Impact of elevated Ca(2+)/Mg(2+) concentrations of reverse osmosis membrane desalinated seawater on the stability of water pipe materials.

Hardness and alkalinity are known factors influencing the chemical stability of desalinated water. This study was carried out to investigate the effect of Ca(2+) and Mg(2+) on corrosion and/or scale formation on the surface of different water distribution pipe materials under tropical conditions. The corrosion rates of ductile iron, cast iron and cement-lined ductile iron coupons were examined in reverse osmosis (RO) membrane desalinated seawater which was remineralised using different concentrations of Ca(2+) and Mg(2+). The changes in water characteristics and the coupon corrosion rates were studied before and after the post-treatment. The corrosion mechanisms and corrosion products were examined using scanning electron microscope and X-ray diffraction, respectively. We found that the combination of Ca(2+) and Mg(2+) (60/40 mg/L as CaCO3) resulted in lower corrosion rates than all other treatments for the three types of pipe materials, suggesting that Ca(2+)/Mg(2+) combination improves the chemical stability of desalinated seawater rather than Ca(2+) only.

Electrocoagulation versus chemical coagulation: coagulation/flocculation mechanisms and resulting floc characteristics.

Electrocoagulation (EC) and chemical coagulation (CC) are employed in water treatment for particle removal. Although both are used for similar purposes, they differ in their dosing method - in EC the coagulant is added by electrolytic oxidation of an appropriate anode material, while in CC dissolution of a chemical coagulant is used. These different methods in fact induce different chemical environments, which should impact coagulation/flocculation mechanisms and subsequent floc formation. Hence, the process implications when choosing which to apply should be significant. This study elucidates differences in coagulation/flocculation mechanisms in EC versus CC and their subsequent effect on floc growth kinetics and structural evolution. A buffered kaolin suspension served as a representative solution that underwent EC and CC by applying aluminum via additive dosing regime in batch mode. In EC an aluminum anode generated the active species while in CC, commercial alum was used. Aluminum equivalent doses were applied, at initial pH values of 5, 6.5 and 8, while samples were taken over pre-determined time intervals, and analyzed for pH, particle size distribution, ζ potential, and structural properties. EC generated fragile flocs, compared to CC, over a wider pH range, at a substantially higher growth rate, that were prone to restructuring and compaction. The results suggest that the flocculation mechanism governing EC in sweep floc conditions is of Diffusion Limited Cluster Aggregation (DCLA) nature, versus a Reaction Limited Cluster Aggregation (RLCA) type in CC. The implications of these differences are discussed.

Fouling mitigation by iron-based electroflocculation in microfiltration: Mechanisms and energy minimization.

High-energy demand presents a major obstacle in the application of advanced water-purification systems. In this work, energy minimization and fouling mitigation by iron-based electroflocculation in dead-end microfiltration were investigated. Highly pure water contaminated with Silica-CMP (chemical mechanical polishing) particles were pretreated by electroflocculation at short operation times and a constant electrical current intensity of 0.4 A, followed by different slow-mixing times and filtration without any sedimentation step. By using a new method for filtration-energy appraisal, we found that an over 90% reduction in filtration energy could be achieved. The improvement was observed at all pH values examined (pH 6-8); pH values below 7 were problematic because the permeate turned yellow as a result of residual iron. The appearance of residual iron was explained by the dependence of Fe(2+) to Fe(3+) reaction rates on pH. Scanning electron micrographs of the fouled membrane surface showed the important role played by the sweep-coagulation mechanism in mitigating fouling. When internal fouling was the dominant mechanism, the amorphous iron-hydroxide solids formed a layer that filtered out the primary particles, protecting the membrane pores from plugging. Iron-hydroxide particles also reduced the hydraulic resistance of the cake when the external fouling mechanism dominated. Significant energy reduction was observed, even without the slow-mixing step, as a result of the local flocculation conditions near the membrane surface. Additional energy savings were obtained due to the significantly higher initial-flux restoration rates (>90%) resulting from electroflocculation pretreatment.

Characterization of alginate-like exopolysaccharides isolated from aerobic granular sludge in pilot-plant.

To understand functional gel-forming exopolysaccharides in aerobic granular sludge, alginate-like exopolysaccharides were specifically extracted from aerobic granular sludge cultivated in a pilot plant treating municipal sewage. The exopolysaccharides were identified by the FAO/WHO alginate identification tests, characterized by biochemical assays, gelation with Ca(2+), blocks fractionation, spectroscopic analysis as UV-visible, FT-IR and MALDI-TOF MS, and electrophoresis. The yield of extractable alginate-like exopolysaccharides was reached 160+/-4mg/g (VSS ratio). They resembled seaweed alginate in UV-visible and MALDI-TOF MS spectra, and distinguished from it in the reactions with acid ferric sulfate, phenol-sulfuric acid and Coomassie brilliant blue G250. Characterized by their high percentage of poly guluronic acid blocks (69.07+/-8.95%), the isolated exopolysaccharides were capable to form rigid, non-deformable gels in CaCl(2). They were one of the dominant exopolysaccharides in aerobic granular sludge. We suggest that polymers play a significant role in providing aerobic granular sludge a highly hydrophobic, compact, strong and elastic structure.

Iron-oxidation processes in an electroflocculation (electrocoagulation) cell.

The processes of iron oxidation in an electroflocculation cell were investigated for a pH range of 5-9 and electric currents of 0.05-0.4A (equivalent current densities of 8.6-69 A/m(2)). At all pH values and electric currents investigated, it was demonstrated and proven that for all practical purposes, the form of iron that dissolves from the anode is Fe(2+) (ferrous). The difference between the amount of theoretical dissolution as calculated by Faraday's law and the amount of observed dissolved iron ions may indicate two phenomena in electrochemical cells. The first is possible dissolution of the anode even without the operation of an electric current; this led to higher theoretical dissolution rates at lower pH. The second is the participation of some of the electrons of the electric current in reactions other than anode dissolution which led to lower theoretical dissolution rates at higher pH. Those other reactions did not lead to an increase in the local oxidation saturation level near the anode and did not affect iron-oxidation rates in the electroflocculation processes. The oxidation rates of the dissolved Fe(2+) (ferrous) to Fe(3+) (ferric) ions in electroflocculation processes were strongly dependent on the pH and were similar to the known oxidation rates of iron in non-electrochemical cells.

Capture of water-borne colloids in granular beds using external electric fields: improving removal of Cryptosporidium parvum.

Suboptimal coagulation in water treatment plants often results in reduced removal efficiency of Cryptosporidium parvum oocysts by several orders of magnitude (J. AWWA 94(6) (2002) 97, J. AWWA 93(12) (2001) 64). The effect of external electric field on removal of C. parvum oocysts in packed granular beds was studied experimentally. A cylindrical configuration of electrodes, with granular media in the annular space was used. A negative DC potential was applied to the central electrode. No coagulants or flocculants were used and filtration was performed with and without application of an electric field to obtain improvement in removal efficiency. Results indicate that removal of C. parvum increased from 10% to 70% due to application of field in fine sand media and from 30% to 96% in MAGCHEM media. All other test particles (Kaolin and polystyrene latex microspheres) used in the study also exhibited increased removal in the presence of an electric field. Single collector efficiencies were also computed using approximate trajectory analysis, modified to account for the applied external electric field. The results of these calculations were used to qualitatively explain the trends in the experimental observations.

Fluorescent dye labeled bacteriophages--a new tracer for the investigation of viral transport in porous media: 1. Introduction and characterization.

A new method for the study of pathogen transport in porous media is presented. The method is based on conjugation of fluorescent dyes to target bacteriophages and application of the modified bacteriophages for tracer studies. We demonstrate that the relevant transport determining properties of Rhodamine and several fluorescein-labeled phages are practically identical to those of the native bacteriophages. The advantages of the proposed method relative to direct enumeration of bacteriophages by plaque forming unit method, turbidity, fluorescent microspheres, and other alternative tracers are discussed. Notable advantages include simple quantitation by optical methods, unbiased signals even when virus aggregates are formed, and the ability to decouple inactivation kinetics from transport phenomena. Additionally, the signal reflects the removal and transport of the studied microorganism and not a surrogate.

Fluorescent dye labeled bacteriophages--a new tracer for the investigation of viral transport in porous media: 2. Studies of deep-bed filtration.

Viral transport in deep-bed sand filters was studied by a new method that enables rapid and simple quantitation of labeled viruses. The residence time distribution (RTD) of viruses in the bed was compared to the RTD of a fluorescein dye under conditions that simulate a filter run. The characteristics of the RTD curves for the free dye and the labeled bacteriophages followed very different trends during the filter run. While the retention time of free dye was practically independent of the filtration stage, the average retention time of the labeled bacteriophage depended in a non-linear way on filtration time. Average virus retention time as well as virus-removal efficiency were minimal at the ripening stage, increased during the operational stage and then decreased again towards the turbidity breakthrough stage. This complex trend reflects two opposing mechanisms that dominate the behavior of the filter. During the ripening stage the accumulation of the kaolin-alum material in the filter increases the adsorption surface area and retards virus mobility. After sufficient kaolin-alum deposit is accumulated in the filter, aging and densification of the alum deposit induces size exclusion phenomenon giving faster apparent mobility of viruses in the filter bed.