What are the effects of chemical phosphorus removal on sewage treatment

     There are two processes for phosphorus removal: chemical phosphorus removal and biological phosphorus removal. Biological phosphorus removal is a relatively economical method for phosphorus removal. However, because the phosphorus removal process is currently unable to guarantee a stable standard of 0.5mg/l effluent, so To achieve a stable effluent standard, it is often necessary to take chemical phosphorus removal measures to meet the requirements. Chemical phosphorus removal uses inorganic metal salts as precipitating agents to react with phosphate substances in sewage to form insoluble phosphorus compounds and flocs to separate soluble phosphates in sewage.
The influence of normal dosage on sewage treatment
　　1. Influence on metal content of effluent
　　The content of metals and chemicals in the effluent of sewage treatment plants mainly depends on the separation of suspended solids. Of course, the dosage of chemicals, β value, pH value, sewage alkalinity and dosing technology also affect it. Iron and aluminum in the effluent of sewage treatment plants are generally insoluble phosphates and hydroxides and exist in a suspended state.
In the case of normal chemical dosage (such as β=1.5, simultaneous precipitation), pH is neutral and there is a good enough secondary sedimentation tank or sedimentation tank, the content of aluminum and iron will generally not exceed 1.0mg/l . Moreover, although the iron in the influent water of the sewage treatment plant often exceeds 1.0 mg/l, the iron or aluminum content in the effluent water of the flocculation filter is generally less than 0.5 mg/l.
　　2. Influence on salt content of effluent
　　The use of metal reagents for phosphorus precipitation will inevitably lead to the increase of the salt (Cl- or SO42- content) in the effluent of the sewage treatment plant. The increase can be determined by calculation:
Example 1: AlCl3 is added to the middle of the cast. Since 1kgAl corresponds to 3.9 (3×35.5/27=3.94)kg of Cl-, the increase of Cl-: 130×3.94=513kgCl-/d The converted concentration is: 513kg/d×1000/ 10000m3/d=51.3mg/l
Example 2: Adding FeSO4 in the C.I. Because 1kgFe corresponds to 1.7((32+4×16)/56=1.71)kg of SO42-, the increase of SO24-: 270×1.7=461.7kgSO24-/d The converted concentration is 461.7 kg/d×1000/10000m3/d=46.2mg/l
The normal effluent Cl-<100mg/l and SO24-<200mg/l of urban sewage treatment plants containing a small amount of industrial wastewater, which means that the salt content in the effluent after the use of metal chemicals increases by 50% and SO42- by 25 %. When the receiving water body has strict requirements, the salt content should be checked. Because bicarbonate is also removed while phosphate is precipitating, the total salt content (conductivity) in the effluent remains almost unchanged.

　　3. Influence on alkalinity
　　The alkalinity of water refers to the amount of HCI required to make one liter of water reach a certain pH value. The alkalinity also refers to the buffering capacity of acid. The alkalinity of the influent of the sewage treatment plant corresponds to the alkalinity of the drinking water in the basin where it is located and the alkalinity produced by ammonium. During phosphate precipitation, as long as iron or aluminum ions enter the aqueous solution, a hexahydrate complex is formed; the general form is Me(H2O)3+6 (Me: metal). This complex can be further hydrolyzed like an acid:
　　Me(H2O)63+→3H++Me(OH)3+3H2O
　　 The reaction is related to the pH value of the solution, and it also reduces the alkalinity of the water. Since the hydroxide precipitates in the form of a poorly soluble complex and does not increase the alkalinity of the sewage, the acid equivalent must be estimated for the precipitation of metal hydroxides, and the same is true for the precipitation of metal phosphates. Separation of phosphate in simultaneous precipitation can only slightly increase the alkalinity of sewage.
　　4. Impact on remaining sludge output
　　 As mentioned above, the removal of soluble phosphorus in sewage results in the production of sludge. Different processes have different locations for sludge removal. For simultaneous sedimentation, the facility is discharged in the form of excess sludge. The surplus sludge output is an important parameter for the design and operation of sludge treatment. With simultaneous precipitation and chemical phosphorus removal, the unit sludge output is composed of the surplus sludge produced by the removal of BOD5 and the precipitation of the simultaneous precipitation and phosphorus removal . For simultaneous precipitation, the sludge produced by chemical phosphorus removal is determined by the type of precipitation agent, the molar ratio of the added metal ions and the phosphorus to be precipitated. When β=1.5, adding 1kg Fe produces 2.5kg dry matter, or adding 1kg Al produces 4kg dry matter.
　　 For simultaneous precipitation and chemical phosphorus removal, the sludge produced is determined by the type of precipitation agent, the molar ratio of the added metal ions and the phosphorus to be precipitated. When β=1.5, adding 1kg Fe produces 2.5kg dry matter, or adding 1kg Al produces 4kg dry matter.
　　 5. Influence on nitrification reaction
　　When using ferric sulfate for simultaneous precipitation, it will hinder the nitrification reaction. In this case, it is recommended to increase the sludge age by 10%. The use of ferric chloride salt has no effect on the nitrification reaction. The following table shows the influence coefficients of various precipitation processes on the nitrification reaction. This influence coefficient refers to the sludge age under the specific process conditions and the conventional process conditions (no phosphorus removal, and under the same nitrification reaction capacity) ) Is the ratio of sludge age.

The influence coefficient of various processes and chemicals on the nitrification reaction
Process name Medicament Influence factor
Conventional process 1.00
Pre-precipitation FeCl3 0.75-0.90
Simultaneous precipitation FeCl3 0.58-1.00
Simultaneous precipitation FeSO4 1.10-1.35
Because the insoluble carbon compounds will also be precipitated during the pre-precipitation, which cannot provide sufficient carbon compounds for the denitrification reaction required for the stability of the nitrogen oxidation process, so the pre-precipitation can also remove nitrogen. Will have a negative effect. The problem that often arises is that many carbon compounds have been removed by one precipitation, which is often not enough for the pre-denitrification reaction, and pre-precipitation aggravates this contradiction.
　　The impact of excessive dosing on sewage treatment
　　 Synchronous chemical phosphorus removal. If the dosage of phosphorus removal agent is insufficient, the phosphorus removal effect will be poor, but it is not that the more the dosage, the better the effect, and the excessive dosage will cause some problems. The effects of excessive addition of phosphorus removal agent are as follows:
1. Increase flocculation. Another identity of the chemical phosphorus removal agent is the polymer inorganic flocculant. Unlike PAM, the influent flocculant will not have reverse dissolution, which can ensure that some sewage treatment plants have poor sludge flocculation. The water is clear.
　　2. Excessive positively charged metal deposits are adsorbed on the surface of the micelles (negative charges), which changes the charge of the micelles and increases the difficulty of pressing mud without changing the PAM.
　　3. Excessive positively charged metal deposits are adsorbed on the surface of the micelles, which weaken the effect of extracellular polysaccharides (EPS) and weaken the flocculation of the micelles, resulting in fine activated sludge flocs.
　　 4. The amount of sludge increased. In wastewater dephosphorization treatment, when the dosage of dephosphorization agent is too large, the sludge production will be too high. This is also its disadvantage compared to the microbial treatment method used in wastewater phosphorus removal treatment. Excessive dosage will cause more metal phosphates in the water and form more sludge.
　　 5. The difficulty of sludge treatment increases. When the phosphorus removal agent is added excessively, in addition to the increase in the amount of sludge, the inorganic metal salt in the sludge will also increase, which increases the difficulty of sludge treatment.
　　6. ​​The water body turns yellow. When the phosphorus removal agent is added excessively, because the commonly used phosphorus removal agent belongs to the iron salt phosphorus removal agent, it forms too much iron or aluminum ion in the water, which will make the water appear yellow or brown.
　　7. The pH value drops. This is because the phosphorus removal agent is a weakly acidic polymer agent. In the treatment of wastewater phosphorus removal, the increase in the dosage of the acid agent will increase the overall water quality and decrease the pH.

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