By E.H.K. Zeiher, B. Ho, and B. Andrews
Efficient operation of reverse osmosis water treatment systems requires scale control, fouling prevention, timely membrane cleaning, and routine membrane replacement, all of which represent system costs.
Ondeo-Nalco has developed a new technology that uses fluorescence-based technology (RO Trasar®) to detect hidden operational problems in RO systems and insure optimal scale and fouling control.
The use of fourescence-based technology for monitoring and controlling chemical dose is well established in cooling and boiler water treatment programs. Many organic molecules exhibit the property of fluorescence, which acts like a bar code on a molecule to track its presence.
Incorporating an inert fluorescent molecule into an antiscalant allows its dosage to be determined accurately, in real time and on line, using a simple fluorescence detector. Recent work has enabled this technology to be used for enhanced monitoring, chemical dosing and troubleshooting in RO systems. It can optimize RO performance and minimize cleaning and chemical costs.
The technology can also detect membrane damage. Laboratory and field results show the fluorescent molecule is rejected 100% by polyamide (PA) membranes. In an undamaged membrane, the tracer does not pass into the permeate. It does pass into the stream through a damaged membrane. This characteristic allows percent recovery to be calculated using mass balance of the fluorescent molecule.
Typically, increased salt passage is considered indicative of damage. However, other factors such as changes in flow, changes in the concentration polarization layer, temperature, fouling, or scaling may also contribute to increased salt passage. The fluorescent molecule's passage into the permeate can only occur when system or membrane damage is present.
The Ondeo RO Trasar technology was used in a trial program by a food processing plant that had experienced difficulties with its RO systems. Two RO units (75-80% recovery, well water supply) produced boiler feedwater. Each unit produced approximately 100 gpm. Water production from the primary RO unit (a 4:2 staged configuration) was supplemented by an identical secondary unit as needed. The RO experienced frequent scaling and iron fouling and required frequent cleaning. Membranes were replaced approximately nine months prior to the trial.
Chemical dosage to an RO unit is generally assumed to be constant. In this case, fluorescent tracing revealed previously unseen variations in the antiscalant dose, including periodic slug dosing conditions.
Although the average chemical dose was 6 ± 0.5 ppm (determined via draw-down), analysis of the data showed that, for half of the operating time, the RO was underfed by more than 22% (median values). The draw-down method of dosage monitoring was not sensitive enough to show the slug dosing.
Analysis of the data showed the time between treatment spikes could be correlated with on/off cycling of a well-water supply pump. The oversized pump supplied water faster than a single RO unit consumed it. The cycling was accompanied by changes in fluid velocity and violent mechanical vibrations, sufficient to cause erratic antiscalant dosing. This led to scaling conditions. The increased mechanical wear was also expected to lead to premature replacement of membranes and system components.
RO experts routinely recommend operating a single unit for as long as possible, minimizing downtime. In this plant, fluorescent tracing technology revealed that constant chemical dosage was maintained best when both RO units operated simultaneously. Process improvements, made based on this study, translated into more consistent operation, longer times between RO cleanings and extended membrane life.
In addition to troubleshooting, RO Trasar technology can control antiscalant feed to an RO system. Feedback control compensates for changes in chemical feed pump performance, process swings, fluctuations in feedwater flow, and other variabilities that can occur during normal system operation.
In this plant, the system was used to optimize antiscalant dosing. Without control, the antiscalant dose drifted from the target dose over time. Minor manual adjustments to pump stroke length and stroke rate compensated for this drift. Using manual control, the variation in chemical dosage was approximately ± 2 ppm. Using Trasar control, an antiscalant concentration of ± 0.5 ppm was achieved.
Improvements in RO performance significantly impacted chemical cleaning and membrane replacement costs. Prior to using traced antiscalant, poor scale and fouling control resulted in frequent membrane cleaning — on average the unit was cleaned every 21 days.
Because of the severe fouling and scaling, the need for multiple cleaning steps and the interruption of plant operation during prolonged cleaning, an off-site cleaning company was used at an annual estimated cost of $62,600 per RO.
Since process improvements were made using the fluorescent tracing technology, the plant estimates that cleaning will only be necessary every six months. Furthermore, the cleanings can now be done in-house at an annual cost of less than $3,000 per RO.
The cost of new membranes was approximately $10,800 per RO. The cumulative effects of fouling and aggressive cleaning had reduced the membrane life to an average of 12 months. Process improvements are expected to allow the membranes to be replaced after 3–5 years of use. The depreciated cost of replacing membranes every 3 years vs. every 5 years is estimated at $1,440 per year for each RO unit.
Total cost savings for the customer for the first year were estimated to be over $84,000 for each RO unit.
About the Authors: E. H. Kelle Zeiher, Ph.D., is a staff scientist with Ondeo-Nalco Co. specializing in water treatment and membrane research. Bosco Ho, Ph.D., is a research scientist with Ondeo-Nalco and has been in the water treatment industry for 24 years. Brett Andrews is the Business Manager for the Ondeo-Nalco PermaCare Group. He has over 14 years experience in membrane treatment applications. This article was adapted from a paper presented at the 63rd Annual International Water Conference.
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