Clogging vs. fouling in immersed membrane bioreactors

Abstract

Whilst the fouling of MBR membrane surfaces has been very extensively explored by the academic community, there is an increasingly widespread recognition by practitioners of the issue of clogging of membrane channels with sludge solids, sometimes termed "sludging". The study undertaken has quantified this phenomenon using a bespoke test cell allowing a flat sheet membrane channel to be viewed directly during operation and the accumulated solids determined by digital image processing. Sludging behaviour has then been correlated both with the sludge properties, from sludge samples taken from both an industrial and municipal MBR, and the permeability decline rate data. The work has revealed the expected trends in fouling propensity, as quantified by the exponent n of the Δp/Δt = m.exp(nJ) correlation from classical flux-step tests. With zero membrane aeration the industrial samples exhibited sludging, the filling of the complete thickness of the membrane channel with sludge solids, whereas for municipal sludge the solids formed a cake layer which did not fill the channel. In the absence of sludging the permeability decline followed the expected pattern of increasing at the elevated soluble COD and capillary suction time values of the industrial sludge, compared with municipal sludge at the same solids concentration range (8-12 g.L). However, there was no evident correlation between fouling (permeability decline without sludging) and sludging: incipient sludging did not appear to influence permeability, though can be assumed to negatively impact on long-term operation, or relate to the sCOD concentration. Sludging instead appeared to depend on the sludge physical properties, and primarily the viscosity: sludge samples at high viscosities were found to exhibit a different air-scour pattern to that at normal MLSS concentrations. Outcomes suggest that sludging is caused by rheological conditions promoting bubble coalescence and bubble stream constriction, reducing the exposure of the membrane surface to scouring air.

The Impact of Mechanically-Imposed Shear on Clogging, Fouling and Energy Demand for an Immersed Membrane Bioreactor

“…Results revealed the viscosity of the sludge, as determined at a shear rate of 120 s −1 in accordance with the benchmark used previously [10], to follow an exponential relationship (Figure 4). This is in keeping with trends reported in a number of previous rheological studies [13,14,15].…”

“…A single A4-sized 6 mm-thick FS panel (Kubota Membranes Europe, London, UK) was used to complete the testing, the complete test cell being based on what has recently been reported [10]. The cell (Figure 2a) comprised a ~500 mm-tall rectangular acrylic tank (130 × 245 mm) of around 20 L total volume.…”

“…The membrane was placed in a conduit formed by a baffle between the tank and the outer cell wall, forming a 6 mm channel either side of the membrane. The membrane surface could then be viewed directly through the test cell wall, allowing sludging (i.e., the accumulation of solids filling the channel) to be measured by digital image analysis [10]. The non-visible side of the membrane was sealed so as to direct the permeate flow solely through the viewable side.…”

“…Filtration behavior was monitored according to the change in transmembrane pressure (TMP) and clogging propensity with time, the latter being recorded as the percentage coverage of the membrane with agglomerated solids ( %C , in h −1 ) according to the method of Buzatu et al [10]. Sludging was equated to the area of the membrane occluded by the sludge solids, as viewed through the transparent wall of the membrane test cell (Figure 2a).…”

Development of Polyvinylidene Fluoride Membrane by Incorporating Bio-Based Ginger Extract as Additive

“…However, physical treatment alone is insufficient to restore the membrane performance from the irreversible fouling, while aggressive chemical treatments using strong acidic or alkaline solutions can potentially damage the membrane material and thus shorten membrane lifetime [ 8 ]. Overall, biofouling phenomena increase the operational and capital costs of a membrane system due to the increase in energy consumption (for biofouling control) and membrane chemical maintenance [ 9 , 10 ]. Therefore, the method for effective control of biofouling must be implemented to prevent severe permeability loss.…”