Many waters in Tanzania exhibit high concentrations of organic matter and dissolved contaminants such as fluoride. Due to bacteria and virus removal, ultrafiltration (UF) is an attractive option for drinking water treatment, and when coupled with adsorbents, may compete with other established processes like nanofiltration (NF) for lower contaminant concentrations. The results presented here examine the characteristics and treatability of tropical natural organic matter (NOM) by UF as a function of seasonal variation. The Tanzanian river Maji ya Chai was sampled monthly during one year. The composition of NOM in Maji ya Chai River is influenced strongly by precipitation. Total organic carbon (TOC), specific ultraviolet absorbance (SUVA) and concentration of allochthonous organics substances (such as humic substances (HS)) are elevated in periods following high precipitation, while TOC is lower and contains more biopolymers in the dry seasons. UF experiments with two regenerated cellulose membranes of different molecular weight cut-off (MWCO, 5 and 10 kDa) were conducted. UF is able to remove 50-95% of TOC with a seasonal variability of 10-20%. Due to the remaining NOM in the water that would contribute to disinfection by-product formation and bacterial regrowth, the physically disinfected water is more applicable for point of use systems than distribution or storage.

Reducing membrane fouling caused by seawater algal bloom is a challenge for regions of the world where most of their freshwater is produced by seawater desalination. This study aims to compare ultrafiltration (UF) fouling potential of three ubiquitous marine algal species cultures (i.e., Skeletonema costatum-SKC, Tetraselmis sp.-TET, and Hymenomonas sp.-HYM) sampled at different phases of growth. Results showed that flux reduction and irreversible fouling were more severe during the decline phase as compared to the exponential phase, for all species. SKC and TET were responsible for substantial irreversible fouling but their impact was significantly lower than HYM. The development of a transparent gel layer surrounding the cell during the HYM growth and accumulating in water is certainly responsible for the more severe observed fouling. Chemical backwash with a standard chlorine solution did not recover any membrane permeability. For TET and HYM, the Hydraulically Irreversible Fouling Index (HIFI) was correlated to their biopolymer content but this correlation is specific for each species. Solution pre-filtration through a 1.2 μm membrane proved that cells and particulate algal organic matter (p-AOM) considerably contribute to fouling, especially for HYM for which the HIFI was reduced by a factor of 82.3.

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Highlights Models of polymer physics were applied to describe HMW SKC-IOM polymeric nature  HMW SKC-IOM polymeric structure showed highly responsive to solution chemistry  Two regions of different nanomechanical properties were observed in HMW SKC-IOM  HMW SKC-IOM showed fully extended chains at low ionic strength  HMW SKC-IOM evidenced compressed structures with increasing salt concentration AbstractIn the current investigation, a rigorous characterization of the high molecular weight (HMW) compounds of Skeletonema costatum (SKC) intracellular organic matter (IOM), including nanomechanical properties, was conducted. HMW SKC-IOM was characterized as a mixture of polysaccharides, proteins, and lipids. Atomic force microscopy (AFM) provided crucial information of this isolate at a nanoscale resolution. HMW SKC-IOM showed highly 3 responsive to solution chemistry: fully extended chains at low ionic strength, and compressing structures with increasing electrolyte concentration in solution. Interestingly, two regions of different nanomechanical properties were observed: a) Region #1: located farther from the substrate and showing extended polymeric chains, and b) Region #2: located <10 nm above the substrate and presenting compressed structures. The polymer length, polymer grafting density, and compressibility of these two regions were highly influenced by solution conditions. Results suggest that steric interactions originating from HMW SKC-IOM polymeric structure would be a dominant interacting mechanism with surfaces. The current investigation has successfully applied models of polymer physics to describe the complex HMW SKC-IOM structural conformation at different solution conditions. The detailed methodology presented provides a tool to characterize and understand biopolymers interactions with surfaces, including filtration membranes, and can be extended to other environmentally relevant organic compounds.

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