How to add chemicals to make chemical phosphorus removal more effective?

The Selection of chemical phosphorus removal agents

In order to produce insoluble phosphate compounds, the main chemical agents are metal salts and calcium hydroxide, which are used for chemical phosphorus removal. Many high valence metal ion reagents will combine with the soluble phosphorus ions in the wastewater to form insoluble compounds. However, the main metal salts used for phosphorus precipitation are Fe3 + salt, Fe2 + salt and Al3 + salt because of economic reasons, which are used in solution and suspension state. In addition to metal salts, calcium hydroxide is also used as precipitation agent to produce water-insoluble calcium phosphate.

Phosphorus removal equation of aluminum salt:

Al2(SO4)3+6H2O→2Al(OH)3+3SO42-+6CO2

Al2(SO4)3+2PO43-→2AlPO4+3SO42-

Under the condition of pH 6.0-6.5, aluminum should be added 1.5-3.0mol per 1mol of phosphorus. If the water is alkaline, the pH should be decreased for reducing the precipitation of Al (OH) 3 before adding aluminum.

Phosphorus removal equation of iron salt:

Fe2(SO4)3+3HCO3-→Fe(OH)3+2SO42-+3CO2

Fe3++PO43-→FePO4↓   pH=5～5.5

Iron (Fe3 +) should be added 1.5-3mol per 1mol of phosphorus, and the optimum pH value is 5.0.

For the 5mg / L phosphorus content secondary treatment water, the phosphorus removal rate of more than 90% can be obtained by adding 100-200mg / L ferric chloride (fecl3.6h2o).

Metal hydroxides can form large flocs, which is beneficial to the flocculation of precipitated products. At the same time, it can also adsorb colloidal substances and fine suspended particles. It should be noted that the precipitation and removal of organic matter is secondary in the chemical precipitation reaction, but the coagulation of organic colloid and suspended solids is the decisive process in the floc.

The precipitation effect is affected by pH value, and the solubility of metal phosphate is also affected by pH value. The optimum pH range is 5.0-5.5 to iron salt , and is 6.0-7.0 to aluminum salt, because the solubility of FePO4 or AIPO4 is the lowest in the above pH range. In addition, the use of metal salt chemicals will bring benefits to sewage and sludge treatment, such as reducing the sludge index of sludge, and conducive to biogas desulfurization.

The content of Cl - or SO42 - ions will increase due to the addition of metal salts in the effluent of wastewater treatment plant. If there is additional acid in the precipitation solution, special attention should be paid.

The alkalinity of wastewater will be reduced by adding metal salts, which may have an adverse effect on the purification. When ferric sulfate is used in the simultaneous precipitation process, the effect on nitrification must be considered.

In addition, if the sludge of sewage treatment plant is used for agriculture, the influence of aluminum or iron load on agriculture must be considered when using metal salt agent to remove phosphorus.

Phosphorus removal equation of lime is as follows:

5Ca2++4OH-+3HPO42-→Ca5OH(PO4)3+3H2O

In order to remove more than 90% phosphorus, it is necessary to adjust the pH value above 10.5 ~ 11.0. The weight ratio of Ca / P is more than 2.2:1.

In the process of precipitation, it is not Ca2 + but OH - ion that plays an important role in the formation of insoluble calcium phosphate. With the increase of pH value, the solubility of calcium phosphate decreases. The required pH value of Ca (OH) 2 for phosphorus removal is above 8.5.

However, when the pH range is between 8.5 and 10.5, not only calcium phosphate precipitation but also calcium carbonate will be produced. It may lead to scaling on the wall of tank, channel and pipe. The reaction of phosphate precipitation with calcium is affected not only by pH value, but also by bicarbonate concentration (Alkalinity). At a certain pH value, the dosage of calcium is proportional to the alkalinity.

For the soft or medium hard sewage, the amount of calcium needed is very small to reach the required pH when calcium precipitation is used. On the contrary, the sewage requires a large amount of calcium with strong buffer capacity.