1. Selection of agents

　　Phosphorus removers generally use aluminum salts and iron salts, aluminum salts include aluminum sulfate, sodium aluminate and polymeric aluminum, among which aluminum sulfate is more commonly used. Iron salts include ferric chloride, ferrous chloride, iron sulfate and ferrous sulfate, among which ferric chloride is the most commonly used. There are also those who use lime to remove phosphorus, which is relatively rare. Generally speaking, the phosphorus removal effect is iron salts> aluminum salts> calcium salts!

　　When phosphorus removal is carried out by aluminum salt or iron salt, insoluble aluminum phosphate or iron phosphate is mainly generated, and the dosage is directly proportional to the total phosphorus content in the sewage. It can be used for pre-, post-, and simultaneous dosing of biological reaction cells. The use of ferrous salt needs to be oxidized to iron salt before it can achieve the maximum phosphorus removal effect, so it is generally not used as a coagulant for post-dosing, and is generally added in the aeration settling tank during pre-dosing, so that the ferrous salt can be quickly oxidized into iron salt.

　　When lime is used to remove phosphorus, Ca5(PO4)3OH precipitation is formed, and its solubility is related to pH, so the amount of lime required depends on the alkalinity of the sewage, not the phosphorus content. Lime as a coagulant cannot be used for synchronous phosphorus removal, but can only be used for pre- or post-phosphorus removal. Lime is used for pre-phosphorus removal, and the pH value of sewage is high, and the pH value needs to be adjusted before entering the biological treatment system. When lime is used for post-phosphorus removal, the treated effluent must adjust the pH value to meet the discharge requirements. Lime can also be used for phosphorus removal in sludge anaerobic phosphorus release tanks or phosphorus-rich supernatants produced during sludge treatment. The amount of sludge used to remove phosphorus is much larger than that of aluminum salt or iron salt, so it is rarely used. A small amount of anionic, cationic, or anionic polymeric electrolyte, such as polyacrylamide (PAM), is added as a coagulant to facilitate the coagulation and precipitation of dispersed free metal phosphate flocs.

　　2. Hydraulic conditions

　　The influence of stirring time on chemical phosphorus removal, after the phosphorus remover is added into the wastewater, it is through precipitation and flocculation reaction to adsorb phosphorus in the wastewater, metal ions and phosphate are a condensation phosphorus removal process, so when the phosphorus remover is added, it must be mixed and stirred immediately with high intensity, otherwise the mixing strength will be insufficient, resulting in hindering the precipitation reaction of phosphate. On the contrary, if the slow stirring time is too long, it will also affect the growth of floc.

　　In order to make as much aluminum (iron) salt as possible react with phosphate to form aluminum (iron) salt and become the core particle of the flocculation reaction, it is required to carry out instantaneous high-intensity mixing and stirring when the agent is put into the agent (mixing time is not more than 1s, and the mixing strength is between G=1500~6000s-1). If the mixing strength is insufficient, the sediment particles may be the first to be encapsulated around the metal ions, thus hindering the formation of aluminum (iron) phosphate salts. If the high-intensity stirring time is too long, it will affect the growth of the floc and is not conducive to sedimentation separation. After the initial high-intensity mixing, in order to promote the further growth of aluminum phosphate (iron) and aluminum hydroxide (iron) flocs, the required mixing and flocculation conditions are the same as those required for general wastewater flocculation and precipitation.

　　3、PH

　　The influence of wastewater PH value range on chemical phosphorus removal, whether aluminum salt or iron salt is used as phosphorus removal agent, the PH value required for the best phosphorus removal effect is 6.5--7.0. In this PH range, aluminum salts and iron salts can play the greatest role in precipitation and phosphorus removal.

　　According to the solubility experiment of aluminum phosphate (iron), the lowest solubility of aluminum phosphate in water is about PH=6.3, while the lowest solubility of iron phosphate in water is about PH=5.3. In practical applications, good results have been achieved when the PH value of wastewater is in the range of 6.5~7.0, and the PH value in this range is suitable for most biological treatment processes. For low-alkalinity wastewater, alkali may sometimes need to be added to maintain the PH value in the range of 5~7.

　　Moreover, in the process of chemical phosphorus removal, whether it is aluminum salt phosphorus removal or iron salt phosphorus removal, the alkalinity in the wastewater will be consumed. When the alkalinity of wastewater is too low due to excessive alkali consumption, it will affect the reaction effect of phosphorus removers, so once this situation occurs, alkali needs to be added immediately.

　　4. Suspended objects

　　The effect of suspended solids (SS) and dissolved solids (TDS) in wastewater on chemical phosphorus removal, the role of phosphorus remover in chemical phosphorus removal is to condense solid particles in wastewater, so if there is too much SS in wastewater, a large amount of phosphorus removal flocculant will be consumed, so to control the conditions of low SS and low TDS, note that this factor is limited to pre-phosphorus removal and post-phosphorus removal, excluding synchronous phosphorus removal!

　　5. Dosing and dosing

　　The influence of the amount of phosphorus removal agent on the phosphorus removal effect, the amount of phosphorus removal agent in the process of chemical precipitation phosphorus removal is in a reverse parabolic shape. In addition, the different dosing points of phosphorus removers directly affect the dosage of phosphorus removers.