Fouling study of inorganic microfiltration membranes for protein solutions processing

Two microfiltration units were designed and constructed during this research. One was a dead-end strirred cell unit and the other was a tubular membrane crossflow unit, both with integrated backwash ability. Initial experiments on cross-flow microfiltration, in a pressure range of 2.5 to 30 psi, are conducted using tubular ceramic membranes of both 1.4 and 0.8 μm nominal pore size and whey protein concentrate solution. Although 1.4 μm membranes do not retain any protein, a possible pore constriction mechanism is identified, as well as cake formation during filtration with 0.8 μm membranes. A potential “optimum” pressure difference is found to be in effect during cross-flow microfiltration and is further investigated in a following set of customized experiments. Permeate flux decline in dead-end microfiltration of whey protein isolate solutions is studied, using disc-type ceramic membranes of nominal pore size 0.8 μm. The tests involve five successive filtration cycles, under fixed filtration pressure, with intermediate backwashing. Flux decline analysis and membrane resistance are employed to determine fouling mechanisms, with respect to applied pressure, in the range 2.5–10 psi. Data interpretation, suggests that more rapid fouling, as well as significant surface-layer “compaction” effects may occur at the higher pressures employed. There is evidence that irreversible fouling effectively develops during the first cycle of the tests (of 30 min duration) and apparently does not significantly increase later on; however, reversible fouling occurs through the entire filtration series, being more intense during the first minutes of each cycle. The effect of protein aggregates is investigated with the aid of DLS measurements and filtration tests with prefiltered solutions; it appears that whey protein aggregates present in the solution, are responsible (almost entirely) for the observed membrane fouling. Microfiltration of whey protein solutions by tubular ceramic membranes, under constant cross-flow and trans-membrane pressure, with periodic backwashing, is investigated using a fully instrumented pilot unit. Relatively large nominal membrane pore size (0.8 μm) insures very high protein transmission, which is desirable in applications such as microbial load reduction. In the first of a sequence of three filtration-backwashing cycles, irreversible and reversible fouling are identified, over the tested pressure range of 5 to 17.5 psi. Permeate and retentate concentration data complement the permeation rate measurements. Both irreversible and reversible fouling is identified for the backwashing mode employed. Early in the first cycle, especially at the higher pressures, a pore constriction / blocking mechanism appears to be responsible for the irreversible fouling. In the other two cycles only the reversible fouling is significant, possibly due to some kind of protein layer formation on the membrane surface. The permeate flux level tends to increase by increasing transmembrane pressure up to a near-optimum value of 10 psi, beyond which pressure has a negative effect. This interesting trend is attributed to the interplay of cross-flow velocity, which tends to reduce fouling by promoting re-suspension and breakage of colloidal protein agglomerates, with the trans-membrane pressure (and related flux) which leads to protein layer formation on the membrane and may impose compressive stresses, thereby increasing its resistance to permeation. An effective membrane cleaning procedure is proposed, combining both typical chemical treatment and ultrasounds. Evidence is also presented on the effect of isotropic nature of the membranes, especially during the first few minutes of filtration