- The Filtration Society - http://www.filtsoc.org -

Volume 14, Issue 1

CLEANING HOLLOW FIBRE MEMBRANES DURING AND AFTER MICROFILTRATION USING ULTRASOUND
Abdolmajid Maskooki (pages 22-30)

Membrane technology is recognised as one of the most promising separation technologies of the past 30 years. The greatest disadvantage of the technology, particularly in microfiltration (MF) and ultrafiltration (UF) processing, is fouling. The fouling phenomenon occurs due to plugging of surfaces and pores of a membrane that is created by organic and inorganic particles and results in the gradual decrease of permeate flux. Therefore, the fouling process must be prevented during a filtration or be removed by cleaning techniques after the separation process has taken place.

In this study, various frequencies (28, 45 and 100 kHz) of power ultrasound were applied during and after crossflow microfiltration in order to evaluate the cleaning efficiency in hollow fibre membranes. Results showed that permeate flux rate for all treated membranes was increased significantly by decreasing the ultrasonic wave frequency. Maximum ultrasound effects were observed in the first five minutes of an MF process and after backwash flushing under the sonication. Maximum flux recovery was achieved when 28 kHz ultrasonic waves were applied.

EFFECTS OF POST-COATING BY GENERATING A THIN SECONDARY
PARTICLE LAYER ON SURFACE FILTRATION
Qian Zhangang and Eberhard Schmidt (pages 30-37)

Periodic cleaning is necessary in surface filtration for dust separation to limit the pressure drop, which increases as more dust is deposited. Higher cleaning efficiency can be achieved with lower adhesion of the dust cake on the substrate on the one hand and higher cohesion in the dust cake on the other. The idea of an innovative conditioning concept for surface filtration, the post-coat filtration (also called post-coating), is introduced and realised on a laboratory scale.

An additional thin particle layer is generated by the filtration of aerosols on the main dust cake before cake discharge is performed. Experiments were done on a surface filter test rig according to VDI guideline 3926-1, Application 1. Results with needlefelts as test filters, limestone as the main dust and several post-coat particles are presented and discussed. The effects of increased formation of larger cake fragments during particle layer detachment, compared to the corresponding results in cases of conventional filtration without post-coating, were observed for selected material combinations and operating conditions. Comparatively positive effects of a selected post-coat on the operating behaviour of the test filters were observed during filtrations over several filtration cycles.

USING MICROWAVES TO ACCELERATE AGEING OF AN ULTRAFILTRATION PES MEMBRANE BY NaOCl TO OBTAIN A SIMILAR AGEING STATE TO THAT FOR MEMBRANES WORKING AT THE INDUSTRIAL SCALE
Murielle Rabiller-Baudry, Cindy Lepéroux, David Delaunay, Houda Diallo and Ludovic Paquin (pages 38-48)

Looking for a laboratory scale protocol that leads to aged membranes that could be representative of the aged state obtained at the industrial scale, this paper proposes a systematic study of the degradation of a polyethersulphone (PES)/ polyvinylpyrolidone (PVP) flat membrane by sodium hypochlorite. Different protocols are compared: ageing by immersion in static conditions with or without the coupling of applied microwaves as well as in dynamic conditions, namely directly on an ultrafiltration (UF) pilot. Regardless of the protocol, the physico-chemical evolution of the pristine membrane always highlights the progressive removal of PVP and slow degradation of the PES matrix itself. The use of microwaves appears a very interesting way to significantly decrease the duration of the treatment required to damage a membrane close to the aged state at the industrial scale. Nevertheless, some limitations are also evident thanks to UF behaviour in terms of both water flux and protein filtration.

ANALYTICAL CENTRIFUGATION: IN-SITU VISUALISATION AND CHARACTERISATION OF SEPARATION BEHAVIOUR FOR FILTER FEED DISPERSIONS
Dietmar Lerche (pages 49-60)

Dispersion behaviour is important in many technical applications and especially in solid/liquid separation. While for some applications sedimentation and sediment formation have to be avoided or minimised (e.g. food products), for solid/liquid separation processing or waste recycling these are essential. Besides volume concentration and rheology of the continuous phases, agglomeration, agglutination or flocculation are crucial in this respect. It is quite common to use zeta-potential to differentiate between flocculated and non-flocculated dispersions. However, this approach is not always applicable, especially in cases of sterically or rheologically stabilised dispersions and products made from soft particles.

Analytical centrifugation based on space and time resolved detection of extinction profiles over the entire sample height was used to characterise the sedimentation and consolidation behaviour as well as particle interaction (degree of flocculation) in dispersions at their original concentration. Depending on initial turbidity of the suspension, NIR- and visible light or mono-energetic X-ray radiation was employed.
Sedimentation and consolidation behaviour of stable or flocculated polydisperse suspensions made from quartz, limestone, mixtures of metallic oxides or magnetic therapeutic carrier and enzyme coated nanoparticles were investigated by in-situ visualisation of the separation process during centrifugation. The state of a dispersion can be easily recognised by the typical fingerprint pattern of the evolution of transmission/intensity profiles. Colloidal stable dispersions (stable against flocculation, agglomeration) show typical polydisperse sedimentation behaviour, whereas flocculated dispersions exhibit zone sedimentation and gradual consolidation. Zeta potential did not always predict the state of dispersion regarding flocculation.

While the packing density for colloidal stable dispersions only slightly depends on the excess pressure acting on the sediment it strongly depends on pressure for flocculated dispersions, and it takes much longer to reach the corresponding equilibrium packing density. Instead of carrying out measurements at different accelerations it is also possible to evaluate the compressibility of sediments (or cream layers) by increasing the acceleration step-wise and recording the kinetics of the sediment (cream layer) height or sediment packing concentration. It was shown in the literature that based on these data filterability and compressive yield stress can be predicted.