Air stripping at alkaline pH was carried out to remove ammonia from the piggery wastewater, and its effects on subsequent anaerobic digestion were investigated in semi-continuous experiments. In ammonia stripping process, three alkalis (NaOH, KOH and CaO) were used for pH adjustment. When using NaOH and KOH, the methane production rate increased more than two folds as compared to the control (no ammonia stripped), but cation toxicity exerted by sodium and potassium ions was observed. When using lime, on the contrary, it was found that volumetric methane production rates (1040-1130 mLCH(4)/L day) and yields (262.3-258.9 mL CH4/g of CODadded) were significantly higher than others. In addition, the organic removal efficiencies (54.2-59.5% of volatile solid, 59.6-64.0% of total COD, 72.1-81.9% of soluble COD and 89.3-98.9% of volatile fatty acid) were also high. Batch toxicity test results confirmed that cations of Na+, K+ were strong methanogenic inhibitors as compared to Ca2+. From these observations, it was concluded that ammonia stripping at alkaline pH is important for anaerobic digestion of piggery wastewater and the alkali types should be chosen cautiously to avoid cation toxicity. (C) 2010 Published by Elsevier B.V.