Synthesis gas fermentation at high cell density: How pH and hydrogen partial pressure affect productivity and product ratio in continuous fermentation

For the first time, syngas fermentation was operated continuously with total cell retention and process pressures up to 4barg in long-term runs of up to 3000 hours. Throughout this time, the process was stable. The measured data have shown that hydrogen uptake and ethanol space-time yield are highest at a slightly reduced pH of 5.7 compared to pH5.9. Even lower pH values lead to higher acetic acid to ethanol product ratios, while C2space-time yields remain constant. Increasing the hydrogen partial pressure to 1.52bar resulted in a significant increase in hydrogen uptake rate and ethanol formation. An ethanol space-time yield of 10mmolL-1h-1 was short-term achieved, being the highest space-time yield measured to date for the wild type of C. ljungdahlii. Hydrogen uptake above a theoretical equilibrium concentration of [Formula: see text] is significantly reduced, indicating an inhibition of an enzymatic reaction.

Biofilm formation as a method of improved treatment during anaerobic digestion of organic matter for biogas recovery

The efficiency of anaerobic digestion could be increased by promoting microbial retention through biofilm development. The inclusion of certain types of biofilm carriers has differentiated existing AD biofilm reactors through their respective mode of biofilm growth. Bacteria and archaea engaged in methanogenesis during anaerobic processes potentially build biofilms by adhering or attaching to biofilm carriers. Meta-analyzed results depicted varying degrees of biogas enhancement within AD biofilm reactors. Furthermore, different carrier materials highly induced the dynamicity of the dominant microbial population in each system. It is suggested that the promotion of surface contact and improvement of interspecies electron transport have greatly impacted the treatment results. Modern spectroscopy techniques have been and will continue to give essential information regarding biofilm's composition and structural organization which can be useful in elucidating the added function of this special layer of microbial cells.

Potential of anammox process towards high-efficient nitrogen removal in wastewater treatment: Theoretical analysis and practical case with a SBR

Anammox process has been widely applied in the wastewater nitrogen removal for its high rate and low cost. However, few researches reported the process potential in treating low-strength nitrogen wastewater. In this study, a sequencing batch reactor (SBR) was taken to explore the feasibility of low-strength nitrogen wastewater treatment by anammox process in theory and practice. The practical operation indicated that the effluent with satisfactory quality (1.90 ± 0.70 mg-TN·L^-1) could be achieved, when the SBR was fed with low-strength nitrogen influent (6.20 ± 0.45 mg-NH₄⁺-N·L^-1 and 7.96 ± 0.59 mg-NO₂^--N·L^-1). The hydraulic retention time (HRT), nitrogen removal efficiency, nitrogen removal rate (NRR) and hydraulic loading rate of SBR were 5.42 h, 86.5%, 0.054 kg-N·m^-3·d^-1 and 4.43 m³·m^-3·d^-1 during the 79-day operation, respectively. The theoretical analysis revealed the potential of anammox SBR. When SBR is stably operated, the maximum NRR would be 0.062 kg-N·m^-3·d^-1 if the effluent nitrogen was required to be as low as 3 mg·L^-1. The NRR value is feasible for engineering. However, considering the lower specific substrates utilization rate in practice, the maximum stable NRR was calibrated and found inefficient afterwards. In order to improve the potential of anammox process, the reactors without back mixing and with periodic bioaugmentation should be taken in priority for the engineering applications. In particular, the bioaugmentation frequency and single addition amount were calculated as 7 d and 0.3 g-VSS·L^-1, respectively. The results may provide guidance for the development of high-efficient and stable nitrogen removal process under low-strength condition.

Immobilization of sulfate and thiosulfate-reducing biomass on sand under haloalkaline conditions

Biological sulfate and thiosulfate reduction under haloalkaline conditions can be applied to treat waste streams from biodesulfurization systems. However, the lack of microbial aggregation under haloalkaline conditions limits the volumetric rates of sulfate and thiosulfate reducing bioreactors. As biomass retention in haloalkaline bioreactors has not been studied before, sand was chosen as a biomass carrier material to increase cell retention and consequently raise the volumetric rates. The results showed that ~10 fold higher biomass concentrations could be achieved with sand, compared to previous studies without carrier addition. The volumetric rates of sulfate/thiosulfate reduction increased approximately 4.5 times. Biomass attachment to the sand was restricted to cavities within the sand particles. Acetate produced by acetogenic bacteria from H₂ and CO₂ was used as carbon source for biomass growth, while formate that was also produced from H₂ and CO₂ enhanced sulfate reduction. The microbial community composition was analyzed by 16S rRNA gene amplicon sequencing, and Tindallia related bacteria were probably responsible for formate formation from hydrogen. The community attached to the sand particles was similar to the suspended fraction, but the relative abundance of sequences most closely related to Desulfohalobiaceae was much higher in the attached fraction compared to the suspended fraction (30% and 13%, respectively). The results indicated that even though the biomass attachment to sand was poor, it still increased the biomass concentration and consequently the sulfate and thiosulfate reduction volumetric rates.

Energy savings and higher volumetric loading rate achieved in the conventional anoxic-oxic process for sewage treatment by enhancing biomass retention in the secondary clarifier

This is the first study to achieve a short HRT (∼2h for the A/O reactor), high MLSS (∼10gL^-1), and high volumetric loading (∼3.7kg CODm^-3d^-1 and ∼0.6kg NH₄-Nm^-3d^-1) in the A/O process for sewage treatment by enhancing biomass retention in the secondary clarifier (∼4h settling time), which we refer to as the high-performance A/O process (HP-A/O) in this paper. Over 258days of continuous operation, with a decrease in HRT from 12 to 2h, remarkable COD (95±3%), NH₄⁺-N (98±2%), TN (79±5%), and TP (74±10%) removals were stably achieved, while the air requirement significantly decreased by 22%. The HP-A/O process offers advantages over the conventional A/O process (6-8h for A/O reactor, 3-5gL^-1MLSS, and ∼1.0kg CODm^-3d^-1) for sewage treatment in terms of its lower energy consumption, smaller footprint and reactor requirements.

Modeling the performance of an anaerobic baffled reactor with the variation of hydraulic retention time

Anaerobic baffled reactors (ABRs) have been widely used in engineering but very few models have been developed to simulate its performance. Based on the integration of biomass retention and liquid-gas mass transfer of biogas into the biochemical process derived in the International Water Association (IWA) Anaerobic Digestion Model No.1 (ADM1), a mathematical model was developed to predict volatile fatty acids (VFAs), chemical oxygen demand (CODCr) and biogas in a 4-compartment ABR operated with variable hydraulic retention time (HRT). The model was calibrated and validated with the experimental data obtained from the reactor when the HRT decreased from 2.0 to 1.0d by stages. It was found that the predicted VFAs, CODCr and biogas agreed well with the experimental data. Consequently, the developed model was a reliable tool to enhance the understanding among the mechanisms of the anaerobic digestion in ABRs, as well as to reactor's designing and operation.

Long-term operation of a pilot scale anaerobic membrane bioreactor (AnMBR) for the treatment of municipal wastewater under psychrophilic conditions

The performance of a pilot scale anaerobic membrane bioreactor (AnMBR), comprising an upflow anaerobic sludge blanket (UASB) reactor coupled to an external ultrafiltration membrane treating municipal wastewater at 18±2°C, was evaluated over three years of stable operation. The reactor was inoculated with a mesophilic inoculum without acclimation. The AnMBR supported a tCOD removal efficiency of 87±1% at hydraulic retention time (HRT) of 7h, operating at a volumetric loading rate (VLR) of between 2 and 2.5kgtCOD/m(3)d, reaching effluent tCOD concentrations of 100-120mg/L and BOD5 concentrations of 35-50mgO2/L. Specific methane yield varied from 0.18 to 0.23Nm(3)CH4/kgCODremoved depending on the recirculation between the membrane module and the UASB reactor. The permeate flow rate, using cycles of 15s backwash, 7.5min filtration, and continuous biogas sparging (40-60m/h), ranged from 10 to 14Lm(2)/h with trans-membrane pressure (TMP) values of 400-550mbar.