Treatment of papermaking wastewater by membrane process

Papermaking industry is a big water consumer, the amount of waste water discharged is very large, and the environmental pollution is also very serious. Therefore, the treatment of papermaking wastewater has been paid great attention by governments and enterprises.

The paper pulp and paper industry has different treatment methods (as shown in Table 1) due to the different components of wastewater produced by each process [1]. The wastewater with large quantity, the most complex composition and the most serious pollution is the wastewater from selection and bleaching processes. At present, most of these wastewater are treated by chemical precipitation, activated sludge, chemical floatation, air floatation and other methods. However, the wastewater treated by these methods often fails to meet the strict discharge standards, especially the activated sludge method. Due to the high temperature in summer months, the efficiency of activated sludge treatment is affected, and it is particularly difficult for the activated sludge plant to digest and treat the aromatic compounds in papermaking wastewater.

In recent years, ultrafiltration (UF), reverse osmosis (RO) and electrodialysis (ED) with semi permeable membrane as separation medium have been widely used in the treatment of pulp and papermaking wastewater at home and abroad [3]. Many valuable chemical products, such as lignin, lignosulfonate, vanillin and so on, can be recovered in the membrane process. The purified water can be reused in the papermaking process. Therefore, for more than ten years, membrane treatment plants have been established and put into operation in the paper industry of many countries in the world. Table 2 lists some operating conditions of membrane devices produced by DDS company in Denmark in the world's paper industry [3, 4]. According to the report [4], by the end of 1980, only DDS's UF and RO membranes had been used in the papermaking industry with an area of about 2787m2, and the by-products (in solid terms) recovered by UF method reached 15000-20000 tons / year, and the recycled water was about 454.6m3/day. It can be seen that the membrane process is a promising new technology for advanced treatment of papermaking wastewater, which has produced amazing social, environmental and economic benefits.

2. Treatment process

① the process flow chart 2 of the membrane system is a convenience of the process flow of the multi-stage continuous filtration membrane system designed according to the concept of Fig. 1 (c).

In Figure 2, after filtration, the waste water is pumped into the components of each section, and then collected to the respective outlets through the liquid and concentrate. The system has a heat exchanger with constant temperature control by steam or cooling water. The flow and solid content of the concentrate are controlled by the refractometer at the end of the membrane system of the production line or by the controller controlling the flow proportion of the feed liquid and the concentrate. Part of the permeate is mixed with the cleaning agent as the cleaning solution for cleaning the membrane regularly.

② as shown in Table 1 of papermaking wastewater treatment process, conventional treatment methods such as coagulation sedimentation, activated sludge, upfloating filtration are generally applicable to wastewater treatment of different papermaking processes. However, in order to carry out efficient treatment, the membrane method should be combined. Figure 3 shows some treatment processes currently used by the membrane method [1, 2, 4].

It can be seen from Figure 3 that some processes are lengthy, with many settings and high investment cost; some processes are inefficient and incomplete. Therefore, although there are many kinds of processes, people are still exploring new high-level processes.

The sludge, inorganic matter, fiber and ash produced in the process of pulp and paper wastewater treatment should be properly disposed. Many effective methods of using the final waste can avoid secondary pollution.

3. Characteristics of membrane treatment

Compared with the conventional method, the membrane method has the following obvious advantages in the treatment of papermaking wastewater:

① the equipment investment is low and the floor area is small.

② simple operation management and convenient maintenance. Because the operation is easy to be automated and the labor input is small, the operators of other posts can concurrently perform regular patrol management. Because of the multi-component and multi-stage design of the membrane system, it is possible to replace a component under the condition of continuous operation of the system. When cleaning a certain section of components, other sections of components do not need to be shut down, which will not affect the continuous operation of the system.

③ the operation environment is healthy. As the membrane process is a closed system, there is no sewage overflow and odor emission.

④ low energy consumption. Membrane method is a phase-change free separation technology, which only consumes the electric energy when pumping the feed liquid. For example, the energy of RO process for treating sulfite waste liquid (SSL) is about 1 / 4-1 / 30 of that of evaporation process (see Table 3 [4]). Obviously, RO is an economical way to condense SSL.

② the treatment efficiency is high. The highest removal rate of c0d by conventional method is 50% by coagulation sedimentation method, 60-70% by coagulation sedimentation + activated sludge method, and 80% by membrane method. The removal rate of chroma by conventional method is 80%, and 95% by membrane method [5].

There is no sludge. Membrane method does not produce a large amount of sludge, so it saves the cost and labor of sludge treatment. As shown in Fig. 3 (g) and Fig. h), the concentrate is used as fuel and the recovered heat is used for evaporation and concentration, which shows the advantages of energy saving of membrane method.

III. examples of membrane treatment of papermaking wastewater

1. Treatment of kraft paper washing wastewater

In June 1981, Japan Dawang Paper Industry Co., Ltd. built the largest tubular membrane ultrafiltration plant in the world to treat kraft paper washing wastewater in Sandao paper mill [2, 5, 6]. The factory is designed by Mitsubishi Chemical Machinery Co., Ltd. of Japan, and uses the tubular membrane and components produced by Japan's Nikko Electric Co., Ltd.

The paper production of Dawang paper mill is about 3000 tons / day, and the waste water discharged is about 220000 tons / day. UF plant processes 4000 tons / day of kraft paper washing wastewater which is the most polluted.

The UF plant consists of two production lines. Each production line is designed according to the concept of Figure 1 (c), which is composed of 6 sections, among which, 1-4 section has 224 membrane modules (56 modules / section), 5-6 section has 98 membrane modules (49 modules / section), 1 85m3 / 30KW · h feed pump, 6 150m3 / 45KW · h UF circulating pumps, 1 4.2m3 / 5.5kw · h booster pump, and the operating pressure of the system is 1.0-0.5mpa Ntu-3508-p18lp is composed of stainless steel pipe and end plate. The effective membrane area of each production line is 740m2 (2.3m2 / module), the daily treated wastewater is 2000m3, and the average permeable capacity is 67gfd. Two production lines share one set of cleaning equipment. The normal operation of each production line is five section membrane module, because one section is used for maintenance or cleaning. The cleaning cycle is 1 time / 2 days, and the cleaning time is 2-3 hours / time. Since the operation of the production line is fully automatic and unmanned, the maintenance and management personnel are the operators of other equipment.

The process flow of UF plant is shown in Figure 3 (g). Before the waste water enters into the membrane module, 40 mesh wire screen is used to filter out the wood fiber and large particles, then the feed pump pressurizes the feed liquid to g.61mpa, and then the circulation pump is used to pump the feed liquid to each membrane module of the system. The UF concentrate is sent to the incinerator, and the UF permeate is sent to the storage tank for mixing with NaOH and cleaning agent for regular membrane cleaning, and then to the activated sludge plant for further treatment. Table 4 lists the design value and actual operation performance of UF plant in treating washing wastewater from kraft paper plant.

As can be seen from table 4, although it has been operating at high temperature and pH for two years. The performance of UF plant has no obvious decline, the average water permeability of the membrane is maintained at 57gfd, and the removal rate of COD, chroma and SS is still high. Due to the use of cleaning technology, the initial stage of the membrane is greatly extended

Standard life. After the UF permeate (mainly dissolved solids and low molecular organic compounds) is treated by the activated sludge plant, the whole kraft paper washing wastewater has met the new environmental discharge rules of the Japanese government (ph5.8-8.0, SS < 38ppm, COD < 80ppm). First, the paper mill has changed the difficult situation that the conventional method can not meet the standard in the past.