By Peter Chawaga
Much is swept up in the march towards progress. Modern demands are pushing everything to move faster, more efficiently, and with fewer environmental consequences than ever before while technical breakthroughs facilitate. Water treatment plants have proven to be no exception.
But these demands can have adverse effects on the equipment tasked with processing more water, faster and cleaner than ever before. New membranes are being equipped with the ability to handle higher flux operation, utilizing technology that allows for low transmembrane pressure, high salt rejection, and the wherewithal to take on more water. But the more a membrane is asked to do, the faster it will become fouled.
“As permeate flux increases, so does membrane fouling by colloidal and particulate solids,” said Lee Durham, technical support director for Avista Technologies. “We have seen increased fouling when permeate fluxes exceed industry-established guidelines.”
Boris Liberman, the VP and CTO for membrane technologies at IDE Technologies, estimates that higher mechanical durability and low transmembrane pressure allows membranes to handle eight times higher flow. This, in turn, leads to eight times more fouling.
“This can be solved in two ways,” explained Liberman. “Clean feed water eight times better, which is extremely expensive, or clean membranes eight times more frequently. The practical way is to change the philosophy of membrane cleaning.”
Liberman proposes putting aside chemical methods for membrane cleaning and embracing more ecologically friendly physical methods, like frequent backwash and membrane micro-oscillation. While physical cleaning is commonly associated with taking the membrane offline in costly and time-consuming fashion, Liberman recommends an online, osmotic backwash method which he claims would take about 10 seconds per day.
“High flux operation unavoidably will request fast and frequent cleanings,” he said. “The membrane cleaning technology has to be developed in this corridor of fast, online, and environmentally friendly.”
Durham, for his part, demurs.
“In our experience, proper, in-situ chemical cleaning is always more effective,” he said.
He warned against any method that involves removing elements from the system and disassembling them as unpractical. Methods that involve micro bubbles or air-water “sparging” also present problems in his view.
“The introduction of air has the potential to reduce the cleaning efficacy by drying out the foulant and reducing the cleaning chemicals’ ability to penetrate the foulant and facilitate its removal from the membrane surface,” he said.
Durham recommends that a treatment plant run diagnostics to determine the primary fouling culprit, then cater the membrane cleaning solution accordingly. This can be achieved through a cleaning study, by installing an analyzer into the system slipstream, or by performing a cleaning study in a small pilot system.
Following the identification of the foulant, a membrane company can offer a best fit solution. A treatment plant is likely to find, as it is while shopping for any solution, that the prescribed antidote is informed by the background of the chosen provider. If the company providing counsel is Avista, this will be a chemical solution. If Dr. Liberman is on the case, there is a higher chance that the proposed solution will be one of the physical methods he champions.
Whichever road membrane fouling leads a plant down, the two camps can agree: faster fouling means higher demand for solutions.