Abstract. Project Summary

Transcription

1 EMERGENCY RAW WATER DESALINATION FOR TEXAS DROUGHT RESPONSE Mark Graves, PE, HDR, 4401 West Gate Blvd, Suite 400, Austin, TX Todd Townsend, PE, HDR, Dallas, TX Abstract In response to a historic drought in Texas, the Palo Pinto CMWD1 (District) and Mineral Wells (City) were faced with a dwindling water supply that would be exhausted in eight months if trends continued. Emergency options to maintain an uninterrupted supply of water were evaluated and implemented. This presentation will detail the unique planning, design, discharge and treatment permitting, and construction procurement/execution implemented to provide a crucial new water supply to the District with 8 months notice from planning to operations. Project Summary Palo Pinto County Municipal Water District No. 1 (District) and the City of Mineral Wells (City) (PWS # ) are faced with a dwindling water supply due to the ongoing drought in Texas. They have responded by seeking out additional emergency water supplies. HDR, Inc. has been hired to assist the District and City in evaluating emergency options to maintain an uninterrupted supply of water to customers. After evaluating several emergency supply options, the decision was made to use water from Possum Kingdom Lake as purchased from the Brazos River Authority (BRA) by Brazos Electric Power Cooperative (BEPC) with a portion of BEPC s contract being temporally assigned to the District. The planned raw water treatment facility will divert water released from Possum Kingdom Lake to the Brazos River and remove secondary contaminants through ultrafiltration (UF) and reverse osmosis (RO) membranes. The secondary contaminants of concern are total dissolved solids (TDS) and chlorides. After removal of the secondary contaminants, the water will be pumped to Hilltop Reservoir (capacity of 1,150 acft) for blending and subsequent treatment at the District s existing Hilltop water treatment plant (Hilltop WTP). There it will undergo conventional water treatment. The temporary raw water treatment facility will be operated until storage in Lake Palo Pinto recovers to a level that will provide an adequate raw water supply. When this level is

2 achieved, the temporary raw water treatment system will no longer be needed, and the rental equipment will be removed. However the remaining facilities will stay in place for the next 5 to 8 years until the District s proposed expansion of Lake Palo Pinto (Turkey Peak Reservoir) is completed and filled. These facilities will remain available for use in the event another drought occurs prior to the Turkey Peak Reservoir being completed. The District s current water source is surface water from Lake Palo Pinto. Water is pumped from behind a channel dam on Palo Pinto Creek to the Hilltop WTP. During the current drought between 20 and 25% of total raw water has been pumped from Palo Pinto Creek downstream of the channel dam back to the existing raw water pump station and blended in the wet well. This water from downstream of the channel dam primarily consists of Brazos River water. The City has an estimated service population of about 16,800. The City also sells treated water to several smaller water systems with an estimated population of about 14,542. The total 2020 estimated service population (from the 2016 Region G Water Plan) of the District is 38,964. The current rated treatment capacity of the Hilltop WTP is 16 MGD. Because Lake Palo Pinto is currently at less than10% capacity, the District and City are in danger of losing their sole source of drinking water. If current trends continue, the District will not be able to use any water from Lake Palo Pinto by July In order to remedy this issue, the District is taking steps to add Brazos River water to provide the full drought water demand. A total raw water supply of 3.5 MGD is needed from the Brazos River in order to meet drought demand. This raw water RO treatment facility will provide 3.5 MGD of Brazos River water that has been treated to remove chloride and TDS to below Secondary Maximum Contaminant Levels (SMCL) for delivery to the existing Hilltop WTP. Once sufficient raw water supply in Lake Palo Pinto is available, the temporary rental treatment facilities will be removed. The raw water RO treatment facility will utilize a combination of existing, new, and temporary facilities to accomplish its goals as shown in Figure 1. Water from the Brazos River will be pumped from an existing intake location on Palo Pinto Creek. Temporary pumps at this existing intake location will provide the necessary raw water. A new piping segment will move this water to the location of the temporary RO treatment facilities. Once the water has been treated, it will flow by gravity through a new pipeline to the wet well of the existing raw water pump station. The existing raw water pump station will convey the water to the Hilltop Reservoir, which feeds the Hilltop WTP. The waste brine will be conveyed through a new brine discharge pipeline to the Brazos River downstream of the raw water take point. 1

3 Figure 1. Temporary Brazos RO Treatment Facility Infrastructure Map After RO treatment, the water will be conveyed to the existing Hilltop WTP where it enters the City s distribution system through existing infrastructure, which is currently adequate. Therefore, pressure requirements do not need to be considered for the temporary facilities since it is a raw water supply that will be conveyed utilized existing infrastructure. Alternatives In response to the severe drought conditions several alternatives to provide Mineral Wells with additional temporary water supply were evaluated. The District is working on a long-term permanent water supply option (the enlargement of Lake Palo Pinto through the construction of Turkey Peak Reservoir) which is anticipated to be completed in about The alternatives evaluated in the initial planning phase included the following. Lake Mineral Wells Raw Water Pipeline This alternative had the lowest cost but would only supply up to 5 months of additional raw water at current drought conditions. Lake Mineral Wells is about 50% full and contains about 3,300 acft of water. This option was not selected due to the limited quantity of water available. 2

4 o Construction Cost = $4,700,000 o Total Cost = $4,700,000 Weatherford Treated Water Pipeline This alternative consisted of a roughly 15 mile treated water pipeline to connect the City of Weatherford potable water distribution system to the City s potable water distribution system. The cost of this alternative went up significantly from the initial estimates due to higher potable water purchase price from the City of Weatherford and an added requirement that the connection to the distribution system be moved to the East side of Weatherford due to existing City of Weatherford distribution system limitations. This option was abandonded after initial negotiations due to the increased pipeline length and associated costs. o Constructon Cost = $8,300,000 o City of Weatherford Outside City Water Customer Rate: 6 months of 3.5 MGD = 630,000 kgallon * $6/kgallon (purchase cost) = $3,780,000 o Total Cost = $12,080,000 Brazos Reverse Osmosis Water Treatment Plant (Selected Option) Option described in this report consisting of rental treatment trailers to provide 3.5 MGD of desalinated raw water from the Brazos River to the existing water treatment plant. o Constructon Cost = $2,600,000 o 6 Months Equipment Rental, Operations, Raw Water, Power = $4,600,00 o Total Cost = $7,200,000 Basis of Design Raw Water Quality Brazos River water downstream of Lake Possum Kingdom has variable concentrations of dissolved solids. Because of this variability, two grab samples were taken 6 months apart. These two samples provided two different water quality conditions that could be entered into the modeling software. This exercise produces a more thorough evaluation and higher confidence in the treatment selection. A grab sample of the Brazos River water was taken on June 17, Water quality parameters were tested by Talem, Inc. (lab ID No ) in Fort Worth, Texas. A second grab sample was taken on November 14, 2014 and tested by LCRA Environmental Laboratory Services (NELAP accredited laboratory # T ). The results of the two samples are summarized in Table 1. 3

5 Table 1. Brazos River Water Quality Parameter 6/17/2014 Sample 11/14/2014 Sample Unit Nitrate 0 mg/l Nitrate-N 0 <0.05 mg/l Aluminum mg/l Barium mg/l Boron mg/l Calcium mg/l Iron mg/l Lead ND mg/l Magnesium mg/l Manganese mg/l Potassium mg/l Silica mg/l Sodium mg/l Strontium mg/l Color Turbidity NTU Alkalinity mg/l Bicarbonate Alkalinity 40 mg/l Calcium Hardness as mg/l CaCO3 Total Hardness as mg/l CaCO3 Specific Conductance µs/cm Total Dissolved Solids mg/l (TDS) Total Suspended Solids mg/l (TSS) Chloride mg/l ph mg/l Sulfate mg/l Dissolved Organic mg/l Carbon Total Organic Carbon mg/l UV /cm Hetertrophic Plate Count 900 Cfu/ml E.coli 11 MPN/100 ml Total Coliform Bacteria > MPN/100 ml Another important raw water quality concern when considering RO treatment is the Silt Density Index (SDI). The water quality of importance is the quality of pre-treated water that is sent to the RO system. The use of ultrafiltration (UF) for pretreatment provides good silt removal and makes silt less of a fouling concern. 4

6 Variability in the raw water quality will dictate the quantity of raw Brazos River water that can blended with RO desalinated water and still meet chloride finished water goals. In addition to the two grab samples listed in Table 1, the City collected daily samples from the main stem of the Brazos River from the period of 5/27/2014 through 1/24/2015 and tested for chloride concentration and conductivity. The average chloride concentration for the sample period was 950 mg/l and ranged from 600 to 1,400 mg/l. The average conductivity concentration for the sample period was 4000 µs/cm and ranged from 2,600 to 5,300 µs/cm. Finished Water Quality Goals The finished water from the temporary treatment plant will serve as raw water for the City s existing WTP. Therefore, it is important to look at RO treatment facility as a part of the entire treatment process. Finished water from the RO treatment facility does not need to meet potable water standards, it only needs to address some specific secondary water quality parameters in the source water that the existing Hilltop WTP is unable to treat. These contaminants of concern are total dissolved solids, chloride, and sulfate. The finished water quality goal will be to reduce these contaminants to below TCEQ s SMCL. The SMCLs can be found in Table 2. Table 2. Finished Water Quality Goals Parameter Finished Water Goal TCEQ Secondary MCL Unit Total Dissolved Solids mg/l Chloride mg/l Sulfate mg/l No other contaminants will need to be addressed as part of the RO treatment process. All other contaminants can be reduced below MCLs at the existing WTP. While the RO treatment process will be able to remove up to 95% of TDS, that level of treatment is not needed. Some raw Brazos River water can be blended with the desalinated water in order to reduce treatment capacity and maintain a stable total blended raw water supply. Process Flow Rates The proposed operating flow rates were calculated by first determining the required amount of water needed at the WTP. Then, secondary MCLs and treatment efficiency were factored in to determine the required balance of flows throughout the treatment facility. The flows are shown in Table 3. These flow rates are based on treatment capacities to meet the chloride finished water goal at the average raw water chloride concentration of 950 mg/l. Table 3. Treatment Facility Process Flows Water Process Location Flow (MGD) Total Blended Water Product Required 3.5 Raw Water Diversion from Brazos River 4.63 UF Feed Rate 3.75 RO Feed Rate

7 RO Permeate 2.62 Concentrated Brine 1.13 Raw Water Bypass 0.88 Design Criteria Process Flow Description Water will be diverted from the Brazos River through the existing temporary pump station. It will be pumped to raw water tanks at the new water treatment plant. A feed pump will deliver water to three ultrafiltration membrane trailers (1,000 GPM capacity each). After passing the UF membranes, filtered water will flow to filtered water balancing tanks. The tanks will provide some storage and balance flow between the UF and RO membranes. Another feed pump will deliver water to three RO membrane trailers (600 gpm capacities each). Desalinated water from the RO trailers will be blended with raw Brazos River water to achieve acceptable blended water goals. This blending takes place in an existing wet well before being pumped to the Hilltop Reservoir. After storage in the reservoir, the water is treated through the conventional surface water treatment process. See Figure 2 for a process flow diagram showing flows at the average chloride concentration of 950 mg/l. Process Drawings and Geotechnical Report Hydraulic profiles of the raw water line, treatment process, backwash handling, and brine discharge were developed along with process drawings showing the proposed site improvements. Process flow diagrams provided by GE Water detailing the treatment components within each rental trailer were developed. A geotechnical report for the water treatment plant site and a slope stability analysis were developed. Raw Water Pump Station and Flood Plain Temporary end suction pumps will pump raw water drawn from Palo Pinto Creek at the Intake Site. The Intake Site is downstream of the existing Channel Dam so the water that will be pumped at the Intake Site will primarily consist of Brazos River water that is drawn up Palo Pinto Creek. The raw water pumps will be rental units mounted on trailers located adjacent to Palo Pinto Creek about 8 above the normal creek water level. The pumps will be located in the flood plain but are mounted on trailers for quick and easy removal and will be hooked up with a simple to remove electrical plug so they can be easily removed if water levels rise. In the event of heavy rains there will be adequate fresh water supplies to feed the existing fresh water treatment systems and the desalination treatment system will not be needed. 6

8 The Brazos RO Water Treatment Plant (Brazos WTP) is located on a hill above the Intake Site well above the flood plain. Desalinated water will flow by gravity to the existing Brazos Pump Station. The RO concentrate will be transferred from the Brazos WTP site through a new 10 HDPE brine pipeline that will be installed mostly above grade in temporary easements. The brine pipeline will discharge into the Brazos River at a point downstream of the confluence with Palo Pinto Creek just upstream of the Highway 281 bridge. There is a shallow rapids area in the Brazos River that separates the location where the Brazos River will flow into Palo Pinto Creek and the point where the brine will be discharged to the Brazos River. This will prevent the brine from recycling into the raw water headed to the RO treatment plant. A permit for discharge of the RO concentrate into the Brazos River has already been obtained. Pretreatment Pretreatment of the water from the Brazos River consists of UF membrane trailers. Three UF membrane trailers provided by General Electric (GE) Water will be used to pretreat the water before RO treatment. The goal of the pretreatment is to remove turbidity and suspended solids. This will protect the RO membranes and prolong the interval needed between RO cleanings. GE KlarAid IC1179P will be added to the feed water for coagulation. Each UF trailer will operate independently and in parallel. Each trailer will have two trains, each capable of producing 500 gpm of filtered water. GE ZeeWeed 1500 membranes will be used. Each trailer holds 80 membrane modules. Backwashes are expected to occur two to five times per hour per train. Membranes will undergo a maintenance cleaning once per day and a recovery cleaning once per month. See the chemical cleaning section for more detail on the UF cleanings. Backwash water will flow into onsite lagoons. Alternating feed to the lagoons will allow for recovery of decant from the backwash lagoons. Filtered water will have a Silt Density Index (SDI) value of less than 3, TSS value of less than 1 mg/l, and a turbidity of less than 1 NTU. RO Treatment The RO treatment will occur in three RO membrane trailers provided by GE Water. The RO membrane trailers will operate in parallel and are all capable of operating independently. Each trailer houses two treatment trains. Each train is operated separately. One train per trailer has a array and the other train has a array. They will be capable of producing 360 gpm and 240 gpm, respectively. GE OSMO-BEV-RO-FF membranes will be used. Individual membranes are 4-inches in diameter by 40-inches long. Each pressure vessel will house six membranes. The treatment will utilize single pass RO, and no concentrate blending with feed water will occur. The RO membrane material is thin film composite membrane (TFM). TFM does not generally tolerate oxidizing agents and the use of an oxidant will be avoided. If chlorine needs to be used on the storage tanks before the RO membranes, a sodium bisulfite product (BetzDearborn DCL32) will be used to dechlorinate the water. RO design parameters and treatment information is summarized in Table 4. 7

9 Figure 2. RO Water Treatment Plant Process Flow Diagram 8

10 Table 4. Membrane Design Parameters Parameter Value Unit Modular Treatment Units 3 Each Capacity (Per Trailer) 600 Gpm Array (2) 9x6x3 and 6x4x2 One array of each per trailer Recovery Percent Flux Gallon/day/ft 2 Feed Pressure 227 Lbs/in 2 Stages 3 Each Membrane Model # OSMO-BEV-RO-FF Membrane Diameter 4 inch Membrane Length 40 inch Membrane Surface Area 365 Ft 2 Operating Temp. Range 2-30 C Maximum Permeate Flux 24 Gallon/day/ft 2 Modeling Software Results GE Winflow (version 3.1.2) software was used to predict RO treatment performance. Three different modeling scenarios were performed. The different scenarios were; 1. Feed water from June 2014 testing and clean membranes 2. Feed water from June 2014 testing and fouled membranes 3. Feed water from November 2014 and clean membranes The modeling projections are based on one array in one trailer. This is the reason that the models have a projection of 360 gpm of permeate, instead of the full 600 gpm capacity of one trailer. The clean and fouled membrane models showed a small difference in membrane performance. With a 75 percent targeted recovery, removal of TDS, chloride, and sulfate is 97.3%, 96.5%, and 98.4%, respectively. The fouled membranes were able to achieve these model results using feed water from the November testing higher salt rejection, but recovery was lower. Removal rates are shown in Table 5. Table 5. Model Projected Removal Rates Parameter Model w/ June 2014 Testing Model w/ June 2014 Testing Model with November 2014 Testing Membrane Condition Clean Fouled Clean Recovery TDS Removal 97.3% 95.0% 99.3% Chloride Removal 96.5% 93.7% 99.2% Sulfate Removal 98.4% 97.1% 99.8% The primary limitation in the feed water is barium sulfate. It has low solubility and is expected to be above its saturation limit. In order to combat this, Betz MPH9300 will be added to the feed 9

11 water. Betz MPH9300 is a sulfuric acid based anti-scalant, that will lower the ph and reduce scaling potential. Hypersperse MDC700, a GE anti-scalant, will also be added to the feed water. Hypersperse is designed to control mineral scaling, including barium sulfate. Post Treatment After RO treatment, post treatment will occur. The desalinated water has a low ph and will need to be adjusted. Post treatment will be accomplished by blending. Blending of raw Brazos River water will accomplish two goals; it will reduce the corrosivity of the desalinated water, and it will allow the City to minimize the amount of water that needs to be treated through the RO process. Additionally, the existing WTP has the capability to adjust ph with caustic if necessary. At average conditions 0.88 MGD of raw water will be blended with 2.62 MGD of desalinated water to get 3.5 MGD of total water. Projected blend water quality is shown in Table 5. Flow meters on both the raw water bypass and the RO treatment units will ensure that proper blending occurs. Table 6. Blend Water Quality Values Parameter Value Unit Total Dissolved Solids 700 mg/l Chloride 250 mg/l Sulfate 100 mg/l Chemical Cleaning Systems The UF membranes will have on-site chemical cleaning systems. The RO membranes will not have an on-site chemical cleaning system because the 6 month run time is projected to be longer than the cleaning interval. If cleaning of the RO membranes is required during the treatment period, a RO cleaning skid will be brought to the site to accomplish cleaning of the RO membranes in place. UF Chemical Cleaning Each UF membrane trailer will undergo a cleaning schedule used by GE Water as follows; UF Maintenance Clean(Daily) o Feed Water ppm free chlorine UF Recovery Clean(Monthly) o Feed Water ppm citric acid o Feed Water ppm free chlorine The chlorine concentration will be achieved by adding sodium hypochlorite (12.5%). The citric acid concentration will be achieved by adding Betz MPH9300, a GE chemical containing citric acid. 10

12 RO Chemical Cleaning Due to the emergency nature of the project, it is not known how long the temporary treatment facility will operate. With UF membrane treatment upstream of the RO membranes, GE Water expects the RO membranes to need cleaning no more frequently than once every 6 months. It is therefore likely that the RO membranes will not become fouled and need chemical cleaning during operation. If cleaning is necessary, typical chemical cleanings usually include high and low ph solution cleanings. High ph solutions work well to remove silt, colloids, organic, and microbiological foulants. Low ph solutions work well to remove inorganic scale and mineral buildup on the membrane surface. The GE trailers are designed to perform clean in place (CIP) chemical cleanings with the addition of a mobile cleaning module. GE will use MCT 405, a high ph(11-12) solution, and MCT 105, a low ph( ) solution. GE recommends a CIP performed in the following sequence; 1. High ph cleaning solution circulated through membranes hour high ph cleaning solution soak 3. Flush with desalinated water. 4. Low ph cleaning solution circulated through membranes hour low ph cleaning solution soak. 6. Flush with desalinated water. Residuals Management The District and the City have obtained a discharge permit to return RO concentrate to the Brazos River. The discharge permit was issued on December 5, The permit is valid until May 1, Because of the variable feed water quality, water quality of the waste stream discharge will be in a range. The RO modeling software has provided two projected concentrate water qualities based on the two different feed water samples that were taken. The projections are shown in Table 6. Table 7. RO Concentrate Waste Characteristics of Concern Parameter 6/17/14 projection 11/14/14 projection Unit Total Discharge MGD Total Dissolved Solids 7,832 11,284 mg/l Chloride 2,711 5,514 mg/l Sulfate 2,197 1,599 mg/l Conductivity 11,496 17,199 µs/cm 11

13 Proposed Operating Procedures Because this facility is temporary and will be used on an emergency basis, the City s water treatment staff will be assisted by trained and experienced operators provided by GE Water. The membrane trailer provider (GE Water) will have a full-time (40 hours per week) operator onsite. The City s certified water treatment operators will collect regulatory samples and submit required information to TCEQ. The RO membrane units have control panels and SCADA systems that will be recording data. Recorded data will include, flow, feed pressure, temperature, feed ph, and feed and permeate conductivity. GE Water and City operators will monitor the UF and RO system data on a daily basis. City staff will monitor the TDS, chlorides, and sulfates of the blend water to ensure the secondary MCLs are being met. Any changes in blended water quality will be detected by the City staff. Adjustments in process operations will then be coordinated with GE operators to ensure blended water quality is maintained. The RO membrane treated water will be blended with raw water and conveyed to Hilltop Reservoir that serves as the raw water source for the existing water treatment plant. Blended water quality will also be monitored in the Hilltop Reservoir which has a storage capacity of 1,150 acre-ft (375,000,000 gallons) which will provide 107 days of balancing/blending storage before the existing water treatment plant. Project Schedule The project schedule is summarized below. The project construction has been procured through the Construction Manager at Risk (CMAR) method. Past Actions 5/2014 District declairs Drought Emergency and authorizes HDR to seek contractor 6/2014 Requests for Statements of Qualifications sent to 3 construction firms for emergency drought response project 7/10/2014 Submitted brine discharge TPDES permit application to TCEQ 8/15/2014 Two contractors are interviewed by District and City staff 8/22/2014 District Board selects Archer Western for contruction of projects 12/5/2014 TCEQ appovals TPDES discharge permit for brine discharge 12/5/2014 District Board approves selection of Brazos WTP project for emergency drought response project to desalinate Brazos River water for raw water supply and approves HDR to authorize Archer Western to initiate procurement of materials 12

14 12/16/2014 City of Mineral Wells Council approves water rate increase for Brazos WTP 12/19/2014 TCEQ submittal of plans and specifications for review and submittal of TCEQ exception to use reverse osmosis for removal of secondary contaminants from Brazos River raw water 1/8/2015 Meeting held with TWDB staff to discuss funding Scheduled Actions 2/2015 Submit application for TWDB loan commitment 2/16/2015 TCEQ review of plans and specifications complete 2/20/2015 Easement agreement for water treatment plant site complete 2/23/2015 Authorize construction start of water treatment plant 3/2/2015 Easement agreements for brine discharge pipeline complete 3/2/2015 TXDOT permit for brine discharge pipeline in ROW complete 3/13/2015 Authorize construction start of brine discharge pipeline 6/1/2015 Start-up water treatment plant 12/1/2015 Decommission water treatment plant if adequate water supply in Lake Palo Pinto (plant infrastructure to remain until expansion of Lake Palo Pinto is completed and filled) 13