Application of Cross-Flow Microfiltration for the Treatment of Combined Sewer Overflow Wastewater

Application of cross-flow microfiltration with and without backpulsing is evaluated for the treatment of dilute primary sewage effluent simulating combined sewer overflow wastewater. Four alpha alumina ceramic membranes of various pores sizes (0.2–5.0?μm) were tested to understand the impact of cross-flow velocity and transmembrane pressure on the permeate water quality and flux rate. The 0.2 and 0.8?μm membranes produced a permeate water quality that is likely to be suitable for surface water discharge. The combination of permeate chemical and biological water quality and long-term flux rates suggest that a 0.2?μm membrane would be the most appropriate membrane for the treatment of combined sewer overflow wastewater within sewersheds.     

Effects of Operating Parameters in Tubular Ultrafiltration

A parametric waste-specific study was conducted to develop a more mechanistic understanding of the tubular ultrafiltration system using a surrogate metalworking (MW) fluid as a model waste stream. An average gel layer concentration of 31% oil was calculated and the gel layer concentration was determined to be independent of transmembrane pressure and cross-flow velocity. The thin-film model adequately described limiting flux data collected in this study, due to the use of discrete cross-flow velocity∕MW fluid concentration experiments; thus, an improved mechanistic understanding was achieved. Mass transfer and thus the pressure-independent “limiting” permeate flux were generally comparable to values observed in a high-shear rotary UF system, for oil concentrations <26%. However, a decrease in net permeate flux was observed in the tubular ultrafiltration (UF) system at high feed oil concentrations; thus, a hybrid system (conventional tubular followed by high-shear rotary UF) is proposed for treatment applications where high concentrations are desired.     

Membrane Pervaporation for Wastewater Reuse in Microirrigation

A novel wastewater microirrigation technology for plants to extract reclaimed water from hydrophilic, homogenous dense membrane modules placed directly in the soil was evaluated. A series of tests were conducted in the laboratory to examine the effects of membrane configuration (hollow fiber (HF) and corrugated sheet (CS) membranes), soil texture (a loam and loamy sand soil), soil water content, feed pressure, and contaminant concentration on water permeate flux. The performance was evaluated in terms of soil water content, soil electroconductivity, water permeate flux and enrichment factor using borate, selenate, sodium chloride and glucose as model compounds. The results showed that the water permeate fluxes ranged from 0.21 to 1.04?L/m2/d for CS modules and from 0.10 to 1.00?L/m2/d for HF modules, respectively. Soil water content and feed pressure were identified as the main controlling factors for water flux. The enrichment factors were found to be less than 0.25 for all the tested contaminants. Thus, it was concluded that this membrane technology holds promise either to treat brackish ground water or to reuse wastewater for agricultural micro-irrigation.     

Pressure-Dependent Permeate Flux in Ultra- and Microfiltration

The effect of applied pressure on the permeate flux in cross-flow ultrafiltration (UF) and microfiltration (MF) was investigated both theoretically and experimentally. In UF and MF processes, the permeate fluxes are controlled by concentration polarization and cake formation over the membrane surface. As a better understanding of concentration polarization and cake formation becomes available, the permeate flux under any pressure can be theoretically predicted. Experiments were conducted in a ceramic tubular cross-flow filter with silica colloids of a narrow size distribution (model colloids). The pressure-dependent flux of the model colloidal suspension in cross-flow filtration was investigated under various experimental conditions. The experimental measurements were compared with the theoretical predictions, and the results showed that the pressure-dependent permeate flux in cross-flow filtration can be adequately predicted. Furthermore, theory and experiments demonstrated that the performance and operating state of UF and MF could be well characterized by the so-called “characteristic pressure” of the process.     

Simulations of Full-Scale Reverse Osmosis Membrane Process

Performance of a two-stage full-scale reverse osmosis (RO) process for a desalination plant in Florida was simulated with a mathematical model based on the principles of membrane transport and mass conservation. In this model, water flux at any point along the filtration channel is calculated locally according to the basic transport theory of RO membranes. The changes in cross-flow velocity and salt concentration along the filtration channel were determined using mass balance principles of water and salt. Simulations of the plant performance were compared with the in-plant observation data over a period of more than 300 days. The results showed that the model could adequately describe the performance of the full-scale RO process based on a few module and operating parameters. The study also revealed that salt rejection of a RO membrane changed with feed salt concentration. The osmotic pressure coefficient that fits best with performance of this plant was substantially lower than the value determined with the “rule of thumb” (i.e., osmotic pressure in psi ≈ 0.01×total dissolved solids in mg/L) and had to be determined specifically for the particular feed water being processed.     

Removal of Copper Ions from Waters Using Surfactant-Enhanced Hybrid PAC/MF Process

This paper deals with the removal of copper ions from aqueous solutions by using surfactant-enhanced powdered activated carbon (PAC)/microfiltration (MF) hybrid process, including the evaluation of process performance and fouling dynamics at various linear alkyl benzene sulfonate (LABS), PAC, and Cu2+ concentrations of feed solution. Although the use of surfactant as an additive material increased the adsorption efficiency in PAC/MF hybrid process, a considerable amount of the flux was lost for surfactant concentration above critical micelle concentration. The process could be employed with a performance of 74.7%, 97.2% and 87?L/m2?h for LABS rejection, Cu2+ rejection and permeate flux at the conditions of 2?g PAC/L, 5?mM LABS, 0.2?mM Cu2+, and 60-min process time. Cu2+ rejection, which increased with increasing of LABS, and PAC amounts decreased with the increase in Cu2+ concentration. It was understood that the increments in LABS, PAC, and Cu2+ concentrations being an indicator for the feed solution quality led to the occurrence of more fouling on the membrane. The analyses of dynamics concerning the fouling behaviors, which were carried out using single and combined pore blocking models, put forward that the cake formation was the main predominant mechanism in the process. It was also determined that the variation of feed contents deduced the presence of rather complex fouling behaviors as a simultaneous function of secondary membrane layer formation and clogging and narrowing of membrane pores by surfactants.     

Ceramic Filter for Small System Drinking Water Treatment: Evaluation of Membrane Pore Size and Importance of Integrity Monitoring

Ceramic filtration has recently been identified as a promising technology for drinking water treatment in households and small communities. This paper summarizes the results of a pilot-scale study conducted at the U.S. Environmental Protection Agency’s (EPA) Test & Evaluation (T&E) Facility in Cincinnati on two ceramic filtration cartridges with pore sizes of 0.05 and 0.01?μm to evaluate their ability to remove turbidity and microbiological contaminants such as bacteria [Bacillus subtilis ( ≈ 1.0?μm) and Escherichia coli ( ≈ 1.4?μm)], Cryptosporidium oocysts (4–6?μm), polystyrene latex (PSL) beads (2.85?μm) (a surrogate for Cryptosporidium), and MS2 bacteriophage ( ≈ 0.02?μm) (a surrogate for enteric viruses). The results demonstrated that the relatively tighter 0.01-μm cartridge performed better than the 0.05-μm cartridge in removing all the biological contaminants and surrogates. For turbidity removal, the 0.01-μm cartridge performed slightly better than the 0.05-μm cartridge; however, the permeate rate in the 0.01-μm cartridge reduced rapidly at higher feed water turbidity levels indicating that a tighter membrane should only be used with adequate pretreatment or at a low feed water turbidity to prolong membrane life. Microbiological monitoring was identified as a more sensitive indirect integrity monitoring method than turbidity and particle count monitoring to ensure effective treatment of water by ceramic filtration. Both PSL beads and B. subtilis showed potential as effective surrogates for Cryptosporidium, with B. subtilis showing higher degree of conservatism. Any opinions expressed in this article are those of the writer(s) and do not necessarily reflect the official positions and policies of the EPA. Any mention of products or trade names does not constitute recommendation for use by EPA. This document has been reviewed in accordance with EPA’s peer and administrative review policies and approved for publication.     

Rotation and Concentration Effects in High-Shear Ultrafiltration

A parametric waste-specific study was conducted to assess the effects of membrane rotational speed and feed oil concentration on the pressure independent “limiting” flux in a high-shear rotary ultrafiltration (HSRUF) system. The limiting flux data were adequately described by the thin-film model. The transition from pressure dependent to pressure independent behavior occurred at lower oil concentrations as membrane rotational speed was decreased and pressure was increased due to an increase in the thickness of the solute boundary layer at the membrane surface. A gel layer oil concentration, OCgel, of 39% was reported, and OCgel was determined to be constant with respect to average transmembrane pressure and membrane rotational speed. The solute mass transfer coefficient increased with membrane rotation and was greater than reported for conventional ultrafiltration systems. The greater mass transfer characteristics determined for the HSRUF system were attributed to the efficient delivery of “cleaning energy” to the membrane surface due to the effective decoupling of feed pressurization from recirculation∕hydraulic turbulence.     

Treatment of Dilute Wastewaters Using a Novel Submerged Anaerobic Membrane Bioreactor

Three 3?L laboratory scale submerged anaerobic membrane bioreactors (SAMBRs) with in situ membrane cleaning due to the bubbling of recycled biogas underneath them were studied for their ability to treat dilute wastewaters. Both Mitsubishi Rayon hollow-fiber and Kubota flat sheet membranes made of polyethylene with a pore size of 0.4?μm were used in this study, and the effect of different substrates (460?mg/L of glucose or synthetic) on chemical oxygen demand (COD) performance in the SAMBR was investigated. It was found that both membranes resulted in similar COD removals (>90% soluble COD at a hydraulic retention time of 3?h), but that the transmembrane pressure across the hollow fiber membranes was higher under similar conditions. Molecular weight analysis of the feed, reactor contents, effluent, and extracellular polymers using high pressure liquid chromatography showed that the membrane filtered out most of the high MW soluble organics, resulting in high COD removals. The experimental results from the SAMBR show the potential benefits of using this novel reactor design in a biological wastewater treatment process to minimize energy use and sludge production.     

Unified View of Sediment Transport by Currents and Waves. II: Suspended Transport

The problem of suspended sediment transport in river and coastal flows is addressed. High-quality field data of river and coastal flows have been selected and clustered into four particle size classes (60–100, 100–200, 200–400, and 400–600?μm). The suspended sand transport is found to be strongly dependent on particle size and on current velocity. The suspended sand transport in the coastal zone is found to be strongly dependent on the relative wave height (Hs/h), particularly for current velocities in the range 0.2–0.5?m/s. The time-averaged (over the wave period) advection–diffusion equation is applied to compute the time-averaged sand concentration profile for combined current and wave conditions. Flocculation, hindered settling, and stratification effects are included by fairly simple expressions. The bed-shear stress is based on a new bed roughness predictor. The reference concentration function has been recalibrated using laboratory and field data for combined steady and oscillatory flow. The computed transport rates show reasonably good agreement (within a factor of 2) with measured values for velocities in the range of 0.6–1.8?m/s and sediments in the range of 60–600?μm. The proposed method underpredicts in the low-velocity range (<0.6?m/s). A new simplified transport formula is presented, which can be used to obtain a quick estimate of suspended transport. The modeling of wash load transport in river flow based on the energy concept of Bagnold shows that an extremely large amount of very fine sediment (clay and very fine silt) can be transported by the flow.     

Surface Properties of Biofouled Membranes from a Submerged Anaerobic Membrane Bioreactor after Cleaning

The surface structural properties of biofouled membranes from a laboratory-scale submerged anaerobic membrane bioreactor (SAnMBR) treating kraft pulping evaporator condensate after cleaning were studied. A flat sheet polyvinylidene fluoride (PVDF) membrane was used for the study. Three different cleaning methods, physical cleaning (PC), maintenance chemical cleaning (MCC), and recovery cleaning (RC) were applied to the fouled membrane surface, and the treated membranes were subject to flux recovery and surface structural analysis by using spectroscopic methods, zeta potential measurement, attenuated total reflectance-Fourier transform infra red spectroscopy (ATR-FTIR), and advanced correlative microscopic methods, including confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Neither PC, MCC, nor RC methods restored the membrane permeability to initial conditions. Adhesion of a thin extracellular polymeric substance (EPS) layer, consisting of proteins and polysaccharides with a thicknesses of 4.0?μm, 5.3?μm, and 7.1?μm and roughness of 190?nm, 236?nm, and 273?nm was observed on RC, MCC, and PC treated membrane surfaces, respectively. Partial flux recovery was achieved with the MCC and RC methods. This was correlated to the reduction of the protein associated with the foulant. Polysaccharides were found to be the most stable and predominant EPS constituent in relation to protein on the biofouled layer of RC and MCC membrane surfaces.     

Modeling Water Movement and Flux from Membrane Pervaporation Systems for Wastewater Microirrigation

A mathematical model to predict the performance of a membrane pervaporation unit directly placed in the soil to reuse wastewater for agricultural microirrigation was presented. The model was formulated by combining the solution–diffusion and the resistance-in-series model for mass transport across the membrane thickness, the Richard’s equation for soil water movement and the van Genuchten function for soil hydraulic properties to predict the water permeate flux for different types of test soil over a wide range of process operating conditions. Its applicability was assessed by comparing to the experimental data collected using both hollow fiber (HF) bundles and corrugated sheets (CS) membrane modules made of a hydrophilic dense polymer. A good agreement was observed between the model predictions and the experimental measurements. Further analysis concluded that the water permeate flux were mainly controlled by the porosity, the particle-size distribution, and the residual water of the soil. The overall mass transfer resistances were estimated to be 1.2×1014 and 5.6×1013?s?Pa/m for the HF and CS modules buried in loam soil, respectively, which are different from those measured in sweeping air pervaporation tests. The soil resistance for water transport was 7.1×1013?s?Pa/m. It is believed that the model could be a valuable tool to refine the design and optimize the operation of the proposed membrane pervaporation system.     

Evaluation of Apparatus for Membrane Cleaning Tests

Membrane cleaning is critical to the operation of membrane processes. This paper studies the impact of using four different types of bench-scale membrane systems to assess the effectiveness of different cleaning steps after the filtration of colored river water. The systems are a stirred ultrafiltration (UF) cell, a SEPA cell, a small cross-flow (CF) cell, and a six-CF-cell-in-parallel system. The effect of cleaning frequency was also investigated. The comparison was implemented in terms of flux recovery, solute removal, solute resistance removal, and changes of contact angles. The stirred UF cell was only reliable and comparable in terms of flux and flux recovery results. The six-cell-in-parallel system requires further development due to their much lower flux. For cleaning at 30-min intervals, the cleaning efficiency of membranes was similar for the three CF systems. For cleaning intervals of 2 and 4 h did not statistically affect the flux recovery for the stirred UF cell and SEPA cell. There was some irreversible fouling that could not be restored completely by clean-in-place method even with rigorous chemical treatment.     

UASB Treatment of Tapioca Starch Wastewater

Feasibility of the upflow anaerobic sludge blanket (UASB) process was investigated for the treatment of tapioca starch industry wastewater. After removal of suspended solids by simple gravity settling, starch wastewater was used as a feed. Start-up of a 21.5-L reactor with diluted feed of approximately 3,000 mg∕L chemical oxygen demand (COD) was accomplished in about 6 weeks using seed sludge from an anaerobic pond treating tapioca starch wastewater. By the end of the start-up period, gas productivity of 4–5 m3/m3r?day was obtained. Undiluted supernatant wastewater with a COD concentration of 12,000–24,000 mg∕L was fed during steady-state reactor operation at an organic loading rate of 10–16 kg COD/m3r?day. The upflow velocity was maintained at 0.5 m∕h with a recirculation ratio of 4:1. COD conversion efficiencies >95% and gas productivity of 5–8 m3/m3r?day were obtained. These results indicated that removal of starch solids from wastewater by simple gravity settling was sufficient to obtain satisfactory performance of the UASB process.     

Process Limits of Municipal Wastewater Treatment with the Submerged Membrane Bioreactor

The submerged membrane bioreactor (SMBR) is a promising technology for wastewater treatment and water reclamation. This paper presents results from two pilot scale SMBR systems operating in parallel on municipal wastewater in San Diego, Calif. The SMBRs were operated to address the limitations and advantages of the SMBR process compared to conventional activated sludge processes. Minimal membrane fouling was observed throughout the year of testing with the exception of the process limitations. Both pilot units provided consistently high quality effluents throughout the study, even when operating at hydraulic retention times as low as 1.5 h. Two sets of experiments were conducted to identify different fouling conditions. The first experiments were conducted to explore operation at high suspended solids concentrations. The SMBR process experienced adverse performance at mixed liquor suspended solids concentrations greater than approximately 20?g/L. The second experiments explored operation at low mean cell residence time (MCRT). At an MCRT of <2 days, membrane fouling was rapid. Chemical cleaning with sodium hypochlorite solution provided full recovery of the membrane permeability.     

Determination of Perchlorate Rejection and Associated Inorganic Fouling (Scaling) for Reverse Osmosis and Nanofiltration Membranes under Various Operating Conditions

This study focused on perchlorate (ClO4?) rejection and flux-decline in bench-scale cross-flow flat-sheet filtration for two reverse osmosis (RO) and two nanofiltration (NF) membranes with a natural water, and addressed estimation of precipitative fouling/scaling with inorganic salts and characterizations of inorganic fouling and antiscalants. Thus the study considered tradeoffs between productivity (increased recovery and flux) versus ClO4? rejection versus membrane fouling/scaling. In this study, the rejection of water quality parameters (cations, anions, dissolved organic carbon, UVA254, total dissolved solids) and flux-decline trends for four different membranes were investigated over a various range of operating conditions (i.e., J0/k ratio and recovery). Inorganic foulants on the membrane surface were analyzed by various methods (i.e., x-ray diffraction and scanning electron microscopy), and demonstrated inhibition effects of antiscalant. With increasing recovery and J0/k ratio, high productivity (flux) was achieved, however, the rejections of perchlorate and other water quality parameters decreased and the precipitative fouling/scaling potential of membranes increased. At the same operating conditions in the presence of an antiscalant, embodying phosphonate functional groups, flux decline trends for the four membranes indicated lower scale formation supported by the results of the fouled membrane characterizations.     

Application of Artificial Neural Networks for Evaluating Pressure Filtration of Coal Refuse Slurries

Bench-scale pressure filtration testing was performed to evaluate the dewatering characteristics of two coal refuse slurries, which were collected from thickener underflow streams of two coal preparation plants. Pressure filtration provides an opportunity to produce drier solids (filter cake) that can be stacked or mixed with the coarse refuse and improved water (filtrate) recovery that can be reused in the plant. This paper examines the effects of some of the major influencing variables such as pressure, slurry pH, feed solids concentration, fines fraction of solids in the slurry, filtration time, and temperature on dewatering thickener underflow slurry. Experimental results indicated that the overall filtrate flux increased with increase in pressure and temperature while it decreased with increase in fines fraction, pH, filtration time, and solids concentration. A total of 82 experimental results were used to develop a feed forward back-propagating artificial neural network (ANN) model. The model had R² values over 0.9 for both the training and the testing datasets, indicating the goodness of fit. Sensitivity analysis performed using the ANN model indicated that filtration time and pH were the most significant variables influencing filtrate flux.     

Hydrodynamic Separation of Particulate Matter Transported by Source Area Runoff

Hydrodynamic separation is a preliminary unit operation frequently utilized in wastewater and more recently in storm water for separation of coarse particulate matter (PM) and gross solids. In order to examine the behavior and separation mechanisms of a screened hydrodynamic separator (HS) not influenced by scour, this study examined the event-based performance of an empty-bed (clean sump and volute) HS for PM fractions transported in eight runoff events from a 1,088?m2 paved source area urban watershed. Influent particle-size distributions (PSDs) {d50?m from 270?to?2,202?μm} and HS particle separation efficiency (PSE) (from 38 to 70% of mass) exhibited variations influenced by hydrology and previous loadings. When examined as PM size fractions, results demonstrate separation of the sediment fraction (>75?μm) ranging from 76 to 94% while for settleable and suspended (1–25?μm) fractions, the PSE was variable and significantly lower; from 3 to 57% and 2 to 43%, respectively. Results demonstrate a correlation between higher influent PM concentration and coarser PSDs, illustrating why higher PM concentrations promote higher PSEs; and why HS performance must be specified at a PM concentration, PSD, and flow rate. Results demonstrate that HS behavior is influenced by influent PSDs coupled with flow rate. Hydrodynamic separation is effective for high-rate gross solids control. However, current HS designs require incorporation of hydrologic control, methods of frequent sludge zone management before scour, and stored runoff management to control interevent redox conditions.     

Utility of Suspended Solid Measurements for Storm-Water Runoff Treatment

In this paper alternative solutions are presented to solve problems associated with the measurement of total suspended solids (TSS) in storm-water runoff. Results revealed that the accuracy of TSS measurement is largely related to sample representativeness, particle size distribution (PSD), sampling pipette position, and sample mixing. In general, when the PSD in the runoff was mostly larger than 75?μm, the most accurate and reproducible results were obtained when samples were collected from a position of mid-depth and midway between the walls of the beaker and the vortex and mixed at speeds in the range of 600–700?rpm. For runoff samples with a PSD smaller than 75?μm, mixing at a higher rpm is not a significant factor. As long as the PSD in the TSS subsample is representative of the original sample, a strong correlation between TSS and suspended solid concentration can be achieved. The results showed that density was largely correlated with the organic content of the particles, and, in general, smaller particles tended to have a lower density. The density results revealed that assuming a single sand size density of 2.6?g/cm3 for storm-water runoff produced a large error in the computation of sediment load and particle settling velocity.     

Experimental Study of Sand and Slurry Jets in Water

This paper presents the results of an experimental study of turbulent sand jets and sand-water slurry jets impinging vertically into a stagnant water body. The jets contained silica sand with a median diameter D50 of 206?μm, and with an initial concentration 0.60 by volume for the sand jets, and 0.055–0.124 by volume for the slurry jets. The jets had densimetric Froude numbers between 2.0 and 5.94. The sand concentration and velocity profiles were measured simultaneously using a novel fiber optical probe, up to a distance of 130do for sand jets, and 65do for slurry jets, where do is the jet diameter at the water surface. The jets were found to have self-similar Gaussian profiles. The centerline sand concentration within the jets was found to decrease rapidly, following trends similar to single phase plumes. The centerline sand velocity profile decreased significantly before reaching a plateau region. The “terminal” centerline sand velocity within this region varies somewhat depending upon sand mass flux, and is between 0.32 and 0.43 m/s. The spreading rates of the jets were found to vary with the particle Froude number. Within the sand jets and the higher Froude number slurry jet, the sand concentration had a smaller spreading rate than the velocity. The other slurry jets had equal concentration/velocity spreading rates. The momentum flux of the sand within the jets was found to decrease sharply, followed by a constant flux below a depth of 25 to 30 jet diameters.