J. A. Howell

Critical flux concept for microfiltration fouling

Abstract Several constant-flux filtration experiments for yeast cell suspensions, yeast cell debris, and dodecane-water emulsion were performed at various operating conditions in both flat-sheet and tubular-membrane systems. The aim of the paper is two-fold. Firstly the relationship between constant-flux behaviour and membrane fouling is discussed. In some cases constant-flux filtration was realized at a constant transmembrane pressure which was below a critical value. In general constant-flux filtration was obtained with moderately increasing transmembrane pressure, and this approach is shown to have some advantages over normal constant-pressure filtration because it clearly provides for the possibility of avoiding over-fouling and so reduces the severity of fouling. Secondly, the concept of critical flux is introduced. Whilst it has long been recognised that low-pressure microfiltration is much more effective than high-pressure microfiltration, the emphasis in this work is upon the possible existence of a critical flux and the desirability of starting filtration operations at a low flux. The critical-flux hypothesis is that on start-up there exists a flux below which a decline of flux with time does not occur. Equations which may enable identification of the appropriate flux level are included.

Sub-critical flux operation of microfiltration

Abstract In the microfiltration of colloids it has become apparent that there is a critical flux below which there may be no fouling of the membrane by the colloids. The literature supporting this conclusion is reviewed and experimental results presented which show that there is no fouling of the membrane in laboratory and full scale systems operated below the critical flux in the region which is termed sub-critical. Operation above the critical flux however causes fouling which reduces flux to the critical value over time. Increasing transmembrane pressure only increases flux for a transient period with the stable flux eventually falling to the critical flux. Reduction of transmembrane pressure causes the flux to fall below the critical region if fouling of the membrane has already occurred. It may be possible once flux is in the sub-critical region for spontaneous cleaning to occur. The value of the critical flux is known to be a function of particle size, hydrodynamics and membrane-colloid interactions.

Separation of concentrated organic/inorganic salt mixtures by nanofiltration

This paper considers nanofiltration (NF) of concentrated organic/inorganic mixtures using the FILMTEC™ NF-200B membrane. Mixtures of salt (up to 17% (w/v)) and lactic acid (2% (w/v)) were used as model solutions. The work centres on the effects of salt concentration, pH and temperature on the flux and rejection of lactate. For all solutions under study, the rejection of salt was low, while the rejection of lactate was maximal at neutral pH, and decreased with salt concentration and temperature. The flux was found to decrease with salt concentration and increase with temperature, the activation energy being higher for low fluxes. The flux for pure water and 2% (w/v) lactic acid was at a maximum at neutral pH, but for salt-containing solutions, it increased with pH in the whole range analysed (pH 3–10). The observed flux and rejection patterns suggest that the effects of skin shrinkage in concentrated salt solutions, and sorption of lactate by the membrane, affect behaviour in addition to the conventional effects of charge, solute size and osmotic difference between the retentate and permeate streams.

An experimental study for the development of a qualitative membrane cleaning model

Studies with single, multistage and formulated cleaning regimes have been evaluated for sintered stainless steel and ceramic microfiltration membranes. Results demonstrate the existence of cleaning agent concentration and temperature optima, whilst the effect of increasing crossflow velocity showed minimal increase in flux, and increasing transmembrane pressure showed a decrease in cleaning performance. A qualitative model has been developed which describes the existence of a three species deposit with each species having different removal characteristics. An initial flux increase during cleaning is explained in terms of the removal of loosely bound material which is readily solubilised by caustic solutions. Subsequent flux recovery is explained in terms of changes in deposit morphology that occur on contact with the cleaning agent. Residual fouling, present after the cleaning procedure, accounts for losses in the pristine permeability and selectivity of the membrane.

Economic assessment of membrane processes for water and waste water treatment

Membrane processes are increasingly being considered as an alternative to conventional water and waste water treatment methods in anticipation of future demands for high standards and reduced environmental impact. However, the use of membranes for these applications is currently limited by the high capital and operating costs with which they are associated. This paper looks at the economics of membrane processes for water and waste water applications. The results of extensive pilot plant trials have been used to determine the cost of treatment using a range of ultrafiltration and microfiltration membranes. The most significant factors influencing the overall cost were found to be membrane cost, membrane replacement frequency and power. The importance of selecting the most suitable membrane and optimum operating conditions for each application is demonstrated. A comparison has been made between the costs of membrane systems and conventional treatment processes. Some application areas have been identified where membranes are cost-effective.

A helical baffle for cross-flow microfiltration

Abstract The use of a helical-shape baffle in a mineral (Carbosep) membrane provided an increase of more than 50% in permeate flux compared with that obtained without a baffle at the same hydraulic dissipated power. Two types of solutions were used: bakers yeast (5% dry weight) and dodecane- water emulsion (0.1%, w/w). The effect of the number of helices with respect to baffle length shows that the permeate volume increases with increasing number of helices but to a lesser degree when the number of helices is more than 4 per 25-mm baffle length. Also, when the baffles maximum diameter was reduced by about 40% a small variation in permeate flux values was observed. More interesting results were obtained with experiments at very low transmembrane pressures (0.1–0.2 bar) when the permeate flux remained almost constant even from the begining of the experiment, indicating that progressive fouling was practically absent. Flow visualization was made with a video camera (VHS) and showed that the flow was rotational around the baffle axis and that the rotational velocity increased the mixing and migration of the rejected particles from the membrane surface. The manufacturing and installation of this type of baffle was found to be easy, and the cleaning time required for membrane regeneration was found to be shorter than after unbaffed steady flow experiments; so it was possible to recover the initial membrane permeability.

Yeast cell microfiltration: Flux enhancement in baffled and pulsatile flow systems☆

Abstract The filtration performance of yeast cell harvesting was greatly improved by using an oscillatory flow mixing technique in both tubular and flat sheet membrane systems; flux increases of several fold were achieved. The effects of various operating parameters such as frequency, amplitude and ratio of net forward flow rate to oscillatory flow rate were investigated. A simple alternative to the dual head pump, which involved the use of solenoid valves for producing flow reversal, was developed. The flux was significantly increased using this new operating mode. As the only increase of power consumption was from inertia loss due to change of flow direction the additional power consumption is minimal.

Controlling fouling in membrane bioreactors operated with a variable throughput

Membrane bioreactors (MBRs) have been used increasingly for municipal wastewater treatment. The current wastewater treatment plants are designed to treat three times the average flow in dry weather (DWF) which covers the expected range of incoming flow rates. If throughput in MBRs can be changed readily by changing the energy input into the system, a smaller plant can be designed. Under varying throughput operation, a high aeration rate is required to generate a high crossflow velocity to minimize fouling. At low flow rates, a low aeration rate is used to minimize energy consumption. The aim of this work is to explore the feasibility of designing smaller membrane plants by varying the throughput. This requires the control of membrane fouling, so that chemical cleaning is not compromised. Fouling is controlled by limiting the membrane flux and also by flushing the surface of the membranes with large air bubbles. Variations of the permeate flux and the aeration rate were varied in this study and their effect on controlling fouling noted. Intermittent permeation while retaining aeration was found to be an effective technique for long-term sustainability of high fluxes.

The effect of pulsed flow on ultrafiltration fluxes in a baffled tubular membrane system

Abstract The filtration performance of an ultrafiltration membrane in tubular form was improved by a factor of about 2.5 by the incorporation of periodically spaced baffles of a doughnut or disc shape within the tubes. Further improvement was noticed when pulsed flow was used and was most marked at the lowest flow rates. The viability of a “snapshot” technique for characterising membrane performance was also demonstrated.

Critical flux in ultrafiltration of myoglobin and baker’s yeast

Abstract Critical fluxes were determined by constant flux ultrafiltration (UF) experiments under laminar flow conditions. The experiments were performed by using hydrophilic C30G and hydrophobic GR51 ultrafiltration membranes and dilute myoglobin solutions and baker’s yeast suspensions as model colloids. Solution concentration, pH and cross-flow were investigated. The critical flux increased with increasing flow velocity and decreasing solute concentration. The regenerated cellulose C30G membrane exhibited higher critical fluxes than the polysulphone GR51 membrane. The highest critical flux was obtained at pH 8 in the presence of repulsive electrostatic forces between the molecules and the surface of the membrane and the lowest at the isoelectric points of the colloids. In the case of baker’s yeast below the critical flux, the flux was about the same as the pure buffer solution flux showing a strong form of the critical flux. This also occurred with the C30G membrane at low concentrations of myoglobin except at pH 6 when a weak form of the critical flux was measured. With the GR51 membrane, the permeate flux deviated from the pure buffer solution flux even at the lowest fluxes.