Effect of Long‐Term Cobalt Deprivation on Methanol Degradation in a Methanogenic Granular Sludge Bioreactor

Abstract

The effect of the trace metal cobalt on the conversion of methanol in an upflow anaerobic sludge bed (UASB) reactor was investigated by studying the effect of cobalt deprivation from the influent on the reactor efficiency and the sludge characteristics. A UASB reactor (30 degrees C; pH 7) was operated for 261 days at a 12-h hydraulic retention time (HRT). The loading rate was increased stepwise from 2.6 g chemical oxygen demand (COD) x L reactor(-1) x d(-1) to 7.8 g COD x L reactor(-1) x d(-1). Cobalt deprivation had a strong impact on the methanogenic activity of the sludge. In batch tests, the methanogenic activity of the sludge with methanol as the substrate increased 5.3 (day 28) and 2.1 (day 257) times by addition of 840 nM of cobalt. The sludge had an apparent K(m) for cobalt of 948 nM after 28 days of operation and 442 nM at the end of the run. Cobalt deprivation during 54 days of operation led to a methanol conversion efficiency of only 55%. Continuous addition of cobalt (330 nM) for 33 days improved the methanol removal efficiency to 100%. In this period of cobalt dosing, the cobalt concentration in the sludge increased 2.7 times up to 32 microg x g TSS(-1). Upon omission of the cobalt addition, cobalt washed-out at a stable rate of 0.1 microg x g VSS(-1) x d(-1). At the end of the run, the cobalt concentration of the sludge was similar to that of the seed sludge.

Cobalt toxicity in anaerobic granular sludge: influence of chemical speciation

“…Table 5 and Fig. 4 show that there is only a small difference between the optimal and toxic free cobalt concentration: the optimal concentration was shown to be approximately 7 lmol/L with an apparent K m value of 0.9 lmol/L (based on total cobalt addition) [46], whereas a free cobalt concentration of only approximately 18 lmol/L is already significantly toxic (50% inhibition). This means that in case only a small amount of complexing agents is present in the medium, a relatively low total cobalt concentration can cause operational problems with anaerobic reactors.…”

“…The SMA with methanol as the substrate was determined in duplicate at 30 (±2)°C using on-line gas production measurements as described by Zandvoort et al [46]. Granular sludge specimens (approximately 1 g wet wt) were transferred to 120 mL serum bottles containing 50 mL basal medium with 4 g chemical oxygen demand (COD)/L of methanol, along with different cobalt concentrations.…”

“…The cobalt concentration was determined with inductively coupled plasma with optical emission spectrometry (Varian, Palo Alto, CA, USA) as described by Zandvoort et al [46]. The total suspended solids and VSS concentrations were determined according to [1].…”

“…Especially cobalt was many times reported as a very important micronutrient, as the cobalt-containing cofactors (vitamin B 12 ) play a key role in the methylotrophic methanogenic pathway [10,11,47]. It has often been reported as a reason for failure of labscale reactors [10,46] and as a limiting factor in full-scale UASB reactors [47]. Trace metals are dosed in excess in practice in industrial wastewater treatment plants to overcome problems associated with the lack of micronutrients.…”

“…Cobalt was chosen as a model for essential trace metals for methanogenesis. The borders between its nutritional requirement [3,46] and toxicity is very narrow; although so far no toxic concentrations have been reported for anaerobic granular sludge. Therefore, the present study aimed to determine cobalt toxicity data for methanol fed anaerobic granular sludge based on changes in specific methanogenic activity (SMA) upon exposure to various cobalt concentrations.…”