Abstract: Analyze the characteristics of waste water and waste gas pollutions in wood-based panel production and propose the corresponding control technology and solutions.
The overall improvement of ecological environment has become one of the goals of well-off society, the government revised the <Environmental Protection Law> in 2015 and issued series of new regulations such as <Prevention and Control of Water Pollution>, < Prevention and Control of Air Pollution> in the same year.
After 30 years of reform and opening up, the wood-based panel industry has developed rapidly but it also lead to waste water and waste gas(Volatileorganiccompounds，VOCs ) pollutions in the process of production. It has become one of the most important problems faced by manufacturing factories. The <Environmental Protection Catalogue> issued by environmental protection department stated clearly that only products meet GB/Tl1718—2009, GB18580—2001 and HJ5712010 standards would not be included in the list of “High pollution, environmental risks”.
To deal with increasingly serious environmental protection pressure, the writer proposed solutions and low cost technologies for reference.
Characteristics of pollution in production process
The pollutant category, quantity and composition depend on the raw materials being used, production capacity, production technology and management level. Different panels will discharge different types of pollutants due to different production technology. This paper mainly emphasizes the pollution control solutions.
1.1Waste water
Waste water will be produced in the production process of fiberboard, particle board and plywood. And the wood cooking wastewater produced during the production of plywood mainly contains VOCs which belongs to biorefractory wastewater. The waste water produced during the production of particle board mainly comes from glue making process. There is a large quantity of wastewater produced during the production of fiberboard and the water quality is complex. (Refer to Table 2)
Wastewater is produced from washing process and hot grinding process during the production of fiberboard. The composition of water quality is similar to that of chemi-machanical pulp wastewatercontains cellulose, hemicellulose, Carbohydrate, resin, tannin, pectin, sediment and tree bark with high content of CODC, BOD5 and SS.
1.2Waste gas
Waste gas pollution in plywood production is caused by the exhaust gas which comes from veneer dryer, and the harmful gas comes from gluing process, and the contaminates comes from sanding process. For drying 1m veneer, the dryer will exhaust waste gas contains 4.6g of solid particle and 250g of HC, the emission is associated with wood species. Waste gas contained in HC is a mixture of terpenes, hemiterpenes and water vapor.
In the particle board production, due to the widely use of resin, formaldehyde
Will escape from the process of gluing, hot pressing and cooling. Dust will be produced during the process of manufacturing, drying and sanding. In addition, the dry tail gas exhausted from drying process also contains certain quantity of VOC and PM.
Currently the fiberboards are commonly manufactured by dry process, large quantity of waste gas will be exhausted from fiber drying process mainly contains VOC and PM.Everycubicmeterofdry tail gas contains 200-600mg dust, 50mg of Formaldehyde, 20mg of phenols, 20-50mg of organic carbon, 200-800mgt of aerosol. Formaldehyde
Will escape from the process of hot pressing and cooling processes.
2.1Waste water control technology
Currently in our country a combination process with physicochemical-biochemical techniques is widely used for waste water treatment such as AO process and ABR-SBR process, but the effect is not good. Some factories take the treated waste water as recycle water, but it would have a big impact on the quality of washing wastewater and thus affect the biochemical treatment system, so most wastewater should be discharged.
Foreign countries have stricter requirement on the wastewater discharge. After the traditional biochemical treatment, more processes have been added for further purification such as multilevel filtration, microfiltration and hyper filtration processes. They attach great importance on the research and development of “Wastewater Zero Discharge” method for the recycling utilization of wastewater.
Technology optimization of wastewater treatment process:
To analyze and evaluate the operation data of wastewater treatment plants, a model has been built by use of system control theory to determine the process operating parameters.
Process quality assurance technology:
Aimed at customers who have requirement for recycle water, make corresponding process system solutions for wood-based panel according to effluent quality.
3.2.2
Waste gas control technology
In order to meet the requirement of emission standard, waste gas should be treated by combining various techniques. For example, about tail gas treatment of fiber boards we need to combine gravity separation, washing, electrostatic cleaning and water treatment techniques together. As shown in figure 2, a tail gas treatment system of German company adopts many techniques such as washing and electrostatic cleaning.
3.3New technology of biochemical treatment
The biochemical technology has the advantages of less investment, high treatment efficiency, low cost, without secondary pollution.
The technology of microorganisms is a circulation process used in sewage treatment plant by cooperation mechanism of microorganisms with different physiological functions. Though these microorganisms are very small, they grow quickly and can carry out material exchange with surrounding quickly. The waste water privies optimal breeding conditions for microorganisms.
The treatment of waste gas by using biochemical process is take microorganisms as carbon source to oxidize and degrade the waste gas into non-toxic organics. The theoretical basis for biodegradation process is to put VOCs into the microorganisms by gas-liquid mass transfer, it would be degraded into carbon dioxide, water and cytoplasm in aerobic condition.
Microorganism is sensitive to environment, has poor resistance, and easily affected by bad environment. The microorganism has simple structure without immune monitoring systemso it can mutate easily. On the other hand, its bad genetic stability makes the bacteria relatively easy to be cultivated.
Key research direction of pollutants in wood-based panel industry:
1)Get dominance bacteria by acclimation and screening, construct superbug by genetic engineering technology to improve the adaptability of autochthonous microflora to process fluctuations.
2)Strengthen the research on microbial engineering technology by combining with physical and chemical method to form a systematic solution.
4Conclusion:
We still have a long way to go to control pollution in wood-based panel industry.Enterprises should play the main role and need to work together, meanwhile cooperate with scientific research institutes to develop forward-looking technology.
