Proc.Intern’l Conf. on Wastewater Treatment and Reuse Adapted to Mediterranean Area 25-28 October 2000, Tunisia (WATRAMA’2000), p.272-275  2000, INRST – Printed in Tunisia

Application of Membrane Technologies in Dairy Industries

Josef Robert, Rolf Kuemmel, and Hans Fahlenkamp Fraunhofer Institute for Environmental, Safety, and Energy Technology Osterfelder Strasse 3 D-46047 Oberhausen, Germany Tel. +49 208 8598-0 e-mail info@umsicht.fhg.de

INTRODUCTION Despite of its appearance, the food-processing industry is generally agreed to be one of the major sources of environmental pollution in industrialized countries. Emissions of gaseous components, aqueous effluents containing heavy loads of biodegradable organics (high BOD values), and solid or pasty residues of different compositions are frequently accompanying phenomena of food production lines. The dairy processing industry is known for high energy and water consumption, in part due to product diversification. The energy consumption in some dairy companies has been significantly reduced through the coupling of heat and power generation. Although the consumption of water and the generation of waste water has been reduced over the past years, it has not been decreased sufficiently. Until now, in the overwhelming majority of process units, end-of-pipe technologies have been preferably applied to protect natural systems against inputs of contaminants, while, on the contrary, the number of studies referring to a more modern process design is rather limited. For a more advanced processing of substances in the dairy industry, there are many potential applications for membrane processes. The different membrane processes include reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), electrodialysis (ED), and membrane reactions (MR). Various improvements have been achieved by utilizing membrane technology in the dairy industry, e.g. for bacteria removal (MF), whey treatment (UF, NF), demineralization (NF, ED), fractionation of milk and whey (UF, NF) and water recycling (RO). More and more, membrane technologies are refining or completely replacing conventional process lines. This results in reduced operating costs and increased capacities of existing plants. UF units are most effectively used to improve the separation of proteins, fats and solids and to yield products that are more uniform in appearance and composition. NF devices are most effective in demineralization processes. Following will be a description of membrane applications in the dairy industry, and an evaluation of an exemplary case study from a medium sized dairy company.

MEMBRANE APPLICATIONS IN THE DAIRY PROCESSING INDUSTRY Nowadays, membrane separations are increasingly used in concentrating multicomponent aqueous mixtures, in recovering valuable materials from process and waste streams, and in all types of downstream processing. In the dairy industry, the term downstream processing describes the entire set of operations starting from materials of their primarily natural origin to preparing the desired final product. It includes liquid-solid separations, the 272

Robert et al. : Membrane technologies

separation of components with respect to different molecular weights, the isolation of valuable components, and the treatment of waste and waste water streams. The partial concentration of diluted aqueous mixtures by means of membrane filtration is one of the most convincing examples, since the energy requirement for the separation of 1 kg of water by RO (100 - 120 kJ/kg) compares favorably with the amount of energy required by conventional evaporation techniques (500 - 700 kJ/kg). The well-known advantages gained from the application of membranes are e.g. lower energy consumption, no thermal stress acting on the feed constituents, recovery of fresh water, reduced waste treatment costs, and relatively low space and capital requirements, whereas a number of disadvantages and problems are the restricted membrane operating life, the attack of chemicals and pH changes on the membrane material, the fouling and scaling phenomena, the limited range of operating pressure, and the product safety as well the need for additional permission documents. As mentioned above, membrane technologies are going to improve or completely replace conventional process lines in the dairy industry. The application of membrane technology is not entirely new in the dairy industry, but the development of new, more economical membranes has opened up a variety of additional application opportunities. The most important applications can be found in water removal and (pre)concentration steps, in non-selective separations of inorganic from organic substances and of low-molecular from high-molecular solutes, and in water treatment and water recycling processes. In this context, the introduction of membrane processes is primarily aimed at reducing the operating costs and at increasing the capacities of existing plants.

WHEY PROCESSING The integration of membrane separation technology working at different pressure levels can be successfully applied to the separation of the disaccharide lactose from process waters of the dairy industry. Today, the annual world production of lactose amounts to about 300,000 tons. The conventional process starts from aqueous whey, which after pasteurization and separation passes a UF unit where a high-molecular protein concentrate is separated. This concentrate is then transformed by successive evaporation and flash drying steps into protein powder. The UF permeate which contains approximately 50 g/l lactose is further concentrated in a multistage vaporizer and finally transferred to a crystallization vessel where solid lactose is caused to precipitate by spontaneous crystallization or addition of seeding crystals. Separation of the crystals, conveyor drying and, if necessary, recrystallization to achieve pharmaceutical quality are the final steps of the process.

An innovative treatment process has been designed which differs from the conventional one by introducing a nanofiltration unit capable of improving the separation of organic and inorganic constituents. It could be demonstrated by laboratory and pilot-scale investigations that NF membranes operating at relatively high pressures show rejection rates of clearly more than 99 % with respect to lactose while lactic acid and citric acid as well as mono-mono-valent electrolytes are allowed to permeate through the membranes. At the same time, due to an extremely extended induction period prior to crystallization, the lactose concentration can be easily increased up to a degree of super-saturation of more than 200 % without immediate crystallization. In a special crystallizer, the addition of seeding crystals initiates the mass crystallization of lactose crystals which can be separated and collected by a decanter system. The mother liquor is recirculated, partly in front of the membrane unit, partly within the crystallization cycle. Optimization of the amount of washing liquid ensures that small

273

Robert et al. : Membrane technologies

crystals that cannot be separated by the decanting equipment do not dissolve completely and are transferred to the crystallizer. A detailed comparison of both processes reveals a number of advantages in favor of the NF technique. The thermal stress acting on the organic constituents is much lower since maximum temperatures of only 40 °C are applied. Therefore, Maillard-type decomposition reactions leading to high-molecular and colored substances can be completely prevented. The product purity is high, whereas electrolyte contamination is considerably lower. The energy requirement reduces by about 50 %, the yield of lactose increases from 70 to 93 %, and the amount of COD released with the process water reduces to nearly a third of the original values.

EXEMPLARY CASE STUDY: REDUCTION OF WASTEWATER AND RECOVERY OF VALUABLE SUBSTANCES IN A MEDIUM-SIZED DAIRY COMPANY A medium-sized dairy company in Northrhine Westphalia specializes in the production of various types of highquality cheese. They process between 100,000 l and 300,000 l milk per day into various cheese products. The production of cheese out of milk inevitably generates a considerable amount of whey as well as a whole range of process-specific milk, whey, or milk-water cheese mixtures, which are generally labeled »white water«. Moreover, the various types and shapes of the high-quality mozzarella cheese produced lead to numerous additional kinds of cheese brine. Because of its high concentrations of organic contents (BOD approx. 32,000 mg/l; COD approx. 60,000 mg/l), unprocessed whey constitutes a significant and serious wastewater problem. Approximately 90,000 liters of whey are generated every day. The wastewater generated by rinsing, cleaning, disinfecting and cooling amounts to 300 cubic meters on average. It usually contains 3,000 mg/l of COD. For every 1000 l of milk processed 0.5 to 3 m3 of waste water with a BOD5 of 0.3 to 4 kg is generated. Like many other companies of this size, they do not have their own waste water treatment facility. The produced waste water is transferred to a retention tank in order to compensate for fluctuations in the waste water composition during the daily production and then sent to the municipal waste water treatment plant. The high fat and nitrous oxide content of the waste water put additional stress on the treatment plant. Over the last few years, the dairy company expanded their cheese production, which in turn also raised their production of waste water. However, the municipal waste water treatment plant does not have the capacity to accommodate for the extra waste water. As a result, the dairy company is faced with fines for the additional waste water they produce and with possible shut down due to restrictions on the allowed daily fresh water intake. A case study done by Fraunhofer demonstrated that by means of an innovative combination of different membrane processes the load of harmful substances in the water can be reduced significantly (COD reduction by 50 to 70 %). It also demonstrated that the deployment of this technology will lead to an effective closing of water circuits and an increased recovery of valuable substances. An innovative ultrafiltration unit, which works below atmospheric pressure on the side of the permeate (VUF), with an integrated hot water cleaning unit is the central element of this technology. One of the main challenges of the plant is the separation of the considerable amount of fat and cheese coagulate from the water. Conception, assembly and operation of a demonstration plant were realized in co-operation with the Fraunhofer Institute for Environmental, Safety, and Energy Technology. The institute is also responsible for the scientific monitoring of the project. One research priority concerns the optimization of the VUF permeate and the separation of solid lactose by means of pilot experiments and concept studies. In this context, the reduction of the process water streams in 274

Robert et al. : Membrane technologies

order to facilitate external reprocessing is of predominant importance. For this purpose an innovative process has been designed which differs from the conventional one by introducing a NF unit capable of improving the separation of organic and inorganic constituents. A detailed comparison of both processes reveals a number of advantages in favor of the new technique, as described earlier. The thermal stress acting on the organic constituents is much lower since maximum temperatures of only 40 °C are applied. Initially, in the course of three months, all individual material streams in the dairy plant were characterized with regard to their behavior in the VUF plant. The consequences of the process for the dairy company as a whole as well as the impact on the composition of the waste water and on the reuse of process water were demonstrated by means of a mobile pilot plant. The process worked as predicted, led to considerable cost savings and, thus, stood the test of functionality. On the basis of these promising results, a plant on an industrial scale was designed and built by Fraunhofer. In conventional processes, approximately one ton of steam is needed for the precipitation/ extraction of proteins out of ten cubic meters of whey. As the amount of whey is reduced by the factor of 20, approximately nine tons of steam, which corresponds to half a ton of heating oil, can be saved per day through the utilization of membrane technology. Furthermore, the addition of acids (such as citric acid) in order to lower the pH value will no longer be necessary. It was shown that the VUF plant, which operates below atmospheric pressure on the side of the permeate, has a better performance per unit of area and, thus, leads to reduced operating costs. Another point of interest during the research period were new cleaning methods for the membrane filtration plants in compliance with the high hygiene standards in the dairy business. The conventional cleaning method for whey ultrafiltration plants consists of a three hour enzyme treatment and a post treatment with a chlorinated solution or a three hour base-acid-base rinse. The enzyme method can only be used at maximum possible temperature of 60 °C because the enzymes start to get damaged at higher temperatures. The chlorine tends to damage the membrane and, thus, shortens its life expectancy. During pilot tests a basic solution with pH 12 at a temperature of 80 °C was tested. This method regenerated the membrane in 15 minutes. At this pH and cleaning temperature, which is above the pasteurization temperature of milk, all water and ground bacteria as well as lacto-bacteria are eliminated. REFERENCES Rautenbach, R.; Albrecht, R.: Membrantrennverfahren. Salle und Sauerländer, Frankfurt,Berlin, München, Aarau 1981 Kümmel, R.; Robert, J.: Application of Membrane Processes in Food Technologies. In: Belafi-Bako et al.: Integration of Membrane Processes into Bioconversions, Kluwer Academic Plenum Publishers, New York 2000, p. 143 - 154 Scott, K.: Handbook of Industrial Membranes. Elsevier Advanced Technology, Oxford 1997

275