Zero discharge of wastewater is called zero discharge of liquid (ZLD) abroad, that is, enterprises do not discharge any form of wastewater into surface water bodies. In GB/T2 1534-2008 "Terminology of Industrial Water Saving" issued by AQSIQ in 2008, the explanation of zero discharge is that the production water system of enterprises or main units has no industrial wastewater discharge. In short, zero discharge is to concentrate industrial wastewater into solid or concentrated solution and then treat it, instead of discharging it into natural water in the form of wastewater.
Zero discharge of wastewater is a systematic project, including two levels. First, take measures such as water-saving technology to improve water utilization rate, reduce production water consumption, and at the same time improve the reuse rate of wastewater as much as possible to maximize the use of water resources; Second, high-efficiency water treatment technology is used to treat high-concentration organic wastewater and saline wastewater, and the unusable saline wastewater is concentrated into solid or concentrated solution, which will not be discharged into natural water in the form of wastewater.
Wastewater treatment mode-saline wastewater treatment
See figure 1 for the overall solution of wastewater zero discharge of typical modern coal chemical enterprises.
The treatment of saline wastewater usually adopts membrane concentration or thermal concentration technology to concentrate the impurities in wastewater, and the clear water is reused in the circulating water system, and the concentrated solution (high-salinity wastewater) is discharged into the evaporation pool for natural evaporation or mechanical atomization evaporation. Membrane concentration technology has the advantages of low treatment cost, large scale and mature technology, but its disadvantages are high requirements for influent quality, easy pollution and low concentration multiple. The main principle of membrane concentration technology is reverse osmosis (RO). The produced clean water has low COD and salt concentration, the recovery rate of clean water is generally 60% ~ 80%, and the high efficiency reverse osmosis (HERO) can reach 90%. Nanofiltration is a pressure-driven membrane separation and concentration process between reverse osmosis and ultrafiltration. Compared with reverse osmosis, its operating pressure and energy consumption are lower, but its application in wastewater treatment is still in the research stage.
Thermal concentration mainly includes multi-effect evaporation, mechanical compression evaporation and membrane distillation. High concentration efficiency, but huge equipment and high energy consumption. Among them, multi-effect evaporation technology is mature and has been applied in many industries, and the recovery rate of clean water is generally around 90%; Membrane distillation can use cheap energy such as industrial waste heat, and the rejection rate of non-volatile components such as inorganic salts and macromolecules is close to 100%, but this method is still in the research stage.
Wastewater treatment mode-concentrated solution treatment
The concentrated solution produced after the treatment of saline wastewater also becomes high salinity wastewater, and the salt content is usually as high as 20% (mass fraction). In China, there are many methods to treat concentrated solution, such as evaporation crystallization, incineration, ash washing, natural evaporation pool, mechanical atomization evaporation and so on. There are also deep well perfusion methods abroad.
Evaporative crystallization is to precipitate the salt in the concentrated solution by crystallization. Vapor compression crystallization technology is a proprietary technology of American General Company, with the highest thermal efficiency. This technology has a large investment in equipment and has been successfully applied to the indirect coal liquefaction projects of Sasol Company in South Africa and Debienskd Coal Mine in Poland. In China, only the coal-to-oil project of Shenhua Group Co., Ltd. adopts this technology to treat a small amount of high-salt wastewater produced in the process of catalyst device, and it is still in operation stage.
Incineration method is to send the concentrated solution to an incinerator for incineration, and the residue produced is mainly salt. This technology has high energy consumption, high anti-corrosion requirements and difficult stable operation, and there is no operation example in domestic coal chemical industry. A coal-to-natural gas project plans to adopt this treatment method, and the preliminary design is currently under way.
Ash flushing method is to send the concentrated solution to coal yard for spraying or boiler slag flushing, and the salt and organic matter in the concentrated solution will eventually enter the ash. Some small coal chemical projects and power plants mostly adopt this disposal method.
Natural evaporation pond method is to build a pond with a large enough area, store the solution, evaporate the water by natural evaporation, and leave salt at the bottom of the pond. In general, the evaporation pool needs to take corresponding seepage control measures. This method is more suitable for coal chemical projects in areas with small rainfall, large evaporation and sparsely populated areas.
Mechanical atomization evaporation is to increase the evaporation speed by adding a mechanical atomization evaporator on the basis of natural evaporation. Parkwater mechanical atomizing evaporator is a patented equipment of British Horizon Group, and it is an efficient high-concentration brine evaporation equipment. The equipment has low land occupation cost and saves investment cost. Take the natural evaporation of 2000mm, concentrated water flow150t/h and annual flow of 8000h in northwest China as an example:
1. evaporation pool scale: the natural evaporation pool needs to cover an area of 65,438+200,000 square meters. If Parkwater mechanical atomizing evaporator is added, the evaporation pool only needs to cover an area of 1 10,000 square meters, with a volume of 400,000 square meters and a pool depth of 4 meters.
2. Investment in the construction of evaporation pond: In addition to the land cost, the construction cost per square meter of natural evaporation pond is about 400 yuan, that is, it needs 480 million yuan. If Parkwater mechanical atomizing evaporator is added, excluding the land cost, the cost per cubic meter is around 400 yuan, that is, * * * * needs 40 million yuan.
3. Ton water treatment cost of evaporation pool: there is no energy consumption in natural evaporation pool, and the energy consumption cost of Parkwater mechanical atomizing evaporator is about 2 yuan.
4. Land cost: Parkwater mechanical atomizing evaporator can save land 165438+ 10,000 square meters, and the land cost is 440 million yuan.
At present, the deep well grouting method has been applied in the United States, Mexico and other countries. This method requires high natural geological conditions, and there is no relevant laws, regulations and standard technical support in China at present.