At high temperature, the wastewater from iron and steel plant is constantly evaporating, which makes the content of organic matter and inorganic ions in wastewater increase continuously, especially various salts, such as chloride, carbonate, iron, sulfate and manganese. Precipitation, and the concentration is increasing. It turns out that when major steel mills treat sewage, the existing salt in the sewage cannot be removed at all, and the salt content and alkalinity of circulating water in steel mills are relatively high. Frequent use of this circulating water will form a thick layer of dirt on the surfaces of various machines, equipment and pipelines for a long time, which will affect heat transfer. Therefore, it is very important to improve the utilization rate of steel plant sewage and reduce the salt content in sewage.
In China, the traditional sewage treatment method of steel works is relatively simple. The main technological process is concrete sedimentation tank+high efficiency clarification tank+filtration tank. After such a simple treatment, the backwater is reused. This traditional treatment method can only obviously remove some pollutants in sewage, such as floating objects and various colloidal substances in sewage, but this treatment method has a great disadvantage, that is, it can not reduce the salt content in sewage. Therefore, in recent years, major steel mills have continuously increased their research and practice on sewage treatment. After repeated experiments and explorations, they summed up the modern sewage treatment technology, and constantly expanded the scope of use, making this technology develop rapidly. There are two main forms of modern sewage treatment technology: one is ultrafiltration+reverse osmosis; The other is multi-media filtration+reverse osmosis. Generally speaking, these two methods have their own advantages and are widely used in major steel enterprises in China. They are relatively advanced, energy-saving, environmentally friendly and efficient sewage treatment technologies at present. Two sewage treatment processes are described in detail below.
1 treatment process of "ultrafiltration+reverse osmosis" for steel plant wastewater
Let's talk about how to deal with the sewage problem of iron and steel plants under the process of "ultrafiltration+reverse osmosis". The practice of related iron and steel enterprises proves that since this sewage treatment method is adopted, all kinds of machinery and equipment have been running very stably, and the indexes of treated water are close to the water use standard. This sewage treatment process mainly includes the following steps: from the inlet tank to the high-efficiency cleaning tank through the lift pump, from the backwashing tank to the intermediate tank through the booster pump, through the processes of disc filter and ultrafiltration, and finally into the recovery tank through reverse osmosis.
1. 1 disc filter
It mainly plays the role of protective film filtration system, which can prolong the service life of the film, reduce the cleaning times of the film and enhance the durability of the film. Disc filter can automatically control the whole backwashing process, mainly through the control of three indicators, one is pressure difference, and the other two are water flow and time. As long as one or all of these three indicators reach a certain standard, the disc filter will start to control and realize automatic backwashing. At this time, the controller in the disc filter will send out the signal of automatic backwashing to realize backwashing of all components.
During backwashing, each unit is carried out alternately. The controller in the disc filter will backwash a unit first, and its main workflow is to change the direction of water flow first, mainly through the valve of water inlet and outlet. When the direction of water changes, the nozzle on the disc will spray water outwards. With the continuous rotation of the disc, the water sprayed from the nozzle can wash away all kinds of dirty things on the filter head and the disc. Repeat this operation to operate the second unit, and then under the control of the controller, the water inlet valve and the water outlet valve return to normal working conditions. The cleaning of the other filter unit can be completed in turn without stopping the circulation.
1.2 ultrafiltration
Its main function is to effectively separate clay, crude oil, polymer, microorganism and suspended matter. The ultrafiltration system is mainly composed of the following four systems: ultrafiltration device body, gas flushing and backwashing, chemical cleaning and adding bactericide system. Under the control of ultrafiltration device, in order to effectively control the growth of bacteria, chlorate should be washed at least twice a day; In order to completely restore the performance of the membrane, chemical cleaning should be carried out at least once every three months.
2 "Multi-media filtration+reverse osmosis" treatment process for iron and steel plant wastewater
The process of sewage treatment in iron and steel plant is mainly that after grit is settled between grids, it enters the backwashing pool through the lift pump, and the dosing system is operated during this period. After that, the sewage is pressed into the intermediate tank by the booster pump, treated by the multi-media filter and the activated carbon filter, and then subjected to reverse osmosis by the high-pressure pump, and then flows into the recycling tank.
2. 1 Multi-media filtration
It is mainly composed of anthracite and quartz sand, and works under pressure, so the filtration is more accurate than ordinary filtration. By adding a certain amount of polyaluminum chloride to the sewage for slight flocculation, the suspended solids in the sewage can be further reduced, thus reducing the turbidity of the sewage and ensuring that the pollution index of the treated effluent water quality can be less than 5. The sand filtration system can realize full-automatic operation, and six valves are installed on it. They are gas washing, water outlet, exhaust, positive washing and drainage, backwashing water inlet and backwashing drainage valves. How often to run can be set in advance, and the equipment can automatically control the operation, and whether to clean can be decided by detecting the value of pollution index or measuring the pressure of inlet and outlet water at regular intervals.
2.2 Activated carbon is used in sewage filtration process.
Activated carbon has an important adsorption effect on oxidizing substances, organic substances, iron ions and so on, and its surface area is relatively large. Activated carbon has been applied to the treatment of domestic sewage and achieved good results. Practice has proved that the effluent pollution index of sewage treated by activated carbon filtration can be controlled below 4. The activated carbon filtration system is also automatically operated, and is mainly controlled by six air valves, mainly including an air inlet valve and an exhaust valve; Positive washing drain valve; Backwashing; Drain valve and exhaust valve. The period of activated carbon filtration can be preset for a certain time, and the cleaning time can be determined by detecting the pollution index of sewage, and the cleaning time can be determined by measuring the pressure.
2.3 reverse osmosis
Reverse osmosis membrane is a semi-permeable membrane with special medium, which has strong reverse osmosis ability. Only water molecules can pass through this membrane, but other solutes in sewage can't. As shown in figure 1, after a certain pressure is applied to the sewage, pure water is continuously filtered out and permeated to the other side of the membrane, and various impurity components are intercepted at the same time, and finally highly concentrated brine and relatively clean pure water are obtained. Figure 1 reverse osmosis principle
Reverse osmosis unit is the most critical link in this sewage treatment process, and its main components include: frame, instrument, pipeline, membrane shell and reverse osmosis membrane. In addition, a variety of chemical reagents, such as scale inhibitor, hydrochloric acid, reducing agent and various fungicides, have been put into the water inlet pipe. In order to reduce the scaling of reverse osmosis membrane, it is necessary to add scale inhibitor and hydrochloric acid; In order to prevent the reverse osmosis membrane from being damaged by adding oxides, reducing agents need to be added; The water temperature in the reverse osmosis system is relatively warm, which is very suitable for the growth and reproduction of microorganisms, so a large number of bacteria will breed on the reverse osmosis membrane. In order to effectively kill and control these bacteria, it is necessary to add non-oxidizing bactericide to the sewage regularly, and the effect is very obvious by adding bactericide to kill bacteria and other microorganisms on the reverse osmosis membrane. The high-pressure pump also realizes automatic frequency conversion control to ensure that the pressure rises slowly and gradually when water enters. In the "multi-media filtration+reverse osmosis" sewage treatment system, the pressure, conductivity and flow rate are mainly measured. , automatically decide how much medicine to add, start and close the main engine, open and close the valve, and determine the cleaning frequency.
3 Conclusion
After advanced treatment, the sewage from iron and steel plant can realize the recycling of water resources, mainly as circulating cooling water, which not only effectively improves the utilization rate of water, but also reduces the corrosion degree of the main equipment. The difference between the two modern sewage treatment processes mentioned above is that one uses ultrafiltration and the other uses activated carbon. The use of ultrafiltration is mainly to remove insoluble impurities in water, and can effectively extend the service life of reverse osmosis membrane; Activated carbon can mainly remove some oily substances from water and can be applied to sewage containing many organic substances. Through the analysis and discussion of the above two sewage treatment processes, major iron and steel enterprises should choose the sewage treatment method suitable for their own enterprises according to their actual conditions, continuously improve the recycling of sewage and reduce the consumption of new water. Under the condition that China's water resources are quite poor, realizing the sustainable development of steel mills not only purifies our environment, but also benefits our future generations.
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