Osmotic backwashing of forward osmosis membranes to detach adhered bacteria and mitigate biofouling

Abstract

The efficiency of osmotic backwashing cleaning to remove bacteria from forward osmosis membranes was systematically studied for the first time under different attachment and osmotic backwashing conditions. It is hypothesised that biofouling is preventable when tackling initial adhesion, i.e. during the reversible stage. Cell removal from the membrane was dependent on both adhesion and backwashing conditions: tests were performed for backwashing solutions of different concentrations and salt type, as well as different filtration durations and Ca 2+ concentrations in the feed solution.Following adhesion of P. putida, a backwashing draw solution (DSobw) of 3 M NaCl was the most efficient, removing 93% of the adhered cells after 1 minute of backwashing. All adhered cells left on the membrane were dead/injured due to osmotic shock. To optimise the cleaning regime, the maximum filtration time for which backwashing is efficient must be determined. This was determined to be 30 minutes, after which backwashing became inefficient, only removing 78% of cells. The addition of 5 mM Ca 2+ to the feed caused a 50% increase in cell surface coverage compared to adhesion without Ca 2+ . This increase in adhesion rendered backwashing inefficient, as cell removal was only 60%. To increase backwashing efficiency by increasing the backwashing flux, DSobw with CaCl2 were used. However, this was inefficient due to interactions between Ca 2+ in the DSobw and the adhered cells, even for just 1 minute: for a 55.8 L.h -1 m -2 flux, 39% of removal was obtained for a 3 M CaCl2 DSobw when compared to 93% removal for 3 M NaCl for a 36 L.h -1 m -2 flux. Therefore, both adhesion and backwashing conditions are important for cleaning of FO membranes.

Alternative methods for cleaning membranes in water and wastewater treatment

“…In this method, water flows back from the permeate side into the feed side driven by osmotic forces. In RO membrane cleaning, the osmotic flow is reversed by introducing aqueous draw solutions that contain high concentrations of salts, mainly NaCl or Na 2 SO 4 and MgSO 4 (Daly et al, 2021; Dana et al, 2019).…”

“…OBW is the most recommended technique for cleaning FO membranes, as the permeate flow can be recovered in less time and the foulants are less adhered to the membranes. The OBW was used to detach adhered bacteria and mitigate biofouling from FO membranes (Daly et al, 2021); a backwashing draw solution of 3 M NaCl was used, and about 93% of the adhered cells were removed after 1 min of backwashing (flowrate of 0.9 L/ min). Moreover, bacterial cells were all dead/injured due to osmotic shock, avoiding the formation of biofouling.…”

Membrane Fouling – The Enemy of Forward Osmosis

“…The initial stage of biofouling driven by Van der Waals forces is reversible. Water permeation flux in this stage remains unaffected [38]. However, the flux has deteriorated when the biofilm grows in the later stage.…”

Growth Inhibition of Sulfate-Reducing Bacteria during Gas and Oil Production Using Novel Schiff Base Diquaternary Biocides: Synthesis, Antimicrobial, and Toxicological Assessment

“…The nature of the bacterial strain, surfaces, concentration of nutrients, and the contact of bacterial cells with their community influence the type of the occurred interaction. The adhering property of bacteria is linked to flagella, lipopolysaccharides, and proteins in their outer membrane . Long-range forces like steric and electrostatic interactions, in addition to short-range forces including van der Waal forces, acid–base, hydrogen bonding, and bio-specific interactions, drive the bacterial adsorption .…”

“…The adhering property of bacteria is linked to flagella, lipopolysaccharides, and proteins in their outer membrane. 6 Long-range forces like steric and electrostatic interactions, in addition to short-range forces including van der Waal forces, acid–base, hydrogen bonding, and bio-specific interactions, drive the bacterial adsorption. 7 Bacteria can be found in two communities: planktonic (freely spread and living in bulk solution) or sessile (attached to a surface or contained inside the boundaries of a biofilm).…”