The UASB reactor which had 30 g/l ZVI added to it, successfully developed granular sludge (ZVI-AGS) composed of bacteria associated with precipitated FeCO3 and FeS within 5 months. ZVI addition enhanced 3,4-DClNB transformation and dechlorination efficiencies under high 3,4-DClNB loads, and further promoted dechlorination of 4-chloroaniline (4-ClAn) to aniline.