1. Introduction to papermaking wastewater and water quality characteristics

Introduction to papermaking wastewater

According to statistics from 1995, the total displacement of China's paper industry is 2.39 billion m3/a, which is the third largest in the chemical industry and the steel industry. The discharge of chemical oxygen demand is 3.214 million t/a, accounting for the whole country. According to statistics from 1994, the total discharge of suspended solids in China's paper industry drainage is 1.284 million t/a. Water pollution control in the paper industry has become a hot spot in the paper industry and the whole society, and also the key to the survival and development of paper companies.

   The paper industry is one of the industries with high energy consumption and high material consumption and serious environmental pollution. Its pollution characteristics are large discharge of wastewater, including high COD, suspended solids (SS) content and serious chroma.

   The paper industry uses wood, straw, reed, rags, etc. as raw materials, and the cellulose is separated by high temperature and high pressure cooking to make pulp. In the production process, the non-cellulosic portion of the final discharged raw material becomes a black liquor for papermaking. The black liquor contains lignin, cellulose, volatile organic acids, etc., which is odorous and highly polluting.

   Papermaking wastewater mainly comes from the two production processes of pulping and papermaking in the paper industry. Pulping is to separate the fibers in the plant raw material, make a slurry, and then bleach, this process will produce a large amount of papermaking wastewater; papermaking is to dilute, shape, press, and dry the slurry to make paper, this The process is also prone to papermaking wastewater.

   The papermaking wastewater produced by pulping is the most polluted. The waste water discharged during washing is dark brown, called black water. The concentration of pollutants in black water is very high. The BOD is as high as 5-40g/L, which contains a lot of fiber, inorganic salt and pigment. The papermaking wastewater discharged from the bleaching process also contains a large amount of acid-base substances. The papermaking wastewater discharged from the paper machine, called white water, contains a large amount of fibers and fillers and compounds added during the production process.

   The middle-stage wastewater from papermaking refers to the wastewater discharged from the slurry after cooking, black liquor extraction, screening, washing and bleaching. The discharge is large, and the dissolved lignin and its derivatives make the wastewater have a series of light brown to dark brown. s color. Due to the slow biodegradation of these compounds, the biochemical treatment effect is poor. According to the literature, the secondary biochemical method can remove up to 30% of the chromaticity of the wastewater, and some biological treatment methods actually increase the chromaticity of the wastewater. . The colored wastewater gives people an unpleasant feeling. The chroma component indicates that the wastewater contains many dissolved components. After the environment is discharged, the natural water is colored, the light transmittance of the water body is weakened, and the photosynthesis of aquatic plants and the growth and reproduction of aquatic animals are affected. Decolorization treatment is one of the key to the discharge of papermaking wastewater. From the current application of wastewater treatment technology, methods for effectively removing the color of papermaking middle-stage wastewater include adsorption method, coagulation method, biological method, chemical oxidation method, electro-flocculation method, and membrane separation method.

   Papermaking wastewater treatment should focus on improving the recycling rate, reducing water consumption and wastewater discharge, and actively exploring various reliable, economical and wastewater treatment methods that can make full use of the useful resources in wastewater. For example, the flotation wastewater treatment method can recover fibrous solid materials in white water, the recovery rate can reach 95%, and the clarified water can be reused. The combustion wastewater treatment method can recover sodium hydroxide, sodium sulfide, sodium sulfate and organic substances in black water. Other sodium salts. The neutralization wastewater treatment method adjusts the pH value of the wastewater; the coagulation sedimentation or flotation method can remove the suspended solids in the wastewater; the chemical precipitation method can decolorize; the biological treatment method can remove the organic matter, and is effective for the kraft paper wastewater; Strict and membrane product technology is becoming more mature and widely used. In recent years, there have been applications in the MBR process for deep processing at home and abroad.

2. Traditional processing techniques and problems faced

2.1 Traditional treatment process of pulp and papermaking wastewater

The traditional process generally adopts a combined process of physical pretreatment + biochemical treatment + chemical treatment.

2.2 Problems faced by traditional processes

Pulp and paper wastewater is recognized as a high-concentration organic wastewater with high pollution threat, complex nature and difficult to treat in the world, especially high concentrations of organic matter and ammonia nitrogen. Nowadays, the environment is becoming more and more important, and emission standards are becoming more and more strict. Traditional crafts are becoming more and more urgent. At present, the MBR technology is gradually replaced by the traditional aerobic process, which can meet the national environmental protection requirements, both in terms of operational stability and water output.

3. Advantages of MBR in treating pulp and papermaking wastewater

Membrane Bioreactor (MBR) is a new type of water treatment technology combined with membrane separation unit and biological treatment unit. The membrane module replaces the secondary sedimentation tank to maintain high activated sludge concentration in the bioreactor. And reduce the amount of sludge by keeping the sludge load low. Compared with traditional biochemical water treatment technology, MBR has the following main features:

3.1 effluent water quality is high quality and stable

Due to the high-efficiency separation of the membrane, the separation effect is much better than the traditional sedimentation tank. The treated water is extremely clear, the suspended matter and turbidity are close to zero, the bacteria and virus are largely removed, and the membrane separation also causes the microorganism to be completely intercepted in the MBR pool. The system can maintain a high microbial concentration, which not only improves the overall removal efficiency of the reaction device, but also ensures good effluent quality, and the reactor has good changes to the influent load (water quality and quantity). Adaptability, impact load, stable access to high quality effluent water quality.

3.2 Less sludge production

The process can be operated under high volume load and low sludge load, and the excess sludge production is low, which reduces the sludge treatment cost.

3.3 Small footprint, not subject to installation restrictions

The bioreactor can maintain a high concentration of microorganisms, the processing device has a high volume load, and the floor space is greatly saved; the process flow is simple, the structure is compact, the floor space is saved, and it is not restricted by the installation place, and is suitable for any occasion, and can be done It is grounded, semi-underground and underground.

3.4 can remove ammonia nitrogen and refractory organic matter

Since the microorganisms are completely shut off in the MBR pool, which facilitates the retention growth of slow-growing microorganisms such as nitrifying bacteria, the system nitrification efficiency is improved. At the same time, it can increase the hydraulic retention time of some hard-to-degrade organic matter in the system, which is beneficial to the improvement of the degradation efficiency of the refractory organic matter.

3.5 easy to operate and manage, easy to achieve automatic control

The process realizes complete separation of hydraulic retention time (HRT) and sludge residence time (SRT), and the operation control is more flexible and stable. It is a new technology that is easy to realize equipment in sewage treatment, and can realize automatic control of microcomputer, thereby making operation management More convenient.

3.6 Easy to transform from traditional processes

The process can be used as an advanced treatment unit of the traditional sewage treatment process, and has broad application prospects in the fields of advanced treatment of sewage treatment plants and upgrading of old factories.