Harvey Winters

Characterisation of algal organic matter produced by bloom-forming marine and freshwater algae

Algal blooms can seriously affect the operation of water treatment processes including low pressure (micro- and ultra-filtration) and high pressure (nanofiltration and reverse osmosis) membranes mainly due to accumulation of algal-derived organic matter (AOM). In this study, the different components of AOM extracted from three common species of bloom-forming algae (Alexandrium tamarense, Chaetoceros affinis and Microcystis sp.) were characterised employing various analytical techniques, such as liquid chromatography - organic carbon detection, fluorescence spectroscopy, fourier transform infrared spectroscopy, alcian blue staining and lectin staining coupled with laser scanning microscopy to indentify its composition and force measurement using atomic force microscopy to measure its stickiness. Batch culture monitoring of the three algal species illustrated varying characteristics in terms of growth pattern, cell concentration and AOM release. The AOM produced by the three algal species comprised mainly biopolymers (e.g., polysaccharides and proteins) but some refractory compounds (e.g., humic-like substances) and other low molecular weight acid and neutral compounds were also found. Biopolymers containing fucose and sulphated functional groups were found in all AOM samples while the presence of other functional groups varied between different species. A large majority (>80%) of the acidic polysaccharide components (in terms of transparent exopolymer particles) were found in the colloidal size range (<0.4 μm). The relative stickiness of AOM substantially varied between algal species and that the cohesion between AOM-coated surfaces was much stronger than the adhesion of AOM on AOM-free surfaces. Overall, the composition as well as the physico-chemical characteristics (e.g., stickiness) of AOM will likely dictate the severity of fouling in membrane systems during algal blooms.

New chloroamine process to control aftergrowth and biofouling in permasepR B-10 RO surface seawater plants

Abstract Surface seawater RO plant using PermasepR B-10 permeators typically use chlorination-dechlorination in the pretreatment system to control biological activity. For such plants, bacterial aftergrowth and biofouling in the B-10 permeators can occur when the water temperature rises above 25°C. The bacterial aftergrowth can be significant, requiring frequent disinfection and cleaning of the permeators which reduces the efficiency to the RO plant. Using bacteria isolated from Middle East B-10 RO plants, aftergrowth in a model seawater system was extensively studied over a pH range of 6 to 8 and a temperature range of 150° to 35°C. Both planktonic (growth in solution) and periphytic (growth on surfaces) studies clearly showed that the degradation of humic acid (as well as other organics) by chloride accelerated aftergrowth. The bacterial aftergrowth was influenced by pH and temperature and was directly proportional to the availability of assimilable organic compounds. Chlorine degradation of humic acid in seawater produced these assimilabl organic compounds which led to bacterial aftergrowth and biofouling. The degree of humic acid degradation by chlorine was dependent on pH, temperature and the concentration of chlorine. Chloramine, a disinfectant which was generated in situ, was extensively examined as an alternative to chlorine. Chloramine was a better disinfectant and did not degrade humic acid. In addtion, significantly less aftergrowth was observed in the chloramine process (chloramine followed by neutralization with sodium bisulfite). B-10 permeators were found to be completely compatible with the chloramine process. Even brief exposure of B-10 permeators to chloramine did not significantly affect the Ro performance. The chloramine process is a significant discovery that should control biofouling in seawater RO plants.

Twenty years experience in seawater reverse osmosis and how chemicals in pretreatment affect fouling of membranes

The author summarizes his twenty years experience in seawater reverse osmosis and describes effects of chemicals in pretreatment on bacterial and organic fouling of RO membranes.

Ultrafiltration behaviors of alginate blocks at various calcium concentrations.

Alginate, a linear copolymer, is composed of 1,4-linked β-d-mannuronic acid (M) and α-l-guluronic acid (G), which are combined into homopolymeric blocks (MM-block and GG-block) and heteropolymeric block (MG-block). It has been widely used as a model foulant in various studies of membrane fouling, thus this study investigated the impacts of calcium ion on MG-, MM- and GG-blocks of alginate and the filtration behaviors of the three types of alginate blocks at different concentrations of calcium ion. Results showed that calcium ion had the most serious effects on GG-blocks and significantly promotes the formation of transparent exopolymeric particles (TEP) from GG-blocks which in turn led to rapid formation of thick cake layer on membrane surface during the filtration of GG-blocks. As for MM-blocks, it was found that the formation of TEP was proportional to the Ca(2+) concentration in MM-blocks solution, while the membrane fouling was enhanced by Ca(2+) in the filtration of MM-blocks solution. Unlike MM- and GG-blocks, MG-blocks were nearly not affected by addition of calcium ion, as the result, there was no significant increase in TEP. The initial fouling rates and the mass of foulants deposed on the membrane surfaces further revealed a close correlation between the TEP concentration and the membrane fouling propensity. The observations by field emission scanning electron microscope (FESEM) and atomic force microscope (AFM) further confirmed the formation process of the cake layer by TEP on the membrane surface. This study offers deep insights into the development of membrane fouling by different alginate blocks in the presence of calcium ion, and suggests that TEP formed from alginate blocks played a very significant role in the fouling development.

Transparent exopolymer particles (TEP) and their potential effect on membrane biofouling

Transparent exopolymer particles (TEP) have been described as a class of particulate acidic polysaccharides, which are large, transparent organic particles, and commonly found in seawater, surface water, and wastewater. Due to their unique physicochemical characteristics, more and more attention has recently been given to the effects of TEP on membrane fouling. In this review, the characteristics and determination methods of TEP as well as its potential effect on membrane biofouling are discussed. It appears that the analytical methods for TEP available in the literature are still debatable, and there is room for further improvement. Nevertheless, evidence suggests that TEP might be involved in the development of membrane fouling, especially at the early stage of biofilm development on membranes.

Control of organic fouling at two seawater reverse osmosis plants

Abstract Organic fouling of seawater Reverse Osmosis (RC) membranes is a phenomenon not well understood; it can result in a loss of membrane productivity and salt rejection properties. Two seawater RO plants using DuPont B-10 hollow fiber permeators had experienced organic fouling and were studied. The two plants used different sources of feedwater; one RO plant at Culebra, Puerto Rico, used open seawater; while the other RO plant at Grand Cayman Island, British West Indies, used a sea well. Both feed water sources possessed high concentrations of soluble organics (40–80mg/1) which were mainly humic acids. In an attempt to remove these organics with in-line cationic polyelectrolyte coagulation, the plants experienced organic fouling which caused excessive loss of productivity and salt rejection; both plants initially failed their acceptance tests. It was discovered that the fouling was actually caused by interactions between the humic acids and in-line cationic, polyelectrolyte coagulants which were not removed by in-dedth and cartridge filtration. Rather than remove the humic acid material, acid addition was initiated and in-line cationic coagulants use discontinued to keep the humic acids soluble. It should be noted that with the open seawater intake ferrous sulfate was still used to remove colloidal material and reduce the SDI. Both plants subsequently have passed their 720-hour acceptance test. Culebra and Grand Cayman plants have now exceeded design specification for both productivity and salt rejection. The aramid hollow fiber permeators on acidified feed have shown 100% rejection of these organics at both 25% and 50% conversion and organic fouling has not been evident.

In-plant microfouling in desalination

Abstract Microfouling of solid surfaces exposed to sea water by primary film forming bacteria is a phenomenon reasonably well known. Microfouling is believed to serve as a percursor to marine macrofouling and corrosion, significantly reduce the efficiency of condenser-heat exchangers, and cause a reduction in efficiency of performance of reverse osmosis membranes. The complex poorly understood microfouling-corrosion process has caused a huge and continuing cost to the desalination industry. Microfouling in desalination is a process in which the rate of increase of the mass of surface film is dependent in large part on the specific character of the environment, most particularly the physical, chemical and biological condition of the intake sea water. The microfouling process occurs in four stages: (1) Chemical Conditioning, (2) Attachment and Colonization by bacteria, (5) Colonization by other microorganisms, and (4) Accumulation. The attachment of bacteria to surfaces is mediated by a glycoprotein polymer. This adhesion-inducing polymeric material has been recovered from filtered sea water, marine bacteria, and algal cultures. The glycoprotein is active in exceedingly low concentration and has been shown to adhere very effectively to solid surfaces. While microfouling is a known phenomenon in reverse osmosis, its role in accelerating corrosion as well as causing a decrease in the overall heat transfer coefficient in distillation has still not been fully studied. Sucessful control of microbial fouling will follow when a complete understanding of the process has been achieved.

Control of biological fouling in seawater reverse osmosis desalination

Abstract Various non-acid pretreatment were studied in a seawater reverse osmosis desalination plant for approximately one year with particular attention to microbial aspects of fouling. The basic mechanisms of microbial fouling are discussed and particular attention is directed specifically at reverse osmosis systems. Both polyacrylate and tannic acid showed some promise as an alternative to acid pretreatment and chlorination. A new assay was developed which holds the promise to predict the potential of any feed water sample to promote microbial fouling.

Influence of desalination effluents on marine ecosystems

Abstract Water, re-entering a marine ecosystem after transit through a desalination plant, may be altered in three major ways. One, its thermal energy may be increased; two, its chemical make up may be altered; and three, its microbial biota may be modified. The effect of thermal enrichment on natural marine ecosystems varies with the change in temperature, the duration of that temperature change, and its geographical extent. If the desalination plant is sited in such a manner so as to allow rapid dissipation of the thermal input, the effect of the temperature change will be minimized. Signs of thermal effect on marine ecosystems could be manifested by changes in community structure (types of organisms), as well as the changes in features of individual species. The most obvious chemical changes in desalination effluents may include increase in salinity, a decrease in dissolved oxygen, an increase in dissolved organics, and an increase in pretreatment chemicals. As with thermal input, the effect of these chemical changes in the effluent on natural ecosystem will depend upon the rate of entrance and dispersion. As the rate of dispersion is increased, the effect of chemical changes on ecosystems is correspondingly decreased. Desalination plants, due to their design, provide surface areas for rapid microbial proliferation. Depending upon the sequence of chemical pretreatment, it is possible that these viable microbes will enter the ecosystem and supplement the existing biota, if conditions permit their continued growth. The environmental impact in several types of desalination plants (distillation and reverse osmosis) on natural marine ecosystems will be discussed. The problems imposed on the environment by these desalination plants will be presented and similarities to problems encountered in other types of condenser-heat exchanger systems (Power plants and ocean thermal energy converters) will be analyzed.

Two seawater reverse osmosis plants operating at high pressure and 50% conversion

Abstract At the Second World Congress on Desalination and Water Re-Use (Bermuda) an evaluation was made on a Reverse Osmosis (RO) plant operating at high pressures (1100-1500 psig) using Middle East conditions; it was concluded that such an RO plant could obtain 50 percent conversion of the feedwater and incur lower capital and operating costs. Two RO seawater plants using DuPont B-10T hollow fiber permeators were studied; these plants, one a 400m 3 /day facility and the other a 115m 3 /day facility, were designed for 50% conversion at a maximum 1200 psig operating, pressure. The plants used direct one to one brine staging and have operated at 1140 psig since their start-up. These plants have continually exceeded design specifications for both productivity and salt rejection. Both the first and second stage permeators combined have maintained 50% conversion with a TDS of the product water less than 350 ppm. These hollow fiber aramid permeators have maintained excellent performance at high pressure despite the fact that the feedwater possessed a very high concentration of organics (humic acid). The capital and operating costs of this plant have been significantly less than that of a comparable RO plant using lower pressure (900 psig). The performance data and cost analysis of this high pressure RO plant support the hypothesis that high pressure RO can provide excellent performance at a lower cost.