Water Consumption, Quality Demanded, and Re-use of the Treated Wastewater for Paper Industry

Transcription

1 Water Consumption, Quality Demanded, and Re-use of the Treated Wastewater for Paper Industry C.Y. Peng 1 1.President Yuen Foong Yu Paper Mfg. Co.,Ltd. Fine Paper & Board Group, c.y.peng@yfy.com Abstract The paper industry has been a basic industry for the nation since long time ago in history. The importance and demand of the paper is not impaired by the technological development. On the other contrary, consumption of the paper has been the key- indicator of a state s economic development and culture. Due to its restricted source of the raw material and high demand of water consumption for the manufacturing process, the paper industry is always considered as one of the most water consumptive industries. According to the statistics of the industrial water consumption at year 2003, the paper industry, only second to the chemical material industry, takes 14.54% of the total water consumption for all industries. Facing this great pressure of water demand, the government cooperating with the academic institutes and the private industries has been promoting programs to reduce the industrial wastes and save the energy. Main papermaking mills through almost ten years efforts had dramatically reduced the water consumption even lower than the international standard. However, it still has a lot of works to do in order to reach zero discharge of the wastewater and totally closed system of the white water recycling. This article presents the current status of the paper industry in Taiwan, the water consumption of paper and pulp mills and the quality requirement of the water used, and discuss possible water saving methods for the manufacturing process and the problems that might be encountered. 1. Introduction to domestic paper industry and its water consumption In response to the demand of market and the impact of globalization, paper industry in Taiwan has gradually diversified its products to accepting small quantity of order. Unstable supply of the raw material, high costs of the environmental protection and keen price competition in the global market have caused some paper mills in Taiwan to move out to mainland China or Southeast Asia. This causes an obvious change in this industry property. The number of Taiwan Paper Industry Association s members was reduced from 164 to 108 from year 1992 to year 2005, evidencing drastic structural change in the industry. Differences in product grades and production scale add complication to the classification of the paper mills. Classification of the paper can be made either by the raw material or the product grades. If classified by the raw material, there are mixed pulp paper, virgin fiber paper and secondary fiber. The mixed paper mills use lumber, waste bamboo, cotton and B1-9

2 flax as the raw material and coarse paper and artificial paper is their main product. Constrained by high production cost and such problem as the black liquor treatment, most materials for the pulp making are imported from abroad. There are only two pulp-making mills, Hua-Lian mill of the Chung Hwa Pulp Corporation and Hsin-Yin mill of the Taiwan Pulp and Paper Corporation, in Taiwan. The secondary fiber mills use the recycled waste paper as the raw material. Their main products are packaging papers, such as Kraft paper, cardboard, corrugated paper and other grades for the industries. The virgin papermaking uses virgin pulp as the raw material. Their major products are tissue paper and fine paper. If classified is by the product, there are tissue paper, fine paper, packaging paper, hand-made paper and coarse paper. The number of the packaging paper mill is the highest among all mills in Taiwan. Taiwan Paper Industry Association surveyed the water consumption for the paper industry respectively in 1993 and Their results are shown on table 1 and table 2 (3). As the papermaking is always listed as one of the main water consumption industries, the industry supported by the government and the private sector has been endeavoring to implement water saving programs for almost 10 years. Each major mill has achieved great reduction of the unit water consumption. For example, the unit water consumption for pulping mill was reduced from 90m 3 /ton-pulp to 48m 3 /ton-pulp. For the packaging paper mill it was reduced from 10m 3 /ton-paper to 7m 3 /ton-paper. For the fine paper mill it is reduced from 35m 3 /ton-paper to 16m 3 /ton-paper. But for the tissue paper mill it was increased from 13m 3 /ton-paper to 16m 3 /ton-paper due to high quality demand. In North America and Europe unit water consumption of the typical paper mills falls in the range of 2~20m 3 /ton-paper. This shows that the pulping or papermaking technology in Taiwan is comparable to the global level (see Table 3). However, as the total water consumption of paper industry is still very high, how to reduce the water consumption or to increase supply of the recycled water in order to reduce the discharge of wastewater, remains the main goal for the paper mills and the government to achieve. Table 1 Unit consumption of water in 1993 (3) Pulp and paper industry in Taiwan Unit: m 3 -water/ton-pulp or paper Item Samples Low High Average Pulping Packing paper Fine paper Tissue paper Special paper Water resource and quality requirement for the paper industry. According to the statistics in 1996, 54.6% water supply to the paper industry are mainly from the recycled water, 33.3% from the ground water and 7.9% from the surface water (see table 4). The weight of the recycled water supply was increased from 26.7% in B1-10

3 1994 to 54.6% more than double in1996. As regards to the distribution of consumption in each unit operation in the paper mill, according to Lu et al 5 investigation and analysis, the process water takes the most at 74.8%, next cleaning shower water at 17%, and the rest 4.3% for de-ionized water, 3.5% for cooling water and 0.4% for tab water. Therefore, the water management should aim at reducing the consumption of process water and cleaning shower water or increasing the recycled water as first priority. Table 2 Unit consumption of water in 2001 (3) Pulp and paper industry in Taiwan Unit:m 3 -water/ton-pulp or paper Item Paper Samples Low High Average Pulp Packaging paper Fine Paper Liner board Fluting paper Coated white board Chip board Kraft paper Art paper Printed writing paper Household paper Table 3 Unit consumption of water in 1971 and 2001 Paper mill in North America and Europe ( 4 ) Unit:m 3 -water/ton paper Item Paper Year 2001 Year 1971 Packaging paper Liner board/fluting Duplex board Fine Paper Art paper Printed writing paper Light weight coated paper Household paper Newsprint Paper Table 4 Water resources for paper mills (3) Unit:m 3 -water/ton paper Year Ground water Surface water City water Buying water Recycled water B1-11

4 3. Characteristics and management of the quality of water for paper industry The water used in most of the pulp and papermaking processes, except the mechanical pulping process, are in direct contact with the raw material. Therefore, the quality of the water used in the process affects tremendously quality of the paper produced. For examples, as pulp adsorbs the dissolved solids and metal ions in the water, when the water is being filtered on the wire section, the suspended solid will be captured by the pulp. When the moisture in the pulp is evaporated in the drying section, the suspended solid remained in the pulp will be affecting the quality of the end product. At wet-end chemicals processing unit the sizing agent, dye, fluorescent and bleaching agent will be reacted with the suspended solid and thus the quality of the product will be adversely affected. In some serious condition, it will convert the pitch of woods to the insoluble material, causing the problem so called pitch trouble. Furthermore, if the quality of the raw water is not good, it will cause other problems such as corrosion of the papermaking equipments, scales build-up inside the pipes, erosion of the facilities, etc. problems. This will directly or indirectly affect the production capacity. Thus, high yield of paper production depends upon good management and treatment of the raw water used. For stability of supply and cost effectiveness, most papermaking mills in Taiwan take raw water from the ground water. As characteristics of the ground water are different from place to place, the treatment process for the raw water is also varied. The most common treatment units are coagulation, settlement, and filtration aiming at removing turbidity, COD, BOD and iron ions. Besides the traditional chemical coagulation treatment, some mills add activated carbon adsorption, ion exchange or membrane filtration to the water treatment process in order to satisfy the special quality demand for water for the specific papermaking process. As quality of the raw water affects directly the process selection and cost effectiveness of its treatment, and the treatment process has substantial influence on the quality of the paper product, in 1970 Japanese government did for the paper industry a general survey of the quality of water used for each different process unit, such as cooling water, clean shower water, water for raw material usage, water for moisture adjustment and water for furnishing (6) and based on American TAPPI standard had proposed to the paper industry the quality guideline for each different grades of paper products as shown in Table 5 (3). Kawano et al (6) had made further study on the quality requirement of the clean water used for the process unit and proposed the quality guideline for each different unit as shown in Table The generation and recycle of the wastewater in the paper mill Pulping and papermaking are the two major manufacturing processes for the paper industry. Though the wastewater generated is from different raw material, manufacturing process, product grade and discharging unit, the characteristics of the discharged water is almost the same with only slight difference. In pulping process, the wastewater is mainly from stock preparation, black liquor evaporation, screening room and bleaching section. In B1-12

5 papermaking process, the wastewater is mainly from the paper machine, and it usually called whitewater because it contains fillers, such as clay, CaCO 3 and talc, or the bleached fibers. Main pollutants of the pulping and papermaking wastewater are the suspended solids, the dissolved organic and in-organic material. Table 5 Water quality standard for different paper products (3) Item Kraft paper Kraft paper GP Fine Alkaline (bleached) (non-bleached) paper paper pulping Turbidity (SiO 2 ) Color (Pt-Co) Total hardness (CaCO 3 ) Ca hardness (CaCO 3 ) Mg hardness(caco 3 ) Methyl orange alkalinity (CaCO 3 ) Fe Mn Residual Cl Chloride Dissolved silica (SiO 2 ) Total dissolved solid Free carbonate (CO 2 ) Reduction of the wastewater and rationalization of the water usage can be achieved by recycling the used process water such as the whitewater, or by reducing consumption of the cleaning water. The totally whitewater closed system developed in recent years is a good solution that minimizes the makeup water for the loss of the evaporation and the water contained in the discharged sludge and reject. At the current stage of shortage of water supply this system is recommendable due to its high rate of the water recycled for reuse and the recovery of large amount of fine fiber and white cellulose, but the derived problems encountered in the daily operation due to the increasing concentration of the dissolving material, which is caused by repeatedly recycling the whitewater, can not be overlooked. These problems are listed below: Table 6 Water standard for papermaking process (3) Item Cooling Shower raw Moisture Product water water material adjustment furnishing Turbidity (NTU) ph Alkalinity (CaCO3, ppm) Hardness (CaCO3, ppm) Evaporated Residual (ppm) Chlorine ion (Cl -, ppm) Fe (ppm) Mn (ppm) B1-13

6 The clogging problem caused by fiber. Equipment subject to higher exposure of corrosion due to the increasing concentration of the dissolving material in the whitewater. Precipitate increased, including in-organics scales and organic slime. Microorganism increased due to higher concentration of the organic and higher temperature. Increased concentration of the dissolved solids (represented as COD) and SS in recycled whitewater The wet-end chemistry in paper machine being affected. System temperature increased Therefore, other treatment units such as clarifier, filtration or evaporation facilities must be added to the closed water recycle system. For the executive to evaluate the economic viability of the investment of the white water recovery and the closed water recycle system, the profit of fiber recovery and reduction of cleaning water shall be weighed against the additional costs of investment for such closed system. In additional to the evaluation of economic viability as mentioned above, following steps of consideration and study are to be taken in order to promote rationalization of the water usage for the paper industry: Balance of the water system: to consider the balance between the stock tank and white water tank, the capacity of the white water tank big to take storage of the stock tank to avoid the overflow of white water and makeup of cleaning water. Separate cooling water system: cooling water to be recycled separately to avoid it mixed with other wastewater to the wastewater treatment system. Pump sealing water closed: As the gland sealing used by most pumps in pulp and paper mills consumes water as high as 15~20L/min, only use the cleaned whitewater recycled as the gland sealing water is insufficient in its amount (see table 7). The balance is to be supplied by the clean water or the treated recycled water. Some measures to save the sealing water, like to install the small check-valve in piping or collect the sealing water and recycle it to reduce the clean water usage, can be considered. Closed sealing water for vacuum pumps: Normally the total amount of sealing water for all vacuum pumps is about 5-10m 3 water/ ton-paper. In order to reduce consumption of the clean water, the dilute whitewater can be considered as the preferred source of sealing water. Table 7 Comparison of sealing water and clean recycled water 7 ph Dissolved solids (mg/l) Suspended solids (mg/l) BOD (mg/l) Scale condition Corrosion Sealing water <250 <10 <2 Minor Minor B1-14 Recycled clean water 6-9 >>250 >50 >25 Medium-high Low-high

7 Press section shower and felt clean: press section shower consume in general m 3 -water/ ton-paper, main consumption are for cleaning press roll, felt and suction roll. The recycled whitewater can be considered as first priority to meeting the large demand of the shower water to clean the rolls. When cleaning the felt normally hot water or clean water is used applied, to avoid causing clogging on the felt and the spray nozzle. But the advanced modern design, which uses low pressure-oscillating shower (pressure around 5-7kg/cm 2 ) on the front side and high pressure oscillating shower (pressure around 15-25kg/cm 2 ) on the back side, will reduce consumption of the clean water effectively. Wire section shower system: The wire section shower system, which needs around 20-25m 3 -water/ ton-paper, is the main water consumption part of the paper machine. For most paper machines it provides opportunity to save consumption of the clean water. In practice, consumption of the clean water can be reduced by reducing the number of shower, adopting advanced new type of the spray nozzle, applying adequate shower pressure and using cloudy white water to replace the clean water. Whitewater recycle and management: In the whitewater recycle system, white water recovery machine, such as disk filter or krafta flotation, is normally used to recover the fiber and clarify the whitewater. The recovered fibers can be recycled to the papermaking system to increase the production yield and reduce the production cost. The clarified whitewater can be returned to the process for reuse depending upon whether its quality is acceptable or not. Development of the biotechnology in recent years has provided new strength to the efforts of water saving for the pulping process. The new technological development of biochemistry and biomass has been rapidly advancing. It has brought revolutionary change to the traditional technology of the paper industry, such as bio-pulping low energy and water consumption technology, and new high-valued derivative products. 5. Wastewater pollution prevention in pulp and paper industry. Although the wastewater discharged from the mills is not harmful to human health directly, it still has some impact on the water receiving body, if it has not been treated adequately. The common impacts are suspended solid settled in the riverbed and oxygen insufficiency, foaming and odor creating of the river caused by oxygen consuming degradation of the organic material. If excess dissolved material enters the river, it will cause microorganism to grow rapidly and thus the river will become anoxic which adversely affect the environment for the fish. Therefore, the wastewater of pulp and paper industry should have been properly treated before they are discharged to the river. The pollutant concentration in the discharged wastewater will be higher if it comes from the recycled or reused wastewater. This shows the importance of wastewater pre-treatment. Since the national standard of the discharging became effective in 1993, the paper industry in order to comply with the regulatory standard set for the industry has installed in all mills the wastewater treatment facilities that work quite well. In the treatment plant chemical coagulation/pressurized flotation is often adopted because its equipment is simple, B1-15

8 it is compact and easy to operate, and it is able to remove most suspended solids effectively. But as the characteristics of one mill s wastewater may be different from another mill s wastewater and the pollutant s strength of each mill s wastewater is not the same, only the primary treatment with chemical coagulation cannot completely remove the pollutants especially the dissolved organic material in the wastewater. Therefore, for those paper mills with high pollutant concentration, the secondary bio-treatment or even the tertiary advanced treatment must be added in order to meeting the existing regulatory standard for discharging the wastewater to river. The two treatment processes that are often chosen by the paper industry are shown as follows 8 : Raw wastewater Chemical coagulation Clarifier/flotation Discharge Partly Recycled Sludge dewatered Raw wastewater Chemical coagulation Sludge cake Clarifier/flotatio Bio-treatment Clarifier Partly Recycled Sludge dewatered Sludge cake 6. The feasibility analysis of the wastewater re-uses In recent years our government has promoted strongly reuse program of the water resource. The government in order to carry out the new policy will impose according to the amended water pollution prevention act the wastewater pollution fee on the industry, so that the discharging pollutant will be constrained by the extra burden of paying the pollutant fee. Accordingly, this enforcement will have great impact on the paper industry. The paper industry in Taiwan so far has invested quite lot of manpower and expenses in reducing the unit water consumption of paper making, and in the water saving measures 9. Though the result is satisfactory to the industry none of these investments is for the purpose to reuse the recycled treated wastewater for the process, except to recycle the treated wastewater that meets the discharging standard for cleaning the dewatering machine or washing the floors. How to recycle the discharging water for the process use remains the task of pilot plant testing and is still at stage of evaluation. By applying the advanced wastewater treatment technology, such as activated carbon adsorption, bio-activated carbon (BAC), membrane filtration, ozone, or evaporation 10, it is possible to upgrade the discharging wastewater which is meeting the regulatory standard to the quality meeting the requirement of the process use, but the paper industry is hesitant to make such investment because the extra costs are too high to be acceptable. To recycle the treated wastewater for reuse will soon become possible because generalization of the advanced treatment technology and cost-down of the initial investment, such as lower B1-16

9 prices of the membranes, are envisioned in the near future, and the paper industry will be encouraged by the economic incentives that are offered by the government. 7. References 1.The Statistics of Taiwan Paper Industry (2006), Taiwan Paper Industry Association 2.Wastewater pollution prevention of paper industry (1995), Industrial Development Bureau, Ministry of economic Affairs 3.Peng, Y.H., Water management in paper industry 4.Paulapuro, H., Ed., Stock Preparation and Wet End, Papermaking Science and Technology Vol. 8, FAPET, Lu, W. C., Du, P.H., (1996), Investigation and analysis of industrial water usage in Taiwan 6.Kawano, Shuzuki, (1979), Rationalization of water usage of pulp and paper industry 7.R.W.J Mckinney (1995) Technology of Paper Recycling, chapter 7, Water and waste water treatment in recycling mills. Blackie Academic & Professional. 8.Wastewater pollution prevention technology for paper industry, (1993), Industrial Development Bureau, Ministry of economic Affairs 9.Lin, W. H., Henamatsu, (1998), The introduction of water saving projects in paper industry, Conference of Industrial Waste Reduction and Sustainable Development 10.Chen W. and Horan N.J. (1997) The treatment of a high strength pulp and paper mill effluent for wastewater re-use, III) Tertiary treatment options for pulp and paper mill wastewater to achieve effluent recycle, Environmental technology, Vol.19, pp B1-17