John E. Tobiason

Fouling indices for low pressure hollow fiber membrane performance assessment.

This study evaluated the use of fouling indices to describe low pressure membrane fouling. One critical aspect of this study was the use of a bench-scale hollow fiber membrane system that imitated full-scale operation (constant flux with automatic hydraulic backwash and chemical cleaning). Fouling indices were based on a resistance-in-series model. Two different hollow fiber membrane types (membrane A and B) were tested with water from two water utilities (A and B) and three other natural sources (oligotrophic, algal bloom impacted, and wastewater impaired). The bench-scale testing included use of the same membrane as utilized at Utility B. Most fouling was reversible by hydraulic backwash and chemical cleaning. Specific flux and fouling indices for the bench-scale system were higher than those determined from full-scale data but fouling index ratios were comparable, suggesting a similar fouling nature. At similar organic loading, fouling was specific to water source and membrane type, i.e., no generalization on the impact of water source was possible. Full-scale data were compared with bench-scale data to validate the use of fouling indices. Fouling indices based on a resistance-in-series are useful tools to describe membrane performance data for both raw and pretreated water, for different water sources, and different membrane types.

CHEMICAL EFFECTS ON THE DEPOSITION OF NON-BROWNIAN PARTICLES

Abstract In this work, the effects of changes in solution chemistry on the deposition of non-Brownian latex particles in beds of glass bead porous media are examined. Experimental results are presented for the effect of varying [Ca 2+ ] at pH 7 on the zeta potential and initial deposition of 4 μm diameter polystyrene particles in packed columns of 0.4 mm diameter glass beads. The experimental results show that initial removal efficiencies can be reduced by approximately two orders of magnitude under unfavorable chemical conditions, i.e., in the presence of repulsive electric double layer (EDL) interactions. As conditions become more unfavorable, deposition is gradually decreased. The effects of chemistry on deposition are modelled by coupling surface chemistry, surface interaction force, and particle transport theories. Particle deposition is modelled using trajectory calculations with Happels sphere-in-cell porous media model. Chemical effects are incorporated by including EDL interaction forces in the trajectory model. The EDL interaction force is calculated from experimentally-derived zeta potentials, assuming that these represent the potential at the onset of the diffuse layer. A comparison of model predictions and experimental results reveals quantitative agreement for favorable chemical conditions. However, the usual failure of deposition models that include EDL interaction effects to correctly predict the efects of unfavorable chemical conditions on deposition is observed. The model predicts an abrupt decrease to zero removal at a critical repulsive force while the results illustrate a gradual decrease in deposition as the conditions become more unfavorable. An analysis of some of the possible explanations for the failure of the ‘chemical model’ is presented. The effects of a stochastic distribution of particle and collector zeta potentials does not account for the discrepancy between theory and experiment. The effects of surface roughness, heterogeneous surface chemistry properties, salvation forces, hydrophobic effects, and dynamic aspects of EDL interactions may explain the observations. Estimates of the importance of some of these effects for the experimental system studied are presented. Using the trajectory model, characteristic times for the interaction of EDLs as a particle is transported towards a collector are calculated. These times can be compared to estimates of characteristic times for the relaxation of diffuse layers, charge transfer in fixed layers, and fluctuations in repulsive interaction energy barriers in examinations of theories to describe the observed effects of chemistry on particle deposition.

Effects of ionic strength on colloid deposition and release

The deposition and release of colloidal contaminants in groundwaters may be important with respect to the environmental fate of contaminants and the remediation of subsurface contamination. The objective of this paper is to present results of laboratory studies that address the effect of ionic strength on model colloid (polystyrene spheres) deposition and release. Solutions of different ionic strength at near-neutral pH were pumped through a horizontally oriented, 40 cm long column of 0.46 mm silica sand at a velocity of 1 m per day. Pulse inputs of 1.09 μm fluorescent particles were made and the column effluent was continuously monitored with a fluorometer. After complete breakthrough of the pulse, the solution ionic strength was reduced and the subsequent effects on particle release were observed. Deposition results show evidence of the effects of hydrodynamic chromatography and retardation. As expected, deposition increases with increasing ionic strength. The extent of release due to ionic strength reduction depends on the magnitude of the change in ionic strength and the extent of prior deposition.

Effect of different solutes, natural organic matter, and particulate Fe(III) on ferrate(VI) decomposition in aqueous solutions.

This study investigated the impacts of buffer ions, natural organic matter (NOM), and particulate Fe(III) on ferrate(VI) decomposition and characterized Fe(VI) decomposition kinetics and exposure in various waters. Homogeneous and heterogeneous Fe(VI) decomposition can be described as a second- and first-order reaction with respect to Fe(VI), respectively. Fe(VI) decay was catalyzed by Fe(VI) decomposition products. Solutes capable of forming complexes with iron hydroxides retarded Fe(VI) decay. Fractionation of the resulting solutions from Fe(VI) self-decay and ferric chloride addition in borate- and phosphate-buffered waters showed that phosphate could sequester Fe(III). The nature of the iron precipitate from Fe(VI) decomposition was different from that of freshly precipitated ferric hydroxide from ferric chloride solutions. The stabilizing effects of different solutes on Fe(VI) are in the following order: phosphate > bicarbonate > borate. The constituents of colored and alkaline waters (NOM and bicarbonate) inhibited the catalytic effects of Fe(VI) decomposition products and stabilized Fe(VI) in natural waters. Because of the stabilizing effects of solutes, moderate doses of Fe(VI) added to natural waters at pH 7.5 resulted in exposures that have been shown to be effective for inactivation of target pathogens. Preformed ferric hydroxide was less effective than freshly dosed ferric chloride in accelerating Fe(VI) decomposition.

Coliform transport in a pristine reservoir : Modeling and field studies

This paper reports on field studies and model development aimed at understanding coliform fate and transport in the Quabbin Reservoir, an oligotrophic drinking water supply reservoir. An investigation of reservoir currents suggested the importance of wind driven phenomena, and that both lateral and vertical circulation patterns exist. In-situ experiments of coliform decay suggested dependence on light intensity and yielded an appropriate decay coefficient to be used in CE-QUAL-W2, a two-dimensional hydrodynamic and water quality model. Modeling confirmed the sensitivity of reservoir outlet concentration to vertical variability within the reservoir, meteorological conditions, and location of coliform source.

Impacts of ferrate oxidation on natural organic matter and disinfection byproduct precursors

This study investigated the effectiveness of ferrate (Fe(VI)) oxidation in combination with ferric chloride coagulation on the removal of natural organic matter (NOM) and disinfection byproduct (DBP) precursors. Twelve natural waters were collected and four treatment scenarios were tested at bench-scale. Results showed that intermediate-ferrate treatment (i.e., coagulation and particle removal followed by ferrate oxidation) was most effective followed by pre-ferrate treatment (i.e., ferrate oxidation followed by coagulation and particle removal (conventional treatment)) or conventional treatment alone (i.e., no oxidation), and the least effective was ferrate oxidation alone (i.e., no coagulation). At typical doses, direct ferrate oxidation of raw water decreased DBP formation potentials (DBPFPs) by about 30% for trihalomethanes (THMs), 40% for trihaloacetic acids (THAAs), 10% for dihaloacetic acids (DHAAs), 30% for dihaloacetonitriles (DHANs), and 5% for haloketones (HKs). The formation potential of chloropicrin (CP) consistently increased after direct ferrate oxidation. Pre-ferrate followed by conventional treatment was similar to conventional treatment alone for NOM and DBP precursor removal. Ferrate pre-oxidation had positive effects on subsequent coagulation/particle removal for THM and THAA precursor removal and may allow the use of lower coagulant doses due to the Fe(III) introduced by ferrate decomposition. On the other hand, intermediate-ferrate resulted in substantially improved removal of NOM and DBP precursors, which can be attributed to initial removal by coagulation and particle removal, leaving precursors that are particularly susceptible to oxidation by ferrate. The Fe(III) resulting from ferrate decay during intermediate-ferrate process was primarily present as particulate iron and could be effectively removed by filtration.

Characterization of Particles from Ferrate Preoxidation

Studies were conducted evaluating the nature of particles that result from ferrate reduction in a laboratory water matrix and in a natural surface water with a moderate amount of dissolved organic carbon. Particle characterization included size, surface charge, morphology, X-ray photoelectron spectroscopy, and transmission Fourier transform infrared spectroscopy. Characteristics of ferrate resultant particles were compared to particles formed from dosing ferric chloride, a common water treatment coagulant. In natural water, ferrate addition produced significantly more nanoparticles than ferric addition. These particles had a negative surface charge, resulting in a stable colloidal suspension. In natural and laboratory matrix waters, the ferrate resultant particles had a similar charge versus pH relationship as particles resulting from ferric addition. Particles resulting from ferrate had morphology that differed from particles resulting from ferric iron, with ferrate resultant particles appearing smoother and more granular. X-ray photoelectron spectroscopy results show ferrate resultant particles contained Fe2O3, while ferric resultant particles did not. Results also indicate potential differences in the mechanisms leading to particle formation between ferrate reduction and ferric hydrolysis.

Manganese Removal from Drinking Water Sources

Manganese (Mn) in drinking water can cause aesthetic and operational problems. Mn removal is necessary and often has major implications for treatment train design. This review provides an introduction to Mn occurrence and summarizes historic and recent research on removal mechanisms practiced in drinking water treatment. Manganese is removed by physical, chemical, and biological processes or by a combination of these methods. Although physical and chemical removal processes have been studied for decades, knowledge gaps still exist. The discovery of undesirable by-products when certain oxidants are used in treatment has impacted physical–chemical Mn removal methods. Understanding of the microorganisms present in systems that practice biological Mn removal has increased in the last decade as molecular methods have become more sophisticated, resulting in increasing use of biofiltration for Mn removal. The choice of Mn removal method is very much impacted by overall water chemistry and co-contaminants and must be integrated into the overall water treatment facility design and operation.

DAF treatment of a reservoir water supply: comparison with in-line direct filtration and control of organic matter

The goal of this paper is to compare the performance of an in-line direct filtration (no flocculation) process with a dissolved air flotation (DAF) and filtration process for drinking water treatment. Both processes were studied at the pilot scale and included biologically active dual media (GAC/sand) rapid filters. Specific attention is given to the fate of organic matter. Organic matter was analyzed by measurements of dissolved organic carbon (DOC), assimilable organic carbon (AOC), disinfection by-product formation potential (DBPFP) and ultraviolet absorbance at 254 nm (UV254). In general, flotation removed a large fraction of organic matter with additional removal provided by biologically active filters. Most of the work occurred with ozonation of the raw water (pre-ozone). Ozone increased the biologically assimilable fraction of the water; filtration decreased this fraction to acceptable levels. Overall, effects of ozone on other organics are relatively small.

High Rate Flocculation, Flotation and Filtration in Potable Water Treatment

A study of the optimisation of the process chain of flocculation — flotation — filtration was performed at Lackareback treatment plant in Gothenburg. The Gota River water is a high quality supply of low turbidity and moderate TOC. In winter time the water is generally cold.