Cobalt toxicity in anaerobic granular sludge: influence of chemical speciation

The influence of cobalt speciation on the toxicity of cobalt to methylotrophic methanogenesis in anaerobic granular sludge was investigated. The cobalt speciation was studied with three different media that contained varying concentrations of complexing ligands [carbonates, phosphates and ethylenediaminetetraacetic acid (EDTA)]. Three fractions (nominal added, dissolved and free) of cobalt were determined in the liquid media and were correlated with data from batch toxicity experiments. The average concentration of cobalt that was required for 50% inhibition of methanogenic activity (IC50) for free Co2+ in the three sets of measurements was 13 mu mol/L with a standard deviation of 22% and a similarity of 72% between the data obtained in the three different media for the range of cobalt concentrations investigated. The standard deviation of the IC50 for the other two fractions was much higher, i.e. 85 and 144% for the added cobalt and dissolved cobalt, respectively, and the similarity was almost 0% for both fractions. Complexation (and precipitation) with EDTA, phosphates and carbonates was shown to decrease the toxicity of cobalt on methylotrophic methanogenesis. The free cobalt concentration is proposed to be the key parameter to correlate with cobalt toxicity. Thus, the toxicity of cobalt to granular sludge can be estimated based on the equilibrium-free cobalt concentration

Quantitative NME microscopy of iron transport in methanogenic aggregates

Transport of micronutrients (iron, cobalt, nickel, etc.) within biofilms matrixes such as methanogenic granules is of high importance, because these are either essential or toxic for the microorganisms living inside the biofilm. The present study demonstrates quantitative measurements of metal transport inside these biofilms using T1 weighted 3D RARE. It is shown that iron(II)-EDTA diffusion within the granule is independent of direction or the inner structure of the granules. Assuming position dependence of the spin-lattice relaxivity, Fick’s law for diffusion in a sphere can be applied to simulate the diffusion within the methanogenic granules under investigation. A relatively low diffusion coefficient of 2.5*10-11 m2·s-1 was obtained for iron diffusion within the methanogenic granul

Role of nickel in high rate methanol degradation in anaerobic granular sludge bioreactors

The effect of nickel deprivation from the influent of a mesophilic (30°C) methanol fed upflow anaerobic sludge bed (UASB) reactor was investigated by coupling the reactor performance to the evolution of the Methanosarcina population of the bioreactor sludge. The reactor was operated at pH 7.0 and an organic loading rate (OLR) of 5–15 g COD l−1 day−1 for 191 days. A clear limitation of the specific methanogenic activity (SMA) on methanol due to the absence of nickel was observed after 129 days of bioreactor operation: the SMA of the sludge in medium with the complete trace metal solution except nickel amounted to 1.164 (±0.167) g CH4-COD g VSS−1 day−1 compared to 2.027 (±0.111) g CH4-COD g VSS−1 day−1 in a medium with the complete (including nickel) trace metal solution. The methanol removal efficiency during these 129 days was 99%, no volatile fatty acid (VFA) accumulation was observed and the size of the Methanosarcina population increased compared to the seed sludge. Continuation of the UASB reactor operation with the nickel limited sludge lead to incomplete methanol removal, and thus methanol accumulation in the reactor effluent from day 142 onwards. This methanol accumulation subsequently induced an increase of the acetogenic activity in the UASB reactor on day 160. On day 165, 77% of the methanol fed to the system was converted to acetate and the Methanosarcina population size had substantially decreased. Inclusion of 0.5 μM Ni (dosed as NiCl2) to the influent from day 165 onwards lead to the recovery of the methanol removal efficiency to 99% without VFA accumulation within 2 days of bioreactor operation

Effect of vitamin B-12 pulse addition on the performance of cobalt deprived anaerobic granular sludge bioreactors

The effect of a pulse addition of vitamin B-12 as cobalt source to restore the performance of cobalt depleted methanol-fed bioreactors was investigated. One upflow anaerobic sludge bed (UASB) reactor was supplied with a pulse of vitamin B-12, and its operation was compared to that of another cobalt depleted UASB reactor to which a pulse of CoCl2 was given. The addition of cobalt in the form of CoCl2 supplies enough cobalt to restore methanogenesis and maintain full methanol degradation coupled to methane production during more than 35 days after the CoCl2 pulse. Similar to CoCl2, pulse addition of vitamin B-12 supplies enough cobalt to maintain full methanol degradation during more than 35 days after the pulse. However, the specific methanogenic activities (SMAs) of the sludge in the vitamin B-12 supplied reactor were around 3 times higher than the SMA of the sludge from the CoCl2 supplied reactor at the same sampling times. An appropriate dosing strategy (repeated pulse dosing) combined with the choice of vitamin B-12 as the cobalt species is suggested as a promising dosing strategy for methanol-fed anaerobic bioreactors limited by the micronutrient cobalt

Developments and constraints in fermentative hydrogen production

Fermentative hydrogen production is a novel aspect of anaerobic digestion. The main advantage of hydrogen is that it is a clean and renewable energy source/carrier with high specific heat of combustion and no contribution to the Greenhouse effect, and can be used in many industrial applications. This review discusses fermentative hydrogen production from various points of view. First, the theoretical principles of the biological processes taking place in hydrogen production, as well as the organisms responsible for this process, are described. Second, practical aspects of fermentative hydrogen production are overviewed. Suitable conditions for the hydrogen-producers (pH and temperature), suitable substrates for hydrogen production and applicable reactor designs are discussed. Finally, the challenges faced by fermentative hydrogen production are discussed. Current research directions are listed together with the most important problems currently constraining full-scale applicatio

Supplementation of cobalt to UASB reactors by pulse dosing: CoCl2 versus CoEDTA(2-) pulses

The effect of chelation on the dosing strategy of cobalt to restore the performance of a cobalt limited methanol-fed bioreactor was investigated. Three upflow anaerobic sludge bed (UASB) reactors (30 degrees C, pH 7.0) were operated with methanol as the substrate at an organic loading rate of 8.5 g COD L-1 d(-1). One UASB reactor was supplied with several pulses of cobalt bound to EDTA, and its operation was compared to that of another UASB reactor to which several pulses Of CoCl2 were given. The addition of cobalt (5 mu moles cobalt per litre of reactor volume) in the form of CoCl2 creates a pool of cobalt in the granular sludge matrix due to the high cobalt retention (around 90%). The methanogens present in the granular sludge are able to use that cobalt pool for stable methane formation from methanol during the subsequent 15 days. When added as Co-EDTA(2-), only around 8% of the cobalt added is retained. The small amount of retained cobalt in case of Co-EDTA(2-) addition supports methylotrophic methanogenesis only a few operational days. Furthermore, the side-effects EDTA has on the granule matrix or microbial cells make EDTA an unsuitable ligand for cobalt dosage in full-scale applications

Zinc deprivation of methanol fed anaerobic granular sludge bioreactors

The effect of omitting zinc from the influent of mesophilic (30 degrees C) methanol fed upflow anaerobic sludge bed (UASB) reactors, and latter zinc supplementation to the influent to counteract the deprivation, was investigated by coupling the UASB reactor performance to the microbial ecology of the bioreactor sludge. Limitation of the specific methanogenic activity (SMA) on methanol due to the absence of zinc from the influent developed after 137 days of operation. At that day, the SMA in medium with a complete trace metal solution except Zn was 3.4 g CH4-COD g VSS-1 day(-1), compared to 4.2 g CH4-COD g VSS-1 day(-1) in a medium with a complete (including zinc) trace metal solution. The methanol removal capacity during these 137 days was 99% and no volatile fatty acids accumulated. Two UASB reactors, inoculated with the zinc-deprived sludge, were operated to study restoration of the zinc limitation by zinc supplementation to the bioreactor influent. In a first reactor, no changes to the operational conditions were made. This resulted in methanol accumulation in the reactor effluent after 12 days of operation, which subsequently induced acetogenic activity 5 days after the methanol accumulation started. Methano-genesis could not be recovered by the continuous addition of 0.5 mu M ZnCl2 to the reactor for 13 days. In the second reactor, 0.5 mu M ZnCl2 was added from its start-up. Although the reactor stayed 10 days longer methanogenically than the reactor operated without zinc, methanol accumulation was observed in this reactor (up to 1.1 g COD-MeOH L-1) as well. This study shows that zinc limitation can induce failure of methanol fed UASB reactors due to acidification, which cannot be restored by resuming the continuous supply of the deprived metal

Effect of sorption kinetics on nickel toxicity in methanogenic granular sludge

This study investigates the effect of nickel speciation and its equilibrium kinetics on the nickel toxicity to methylotrophic methanogenic activity. Toxicity tests were done with anaerobic granular sludge in three different media containing variable concentrations of complexing ligands. A correlation between nickel toxicity and the free nickel concentration failed, because not the equilibrium conditions, but the kinetics of the speciation processes taking place in the medium (precipitation, sorption, liquid speciation, etc.) determine nickel bio-uptake and its toxic effect. The latter was confirmed with an F-test (p-value always lower than 0.1). It was shown that the biological activity (methane production) took place within 3-20 days upon the start of methanogenic experiments, i.e. prior the chemical-physical equilibrium of nickel speciation was established in the methanogenic medium (10-20 days). The process of nickel sorption in the methanogenic granular sludge was limited by intra-particle diffusion and the experimental data fitted to the Weber-Morris sorption model. The other sorption kinetic models applied (pseudo-first order sorption kinetics, pseudo-second order sorption kinetics and first order reversible reaction kinetics) did not fit the experimental data satisfactorily