Laser Nephelometers Provide Ultra-Sensitive Membrane Performance Monitoring
When the San Patricio (Texas) Municipal Water District (SPMWD) was planning a new water treatment facility in the late 1990s, conventional settling followed by microfiltration was ultimately determined to be the best solution to providing high-quality water for both residential users and a new cogeneration facility. That decision has turned out to be the right one. Not only does the 7.8-mgd advanced membrane plant, completed in 1999, provide a very high level of filtration, it is also relatively easy to expand, which is very important in this area, where rapid growth and the demand for water quality and quantity are ongoing concerns.
The SPMWD is located in Ingleside, Texas, less than 20 miles northeast of the city of Corpus Christi. SPMWD, a water wholesaler created by the Texas Legislature in 1951, today supplies water to the cities of Odem, Taft, Portland, Gregory, Ingleside, Aransas Pass, Port Aransas and Rockport. Six of these cities receive water treated at the water district’s membrane filtration plant near Ingleside.
Raw water quality from the Nueces River, then the plant’s only raw water source, had ranged from 5 to 200 NTU, and averaged 25 NTU. The district later constructed a 200 million gal reservoir and acquired an additional raw water source from Lake Texana. Lake Texana water has higher turbidity levels, but the addition of the reservoir allows for a degree of settling prior to entering the plant. Raw water turbidity now averages 20 NTU and remains fairly consistent.
But, given the raw water’s variable and silt-laden turbidity at the time of the plant’s design, the district and its engineering consultants, Malcolm Pirnie, realized that effective pretreatment would be critical to good membrane performance as well as for maximizing membrane flux, making the system economically competitive to conventional treatment. Pre-membrane treatment includes chlorine dioxide treatment for TTHM and HAA5 reduction, flocculation and sedimentation.
The Pall microfiltration system contains six membrane racks with 50 microfiltration modules per rack, which provide 15,525 sq ft of filter area per rack. Membrane flux is 109 gal per sq ft of membrane fiber area per day. The plant is equipped with a PLC control and a SCADA workstation, giving the district precise control over the microfiltration system.
The membranes currently are backwashed every 30,000 gal with an air scour. The plant does not recycle its backwash water. Instead, it is sent to a reservoir used by two nearby industrial customers.
The pre-membrane portion of the plant typically reduces raw water to a range of 1 to 2 NTU. Combined membrane permeate turbidity typically measures less than 10 milliNTU, denoted as units of mNTU (0.01 NTU). The plant has a 0.3 NTU effluent turbidity limit.
Turbidity monitoring essential
Accurately monitoring membrane filter effectiveness at ultra-low detection levels of turbidity is essential to the district for protecting public health and meeting increasingly stringent regulatory requirements. Turbidity has no health effects but can provide a medium for microbial growth.
The plant initially installed conventional turbidimeters to monitor the filtrate of each rack and the combined effluent; however, the district soon found that turbidity monitoring of membrane performance requires detecting a fine integrity loss beyond the capabilities of basic detection methodologies. Conventional turbidity instruments cannot discern an actual turbidity change from “instrument noise” until the change reaches 20 to 30 mNTU (0.020 to 0.030 NTU).
The state of Texas requires membrane treatment plants to measure the turbidity of permeate from each membrane module rack. The Texas Commission on Environmental Quality guidance document defining the monitoring, operating and reporting requirements for microfiltration or ultrafiltration installations now specifies the performance of each membrane rack be monitored by either a particle counter or a laser nephelometer. Earlier experiences with using particle counters prompted the SPMWD to exclude these instruments from consideration for use at its membrane plant.
“We had purchased two water particle counters for our conventional treatment facility a few years earlier, but we found them to be maintenance intensive, requiring continuous calibration. They were subject to plugging and providing erroneous data due to fouling,” said Jake Krumnow, supervisor of operations and maintenance for the facility.
New laser-based techniques in turbidity analysis have led to new and better methods for monitoring membrane filter performance, enabling membrane system operators to detect filtration integrity problems much earlier and at much lower detection levels. To that end, following its participation in a beta study of new laser nephelometers, the SPMWD opted to switch to Hach FilterTrak 660sc Laser Nephelometers to monitor the filtrate of each rack and the combined effluent on a continuous basis. The Hach FilterTrak 10133 method, applied by the manufacturer’s FilterTrak 660 nephelometer, is a U.S. EPA-approved method.
The Hach FT660sc is designed specifically to detect changes in turbidity as low as 0.3 mNTU (0.0003 NTU). Using advanced laser optics and signal processing, the instrument detects increased concentrations of submicron-sized (<0.1 µm) particles that are a precursor to larger particles. This allows for early filter deterioration detection that meets or exceeds that of particle counters.
“We always have a true turbidity reading,” Krumnow said. “Right now, I’m looking at our laser nephelometer monitoring our combined effluent, and it’s reading 9 mNTU [0.009 NTU]. There’s no way our conventional turbidimeters could have measured turbidity anywhere that low.”
The method used by these ultra-sensitive nephelometers is based on a comparison of the intensity of light scattered by the sample under defined conditions with the intensity of light scattered by a standard reference suspension. Sample continuously flows into the FilterTrak sensor, first through a bubble trap that removes entrained air and then through a central column where it rises into a measuring chamber. A 35-mW solid state laser diode projects a beam at 660 nm through the water sample. This beam is highly collimated and monochromatic so that stray light is virtually eliminated. The light scattered by particles in the sample is collected at 90 degrees relative to the projection of the laser beam and carried through fiber optic cable to a remote high-sensitivity detection system. The amount of light detected is directly proportional to the turbidity of the sample.
At the San Patricio filtration plant, the laser nephelometers are connected to Hach sc100 Digital Controllers with plug-and-play capability; they also provide direct digital communication and six-month data logging capabilities. The district downloads this data for regulatory reporting. Although the ultra-sensitive nephelometers provide turbidity detection capabilities equivalent to or exceeding that of particle counters, the district has found the units easy to set up, calibrate and maintain.
“Trimming the outputs into our SCADA system was fairly simple,” said Chris Garza, instrument technician for the district. “We flush and clean the nephelometers and sample lines biweekly, and this is also a pretty straightforward procedure. We calibrate the units quarterly using a prepared, stabilized formazin primary standard solution we get from Hach.”
StablCal Certified formazin turbidity standards are formazin polymer suspensions that have long-term stability and are assayed to the nearest mNTU.
“The units measure permeate turbidity every 90 seconds,” Garza said. “We’re highly confident in the measurements they provide, as well as the stability of these measurements.”
Krumnow agreed: “We’ve never had a problem with these meters. Plus, I don’t think there’s anything else available that could read turbidity levels this low on a reliable, continuous, long-term basis.”
New & better methods
New laser-based techniques in turbidity analysis have led to new and better methods for monitoring membrane filter performance, enabling membrane system operators at membrane plants to detect filtration integrity problems much earlier and at much lower detection levels. Process operators armed with these sensitive measurements receive real-time assessments of ultra-low particle content changes during filtration.
This new ability should be of increasing importance to water treatment operations across the country as they begin to comply with the Long Term 2 Enhanced Surface Water Treatment Rule, designed to provide more uniform public health protection by linking the level of required treatment to the level of source water contamination.
The SPMWD has recently expanded its facility by adding 20 new membrane modules to each of the existing racks and using the existing six-laser nephelometers for performance monitoring and reporting at ultra-low detection levels. The expansion brought the district’s total microfiltration capacity to 11.5 mgd.
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