No abstract

H2 thresholds, concentrations below which H2 consumption by a microbial group stops, have been associated with microbial respiratory processes such as dechlorination, denitrification, sulfate reduction, and methanogenesis. Researchers have proposed that observed H2 thresholds occur when the available Gibbs free energy is minimal (DeltaG approximately 0) for a specific respiratory reaction. Others suggest that microbial kinetics also may play a role in controlling the thresholds. Here, we comprehensively evaluate H2 thresholds in light of microbial thermodynamic and kinetic principles. We show that a thermodynamic H2 threshold for Methanobacterium bryantii M.o.H. is not controlled by DeltaG for methane production from H2 + HCO3-. We repeatedly attain a H2 threshold near 0.4 nM, with a range of 0.2-1 nM, and DeltaG for methanogenesis from H2 + HCO3- is positive, +5 to +7 kJ/mol-H2, at the threshold in most cases. We postulate that the H2 threshold is controlled by a separate reaction other than methane production. The electrons from H2 oxidation are transferred to an electron sink that is a solid-phase component of the cells. We also show that a kinetic threshold (S(min)) occurs at a theoretically computed H2 concentration of about 2400 nM at which biomass growth shifts from positive to negative.

Physical disintegration of representative toilet papers was investigated in this study to assess their disintegration potential in sewer systems. Characterization of toilet papers from different parts of the world indicated two main categories as premium and average quality. Physical disintegration experiments were conducted with representative products from each category according to standard protocols with improvements. The experimental results were simulated by mathematical model to estimate best-fit values of disintegration rate coefficients and fractional distribution ratios. Our results from mathematical modeling and experimental work show that premium products release more amounts of small fibers and disintegrate more slowly than average ones. Comparison of the toilet papers with the tampon applicators studied previously indicates that premium quality toilet papers present significant potential to persist in sewer pipes. Comparison of turbulence level in our experimental setup with those of partial flow conditions in sewer pipes indicates that drains and small sewer pipes are critical sections where disintegration of toilet papers will be limited. For improvement, requirements for minimum pipe slopes may be increased to sustain transport and disintegration of flushable products in small pipes. In parallel, toilet papers can be improved to disintegrate rapidly in sewer systems, while they meet consumer expectations.

H2 is a key electron donor for many anaerobic microorganisms; thus, keen competition for H2 occurs among H2-utilizing microbial groups. Monod kinetic parameters provide essential information for kinetic analysis of competition for H2. In this study, we estimated Monod kinetic parameter values for a methanogen that consumes only H2 as its electron donor, Methanobacterium bryantii M.o.H. Utilization of a single electron donor is an advantage in this study, because complications from alternate metabolic pathways are avoided. Using a set of batch experiments designed to provide the best estimates of each parameter, we obtained these values: maximum specific growth rate (mumax) = 0.77/ day, maximum substrate consumption rate (qmax) = 2.36 mol-H2/gcells/day, true yield (Y) = 0.325 gcell/mol H2, fraction of donor electrons to synthesis (fs degrees) = 0.03 e-cell/e- donor, half-maximum-rate substrate concentration (Ks) = 18 000 nM = 18 microM H2, and endogenous decay rate (b) = 0.088/ day. This self-consistent set of parameters indicates that, when H2 is not limiting, M. bryantii M.o.H. is a slow grower (low mumax) compared to other H2-oxidizing methanogens and sulfate reducers, and this is mainly due to its low true Y, not a low qmax. The relatively high Ks and b values suggest that M. bryantii also may not be a strong competitor when H2 is limiting.