In this study, ammonia stripping was optimized for pretreating anaerobic digestion effluent from an anaerobic digestion plant, and the possibility of using CO2 stripping and biogas injection for adjusting the pH of the effluent before and after the ammonia stripping process was also investigated. For ammonia stripping, the results showed that an overdose of calcium hydroxide, i.e., 27.5 g/L wastewater, achieved higher ammonia, phosphorus, chemical oxygen demand, suspended solids, and turbidity removal efficiency. An air flow rate of 5 L/min for I L of wastewater was thought as suitable for engineering application. The pH of the anaerobic digestion effluent can be increased from about 7 to about 9 by CO, stripping, however which is insufficient for ammonia stripping. For I L of wastewater treated after ammonia stripping, the pH can be neutralized to about 7 from greater than I I through biogas injection at I L/min for less than 30 min, and continuous injection does not decrease the pH. It was roughly estimated that 43 m(3) of biogas (CH4:CO2 approximate to 60%:40%) produced daily could be purified to CH4:CO2 approximate to 74%:26% by neutralizing the pH of the 5 m(3) anaerobic digestion effluent pretreated by ammonia stripping. (c) 2006 Elsevier B.V. All rights reserved.