Recirculation of H2, CO2, and ethylene improves carbon fixation and carboxylate yields in anaerobic fermentation

Anaerobic fermentation with mixed cultures has gained momentum as a bioprocess for its promise to produce platform carboxylates from low-value biomass feedstocks. Anaerobic fermenters are net carbon emitters and their carboxylate yields are limited by electron donor availability. In a new approach to tackle these two disadvantages, we operated two bioreactors fed with acetate and lactate as a model feedstock while recirculating H2/CO2 to stimulate concomitant autotrophic activity. After 42 days of operation, hydrogenotrophic methanogenesis was predominant and ethylene (≥1.3 kPa) was added to one of the reactors, inhibiting methanogenesis completely and recovering net carbon fixation (0.20 g CO2 L-1 d-1). When methanogenesis was inhibited, exogenous H2 accounted for 17% of the consumed electron donors. Lactate-to-butyrate selectivity was 101% (88% in the control without ethylene) and lactate-to-caproate selectivity was 17% (2.3% in the control). Community analysis revealed that ethylene caused Methanobacterium to be washed out, giving room to acetogenic bacteria. In contrast to 2-bromoethanosulfonate, ethylene is a scalable methanogenesis inhibition strategy that did not collaterally block i-butyrate formation. By favoring the bacterial share of the community to become mixotrophic, the concept offers a way to simultaneously increase selectivity to medium-chain carboxylates and to develop a carbon-fixing chain elongation process.

Full-scale on-line assessment of toxic wastewaters causing change in biodegradation model structure and parameters

Detecting wastewater toxicity in due time is essential for protection of a sewage works and the receiving waters. A respirometric method is presented that performs short batch experiments, so-called In-Sensor-Experiments for toxicity detection. Two types of wastewater samples can be added to the reactor in the device: either the potentially toxic wastewater entering the plant, or, a defined mixture of acetate and ammonia. From the latter experiments models are identified that describe the heterotrophic and autotrophic activity of the sludge. Since these ‘calibration’ experiments are alternated with experiments in which wastewater is injected, the effect of the wastewater on the sludge can be quantified unequivocally. Full-scale toxicity detection (and the corresponding effluent quality) results are reported for a plant treating a mixture of hospital and municipal wastewaters. The respirometer was installed at the influent line of the plant. It was evaluated during a 6-month period for its on-line toxicity detection capacity. Both deliberate and accidental intoxications were recorded and compared with off-line toxicity measurements. Inhibitory wastewaters affected the nitrification activity of the sludge. This was confirmed by the concomitant increase in NH4+ discharge of the treatment plant. To evaluate the efficiency of control actions, the deliberate addition of toxicant was interrupted at the time a toxicity alarm was triggered by the respirometer. It was observed that plant performance then remained unaffected for all monitored criteria.

An experimental study on the effect of elevated temperature on the heterotrophic and autotrophic food resources of aquatic insects in a forested stream

The effect of temperature on the heterotrophic and autotrophic food resources of aquatic insects was investigated in outdoor experimental channels and in the laboratory. Elevated temperatures in outdoor channels resulted in accelerated decay rates and accumulation of organic layers on introduced tiles. Heterotrophic activity (substrate respiration) increased with temperature in the laboratory for leaf strips, flocculent organic matter, and stone surface organic layers. Autotrophic activity (substrate photosynthesis) was greater in the laboratory for flocculent detritus (per milligram ash-free dry weight) than for stone surface organic layers. These results indicate that the primary food resources of shredder, collector, and scraper functional guilds are enhanced by elevated temperature, but also suggest that coarse particulate organic matter may disappear from the stream more quickly. The results also point out that substantial autotrophic activity occurs within flocculent organic detritus.