Michael R. Bird

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.

The chemical cleaning of polymeric UF membranes fouled with spent sulphite liquor over multiple operational cycles

Abstract Whilst permeate flux is an important parameter in characterising synthetic membrane performance, it is a poor indicator of surface condition. Membrane pores may be fouled, but the charge of the fouled surface is critical in determining performance. Polyethersulphone (PES) and polysulphone (PSf) ultrafiltration membranes were fouled with spent sulphite liquor and cleaned using sodium hydroxide and Ultrasil 11 over several operating cycles. The Osmonics PSf membrane displayed a greater relative flux decline over several cycles than the Nadir PES membrane. After 15 fouling and cleaning cycles, the relative flux decline for the PSf membrane was 70% and 55% when cleaning with NaOH and Ultrasil 11, respectively. The corresponding relative flux decline figures for the PES membrane after 15 cycles were 45 and 30% for NaOH and Ultrasil 11 cleaning, respectively. Performance and zeta-potential graphs are presented that demonstrate the strong relationship between the fouling and cleaning history, the surface charge and the performance of the membranes in terms of flux recovery.

Measuring and modelling flux recovery during the chemical cleaning of MF membranes for the processing of whey protein concentrate

The microfiltration of milk based products is used industrially as a partial sterilisation technique and to facilitate selective separation of components. This paper describes (i) an experimental apparatus, protocol and key results and (ii) the development of a mathematical model, describing the flux recovery occurring when a 2.0 μm sintered stainless steel flat-sheet membrane fouled with reconstituted whey protein concentrate (WPC) powder is cleaned using aqueous sodium hydroxide. The model describes the unsteady-state hydraulic resistance variation which occurs when surface and in-pore bound material undergoes morphological changes during caustic cleaning. The model relies on simultaneous removal and swelling processes occurring within the surface and in-pore deposits, respectively. The validity and applicability of the model is discussed, along with possible future modelling developments.

Beer Clarification by Cross-Flow Microfiltration: Fouling Mechanisms and Flux Enhancement

An experimental study of beer microfiltration has been carried out on ceramic membranes with the eventual aim of carrying this process through to the commercial scale. Enhancement of surface hydrodynamics through flow pulsation had little impact on flux suggesting that pore blocking by in-depth adsorption/deposition was the dominant factor and this was indeed found to be so. The nature of the foulants has been determined by studying the filtration rates of beer treated with various enzymes which degrade potential foulant species. Specific classes of carbohydrates and minerals have been identified as foulants. In particular, pentosans (carbohydrates composed of 5 numbered sugar rings) make a major contribution. A multi-stage backflush programme was developed and optimized in an attempt to achieve maximum pore clearance with minimal use of permeate and time. Moreover, when backflush (BF) was employed, staged increases in trans-membrane pressure had a more positive impact on flux improvement. The effect of membrane pore size on product quality and flux was also investigated in this work. Use of the BF programme achieved a flux improvement of 400%.

Treatment of UF membranes with simple and formulated cleaning agents

Both hydrophilic (C 30F) and hydrophobic (PA 50H and PES 50H) ultrafiltration membranes were fouled with (i) a 3.5 wt% whey protein solution or (ii) ground wood mill circulation water from an integrated pulp and paper mill. The membranes were subsequently treated with different cleaning agents (NaOH, HNO3, Ultrasil 11 and Libranone 960). For whey protein fouled membranes, the effectiveness of the cleaning protocol was a strong function of the sodium hydroxide concentration used. After treatment with Libranone 960, membranes fouled with ground wood mill water displayed a substantial increase in water permeability. The less hydrophilic the membrane surface, the greater the observed flux increase following cleaning. After a short period of cleaning with Libranone 960, the pure water permeability decreased as the surfactant desorbed from the membrane surface. No such trend was seen when cleaning with Ultrasil 11. FTIR, SEM and AFM techniques were used to investigate the nature of the membrane surface before and after fouling and cleaning. These techniques confirmed the efficacy of the Libranone 960 cleaning protocol.

Membrane Cleaning: Chemically Enhanced Removal of Deposits Formed During Yeast Cell Harvesting

Cleaning strategies have been evaluated for a microfiltration membrane fouled during cell harvesting. The membrane selected was a flat sheet of hydrophilic polyethersulphone with a nominal pore size of 0.1 μm. Baker’ s yeast was selected as a model micro-organism. During microfiltration, a constant transmembrane pressure of 2 bar, cross-flow velocity of 1ms−1, and suspension temperature of 34°C produced a steady state flux within 90 minutes. A water rinse removed the majority of the cellular cake revealing tenacious non-cellular and cellular deposits. Chemically enhanced cleaning of the rinsed deposit was investigated using dilute solutions of sodium hydroxide, nitric acid and P3 Ultrasil 11. Cleaning temperature effects were investigated over the range 30°C to 60°C and laminar and turbulent flow regimes were compared. A combination of measured flux recovery and microscopic visualization was used to assess cleaning performance. Effective two-stage cleaning was achieved with sequential alkali and acid treatments while effective single-stage cleaning was achieved using the formulated detergent P3 Ultrasil 11.

The influence of multiple fouling and cleaning cycles upon the membrane processing of lignosulphonates

Although the application of pressure-driven membrane technology continues to grow, fouling remains a major unsolved problem. Despite the success achieved with mechanical cleaning methods such as backfluushing and backshocking, chemical cleaning-in-place remains the major way to tackle fouling 1–3 . This paper investigates the relationship between fouling and cleaning processes, and how they influence each other over a number of operational cycles. Results are reported for the ultrafiltration of lignosulphonates, which simulates a real industrial filtration problem in the pulp and paper industry. Cleaningwas undertaken with both an acid and a commercial aqueous cleaning formulation. Results are presented which show that cleaning performance is strongly dependent upon the nature of the foulant. Results also illustrate the importance of examining membrane performance over several fouling and cleaning cycles.

Peroxygen disinfection of pseudomonas aeruginosa biofilms on stainless steel discs

The disinfection of Pseudomonas aeruginosa biofilms on stainless steel (AISI 304) by a formulation of peracetic acid and hydrogen peroxide [Proxitane®5] has been studied using a laboratory scale rig incorporating a modified Robbins device. The influence of disinfectant temperature, flow rate and concentration has been characterised and comparisons made between biofilms established under static and nutrient flow conditions. Disinfection was enhanced with increased concentration of disinfectant (1–25 ppm peracetic acid, PAA) at all flow rates tested. The greatest reduction in viable cell number occurred at the highest temperature tested (50°C). At 30°C, the transition from laminar to turbulent flow (at 0.768 ms‐1) was accompanied by a 99.7% increase in disinfection at 15 ppm PAA. This effect was not attributable to dislodgement of cells by increased fluid mechanical shear alone. Increased flow velocity (0 to 0.993 m s‐1) and moderate (10°C) increments in temperature enhanced disinfection effectiveness in th...

Application of Fluid Dynamic Gauging in the Characterization and Removal of Biofouling Deposits

Green cleaning is generally defined as cleaning of a surface by consuming minimal resources in order to lessen the impact on human health and environmental quality. The main aim of this study is to perform cleaning studies of Escherichia coli biofilms grown on (i) polyethylene, (ii) stainless steel, and (iii) glass, to observe their removal behavior under controlled hydrodynamic conditions. The biofilms grown on the three different substrates were tested using the technique of fluid dynamic gauging, which allows for the estimation of the cohesive (within the biofilm structure) and adhesive (between biofilm and substrate) strength of the deposits. The results show that the thickness of biofilm on all substrates increases with time and plateaued at 14 days. Mature biofilms grown on glass have a stronger surface attachment than those on stainless steel and polyethylene. The results also suggest structural weakening after 21 days, implying either the death of cells or the weakening of the extracellular polymer matrix structure.

The effect of ethanol pre-treatment upon the mechanical, structural and surface modification of ultrafiltration membranes

ABSTRACT The in situ ethanol pre-treatment of commercially available polysulfone (PSU) ultrafiltration (UF) membranes resulted in a threefold increase in the pure water flux (PWF) values achieved. Techniques that lead to an increase in flux are of both academic and commercial interest. It is postulated that the mechanisms for performance improvement can be attributed to swelling of membrane skin-layers, as demonstrated by changes in thickness measurements, and consideration of polymer solubility parameters, giving a degree of polymer plasticisation. The modification is accompanied by a hydrophobicity increase – this parameter is linked to a greater fouling tendency. Increases in hydrophobicity contrast with the usual effect of ethanol contact, by enhancing the removal of membrane preservatives and polyvinylpyrrolidone (PVP), a common pore-forming agent. Mechanical property changes were not readily detected, whilst the apparently unaltered sub-layer masked more subtle changes occurring within the dense skin-layer. Directing analysis specifically at the skin layer using colloidal AFM probes allowed a decoupling of changes against the support, showing that the elastic modulus was reduced as a consequence of PVP removal and plasticisation. Moreover, regional elasticity probing allowed observation of spatial inhomogeneities in elasticity, occurring due to the removal of the previously unevenly distributed PVP and leading to pitting. Consequently, the effects of pre-treatment with ethanol are shown to offer advantages by maximising the performance of commercial membranes, though such methods must be used with caution. Elasticity changes that occur may be detrimental to performance if carried out at high transmembrane pressures, where compaction could be assisted.