Filtration analysis and fouling mechanisms of PVDF membrane for POME treatment

Palm oil mill ef ﬂ uent (POME) is a hazardous wastewater which contains high organic constituents and salt concentrations. The ultra ﬁ ltration (UF) process is a promising treatment design used for secondary treatment such as POME. However, membrane fouling is the major problem which limits the performance of the UF. This paper describes a detailed investigation of polyvinylidiene ﬂ uoride (PVDF) membrane for the treatment of POME. The fouling behavior was analyzed by water ﬂ ux, fouling mechanism, scanning electron microscopy (SEM), particle size distribution (PSD) and Energy Dispersive X-ray (EDX). It was found that a signi ﬁ cant reduction in the permeate ﬂ ux was caused by the build-up of a fouling layer. Study on the fouling mechanism shows that cake ﬁ ltration dominated the fouling activities on the membrane surface, compared to standard blocking, intermediate blocking, and complete blocking. This result is supported by membrane autopsy through SEM, PSD and EDX.


GRAPHICAL ABSTRACT INTRODUCTION
Malaysia is known as one of the largest exporters of palm oil with an average production of crude palm oil of more than 13 million tonne per year (Subramaniam et al. ).
However, the production of wastewater from the palm oil mills, known as palm oil mill effluent (POME), contributes to the highest pollution load that is discharged into rivers all over the country (Taha & Ibrahim ; Ghani et al. ). There are three major processes contributing to the production of POME which is hydrocyclone, sterilizer condensate, and oil clarification in a ratio of 1:9:15, respectively (Wu et al. ). POME is a thick brownish colloidal with a mixture of oil, suspended solids and water, that is discharged at a temperature of 80-90 C, and considered as a non-toxic wastewater because no chemicals were used during the extraction processes (Alrawi et al. ). The properties of POME are 4-5% of total solids, 0.6-0.7% of oil and grease and 95-96% water. Even though POME is a non-toxic wastewater, it contains soluble elements with different types of liquids, dirties, residual oil and suspended solids that are very unsafe to the environment, either in the form of soluble gases (such as ammonia (NH 3 ), sulphur dioxide (SO 2 ) and methane (CH 4 )), soluble solids or liquids, with concentrations exceeding the threshold limit values (Mohammed & Chong ). When the water quality declines due to untreated wastewater such as POME, it often leads to outbreaks or waterborne infectious diseases (Pons et al. ). It is noted that 85% of the palm oil mill in Malaysia uses a ponding system as a treatment method for POME, but it requires a high retention time and large area of land (Abdurahman & Azhari ).
The application of membrane separation processes have increased significantly in recent years and it has been adopted in many industries because of its ability to treat water and wastewater efficiently, it is economical and easy to operate (Shad et al. ). Generally, a membrane can be identified as a thin layer of semi-permeable material which separates substances and allows one part of a mixture to permeate the membrane, while inhibiting the other when a driving force is employed (Hai et al. ). Up to now, there are four different pressure-driven membrane types used in water and wastewater treatment, namely microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). UF is a cross-flow membrane process which is able to eliminate all microbiological species, viruses and humic materials (Daisuke et al. ). Previous studies show that UF membranes are able to treat wastewater and maintain the turbidity value of the effluent below 0.1 NTU, and also eliminate particulate metals such as iron and manganese effectively (Ioannis et al. ). In the treatment of bilge wastewater, Grypta et al. () found that UF membranes are able to reduce the oil content below 5 ppm and completely remove the oil pollutants when the second stage of filtration is applied. In the treatment of oil-water/ wastewater, the UF membrane has been validated as a promising filtration technique on account of the appropriate pore sizes (usually in the range of 2-50 nm) and the ability of eliminating emulsified oil beads with no de-emulsification forms (Racar et al. ).
In the treatment of wastewater using UF as a filtration system, PVDF has been chosen to fabricate UF membranes compared to other polymers because of strong chemical resistance and mechanical properties (Kim et  that PVDF with a surface area of 12.56 cm 2 and an influent concentration of 224.5 mg/L was able to treat 13.9 L of lead contaminated water (equivalent to 73,000 of bed volumes) that meet the maximum contaminant level of 15 mg/L. It shows that the PVDF membrane was able to remove lead with significant results and had a better reusability in its applications (Zhao et al. ). The use of UF membrane is very effective in water treatments, however the major drawback of the system performance is the fouling behavior during the treatment processes (Kasi et al. ; Lee et al. ). Membrane fouling can be defined as the accumulation of foulant on the surface or pore of the UF membrane. In terms of fouling mechanism, basically there are four types of fouling model which have been used to analyze the fouling of membranes by different types of water reclamation with complex compositions (Huang et al. ). Those four types of fouling models are cake filtration, standard blocking, intermediate blocking and complete blocking. If the particle size of the foulant is larger than the size of the membrane pore, the deposition of this particle on the membrane surface will contribute to the growth of cake layer. On the other hand, if the particle size of foulant is smaller than the size of membrane pore, particles can enter the membrane pore and block the pores. The fitting equation of a fouling mechanism are shown in Table 1 (Iritani ). The main objective of this study was to investigate the PVDF performance in the treatment of POME and fouling behavior during the treatment processes. The equation in Table 1 was used to determine the type of fouling mechanism. A 0.0125 μm pore size PVDF membrane with a molecular weight cut-off (MWCO) of 200 kDa was chosen for the treatment process. The sample used was from the final discharged pond effluent of palm oil mill.

MATERIAL AND METHODS
Feed solution POME was obtained from the Seri Ulu Langat Palm Oil Mill, Dengkil, Malaysia. The effluent sample was collected at the point of discharge to the anaerobic ponding system. Initially, the visual inspections were carried out at multiple points of the sampling site in order to evaluate the presence of a constitution, sample color, odors, etc. After collection, the sample was then tested for COD, TSS, pH, color and turbidity according to the standard methods. The sample was then restored at 4 C for further use. The percentage Particles deposit on each other and settle down on the surface of membrane, blocking the membrane pore and form a cake layer Small particles attach on the interval walls of the membrane pores Intermediate blocking Approaching particles contact with the existing particles on the surface of membrane and block the membrane pores Complete blocking Q ¼ Q i À K c V Particles deposit on the membrane surface that is larger than the membrane pores a K c is blocking filtration constant, V is permeate volume, Q is the volumetric permeate flow rate, and Q i is the initial volumetric permeate flow rate.
reductions of the sample parameters were calculated by the following equation: where C b is the concentration of the permeate solution and C a is the concentration of the feed sample.

UF materials and procedures
The filtration system was connected to two tubular crossflow UF membrane modules (

Investigation on membrane fouling
The fouling experiments were carried out using a laboratory scale cross-flow membrane filtration unit as shown in   Table 1 were performed in describing and quantifying the fouling mechanism controlling the membrane processes.
At the end of each experiment, each of the fouled membranes were taken out for further studies.

Fouled UF membrane analysis
After fouling, the UF membrane was taken out from the membrane module. The membrane surface was wiped with a plastic sheet to physically get rid of the cake from the membrane surface, and later this fouled membrane was soaked in an alkaline solution (1M NaOH) for 24 h in order to complete the desorption of the remaining foulants.

SEM analysis
After the experimental works, a small part of the membrane was cut from the middle for SEM analysis. Before the viewing procedures, the samples were coated with gold using a sputter coater (BAL-TEC SCD005; Bal-Tec Co., Balzers; Vaduz, Liechtenstein). Later, the coated membranes were analyzed using a scanning electron microscopy (Hitachi S-3400N, Tokyo, Japan).

EDX analysis
The chemical compositions of the fouling layer were analyzed using an EDX spectrometer (Thermo electron corporation Instrument, USA) attached to an SEM (Hitachi S-3400N, Tokyo, Japan).

Membrane cleaning
The cleaning procedure was executed at the end of each experiment for the prevention of membrane drawback such as fouling and compaction. The membrane module was circulated with distilled water in order to flush out the remaining POME in the membranes. Later, the membrane was circulated with a chemical solution mixed with 1% w/w NaOH and 0.6% w/w NaClO for 30 min. The procedure was performed at room temperature with a constant pressure of 100 kPa and crossflow velocity at 1 m/s. Afterwards, the membrane was soaked in an acidic solution (HCl 0.5%) for 24 h, and later washed again with distilled water for 30 min. Finally, the water filtration of the cleaned membrane was calculated before and after cleaning for the verification of the cleaning effectiveness.

RESULTS AND DISCUSSION
Characteristic of POME As shown below, Table 2 presents the POME samples which were collected from Seri Ulu Langat Palm Oil Mill, Dengkil, Malaysia. It is noted that the POME samples collected from this mill contain high COD, turbidity, TSS, and the color was also unsatisfactory.

Effect of applied pressure on permeate flux
The investigations on the relationship of flux-pressure are presented in this section. The effect of applied pressure on the filtration performance with respect to the POME treatment is given in Figure 2.
As observed in Figure  This finding was potentially a result of the formation of a

Mechanism of membrane fouling
The ability to predict the fouling phenomenon on the UF membrane could be a key factor which can reduce or control fouling behavior during a plant operation or running at a design stage. This is because many factors may affect the existence of membrane fouling such as retention of smaller matters, formation of cake layer, plugging on the membrane pore, and concentration polarization (   As shown from the plotted graphs above, the cake filtration was governed by the fouling mechanism of PVDF membrane with the greatest value of R 2 coefficient. The data from Table 3   on the fouling mechanism and found that the cake filtration was proven to be the dominant mechanism. The studies carried out by Said et al. () found that the optimum conditions of the UF membrane to treat POME were at applied pressure of 5 bars, 40 C and pH of 9.05. At this stage, they also noticed that the cake layer was the best fit for the fouling mechanism with a R 2 value of 0.974, followed

Scanning electron microscopy (SEM) analysis
The SEM images of the surface of the PVDF membrane specimens were taken in order to describe the morphology on the membrane surfaces, which are given in Figure 8.
The comparison of the new membrane and fouled membrane can be seen through the SEM images which explain the fouling phenomenon that occurred on the surface of the membranes.
In the SEM images of Figure 8

Particle size distribution (PSD) analysis
The PSD measurements of foulant liquid of the PVDF membrane are presented in Figure 9. Based on Figure 9, the particle size distribution analysis showed that the particles of the foulants were dominated by the particle size in the range of 10-50 μm with a PSD percentage of 47.39%. This data indicates that smaller particles tend to foul or deposit on the membrane surface compared to larger particles. Generally, one of the main factors that affected the fouling of the membranes depended on the particle size (Bae & Tak ). This result was also in agreement with Hwang et al. () as they monitored smaller particles which tend to accumulate on the membrane pore compared to larger particles.
Similar results were also reported by Liu et al. () as they had studied the effects of particle-size based foulants which caused the fouling phenomenon in the treatment of polluted raw water using UF membrane.
Energy dispersive X-ray (EDX) analysis The chemical compositions of the fouling layer were analyzed by the EDX analysis as shown in Figure 10.

Analysis on membrane cleaning
The application of chemical cleaning was able to bring back the quality of UF membrane used in this project to the initial flux with 95% restoration of distilled water flux. It was also noticed that the NaOH solution was able to control the fouling of the UF membrane for the next filtration processes into its initial active performance with flux recovery (FR) higher than 90%. The study by Madaeni et al. (2001) proved that the application of NaOH as a chemical cleaning agent for fouled membrane by inorganic matters had recovered the flux efficiently. Hence, the application of cleaning procedures on the UF membranes after the filtration process may lengthen the membrane lifetime, reduce maintenance operation and also repair cost.

CONCLUSIONS
The aim of this research was originally to study the effect of PVDF membrane in the treatment of POME. The laboratory work of filtration experiments provided excellent results in the permeation flux rate and removal of particulates which are present in POME. Study on membrane fouling found that an increase in applied pressure can seriously affect the membrane condition and continuous treatment may cause a reduction of the membrane performance. The rejection of pollutants from the membrane may also decrease as the fouling layer becomes the membrane protection and blocks other pollutants that come through.
Based on the fouling model, SEM, PSD and EDX analysis, the fouled membranes has been dominated by the cake layer which was caused by small particles (ranges of 10-50 μm) from POME as they tend to accumulate on the membrane pores compared to larger particles. However, further treatment by membrane cleaning may return the membrane to the initial performance. Hence, further studies should get more attention to increase our knowledge on the relation between fouling behaviors and membrane performance, especially in the treatment of POME.