"Zero discharge" of wastewater means that after repeated use of industrial water, this part of the salt content and pollutants are highly concentrated into wastewater (more than 99%) for recycling and reuse, and no waste liquid is discharged from the factory. The salts and pollutants in the water are concentrated and crystallized to be discharged in solid form and sent to the landfill of the waste treatment plant or recycled as useful chemical raw materials.

Existing ways to achieve zero emissions are mainly: RCC technology, HERO technology, DTRO membrane technology, electrodialysis technology.

1. RCC technology

The core technologies of RCC are "mechanical vapor recompression cycle evaporation technology", "seed crystal technology" and "mixed salt crystallization technology".

The so-called mechanical vapor recompression cycle evaporation technology, according to the principle of physics, the same amount of material, from liquid to gaseous process, need to absorb a certain amount of heat energy. When a substance changes from a gaseous state to a liquid state, it releases the same amount of heat energy. According to this principle, when using this evaporator to treat waste water, the heat energy required to evaporate the waste water is provided by the heat energy released during steam condensation and condensed water cooling. During operation, there is no loss of latent heat. What is consumed in the operation process is only the electric energy consumed by the water pump, steam pump and control system that drive the circulation and flow of waste water, steam, and condensed water in the evaporator. In order to resist the corrosion of wastewater to the evaporator and ensure the service life of the equipment, the main body of the evaporator and the internal heat exchange tubes are usually made of high-grade titanium alloy. Its service life is 30 years or more.

If the waste water contains a large amount of salt or TDS, when the waste water evaporates in the evaporator, the TDS in the water is easy to adhere to the surface of the heat exchange tube, which will affect the efficiency of the heat exchanger, and the heat exchange tube will be blocked when the amount is severe. Solving the scaling problem of heat exchange tubes in the evaporator is the key to whether the evaporator can be used to treat industrial wastewater. Seed crystal technology can solve the problem of fouling of evaporator heat exchange tubes. The "seed method" is based on calcium sulfate. There must be calcium and sulfide in the wastewater. Before the concentrator starts to operate, if the content of calcium and sulfide ions naturally existing in the wastewater is insufficient, it can be artificially supplemented. Calcium sulfate seeds are added to the wastewater to make the calcium and sulfide ions in the wastewater reach an appropriate level. When the wastewater begins to evaporate, the calcium and calcium sulfate ions that begin to crystallize in the water attach to these seeds and remain suspended in the water without scaling on the surface of the replacement tube. This phenomenon is called "selective crystallization". Brine concentrators can usually be operated continuously for up to one year or more before regular cleaning and maintenance are required. In general, seeding is not required during normal operation, except for the possibility of "seeding" at the start-up of the concentrator.

General productive chemical crystallization process, such as sodium chloride, sodium sulfate and other chemical commodities production, only need to deal with the crystallization of a salt, this kind of single salt brine crystallization process, relatively easy to master, but the salt contained in industrial sewage, complex types, and even contains two kinds of salt composition of double salt. There are a variety of salt co-existence of the brine will produce foam in the crystallizer and has a strong corrosive, while the existence of a variety of different salts, will cause the brine different boiling point increase. Different degrees of fouling have different effects on the heat transfer coefficient of the equipment. The crystallizer for the crystallization of mixed salts can be driven by steam or by an electric steam compressor, which is a more energy efficient system.

2. HERO Technology

HERO is short for High Efficiency Reverse Osmosis. The pretreatment steps of the HERO process are customized according to the water chemistry and site specific design specifications. One step that is constant is that RO is run at high pH.

In order for RO to operate at high pH conditions, all hardness and other cationic components that cause membrane fouling must be removed. Suspended solids should be reduced to near zero to avoid membrane clogging, and carbon dioxide should be removed to a certain extent to reduce the buffering of water. Silicon is highly soluble at high pH and therefore does not limit the recovery of RO. Theoretically, after pretreatment, the recovery ratio will only be limited by the osmotic pressure of the concentrate. This process can achieve a recovery rate of 95%. In most applications of electronic ultrapure water, the recovery rate will be higher.

3. DTRO membrane

The special membrane is mainly composed of a filter diaphragm, a guide disc, a central pull rod, a membrane shell, flanges at both ends, various seals and connecting bolts. The filter diaphragm and the diversion disc are stacked alternately, and the central tie rod string film core is placed into the high-pressure vessel, and the flanges at both ends are fixed, and then the tie rod is combined to form. The raw water reaches the bottom of the membrane element through the gap between the membrane core and the high-pressure vessel, uniformly flows into the diversion disk, flows in a turbulent manner on the surface of the diversion disk, and the produced water is discharged from the membrane element through the central pipe.

The disc tube reverse osmosis (DTRO) membrane completely separates organic matter, salinity and water in the concentrated water of the membrane process, and the quality of the permeate is better. The removal rates of COD and salinity can reach more than 90%. Therefore, the permeate can be directly discharged or enter the biochemical treatment process for further treatment. The concentrated liquid of the disc tube reverse osmosis (DTRO) membrane enters the MVR evaporation system for zero emission treatment of evaporation crystallization.

MVR is a mechanical compression evaporation technology, which maximizes the use of the latent heat of evaporation in the secondary steam. With the action of the MVR pump, low-grade steam can be compressed to high-grade steam with a higher saturation temperature with less energy input, so that the steam can be recycled. This will save a lot of energy than a multi-effect evaporator. Using the secondary steam generated in the evaporation process to compress, increase the temperature and then return to use as an evaporation heat source, can greatly reduce steam consumption. After MVR treatment, most of the water in the concentrated liquid enters the condensate, and a large amount of salt and organic matter precipitate into residue, thus completing the complete separation of high concentration of various pollutants from the water phase.

In principle, the combined process of "disc tube reverse osmosis (DTRO) membrane technology MVR evaporation" has a very ideal removal effect on the organic pollutants and salinity of traditional membrane process wastewater. Most of the pollutants and salinity finally enter the residue of MVR evaporation unit. Therefore, the permeate of disc tube reverse osmosis (DTRO) membrane and the condensate of MVR evaporation unit have good water quality and can be directly used or after simple advanced treatment.

4. Electrodialysis Technology

Under the action of the applied DC electric field, the permeability of the ion exchange membrane (that is, the positive membrane only allows cations through, the negative membrane only allows anions through), so that the anion and cation in the water for directional migration, so as to achieve a physical and chemical process of ion and water separation in the water.

The principle is: between the cathode and the anode, a number of alternately arranged anode membranes and cathode membranes are placed, allowing water to pass through the two membranes and the compartments formed between the two membranes and the two poles. After the electrodes at both ends are connected to the straight-through power supply, the cathode and cation in the water migrate to the anode and cathode respectively. Due to the selective permeability of the anode membrane and the cathode membrane, A light chamber in which the ion concentration decreases and a rich chamber in which the ion concentration increases alternately are formed. At the same time, an oxidation-reduction reaction, that is, an electrode reaction, also occurs on both electrodes. As a result, the cathode chamber is fouled due to the alkaline solution, and the anode chamber is corroded due to the acidic solution. Therefore, in the process of electrodialysis, the consumption of electric energy is mainly used to overcome the resistance and electrode reaction when the current passes through the solution and the membrane.