Biofouling control of membrane distillation for seawater desalination: Effect of air-backwash and chemical cleaning on biofouling formation.

This study explored the applicability of chemical cleaning and air-backwash to alleviate biofouling on seawater membrane distillation (SWMD). Membrane performance and wettability properties maintained at optimum duration and frequency of the treatments, as indicated by low permeate conductivity throughout the tests. The cleaning of the membrane using 2% NaOH by immersing the membrane for 30 min after 240 min operation removed the biofouling layer, indicated by low permeate conductivity of 370 µScm-1 after cleaning. However, more frequent membrane cleaning led to membrane damage, more severe wetting, and membrane hydrophobicity reduction. Ten-second air-backwash after 240 min of operation was also effective in controlling the biofouling, particularly when conducted at air pressure of 1 bar. More frequent air-backwash resulted in more aggravated inorganic fouling and accelerated biofouling formation due to the recurring introduction of air, leading to rapid membrane wetting.

Ferrous ion as a reducing agent in the generation of antibiofilm nitric oxide from a copper-based catalytic system.

The work found that the electron-donating properties of ferrous ions (Fe2+) can be used for the conversion of nitrite (NO2-) into the biofilm-dispersing signal nitric oxide (NO) by a copper(II) complex (CuDTTCT) catalyst, a potentially applicable biofilm control technology for the water industries. The availability of Fe2+ varied depending on the characteristics of the aqueous systems (phosphate- and carbonate-containing nitrifying bacteria growth medium, NBGM and phosphate buffered saline, PBS at pH 6 to 8, to simulate conditions typically present in the water industries) and was found to affect the production of NO from nitrite by CuDTTCT (casted into PVC). Greater amounts of NO were generated from the CuDTTCT-nitrite-Fe2+ systems in PBS compared to those in NBGM, which was associated with the reduced extent of Fe2+-to-Fe3+ autoxidation by the iron-precipitating moieties phosphates and carbonate in the former system. Further, acidic conditions at pH 6.0 were found to favor NO production from the catalytic system in both PBS and NBGM compared to neutral or basic pH (pH 7.0 or 8.0). Lower pH was shown to stabilize Fe2+ and reduce its autoxidation to Fe3+. These findings will be beneficial for the potential implementation of the NO-generating catalytic technology and indeed, a 'non-killing' biofilm dispersal activity of CuDTTCT-nitrite-Fe2+ was observed on nitrifying bacteria biofilms in PBS at pH 6.

Iron Complex Facilitated Copper Redox Cycling for Nitric Oxide Generation as Nontoxic Nitrifying Biofilm Inhibitor.

In this study, we developed poly(vinyl chloride) (PVC)-solvent casted mixed metal copper and iron complexes capable of catalytic generation of the antibiofilm nitric oxide (NO) from endogenous nitrite. In the absence of additional reducing agent, we demonstrated that the presence of iron complex facilitates a redox cycling, converting the copper(II) complex to active copper(I) species, which catalyzes the generation of NO from nitrite. Assessed by protein assay and surface coverage analyses, the presence of the mixed metal complexes in systems containing water industry-relevant nitrite-producing nitrifying biofilms was shown to result in a "nontoxic mode" of biofilm suppression, while confining the bacterial growth to the free-floating planktonic phase. Addition of an NO scavenger into the mixed metal system eliminated the antibiofilm effects, therefore validating first, the capability of the mixed metal complexes to catalytically generate NO from the endogenously produced nitrite and second, the antibiofilm effects of the generated NO. The work highlights the development of self-sustained antibiofilm materials that features potential for industrial applications. The novel NO-generating antibiofilm technology diverts from the unfavorable requirement of adding a reducing agent and importantly, the less tendency for development of bacterial resistance.

Copper Complex in Poly(vinyl chloride) as a Nitric Oxide-Generating Catalyst for the Control of Nitrifying Bacterial Biofilms.

In this study, catalytic generation of nitric oxide by a copper(II) complex embedded within a poly(vinyl chloride) matrix in the presence of nitrite (source of nitric oxide) and ascorbic acid (reducing agent) was shown to effectively control the formation and dispersion of nitrifying bacteria biofilms. Amperometric measurements indicated increased and prolonged generation of nitric oxide with the addition of the copper complex when compared to that with nitrite and ascorbic acid alone. The effectiveness of the copper complex-nitrite-ascorbic acid system for biofilm control was quantified using protein analysis, which showed enhanced biofilm suppression when the copper complex was used in comparison to that with nitrite and ascorbic acid treatment alone. Confocal laser scanning microscopy (CLSM) and LIVE/DEAD staining revealed a reduction in cell surface coverage without a loss of viability with the copper complex and up to 5 mM of nitrite and ascorbic acid, suggesting that the nitric oxide generated from the system inhibits proliferation of the cells on surfaces. Induction of nitric oxide production by the copper complex system also triggered the dispersal of pre-established biofilms. However, the addition of a high concentration of nitrite and ascorbic acid to a pre-established biofilm induced bacterial membrane damage and strongly decreased the metabolic activity of planktonic and biofilm cells, as revealed by CLSM with LIVE/DEAD staining and intracellular adenosine triphosphate measurements, respectively. This study highlights the utility of the catalytic generation of nitric oxide for the long-term suppression and removal of nitrifying bacterial biofilms.