Little more than five years ago when the Bexar Metropolitan Ultra-filtration Water Treatment System went on line in San Antonio, TX, it was the largest membrane ultrafiltration treatment plant in the United States at 9 mgd. Designed and built by United Water Services in a span of 16 months, it was the first contracted design/build/operation (DBO) water treatment project in Texas.
In 2002, the Bexar (pronounced “Bear”) Metropolitan Development Corporation, known as BexarMet, requested that United Water determine warm water (maximum) plant production capacity. United Water demonstrated, through pilot and full-plant testing, that leaving two standby membrane filter racks in service - for a total of seven full-time membrane racks - allowed production of 10.8 mgd in winter and a firm 14.5 mgd in summer. This minimum capital investment allowed BexarMet to optimize production in a facility footprint three times smaller than that of a conventional treatment plant realizing the same throughput.
More than just procurement pioneers, BexarMet and United Water have demonstrated remarkable efficiency of ultrafiltration treatment of surface water. Today, this trail-blazing facility serves a population of 110,000 with four full-time United Water employees, at a total cost of $1.25 per 1,000 gallons that reflects $0.65 of debt service, $0.20 of capital replacement cost and $0.40 operating costs.
Pretreatment Increases Efficiency
According to Joseph Thaxton, Project Manager for United Water, pretreatment is critical in maintaining good membrane performance. United Water installed a Superpulsator® Clarifier (Infilco Degremont), customized by adding settler tubes to create an “ultrapulsator.” This upflow-process clarifier applies vacuum pulsation to facilitate settling of particles and removal of suspended solids, organics, and pathogens.
“We typically reduce Medina River raw water from a turbidity of 10 to 40 NTU to a normal range of 0.6 to 1.0 NTU. The pulsator has seen spikes in incoming turbidity as high as 2,000 to 6,000 NTU after a rain event. Yet, we use only ferric sulfate as a coagulant. No polymers are needed,” Thaxton said.
Clarifier effluent is sent to the ultrafiltration membranes (Aquasource™ Ultrafiltration Membrane, from Infilco Degremont) with long, 0.01-micron hollow cellulose acetate fibers that remove pollen, algae, parasites, bacteria, viruses, fungi, and cysts. Combined membrane permeate turbidity typically measures 10 to 20 milli-NTU (0.01 to 0.02 NTU). The ultrafiltration membrane units are allowing 3-log removal credit for Giardia lamblia cysts and a 2-log removal credit for viruses.
The disinfection process at the BexarMet facility realizes benefits, too, from effective ultrafiltration. Minimal chlorine demand of membrane permeate, about 0.3 to 0.4 mg/L, reduces sodium hypochlorite application - and the potential of disinfection byproducts. Operators typically dose filtration effluent at a rate of 3.5 to 4.0 mg/L free chlorine to achieve about 3.0 to 3.5 mg/L residual prior to temporary storage and post-treatment chloramination prior to distribution.
“This plant has a very good record of compliance for both turbidity and disinfection by-products,” said Don White, Environmental Investigator IV of the Public Water Supply Division of the Texas Commission on Environmental Quality (TCEQ). “You’ve got to hand it to United Water and BexarMet for putting these two high-performance components together. The clarifier upstream of the membranes is where they’re removing THM-precursors.”
Another element contributing to the plant’s operating efficiency is the United Water staff itself. The instrument technician and mechanical technician are also certified Texas ‘C’ surface water treatment operators, as is the laboratory technician. Thaxton himself holds Texas ‘A’ Water and ‘A’ Wastewater licenses. Finally, the plant utilizes programmable logic controller (PLC)-based microprocessor control coupled with Intellution® human machine interface (HMI) for automated control.
Low-Maintenance System
Thaxton emphasized that upflow clarification prior to ultrafiltration helps extend membrane life. Nominal trans-membrane pressure (TMP) of 8 to 10 psig impacts the membranes minimally and controls pumping costs. Operators typically backwash the membranes, with no air scour, about every 60 to 90 minutes in the summer and every 45 minutes in the winter.
“We see recycle rates at this facility of about 5% in the summer and 8-9% in the winter,” he added.
The plant staff performs a chemical clean every 10 days, with a weak solution of citric acid and a proprietary detergent, to prevent degradation of TMP.
In January 2005, United Water took the BexarMet Ultra-filtration Water Treatment System off line for two weeks to replace all membrane modules deployed when the plant initiated treatment in December 1999. The approximate replacement cost was $1.1M.
Laser Nephelometer Surveillance
Thaxton also gives a nod to the performance of the ultra-sensitive nephelometers that continuously monitor the BexarMet membrane filtration process. Eight units of the FilterTrak 660™ Nephelometer (Hach Co.) continuously measure membrane effluent turbidity, one unit on each membrane rack and one on the combined effluent.
The nephelometers employ a laser optical system capable of measuring turbidity from 0 to 5,000 milli-NTU (0.0005 to 5.000 NTU) with resolution of 0.003 NTU - the range of interest to United Water operators who must comply with the plant’s 0.1 NTU effluent turbidity limit.
Similar in intent to the federal Interim Enhanced Surface Water Treatment Rule (IESWTR) for conventional filtration plants, the state of Texas requires membrane treatment water plants to measure the turbidity of permeate from each membrane module rack. Specifically, the TCEQ guidance document defining the monitoring, operating, and reporting requirements for microfiltration or ultrafiltration installations specifies the performance of each membrane rack be monitored by a particle counting or monitoring device or the laser nephelometer, which applies USEPA-approved Method 10133.
“Absolutely, these instruments are vital for our operation,” Thaxton said.
While the nephelometers provide the breakthrough detection ability of particle counters, Thaxton reported they are easy to clean, calibrate and maintain. His staff flushes and cleans the nephelometers and sample lines once a week.
“The cleaner we keep our equipment, the better off we are. We’re manned one shift a day from Monday through Friday, so we rely on reliable monitoring and SCADA operation and alarming to support plant performance. If we take care of the instruments, the instruments take care of us.”
His fastidious approach applies to nephelometer calibration, too. United Water uses prepared, stabilized formazin primary standard solution from Hach to calibrate all eight nephelometers once a month rather than quarterly as prescribed by the TCEQ regulation.
“We are committed to providing superb water, and we need to know the nephelometers are telling us the truth and are not drifting. We start investigating the process if the nephelometer readings exceed 30 milli-NTU,” said Thaxton.
Benefits of innovation at the BexarMet Ultra-filtration Plant are many. The progressive facility consistently produces superior-quality water. It is well positioned to meet current and future water quality regulations. And, its ability to efficiently treat surface water is a major step in reducing demand on the Edwards Aquifer - a benefit felt deep in the heart of Texas. WW
Operations and Maintenance Technician Roy Dugosh calibrates one of eight laser nephelometers monitoring permeate from the membrane filtration process.
In January, United Water replaced the 48 membrane modules of each of seven membrane racks at the BexarMet Ultra-Filtration Water Treatment System in San Antonio. Backwash and recirculation pumps are located in the center of each rack.
The BexarMet Ultra-filtration Plant typically realizes production of 14.5 mgd in the summer and 10.8 mgd in the winter using 336 membrane modules like the one shown here.
In April 2004, United Water replaced a conventional turbidimeter monitoring combined membrane rack effluent with a laser-optics nephelometer to achieve the sensitivity required to monitor its ultrafiltration process.
