A study of the effect of embedding ZnO-NPs on PVC membrane performance use in actual hospital wastewater treatment by membrane bioreactor

https://doi.org/10.1016/j.cep.2018.06.019Get rights and content

Highlights

  • New configuration of UCT-MBR for Actual hospital wastewater treatment.

  • Embedded of ZnO-NPs in PVC membrane resulted in a significant decrease in the CA value by 17.775°.

  • Cake layer was reduced from 52.8 to 10.42 μm and PWP is enhanced by 315%.

  • The long-term of the membrane in the UCT-MBR process was improved 500%.

  • COD removal efficiency of the UCT-MBR process was around 73.5% for all membranes.

Abstract

In this work, an anti-biofouling polyvinyl chloride/zinc oxide (PVC/ZnO) membrane was prepared using the phase precipitation method for application in a University of Cape Town membrane bioreactor-submerged membrane bioreactor (UCT-MBR) for treatment of actual hospital wastewater. Effects of various ZnO nanoparticle (NPs) amounts (i.e. 0.1, 0.2, 0.3, and 0.4 g) on membrane properties were studied. The hypothesis of this effort was that ZnO would act as an anti-biofouling material, thus overcoming the formation of a bio-cake layer on the PVC-ZnO membrane surface, which in turn greatly extends the long-term of the membrane. The performance of the PVC membranes with ZnO-NPs in a submerged membrane bioreactor (SMBR) was systematically investigated. The characteristics of PVC/ZnO membranes were inspected via scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle measurement, and pollutant removal efficiency. It was found that the ZnO nanoparticles clearly influenced the structural morphology of the membranes. The addition of 0.1 g of ZnO nanoparticles resulted in a significant increase in the mean roughness by about 140%, with smaller mean pore size and narrow pore size distribution. The addition of ZnO nanoparticles, up to 0.3 g, had a positive effect on the hydrophilicity of the PVC/ZnO membrane with decreasing the contact angle (CA) value by 17.775°. The pure water permeability (PWP) of the membrane improved by 315% with addition of 0.1 g of ZnO. The cake layer build-up on the membrane surface was reduced from 52.8 to 10.42 μm with an increase of ZnO nanoparticles up to 0.3 g, as 0.4 g ZnO had no further effect on the cake layer thickness. The long-term of PVC-0.3 g NPs was improved up to 70 days before membrane cleaning compare with 29 days for neat PVC membrane. Chemical oxygen demand (COD) removal efficiency of UCT-MBR process was approximately similar and around 73.5% for all membranes.

Introduction

Wastewater discharge containing abnormally high amounts contaminants, i.e., suspended or dissolved materials can lead to serious problems of water resources. Several separation techniques have been used and developed to remove these contaminants from wastewater, which is critical to environmental protection such as biological nutrient removal processes (BNR), activated sludge (CAS), and Membrane bioreactor (MBRs), etc. [[1], [2], [3], [4], [5]].

Among a challenges that affects the efficiency of MBRs process is membrane fouling, which decreases the permeate flux. This phenomenon imposes problems in the separation process efficiency and leads to frequent maintenance during membrane operations [6]. A variety of researches has been performed to overcome fouling issues. Several membrane modification methods have been widely practiced to improve membrane performance, i.e. development of composite membranes via interfacial polymerization, UV-initiated grafting, and incorporation of nanoparticles or antifouling agents [[7], [8], [9]].

During the phase inversion process and modify membranes structure and performance such as blending polymeric systems [10] and incorporation of nanoparticles in membranes [[11], [12], [13]] and membranes properties are a trade-off between thermodynamic enhancement and kinetic hindrance during the phase inversion process [14]. Different polymers have been used for membrane preparation via phase inversion for MBR application, such as polyethersulfone (PES) [[15], [16], [17]], polysulfone (PSF) [18], polyvinylidene fluoride (PVDF) [19,20], polyacrylonitrile (PAN) [21], polyvinyl chloride (PVC) [10,22,23], and cellulose acetate (CA) [24]. However, of these materials, PVC has attracted great attention because of its interesting mechanical and chemical resistance (to acids, halogens, or oxidants), lower cost, good stability at high temperatures and solubility in different solvents. The most important drawback of PVC is its hydrophobicity, which likely results in more fouling during ultrafiltration (UF). To resolve this problem, Peng and Sui [25] added poly (vinyl butyral) (PVB) to a PVC polymeric solution, and they observed improvement in surface hydrophilicity of the membranes, which indicates better fouling resistance compared to the neat PVC membranes. Mei et al. [22] found that increments of poly (vinyl pyrrolidone) PVP, poly(ethyleneglycol) (PEG), and sucrose to a PVC/dimethylacetamide DMAc system lead to a decrease in the thermodynamic stability of the dope solution in contact with water. In addition, they reported that the velocity of solvent-non-solvent exchange and membrane formation increases with these additives, and the increment of PVP needed is greater than that for PEG, thereby considerable changes in membrane morphology were observed due to changes in kinetic and thermodynamic properties.

To date, NPs made of numerous types of metal or metal oxides, such as silver (Ag), iron (Fe2O3, Fe3O4), silica (SiO2), aluminum (Al2O3), titanium (TiO2), magnesium oxide (MgO), and zirconium dioxide (ZrO2), have reportedly been used in membrane applications [9]. Some of these metal oxide NPs are quite expensive, and thus efforts have been focused on lower cost metal oxide options. One of the popular low-cost metal oxides is zinc oxide (ZnO), which has been used as a new alternative for titanium oxide [26]. ZnO-NPs are gaining increased attention in various industries such as biomedical, optics, and electronics and recently in the development of membrane technology, owing to their superb antimicrobial, anti-corrosive, and thermal and mechanical stability properties [27]. Studies have been reported on the incorporation of various concentrations of ZnO-NPs into different polymer matrices such as PSF, PES, and PVDF. These studies reported on the formation of membranes with improved membrane performances such as higher permeability, rejection capability, porosity, and hydrophilicity and enhanced antifouling properties [[28], [29], [30], [31]]. In light of the aforementioned issues, however, studies have reported the effects of nanoparticles on the several polymers are shown in Table 1.

For example, Alsalhy et al. [32] reported that the hydrophilicity, mean pore size and mean roughness of the Polyphenylsulfone PPSU/ZnO-NPs membranes were increased with the increase of ZnO-NPs concentration. The authors observed that with addition of 0.025 wt.% ZnO-NPs, no important change in solute rejection with significant enhancement in the PPSU-NPs flux (i.e., 76–107 (L/m2 h bar)). Hong and He [31], found that the hydrophilicity of the PVDF microfiltration with different concentrations of ZnO-NPs in casting solution (i.e., 0–1%) was considerable improved. Moreover, the results of Hong and He [36], showed that the hydrophilicity and the average pore diameter of PVDF membranes improved when the concentration of ZnO-NPs increased up to 1.5%. Also, they found that the COD removal efficiency reduced strongly when the concentration of ZnO-NPs surpassed 0.1% because of the increased pore diameter, whereas COD removal efficiency enhanced a little when the concentration of ZnO-NPS increased up to 1.5%, because of the hydrophilic character of the membrane.

In this work, PVC membranes were prepared to study the effects of various contents of ZnO-NPs as an inorganic additive on the performance of UCT-MBRs process for the first time for the treatment of actual hospital wastewater. The hypothesis of this effort was to use the ZnO as an anti-biofouling material to overcome the formation of a bio-cake layer on the PVC membrane surface, which in turn greatly extend the long-term of the membrane. Furthermore, in biological systems, microorganism contamination may take place in the membrane module, which affects the membrane performance and therefore the product quality. The efficiency of the flat-sheet membranes was measured in terms of permeation flux, and removal of chemical oxygen demand (COD) of the wastewater from an Iraqi hospital. The PVC/ZnO membrane structure was studied via scanning electron microscopy (SEM) and atomic force microscopy (AFM). In addition, modification of the hydrophilicity of the membrane surface was investigated using acontact angle measurement (Table 2).

Section snippets

Materials

The PVC resins (65 kg/mol) were obtained from the Georgia Gulf Company (Georgia, USA), and the DMAc solvent was supplied by Sigma-Aldrich, Germany. ZnO nanoparticles (99%, 10–30 nm, product no. 8411DL) were purchased from SkySpring Nanomaterials, Inc, company, USA.

Membrane preparation

The polymer material PVC was dried at 60 °C for 4 h in an oven device to remove its moisture content. The casting solution was prepared by adding dried 13 wt.% PVC to 87 wt.% DMAc solvent. After the PVC solution was homogeneous owing

Analysis of EDX

The EDX analysis was rather important for verifying the elements present in the membrane matrix, therefore all the membranes were examined for the presence of the zinc components. Fig. 2 shows the EDX photos of the PVC/ZnO NPs membranes prepared from various ZnO loadings of the casting solution. The corresponding Zn element mappings indicated scattering of nano-ZnO among the whole membrane. It can be seen that the best dispersion of the ZnO NPs in the membrane was obtained with addition of

Conclusions

In this effort, treatment of actual hospital wastewater was carried out by using UCT-MBR configuration process. A developed anti-biofouling PVC/ZnO flat-sheet membrane was prepared and used in an UCT-MBR. Based on this study, the following conclusions were made:

  • The addition of ZnO NPs in PVC casting solution changed the morphological structure of the membranes.

  • Addition of 0.1 g of ZnO NPs significantly increased the mean roughness by about 140%, with smaller mean pore size and narrow pore size

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