Development of a low flow gas meter and the effect of reactor headspace pressure and mixing on biological hydrogen production via anaerobic fermentation

Accurate measurement of gas production from biological processes is important in many laboratory experiments. A gas metering system, consisting of an embedded controller operating three gas meters, was created to measure volumetric flows between 0 and 10 mL min-1 (1 atm, 273.15 K). The gas meter builds on earlier pressure based designs by adding a reference chamber to avoid errors associated with atmospheric pressure fluctuations. A graphical user interface was developed to download data from the controller to a personal computer and to plot real time data. Each gas meter was calibrated using a bubble meter as the standard. The error of flows predicted with the calibration equation is expected to be within +/- 0.328 mL min-1, 95% of the time. With a more precise calibration flow source, this can be reduced to +/- 0.165 mL min-1.;To ensure accurate gas flow rate measurements, the reference and measurement chambers of the gas meter should be at the same temperature. A modeling effort was undertaken to determine the length of tube needed to cool gas from a mesophilic (35°C) reactor to ambient temperature so that this condition is met. The heat transfer model estimated that over 2.5 meters of tubing should be used to connect reactors to the gas meter.;Anaerobic digestion of organic material is an important renewable energy technology with environmental benefits. Enhancing hydrogen production during anaerobic digestion can improve digester economics, but any new technology must be practical to implement at the industrial scale. Two process parameters that can potentially increase hydrogen production in large scale anaerobic digesters, low reactor headspace pressure and mixing, were chosen for further investigation. These parameters have the potential to lower liquid phase hydrogen concentrations and allow for enhanced hydrogen production via an NADH:ferredoxin oxidoreductase and hydrogenase pathway.;To determine the effect of reactor headspace pressure and mixing on hydrogen production, an experiment with many replicates was planned. The gas metering system developed could only measure gas production from three reactors simultaneously so a series of experiments needed to be conducted, which necessitated inoculum storage. The effect of storage time on anaerobic inoculum was evaluated and was determined to be significant. To account for this effect, a split plot experiment was designed with a time block.;The effect of reactor headspace pressure and mixing on: biogas yield, maximum production rate, and lag time; and ethanol, acetate, and butyrate production was determined with statistical analysis of the split plot experiment. No significant difference was found between reactors vacuumed to 60, 80 and 100 KPa for any of the dependent variables. Mixing (≈ 500 rpm) significantly increased average hydrogen and total biogas yields from 87.4 to 121.4 mL gVS -1 and 219.8 to 306.4 mL gVS-1, respectively. Hydrogen and total biogas maximum production rates increased, and lag times decreased. Acetate to butyrate ratios increased during mixing from 0.44 to 0.68 (p = 0.1107).;Increases in gas production with mixing cannot solely be attributed to improved substrate availability because increased hydrogen yields, larger headspace hydrogen concentrations, and a shift in fermentation products implies metabolic changes. This shift was probably a result of lowered hydrogen concentrations in the liquid phase. This work indicates that mixing hydrolysis reactors following substrate loading in two stage anaerobic digesters has the potential to significantly increase biogas yields by favoring hydrogen production in the first stage and by producing more acetate to feed methanogens in the second stage.