The importance of pretreatment tailoring on the performance of ultrafiltration membranes to treat two-phase olive mill wastewater

In this work, the performance of an ultrafiltration (UF) membrane in the treatment of the effluents by-produced by olive mills is addressed by applying different pretreatments on the raw effluents. By conducting a photo-catalytic process (UV/TiO2 PC) after pH-temperature flocculation (pH-T F) higher threshold flux values were observed for all feed stocks than by applying solely the pH-T F process, with an 18.8–34.2% increment. In addition, the performance of the UF membrane was also improved in terms of rejection efficiency, such that higher rejection values were yielded by the membrane for the organic pollutants (RCOD) by 48.5 vs. 39.9% and 53.4 vs. 42.0%. The UF membrane performance was also improved in terms of the volume feed recovery factor (VFR), achieving up to 88.2 vs. 87.2% and 90.7 vs. 89.3%. Results in the same line were also observed when the highly polluted olives oil washing wastewater raw stream was previously mixed with the effluent stream coming from the washing of the olives. This permits the UF to permeate, achieving the standard limits to reuse the purified effluent for irrigation purposes (COD values below 1000 mg·L−1), which makes the treatment process cost-effective and results in making the olive oil production process environmentally friendly.En este estudio se aborda el rendimiento de una membrana de ultrafiltración (UF) para el tratamiento de los efluentes generados por la industria oleícola, mediante la aplicación de distintos pretratamientos. Tras aplicar un proceso fotocatalítico (UV/TiO2 PC) después de una floculación pH-temperatura (pH-T F) se observaron flujos límite para todos los efluentes mayores que tras la aplicación únicamente del proceso pH-T F, con incrementos del 18.8–34.2 %. Además, el rendimiento de la membrana de UF mejoró en términos de eficiencia de rechazo, con mayores valores de rechazo respecto de los contaminantes orgánicos (RCOD), 48.5 vs. 39.9 % y 53.4 vs. 42.0 %. El rendimiento de la membrana mejoró también en términos de recuperación de volumen de alimentación (VRF), alcanzando hasta un 88.2 vs. 87.2 % y 90.7 vs. 89.3 %. Se observaron resultados en la misma línea cuando las aguas residuales del lavado del aceite, altamente contaminadas, fueron previamente mezcladas con el efluente generado en el lavado de las aceitunas. Esto permite que el permeado de la UF cumpla con los límites estándar para la utilización del efluente para riego (valores de la DQO inferiores a 1000 mg L−1), favoreciendo la eficiencia económica del proceso de tratamiento y permitiendo que el proceso de producción del aceite de oliva pueda ser respetuoso con el medio ambiente.The membrane pilot plant was constructed in the framework of the European project PHOTOMEM (contract no.FP7-SME-2011, grant 262470) and revamped under the European project ETOILE (contract no. FP7-SME-2007-1, grant 222331). Funding by the EC is gratefully acknowledged. The Spanish Ministry of Science and Innovation is also gratefully acknowledged for having funded the projects CTQ2007-66178 and CTQ2010-21411, as well as the University of Granada

Optimal design of membrane processes. A problem of choices between process layout, operating conditions and adopted control system

The development of membrane processes as a technology for environmental treatment applications and in particular for the purification of wastewater streams has significantly increased in the last decades. Fouling on membranes appears to be one of the main technical limit of this technology. This phenomenon causes the unavoidable deposition of particles on the membrane surface, building a resistive growing layer to permeability. Sensible fouling of the membrane leads to a significant reduction of the performances, a decrease of the operating life and, as a consequence, the increase of the operational costs due to the replacement or cleaning of the exhausted membrane modules. The presence of the fouling phenomena makes the proper design and control of membrane systems a difficult task. Optimal design of the membrane processes will be here discussed. The procedure requires to determine the optimal process layout given the input data and target requirements. At the end, the required membrane area is calculated. This latter property is strictly dependant of the adopted operating conditions, most importantly by the adopted value of transmembrane pressure (TMP). Moreover, it depends if the value of TMP remain fixed as a function of time or is variable (as in case of fixed permeate flow rates). Therefore, the optimal design of the system may occur only if the adopted control strategy is defined a priori. As a consequence, design choices of the membrane process layout, operating condition and adopted control system are strictly dependant, and connections between these different aspects should not be neglected during the engineering and P & I development stage of membrane systems. This paper will start from the theory of the boundary flux, in order to describe a novel design approach to membrane systems. Parallel to this, the development of an advanced control system, that allows to limit fouling formation during operation, is presented. The advanced control system relies on a suitable simulation software capable to predict the boundary flux, that changes the controller's set-points accordingly. Finally, the paper will merge all elements together, and report about the optimal design of membrane processes equipped with the advanced membrane process control system; validation of the proposed approach will be based on the use of a custom simulation model in ASPEN HYSYS and by experiments on lab scale

Optimization study of the fouling build-up on a RO membrane for pretrated olive mill wastewater purification

Even though membranes are considered in many aspects a mature technology, a range of features are still in development and under investigation. Regarding this, the main handicap of this technology is inevitably membrane fouling. Fouling issues have investigated by many research groups in the last years to convince investors to implement membranes as substitutes of a range of unit operations at industrial scale. In the wastewater treatment field, this is especially problematic, given the low economic value of the product, that is, treated water. On another hand, the management of the effluents generated by olive oil industries, olive mill wastewaters (OMW), is a task of global concern not anymore constrained to a specific region. These wastewaters represent an ever-increasing problem still unresolved. The present work was aimed for the modelling and optimization of a reverse osmosis (RO) membrane operation for the purification of pretreated olive mill wastewater, with a focus on the dynamic fouling development minimization on the selected membrane as a function of the set-up of the operating conditions. For this goal, beforehand a factorial design was implemented for the optimization of the RO treatment of the OMW stream. The results gathered were thereafter interpreted by means of the response surface methodology. A significant impact was noted to be driven by the operating pressure and the tangential velocity on the fouling rate on the RO membrane. The response surfaces withdrawn from the experimental data support the previous results, and the optimised parameters - ambient temperature range (24 - 25 °C), moderate operating pressure (25 - 30 bar) and turbulent tangential flow (3.1 - 3.5 m s -1 ) - were found to provide a stable permeate flux of 32.3 - 38.5 L h -1 m -2 . These results reveal the proposed process could be operated successfully at ambient temperature conditions and medium operating pressure, boosting the economic efficiency of the RO purification of this effluent. Finally, the parametric quality standards stablished to reuse the purified effluent for irrigation purposes were checked and found to be satisfactory

A focus on fouling of nanofiltration membranes in the treatment of two-phase olive mill wastewater by boundary flux and pore blocking theories

The implementation of membranes in water and wastewater treatment processes has significantly increased in the last decades. However, membrane fouling leads to increased expenses if not properly examined and considered, and this is especially problematic in wastewater treatments. For this reason, fouling minimization represents the key factor to make those processes feasible. The use of NF membranes is especially problematic regarding fouling problems. In first place, adequate fouling inhibition methods should be designed upstream the membrane operation, in order to make the downstream membrane processes for wastewater treatment technically and economically feasible. In the present work, fouling build-up on a nanofiltration (NF) membrane during the treatment of olive mill wastewater coming from Spain (OMW-S) is addressed by the boundary flux theory, and the results were compared and complemented by using the pore blocking models. Fouling mechanisms are important to fully understand what is happening between the membrane and the effluent, to take the adequate decisions with respect to the design of the membrane plant and set-up of optimized operating conditions. The goal is to operate membranes modules by avoiding irreversible fouling for a long period of time, that is, several years of service lifetime. Thereafter, the operating parameters should be carefully chosen to avoid working beyond the conditions that the selected membrane can stand for the specific feedstream. The followed strategy allows the operation of the membranes system in a controlled framework that permits the stable operation of the plant. Moreover, the required membrane area is minimized and the constancy of the permeate productivity is also narrowed by following the proposed methodology

Analysis of fouling resistances under dynamic filtration of pretreated olive mill wastewater on a loose reverse osmosis membrane

In this work, a loose reverse osmosis (RO) membrane (Osmonics AK model), capable of offering beforehand higher fluxes under lower operating pressure than typical tight reverse osmosis membranes but still offering similar rejection, was used for the final purification of olive mill wastewater. However, the output that a membrane may offer when it is virgin and readily used will change in time due to membrane fouling. If not properly considered, the advantages that a chosen membrane may offer in contrast with others would quickly and often irreversibly vanish, with the consequences in terms of capital expenses that this will represent. One approach to meet the investor's needs to trust membrane technology is to guarantee that fouling will be inhibited as much as possible, but to overcome the loss of performance that fouling carries engineers overdesign the membrane plants by using too wide safety margins that trigger the costs sensibly. Since the mechanisms by which fouling phenomena are triggered are always complex, the osmotic-pressure resistances-in-series model can be a simple but reliable model to describe the membrane response and predict its performance in time. In this context, the normalized fouling measured on the examined RO membrane was found to be minimum in the operating pressure range between 5 and 8 bar (0.65-0.98, respectively), and it decreased down to 0.51 upon increasing the crossflow up to 5.09 m s-1, avoiding irreversible fouling. Moreover, significantly minor fouling (0.33) was attained at the lowest temperature, regularly experienced during the olive oil production campaign. On another hand, the rejection towards organic solutes was maintained above 97%

Study on fouling behaviour of ultrafiltration and nanofiltration during purification of different organic matter polluted wastewaters

The boundary flux concept is a profitable tool to analyse fouling issues in membrane processes. The boundary flux value separates an operating region characterized by reversible fouling formation from irreversible one. Boundary flux values are not constant, but function of time, as calculated by the sub-boundary fouling rate value. The knowledge of both parameters may fully describe the membrane performances in sub-boundary operating regimes. Many times, for wastewater purification purposes, ultrafiltration and nanofiltration membranes are employed to treat different wastewater streams. This appears to be feasible from both technical and economical point of view many times. Whereas initial productivity and selectivity to reach the desired purification targets are generally guaranteed, key to reach process feasibility is that the membrane must resist to fouling issues, with a limited reduction of the performances as a function of time. In other words, longevity of the membranes must be that high to minimise their substitution and, consequently, operating (consumable) costs for the replacement. In this work, after a brief introduction to the boundary flux concept, for many different wastewater, the boundary flux and sub-boundary fouling rate values of different microfiltration and ultrafiltration membranes will be discussed and compared. By this approach, it will be possible to separate those systems where the use membranes for their treatment results successfully from those that represent a challenge (from a technical and/or economic point of view). This will depend sensibly of the feedstock characteristics and, in detail, on the particle size of the suspended matter and guidelines for process designers will be discussed. In most cases, it will be shown that membranes appear to perform very well, making this technology very interesting for many case studies

About the limits of microfiltration for the purification of wastewaters

In the past, microfiltration was widely used as a pretreatment step for wastewater stream purification purposes. Experiences performed during the last years shows that microfiltration fails to maintain its performances for longer period of times. Many case studies demonstrate that the adoption of microfiltration leads to the failure of the overall process; the severe fouling of the microfiltration membranes leads to high operating costs with the consequence to make the treatment of the wastewater economically unfeasible. The boundary flux concept is a profitable tool to analyze fouling issues in membrane processes. The boundary flux value separates an operating region characterized by reversible fouling formation from irreversible one. Boundary flux values are not content, but function of time, as calculated by the subboundary fouling rate value. The knowledge of both parameters may fully describe the membrane performances in sub-boundary operating regimes. Many times, for wastewater purification purposes, ultrafiltration membranes appear to be suits better to the needs, even they exhibit lower permeate fluxes compared to microfiltration. Key to this choice is that ultrafiltration appears to resist better to fouling issues, with a limited reduction of the performances as a function of time. In other words, it appears that ultrafiltration exhibit higher boundary flux values and lower sub-boundary fouling rates. In this work, after a brief introduction to the boundary flux concept, for many different wastewater streams (more than 20, produced by the most relevant industries in food, agriculture, manufacture, pharmaceutics), the boundary flux and sub-boundary fouling rate values of different microfiltration and ultrafiltration membranes will be discussed and compared. The possibility to successfully use microfiltration as a pretreatment step strongly depends on the feedstock characteristics and, in detail, on the particle size of the suspended matter. In most cases, microfiltration demonstrates to be technically unsuitable for pretreatment purposes of many wastewater streams; as a consequence, the adoption of microfiltration pushes operators to exceed boundary flux conditions, therefore triggering severe fouling, that leads to economic unfeasibility of the process in long terms

Optimal design of membrane processes. A problem of choices between process layout, operating conditions and adopted control system

The development of membrane processes as a technology for environmental treatment applications and in particular for the purification of wastewater streams has significantly increased in the last decades. Fouling on membranes appears to be one of the main technical limit of this technology. This phenomenon causes the unavoidable deposition of particles on the membrane surface, building a resistive growing layer to permeability. Sensible fouling of the membrane leads to a significant reduction of the performances, a decrease of the operating life and, as a consequence, the increase of the operational costs due to the replacement or cleaning of the exhausted membrane modules. The presence of the fouling phenomena makes the proper design and control of membrane systems a difficult task. Optimal design of the membrane processes will be here discussed. The procedure requires to determine the optimal process layout given the input data and target requirements. At the end, the required membrane area is calculated. This latter property is strictly dependant of the adopted operating conditions, most importantly by the adopted value of transmembrane pressure (TMP). Moreover, it depends if the value of TMP remain fixed as a function of time or is variable (as in case of fixed permeate flow rates). Therefore, the optimal design of the system may occur only if the adopted control strategy is defined a priori. As a consequence, design choices of the membrane process layout, operating condition and adopted control system are strictly dependant, and connections between these different aspects should not be neglected during the engineering and P & I development stage of membrane systems. This paper will start from the theory of the boundary flux, in order to describe a novel design approach to membrane systems. Parallel to this, the development of an advanced control system, that allows to limit fouling formation during operation, is presented. The advanced control system relies on a suitable simulation software capable to predict the boundary flux, that changes the controller's set-points accordingly. Finally, the paper will merge all elements together, and report about the optimal design of membrane processes equipped with the advanced membrane process control system; validation of the proposed approach will be based on the use of a custom simulation model in ASPEN HYSYS and by experiments on lab scale

About merging threshold and critical fux concepts into a single one: the boundary flux

In the last decades much effort was put in understanding fouling phenomena on membranes. One successful approach to describe fouling issues on membranes is the critical flux theory. The possibility to measure a maximum value of the permeate flux for a given system without incurring in fouling issues was a breakthrough in membrane process design. However, in many cases critical fluxes were found to be very low, lower than the economic feasibility of the process. The knowledge of the critical flux value must be therefore considered as a good starting point for process design. In the last years, a new concept was introduced, the threshold flux, which defines the maximum permeate flow rate characterized by a low constant fouling rate regime. This concept, more than the critical flux, is a new practical tool for membrane process designers. In this paper a brief review on critical and threshold flux will be reported and analyzed. And since the concepts share many common aspects, merged into a new concept, called the boundary flux, the validation will occur by the analysis of previously collected data by the authors, during the treatment of olive vegetation wastewater by ultrafiltration and nanofiltration membranes

Comparison of the performance of two reverse osmosis membranes for the final purification of olive mill wastewater

Two quite different reverse osmosis (RO) polymeric membranes were examined for the final purification of olive mill wastewater from two-phase olive mills (OMW2): the first one is a thin-film composite (TFC) membrane consisting of polyamide active layer on polysulfone ultrafiltration support, whereas the other one is a low-pressure membrane made of asymmetric polyamide. A net operating pressure (PTM) of 25 bar was found as the target for the TFC membrane, whereas for the asymmetric one a PTM of 8 bar was chosen, given that similar flux decay but still significant productivity was observed by increasing the PTM for this membrane. These results are confirmed by the fouling index (b) values calculated for each membrane. Complete removal of suspended solids, phenolic compounds and iron was achieved by both membranes. Otherwise, the asymmetric membrane ensured slightly higher organic matter (COD) and electroconductivity (EC) reduction, leading to a COD concentration in the permeate stream equal to 3.7 mg L-1 and 1.4 mg L-1 (TFC vs. asymmetric), whereas the EC values were 97.0 and 31.0 μs cm-1, respectively. This would permit reusing the purified effluent provided by both membranes in the production process and close the loop at industrial scale. Moreover, the asymmetric membrane provides a steady-state flux value of the same order of that yielded by the TFC membrane upon more than three times less PTM (14.9 L h-1m-2 at PTM = 8 bar vs. 15.2 L h-1m-2 at PTM = 25 bar), implying a reduction of the specific energy consumption above 50 %, from 0.30 ? m-3 for the TFC membrane to 0.14? m-3 for the asymmetric one. Copyright © 2015, AIDIC Servizi S.r.l