Role Model Water Recycling Program

WEFTEC®.06

ROLE MODEL WATER RECYCLING PROGRAM

Adel Bassyouni, HDR Engineering, Mandira Sudame, HDR Engineering, Don Mc Dermott, Viejas Public Works Department

8690 Balboa Avenue, Suite 200 San Diego, CA 92123

ABSTRACT

The Viejas Band of Kumeyaay Indians, a federally recognized tribe, vested with authority by the U.S. Environmental Protection Agency (USEPA), form the Tribal Government to regulate all activities concerning water quality issues within the reservation boundaries. The Reservation has 400 non-transient residents and an average of 5,000 transient residents. The transient population visits the Viejas Casino, an Outlet Mall and a Recreational Vehicle (R.V) Park. In 2001, the Viejas Tribal Government completed the Reservation Water and Sewer Master Plan. The Master Plan recommended improvements to the water distribution system a new wastewater collection system and a backbone recycled water distribution system. Most of the recommended water and sewer improvements have been designed, constructed and are in operation. The Master Plan water system improvements included new potable water reservoirs designed to operate at the same hydraulic grade line and with sufficient capacity to serve both the Viejas Indian Reservation and the casino and Outlet Mall. Wastewater generated within the Reservation is treated at the Reservation’s Viejas Water Reclamation Plant (VWRP). The VWRP uses ultra filtration membrane technology and disinfection using sodium hypochlorite to produce tertiary quality effluent. The VWRP effluent meets and exceeds all recycled water quality standards for unrestricted landscape irrigation. Until late last year, VWRP effluent was used to meet in-plant washwater demands and to irrigate fields in the vicinity of the plant. Excess VWRP effluent exceeding irrigation demands was conveyed to percolation ponds for disposal.

Two years ago the VWRP was expanded from 200,000 gallons per day (GPD) to 300,000 GPD and additional facilities were designed and constructed to expand the recycled water distribution system. Construction of these facilities would allow Viejas Tribal Government to expand their water conservation plan by using high quality tertiary effluent to irrigate landscaped areas with recycled water.

To reduce the capital cost to expand the recycled water distribution system and to minimize disruption to the Tribal members and transient population, the Master Plan recommended integrating into the upgraded recycled water system existing facilities that have been abandoned as result of recent water and sewer system improvements. The existing recycled water (RW) distribution system includes 8-inch pipelines located on the south side of Browns Road and crossing Viejas Creek utility bridge and terminating on the eastern side of the Viejas Creek at the VWRP. Existing wastewater facilities to be integrated into the recycled water distribution system include the 8-inch wastewater force main that used to convey the casino wastewater to the VWRP. The force main was abandoned when a new pump station and force mains to serve the casino started operation. Other facilities were incorporated into the recycled water distribution

system include the two abandoned 400,000-gallon water reservoirs, booster pump station, and engine driven fire pump station that used to serve the Casino and Outlet Mall loop. These pump stations and reservoirs were abandoned when the new water reservoirs and water distribution system were completed. The Public Works Department relocated the engine driven fire pumps from the business enterprise site to an area next to the 50,000-gallon recycled water storage tank located at the VWRP

The total acreage being served by the upgraded recycled water distribution system is 31 acres with a potential of serving fifteen acres in the future. The recycled water system and proposed improvements had been modeled using H2ONet Modeling software to confirm that both abandoned water distribution system pipelines and booster pump station, and wastewater facilities can be integrated with the existing recycled water facilities into a single system. New pipelines have been added to better serve the distribution network. A 6-inch recycled water line 900 feet long is provided to supply landscape irrigation system in the Outlet Mall. The pipeline has been constructed using directional drilling to minimize any inconvenience or shut down affecting the customers and the residents. Landscape irrigation system modifications also includes cross connections inspection and testing to ensure safety and compliance with Health Service Department requirements. Existing irrigation system controllers, irrigation system stations and irrigation heads have been converted to comply with RW regulations. Approximately 15 irrigation controllers have been retrofitted. On the average each system controller operates 26 irrigation system stations and each station serves 20 irrigation heads. Now Viejas is recycling 100% of the water collect in the sewerage system for landscape irrigation and utilizing available storage to avoid the use of potable water for irrigation any where in the reservation.

KEYWORDS

Membrane Bio Reactor; Recycled Water, Wastewater Treatment, Master Plan.

INTRODUCTION

The Viejas Band of Kumeyaay Indian is a federally recognized tribe and is represented by a Tribal Government. The Viejas Reservation, located in Alpine, California, in the eastern part of San Diego County, has an ultimate permanent population of about 750. The Reservation has a 210,000 square feet casino, an outlet mall with more than 50 stores and an RV Park. The Tribal Government is vested with authority by the United States Environmental Protection Agency (USEPA) to regulate all activities concerning water quality issues within the reservation boundaries. Sewer lines, lift stations and an extended aeration package plant installed in 1996 provide sewage collection and treatment for the casino and outlet stores. The residences and RV Park are connected to septic tanks.

The original Viejas extended aeration wastewater treatment plant was designed for tertiary treatment of 150,000 gallons per day (gpd). The plant included secondary clarification, flocculation, sand filtration and chlorination. The plant however, could not attain tertiary effluent quality with flows over 70,000 gpd. In 1999, the Tribal Government hired HDR Engineering (HDR) to undertake the task of evaluating the treatment process and designing temporary improvements to the plant to ensure compliance with Tribal Government goals. In a second phase, in 2000, HDR designed process changes to increase treatment capacity to 200,000

gpd to accommodate commercial enterprise expansions. In this phase, Immersed Membrane Bioreactors were retrofitted in the existing plant. Finally, as part of a third phase, HDR completed the plant modifications to increase the plant capacity to 300,000 gpd, to accommodate commercial growth and connect residences and the recreational vehicle (RV) Park. Additionally, design is now completed to use reclaimed effluent for landscape irrigation at the casino and outlet mall.

This paper discusses the engineering aspects of the planning, implementation and commissioning of the water wastewater and recycled water infrastructure program in Viejas. The program included the upgrade of the water distribution and storage system, water reclamation plant modifications, new sewage collection system and recycled water for landscape irrigation.

WATER DISTRIBUTION AND STORAGE SYSTEM UPGRADE

The water supply system for Viejas and its upgrade phases are summarized below:

• Water Supply Source-Groundwater • Aquifer is a broad basin sloping southwest • Water systems – Business Enterprise – Community – Ma-Tar-Awa R.V. Park – Alpine Springs-Trailer Park • Water Well System – Total 10 Wells – Seven Active Wells – Pump depths vary from 130 ft to 670 ft – Well capacities vary from gpm to gpm • Water Disinfection – 12.5% Sodium Hypochlorite (PAST) – MIOXX system ( PRESENT) • Water Distribution System was divided into three systems – Community and Ma Ta Ra Wa – R.V. Park – Business Enterprise • Outlet Center • Casino • The three water distribution systems were combined in one system • New system provided adequate flow and pressure for fire protection throughout the reservation. • Viejas abandoned two old water tanks after installing two new tanks. • Three tanks connected to the water distribution system. Total storage capacity of 1.3 million gallons. • The system upgrade was completed before the second wastewater treatment plant expansion.

The system built out layout is shown in the figure below.

Ground Water

Drinking Water Storage

Potable Water Pipe

WASTEWATER COLLECTION SYSTEM

The water supply for Viejas described in the following summary:

• Originally the Wastewater Treatment Facilities included – Treatment Plant Serving Business Enterprise – Community served by septic tanks • The collection system was expanded to serve: – Reservation residents – Ma-Tar-Awa R.V. Park • System Improvements included: – 6.5- miles of gravity sewers – Lift station from low point in the collection system to the treatment Plant – Two 8-inch PVC force mains • The system construction was completed at the same time with the second wastewater treatment plant expansion to accommodate the added flows.

The wastewater collection system is shown in the figure below.

REGULATIONS GOVERNING RECYCLED WATER USE IN VIEJAS INDIAN RESERVATION

Use of recycled water on the reservation is regulated by the Tribal Government and based on the following rules and regulations: • Viejas is Independent and Sovereign Nation • Under USEPA jurisdiction • Viejas Elected to follow Rules and Regulations: – Tribal Government Ordinances – State of California RWQCB – California State Department Of Health Services (DHS) • Compliance With RW Production Requirements – Managing your VWRP • Compliance with Title 17 Requirements – Ensuring correct equipment and installation – Maintaining ongoing inspection and testing program • Compliance With Title 22 Requirements – Ensuring proper handling of RW

– Ensuring proper management of RW use area

WASTEWATER TREATMENT PLANT MODIFICATIONS

PHASE I IMPROVEMENTS

The Viejas Casino wastewater treatment plant was constructed in 1996 to treat 150,000 gpd of commercial wastewater from the gaming facilities. The package treatment plant included a bar rack, flow equalization, activated sludge process using extended aeration, secondary clarification, flocculant addition and flocculator, effluent sand filtration, chlorine disinfection and aerated sludge holding tank. Because the plant was unable to meet effluent discharge standards consistently and reliably, the first phase of treatment process upgrade was implemented.

The first plant modifications, which took place in 1999, consisted of:

• Installing a 150,000-gallon flow equalization concrete tank with two compartments of equal volume. The tanks receive wastewater from the bar rack. Three blowers, two on duty and one standby were re-piped from the original aerated basin of the package plant, to provide compressed air to the equalization basin for mixing and solids suspension. Three 75 gallons per minute (gpm) submersible pumps with variable frequency drives were installed to transfer equalized flow to the biological treatment tanks. With this six-fold increase in equalization volume, the influent flow peaks and large variations in quality were significantly attenuated. • The original 25,000 gallon flow equalization tank part of the package plant was converted into a sludge holding tank and the 37,700 gallon sludge holding tank was converted into anoxic biological treatment zone for denitrification of the influent. Based on the actual plant capabilities to handle the organic, nutrient and hydraulic loads of the influent it was necessary to de-rate the plant capacity from 0.15 million mgd to 0.1 mgd.

With the above modifications, the plant was able to meet tertiary effluent standards. It performed reliably for over one year, treating average flows of 70,000 gpd. Effluent disposal was done with two one-acre percolation ponds. Sludge disposal was done hauling thickened sludge to a pumping station of the sewerage system of the City of San Diego.

CAPACITY EXPANSION—PHASE II IMPROVEMENTS

The expansion of the Viejas Casino and Outlet Mall, opened in July 2000, required higher and more reliable wastewater treatment capacity. Modifications to the treatment facility were implemented to improve the WWTP effluent quality and augment its treatment capacity. The rated capacity of the facility after Phase II improvements was 200,000 gpd average flow and a peak flow rate of 270,000 gpd. The actual monthly average flow rate since the Phase II plant began operation in July 2000 has been 116,000 gpd, with a maximum day flow rate of approximately 217,000 gpd.

The selection of the treatment technology and the design of Phase II improvements faced several challenges:

• The plant should consistently achieve reclaimed effluent quality meeting California Administrative Code Title 22 requirements, while still receiving a wide range of hydraulic, organic and nutrient loadings. Portion of the effluent was intended for landscape irrigation in casino and outlet mall.

• The capacity increase should be implemented in an accelerated schedule, in time for the opening of the casino expansion, without taking the original plant off-line during construction. • Use of existing tanks and designated land needed to be maximized allowing for future capacity expansion to 300,000 gpd without extensive tank additions. • One single treatment technology should be used for the design flow and future expansion to 300,000 gpd. • The improvements needed to be cost effective in capital expenditure and operating cost.

• The following alternatives were evaluated to meet effluent quality requirements.

• Increasing the capacity of the extended aeration plant by installing new similar treatment train in parallel using steel tanks (package plant) • Building a new conventional activated sludge plant for the entire design flow using concrete tanks • Use existing tanks to built a Sequential Batch Reactor • Carrousel oxidation ditch with anoxic zone for nutrient removal; and • Membrane Bioreactor (MBR), by immersing ultrafiltration membranes in existing aerated tanks

Increasing treatment capacity with a 100,000 gpd extended aeration train in parallel, required tanks for anoxic and aerobic zones, secondary clarifier and sludge storage. A new flocculator and sand filter were also required to handle the additional flow. New blowers and diffuser system for aeration and sludge storage tanks were also needed.

The use of the existing tankage to accommodate a sequential batch reactor (SBR) required larger flow equalization to account for batch operation. Additionally, the operation was considered difficult and the automation requirements extensive. The addition of tertiary treatment units, such as added flocculation and filtration capacity were also required with an SBR. Added sludge storage capacity was also required.

Construction of an oxidation ditch would have required extensive earthwork, increased construction area, and abandoning the existing plant and auxiliary equipment.

A detailed evaluation was conducted for Alternatives A, B, and C. Although the equipment and construction cost was approximately $880,000 for all of them, the annual operating cost of the MBR was $100,000 lower than the other two alternatives. Additionally, this alternative offered the highest flow rate, highest and most consistent effluent quality, the shortest construction time and the lowest use of additional space.

The selected alternative was membrane bioreactors (MBR). This alternative consisted of retrofitting the package plant by immersing ultrafiltration membrane cassettes in the existing aeration tanks. MBR enabled capacity increase without adding tank volume, even to reach the future 300,000 gpd. Two 800 gpm submersible pumps were installed in the aeration basins to recycle mixed liquor to the anoxic zone. Equalized inflow fed to the anoxic zone mixes with recycled mixed liquor. Thus mixing is done hydraulically. Two lobe-type positive displacement blowers were installed to provide oxygen and mixing to the two original 88,500-gallon aeration tanks. These blowers substituted the three original blowers diverted to the equalization tanks. Higher mixed liquor suspended solids (MLSS) concentrations and longer solids retention times (SRT), typical in these systems, enabled the application of higher hydraulic, organic and nutrient loading rates and lower biological solids generation. Additionally, since the membranes perform solids separation, clarifiers and sand filters were not needed.

MBR Alternative Implementation

The Viejas plant retrofit with immersed ultrafiltration membranes was the first full-scale wastewater treatment plant installation of this type in California. The retrofit included modifications to the plant pretreatment section, such as the installation of a rotary drum wedge- wire microscreen prior to the flow equalization tanks and a screenings screw compactor to compress and elevate screenings to a storage dumpster, for disposal as solid waste. In the biological treatment section the modifications consisted of immersing two membrane cassettes in each aeration tank; an overhead steel structure and crane to remove the cassettes; air lines to supply coarse bubble diffusers integral to the modules for membrane scouring; permeate and backpulse pumping system and piping modifications to bypass final clarifiers, flocculator tank and sand filter. All these modifications were done in 4 months at a construction cost of $875,000.

MEMBRANE BIOREACTOR DESIGN, INSTALLATION AND OPERATION FEATURES

Ultra-filtration Membranes. The process selected to retrofit the plant was ZenoGem, by Zenon Environmental, Inc. from Canada. 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. Self-priming 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 (Zimmerman, B. personal communication; Thompson, D., 2000).

Table 1 ZeeWeed 500 Membrane Data Parameter Design Membrane Type Hollow Fiber Structural Support for Membranes (inside fiber) Perforated Nylon String Membrane Area 500 ft2/module Modules per cassette 8 (4,000 ft2) Median Pore Size 0.035 microns

Maximum Pore Size 0.1 microns Trans-Membrane Pressure Range 1.0 to 8.0 psig Maximum Chlorine Exposure 2000 mg/l Flow Type Outside/In Current Operating Flux (Gross) 13.5 gpd/ft2

Design Net Average Flux (including backpulses) 12.5 gpd/ft2 Design Net Peak Flux 18.0 gpd/ft2 Backpulse water daily volume 2,500 gpd per cassette Backpulse instantaneous flow 80 gpm per cassette Membrane Scouring Air (continuous coarse bubble) 3 scfm/100ft2 (15 cfm/module)

Two cassettes were installed inside each of the two 88,500 gallon aerated tanks. The cassettes measure 2.5’x 7’x 7’ and weight approximately 1,500 lb when wet. Each cassette has a total filtration area of 4,000 ft2.

Permeate/Backpulse Pumping and Membrane Maintenance. Three 3-inch flexible tubing lines with quick disconnect fittings are attached to the top of each cassette. Two of the lines are connected to a permeate/backpulse manifold and the third one is an air supply line, which feeds coarse bubble diffusers built-in at the bottom of each module (120 cubic feet per minute per cassette). Two 3 horsepower (HP), 3” x 3” self-priming centrifugal pumps operated with variable frequency drives are used to pump permeate across the membranes (one for each train). The pressure drop through the membranes has increased from 1.3 to 1.7 psig (before and after backpulse) at startup to 2.0 to 2.8 psig after 7 months without removing the membranes from the aeration tanks. This pressure drop occurs at a set flow of 75 gpm per basin (37.5 gpm gross flux per cassette), which results in 13.5 gpd/ft2, about 70 percent of the design gross peak flux. When backpulse time is factored in, the net flux of permeate is 93 percent of the gross flux.

Back pulses occur every 15 minutes for 20 seconds. A programmable logic controller (PLC) operates pneumatic three-way valves at the discharge of the permeate/back pulse pumps. Backpulses are alternated between aeration basins at a flow rate of 160 gpm per basin (two cassettes) for the 20 seconds. Backpulses are done with a 2 mg/L sodium hypochlorite solution in permeate water. Besides intermittent backpulses, an extended 60-minute backpulse is done at night. The manufacturer recommends quarterly cassette soak cleaning in a stronger sodium hypochlorite solution (below 200 mg/L). Such maintenance procedure was completed a week ago resulting in a trans-membrane pressure recovery to the original condition.

The permeate pumps operate at a set point of 75 gpm each (per basin). As the pressure drop through the membrane increases causing flow to decrease, the VFD increases the RPM of the pump to compensate for flow pressure losses and maintain the set flow. In low flow periods, a liquid level sensor inside the aeration basins shuts off the pumps at low water mark. Controls are available to increase drive velocity and pumping rate in the event of continuously rising water level. In the case of the Viejas WWTP this feature is not necessary due to high flow equalization volume available.

Pretreatment Requirements. The manufacturer recommends grit removal and fine screening at least below 3 mm. For applications over 2 mgd, 0.010” (0.25 mm) screening is required (Thompson, D., 2000). The Viejas WWTP has a 0.5 mm opening rotating drum microscreen, designed for an average flow of 200,000 gpd. No grit removal is provided, but since the collection system is direct from the casino and outlet stores, with minimal exposure to grit access, grit removal is not needed.

Operating Parameters of Biological Units. The design flow of the MBR plant is 200,000 gpd. The peak flow the system can handle is 270,000 gpd. Without tank additions, the MBR system could be expanded to 300,000 gpd. Since plant start-up in July 2000, the average daily flows have been 116,000 gpd. 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, BOD5 (mg/L) 630 Soluble (mg/L) 430 Suspended (mg/L) 200 Total Kjeldahl Nitrogen, TKN (mg/L) 130 Ammonia Nitrogen, NH3-N (mg/L) 100 Temperature, ºF Maximum 80 Minimum 68 pH units 7.5

Grab samples as high as 1,700 mg/l BOD5 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 BOD5/day and the ammonia loading 63 lb NH3- 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.

Anoxic Zone. Two submersible pumps transfer equalized flow to the anoxic tank. The anoxic tank has a volume of 37,700 gallons and provides 7 hours of HRT to the sewage, at average flow. Recycle mixed liquor from the aerated tank is pumped to the anoxic tank at a rate of 1,600 gpm (two 800 gpm submersible pumps). The mixed liquor recycle ratio is currently 11.4 x Q at average flow (8 x Q for the design flow).

Considering the mixed liquor recycle rate, the sludge retention time in the anoxic zone is 24 minutes. Inflow from the equalization tank and recycled liquor are mixed hydraulically in this tank. Two submersible pumps provide additional mixing in the anoxic chamber. Dissolved oxygen (DO) concentrations in this chamber range from 0.2 to 0.4 mg/l, probably as a result of high recycle rates and high DO concentrations in the mixed liquor.

Aerobic Zone. The aerobic zone of the MBR has an extremely long retention time, as it was originally an extended aeration package plant. The two aerated basins have a total volume of 177,180 gallons. This volume provides 34 hours of HRT in average flow conditions. MLSS concentrations in the aerobic tanks range between 8,200 mg/l and 12,000 mg/L. At average flow, and estimating 70 percent VS in the mixed liquor, the estimated F/M ratio is 0.08 lb BOD5/lb MLVSS in the aerated tank. These low F/M ratios enable the plant to handle hydraulic and organic peaks without detrimental effects on effluent quality. The high inventory of active biomass introduces great process stability.

At this low F/M ratio and long SRT the microorganisms are in endogenous phase (Lesjean, B. et al, 1999). Under these conditions bacteria begin to consume their own protoplasm to obtain energy for cell maintenance reactions and cell reproduction is diminished (Metcalf & Eddy, 1991). As a result less solids are generated and the solids produced are somewhat stabilized by aerobic sludge digestion taking place in the aerated tank. In order to maintain a constant MLSS, sludge from the aerobic zone is periodically wasted using airlift pumps to an aerated sludge holding tank.

Sludge wastage in the MBR process is not controlled by mixed liquor settleability, F/M ratio or sludge level in secondary clarifiers as it is done in conventional activated sludge plants. The only parameter that controls wastage in this process is MLSS concentration. This greatly simplifies the operation.

Two 75 HP positive displacement lobe-type blowers are used to supply air for mixing and oxygen transfer to the aerated tanks. Fine bubble diffusers are used for increased oxygen transfer. The same blowers also supply 120 cfm of continuous aeration to each cassette for coarse bubble membrane scouring. DO readings in the aerated tank range from 2.0 to 4.8 mg/l, depending on organic loading.

Sludge Holding Tank

A 25,000-gallon sludge holding tank provides aeration for partial sludge digestion. An average hydraulic retention time of 19 days provides further aerobic sludge digestion. Due to the long solids retention time in the aerated basin and the high availability of dissolved oxygen, partial solids digestion takes place in the mixed liquor tank. The aerobic digestion process is continued in the holding tank. Although no digestibility tests have been conducted with the sludge as

wasted or as removed from the sludge holding tank to determine the degree of stabilization, noticeable volume reduction by digestion takes place through the sludge holding tank. Periodically, the aeration in the sludge holding tank is turned off to allow the sludge to settle. Although there is very little settling, with this procedure, the sludge is thickened from about 0.8 percent to 1.5 percent solids. Supernatant from this procedure is pumped into the anoxic tank. Aeration is then restarted for further digestion.

When the sludge holding tank is full with sludge that cannot be further decanted, the liquid sludge is removed from the tank and hauled off-site for disposal. The sludge in the holding tank averages about 1.5 percent solids before final disposal. Approximately 40,000 gallons per month of 1.5 percent solids sludge are hauled out for disposal to one of the pump stations in the sewerage system of the city of San Diego. Comparing the mass of solids pumped into the sludge holding tank and the mass hauled out for disposal, the estimated total solids reduction in the holding tank is 28 percent.

Disinfection

Permeate from the membranes is pumped to a chlorine contact tank. Effluent is disinfected using commercial 12.5 percent concentration of sodium hypochlorite. Chlorination equipment includes two metering pumps and a 100-gallon polyethylene storage tank housed in a weatherproof enclosure. The chlorine contact tank is equipped with a chlorine residual analyzer. Sodium hypochlorite is injected at the upstream end of the contact tank. The tank is fitted with four baffle walls to increase mixing and contact time. The disinfected water is discharged at the downstream end of the chorine contact tank, into the effluent dosing tank, through a v-notch weir.

Effluent Quality and Disposal

The quality of the effluent is very consistent. The membranes remove all volatile and non- volatile 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.

Table 3 Effluent Quality Parameter Value BOD < 1 mg/l TSS 0 mg/l Ammonia-N < 0.2 mg/l Nitrate 5-11 mg/l Total Coliform < 2.2 MPN/100 mL Turbidity < 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 ºF, only a very small fraction of the ammonia is volatilized. If it is considered that the average ammonia nitrogen influent concentration is 75 mg/l and the influent nitrate concentrations are negligible, average nitrate effluent readings of 5.5 mg/l indicate that greater than 90 percent denitrification

is being achieved in the anoxic zone. This is a remarkable achievement considering the minimal control over the dissolved oxygen concentrations in both the anoxic and aerobic zones. 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. Such improvements are contemplated as part of the Phase III modifications of the Viejas WWTP.

Disinfected water is pumped to the percolation ponds, reclaimed water storage tank or to the process water system (yard water for plant uses). The facility has two percolation ponds, a 0.7- acre and a 1.0-acre. The reclaimed water storage tank is a 50,000-gallon circular tank used as the wet well for a temporary pumping station used for spray irrigate of open fields. The last use had not been implemented pending modifications in the chlorine contact tank, to comply with baffling and contact tank aspect ratio guidelines of California Code of Regulations, Title 22. Shortly after, the chlorine contact tank was upgraded and made the Viejas plant effluent meets California tertiary effluent limitations for water reclamation plants. The effluent limitations of the California Code of Regulations, Title 22 are presented in Table 4.

Table 4 Viejas Water Reclamation Plant Tertiary Effluent Limitationsa Constituent Units Limit Remarks Biochemical Oxygen O Demand (BOD5 20 C) mg/L 10 Shall be less than Total Suspended Solids mg/L 10 Shall be less than Total Organic Carbon mg/L 20 Shall not be exceeded Nitrate (as N) mg/L 10 Shall not be exceeded MPN/100 2.2 Shall not be exceeded by median concentration of Total Coliform mL last 7 days 23 Shall not be exceeded more than once in 30-day period 240 Shall not be exceeded 0.2 Shall not be exceeded more than 5% of the time within 24 hrs. Turbidity NTU 0.5 Shall not be exceeded a) Based on water quality standard requirements for reuse or disposal of tertiary treated effluent. Effluent used for irrigation purposes shall conform with standards equal to all applicable provision of California Code of Regulations, Title 22, Division 4, Chapter 3 (Reclamation Criteria) in its present form or as it may be amended.

Membrane Performance

Since January 2001 the plant has been operating at a fairly constant flux of 13.5 gfd and the transmembrane pressure has never been above 3.7 psi, well below the maximum design transmembrane pressure of 8.0 psi. Approximately every 6 months, the membranes are soaked in a concentrated chlorine solution in order to maintain membrane permeability above 5 gfd/psi.

15 Flux (gfd) 10 Permeability (gfd/psi)

0 01-Jan-01 11-Apr-01 20-Jul-01 28-Oct-01 05-Feb-02 16-May-02 Date Figure 1 – Train 1 Flux and Permeability

PLANT CAPACITY EXPANSION, PHASE III IMPROVEMENTS

The Viejas Indian Reservation completed the construction phase of upgrading the Viejas Water Reclamation Plant (WRP) by expanding its treatment and effluent disposal capacities from 0.2 mgd to 0.3 mgd average daily flow. This expansion is intended to accommodate residential wastewater flows collected from the Reservation as well as flow from their Casino/Commercial Complex.

It is estimated that the average influent rate upon tie in of the residential flows will be approximately 0.145 mgd, with a peak flow of 0.346 mgd. As a result of the combination of commercial flow with domestic sewage, the strength of the wastewater is expected to decrease. Table 5 presents estimated combined influent quality parameters.

Table 5 Expected Combined Influent Quality Parameter Value Total Suspended Solids, TSS (mg/L) 250 Biochemical Oxygen Demand, BOD5 (mg/L) 450 Soluble (mg/L) 150 Suspended (mg/L) 300 Total Kjeldahl Nitrogen, TKN (mg/L) 100 Ammonia Nitrogen, NH3-N (mg/L) 80 Temperature, ºF Maximum 77 Minimum 68 PH units 7.5

The MBR plant had been upgraded to accommodate an average annual flow of 0.3 mgd and a peak annual flow of 0.45 mgd. Table 6 summarizes current and Phase III influent and effluent loading.

Table 6 – Hydraulic and Organic Loading Current Design Expansion Parameter Influent Effluent Influent Effluent Hydraulic Loading, mgd Annual average daily flow 0.2 0.2 0.3 0.3 Peak hourly flow 0.706 0.706 1.014 1.014 Organic/Nutrient loading, lbs/day BOD 5 751 8 1,126 13 TSS 417 8 626 13 Total Kjeldahl Nitrogen, TKN 167 17 250 25

The improvements included an additional 0.3 mgd rotating drum microscreen and screenings compactor, odor control system for headworks, modification to equalization basin and biological process, changes to the chlorine contact tank and the reclaimed water storage tank.

Equalization Basin. Improvements in the equalization basin include increasing diameter of gravity line feeding the basins with screened sewage to accommodate increased flow, and adding a stand-by submersible pump to feed the anoxic tank.

Biological Process. The total volume of the biological process tank is adequate for the 0.3 mgd average flow. The volume of the anoxic zone has been increased to improve denitrification and accommodate the new flow. To avoid any major structural modifications in the tank, the first third of the aeration tanks will be used as an extension to the anoxic zone. This will be achieved with baffle curtains and by shutting down the airflow to the front part of the aeration basin. About 100 new diffusers will be added to the aerobic zone and the aeration system capacity will be increased by installing a new 75 HP blower, to support air, mixing and membrane scouring requirements of the aerobic zone.

Two new Zenon ZeeWeed 500 membrane cassettes have been fitted in the existing aerated tanks (one in each). The existing permeate pumps (two operating and one stand-by) are sufficient to handle the increased permeate and backpulse flow requirements.

Disinfection. Due to insufficient length of the chlorine contact zone and contact time the effluent does neither conform to Title 22 specifications nor is it capable of handling increased flow of 0.3 mgd. In order to provide more contact time and accommodate the new flow, a sand filtration unit (unused since MBR implementation) upstream of the chlorine contact tank will be removed and modified with baffles, to be incorporated as part of the contact chamber. The disinfected effluent will be collected in the existing effluent dosing tank. The disinfected tertiary effluent will be compliant with Title 22 specifications and hence will be used for unrestricted irrigation. The effluent dosing tank will be fitted with one additional reclaimed water storage pump.

Sludge Disposal. The sludge holding tank will be maintained in the current operation. It is expected that liquid sludge disposal will be eliminated from the City of San Diego County in the near future. At that time a centrifuge dewatering system will be installed to dewater the sludge

Copyright © 2006 Water Environment Foundation. All Rights Reserved 6185 WEFTEC®.06 to 15 to 20 percent solids. These dewatered solids would be disposed off in a landfill or at a land application facility.

EFFLUENT DISPOSAL AND RECYCLED WATER SYSTEM

The master plan prepared after the first plant improvements outlined the planned uses for treated wastewater as: process water for the plant, landscape irrigation, spray field irrigation and disposal to the percolation ponds. Effluent in excess of reclaimed water demand will be disposed off in the percolation ponds. The water supply system for Viejas and its upgrade phases are described in the following summary:

• After the second upgrade of the water reclamation plant Viejas started to build the infrastructure for the recycled water system by preparing the design for the distribution system. • The main distribution loop was installed with the installation of the upgrades for the water distribution system with a connection to the reclamation plant site. • Using a temporary pumping system drawing reclaimed water from the 50,000 gallon tank at the plant and pumping to the spray fields; Viejas started the beneficial use of the reclaimed water. • The spray fields were design for grazing cattle with several sections to allow rotation of the grazing and cultivation cycles. • The spray fields were provided with portable distribution piping network fitted with spray nozzles to move it between sections. • After the plant expansion, Viejas decided to complete the recycled water use program by implementing the unrestricted landscape irrigation throughout the business enterprise area and to allow for future hook ups for gymnasium, school and the public buildings. • As a design build project Viejas completed the recycled water system and to convert the irrigation system in the Casino and outlet center to use recycled water. • The system utilized the abandoned pipelines from the water distribution system upgrade, and collection system upgrade to be part of the recycled water system as applicable. • The system converted two 400,000 gallon storage tanks, .formerly used to serve potable water system to be the elevated storage tanks for the recycled water system. • The system reused the irrigation system booster Pump to serve the converted outlet center irrigation System. • The system included twelve recycled water connections – 6 connections to Viejas outlet center irrigation system – 6 connections to Casino irrigation system • The program included cross connection testing before the start up of each section. • The system was provided with signs to comply with the signage requirements stipulated in the title 22 regulations. • A full day training session was conducted for the operation and maintenance staff to train them for the system operation and maintenance requirements and educate them on the recycled water use rules and regulations.

CONCLUSIONS

The following conclusions are presented as the benefits for the planning and implementation of a recycled water program:

1. Viejas Tribal Government has the desire to protect and conserve of the groundwater basin underlying the Reservation as it is the sole source of water supply at Viejas. 2. Viejas prepared a master plan for the water, wastewater and recycled water facilities to plan and integrate as much as possible the entire infrastructure upgrade and beneficial uses. 3. The master plan addressed the upgrade required for each system independently and then planned to incorporate abandoned facilities in other uses if applicable. 4. The master plan recommended a phasing process for implementing the program to establish the sequence required for successful operation. 5. Viejas authorized the budget and schedule for the program for completion in five years and within a budget of 12 million dollars. 6. The program included design and construction management by engineering firm and qualified inspectors. 7. The program included the start up testing and the training requirements to assure successful operation of the system for years to come.