Aerobic digestion is commonly used to achieve secondary sludge reduction in the small-size wastewater treatment plants. Nevertheless, secondary sludge degradation is usually restricted by the slow hydrolysis rate and low degradable percentage of secondary sludge. Here, we present an innovative approach using pretreatment of free ammonia (FA, i.e. NH3), a renewable chemical from wastewater, to improve the degradation of secondary sludge during aerobic digestion. The secondary sludge was degraded by 36 +/- 4% (volatile solids (VS) basis) within 15 days of aerobic digestion while being pretreated at 300 mg NH3-N/L (pH 9.0; total ammonia nitrogen = 800 mg N/L) for 24 h, whereas only 23 +/- 3% (VS basis) of the secondary sludge without FA pretreatment was degraded over the same period. Similarly, the production of inorganic nitrogen also increased from 27 +/- 2 to 38 +/- 2 mg N/g VS after implementing FA pretreatment, corroborating the idea that degradation of secondary sludge was effectively improved by FA pretreatment. Further analysis by model revealed that the improved hydrolysis rate and increased degradable percentage of secondary sludge were responsible for the enhanced sludge degradation in aerobic digestion. It was also found that FA pretreatment would produce an aerobic digestate with a better stability and dewaterability, as indicated by the lower degradable percentage of digestate and the decrease of capillary suction time from 38 +/- 1 to 34 +/- 1 s, respectively. Economic analysis indicates that the FA pretreatment approach would be economically favorable when the sludge transport and disposal cost is higher than $40/wet tone.