Brightwater: The Design Challenges of a 39 mgd (150 MLD) Membrane Bioreactor

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1 Brightwater: The Design Challenges of a 39 mgd (150 MLD) Membrane Bioreactor J. Komorita 1, P. Burke 2, B. Youker 2, and G. Crawford 2 1 King County, 2 CH2M HILL ABSTRACT Many Membrane Bioreactor (MBR) facilities have been constructed in North America, however to date the largest operating MBR facility in Traverse City MI has had a maximum month capacity of 8.5 mgd (32 MLD). The Brightwater MBR plant, now under construction in King County, Washington, is designed to have a maximum continuous capacity of 39 mgd (148 MLD), with a peak MBR treatment capacity of 57 mgd (216 MLD). The design of this facility, the largest under construction in the world, faced many challenges associated with the design and control of such a large facility. One challenge was the control and treatment of peak wet weather flows in excess of the MBR capacity by chemical-enhanced primary clarification. A second challenge was the implementation of a competitive procurement process for membrane equipment for the facility. Foam management, aeration system design, and internal recirculation system sizing were also key design issues. KEYWORDS MBR, Membrane Bioreactor, Design, Large Plant. INTRODUCTION The Brightwater Treatment Plant is a key element of King County s Brightwater Regional Treatment System (Brightwater), a $1.4 billion wastewater program. Brightwater involves 14 miles of conveyance tunnel, an advanced treatment plant incorporating membrane bioreactor technology, a marine outfall, and facilities for delivery of Class A reclaimed water. Construction of the treatment plant began in 2006; plant start-up is scheduled for late The Brightwater membrane bioreactor (MBR) balances the need for high quality effluent with the economic reality of treating peak wet weather flows that can exceed four times average flow. Membrane procurement activities analyzed membrane equipment systems configured for both base loaded and flow peaking scenarios, and selected a system that will accommodate limited peaking. The Brightwater concept ensures that the MBR will treat 98% of the flow in any year, producing an effluent of 2 mg/l BOD and Total Suspended Solids (TSS). Sidestream chemical enhanced primary treatment will treat portions of extreme wet weather flows, not to exceed 2% of the flow in any year. Using this MBR-based split flow concept, the annual discharge of BOD and TSS will be significantly less than the discharge from a conventional activated sludge process treating 100% of the flow. The MBR-based split flow concept is illustrated in Figure 1, and is discussed in the next section of this paper. The design of the Brightwater MBR is nearly complete, and the membrane equipment package has been awarded under a competitive procurement. Several key design challenges were overcome, many of which were related to the significant size of this MBR, approximately four times the capacity of any other operating MBR in North America. A later section of this paper discusses some of the design challenges related to peak flow management; process control; aeration, 1897

2 permeate, chemical, recirculation system sizing, and foam management. Procurement of the membrane equipment, including the timing of the procurement within the design process and the Owner s procurement process, is also discussed. THE MBR-BASED SPLIT FLOW CONCEPT Sizing the Brightwater MBR process to treat the peak hour flow, which has a 1 in 20-year recurrence interval, would require construction of treatment units that would see limited use. Alternately, optimizing the MBR process for efficient treatment of non-peak flows and providing another means of treatment for extreme storm-influenced flows offers opportunities to reduce pollutant loads, capital costs, and operating and maintenance costs without compromising the ability to meet projected NPDES Permit requirements. This concept, referred to as split flow treatment, produces a net environmental benefit compared to conventional activated sludge, allowing membrane bioreactor (MBR) technology to be used cost effectively. Figure 1 illustrates the concept. The split-flow concept was developed, evaluated and selected for implementation by the full project team including King County, Brown and Caldwell, and CH2M HILL. The split flow treatment analysis identified the split stream threshold as 39 mgd (148 MLD) for maximum month flow conditions. Wastewater flows at or below the threshold will be treated by the MBR. Wastewater flows in excess of the threshold will be treated via chemically enhanced primary clarification (CEPC), blended with MBR effluent, disinfected, and discharged through the marine outfall. Table 1 summarizes the intended flow splitting program and the projected effluent quality resulting from the split flow system analysis. Figure 1. MBR-Based Split Flow Treatment Concept Split Stream: Flows above 39 mgd (148 MLD) Chemically Enhanced Primary Treatment Permeate Influent Effluent Conventional Primary Treatment and MBR: Flows up to 39 mgd (148 MLD) Flow Blending Structure Reuse 1898

3 Under the split flow concept, the membrane equipment within the MBR must be able to produce 39 mgd (148 MLD), the split flow threshold, for prolonged periods. Additionally, it is desired that the MBR process peak week and diurnal peak flows above the threshold flow. Table 2 summarizes the membrane equipment capacity requirements that were considered and then incorporated into the membrane equipment procurement process. Two alternative flow splitting scenarios were considered, and the Peaking Alternative was selected for installation. All flows are treated by primary clarification. Algorithms within the plant control system continuously predict the projected near term and long term flows that will be received at the plant, and these projections are compared to the long term and short term MBR capacities to proactively determine the need for flow splitting. Flow splitting is initiated by two actions: the effluent flow from one or more primary clarifiers is diverted directly to the MBR effluent; and coagulant chemicals are added to the influents of those specific primary clarifiers. Table 1: Brightwater Split Flow Treatment Summary Parameter Split Stream Threshold, Maximum Month (MLD) a Split Stream Threshold, Maximum Hour (MLD) Maximum Hour Split Stream Flow (MLD) Maximum Hour Total Flow (MLD) Average Split Flow Frequency (days/yr) b Average Split Flow Volume (ML/yr) Value 39 mgd (148 MLD) 57 mgd (216 MLD) 73 mgd (276 MLD) 130 mgd (492 MLD) MG (785 MLD) Peak Month Conditions Blended Effluent BOD 5 (mg/l) Blended Effluent TSS (mg/l) BOD 5 Removal Efficiency (%) TSS Removal Efficiency (%) a Nominal capacity of secondary process b Indicates number of days during which split flow occurs; events may be less than 24 hours in duration. Table 2: Membrane Equipment Capacity Requirements Condition Capacity, Base Loaded Capacity, Peaking 1899

4 Alternative Alternative Annual Average 31 mgd (117 MLD) 31 mgd (117 MLD) Maximum Month 38 mgd (144 MLD) 39 mgd (148 MLD) Maximum Week 38 mgd (144 MLD) 45 mgd (170 MLD) Maximum Day 38 mgd (144 MLD) 45 mgd (170 MLD) Maximum Hour 47 mgd (178 MLD) 57 mgd (216 MLD) MEMBRANE EQUIPMENT PROCUREMENT A membrane equipment procurement process for MBR projects that considers both financial and non-financial criteria has previously been presented (Crawford et al, 2002) and has previously been successfully employed on other MBR projects (Crawford and Lewis, 2003; Crawford and Lewis 2004). The procurement process for Brightwater needed to be adapted to achieve several objectives, including the creation of a competitive process, having multiple capable bidders, conforming with King County procurement procedures and requirements, and recognizing the value of a strong corporate commitment by the successful bidder who will become a major partner in this project. An additional objective was to consolidate multiple membrane system procurements into a single process at the time of the Brightwater procurement King County was also procuring a membrane system for the 0.44 mgd (1.66 MLD) Carnation MBR plant. The first step was to complete the process design and preliminary plant layout, so that the parameters and constraints for the membrane equipment systems could be identified. The process schematic, system solids retention time, internal recirculation rates, number of parallel trains, redundancy, and sizes of each anoxic and aerobic zone were determined. The bioreactors and membrane tanks were laid out on the site, recognizing that the relative proportions of the bioreactors and membrane tanks may change depending upon the type of membrane equipment selected. A decision was made to pump the mixed liquor from the end of the bioreactors up to the membrane tanks, with gravity return of the membrane tank overflows to the bioreactors. That decision ensured that all major membrane equipment manufacturers would be able to bid on the project, including both hollow fiber and flat sheet membrane configurations. The design at this point was slightly more advanced than what is often referred to as the 30% design milestone. A key discussion and decision was made related to qualification requirements for the membrane equipment manufacturers. Both a two step and a one step procurement process were considered. Under the two step process, a prequalification step would be used to identify qualified bidders, after which those bidders would be invited to submit a formal quotation for the supply of the membrane equipment. Under the one step process, any and all interested bidders would submit a package that included both non-financial (qualifications-type) information and financial (quotation-type) information. King County was concerned that the two step process could prevent the consideration of non-financial information, such as the relative experience of the manufacturer, during the bid evaluation stage of the second step of the process, on the basis that once a prequalification step had been completed then all bidders are qualified. The one-step process was therefore seen to be 1900

5 advantageous, in that both financial and non-financial criteria could be used in the evaluation, and the relative qualifications and experience of the various bidders could be considered and scored as part of the evaluation and selection process. The one-step process, however, allows any firm to submit a bid that must then be fairly and fully evaluated, and therefore the evaluation, scoring and selection process becomes critical to the success of the procurement. Table 3 presents the basic criteria and relative allocation of scoring points used for evaluation of the bids received. This table was shown in the bid documents. Prior to the submittal date for the bids, King County developed sub-criteria for each category, and additionally written descriptions were established for scoring of the non-financial criteria and categories. These written descriptions provided guidelines to the bid evaluation team members to be used to establish point scores for the identified criteria based upon the information provided by each membrane manufacturer in their bid submittal. The Schedule Location refers to the bid schedule, to be completed by the bidder, within which the information to be used for evaluation is to be presented. The specific schedules are identified in Table 4. Of the 1100 available scoring points, 690 points were allocated for financial criteria, including both capital and operating costs. 80 of these financial points were allocated to the capital cost for the Carnation system component, while the remaining 610 points were allocated to the Brightwater system. The remaining 410 points were allocated for non-financial criteria that were similar to the those used in the previously-mentioned references however consolidated into two categories related to the project team and approach (Project Implementation) and to corporate commitment to the market and prior experience (MBR Experience and Corporate Responsibility). Prior to the bid phase, the project was publicly advertised and membrane equipment manufacturers were invited to submit a letter of interest to King County. Those who responded were provided with an opportunity to review draft versions of the bid documents, and to provide comments and recommendations to King County. All comments were carefully considered, to ensure that the bidding process would be competitive and fair to all manufacturers. Schedule B requires the bidder to provide a minimum 3 year extended warrantee on the membranes, and additionally to provide an annual cost for each subsequent year up to year 20, for which the bidder agrees to provide a full cost repair and replacement warranty for the membranes. This type Table 3: Bid Evaluation Criteria and Points Allocation 1. Schedule Element Schedule Location Points Total Capital Cost Carnation Total Capital Cost A, H, K 2. Annual Cost B O&M Considerations C, G, J Project Implementation D MBR Experience and Corporate Responsibility Proposal For Alternative Brightwater Design Conditions E, F 60 I 100 Total 1,

6 Table 4: Bid Submission Schedules Schedule A Breakdown of Lump Sum Proposal Price Schedule B Supplemental Pricing and Warranty Schedule C Operations and Maintenance Considerations Schedule D Project Implementation Schedule E Membrane Bioreactor Experience Schedule F Corporate Representation Schedule G Brightwater Equipment and Technical Data Schedule H Brightwater Alternative Equipment Schedule I Brightwater Proposal for Alternative Brightwater Design Conditions Schedule J Carnation Equipment and Technical Data Schedule K Carnation Alternative Equipment of extended warranty for membrane equipment systems has previously been described (Crawford and Lewis, 2003). One cost component of particular importance was that related to the differences in membrane tank sizes associated with each manufacturer. The total volume of the system remains the same regardless of the manufacturer, therefore the total volume of bioreactors plus membrane tanks is the same for all bidders. Any increase (or decrease) in membrane tank volume associated with a vendor relative to an assumed base volume is offset by a decrease (or increase) in bioreactor tank volume. Any cost differences would therefore include both bioreactor and membrane tank components. The procurement process was successful. Multiple, competitive bids were received. A bid evaluation team comprised of a variety of King County staff evaluated and scored each bid, with technical support provided to the evaluation team by the County s engineering staff and consultants. After selection of the highest-ranked bidder (ZENON Environmental), the design team completed the facility design based upon the use of ZENON membrane equipment. Subsequent construction and installation contract documents were prepared reflecting information submitted by ZENON during the final design phase. DESIGN CHALLENGES The Brightwater MBR faced design challenges related to the size of the facility, such as the need for large permeate, backwash, chemical feed, vacuum, mixed liquor (ML) and return activated sludge (RAS) piping systems. Utilizing a pumped system to feed ML to the membrane trains with gravity flow of RAS, the ML feed system has a firm capacity of approximately 180 mgd (681 MLD) using five vertical axial-flow pumps, and gravity flow of RAS sized for 135 mgd (511 MLD) with flow splitting capabilities to in-service basins. Chemical feed systems typically sized for about 15 gph (55 Lph), are required to deliver approximately 50 gpm (190 Lpm). The aeration system delivers 45,000 scfm (1,275 scmm) of process air to the aeration basins and 56,000 scfm (1,600 scmm) of agitation air to the membrane basins. Flow management to the MBR poses challenges to ensure that warranty-based flows and durations are not exceeded and that chemically-enhanced primary treatment is automatically started as the split flow threshold is approached. 1902

7 Foam management is always a key design feature for MBRs. The Brightwater design is based upon the bioreactor discharge being pumped up to the membrane tanks, with gravity flow to return the mixed liquor overflowing from the membrane tanks back to the bioreactors. Foam is allowed to flow from zone to zone within the bioreactors, and therefore foam removal is required in the area of the membrane tank feed pumps. Due to the need for flow splitting to the multiple bioreactors, foam may also accumulate at the downstream end of the mixed liquor gravity return channel, and therefore foam removal is also provided at this location. The ZENON membrane equipment system included a vacuum priming system to remove air from the piping systems between the membrane assemblies and the suctions of the permeate pumps. A centralized vacuum system was proposed by ZENON, with air separators and vacuum relief valves provided on the suction piping system for each train. The design team discussed this system with ZENON, including concerns with the use and reliance on vacuum piping systems for membrane equipment operation, and with the maintenance requirements for the vacuum relief valves that would be operating nearly continuously. An improved design was developed that includes solenoid valves at each vacuum relief valve. The process control system was then modified to allow the intermittent operation of the vacuum relief valves. This change is expected to significantly reduce the maintenance requirements associated with the vacuum relief valves, and further provides some protection against a loss of prime should a leak develop in the main vacuum piping system. Air is required for two functions: to satisfy process aeration requirements in the bioreactors, and to provide a coarse bubble air flow to scour the membrane fibers and thereby maintain desired membrane performance. The air scour blowers were included within the membrane equipment manufacturer s scope, and will be multistage centrifugal type. A separate system will be provided for process aeration, and these blowers will be the single-stage centrifugal type. CONCLUSIONS Construction has begun for the 39 mgd (148 MLD) Brightwater MBR facility, anticipated to be the largest MBR facility in the world when construction is anticipated to be complete in Several aspects of the design of large MBRs were developed for the project, including: optimization of the treatment process to manage peak wet weather flows; resolution of specific design challenges related to this extremely large MBR facility; and the competitive procurement of a large, complex membrane equipment system. REFERENCES Crawford, G., Fernandez, A., Shawwa, A., and Daigger, G. (2002). Competitive Bidding and Evaluation of Membrane Bioreactor Equipment Three Large Plant Case Studies. Proceedings of the Water Environment Federation 75 th Annual Conference & Exposition, Chicago IL, CD-ROM, Oct, Crawford, G. and Lewis, R. (2003). Traverse City Membrane Bioreactor Facility: the Largest in North America and a Sustainable Solution for the Future. Proceedings of the Water Environment Federation 76 th Annual Conference & Exposition, Anaheim, CA, CD-ROM, Oct, Crawford, G. and Lewis, R. (2004). Traverse City: the Largest Operating MBR Facility in North America. Proceedings of the Water Environment Federation 77 th Annual Conference & Exposition, New Orleans, LA, CD-ROM, Oct,