How to treat high concentration ammonia nitrogen wastewater？

1. How did it come?

High ammonia nitrogen wastewater mainly comes from landfill leachate, monosodium glutamate production, coal chemical industry, non-ferrous metal smelting and other industries, and its ammonia nitrogen content reaches 1000~10000mg/L.

2. How to deal with it?

High ammonia nitrogen wastewater has complex composition and strong toxicity, and cannot be treated by biological methods and soil irrigation methods. The main treatment technologies are as follows.

1. Precipitation method of magnesium ammonium phosphate
a. Principle
In the case of weak alkali, add chemicals containing Mg2+ and PO43- to the wastewater containing high concentration of ammonia nitrogen, so that the ammonia nitrogen and phosphorus in the wastewater are precipitated in the form of struvite (magnesium ammonium phosphate), and the wastewater in the wastewater is recovered. Nitrogen and Phosphorus. The reaction process is as follows:
Mg2++NH4++HPO42-+6H2O→MgNH4PO4·6H2O+H+(KSP=2.5×10-13,25℃)
Theoretically, 17.5g of MgNH4PO4·6H2O will be precipitated for every 1g of NH4+-N removed.
b. The main influencing factors of this reaction are: suitable magnesium salt, phosphate, and suitable pH.
MgCl2·6H2O and Na2HPO4·12H2O are mostly used as precipitants, and magnesium ammonium phosphate is an alkaline salt. In the solution environment of pH>9.5, the crystals will dissolve. Therefore, it is very important to control the reaction pH.
c. Features
At present, the MAP method is mostly used in the pretreatment of landfill leachate. It is not affected by temperature, is simple to operate, and has low investment and design cost, and can be applied to the treatment of various concentrations of ammonia nitrogen wastewater.
The operating cost is mainly the added magnesium salts and phosphates. If the company can source materials according to local conditions and find cheap precipitants, such as magnesium-containing or phosphorus-containing wastewater, and make waste from waste and comprehensively utilize it, the processing cost can be greatly reduced.
If a precipitant is added alone, excess magnesium and phosphorus will remain after the wastewater is precipitated, which not only increases the treatment cost, but also introduces phosphorus pollutants, which is likely to cause secondary pollution. The generated ammonium magnesium phosphate precipitate may entrain organic matter and heavy metals in wastewater. Whether it can be used as a compound fertilizer needs further research, and its application value has yet to be developed.
Therefore, two key problems must be solved for the MAP method to be widely used in production:
(1) Cheap precipitant
(2) Purify the ammonium magnesium phosphate precipitate to meet the use standard of compound fertilizer, popularize and apply

2. Stripping method / stripping method
a. Principle
The stripping method has been widely used in fertilizer plant wastewater, landfill leachate, petrochemical, oil refinery and other ammonia nitrogen-containing wastewater. The stripping method is used to remove ammonia nitrogen from water.
That is, the gas is poured into the water, so that the gas and liquid are in full contact with each other, so that the free ammonia dissolved in the water passes through the gas-liquid interface and transfers to the gas phase, so as to achieve the purpose of removing ammonia nitrogen.
Air is commonly used as the carrier (if water vapor is used as the carrier, it is called stripping).
The stripping tower is often operated in countercurrent, and the tower is equipped with a certain height of packing to increase the gas-liquid mass transfer area and facilitate the desorption of ammonia from the wastewater.
Commonly used fillers are Raschig rings, polypropylene Pall rings, polypropylene polyhedral hollow spheres, etc. The wastewater is lifted to the top of the packed tower and distributed to the entire surface of the packing, flows down through the packing, and flows in the opposite direction to the gas. The partial pressure of ammonia in the air increases with the increase of the removal degree of ammonia, and decreases with the increase of the gas-liquid ratio. .
pH is one of the main factors affecting the percentage of free ammonia in water. When the pH is greater than 10, the dissociation rate is above 80%, and when the pH reaches 11, the dissociation rate is as high as 98%.
b, the main influencing factors
The key factors to control the stripping efficiency are water temperature, gas-liquid ratio and pH.
When the water temperature is 25℃, the gas-liquid ratio of stripping is controlled at about 3000~3800, and the pH is controlled at 10.5, which can make the stripping efficiency greater than 90%. waste water.
The temperature will also affect the stripping efficiency. When the water temperature of the stripping method is low, the treatment efficiency is very low, and it is not suitable for use in cold winter. The temperature of the wastewater increases, the proportion of free ammonia increases, and the treatment efficiency increases.
Therefore, the stripping method is an improved version of the stripping method. It uses steam as the carrier to improve the ammonia nitrogen treatment efficiency. The stripper is more suitable for treating wastewater with ammonia nitrogen of 2000~4000mg/L. However, after the stripper runs for a period of time, the stripper will be fouled, which will affect the treatment efficiency.
c. Advantages and disadvantages
The stripping method and the stripping method have simple process, stable effect and low investment; however, the energy consumption is large and the treatment cost is high, and the treatment cost is about 20~30 yuan per ton of water. The effluent ammonia nitrogen is about 50~200mg/L, which cannot meet the discharge requirements, and subsequent advanced treatment must be added to meet the discharge standards.
The ammonia gas blown out is absorbed by water leaching, the concentration of ammonia water is low (about 1%), the reuse value is low, it is volatile, and it is easy to cause secondary pollution; it is absorbed by acid solutions such as sulfuric acid, and other ammonium salts such as ammonium sulfate are generated. Further treatment is required, the technological process is long, and the investment cost is bound to increase, and the final ammonium sulfate product produced is low in price and difficult to sell.

3. Stripping rectification method

The treatment of ammonia nitrogen wastewater based on stripping and simple stripping methods has problems of secondary pollution and high operating costs. At this stage, many environmental protection equipment research and development institutions have improved and adopted the method of rectifying ammonia to recover ammonia water, which is widely used in the treatment of ammonia nitrogen in production. waste water.
a. Principle
There is a difference in the relative volatility of ammonia and water molecules. By carrying out multiple gas-liquid phase balances in the rectification tower, ammonia nitrogen is separated from water in the form of molecular ammonia, and then discharged from the top of the tower in the form of ammonia water or liquid ammonia, and is The condenser is cooled to room temperature to become high-purity ammonia water for recycling, which can be reused for production or direct sales.
The pH of the effluent from the tower kettle is controlled to be above 10, and the ammonia nitrogen concentration of the wastewater after deamination can be reduced to below 10 mg/L, which can be directly discharged or reused for production after treatment.
b, the cost of stripping and rectifying ammonia water recovery method
Investment cost: 1.2 to 6 million yuan, the concentration of recovered ammonia water: 16% to 22% concentrated ammonia water. Operating cost: 5~10 yuan/ton. The operating cost is greatly affected by the ammonia nitrogen concentration and pH of the raw water. For wastewater with high ammonia nitrogen and high pH, the more ammonia water is recovered, the lower the operating cost.
c. Advantages and disadvantages
The investment cost and operating cost of this method are at a moderate level, but the concentration of recovered ammonia water is relatively high, which can be reused for production or sold according to the situation of the enterprise. The profit from ammonia water reuse or outsourcing can basically offset the operating cost of the process equipment, and the effluent effect is good, and the ammonia nitrogen concentration can be reduced to below 10mg/L, eliminating the need for the investment and operation of secondary deamination in order to meet the emission standards. cost.
The disadvantage is that in order to ensure that the effluent reaches the standard, the pH of the effluent must be controlled above 10, resulting in a waste of alkali, and acid must be added to return to neutrality before the discharge can meet the standard. In addition, this method is especially suitable for high-concentration ammonia nitrogen alkaline wastewater with ammonia nitrogen concentration above 7000mg/L. Otherwise, the ammonia nitrogen concentration is low, and under the same conditions, less ammonia water is recovered, and the benefit of ammonia water reuse or take-out is low, and the overall operating cost is low. will rise.

4. Gaseous membrane method

Gaseous membrane, also known as support membrane, membrane absorption. It has been used in the removal, recovery, enrichment and purification of volatile reactive solutes such as NH3, CO2, SO2, H2S, Cl2, Br2, I2, HCN, amine, and phenol in aqueous solutions.
The gaseous membrane has the advantages of specific surface area, high mass transfer driving force, large operating flexibility, high ammonia nitrogen removal efficiency, and no secondary pollution.
The gaseous membrane deamination technology uses a hydrophobic hollow fiber microporous membrane as a barrier for ammonia-containing wastewater and absorption liquid. At this time, one side of the membrane is the ammonia nitrogen wastewater to be treated, and the other side is the acid absorption liquid. The hydrophobic microporous structure is in A thin gas film structure is provided between the two liquid phases.
The free NH3 in the wastewater diffuses to the surface of the hydrophobic microporous membrane through the concentration boundary layer on the wastewater side. Then, driven by the partial pressure difference of NH3 on both sides of the membrane, NH3 vaporizes into the membrane pores at the interface between the wastewater and the microporous membrane, and then diffuses. It enters the absorbing liquid side and reacts rapidly and irreversibly with the acidic absorbing liquid, so as to achieve the purpose of ammonia nitrogen removal.
Gaseous membrane deamination generally uses dilute sulfuric acid as the absorbent, but for many enterprises, the generated ammonium sulfate has problems such as low sales price, which is not an ideal recycling product, and many enterprises prefer to recover a certain concentration of ammonia water for personal use or sales.
Therefore, the combination of gaseous membrane + rectification technology has attracted attention. The principle is mainly to use a renewable absorbent to absorb ammonia on both sides of the membrane, and the saturated absorbent is rectified by rectification to recover 15~18% ammonia water. Ammonia nitrogen can reach below 15mg/L, and the absorbent can be reused.
This method is most suitable for the treatment of ammonia nitrogen in wastewater between 3000 and 6000 mg/L. Saturated absorbent can increase the concentration of ammonia nitrogen to more than 10000 mg/L, the steam consumed by rectification is greatly reduced, and the treatment cost is the lowest compared with other treatment methods. Highest benefit.
For the wastewater with ammonia nitrogen above 8000mg/L, the gaseous membrane method has no obvious cost advantage.