Based on a theoretical analysis for every items of alkali needed in phosphorus recovery process by struvite

a formula for calculation of alkali amount needed was established

and used for estimating the alkali amount at different process conditions.The values calculated were compared with the experimental one measured and obtained in a double pipes/CO2 stripping packed layer phosphorus recovery pilot unit

and the results showed that average relative error between them was 18.73%

indicating that the theoritical calculation is reliable and the formula can be used for engineering estimation of alkali needed. Due to CO2 and NH3 stripping with packed layer and air

alkali amount needed saved about 20% for the double pipes/CO2 stripping packed layer phosphorus recovery unit.The alkali needed increases linearly with pH

ammonia-nitrogen/phosphorus molar ratio and the initial concentration of phosphorus.However

it should be noted that the effect of magnesium/phosphorus molar ratio was weak.The optimal operation conditions for reducing treatment costs could be as:pH9.20—9.30

molar ratio of ammonia-nitrogen/phosphorus and magnesium/phosphorus 3.0—5.0 and 1.1—1.2 respectively.Reagents consumption for phosphorus recovery process depends on initial concentration of phosphorus in wastewater and process conditions.For example

when initial concentration of phosphorus is 77.5 mg·L-1

N∶Mg∶P=5∶1.2∶1

pH=9.20

alkali amount needed is 0.3315 kg and magnesium chloride containing six crystallization water is 0.6099 kg for treating 1 ton of wastewater

the cost about ￥0.828 and ￥ 0.396 respectively

and ￥1.224 as total

in which cost of alkali takes about 67.6%.