this equation, C 10 H 19 O 3 N` represents the organic matter present in the wastewater. This organic matter is necessary in denitrification to provide a carbon source for the denitrifiers and serves as the electron donor. The nitrate serves as the electron acceptor. In summary, denitrification will not occur at a significant rate if oxygen is present and/or a source of carbon (organic matter or BOD) is not available. As the equation for denitrification shows, alkalinity (in the form of OH - ) is produced during denitrification. The theoretical amount of alkalinity produced is 3.57 mg as CaCO 3 per mg of nitrate nitrogen reduced to nitrogen gas. Actual alkalinity production has been measured to be somewhat lower, around 2.9 mg as CaCO 3 per mg of nitrate nitrogen reduced (Horstkotte et al., 1974). This difference is most likely due to over-simplification of the biological processes involved. Denitrification tends to help offset the effects of nitrification by providing alkalinity and raising the ph. However, the alkalinity produced during the denitrification is only about one-half of the amount lost during nitrification. By controlling the activated sludge process to allow denitrification to occur, the benefits of recovered alkalinity, nutrient removal, and energy savings are achieved. Carbon Source requirement One of the main control factors for denitrification process is the availability of the carbon source. According to different authors, the denitrification kinetics vary with the nature of the carbon source (Paul et al., 1989 ; Kristensen et al., 1992 ; Henze et al., 1994). The denitrification efficiency (ratio of consumed carbon and reduced nitrate) depends only on the carbon source. In wastewater treatment, the most common carbon source used is methanol, but the wastewater itself can provide the carbon source: it is called combined denitrification. If the nitrates are the limiting factor during the denitrification, excessive COD will be observed in the effluent (Henze et al, 1978). The composition of the organic matter in the wastewater is very complex. Except for the acetate which represents 5 to 10% of the total COD, all the other compounds are in very small concentrations (Henze et al., 1994). All these compounds, as a whole, are important for the kinetics and removal efficiency. For toxic compounds, even very small concentrations can have an impact. The denitrification rate is, in most cases, limited by the rate of hydrolysis of easily and slow biodegradable COD. Acetate increases the denitrification rate by a factor of 3-4 (Henze et al., 1994). When all the acetate is metabolized, the denitrification rate decreases to the limiting hydolysis rate (Kristensen et al., 1992 ; Henze et al., 1994). The denitrification rate is very dependant on the wastewater composition and the nitrogen removal efficiency requirement. Denitrification is affected by pretreatment since they changed the COD composition, in particular the directly assimilated fraction vs. easily degradable fraction vs. slowly degradable fraction. The detailed composition of the organic matter of 7077

9 wastewater changes the denitrification rate. The directly assimilated fraction and easily biodegradable fraction induces higher kinetic rates (see table 4) but these compounds are in small quantity which means that their influence on the overall kinetic rate is small. Table 4 Denitrification rate at 20 o C for different wastewater carbon source fractions (Henze and al., 1994) Fraction g N/kg MLVSS/h Directly assimilated Easily degradable 2 4 Slowly degradable Based on table 4, a small quantity of heterotrophs indicates the easily assimilated fraction of the carbon source is not available anymore (Henze et al., 1994 ; Surmacz-Gorska, 1996). Consequently, there is significant variations of denitrification rate during the day depending on the wastewater quality entering the plant (Kristensen et al., 1992). Similarly, the denitrification rate varies from one type of wastewater to another (Henze, 1986). In general, 40 to 60% of the nitrogen in the wastewater is denitrified. The rest in assimilated in the sludge (20%) or is in the effluent (Henze et al., 1994). Another important parameter for the denitrification rate is the ratio COD/N. There is a critical ratio below which denitrification is limited by the carbon source and above which denitrification efficiency is not improved even if the COD supply is increased (Goronszy et Barnes, 1982). For wastewater, the minimum COD/N ratio is 4.5 mg COD consumed/mg N-NO 3 reduced (Goronszy et Barnes, 1982). Kristensen (1974) gives critical ratio of 5 mg BOD 7 /mg N based on laboratory studies and 4.6 for full scale plants. These studies were realized with a F/M ratio less than 0.1 and a hydraulic residence time varying from 16 to 45 hours. In other studies, Kristensen et al. (1977) conclude that the critical ratio depends on the process and is about 4 to 6 mg BOD 7 /mg N. For Henze et al. (1994), COD/N ratio is about 3.5 to 4.5 for a denitrification process where none of the carbon source is lost by oxydation of oxygen. For Ceçen et Gonenç. (1995), 5 is the optimum for the COD/N ratio. If this ratio is above 5, the nitrogen removal efficiency is not improved (Ceçen, 1990). This is due to mechanism of COD consumption more than denitrification (Ceçen et Gonenç., 1995). The minimum and maximum kinetic rates given by these different authors for wastewater carbon source are 2-3 and mg N/g MLVSS/h at 20 C (Goronszy et Barnes, 1982). Barnard (1975) gives mg N-NO 3 /g MLSS/h at 20 C for denitrification rate with a F/M ratio less than 0.1. The different values given in the literature for the denitrification rate are due to different operating conditions and in particular to different organic loads (Goronszy et Barnes, 1982). 7078

10 RESULTS Performance Test Data The MBR supplier (USFilter) agreed to perform a 30 day process test for the MBR equipment as part of the procurement required for this project. The performance test was to be accomplished during the peak Christmas ski season. The test was schedule for one year following startup, however the winter of 2004/2005 saw almost no snow at the ski facility and the ski season was canceled for all practical purposes. For this reason, the performance test was accomplished, this past winter in 2006, from the 27 th of December 2005 to the 12 th of January Performance with respect to permitted parameters was excellent. Effluent nitrate is not itself subject to permit limitations. However a target goal for effluent nitrate was set as part of the performance test because alkalinity recovery is an issue at the facility. The target goal for nitrate (as N) in the effluent is 25 mg/l. The summary data (see table 5) was developed as part of the abbreviated performance test. As shown in table 5, the fraction of reduced nitrogen entering the plant which is ultimately denitrified is quite low, in the range of 30 to 40%. This low denitrification rate is observed in the presence of near complete nitrification and adequate recycle from the anoxic to aerobic basins. It should be noted that the COD to N ratio appears to be in a range where a greater degree of denitrification would seem to be feasible. Adequate alkalinity is also present. MLSS of the process was about 6,000 mg/l. Note however, the ratio of the carbonaceous BOD/N is quite low, indicating that the biodegradable reduced carbon is in short supply. It appears that the biodegradable organic carbon, as measured by the BOD test, is probably a better indicator of the available carbon source than the COD test, particularly when reduced nitrogen is unusually elevated as in the present case. An evaluation of design loading rates of nitrate to the anoxic basin was performed to determine the kinetic limits on substrate utilization. The operating temperature over the winter season is in the range of 10 C. Assuming a theta value of 1.09, table 6 was derived. Comparing table 6 to table 4, the carbon source present at the Stevens Pass wastewater treatment plant seems to be easily degradable. Consequently, denitrification kinetics is not limited by the carbon source degradation properties. There has been some concern expressed by the supplier (USFilter) that the dissolved oxygen in the anoxic basin is too high. The concern is that the elevated levels in the aerobic basin are brought into the anoxic basin via the recycle flow. The aeration blowers however are at minimum speed already and we were not able to turn the air down to test this theory. We anticipate that further testing will be accomplished this coming winter of 2006/2007. In fact, one of the reasons for abbreviating the performance test was to limit costs in anticipation of additional testing the following year. With respect to nitrate removal, the primary purpose was alkalinity recovery. The cost of the additional alkalinity purchased by the district has not been significant. Therefore, denitrification performance will be pursued primarily for its academic 7079

11 interest. It should be noted that there were several unusual circumstances in plant operation which occurred just prior to the testing. Therefore this anomaly may clear itself up under more normal circumstances. 7080

12 Table 5 - Summary of Performance Test Results for the Stevens Pass Sewer District Winter 2006 Sample Location Flow DO Aeration Basin DO Anoxic Basin Alkalinity Dose (As CaCO3) TSS TKN NH3 BOD (with nitrification inhibitor) COD Soluble COD Alkalinity (Total) as CaCO3 ph Nitrate Percent Denitrification COD/N BOD/N Ratio /27/2005 Influent /27/2005 Effluent /28/2005 Influent % /28/2005 Effluent < /30/2005 Influent /30/2005 Effluent /03/2006 Influent , /03/2006 Effluent /02/2006 Influent , % /02/2006 Effluent 2 < /04/2006 Influent % /04/2006 Effluent < /09/2006 Influent /09/2006 Effluent 01/12/2006 Influent , % /12/2006 Effluent < Table 6 - Design and Observed Substrate Loading Rates Condition Substrate Utilization Rate 10 C Theta Value Adjusted to 20 C. Design condition 1.01 g N/kg MLVSS/h g N/kg MLVSS/h Observed Loading 1.13 g N/kg MLVSS/h g N/kg MLVSS/h WEFTEC.06

13 Annual Process Startup Summertime flows at the plant are so low that the biological processes are essentially nonexistent over the summer low flow (mode 12 operation). Therefore the biological process must be essentially started up each year in the fall. Timing is critical because the biological process must be built up and maintained just prior to the opening of the ski area. The startup period is necessary because the flows go from near zero to design flows overnight, when the ski area opens. Biological startup is accomplished by feeding a single tank with powdered milk, just prior to opening of the ski area. Fifty pounds of the milk powder per day is approximately representing BOD for 1,000 ski visits per day. The aim is to be able to go from a couple hundred people to a few thousand people on very little notice. The district brings in a mature seed, about 3,500 gallons from another plant at the beginning of November in order to start the culture. The intent is to feed it with milk powder on a daily basis to grow a biological culture and nitrifying bacteria prior to the ski area s opening day. The district has never been totally successful in this startup procedure due to variables beyond its control such as: In the winter of 2003/2004, startup of the process was complicated by the simultaneous commissioning of the plant, In the winter of 2004/2005, there was almost no snowfall (a record low). As a consequence, the ski area was only open for a couple of weekends. The membranes were replaced just before startup in the winter of 2005/2006. As a result, the ski area opened after less than two weeks of starting feeding powdered milk. We have found that it takes about three or four weeks to establish a nitrifying culture at the plant. However, we believe that given a normal start-up scenario the yearly startup approach would be successful in being in compliance, especially with BOD and ammonia, soon after the ski area opens. Membrane Performance through the different season and modes of operation Figure 2, 3 and 4 show the membrane performance data for both membrane tanks for flux, TMP and permeability. The mode of operation is also represented on all these graphs to give an idea of when the ski season opens and closes and consequently when the flows and loads are getting higher, mode 12 being the summer mode and mode 7 or 8 being the winter mode. It can be observed from all these graphs that during the summer season the effluent discharge is much less. Mode 12 is a batch mode during which effluent is filled in the aeration basins. When the water level in the aeration basins reach the high level setpoint, the effluent is discharged. On an average, effluent is discharged once a week to once every two weeks in summer. The membrane flux varies between 6 and 14 LMH with an average of 9 LMH. These flux seems very conservative but the plant did not reach design flow for winter 2007 conditions yet. The 7082

14 corresponding TMP varies between 2.6 and 9.5 kpa with an average of 5.5 kpa. No recovery cleanings have been performed since startup in February Maintenance cleans are performed as shown on graphs once a week to once every 2 weeks depending on summer or winter operation. Figure 2 Flux and TMP data for membrane tank # Flux at 20oC (LMH) TMP (kpa) Mode of Operation /1/ /9/2004 2/17/2005 5/28/2005 9/5/ /14/2005 3/24/2006 7/2/2006 Day Flux TMP Maintenance Clean Mode of Operation Figure 4 shows permeability data for membrane tank #1 and #2. Permeability dropped from 230 LMH/bar to 140 LMH/bar during winter 2004/2005 operation (first winter). At startup, membranes permeability always drops to reach its stable operating conditions. It took about 2 months to reach these conditions. During summer 2005, the membranes stayed stable at 140 LMH/bar. Then beginning of November 2005, permeability jumped to more than 250 LMH/bar. This jump is due to the fact that all the membranes were replaced because of the malfunction of the screen. The 2 mm perforated screen installed on the influent stream has been bypassed since startup (February 2004). Consequently, fibrous materials, hairs, debris of all sorts accumulated and got entangled on the membrane filters to the point where half of the membranes were covered with debris. Even with this terrible screen malfunction, it should be noted that the membrane performances were still very good (140 LMH/bar) and stable. The owner requested fixing the screen bypass and, when done, changing the membranes. This happened in November Since then the membrane permeability of membrane tank 1 and 2 is slightly different. This could be due to different membrane batch during manufacturing inducing different membrane performances. Permeability dropped from about 275 LMH/bar to 210 LMH/bar in 2 months on train 1 and 200 LMH/bar to 150 LMH/bar for train 2. Figure 3 - Flux and TMP data for membrane tank #2 7083

15 Flux at 20oC (LMH) TMP (kpa) Mode of Operation /1/ /9/2004 2/17/2005 5/28/2005 9/5/ /14/2005 3/24/2006 7/2/2006 Day Flux TMP Maintenance Clean Mode of Operation Figure 4 - Permeability data for membrane tank #1 and # Permeability at 20oC (LMH/bar) Mode of Operation /1/ /9/2004 2/17/2005 5/28/2005 9/5/ /14/2005 3/24/2006 7/2/2006 Day MT1 MT2 Maintenance Clean Mode of Operation CONCLUSIONS 7084

16 The MBR process at Stevens Pass met all the effluent requirements (BOD < 10 mg/l, TSS < 10 mg/l, NH4-N < 3 mg/l). The effluent nitrates are relatively high (only 30-40% reduction) possibly due to an elevated dissolved oxygen concentration in the anoxic basin that partly inhibits denitrification. Further testing will be performed next winter (2006/2007) to try to improve the denitrification performance and consequently the alkalinity recovery. It is important to note however, that the availability of readily biodegradable organic carbon is probably low in this system. It appears to be adequate based on the COD test, but inadequate based on the BOD test results. Since the COD test would include in its measure, the oxidation of reduced nitrogen, it probably overestimates the available reduced carbon. Lack of availability of adequate carbon sources is probably the limiting factor in denitrification performance at this facility. The fine screen on the influent was bypassed since startup resulting in increased debris accumulation on the membrane filters. The membranes were still performing well and did not require any chemical cleaning. The screen eventually was repaired and the membranes were replaced. Since then, the membranes have been performing well. The MBR process has been a good fit for Stevens Pass Sewer District. There are two basic reasons for selecting this technology at the site. Constraints on construction of new basins to achieve a more advanced level of treatment were a primary factor. Snowfall at the site can be as high as 50 to 60 feet per year. For this reason, the entire treatment plan is enclosed in a large existing concrete structure. Construction of additional basins would have required additional structures to house the new basins. These types of structures are prohibitively expensive. With the MBR technology, we were able to get additional treatment out of the existing basins. Another major factor in favor of the MBR technology was achieving fairly strict effluent requirements exceeding secondary treatment levels. The several modes of operation of this facility were necessary to implement due to the following factors: 1. Process efficiency in the initial years of the project was better with single train operation, 2. Extreme low flows in the summer are matched by a simple operational mode, requiring little operator input, 3. Commissioning of the plant to include dual train operation was considered necessary to ensure that the control system operated properly. In practice, it was found, however that the several modes of operation substantially complicated the programming process for this project. This issue was accentuated by the high level of reliability required at this facility. For example, each treatment train had an independent PLC and control system in order to ensure that the entire plant was not vulnerable to failure. However, there was one crossover valve common to both trains that needed to be controlled alternatively, by control system number one and control system number two, depending on which treatment train was in failure mode. The programming to control a single valve with alternative control systems, depending on particular circumstances, at a given point in time was complicated. Prior to the winter of 2002/2003 the plant experienced major and continuing violations of its NPDES permit each year at the start of the ski season. Our experience in startup of a 7085

17 nitrifying treatment culture prior to the ski season opening has indicated the potential for success using high-protein powdered milk. We know that the process can be successful, as we learned in the winter of 2002/2003, prior to upgrading the plant to the MBR technology. At that time, dog food in lieu of powdered milk was used. Since dog food has a higher content of grease, which could eventually coat and foul the membranes, powdered milk was chosen after the plant upgrade. Nothing should interfere with the startup process next winter season (2006/2007). This process will consequently be followed and monitored to try to identify appropriate duration and quantity of powdered milk to use and enhance the procedure. REFERENCES Barnard, J. L. (1975) Biological nutrient removal without the addition of chemicals. Water Research, 9, 485. Cecen, F. (1990) Nitrogen removal from high-strength wastewaters by upflow submerged nitrification and denitrification filters. Istanbul Technical University Thesis. Cecen, F.; Genenc, I.E. (1995) Criteria for nitrification and denitrification of high-strength wastes in two upflow submerged filters. Water Environment Research, 67 (2), 132. Goronszy, M. C.; Barnes, D. (1982) Nitrogen removal in continuous-flow sequentially aerated activated sludge systems. Process Biochemistry, Jan/Feb. Henze, M. (1986) Nitrate versus oxygen utilization rates in wastewater and activated sludge systems. Water Science and Technology, 18 (6), 115. Henze, M.; Kristensen, G. H.; Harremoes, P. (1978) Nitrification and denitrification in wastewater treatment. Henze, M.; Kristensen, G. H.; Strube, R. (1994) Rate-capacity characterization of wastewater for nutrient removal processes. Water Science and Technology, 29 (7), Horskotte, G. A; Niles, D. G.; Parker, D. S.; Caldwell D. H. (1974) Full-Scale TESTING OF A Water Reclamation System. JWPCF, 46 (1), Kristensen, G. H. (1974) Denitrifiation of sewage by alternating process operation. 7 th IAWPR Conference in Paris, September. Kristensen, G. H.; Harremoes, P.; Jensen, O. R. (1977) Combined sludge denitrification of sewage utilizing internal carbon sources. Prog. Water Tech., 8, 589. Kristensen, G. H.; Jorgensen, P. E.; Henze, M. (1992) Characterization of functional biomass groups and substrate in activated sludge and wastewater by AUR, NUR and OUR. Water Science and Technology, 25 (6), 43. Paul, J. W.; Beauchamp, E. G.; Trevors, J.T. (1989) Acetate, propionate, butyrate, glucose and sucrose as carbon sources for denitrifying bacteria in soil. Canadian Journal of Microbiology, 71, Surmacz-Gorska, J.; Gernaey, K.; Demuynck, C.; Vanrolleghem, P.; Verstraete, W. (1996) Nitrification monitoring in activated sludge by oxygen uptake rate (OUR) measurements. Water Research, 30 (5),