I, the current situation of water pollution control
70s, China's industrial pollution control is mainly concentrated in the point source, to control industrial "three wastes" as the main; in the 80s, through the adjustment of irrational industrial layout, industrial structure and product structure, combined with technological transformation, and strengthen the environmental management and other measures, industrial pollution comprehensive prevention and control, and carried out on a small scale comprehensive regional environmental management; into the 90s since the strengthening of industrial pollution control while carrying out large-scale regional environmental comprehensive improvement of key cities, watersheds, and regional environmental comprehensive management. In the 1980s, through the adjustment of irrational industrial layout, industrial structure and product structure, combined with technological transformation, strengthening of environmental management and other measures, industrial pollution was comprehensively prevented and controlled, and comprehensive regional environmental management was carried out on a small scale; since the 1990s, while strengthening the prevention and control of industrial pollution, large-scale comprehensive regional environmental improvement of key cities and watersheds and prevention and control of surface pollution in rural areas have been carried out in a bid to improve the quality of the environment of some cities and regions. The prevention and control of water pollution in the "Three Rivers" and "Three Lakes" identified by the State is in full swing.
In 1998, the total amount of wastewater discharged in China was 39.5 billion tons, and the chemical oxygen demand (COD) emission was 14.99 million tons, which was 5.0% and 14.7% lower than that of the previous year, of which 20.1 billion tons of industrial wastewater was discharged, and the COD emission was 8.06 million tons, which was 11.5% and 24.9% lower than that of the previous year; the amount of domestic sewage discharged was 19.4 billion tons, and the amount of COD emission was 1.4 billion tons, and the amount of domestic sewage discharged was 1.4 billion tons. , chemical oxygen demand (COD) emissions of 6.93 million tons, respectively, an increase of 2.6% and 1.3% over the previous year (Table 7-3).
Table 7-3 Comparison of Wastewater and Chemical Oxygen Demand Discharge in 1998 and 1997
The industrial wastewater treatment rate and discharge compliance rate of industries in counties and above counties and key townships were 87.4% and 65.3% respectively, which were 8.5% and 10.9% higher than the previous year. The country has built and operated 266 urban sewage plants, with a daily treatment capacity of 11.36 million tons, urban sewage centralized treatment of more than 20%.
Second, the current situation of water pollution control technology
1. Conventional sewage treatment technology
Sewage treatment technology, according to its principle of action can be broadly divided into three categories of physical method, chemical method and biological method. Physical method is the use of physical action to make the pollutants in the conditions of its chemical nature does not change the separation of methods from sewage, such as precipitation, filtration, flotation, adsorption, evaporation, etc., mostly used for sewage pre-treatment and post-treatment; chemical method is the use of chemical reaction to separate, recover the pollutants in sewage, or to make it into harmless substances in the method, such as coagulation, neutralization, oxidation reduction, electrolysis, etc., mostly used for higher concentrations of pollutants, more toxic, more toxic. Used for industrial wastewater treatment with higher concentration of pollutants, greater toxicity or microbial degradation; biological method is to use microbial metabolism to make the sewage organic pollutants in the dissolved and colloidal state of the degradation and transformation of harmless substances, mostly used for organic wastewater treatment with good biochemical properties. Biological method according to the microbial demand for oxygen can be divided into two categories of anaerobic and aerobic method, and at the same time according to the existence of biological flocs in the reactor is divided into activated sludge and biofilm method. Anaerobic method is mostly used to treat high concentration of sewage, and aerobic method is mostly used to treat lower concentration of sewage.
Domestic and foreign sewage treatment processes are mainly based on biological technology, the reason is that the biological method has a wide range of sources of microorganisms, species, reproduction, easy to domesticate and mutation of the characteristics of the treatment cost is relatively low. Especially in recent years, with the deep understanding of the microbial purification mechanism and reaction kinetics, biological technology is more mature and perfect, not only the purification efficiency and effluent effect has been greatly improved, the operating cost has also been substantially reduced. Although China's sewage treatment started late, but in the digestion and absorption of foreign technology on the basis of active research and development, has also formed a series of new technologies, new techniques suitable for our national conditions.
At present, the sewage treatment processes commonly used at home and abroad include: aerobic traditional activated sludge method, oxidation ditch method, A-B method, biological contact oxidation; anaerobic sludge bed method, two-phase anaerobic digestion method; anaerobic, aerobic combination of A-O method, A-A-O method.
(1) Traditional activated sludge method. Traditional activated sludge method since 1914 in Manchester, England, built the first pilot plant has a history of more than 80 years, through years of production practice in the continuous improvement, is now very mature, and in many countries, large and medium-sized sewage plant applications. Its purification mechanism focuses on the use of microbial logarithmic growth period of high activity, so it has a high purification efficiency.
The advantages of this process are high purification efficiency, good water effect; the disadvantage is that the sludge production, nitrogen and phosphorus removal capacity is poor, operation and management of the technical requirements of strict, once the negligence and management of the activated sludge will cause anomalies (such as bio-poisoning, sludge expansion, etc.), and re-establishment of the need for a longer period of time; therefore, this process has been gradually replaced by other processes.
Turkey's Anchengla sewage treatment plant, the daily water treatment capacity of 765,000 tons, operating parameters such as Table 7-4. China's Beijing (Gaobeidian) municipal sewage treatment plant, Tianjin (Jizhuangzi) municipal sewage treatment plant also belongs to this category.
Table 7-4 Turkey Ankara Wastewater Treatment Plant effect
(2) oxidation ditch method. Oxidation ditch, also known as circulating aeration tank or oxidation channel, developed by the Netherlands Institute of Sanitary Engineering in the 1950s, is a variant of the activated sludge method, and the purification mechanism is slightly different. The purpose of the development is to meet the requirements of sewage treatment at the same time, so that the amount of residual sludge is further reduced, thereby reducing the cost of sludge treatment. The principle is to extend the aeration reaction time, and the use of circular circular aeration pool, so that the sewage in the pool has a longer cycle period, in the section closer to the aerator dissolved oxygen concentration is high, the microorganisms are in a high activity period, is conducive to the degradation of organic matter; away from the aeration area, the concentration of dissolved oxygen is low, the microorganisms are in the anoxic environment, which is conducive to improve the system's sludge self-digestion and denitrification and phosphorus capacity. Oxidation ditch method in terms of its structural form, and there are many variants, of which the more typical are O'Bail type (orbal), Carrousel type (cor-rousel) and alternate.
The advantages of this process are simple process, good effluent water quality, nitrogen, phosphorus removal rate is high, less sludge production. The disadvantage is that the sewage retention time is long, large infrastructure investment, and at the same time, because the method uses the surface turn butterfly aeration, the water channel is shallow, not only the aeration efficiency is low, the ambient temperature requirements are also high.
Project example: Handan City, Hebei Province, the introduction of Danish technology to build a three-ditch alternating oxidation ditch system, water treatment scale of 100,000 m3 / d, the operational indicators are better than the design requirements (Table 7-5). Beijing Yanshan Petrochemical Company Petrochemical Sewage Plant (60,000m3/d), Kunming Orchid Gully Sewage Plant (55,000m3/d) are used respectively in the O'Bail type and the Carrousel type oxidation ditch, the operating effect is also very satisfactory.
Table 7-5 Handan City Wastewater Treatment Plant Operation Effect
(3) A-B activated sludge method. A-B activated sludge method, also known as adsorption-biological oxidation method, is the University of Aachen, Germany, Prof. Bink in the mid-1970s, in order to effectively remove the nitrogen, phosphorus and difficult to degrade organic matter in the sewage and designed. The method divides the sewage treatment process into two stages according to the principle of microbial reaction, with the first stage (A) being the high-load adsorption-synthesis section and the second section (B) being the low-load bio-oxidation regeneration section. Through segmentation, the ecosystems in each segment are relatively independent, avoiding mutual interference between organisms, giving full play to the purifying effect of different biological groups themselves, and improving the overall purifying capacity of the system. In section A, due to the strong adsorption effect of microorganisms and their own synthesis ability, not only can it remove biodegradable pollutants to a high degree, but also remove a large number of difficult to degrade pollutants; in section B, due to the operation in a low load environment, it is conducive to the propagation of digestive bacteria and phosphorus-polymorphic bacteria, and therefore also improve the system's ability of denitrification and phosphorus removal.
The process is characterized by: high purification efficiency, capital investment, impact resistance, good water quality. The disadvantage is that the process is more complex, operation and management is inconvenient.
Project example: Germany Krefeld sewage treatment plant, the daily treatment of sewage 24,000m3 / d, influent water quality BOD5400mg / L, COD800mg / L, TKN45mg / L, TP10mg / L, the results of its treatment are shown in Table 7-6.
Table 7-6 Germany Krefeld wastewater treatment plant operation results
(4) Biological contact oxidation method. Biological contact oxidation method is a kind of biofilm method, first created by Japan in the 70s. The principle is to load the filler (or carrier) in the biological reactor, the use of microorganisms own attachment role in the formation of biofilm on the surface of the filler, so that the sewage in contact with the biofilm in the process of purification. The method due to the presence of filler, on the one hand, increase the biomass in the reactor, improve the purification capacity of the system; on the other hand, also due to the biofilm is composed of aerobic bacteria, anaerobic bacteria, parthenogenetic bacteria, fungi, protozoa and higher aquatic animals *** with the composition of the high density of ecosystems, and therefore has a good purification effect.
Bio-contact oxidation method of oxygen supply more than the bottom of the aeration method, air bubbles in the filler meandering upward floating process, on the one hand, can be in full contact with the biofilm, improve the utilization of oxygen, reduce the consumption of air power. On the other hand, due to the bubble stirring effect, can also promote the renewal of biofilm, improve biological activity.
The advantages of this method are high purification efficiency, strong shock resistance, low sludge yield, easy operation and management, low power consumption, nitrogen and phosphorus removal ability. The disadvantage is that for the larger sewage plant needs too much filler, not easy to transport and filling. In recent years, China has developed a two-stage biological contact oxidation process on the basis of ordinary biological contact oxidation process according to the principle of biological adsorption, which further improves the purification efficiency of the technology and the ability to remove nitrogen and phosphorus, and broadens the scope of application.
Examples of projects: Beijing Yanshan Petrochemical Company Star City sewage treatment plant (20,000m3/d), Inner Mongolia Dongsheng City comprehensive sewage treatment plant (30,000m3/d), Hebei Handan Congtai Brewery (2,000m3/d), Hebei Jinzhou City Printing and Dyeing Plant (2,000m3/d) and so on are used in the two-stage biological contact oxidation process, and the operating results are good.
(5) upflow anaerobic sludge bed method. Anaerobic method, also known as anaerobic digestion, is in the anaerobic conditions, through the parthenogenetic bacteria and specialized anaerobic bacteria on the organic matter of digestion and degradation, so that the sewage is purified method. The digestion and degradation process can be divided into two stages: acidic hydrolysis and alkaline digestion (alkaline fermentation or methane digestion). In the first stage, through the action of acid putrefactive bacteria or acid-producing bacteria, the end products are organic acids including butyric acid, propionic acid, acetic acid and formic acid, as well as alcohols, ammonia, CO2, hydrogen sulfide, hydrogen, and bioenergy, which prepare for the next stage of methane digestion; in the second stage, under the action of methanotrophic bacteria, the products of the first stage are further decomposed into digestive gas, and the main products are methane (CH4) and carbon dioxide (CO2).
The upflow anaerobic sludge bed method is produced under the principle of anaerobic reaction. The so-called sludge bed, refers to the artificial culture or domestication conditions, anaerobic sludge in the form of granular floc deposited at the bottom of the reactor formed by the high concentration of sludge layer. When sewage flows upward and first passes through the sludge layer, it converts most of the organic matter into digestive gas. Therefore, the key to the success of the upflow anaerobic sludge bed method is to form a granular highly activated sludge with good settling performance.
The advantages of this method are high organic load, strong purification capacity, able to directly treat higher concentrations of organic wastewater, and can generate new energy. The disadvantage is that the reaction temperature requirements are strict, when the sewage sulfur compounds in the higher concentration of the reaction will have an inhibitory effect.
At present, this method has been widely used in the treatment of high-concentration organic wastewater, such as beer, pharmaceutical, food, meat processing, alcohol and other industries.
(6) two-phase anaerobic digestion. As mentioned before, the anaerobic digestion process includes two phases of acidic hydrolysis and alkaline digestion, each of which has its own independent microbial community involved in its reaction. Due to the two types of microbial groups on the requirements of environmental conditions are very different, the reaction rate of the substrate is not the same, the entire reaction process is controlled by the rate of alkaline digestion. Therefore, when these two types of microbial groups living in the same digester, it is inevitable that they are not in the best state of growth and reproduction, to play their respective roles. Two-phase anaerobic digestion is based on this principle, the two reaction phases were completed in two digesters, so that each section of the biological organisms are growing in their own optimal environmental conditions, which greatly improves the digestion efficiency, the degree of decomposition of organic matter and the organic load of the system, but also improves the purity of methane in the digested gas. In addition, due to the acidification and hydrolysis section of the microbial population on the reaction environment (water quality, temperature, pH value, etc.) is much wider than the second section of the requirements of the scope of application of the process is also enhanced, especially for difficult to degrade organic pollutants, through the first section of the pre-treatment, can be a greater degree of improvement in the performance of sewage biochemistry, laying a good foundation for the subsequent treatment.
At present, due to the two-phase anaerobic first section of the special purification effect, people also use this point, so that it is directly connected with the aerobic process, the development of a new process such as A-O and A-A-O method, to overcome the aerobic method does not dare to dabble in the high concentration of wastewater or poor biochemical sewage shortcomings.
2. Sewage treatment unconventional technology
Sewage secondary treatment, and can not completely solve the pollution of the environment, the treated sewage can not be directly used back to industrial and agricultural production. With the sharp shortage of water resources in all countries of the world, pure sewage secondary treatment has been far from meeting the needs of economic development, so in the western developed countries have gradually begun the tertiary treatment of sewage and in-depth treatment, but in actual operation, its expensive infrastructure investment and operating costs make many water plants in trouble. In order to seek both economic and energy-saving treatment technology, people break the routine, and back to the old way of natural purification. Such as in recent years to promote the application of land treatment, artificial wetlands and oxidation pond technology, known as innovation and alternative technology (Innovative and Alternative Technology), referred to as IA technology. At present, IA technology has been widely used in the deep treatment of sewage in developed countries, such as the United States, Britain, Canada, Israel and other countries. In the secondary treatment of sewage, developing countries have also made bold attempts and achieved good treatment results, and has been popularized and applied in many countries, such as China, India, Brazil and other countries.
The advantages of IA technology are low capital and operating costs, simple operation and management, easy maintenance, and good quality of treated water. The disadvantages are lower purification efficiency, large floor space, and unsuitable for application in cold regions.
(1) Land treatment system. Land treatment system is in the artificial control of the conditions of the sewage cast in the land, through the soil adsorption, microbial decomposition and plant root absorption, so that the sewage is naturally purified sewage treatment technology. The basic processes include: diffuse infiltration, rapid infiltration, ground surface diffusion and wetlands. In order to better improve the purification efficiency and economic benefits, some also specialize in the introduction of plants or forest trees with high economic value, strong purification ability and adaptability, called artificial land treatment system. The most representative country utilizing this technology is Israel. In this country, after secondary treatment of sewage almost 80% of more than to be further purified by land treatment, and then back to industrial and agricultural production.
China in recent years has also carried out research in this area, and in the secondary treatment of sewage has been widely used, such as Beijing Yanshan Petrochemical Company's artificial wetland and Inner Mongolia Ningcheng old cellar distillery land rapid percolation system.
(2) Stabilization pond (commonly known as oxidation pond). Stabilization pond is the use of natural or artificially modified, constructed ponds, through such as anaerobic, aerobic, parthenogenetic biological decomposition and absorption of aquatic plants and other natural purification capacity, complete the sewage purification treatment technology. Stabilization ponds can be divided into anaerobic ponds, aerobic ponds and parthenogenetic ponds according to the different dissolved oxygen content in the ponds, and are used individually or in combination according to the functions and efficacies of the various ponds in practical applications. The treatment load is limited to the self-purification capacity of the system. In order to further improve the purification capacity of the system, but also to add many artificial measures, which in turn gave rise to such as enhanced aeration ponds, aquatic plant ponds and ecosystem ponds.
Engineering applications: such as the Federal Republic of Germany Ohlstadt City of Biolak aeration pond, Brazil Bahia Province of water hyacinth ponds and China's Beijing Yanhua company NiuKouYu reservoir ecological ponds.
(3) Ecosystem approach. Ecosystem approach is based on the ecological point of view and systematics theory, through a variety of cheap treatment technology optimization combination, to achieve the technical method of sewage purification. The method in 1990 in China's Beijing Yanshan Petrochemical Company NiuKouYu reservoir petrochemical sewage treatment (55,000m3 / d) was first applied, not only achieved good results in the treatment, but also quickly restored the ecological reservoir area, has been highly evaluated by domestic and foreign environmental protection experts, is called both symptomatic and curative treatment of the root cause of the "Chinese medicine therapy".
The adopted ecosystem includes:
-Organic matter→bacteria, algae→protozoa→advanced aquatic animals (biological purification system)
-Organic matter→microorganisms→aquatic and terrestrial plants (plant purification system)
Organic matter→biological bed→soil layer (soil purification system)