Use of submerged membrane technology for the treatment of olive mill wasterwater: fouling study and process performance

Abstract

The objective of this research work was to study the performance of an immersed (or submerged) membrane system for the treatment of vegetative waste water, coming from the production of olive oil (or Olive Mill WasteWater OMWW). To this end, a prototype has been built up on a bench‐scale capacity of 5 L, which employs a bundle of polymeric hollow fiber membranes. The approach of the study has been to divide the survey on three fronts: the first aimed at studying the waste water matrix in order to identify a pre‐treatment method capable of favoring the membrane filtration processes limiting the fouling (fouling ); the second involved the study of fouling by adsorption of the components present in the waters using different polymeric membranes, in order to identify the most suitable materials for the process; the third and final concerned the construction of the immersed membranes system and to the study of its performance as‐a‐function‐of‐process‐parameters. The chemical/physical analysis on the vegetation water evidenced range of values affected by different parameters related to the production of olive oil. Parameters such as the collection period, maturation of the fruit, the climate and soil can significantly vary the chemical concentration of a compound, which can become more than double in certain condition. This means that the effluent to be treated needs a flexible process to cope with such variations. One of the properties of the waste water that does not vary is the Z potential of the solution. The post production vegetation waters have a potential value of about ‐30 mV, which defines a stable solution, and the inability of the particles in solution to undertake processes of aggregation and or flocculation. On the basis of this finding it has been studied a treatment that provided for the destabilization of the solution to values of Z potential between ± 5 mV, in such a way as to favor the attraction between the particles in solution and subsequent sedimentation. Once removed the deposit of material, is obtained an effluent easier to treat with the processes of submerged membrane filtration compared to the original effluent. The interaction of the components that cause fouling on the membrane surface was studied using different membranes, which differed in composition of material and pore size. In order to understand the behavior of fouling, three different systems to put in contact vegetation water with membrane surface were used. The three different systems were intended to verify the different contributions to fouling by adsorpition of molecules on the surface of membranes and/or the intrusion of molecules within the pores due to the even minimum values of the hydrostatic pressure of the liquid column which is in contact with the membrane. By means of membrane ultrapure water permeability measurements, before and after contact with the waste water, and by morphological analysis of the surfaces of the same membranes, by atomic force microscopy (AFM), it was possible to define the degree of fouling and the mechanism for‐different‐types‐of‐membranes. The construction of a system immersed membrane system on a banch‐scale was obtained using hydrophilized polymeric hollow fiber membranes. The study of the influence of operating conditions on the efficiency of the process permitted to identify the parameters that make competitive the treatment of vegetable waste by means of immersed‐hollow‐fiber‐microfiltration. The membrane module was constructed with a bandle of about 50 polymeric hollow fibers of polyethylene having 0.4 μm pore diameter and the length of 20 cm. At the base of the module a system for the production of air bubbles was inserted connected to an air line with adjustable flow. The membrane module was installed inside a cylindrical tank with a capacity of 5 L and connected to an adjustable peristaltic pump. The lumens of the hollow fibers is occluded from the upper and immersed in the solution (in which the fibers are free to sway) while it is open from the end secured to the base of the module. The peristaltic pump creates a depression inside the fibers, which promotes the permeation of water through the membrane. A pressure gauge positioned along the connection line between the module and diaphragm pump measured the pressure downstream the fibers. The intent was to find a modus operandi that would allow the system to work continuously at a constant flow for 8 hours (the equivalent of a working day). The operating conditions studied include the influence of the transmembrane pressure, frequency and flow rate of air and the frequency of back‐flushing on the progress‐of‐the‐permeation‐flux‐over‐time. The studies have been conducted with various vegetative waters differing in pH and solids content. Results confirmed that a flexible system for the treatment of vegetative waste water was identified. In fact, the immersed membranes system was efficient in clarifying these waters in terms steady state permeate flux, product quality and reusability. It should be noted that the low transmembrane pressures employed determine a permeate flow through the immersed membranes lower compared to that usually obtained with side‐stream membranes. However, the lower power consumption and less tendency to fouling of immersed polymeric membranes makes the latter competitive for the first stage of water treatment with high pollutant load such as the vegetative waste water.