Role of dissolved air flotation (DAF) and liquid ferrate on mitigation of algal organic matter (AOM) during algal bloom events in RO desalination

Boosting algicidal efficiency of Alteromonas sp. FDHY-CJ against Skeletonema costatum through fermentation optimization

Algicidal bacteria exhibit promising potential against harmful algal blooms (HABs); however, their application has been limited due to their limited algicidal activity. This study demonstrates the enhanced algicidal activity of Alteromonas sp. FDHY-CJ bacteria against harmful Skeletonema costatum using a 5 L fermenter. Utilizing this refined framework increased the OD600 value and algal cell mortality by 6.50 and 2.88 times, respectively, compared to non-optimized culture cultivated in a flask using marine broth 2216E medium. The mechanism of action involves significant inhibition of algal photosynthetic efficiency with concurrent degradation of photosynthetic pigments. Relative to the non-optimized group, the optimized bacterial treatment led to a significant increase in H2O2 and MDA (malondialdehyde) by 19.54 and 4.22-fold, respectively, and resulted in membrane damage. The culture optimization procedure yielded effectual algicidal substances capable of considerably reducing the severity of S. costatum HABs through cell membrane disruption.

Novel use of ferrous iron/peroxymonosulfate for high-performance seawater desalination pretreatment under harmful algal blooms

Marine harmful algae bloom (HAB) is a growing threat to desalination plants worldwide. This work proposes ferrous iron/peroxymonosulfate (Fe2+/PMS) as a novel pretreatment technology for seawater reverse osmosis (SWRO) under HAB. Herein, Fe2+/PMS achieved a significantly higher reduction of negative charge of algae-laden seawater as compared to conventional coagulation (i.e., coagulant is Fe3+), which thereby facilitated improved flocculation to remove algal cells, turbidity and algal organics matters (AOMs), and marine Ca2+ (∼430 mg/L) could partially contribute to the enhanced coagulation performance. A new understanding of the improved coagulation efficiency achieved with Fe2+/PMS in seawater has been proposed as compared to freshwater: seawater matrix (e.g., 504 mM Cl-) was demonstrated to significantly enhance the generation of high-valent iron (FeO2+) as the main reactive intermediate instead of the long-recognized Fe3+ and free radicals, as revealed by methyl phenyl sulfoxide (PMSO) probe, radicals scavenging analysis and electron spin resonance (ESR) spectra. This new mechanism is expected to provide valuable insights for the development of more novel oxidative seawater treatment technologies. Of note, while trade-off between particles and AOMs played an important role in membrane fouling reduction by different dosages of Fe2+/PMS, Fe2+/PMS with an optimal dosage of 0.1 mM/0.05 mM achieved an unprecedentedly higher reduction (95.26%) of modified fouling index (MFI) as compared to conventional coagulation (13.28%-42.36% with 0.1-0.2 mM of Fe3+). Optical-photothermal infrared spectromicroscopy with sub-micron spatial resolution was employed to analyze membrane foulants for the first time, and Fe2+/PMS was found to mainly cause reduced cake layer resistance, which was attributed to the collectively reduced concentration of algae cells, micro-particles with sizes from 2 to 10 µm, humic substances and biopolymers. Moreover, Fe2+/PMS resulted in lower dissolved Fe3+ (<0.027 mg/L) in ultrafiltration (UF) permeate, which would make it more reliable for SWRO operation as compared to conventional coagulation. When energy-intensive dissolved air flotation (DAF) was employed to withstand HAB, Fe2+/PMS outperformed it and was instrumental in achieving reduced MFI with 56.4% lower operational cost. In this context, Fe2+/PMS would facilitate a high-performance and low-cost pretreatment technology for seawater desalination plants under HAB.

Design and application of gel coagulation-spontaneous flotation integrated process in water treatment: "Clouds in water"

The gel coagulation-spontaneous flotation (GCSF) process designed in this paper mainly rely on dissolved gas in water rather than auxiliary gas equipment to achieve spontaneous flotation. Compared with the traditional coagulation-dissolved air flotation method, GCSF has more stable flotation efficiency and shorter operation cycle under conventional hydraulic conditions. In this study, the GCSF scheme was applied for surface water treatment, and its operating efficiency, mechanism of action, and environmental implications were explored systematically. The results illustrate that the dosage ratio of sodium alginate (SA) to aluminum sulfate (AS) should be controlled in the range of approximately 1.5:1-2.5:1, and SA should be added 15∼120 s before AS during the coagulation process. Under these conditions, the adsorption cross-linking between SA and Al3+ promoted the generation of gel flocs and effectively encapsulated the dissolved gasses, thereby achieving a stable spontaneous flotation process and 80%-95% removal of pollutants. The purification efficiency of GCSF was positively correlated with pH 4-9, which was attributed to the enhanced hydrophobicity of the chains of organic polymer groups. The residual SA and aluminum concentration in effluent were lower than 1 and 0.05 mg/L, respectively, which guarantee the ecological security of GCSF application. In addition, the results of density functional theory calculations revealed that -OH and -AlO6 in cross-linked flocs could adsorb dissolved oxygen synergistically, while -OH combined with oxygen had a stronger binding energy and stable adsorption.

Can Aggregate-Associated Organisms Influence the Fouling in a SWRO Desalination Plant?

This pilot study investigates the formation of aggregates within a desalination plant, before and after pre-treatment, as well as their potential impact on fouling. The objective is to provide an understanding of the biofouling potential of the feed water within a seawater reverse osmosis (SWRO) desalination plant, due to the limited removal of fouling precursors. The 16S and 18S rRNA was extracted from the water samples, and the aggregates and sequenced. Pre-treatment systems, within the plant remove < 5 µm precursors and organisms; however, smaller size particles progress through the plant, allowing for the formation of aggregates. These become hot spots for microbes, due to their nutrient gradients, facilitating the formation of niche environments, supporting the proliferation of those organisms. Aggregate-associated organisms are consistent with those identified on fouled SWRO membranes. This study examines, for the first time, the factors supporting the formation of aggregates within a desalination system, as well as their microbial communities and biofouling potential.

Controlling harmful algal blooms (HABs) by coagulation-flocculation-sedimentation using liquid ferrate and clay

Harmful algal blooms (HABs) occur worldwide and threaten the quality of marine life, public health, and membrane facilities in Seawater Reverse Osmosis (SWRO) desalination plants. The effects of HABs on seawater desalination plants include extensive membrane fouling, increased coagulant consumption and plant shutdown. To determine how to mitigate such effects, this study assessed if low doses (0.01 mg/L, 0.10 mg/L, and 1.00 mg/L) of liquid ferrate (58% yield) and kaolin or montmorillonite clays alone could remove algal organic matter in coagulation-flocculation-sedimentation (CFS) pretreatment desalination systems. Results showed that 0.01 mg/L of liquid ferrate coagulant removed 42% of dissolved organic carbon (DOC), 52% of biopolymers (BP), 71% of algal cells, and 99.5% of adenosine triphosphate (ATP). At a dose of 0.01 mg/L, clays exhibited high removal of turbidity (up to 88%), BP (up to 80%) and algal cells (up to 67%). The combination of liquid ferrate (58% yield) as a coagulant with kaolin or montmorillonite clays as coagulant aids in CFS pretreatment led to 72% removal of DOC, 86% of BP, and 84% of algal cells with a fixed dose of 0.01 mg/L for each. Findings from this study can help SWRO plants improve the performance of pretreatment systems during algal bloom events by reducing the consumption of coagulants while also maintaining high removal efficiencies.

Membrane Fouling in Algal Separation Processes: A Review of Influencing Factors and Mechanisms

The use of algal biotechnologies in the production of biofuels, food, and valuable products has gained momentum in recent years, owing to its distinctive rapid growth and compatibility to be coupled to wastewater treatment in membrane photobioreactors. However, membrane fouling is considered a main drawback that offsets the benefits of algal applications by heavily impacting the operation cost. Several fouling control strategies have been proposed, addressing aspects related to characteristics in the feed water and membranes, operational conditions, and biomass properties. However, the lack of understanding of the mechanisms behind algal biofouling and control challenges the development of cost-effective strategies needed for the long-term operation of membrane photobioreactors. This paper reviews the progress on algal membrane fouling and control strategies. Herein, we summarize information in the composition and characteristics of algal foulants, namely algal organic matter, cells, and transparent exopolymer particles; and review their dynamic responses to modifications in the feedwater, membrane surface, hydrodynamics, and cleaning methods. This review comparatively analyzes (i) efficiency in fouling control or mitigation, (ii) advantages and drawbacks, (iii) technological performance, and (iv) challenges and knowledge gaps. Ultimately, the article provides a primary reference of algal biofouling in membrane-based applications.