In situ visualization of organic fouling and cleaning mechanisms in reverse osmosis and forward osmosis

Abstract

Fouling models rely on knowledge of foulant accumulation and removal mechanisms. In this study, a fouling visualization apparatus is developed to elucidate centimeter-scale mechanisms of organic fouling and cleaning in reverse osmosis (RO) and forward osmosis (FO). Alginate is used as a model organic foulant and dyed with methylene blue, which is shown not to affect fouling or cleaning, and to sufficiently highlight the gel for visualization at low salinity (up to 1% NaCl). When cleaning by increasing the cross-flow velocity, with or without reverse permeation, foulant peels off the membrane in discrete pieces in both RO and FO. Videos of cleaning show that foulant cake swelling and wrinkling can facilitate gel detachment and removal. Despite their effectiveness in slowing fouling, spacers can hinder removal of detached foulant pieces by obstructing their path. Finally, photographs point to a new mechanism of internal fouling in FO: vapor formation due to sub-atmospheric pressure within the membrane. Awareness of these mechanisms allows for better modeling of fouling and motivates optimization of swelling-inducing cleaning procedures.

Quantifying osmotic membrane fouling to enable comparisons across diverse processes

“…Negative pressures have been shown to exist in tree xylem, another liquid-filled nanoporous material subjected to suction [39]. However, alginate fouling has been shown to cause nucleation in the larger, micro-scale pores of the feed-facing support layer of FO membranes in PRO orientation [15]. Large pores or hydrophobic materials in the foulant layer could theoretically act as nucleation sites regardless of membrane orientation, although this has not to our knowledge been demonstrated.…”

“…Three nested O-rings, which contact each side of the membrane and outside the membrane, rest in grooves designed to minimize membrane damage and provide sealing that ensures very good overall salt rejection (see Appendix A.2) for a membrane module of this size. Details of module design are provided in [15]. Rather than controlling draw and feed concentration, gradual variation is allowed and accounted for in the feed and draw salinities input to the model, as we discuss in Appendix A.4.…”

“…Pore blocking (described in [4]) and active layer damage are not modelled. The PRO orientation (active layer facing the draw) and its unique fouling mechanisms (e.g., suction-induced membrane dryout [15])…”

Unpacking compaction: Effect of hydraulic pressure on alginate fouling

“…Changes in calcium and sodium ion concentration within the gel, such as could occur when it is rinsed or placed in a dye solution, can cause it to shrink or swell [23]. In situ visualization of FO and RO foulant layers has also been used to compare mechanisms of foulant removal [16].…”

“…[7,8,10,11,12,13,14,15], attribute differences between RO and FO membrane fouling to foulant compaction by high hydraulic pressure. The most compelling evidence comes from studies that show a marked difference in the effectiveness of physical cleaning between identical membranes fouled under identical hydrodynamic conditions at the same initial flux in RO and FO [7,8,14,15,16]. According to the theory that foulant cake density increases with feed hydraulic pressure, the less-compact cake layer formed near atmospheric pressure in FO should be easier to remove.…”