BACKGROUND: Zero-valent iron (ZVI) is increasingly being applied in biological wastewater treatment to enhance the conversion of various contaminants. The objective of this present study was to investigate the effect of ZVI on the anaerobic biotransformation and dechlorination of chloronitrobenzenes (3,4-DCINB and 4-CINB). Experiments were conducted in two upflow anaerobic sludge blanket (UASB) reactors, one (R2) with 30 g L-1 ZVI added, and the other (R1), serving as control reactor. RESULTS: ZVI-based anaerobic granular sludge (ZVI-AGS) composed of bacteria associated with precipitated FeCO3 and FeS was successfully developed within 5 months in reactor R2. ZVI addition obviously enhanced 3,4-DCINB transformation and dechlorination efficiencies under high 3,4-DCINB loads, and further promoted dechlorination of 4-chloroaniline (4-ClAn) to aniline. Compared with the AGS formed in R1 reactor, iron and its corrosion products were observed and colonized with anaerobes such as methanothrix in ZVI-AGS, and the specific transformation rates of 3,4-DCINB and 4-CINB using ZVI-AGS were improved by 34.0% and 64.4%, respectively. Furthermore, ZVI-AGS provided higher 3,4-dichloronailine and 4-ClAn dechlorination efficiency than AGS. Abiotic transformation of CINBs by ZVI, appropriate concentration of iron corrosion products, lower redox potential and greater hydrogen production were the main factors providing enhanced transformation and dechlorination of CINBs in the UASB reactor. CONCLUSION: Addition of ZVI to a UASB reactor enhanced the reductive transformation and dechlorination of CINBs. It provides a feasible proposal for the design and optimization of a high-rate anaerobic wastewater treatment technique for industrial wastewater. (C) 2010 Society of Chemical Industry