Abstract
The effect of long‐term deprivation of all trace elements or solely cobalt on methanol conversion in upflow anaerobic sludge bed (UASB) reactors inoculated with a nickel‐limited sludge was investigated. Two UASB (30 °C, pH 7) reactors were operated at organic loading rates of up to 17.6 g chemical oxygen demand (COD) L^−1^ day^−1^, one reactor (R1) with no trace metal supplementation to the influent and another reactor (R2) to which all trace elements except cobalt were supplied. The cobalt content of the sludge in both R1 and R2 decreased at a rate of 0.1 µg g^−1^ total suspended solids (TSS) day^−1^, while the nickel content of the R1 sludge did not decrease despite nickel being absent in the feed. Cobalt limitation of the methanogenesis from methanol was induced in both reactors within approximately 55 days of operation. The specific methanogenic activity (SMA) of the inoculum sludge with methanol as the substrate could be increased by about 50% by adding solely nickel (5 µmol L^−1^) to the batch medium. After 55 days of operation the SMA of the R2 sludge increased by 49% upon the addition of solely cobalt (5 µmol L^−1^) to the batch medium. This indicates that a cobalt limitation was induced in the R2 sludge by omitting cobalt from the feed and that the initial nickel limitation was overcome by supplementation of nickel to the feed. Copyright © 2006 Society of Chemical Industry