BACKGROUND: The efficiency of H 2 production by dark fermentation described in the literature is around 50% of the maximum theoretical value and the experiments are mostly carried out over short-term operations. This present study evaluated H 2 production in a UASB (upflow anaerobic sludge blanket) reactor operated at 55 ∘ C for 366 days using a synthetic substrate based on sucrose. The reactor response to an increased organic loading rate (OLR) was assessed by applying three different hydraulic retention times (HRTs): 12 h (211 days), 6 h (71 days), and 2 h (84 days). RESULTS: CH 4 was observed in the biogas during startup. The highest average values of the molar H 2 flow rate (MHFR), volumetricH 2 production rate (VHPR), and H 2 yield (HY) were achieved at a HRT of 2 h, and were, respectively, 14.00 ± 5.24 mmol h −1 , 75.33 ± 24.65 mL H 2 L −1 .h −1 , and 1.73 ± 1.14 mol-H 2 mol-suc −1 . Considering only the unstable steady-state period, HY average at a HRT of 2 h was 2.56 ± 0.84 mol-H 2 mol-suc −1 . CONCLUSION: Operational conditions were found to be the main factors responsible for removing methanogenic microorganisms from the system. Increase in the OLR enhanced H 2 production and the ethanol pathway proved to be favorable for H 2 production.

This study evaluates the trace metal composition and fractionation in sludge samples from anaerobic sewage treatment plants from six cities in Brazil. Ten metals were evaluated: Ni, Mn, Se, Co, Fe, Zn, K, Cu, Pb and Cr. Specific methanogenic activity of the sludge was also evaluated using acetic acid as the substrate. Among the essential trace metals for anaerobic digestion, Se, Zn, Ni and Fe were found at a high percentage in the organic matter/sulfide fraction in all sludge samples analyzed. These metals are less available for microorganisms than other metals, i.e., Co and K, which were present in significant amounts in the exchangeable and carbonate fractions. Cu is not typically reported as an essential metal but as a possible inhibitor. One of the samples showed a total Cu concentration close to the maximal amount allowed for reuse as fertilizer. Among the non-essential trace metals, Pb was present in all sludge samples at similar low concentrations and was primarily present in the residual fraction, demonstrating very low availability. Cr was found at low concentrations in all sludge samples, except for the sludge from STP5; interestingly, this sludge presented the lowest specific methanogenic activity, indicating possible Cr toxicity.

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