How to solve the dilemma of sewage treatment in the development of coal chemical industry
——Interview with Xu Yanhua, a well-known expert in chemical pollution control and Dean of School of environment, Nanjing University of Technology
Xu Yanhua, Dean of the school of environmental science and engineering, Nanjing University of technology, doctoral supervisor, distinguished expert of the national "thousand talents plan", member of the national coal chemical industry professional committee, director of the National Sustainable Development Research Association, director of the clean coal conversion, water saving and emission reduction engineering laboratory in China's petroleum and chemical industry, Director of Jiangsu chemical pollution control and accident emergency engineering technology research center, mainly engaged in the innovative R & D and engineering application of industrial water pollution control, toxic and odor industrial waste gas treatment, new technologies, new materials and new equipment for industrial solid waste recycling, and presided over and completed a number of national "863" major special projects, national major water special projects National major science and technology support program topics, won 5 provincial and ministerial science and technology progress awards (including 2 first prizes), and is a well-known expert in chemical pollution control and industrial water conservation and emission reduction in China.
Since this year, the EIA of many coal chemical projects has been rejected by the Ministry of environmental protection. In order to develop coal chemical projects, environmental problems can not be bypassed. What are the difficulties of environmental problems in coal chemical industry? What effective solutions are there? With such questions, the reporter interviewed Professor Xu Yanhua, a well-known expert in chemical pollution control and industrial water conservation and emission reduction in China and President of the school of environment of Nanjing University of technology.
Do not agree to ban evaporation ponds across the board
Reporter: what do you think of the current environmental problems of coal chemical industry?
Xu Yanhua: a while ago, I participated in the special investigation of coal chemical enterprises in four western provinces organized by China Petroleum and Chemical Industry Federation, focusing on the environmental problems faced by the coal chemical industry. After 7 or 8 days of research, my overall feeling is mixed. Fortunately, many coal chemical enterprises have made obvious breakthroughs in the localization and large-scale of key technologies and equipment such as gasification and transformation. Some coal chemical industry concentration areas have made active and fruitful exploration in Coal Power Polygeneration, extension and expansion of coal based products and complementary symbiosis of coal chemical industry and petrochemical industry. The worry is that the environmental problems faced by the industry are very prominent, and water shortage and water pollution have become the key constraints on the development of the industry. In some areas that are vigorously developing coal chemical industry, on the one hand, there is a serious shortage of water resources, on the other hand, the ecology is fragile and there is no environmental capacity. In the summary report, some enterprises claimed that the sewage had achieved zero discharge, but the actual specific treatment process and reuse destination were unclear and could not withstand scrutiny. Due to the lack of effective monitoring and supervision means, the actual sewage treatment effect, reclaimed water reuse rate and water consumption per unit product of the enterprise are difficult to grasp, which is worrying.
Reporter: the pollution of Tengger desert has made the evaporation pond a target of public criticism. It is said that some coal chemical projects have been rejected because of the setting of evaporation ponds. What do you think of this?
Xu Yanhua: the initial purpose of setting evaporation pond is to receive sewage with high salinity and low chemical oxygen demand. In Northwest China with large evaporation capacity, the natural evaporation and concentration of sewage by using solar energy can greatly save the investment of enterprises in building evaporation devices and the operation cost of evaporation crystallization. For coal chemical projects with large investment and low benefit, the setting of evaporation pond provides a practical solution for enterprises to treat high salt wastewater at low cost. Unfortunately, some enterprises mistakenly regard the evaporation pond as a temporary sewage storage tank and accident tank, and directly discharge a large amount of high concentration sewage. Some even cause environmental pollution accidents, which completely violates the original intention of the government to approve the construction of evaporation pond.
Some local governments have therefore banned evaporation ponds across the board, requiring enterprises to achieve absolute zero discharge of sewage. I personally believe that there are many reasons for pollution accidents in the evaporation pond. The fault is not in the evaporation pond itself. Inadequate monitoring and supervision and policies are important reasons. Generally, the sewage with low COD, low toxicity and high salt in the evaporation pond will not have a serious impact on the ecological environment. Therefore, there are some problems in the scientificity, rationality and practical feasibility of the one size fits all decision-making of evaporation pond.
How to strengthen monitoring and supervision to ensure that the quality and quantity of sewage discharged into the evaporation pond meet the set requirements is the key. In addition, how to introduce reasonable supporting policies, increase the pollutant discharge fee or give certain punishment to the enterprises that exceed the standard and exceed the amount of pollutant discharge, give corresponding subsidies and rewards to the enterprises that meet the emission reduction standard, and implement cascade charges for the enterprise's fresh water consumption under the approved unit water consumption. These measures will help to reverse the abuse of evaporation pond, water and water consumption in coal chemical enterprises There is neither pressure nor motivation for water conservation and emission reduction.
It is more scientific and reasonable to achieve near zero emission
Reporter: at present, there are still disputes about zero emission and near zero emission in the industry. What's your point of view?
Xu Yanhua: zero discharge of sewage means that no wastewater and waste liquid are discharged outside the plant, that is, concentrated brine can not be discharged, and all highly concentrated brine must be evaporated and crystallized. However, it is difficult for enterprises to bear the investment and operation cost by relying on evaporation devices for evaporation and desalination, and there is no good way out for crystalline salt at present. According to the existing environmental protection regulations, this kind of salt residue belongs to hazardous waste and needs to be disposed of by a special hazardous waste disposal unit. Due to the high disposal cost, not only the economic burden of enterprises is heavy, but also due to the serious shortage of hazardous waste disposal capacity in many places, it is difficult to realize on-site disposal.
Recently, some units are stepping up the research on the quality separation and crystallization technology of high salinity sewage to seek the resource utilization of crystalline salt. However, these studies are still in the stage of small-scale test and pilot test, and their technical economy and the feasibility of practical engineering application need to be further investigated.
In view of the above actual situation, taking into account the needs of ecological and environmental protection, the actual pollution control capacity of coal chemical enterprises and the existing environmental protection technology level, I personally believe that on the basis of strict implementation of sewage classification collection, quality treatment and whole process optimization, the concentrated sewage with a small amount of up to standard sewage with high salt content and low chemical oxygen demand will be discharged into the evaporation pond to realize nearly zero discharge of sewage, More scientific, reasonable and practical.
Technology is the key to achieving near zero emission
Reporter: do you have any suggestions on achieving near zero discharge of coal chemical wastewater?
Xu Yanhua: at present, there are still many problems in the near zero discharge of sewage in coal chemical projects.
First of all, the sewage treatment design and the main process design are usually not completed by the same unit, and the design work is disconnected from each other without seamless connection. For a near zero discharge project, there is a high correlation and coupling between the sewage treatment system and the main production unit. Water use point, sewage production point, reclaimed water reuse point and sewage collection, treatment and reuse should be an organic whole. Only when the overall consideration and synchronous design between sewage treatment and main process design can we realize near zero discharge in the real sense.
Secondly, there are common problems in the pertinence and effectiveness of the existing sewage treatment processes. Coal to gas (Lurgi furnace or British liquid slag removal Lurgi Gasifier), coal to oil (direct liquefaction) and coal to blue carbon projects generally produce toxic sewage containing high concentration phenol ammonia. The traditional "simple pretreatment + biochemical" treatment process is often difficult to operate stably and effectively. Even if the subsequent membrane treatment and evaporative desalination are complete, it is difficult to achieve near zero emission. A prominent technical problem is the lack of necessary pretreatment. Selecting effective and economical enhanced pretreatment technology to efficiently degrade a large number of toxic organics in sewage can significantly improve the effectiveness and operation stability of the follow-up biochemical treatment system. The above sewage often contains not only high concentration organic poisons, but also a large number of oily substances and suspended solids (coal dust and pulverized coal), which often leads to scaling and blockage of phenol ammonia recovery unit, so that the system can not operate stably. Therefore, effective and economical pretreatment technology must be adopted to effectively remove a large amount of oily substances and suspended solids in the sewage before the phenol ammonia recovery unit, so as to ensure the efficient and stable operation of the phenol ammonia recovery unit and the subsequent sewage treatment system.
Thirdly, there are often problems of technical fragmentation and low integration in the current sewage treatment process. The existing coal chemical projects are implemented by multiple units in sections, and there are poor matching degree, difficult to effectively connect and low integration between each unit technology. Some enterprises do not pay attention to the end treatment and ignore the process water saving. Personally, in order to achieve water conservation and emission reduction, we should strengthen the optimization of the whole process of sewage treatment, apply the principle of system engineering, actively introduce the "water pinch" technology, and vigorously promote the cascade utilization and quality reuse of water. On this basis, the water balance of the whole plant is established, the water island is built, and the flexible dispatching of reuse water of the whole plant is realized.