Room air insufflated during colonoscopy cannot be completely suctioned, is not easily absorbed and remains in the bowel for quite some time, resulting in prolonged bowel distension with the discomfort of bloating. Sufferers often experience a sensation of fullness and abdominal pressure, relieved only after expulsion of the residual gas, often accompanied by colic pain. This can be a lengthy process, and some patients continue to report pain as long as 24 hours after the procedure. Abdominal discomfort after colonoscopy is an adverse event commonly reported by patients, and definitely associated with the procedure. Published reports show that the use of carbon dioxide (CO2) insufflation significantly decreases bloating and pain up to 24 hours post-procedure. Preliminary results of the investigators' previous study about on-demand sedation colonoscopy in diagnostic patients showed that, compared with CO2 insufflation, the water exchange group (WE, infusion of water to distend the lumen during insertion; suction of infused water, residual air pockets an feces predominantly during insertion) achieved significantly lower real-time insertion pain scores. Moreover (insertion-withdrawal method) WE-CO2 had the lowest bloating scores just after the procedure and at discharge, comparable with those achieved by CO2-CO2. Compared with WE-CO2, the use of WE-air insufflation (AI) showed significantly higher bloating scores just after the procedure and at discharge; compared with CO2-CO2 differences were significant only at discharge. The investigators decided to conduct a prospective randomized controlled trial comparing WE-CO2, WE-AI and CO2-CO2. The investigators will test the hypothesis that patients examined by the combination of WE-CO2 will have significantly lower bloating scores at specific time points after colonoscopy than those examined using WE-AI or CO2-CO2. The investigators will also assess the impact of these three methods on patients comfort and activities in the post-procedure period.