oa Cross-Flow Microfiltration of a High Strength Industrial Wastewater: Modelling of Membrane Fouling Mechanisms

Freshwater has become the blue gold of the 21st century due to its continuous depletion resulting from expanding population, shifting climates, urbanization and industrialization, and waterways pollution. Water reclamation is an orthodox alternative drought-proof source of water, and utility of membrane technology seems indispensable to every effective reclamation and reuse program due to its excellent features. However, fouling is a limiting factor of membrane separations, and in order to control particulate fouling at the design stage and plant operation monitoring, tools utilized in evaluating the particulate content of feed-water in predicting membrane fouling are vital. Subsequent to an adsorption process, a downstream cross-flow microfiltration (MF) membrane process was carried out on the final discharge effluent of palm oil mill industry at constant transmembrane pressures (TMPs) of 40 kPa, 80 kPa, and 120 kPa for 60 min filtration time using MF membranes of 0.1 and 0.2 μm pore sizes. Darcy's law and Hermia's blockage models were fitted to the experimental data and it was observed that cake filtration could be best used to explain the fouling mechanisms of effluent on the membranes based on the R2 values generated for the two MF membranes at all TMPs, however, not without the complete, standard and intermediate blocking mechanisms contributing at the onset of the filtration process. This demonstrates that the fouling was as a result of gradual reversible cake deposition which could easily be removed by less onerous cleaning methods. This data could serve as requisite for future upscaling of membrane processes for characteristically similar effluents with the aim of achieving sustainable fluxes.

Introduction

In the past few years, membrane filtration has been applied in the treatment of wastewaters especially in situations where cleaner water quality are sought. The populous utility of the membrane technology is due to its ability to stand up to scrutiny in providing uniformly quality water. Many of the wastewater treatment processes developed so far span across the use of sole membranes or with the integration of other treatment technologies be it for upstream or downstream treatment depending on the quality of the wastewater and the quality of permeate being pursued. However, there has always been the problem of membrane pore blocking or fouling. This phenomenon usually lead to the reduction of permeate flux which is equivalent to productivity. These blocking phenomena are usually categorized into four mechanisms; complete blocking, intermediate blocking, standard blocking and cake filtration. In order to control particulate fouling at the design stage, as well as for monitoring during a plant operation, methods utilized in evaluating the particulate content of feed-water in predicting membrane fouling are essential. The fouling tendency, usually represented by silt density index (SDI) and modified fouling index (MFI), involve a constant-pressure membrane filtration tests, thus, the indices are calculated from the experimentally determined relationship between filtration time and cumulative permeate volumes (Boerlage et al., 2002; Huang et al., 2008). In both tests, feed-water is filtered through a 0.45 μm microfiltration membrane in a dead-end flow at constant pressure. In order to accurately measure and predict particulate fouling, it has been proposed that specific MFI be investigated with respect to specific membranes since MFI0.45 cannot represent all membrane types. This is due to the fact that principal factors such as retention of smaller particulates, proof of cake filtration, membrane pore size, surface morphology, and material must be considered in such investigations (Boerlage et al., 2002). In this study, an optimized adsorption process operation condition was applied to a high strength wastewater (final discharge effluent of the palm oil mill) to reduce the strength of the wastewater and cut down its fouling potential. Fouling mechanism on microfiltration membranes after the upstream adsorption process was characterized at different TMPs, and the results were analysed in suggesting the sustainability of the membrane productivity and cleaning procedure. The results of the study could be applied to any feedwater of similar physicochemical make up.

Methods

Subsequent to an upstream adsorption process, explained elsewhere (Amosa, 2015; Amosa et al., 2015a; Amosa et al., 2015b), with the aim of reducing the blocking tendency of the industrial effluent, a laboratory-based bench scale system using a cross-flow membrane filtration process was operated. The system was equipped with two polyethersulfone (PESU) microfiltration membranes of 0.1 and 0.2 μm pores, and operated at constant transmembrane pressures (TMP) of 40, 80 and 120 kPa similar to previous reports (Springer et al., 2013). Permeate volume V at varying filtration time t data were recorded at each TMP, and the flux-time plots were used to evaluate the fouling propensities of the feed-water. The fouling mechanism of the feed on the membrane was further evaluated using the Hermia's revised blocking filtration models (Hermia, 1982) for complete, standard, intermediate and cake filtration as respectively represented in Eq. 1.0, 2.0, 3.0, and 4.0 were applied in describing and quantifying the blocking mechanisms that controls the membrane filtration at each TMPs. [1.0] [2.0] [3.0] [4.0] where Ac, B, Ai and C represent respective model constants, while J is the flux (Iritani, 2013).

Results and Discussion

Figure 1. Cake filtration model fitted to the 0.1 μm MF membrane filtration data at 40 kPa Fig. 1 shows the domineering cake filtration with the highest coefficient of determination (other plots shown in attached file). With this, it will be intuitive that the intermediate blocking model should be the closest in stability to that of cake filtration model from the principle, and this will serve as a confirmation for the establishment of cake layer formation in the membrane filtration. The closest R2 value after the cake filtration appeared to be that of the intermediate blocking mechanism with a value of 0.9336. This indicates that intermediate blocking had a strength of impact close to that of the cake filtration. This is because, from principle, there exist a region where the intermediate blocking plays a role between the transition phase of the complete and standard blocking, and that of the cake filtration mechanism. The intermediate blocking plot suggests that the transition phase is between 12 and 36 minutes of filtration time which serves as the best straight line of the plot. The complete and standard blocking models are expected to be the dominant mechanism at the onset of the filtration experiment. It is evident from their plots that the two mechanisms dominated the onset of the filtration as they both exhibited their somewhat best straight lines between 0.3-1.2 L, and 1.4-2 L of filtered volumes. This can also be observed from flux-time plot for 0.1 μm MF membrane in Fig. 1. These results followed the cake filtration mechanism as reported in earlier filtration modelling investigations (Boerlage et al., 2003; Boerlage et al., 2002). Table 2 shows the stability of the 0.1 μm MF membrane at various TMPs assessed by the determination of coefficients (R2) of the filtration models. Table 2. Fitting of filtration models to 0.1 μm MF membrane TMP (kPa) R2 of Filtration models Complete Blocking Standard Blocking Intermediate Blocking Cake Filtration 40 0.8357 0.8864 0.9336 0.9835 80 0.7521 0.8022 0.8676 0.9194 120 0.8032 0.8466 0.8967 0.9388 Fig. 2. Cake filtration model fitted to the 0.2 μm MF membrane filtration data at 40 kPa Table 3 below shows the results of filtration models as fitted to the filtration experiments performed using the 0.2 μm MF membrane. Table 3. Fitting of filtration models to 0.2 μm MF membrane TMP (kPa) R2 of Filtration models Complete Blocking Standard Blocking Intermediate Blocking Cake Filtration 40 0.6801 0.7537 0.8605 0.9167 80 0.5578 0.6346 0.7943 0.8388 120 0.561 0.6746 0.886 0.909 Summarily from Tables 2 and 3, it was observed that the operation at TMP of 40 kPa gave the best fitting models overall, and it also showed that cake filtration was dominant in the mechanism describing the filtration process with highest R2 values of 0.9835 and 0.9167 for 0.1 and 0.2 μm MF membranes, respectively.

Conclusion

A general model of cross-flow microfiltration treatment of final discharge of palm oil mill was successfully studied to elucidate the fundamental mechanisms involved in flux decline and fouling of the membrane. The cake deposition resulting from particulates aggregation accounted for the major flux decline. The cake filtration model dominated all other fouling mechanisms in terms of R2 values for both MF membranes and this indicates that the fouling involved did not affect the pore structure of the membrane. Both MF membranes were much stable at the lowest TMP of 40 kPa which amounts to lower energy consumption of the process. However, the 0.1 μm MF membrane exhibited higher stability in terms of its higher R2 value of 0.9835 as against 0.2 μm MF with a R2 value of 0.9167. Furthermore, with cake layer formation, it could be predicted that steady state filtration will be sustainably attained at longer filtration times without complete blocking. Also, fouling due to cake layer formation is reversible, this gives an idea of cleaning methods that may be necessary during process design. This data could be utilized for upscaling design in predicting the fouling behaviour of such effluents when subjected to microfiltration process.

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