21 Treatment of Paper-Plant Wastewater by Ultrafiltration A Case History H. O K A M O T O 1 ,M.MIZUHARA2, Y. NUMATA2, K. NAKAGOME3, T. OCHIUMI3, and T. KURODA3 1
Taio Paper Industrial Company, Ltd., 2-60, Kamiya-cho, Iyomishima, Ehime, Japan Mitsubishi Kakoki Kaishi, Ltd., 4-28, 1-Chome, Mita, Minato-ku, Tokyo, Japan 3 Nitto Electric Industrial Company, Ltd., 1-2, 1-Chome, Shimohozumi, Ibaraki, Osaka, Japan
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The design and operation of the world's largest tubular membrane ultrafiltration unit at a Japanese Kraft paper plant is described. The unit treats the plant's KP-El effluent stream. By reducing the effluent load to the plant's activated sludge system, it has brought the plant into compliance with environmental regulations. The unit treats more than one million gallons of feed per day to produce a concentrate stream of 52,000 gallons per day that contains 79% of the COD content of the feed. An average membrane flux of 57 gallons/ft2/ day has been maintained. The Taio Pulp Company has recently installed the world's largest tubular membrane ultrafiltration unit at its paper plant on Shikoku Island, Japan. The plant was designed by Mitsubishi Kakor Kaisha, Ltd. Company and uses Nitto Electric Industrial Company's tubular membranes and modules. The Taio paper plant produces approximately 3,000 tons of paper per day. During the production process, approximately 220,000 tons (58 million gallons) of effluent water are also produced. In the past, all of the plant's effluent streams were pooled and treated sequentially by conventional sedimentation, activated sludge, coagulation precipitation, and sand filtration. However, recently the government's environmental discharge regulations were changed. The new regulations require a discharge of pH of 5.8 to 8.0, suspended solids (SS) of less than 38 ppm, and chemical oxygen demand (COD) of less than 80 ppm. The old treatment processes had difficulty in meeting these discharge limits, particularly during the summer months when high temperatures reduced the efficiency of the activated sludge processes. Membrane processes have been suggested as a solution to this type of problem by a number of workers, (1-3) and thus a membrane ultrafiltration system was developed and installed. To reduce the load on the activated sludge plant, an ultrafiltration unit was installed to treat one of the most polluted streams from the plant, the KP-El stream from the Kraft pulping processes. 0097-6156/85/0281-0273$06.00/0 © 1985 American Chemical Society
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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This stream has a COD of 1250 to 1900 ppm and a volume of 0.8 to 1.0 million gallons per day. It was anticipated that a reduction of the COD of this stream would bring the plant into compliance.
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Pilot Plant Results The composition of the initial KP-E1 feed solution is shown in Table I. The solution contains a total of 0.2 to 0.3% dissolved and suspended solids; this solution must be concentrated tenfold to 2 or 3%. Because of the highly fouling nature of the feed solution, only tubular ultrafiltration membranes appeared to be suitable for this application. The high temperature and pH of the feed also mandated the use of chemcially resistant membranes. Thus, Nitto's 3508 polysulfone membranes were chosen. These membranes have a nominal molecular weight cutoff of 8000 and can be used at temperatures of up to 60 degrees centigrade.
Table I. Composition of Taio Paper KP-E1 Effluent Stream after Treatment with the Nitto NTU-3508 Membrane Feed Temperature (°C) COD (ppm) pH SS (ppm)
45-55 1,250-1,900 10-11.5 20-110
Concentrate 45-55 18,000-32,000 10-11.5 300-1,800
Permeate 45-55 300-700 10-11
Figure 1 shows gel permeation traces of the feed, permeate, and concentrate from the 3508 membrane. The feed solution contains four peaks labeled A, B, C, and D in this figure. Experience has shown that substances represented by B are highly aromatic and are thus particularly difficult for the activated sludge plant to digest. Ulfiltration through the 3508 membrane produces a permeate that is essentially free of the substances represented by A and B peaks, which are retained in the concentrate solution. The effect of UF treatment followed by activated sludge and coagulation on the product water quality of the system is shown in Table II, compared to normal activated sludge coagulation treatment. As this table shows, the conventional process is rather inefficient at removing COD, and only 51% of the total COD is removed. However, ultrafiltration followed by conventional treatment removes 97% of the COD. This improvement is obtained by the ultrafiltration membrane. In addition, the low molecular weight solutes that permeate the membrane are more easily digested in the activated sludge process than are the larger molecules retained by the membrane. As a result, the efficiency of the activated sludge process also increases.
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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Treatment of Paper-Plant Wastewater by UF
Figure 1. Gel permeation chromatography plots obtained on the KP-E1 feed, permeate, and concentrate (20x) solutions after ultrafiltration with Nitto polysulfone NTU- membrane.
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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Table II. COD Removal from the KP-E1 Stream by Conventional Treatment with and Without Ultraflitration
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Treatment Step
Total COD Removed (%) Conventional UF + Conventional Treatment Treatment
Ultrafiltration Activated Sludge Coagulation Plus Sedimentation
— 40
83 13
11
1
Total
51
97
Design of the Plant Figure 2 shows a plot of water flux vs. concentration factor for the KP-E1 feed solution. As this figure shows, the flux through the membrane falls substantially as the feed solution becomes more concentrated. Several design options are possible in this situation. These are illustrated in Figure 3. The most efficient design is the single-pass system in which the solution is pumped through a number of modules in series. The flux obtained is the average value of the flux vs. concentration factor curve shown in Figure 2. This method of operation is commonly used in reverse osmosis plants, where 5 to 20% of the feed solution is removed as the solution passes through the module. Therefore, only 5 to 10 modules in series are normally required to produce the appropriate concentration. In ultrafiltration systems, however, only 0.1 to 1% of the feed solution is removed per pass through a module. Thus, an impossibly large number of modules would be required to achieve the desired concentration. Many small ultrafiltration systems therefore operate in the onestage feed-and-bleed mode shown in Figure 3b. These systems only contain a few modules, and, thus, the feed solution in the loop is allowed to build up until it reaches the required final concentration. This is an inefficient mode of operation because the membrane is always in contact with the most concentrated solution. A final mode of operation is a multi-stage feed-and-bleed system, as shown in Figure 3c. In this system, the concentration is allowed to build up to the maximum concentration in stages. The Taio paper plant is a very large installation, and it was economical to consider this type of multi-stage design. The system installed consists of two lines of six stages. Five stages of each line are in use at any one time while the sixth stage is being cleaned or maintained. The design of each individual stage is shown in Figure 4. Each stage consists of seven banks of membranes, each bank consisting of seven or eight modules in a series. Thus, each stage contains 49 to 56 modules. The design of the individual modules is shown in Figure 5. Each module consists of 18 J^-inch tubular membranes manifolded inside a stainless steel tube and end pieces. The surface area of each module is 247 sq. ft.
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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Figure 2. Flux vs. concentration factor for NTU-3508 membranes operating on KP-El feed solutions. The average operating pressure was 115 psi.
Figure 3.
System design options.
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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