This paper describes the effect of the cationic chelating agent penta sodium dithylene triamine penta acetate (DTPA) on anaerobic granular biomass. Batch and continuous tests indicated that acidogenic and methanogenic activity were relatively unaffected by short-term exposure to DTPA. However, long-term exposure to high concentrations (> 1800 mg/L) and low concentrations (180 mg/L) in the absence of a sacrificial trivalent cation eventually led to inhibition of hydrogen consuming and acetoclastic methanogens. An anaerobic sludge bed reactor exposed to DTPA which had been complexed to sacrificial Fe3+was not inhibited.Key words: anaerobic, wastewater, pulp, paper, DTPA, penta sodium diethylene triamine penta acetate, cationic chelator.