Experimental performance evaluation of polymeric membranes for treatment of an industrial oily wastewater
Introduction
A recent research regarding generation of the oily wastewater in Iranian industries (e.g. Tehran refinery) indicates that there are over 100 plants in Iran discharging over 1000 m3/day each. However, some industrial oily wastewaters may be less in volume but contain much higher concentration of pollutants. Industries like oil refinery, petrochemical, oil distribution and textile have high levels of oil-grease in their effluents. Oily wastewaters which are daily produced in Iran's oil industries have become a big threat for water sources and environment that must be solved urgently.
Conventional approaches to treat oily wastewaters include gravity separation, API unit and skimming, dissolved air flotation, de-emulsion coagulation and flocculation [1], [2], [3]. The gravity separation process followed by skimming is fairly effective to remove free oil from wastewater. API unit has been widely accepted as an effective, low cost, primary treatment step. However these methods are not effective for removing smaller oil droplets and emulsions. The oil that adheres to the surface of solid particles could be effectively removed sedimentation. Dissolved air flotation process uses air to increase the buoyancy of smaller oil droplets for the enhancement of separation rate. The emulsified oil in the influent of dissolved air flotation process is removed by de-emulsification with chemicals, thermal energy or both. Dissolved air flotation units typically employ chemicals to promote coagulation and to increase flock size to facilitate separation [1], [4].
All these conventional systems based on physical and chemical principles indeed cannot give an absolute guarantee in terms of separation efficiency and effluent quality. However, high consumption of chemicals in coagulation makes these processes costly and even sometimes the chemicals not reacted are also found in the final wastewaters. Treatment of the oily wastewaters according to the environmental discharged standards (oil content less than 5 ppm) requires various oil treatment systems [5], [6], [7].
Membrane based separation processes, especially micro- and ultra-filtration processes are proving to be promising alternatives for conventional industrial separation methods, since they offer numerous advantages (e.g. high selectivity, easy separation, mild operation, continuous and automatic operation, economic and fast operation, as well as relatively low capital and running investment) [1], [3], [7]. Previous studies [8], [9], [10], [11], [12], [13] showed that treatment of domestic, industrial, oil-in-water and oily wastewaters using MF and UF processes satisfied the environmental standards and reuse of the wastewater.
The major problem in application of polymeric membranes for treatment of oily wastewaters is membrane fouling. The fouling of polymeric membranes typically forms by inorganic and organic materials present in the wastewaters, adhering to the surface and pores of the membranes and resulting in deterioration performance (reduction of the permeation flux) with a consequent increase of energy and membrane replacement costs.
In this work, oily wastewater of Tehran refinery was treated by micro- and ultra-filtration processes employing polymeric membranes. In addition, cleaning of the fouled membranes was investigated using chemical agents such as chelating and surfactants. Finally, the fouling mechanisms in the present cases were assessed by fitting the experimental data to various filtration-fouling models.
Section snippets
Feed
Synthetic and industrial oily wastewaters were used to investigate the performance of polymeric membranes. Oil-in-water emulsions were prepared by mixing commercial grade gas–oil (Tehran refinery, IRAN), deionized water and surfactant (Triton X-100 from Merck). The surfactant was dissolved in water 10 min before addition of gas–oil. Surfactant was added as emulsifier to the mixture to stabilize the emulsions. A blender was used to mix the mixtures at high shear rates (6000 rpm) for 30 min. The oil
Theoretical background
The ability of a simple cake filtration analysis to predict variation of permeation flux with time during cross-flow filtration has led to various fouling mechanisms to be proposed to better characterize the permeation flux performance. The various fouling mechanisms widely used are cake formation model, intermediate pore blocking, standard pore blocking and complete pore blocking [17]. By combining various developments on the filtration models [18], [19], [20], [21], [22], [23], various
Performance of micro- and ultra-filtration in oily wastewater treatment
Performance of the MF and UF polymeric membranes for industrial and synthetic oily wastewaters at the best operating conditions were investigated. Fig. 3 shows variation of permeation flux for the MF and UF polymeric membranes with time for the synthetic oily wastewater and the industrial oily wastewater. The results showed that permeation flux for the synthetic oily wastewater is higher than that for the industrial oily wastewater. This is obviously because the industrial oily wastewater has
Conclusion
In this work, treatment of oily wastewater with some polymeric membranes was investigated. According to the results, it can be concluded that micro- and ultra-filtration processes are feasible and advantageous methods for treatment of oil refinery wastewater.
The results indicated that the PAN (100 kDa) membrane performed suitable rejection (97.2% oil-grease content), high permeation flux (96.2 L/m2 h) and medium fouling resistance (60%) in comparison with the other membranes. The results also
Symbols used
A [m2] membrane area J [L/(m2 h)] filtration flux J0 [L/(m2 h)] initial filtration flux Jwi [L/(m2 h)] initial water flux Jww [L/(m2 h)] water flux after fouling Jwc [L/(m2 h)] water flux after cleaning Kb [1/s] complete pore blocking model constant Kc [s/m6] cake filtration model constant Ki [l/m3] intermediate pore blocking model constant Ks [1/s3] standard pore blocking model constant TMP [bar] transmembrane pressure RC [1/m] resistance of the chemically cleaned membrane Rf [1/m] resistance of the chemically fouled membrane t [s]
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