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An air-recirculated stripping involved two processes and did not require any pretreatment.First,stripping CO2 decreased the buffer capacity of the anaerobic digestate,thereby reducing the amount of lime used to achieve a high pH.Second,lime was added to increase pH and remove ammonia from the anaerobic digestate of pig manure.pH increased from 8.03 to 8.86 by stripping CO2 in the first process(gas-to-liquid ratio = 180)and further reached 12.38 in the second process(gas-to-liquid ratio = 300).During process optimization,the maximum ammonia removal efficiency reached 96.78%with a lime dose of 22.13 g.The value was close to 98.25%,which was the optimal result predicted by response surface methodology using the software Design-Expert 8.05b.All these results indicated that air-recirculated stripping coupled with absorption was a promising technology for the removal and recovery of nitrogen in the anaerobic digestate of pig manure.