Crossflow microfiltration of low concentration-nonliving yeast suspensions
Introduction
Heavy metals in wastewater are hazardous to the environment and human health. The specific problem associated with heavy metals in the environment is due to the accumulation of the metals in the food chain and their persistence in the nature. In order to overcome the heavy metal pollutions, many processes have been developed. One of these processes is adsorption. In this process, metal ions from wastewater can be concentrated on the high surface area of substrates and subsequently filtered using a conventional crossflow filtration technique. These substrates include surfactant assemblies such as micelles or multi-lamellar vesicles [1]. It is also possible to use small particulate substrate for the same process.
It is well known that the yeast, Saccharomyces cerevisiae, can uptake heavy metals from aqueous solution [2], [3], [4], [5]. The technique of selective bioaccumulation of heavy metals by living yeast cells systems offers an alternative approach for water decontamination [6]. Another microbial system is biosorption and uses nonliving yeast cells, which can be obtained from numerous fermentation industries as an inexpensive carrier material. The recovery of yeast with bound metal ions from wastewater has been studied using crossflow microfiltration using dilute and concentrated yeast systems [2], [7], [8], [9], [10]. It was shown that crossflow microfiltration was an effective low-cost method for this purpose [7], [8], [9], [10]. At a previous study [7], it was seen that nonliving yeast cells formed aggregates at high yeast concentration. The efficiency of the metal removal did not increase with increasing yeast concentration which was a result of aggregation of yeast and therefore a decrease in the effective surface area. At another study [8], the effects of various metal ions on the transient and steady-state crossflow microfiltration behavior of dilute nonliving yeast cell suspensions were investigated. It was also observed that there was no visible cake on the membrane at low yeast concentration.
It is clear that the crossflow microfiltration behavior of nonliving yeast cells is important. There is no known study on the crossflow filtration of nonliving-low concentration yeast cells in literature. Many studies in literature have involved separation of microbial cells from broth in fermentation process. The crossflow microfiltration is an important technique to separate particulate matters from the aqueous solutions for various fields including biotechnology, water and wastewater treatment and also mineral processing. In fermentation processes, crossflow filtration are used to separate microbial cells from broth, however the permeate flux is still insufficient. Thus, understanding of membrane fouling mechanisms is important. There are many studies to clarify the membrane fouling mechanisms using direct observation technique in literature [11], [12]. Li et al. [11] investigated direct observation of particle deposition (yeast) on the membrane surface during crossflow microfiltration. Authors described that below a critical flux, the particle deposition was negligible; however, the particle deposition was significant near the critical flux and particle layers formed on the membrane surface. The particle size distribution of the deposit changed with the crossflow velocity: the smaller particles deposited on the membrane at higher crossflow velocity. Comparison of the normalized flux with the membrane area coverage by the particles revealed that the flux percentage was significantly less than the uncovered area percentage for filtration of yeast due to the deposition of smaller cell debris. Mores and Davis [12] investigated direct visual observation of yeast deposition end proposed a model for flux recovery.
As the feed concentration increases, the steady-state permeate flux decreases and may became zero. The feed concentration at zero flux is known as gel concentration (Cg). Cg is determined from a plot of permeate flux versus logarithm of feed concentration after extrapolating of the flux to zero. In some cases, this semi-logarithmic plot is not linear and the permeate flux tends to approach to a constant value, as observed in surfactant mediated metal ion removal [1] or during the electro-filtration of surfactant or macro-molecular dispersions [7], [13], [14]. In the non-linear variation, the extrapolation is based on the linear part of the plot; permeate flux versus logarithm of feed concentration and the extrapolated feed concentration is known as pseudo-gel concentration (). The pseudo-gel concentration has some unique properties. It does not depend on the processing conditions but is dependent on the membrane/solute interactions [15]. Therefore, the pseudo-gel concentration can be used to characterize the physical chemistry of the filtration system.
The purpose of the current work was to investigate factors affecting the specific resistance of non-visible and nonliving yeast cake as well as performance of the crossflow microfiltration of yeast suspension at low concentrations (0.1–5 g/l). On the basis of the experimental findings, the permeate flux was monitored during the microfiltration process. The effects of process parameters on specific cake resistance such as yeast cell concentration, membrane pore size, transmembrane pressure drop and crossflow velocity were investigated.
Section snippets
Materials
The yeast used in the experiment was supplied from Pakmaya Company (Izmit, Turkey). The cells were washed three times, centrifuged and dried at 80 °C for 6 h in order to deactivate the cells. This product is called as instant yeast. The cells were then suspended in distilled water. The conductivity of the suspensions was 20 μS/cm at 30 °C. The cell concentration was in the range of 0.1–5 g/l. The membranes (Schleicher and Schuell) were anisotropic cellulose acetate membranes with pore size of 0.2
Crossflow filtration
The relationship between J(t)–t, t/V–V and V–t are shown on the same graphic in Fig. 3 for crossflow filtration of yeast cells. Clearly, as seen from the variation of J(t) with t, the permeate flux decay, J(t), can be divided into three distinctive regions. These regions for the permeate flux decay are given as:
- (I)
Constant flux period (CFP).
- (II)
Rapid flux decay period (RDP).
- (III)
Slow flux decay period (SDP).
As seen in Fig. 4, when the concentration of yeast cell increased, the CFP became shorter. It could
Conclusion
The crossflow microfiltration of the low concentration-nonliving yeast suspensions was investigated in a crossflow membrane reactor. The effects of the yeast concentration, transmembrane pressure drop and crossflow velocity on the permeate fluxes were studied. Consequently, it was shown that it was not possible to apply a single filtration model to the experimental data at lower yeast concentrations. It was concluded that flux decline model fits the intermediate blocking model at the beginning
Acknowledgements
We are grateful to the British Council and Research Council of Ataturk University.
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