Microbubble RO membrane cleaning reduces fouling on WWRO plant

Abstract

A B S T R A C TOver the last 10 years, there have been significant developments in new devices for energy recovery, new membrane materials and new sizes and orientations of reverse osmosis (RO) plants, all designed to reduce costs and improve efficiency. The fundamental issue of keeping membrane surfaces clean to ensure efficient RO plant operation has seen relatively few recent developments. This is surprising as any fouling of the membrane surface will have a dramatic effect on energy consumption and plant efficiency. Many researchers have focused on identifying and studying the foulants in great detail, but there have been few studies in how to remove it. Commodity acid and alkali compounds are still widely used due to the perceived lower application costs. Specially blended cleaning chemicals incorporating detergents, surfactants and chelants are also in wide use and are increasingly accepted by the market to be economically and environmentally viable. "Strategically pairing chemical agents that have complementary cleaning mechanisms so a higher cleaning efficiency can be attained" has been described by Wui. The authors have established a research project to explore in detail the use of novel physical and chemical cleaning methods. These included effervescent chemicals, physically generated bubbles and high ionic strength cleaners designed to agitate the cake layer on the membrane surface, assisting deposit removal. A series of experiments using flat sheet test rigs and pilot plant have been completed and the results presented in papers at IDA Tianjin 2013. This paper explains how the multiple cleaning mechanisms remove foulants and presents new data from a food processing plant which recycles wastewater through an RO plant is presented. Historically, cleans were conducted every one to two weeks due to the very high fouling rate. An air inductor and specially formulated cleaning compound cleaners A & B incorporating effervescent and high ionic strength demonstrated that the plant could be cleaned more efficiently and in a shorter timescale than using conventional cleaners. The presence of microbubbles has a dramatic effect on cleaning efficiency as a result of agitation of deposits on the membrane surface. The differential pressure of the first-stage plant was reduced for 4.5 bar to consistently less than 1 bar. The quantity of permeate produced increased from 15 to 24 m 3 /h. The underlying fouling rate was dramatically reduced so cleans are now conducted on a 6-8 week cycle rather than a 10-day cycle. These improvements occurred because cleaning using microbubbles has been more effective and the membrane surface much cleaner than previously. A clean membrane surface does not foul as quickly as a dirty membrane surface due to surface roughness. The potential for enhanced membrane cleaning by combining different chemical and physical mechanisms is an exciting area of research.

Microbubble-Assisted Cleaning-in-Place Process for Ultrafiltration System and Its Environmental Performance

“…MBs have been reported to aid deposit removal by increasing momentum transfer due to bubble scrubbing [42] as well as energy released and shear flow generated by bubble collapse [24]. Moreover, in our previous study on MB-assisted cleaning of fouled heat transfer surface [21], MBs were observed to attach to milk foulant, resulting in enhanced removal.…”

“…MBs have been applied in various food-related areas, such as increasing soil removal from baby spinach during washing [17], promoting inactivation of Escherichia coli and Salmonella Typhimurium on leafy vegetables [18], enhancing salt diffusion into pork meat during brining [19], and intensifying yeast production [20]. Due to the hydrophobic surfaces, MBs have also proven effective in the cleaning of organic deposits and pollutants, including milk deposit removal from heat transfer surfaces [21], biofilm detachment from the membrane surface [22], food oil deposit removal from the microfiltration membrane [23], fouling reduction on the reverse osmosis membrane [24], and biodiesel removal from sand [25].…”

Microbubble-Assisted Cleaning To Enhance the Removal of Milk Deposits from the Heat Transfer Surface

“…Microbubbles (MBs) are fine bubbles with a diameter of 10–50 μm. − Compared to regular sized bubbles on the order of millimeter and beyond, MBs have several unique physicochemical properties when they are introduced into water, including (i) long residence time due to smaller buoyant force, (ii) hydrophobic gas–liquid interface, and (iii) larger surface-to-volume ratio. − These properties enable MBs to adsorb water-insoluble organic molecules and thus have been used as cleaning and separation agents for different industrial applications, including cleaning of metallic parts, food and vegetable sanitation, separation of oil from sand, − and flotation for wastewater treatment. − MBs have also been applied to reduce fouling on reverse osmosis membranes for wastewater treatment and double the cleaning efficiency of colloidal matters fouled on the MF membrane . Furthermore, MBs were found effective in enhancing the detachment of fixed biomass from nylon by 78% and the removal of biofilms from the steel surface by 1.0 log and from the polypropylene surface by 0.9 log .…”

“…9−11 These properties enable MBs to adsorb water-insoluble organic molecules and thus have been used as cleaning and separation agents for different industrial applications, including cleaning of metallic parts, 10 food and vegetable sanitation, 12 separation of oil from sand, 13 flotation for wastewater treatment. 16−18 MBs have also been applied to reduce fouling on reverse osmosis membranes for wastewater treatment 19 and double the cleaning efficiency of colloidal matters fouled on the MF membrane. 20 Furthermore, MBs were found effective in enhancing the detachment of fixed biomass from nylon by 78% 21 and the removal of biofilms from the steel surface by 1.0 log and from the polypropylene surface by 0.9 log.…”

Direct observation of flow and bubble behavior in flat sheet modules with a distributor

“…15 However recently, many researchers have turned their attention to microbubbles (tens of micrometers in size) as they have peculiar characteristics to eliminate membrane fouling. 31,32 However, on account of the different cleaning mechanism, they are not discussed herein. It is worth mentioning that, the coalescence behavior of bubbles could be drastically affected by a minute addition of surface-active materials.…”