Background and overview[1-2]
At present, phosphorus removal technology is constantly improving and developing, and there are more and more types of phosphorus removal agents. Commonly used phosphorus removal methods are roughly divided into four types, namely chemical methods, crystallization methods, biological methods and adsorption methods. Among them, chemical phosphorus removal is the earliest used phosphorus removal method. Just adding chemicals can make the effluent reach stable standards. However, the large consumption of chemicals leads to generally high treatment costs, and a large amount of secondary sludge is produced, which is not conducive to subsequent treatment. ; As a contact dephosphorization method, the crystallization method has low cost and is easy to control, but has high maintenance costs and is prone to clogging; the biological method has been widely used in various sewage treatment projects because of its low cost and environmental friendliness, but the treatment effect is not good. Stable and highly dependent on the concentration of organic matter in water. Single biological treatment technology cannot meet emission standards, so the combination of biotechnology and chemical technology has gradually attracted attention and been promoted. However, there are still few efficient, stable, and environmentally friendly phosphorus removal agents. Research and development of microbial strains with good phosphorus removal effects is the key. In contrast, adsorption phosphorus removal technology has the advantages of high selectivity, stable performance, desorption and regeneration, and no secondary pollution, and has become a research hotspot in the field of phosphorus removal at this stage.
Types and phosphorus removal mechanism[2]
1. Physical and chemical phosphorus removers
At present, common physical and chemical phosphorus removal agents mainly include aluminum salts, iron salts, and calcium salts. Among them, there are three main types of aluminum salt phosphorus removal agents, namely aluminum sulfate (Al2 (SO4) 3), aluminum chloride (AlCl3), and polyaluminum chloride (PAC). It is generally believed that its phosphorus removal mechanism mainly includes two processes: the reaction of metal ions and phosphate ions to remove soluble phosphorus and the hydrolysis of metal ions to coagulate organic phosphorus and poorly soluble phosphorus in sewage. In order to confirm whether the aluminum salt phosphorus removal agent is mainly based on adsorption or chemical reaction precipitation, it was found that the proportion of aluminum phosphate in the phosphorus removal precipitate of this type of agent is relatively small, and contains a large amount of adsorbed aluminum salt and aluminum hydroxide. , the process is mainly based on adsorption and co-precipitation. And by using domestic sewage as the treatment object to conduct relevant experiments and compare these three phosphorus removal agents, it was found that the dosage of PAC is lower, the effect is better, and the effluent is stable. When the dosage is 60mg/L, the expected phosphorus removal effect can be achieved. Commonly used iron salt phosphorus removal agents on the market mainly include ferric chloride (FeCl3·6H2O), ferrous sulfate (FeSO4·7H2O), and polyferric sulfate (PFS). The mechanism is similar to the two processes of aluminum salts, but there is no literature to verify whether the coagulation or chemical action plays the dominant role.
FeCl3·6H2O is rarely used alone as a phosphorus removal agent for advanced treatment of sewage. It can be directly added to the biochemical system to improve phosphorus removal efficiency. FeCl3·6H2O was added to the SBR system, and the average phosphorus removal efficiency increased from 76% to 90%, but it has a certain toxic effect on microorganisms. Adding FeSO4·7H2O to the biochemical tank can quickly improve the phosphorus removal effect, but it has no effect on the properties of activated sludge. An environmental protection company used FeSO4·7H2O and PFS to compare the phosphorus removal effects of low-concentration phosphorus-containing wastewater and found that at the same dosage, the removal rate of FeSO4·7H2O was significantly higher than that of PFS because the cost of FeSO4·7H2O was much lower than that of PFS. , so when the TP concentration is low, the treatment effect of FeSO4·7H2O can be experimentally determined. Calcium salts generally refer to two types: lime (Ca(OH)2) and calcium chloride (CaCl2), which are cheap. It mainly adds calcium salt directly to the wastewater to remove phosphorus from the wastewater by generating calcium and phosphorus precipitates. At present, various researchers are still controversial about the appropriate conditions for pH value in the phosphorus removal process. For example, when treating a certain phosphating wastewater, it was found that the phosphorus removal rate is the best when the pH value is between 10 and 11; at the same concentration, the phosphorus removal effect is best when the pH value is between 8.5 and 9.5. In order to better select the appropriate pH value to improve the efficiency of calcium salt phosphorus removal, the control mechanism of the phosphorus removal products was explored based on the role of pH value and OH-, and it was found that the optimal pH value is only related to the initial pH value of the wastewater and phosphoric acid. Dependent on salt concentration.
2. Microbial inoculants
The preparation of microbial inoculants is based on the biological phosphorus removal mechanism of microorganisms’ anaerobic phosphorus release and aerobic phosphorus uptake. The target microorganisms (phosphate-accumulating bacteria) are selected and multiplied through industrial production, and then compounded with biological enzymes. Active microbial agents formed from substances such as , nutrients and catalysts can effectively reduce the phosphorus content in wastewater, and through compounding, the effect is better than a simple biological treatment process. As the advantages of microbial inoculants are highlighted, the number of companies developing and preparing various microbial inoculants is also increasing year by year, and the usage is also expanding year by year. For example, the sewage treatment biological strains of Shanghai Bi Laiqing Biotechnology Co., Ltd. are composed of yeast, lactic acid bacteria, nitrifying bacteria, sulfur bacteria, Bacillus subtilis and other beneficial bacteria; the phosphorus removal product produced by Guangzhou Yusali Environmental Protection Technology Co., Ltd. Special inoculants are prepared from plant sources Bacillus subtilis, high-efficiency functional bacteria, flocculating bacteria, short-chain fatty acids SCFA, enzyme preparations, etc.; Biwofeng Biotechnology Co., Ltd.’s Biwofeng phosphorus removal products are made from It is prepared from phosphate-removing bacteria, biological enzymes, nutrients, catalysts, etc. Compared with physical and chemical agents, although microbial agents have the advantages of long duration, low sludge volume, and no secondary pollution, the treatment rate is relatively slow and generally requires activation before addition.
3. New composite phosphorus removal agent
The new compound phosphorus removal agent is evolved on the basis of the first two types of phosphorus removal agents.During the research process, it was found that the combination of two or more phosphorus removal agents has a better phosphorus removal effect than a single phosphorus removal agent. Therefore, new phosphorus removal agents have been applied. Early research mainly focused on several types of phosphorus removal agents such as aluminum salts, iron salts, calcium salts and polyacrylamide (PAM), which were mixed by some means of stirring or even added one after another. The organic-inorganic composite bentonite adsorbent was prepared for the treatment of phosphorus-containing wastewater, and the phosphorus removal rate reached 97.55%; the composite technology of phosphorus locking agent and microbial inoculants was used to treat eutrophic water bodies, and the phosphorus removal effect was obvious and long-term. With the improvement of technology, composite phosphorus removal agents have developed from simple mixing to cross-linking, co-precipitation, and encapsulation of several agents. A new type of iron-copper phthalocyanine pigment composite adsorbent with good adsorption effect on phosphorus was prepared by co-precipitation method, which has good application prospects. Protonated chitosan/magnetic composite materials were prepared using the reverse suspension cross-linking method, and it was revealed that the adsorption process of the material was mainly chemical adsorption. However, most of these new composite phosphorus removal agents are still in the experimental stage and have not been widely promoted and applied.
Apply[2]
In terms of projects, as environmental protection policies become more and more stringent, it is difficult for the pure biological phosphorus removal process to make the effluent meet the national discharge standards; while the chemical phosphorus removal process of simply imported polymer flocculants will Increase the cost and increase secondary pollution; in order to improve the treatment efficiency without increasing the cost, the “biological + chemical” combined phosphorus removal process is adopted. After adding the chemical agent PFS to a certain urban sewage treatment plant based on the oxidation ditch process, the effluent was stable with TP ≤ 0.25 mg/L. When examining the effluent from the AB process treated with FeCl3·6H2O, when the dosage is 75×10-6, the effluent is stable with TP <0.5 mg/L. Further research found that the order of dosing will affect the phosphorus removal effect. Using PAC to remove phosphorus in sewage, the phosphorus removal effect of two dosings of the same concentration of phosphorus removal agent is significantly better than one dosing. The effluent TP is <0.5mg/L. . At present, in the phosphorus removal process of various sewage treatment projects, chemical phosphorus removal is used to enhance the treatment effect of biological treatment to meet the corresponding discharge standards.
