The effect of sub-optimal trace metal concentrations on the conversion of methanol in an upflow anaerobic sludge bed (UASB) reactor was investigated by studying the effect of decreased influent trace metal concentrations on the reactor efficiency, methanol conversion route and sludge characteristics. An UASB reactor (30degreesC; pH 7) was operated for 261 days at a 12-h hydraulic retention time (HRT) and at organic loading rates from 2.6 to 7.8 g chemical oxygen demand (COD) l(-1) reactor(-1) day(-1). Methanol was fully converted to methane (CH4) for 92 days. Thereafter, the reactor efficiency suddenly deteriorated and both methanol and volatile fatty acids (VIA) accumulated in the effluent. The methanogenic activity of the sludge with methanol as the substrate dropped from 1517 mg CH4-COD g volatile suspended solids (VSS)(-1) day(-1) (after 28 days) to 152 mg CH4-COD g VSS-1 day(-1) (after 111 days of operation). Moreover, the sludge lost its methanogenic activity with acetate as the substrate, explaining the acetate build-up in the reactor. Testing the response of the maximum methanogenic activity to individual metals (iron, nickel and cobalt) showed that only iron had significant effects on the methanol degradation rate. Addition of iron at a concentration of 10 muM almost doubled the methanogenic activity of the sludge sampled at day 111. Due to the faster iron-induced methane formation, less acetate accumulated in the batches. Therefore, the iron influent concentration was increased from 1 to 10 muM to restore the reactor performance. The response was, nevertheless, less pronounced as with the batch tests, most probably due to partial washout of the extra dosed iron (which formed colloids with reactor mixed liquor constituents) via the effluent. (C) 2003 Elsevier Science Inc. All rights reserved.