Journal of Membrane Science & Technology

Reuse of wastewater is an essential part of sustainable urban development due to growing world population and rapid industrial development. Membrane processes have been of particular interest among the various technologies for wastewater reuse [1]. The membrane technologies have been applied in diverse areas of water treatment and reuse of urban wastewater or as pretreatment in reverse osmosis systems due to their various advantages including smaller footprint, high solid liquid separation efficiency, and their compactness [2-4]. Additionally, there are several techniques which can be used in conjunction with membrane separation in order to achieve sustained process intensification [5].

Despite the advantages, of membrane processes, the fouling limits its wider application. Membrane biofouling reduces performance of the process and increases the operation and maintenance cost by causing severe flux decline that requires frequent membrane cleaning or replacement [6]. Various mechanisms were reported in the literature for the fouling of membranes. The membrane biofouling occurs not only because of pore blocking but also microorganism abrasion of membranes that causes the formation of mature biofilms on the membrane surfaces [7]. Some previous studies have shown an important relationship between membrane fouling and presence of Extracellular Polymeric Substances (EPS) fraction of sludge supernatant. EPS forms complex slime layers on membrane surfaces or pores and serve as a condition affecting membrane attachment of other biomasses [8−10].

Ultrafiltration (UF) has been used to improve the effluent quality from municipal wastewater treatment plants for reuse applications. Low–pressure ultrafiltration (UF) and microfiltration (MF) membranes can significantly improve the quality of domestic wastewater for levels which would be acceptable for discharge to surface waters or reuse. As simple and reliable treatment process membrane filtration of domestic wastewater is of increasing importance coping with the problems of water scarcity while the treated wastewater can be utilized as a reliable water resource [11]. However, the decline of productivity in low− pressure membrane filtration due to fouling phenomena is a major drawback and limits its application [12,13]. Although the average pore size of biological suspension is much greater than the membrane pore size, there are smaller colloids in the mixture that cause severe pore blockage and fouling. The pore fouling increases as the pore size of the membrane increased. Therefore, relatively smaller pore sized UF membranes was tested to reduce the pore fouling which is relatively more problematic in recovery of initial flux after simple backwash.

The main objective of this study was to investigate the biofouling of various ultrafiltration membranes with different pore sizes using activated sludge suspensions. A cross flow ultrafiltration (CF-UF) system was used to separate the solid and liquid phase. Two different membrane materials (cellulose and polyethersulfon) with three different pore sizes (5, 10, 30 kDa for UC and 5, 10, 20 kDa for UP) were used in the experiments. The effects of the membrane types and pore sizes on the filtration flux and soluble microbial products (SMP) as protein and carbohydrate rejection were investigated. SMP rejections by the membranes were also investigated by analyzing the supernatant with time. Morphological assessments of the membranes were performed by Atomic Force Microscopy (AFM).

Experimental set−up

Activated sludge with Mixed Liquor Suspended Solids (MLSS) of around 3400 ± 250 mg/L was obtained from a pilot activated sludge plant operated in our labs. The pilot plant was fed synthetically with COD content of 650 mg/L. A cross flow ultrafiltration unit was used in the filtration experiments. The detailed information was given in another study [14].

Two types of membranes (cellulose and polyethersulfone) were used in the experiment each with various MWCO (5, 10, 30 kDa for UC and 5, 10, 20 kDa for UP). All membranes were obtained from Microdyn Nadir GmbH. Membranes were rinsed with distilled water prior to use. The properties of membranes used in the experiments are given in table 1.

Table 1: Ultrafiltration membrane properties.

Physico−chemical analysis

Measurements of Chemical Oxygen Demand (COD) and mixed liquor suspended solids (MLSS) were performed according to APHA Standard Methods [15]. However, soluble COD samples of biological suspension were obtained by filtering the mixed liquor through a filter paper (cellulose acetate) with mean pore size of 0.45 μm.

The measurement of protein content was carried out according to Lowry methods [16]. BSA was used as a standard and the results expressed in mg equivalent of bovine serum albumin (BSA) per liter. Polysaccharides were determined of the phenol−sulfuric acid method of Dubois et al. [17]. Glucose was used as a standard and the results expressed in mg equivalent of glucose per liter. All samples determined the concentrations using a UV–vis spectrophotometer (GBC−Cintra−20) at the wavelength of 660 nm for protein or at the wavelength of 490 nm for polysaccharide

Atomic force microscopy (AFM, Digital Instruments) was employed to determine the surface morphology of the ultrafiltration membranes. Before AFM observations, both the used membranes and clean one were gently washed with deionized water, followed by drying at room temperature. The membrane samples were fixed on a slide glass and scanned over 10.0 μm×10.0 μm. AFM was performed under tapping mode with a scanning rate of 6.104 Hz. Obtained data were analyzed with the software of Nanoscope 3.0; images were in the height mode.

Flux behaviors of the UF membranes

The flux values of activated sludge filtration obtained from CF-UF process for UC and UP membranes with various MWCO are shown in figure 1 as a function of time. The initial and pseudo steady-state flux values for each membrane are also presented in table 2. UC 030, UP 020 and UP 010 showed rapid declines in the flux; whereas, UC005, UC010 and UP005 membranes yielded almost constant flux over time. The most fouling was observed in UC030 membrane for which the initial flux and the final flux values were 205 and 89 L/m²/h, respectively. Higher porosity caused greater initial flux that transported colloids and SMP fractions to the surface filing up the pores or pore openings and causing more fouling. Flux values decreased from 124 to 83 L/m²/h for UP010 and 150 to 96 L/m²/h for UP020. After the initial drop, the flux values gradually reduced and reached a pseudo steady state condition within few hours. In this state, attachment of biofoulants to the surface of membranes was equilibrated with detachment of biofoulants from the membrane surface because of the shear force by cross flow velocity and back diffusion by concentration gradient [18]. Almost no drop was observed in flux values of the membranes of UC005, UC010 and UP005, indicating that almost no fouling were occurred for these membranes. This was likely a result that the relatively smaller pore openings did not allow foulants to accumulate in the pores or block the pore openings.

Table 2: Values of initial and steady-state flux for different UF membranes.

Figure 1: Variation of fluxes with time at different membranes with a different MWCO (ΔP: 100 kPa, MLSS: 3400 mg/L).

Despite the low fouling, the steady state permeates flux values for UC005, UC010 and UC030 were 26, 48 and 89 L/m²/h, respectively. However, the steady state permeates flux values for UP005, UP010 and UP020 were 23, 83 and 96 L/m²/h, respectively (Table 2). As expected, the larger pore sized membranes had the greatest steady-state flux. UP 020 yielded the greatest steady state flux value followed by UC 030, UP 010 and UC 010, while UC 005 and UP 005 membranes had the lowest steady state flux values. The difference between UC 005 and UP 005 membranes was relatively insignificant. The difference among membrane materials was a result of morphological differences. The roughness and the pore structure are important factors.

SMP rejection versus of time

The UF membrane filtration efficiencies with different MWCO were evaluated versus of SMP removal as soluble carbohydrates (SMP_c) and soluble protein (SMP_p). Figures 2 and 3 present the SMP concentrations in the UC and UP membranes filtrate, respectively. It can be seen clearly from the figures 2 and 3, both protein and carbohydrate contents of permeates decreased throughout filtration time. Both UC and UP membranes retained the carbohydrate content slightly than protein (Figures 2b and 3b). The decrease in carbohydrate was similar in all membranes. Reduction in both protein and carbohydrate concentrations suggested the seperation of these products on the membrane surface. This is most likely a result of accumulated foulants (bacteria, EPS etc.) on the surface of the membrane acting as a secondary membrane. The soluble carbohydrate levels in the filtrates from the UC membranes ranged from 0.08 to 0.77 (Figure 2a) and for the UP membranes they were between 0.40 and 0.55 (Figure 3a) after 4 h filtration. The relative soluble protein levels in the filtrates from the UC membranes ranged between 0.23 and 0.60 (Figure 2b) and for the UP membranes it ranged from 0.46 to 0.88 (Figure 3b) after 4 h filtration.

Figure 2: SMP changes with time in the UC membrane permeates (a. SPM_c and b. SPM_p rejection with time).

Figure 3: SMP changes with time in the UP membrane permeates (a. SPMc and b. SPMp rejection with time).

Morphological characteristics

The surface roughness is an important parameter for membrane studies and it may influence the degree to which the foulants interact with membrane surface [19]. The AFM images of fouled and unfouled UC 010 and UP 010 membranes for cross flow filtration are presented in figure 4. Slight changes in surface morphology were observed after cross flow filtration. The mean roughnesses (Ra) of membrane surface are presented in table 3.

Table 3: The mean roughness (Ra) values of new and fouled membranes (10.0 μm×10.0 μm surface area).

Figure 4: AFM images of the unfouled and fouled UC 010 and UP 010 membranes.

A laboratory scale cross flow ultrafiltration system was used to investigate the performance of removing of soluble microbial products (SMP) (as proteins and carbohydrates) and fouling mechanisms. The flux decline behavior of activated sludge filtration were studied using two different membrane types (UC and UP) with three different MWCO (5, 10, 30 kDa for UC and 5, 10, 20 kDa for UP). It was found that UC membrane with MWCO 30 kDa and UP membrane with MWCO 20 kDa showed the most rapid decline in the flux among all membranes. Almost no fouling was observed in UC005, UC010 and UP005; however, flux values of these membranes were lower than the other three membranes due to their smaller pore sizes. It can be concluded that fouling may be reduced by reducing the pore size (or MWCO) of the membranes.

There was significant difference in the removal effectives of SMP for both the UC and UP membranes with different MWCO characteristics. Morphological examination of the membranes by AFM showed a little change was occurred on the membrane surface morphology. The results indicated that the UF-CF filtration may be used effectively as an external submerged membrane bioreactor.

This study was financially supported by the TUBITAK, the Scientific and Technological Research Council of Turkey (Project No: 108Y129).