SMALL COMMUNITY TREATMENT PLANT EXPANSION WHILE IN OPERATION USING ADVANCED TECHNOLOGY

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1 SMALL COMMUNITY TREATMENT PLANT EXPANSION WHILE IN OPERATION USING ADVANCED TECHNOLOGY Adel Bassyouni, HDR Engineering, Guillermo Garcia, HDR Engineering, Don Mc Dermott, Viejas Public Works Department Abstract 8690 Balboa Avenue, Suite 200 San Diego, CA The Viejas Indian Reservation in Eastern San Diego County owns and operates a gaming casino and outlet shopping mall that recently underwent a large expansion. At the same time, the Viejas Reservation Tribal Government completed the infrastructure project to provide a new water supply distribution and wastewater collection system to serve the entire reservation in addition to the casino and outlet center. The added residential flows required additional capacity in the existing Membrane Bioreactor Treatment Plant. The treatment plant is the only facility serving the casino and outlet center and can not be taken off line for an extended period of time. The maximum duration for a plant shut down is 24 hours equivalent to the capacity of the existing equalization tanks. To keep the treatment plant on line and augment its treatment capacity, various options were evaluated for their reliability and capital cost including the following: 1) Add a new treatment module in new separate tank. 2) Maximize use of existing facilities and available tankage. 3) Haul away wastewater flows for treatment off site. Based on the economical analysis Viejas Reservation selected to meet the challenge with the design consultant and expand the Immersed Ultra-filtration Membrane Bioreactor to meet their needs while it is in operation. This treatment facility is the first of its type to be in operation in California. It has been successfully operating since July A design/build project is underway to use plant effluent as recycled water for landscape irrigation at the casino, outlet mall and the reservation recreation center. Keywords Small Community, Wastewater Treatment, Membrane Bio Reactor; Recycled Water, Master Plan. Introduction The Viejas Indian Reservation in Eastern San Diego County owns and operates a gaming casino and outlet shopping mall that recently underwent a large expansion. At the same time, the Viejas Reservation Tribal Government completed several projects to provide a new water supply distribution and wastewater collection systems to serve the entire 3018

2 reservation in addition to the casino and outlet shopping mall. The added residential flows required expansion of the existing Membrane Bioreactor Treatment Plant capacity. The treatment plant is the only facility serving the casino and outlet center and can not be taken off line for an extended period of time. The maximum duration for a plant shut down is 24 hours equivalent to the capacity of the existing equalization tanks. To keep the treatment plant on line and augment its treatment capacity, we evaluated various options in regards to their capital cost and reliability to meet effluent discharge requirements during construction. We evaluated the following options: 1) Add a new treatment module in new separate tank. 2) Maximize use of existing facilities and available tankage. 3) Haul away wastewater flows for treatment off site. Based on economics HDR recommended and Viejas Reservation agreed to meet the challenge to expand the Immersed Ultra-filtration Membrane Bioreactor while in operation. This treatment facility, was the first of its type in California, has been operating successfully since July TECHNICAL BACKGROUND The original Viejas extended aeration plant was designed to produce tertiary effluent suitable for unrestricted landscape irrigation at an average flow rate of 150,000 gallons per day (gpd). The package treatment plant included flow equalization, extended aeration, secondary clarification, flocculation, sand filtration and chlorination. However the facility could not produce tertiary effluent quality with flows over 70,000 gpd. In 1999, CGvL Engineers, now part of HDR Engineering, evaluated the treatment process and designed temporary improvements to improve the package treatment plant operation including a 50,000 gallon equalization tank divided into two compartments each with separate feed pumps to the aeration basin. The pumps are driven by VFDs and the tanks are provided with aeration system for mixing. In a second phase, in 2000, we designed process modifications to increase treatment capacity to 200,000 gpd to accommodate commercial enterprise expansions. In this phase, the decision was made to retrofit the package treatment plant with Immersed 3019

3 Membrane Bioreactors and to revise the process to include an anoxic zone, adding internal recirculation and abandoning the secondary clarifiers. This paper will present the engineering aspects of the plant third expansion to 300,000gpd to accommodate the added residential flow to the commercial flows based on the performance data for over three years of operation (operating parameters, effluent quality and maintenance). MEMBRANE BIOREACTOR DESIGN AND OPERATION The process selected to retrofit the plant was ZenoGem, by Zenon Environmental Inc. The membrane modules are ZeeWeed 500 hollow fiber membranes. These membranes are mounted in modules with 500 square feet of membrane area each. Eight modules are assembled together in a self-supported frame (cassette). Two cassettes were immersed towards the end of each existing aeration tank. Selfpriming centrifugal pumps provide energy required to overcome the trans-membrane pressure differential responsible for the flux through the membranes. Table 1 presents basic design parameters for Zenon ZeeWeed 500 membranes. Three cassettes were installed inside each of the two 88,500 gallon aerated tanks. Each cassette has a total filtration area of 4,000 ft 2. The design flow of the MBR plant is 300,000 gpd. The peak flow the system can handle is 450,000 gpd. Without tank additions, the MBR system was expanded to handle 300,000 gpd. Table 1 ZeeWeed 500 Membrane Data Parameter Design Membrane Type Hollow Fiber Structural Support for Membranes (inside fiber) Perforated Nylon String Membrane Area 500 ft 2 /module Modules per cassette 8 (4,000 ft 2 ) Median Pore Size microns Maximum Pore Size 0.1 microns Trans-Membrane Pressure Range 1.0 to 8.0 psig Maximum Chlorine Exposure 200 mg/l Flow Type Outside/In Current Operating Flux (Gross) 13.5 gpd/ft 2 Design Net Average Flux (including backpulses) 12.5 gpd/ft 2 Design Net Peak Flux 18.0 gpd/ft 2 Backpulse water daily volume 2,500 gpd per cassette Backpulse instantaneous flow 80 gpm per cassette Membrane Scouring Air (continuous coarse bubble) 3 scfm/100ft 2 (15 cfm/module) 3020

4 Two submersible pumps transfer equalized flow to the anoxic tank. The anoxic tank has a volume of 37,700 gallons and to provide 7 hours of HRT at average flow the aeration system in the beginning of the aeration tanks has been modified to provide an extension to the anoxic zone. The aeration system provided with valves to facilitate changing the the anoxic zone working volume as required by plant operation. Recycle mixed liquor from the aerated tank is pumped back to the anoxic zone at a rate of 1,800 gpm (6 X Q for the design flow). The aerobic zone of the MBR has a long retention time, as it was originally the extender aeration package plant. The two aerated basins have a total volume of 177,180 gallons, which provides 23 hours of HRT at average flow conditions. Two 75 HP PD blowers supply air for mixing and oxygen transfer to the aerated tanks. The same blowers also supply 120 cfm of continuous aeration to each membrane cassette for membrane scouring. DO readings in the aerated tank range from 2 to 5 mg/l, depending on organic loading. Grab samples as high as 1,700 mg/l BOD 5 and 113 mg/l of ammonia have been taken at the plant prior to screening and flow equalization. Based on average flow and concentrations, the actual organic loading to the plant is 657 lb BOD 5 /day and the ammonia loading 63 lb NH 3 -N/day. After screening, the inflow is equalized in two tanks. The hydraulic retention (HRT) time in the equalization tanks at average flow is 28 hours. Diffused aeration is provided in the equalization tanks avoid solids settling. Table 2 shows average influent quality parameters. Table 2 Average Influent Quality (Equalized Flow) Parameter Value Total Suspended Solids, TSS (mg/l) 250 Biochemical Oxygen Demand, BOD 5 (mg/l) 630 Soluble (mg/l) 430 Suspended (mg/l) 200 Total Kjeldahl Nitrogen, TKN (mg/l) 130 Ammonia Nitrogen, NH 3 -N (mg/l) 100 Temperature, o F Maximum 80 Minimum 68 ph units

5 The quality of the effluent is very consistent. The membranes remove all volatile and nonvolatile suspended solids, most colloidal organic matter and achieve up to 4 log orders of total coliform bacteria removal. The permeate turbidity is extremely low. Table 3 shows average final effluent characteristics over the monitoring period. BOD TSS Ammonia-N Nitrate Total Coliform Turbidity Table 3 Effluent Quality < 1 mg/l 0 mg/l < 0.2 mg/l 5-11 mg/l < 2.2 MPN/100 ml < 0.1 NTU The low ammonia effluent readings are an indication of extremely efficient nitrification taking place in the aeration tank, since at the average plant ph of 8.2 and temperature of 68 o F, only a very small fraction of the ammonia is volatilized. If it is considered that the average ammonia nitrogen influent concentration is 60 mg/l and the influent nitrate concentrations are negligible, nitrate effluent readings of 5 to 11 mg/l (8 mg/l average) indicate that about 80 to 85 percent denitrification is being achieved in the anoxic zone. The de-nitrification process could still be improved by increasing the volume of the anoxic zone and slightly reducing internal sludge recycle to achieve lower DO levels and a more reducing environment (lower ORP) in the anoxic zone. The plant design parameters are included in table 2 based on data collected from plant operation for two years. Equipment pre-selection, construction sequence and reliable operation during construction are the main factors behind the successful wastewater treatment plant expansion. Construction and operation challenges during the plant expansion are discussed in the following points including photos for the plant 3022

6 components in operation in each phase and the operation parameters implemented at that time. 1. The package plant consisted of two side by-side treatment trains. The plan was to develop operating procedures to handle flow with only one tank in service. 2. Plans and specifications required the Contractor to accelerate procurement of ancillary mechanical equipment. The Reservation pre-selected the membrane supplier to be consistent with exiting membrane and to expedite the installation process. 3. The original plan was to take one process train out of service for two weeks to allow the Contractor installation of additional air diffusers and air distribution piping. 4. The new aeration piping was connected to the existing aeration main by taping sleeves and isolation valves. 5. The aeration diffusers were mounted on the air distribution headers out side the tanks to be ready for installation in the tank when available to save time for the train shut down. 6. Steel was used to construct the original package treatment plant. Early on we noted that the steel tank protective coating was failing and required the contractor to sandblast and coat the interior of the tank during the train shut down period. The surface preparation and tank coating was completed while the contractor was installing the aeration distribution pipe supports and then the piping installation. 7. Maximum day flows to the plant occur during weekends and holidays when the Casino has the highest number of patrons. On this basis construction had to be scheduled to avoid any shut down during holidays. 8. The contractor and the project team had to closely coordinate with the treatment plant operator to make sure that the flow equalization tank was empty at the beginning of each construction phase. We defined work to be done in each process train as a phase. 9. To accommodate the added capacity of the plant; we increased the size of the feed pumps from the flow equalization tank to the treatment process tanks. 10. Plant improvements included: a larger rotating drum micro-screen sized for the peak flow of the new plant capacity. The existing smaller capacity micro-screen was left in place and piped to provide partial back up capacity to the larger micro-screen 11. Provided larger pumping equipment to convey circulated flow to the anoxic zone at the beginning of the treatment process. 12. Added an additional aeration blower to meet process dissolved oxygen and membrane agitation air requirements. 13. Each process train required a new 3023

7 membrane cassette to process the added capacity. The cassette supporting frames were assembled outside the tank and installed during the train shut down. 14. The membrane cassette was installed after the train had been put back in service and connected to the aeration and permeate piping while the train was in operation The anoxic zone mixing pumps were replaced with larger size pumps on the same rail system while the plant was in operation. 16. Converted the old sand filter to be a chlorine contact tank to support the added capacity and to be part of the existing chlorine contact tank. 17. Retrofitted the entire chlorine contact tanks with additional partitions to provide the required detention time by Title 22 of the California Code and Regulations. 18. The length and configuration of the added baffle walls were dictated by the existing tank dimensions and were designed to eliminate the potential for short circuiting. 19. The partition material was selected to avoid welding to existing steel walls and to utilize corrosion resistant components. We used corrugated fiberglass panels for the tank partitions. 20. The effluent from the plant was being sent to the percolation ponds without disinfection during the construction period of the chlorine contact tank modifications. After the modifications were completed the tank utilized to produce the disinfected effluent incompliance with Title 22 for unrestricted use on landscape irrigation. 21. The process control panel and SCADA system were upgraded to reflect the added equipment and plant modifications within the construction period and without extended interruption to the plant operation in auto mode. The plant expansion was completed within budget and on schedule. The plant s tertiary effluent is being utilized for unrestricted landscape irrigation at the Casino, Outlet Center, gymnasium and ball park. This facility allows Viejas to recycle 100 percent of the wastewater generated by the Reservation Casino and outlet mall operations and domestic residential flows. To our knowledge this is the first Indian Reservation in California to reuse all of the recycled water for landscape irrigation. 3024