Completely stirred tank reactor behavior in an unmixed anaerobic digester: the induced bed reactor.

The induced bed reactor (IBR) was developed to apply high-rate anaerobic digestion techniques to high suspended solids substrates (6 to 12% total solids). This technology has been implemented at multiple full-scale installations in the United States and Canada. Residence time distribution studies for 58-L laboratory-scale reactors operated at a 3.8-day hydraulic retention time were conducted at 35, 45, and 55 degrees C under control and active digestion conditions. Rhodamine WT and lithium ion were used as tracers. The results show that the IBR most closely approximated completely stirred tank reactor behavior when operated under the study conditions. Mixing was likely a result of a (1) combination of energy inputs from thermal gradients induced by heat flux through the reactors, and (2) shear induced by gas evolution in the sludge bed.

Translocation and cross-contamination of E. coli O157 in beef eye-of-round subprimal cuts processed with high-pressure needleless injection.

UNLABELLED: High-pressure needleless injection (HPNI) is an emerging enhancing process where small-diameter, high-velocity bursts of liquid penetrate soft foods at pressures up to 69 MPa. The incidence and depth of translocated surface-inoculated E. coli O157 in HPNI-processed beef eye-of-round subprimal cuts was determined. HPNI translocated E. coli O157 from the surface to the interior of the eye-of-round subprimal cuts with incidence of 40% (± 7%), 25% (± 8%), and 25% (± 8)% for subprimals that had been surface-inoculated with a 4-strain cocktail at 0.5, 1, and 2 log10 CFU/cm² , respectively. The run-off water was collected and found to contain 2, 2, and 3 log10 CFU/mL E. coli O157. The runoff was reused for HPNI of additional subprimals, and this resulted in a cross-contamination incidence of 83% (± 4%), 60% (± 15%), and 37% (± 6)%. Incidence of translocation and cross-contamination was similar at 0 to 1, 1 to 2, 2 to 3, 3 to 4, 4 to 6, and 6 to 8 cm below the inoculated surface. Results indicate that surface microbiota on beef will be carried to the interior of HPNI-processed beef by initial translocation from the surface with the injected fluid and by cross-contamination with recycled fluid. PRACTICAL APPLICATION: This research has practical relevance for the beef enhancement process called high-pressure needleless injection. The process's effect on surface bacteria on beef was studied.

Method for evaluating anaerobic digester performance.

The degradation rate and efficiency of digestion processes is typically measured by introducing a substrate or pollutant into a digester and then monitoring the effluents for the pollutant or substrate, a costly and slow process. A new method for rapid measurement of the rates and efficiencies of anaerobic degradation of pollutants and lignocellulose substrates from various pretreatments is described. The method uses micro-reactors (10-30mL) containing a mixed culture of anaerobic bacteria obtained from a working anaerobic digester. The rate of degradation of pollutants and metabolic heat rate are measured in parallel sets of micro-reactors. Measurement of metabolic rate and pollutant degradation simultaneously is an effective means of rapidly examining pollutant degradation on a micro-scale. Calorimetric measurements alone allow rapid, relative evaluation of various substrate pretreatment methods.

Effect of feeding strategy on the stability of anaerobic sequencing batch reactor responses to organic loading conditions.

The goal of this study was to examine the effect of feeding strategy on the capability for treatment and the stability of an anaerobic sequencing batch reactor (ASBR) under increasing organic loading. The lab-scale ASBR systems were operated at 35 degrees C using synthetic organic wastewater under both batch and fed-batch operational modes with different feed to cycle time (F:C) ratios. Experimental studies were conducted over a wide range of volumetric organic loading rates (VOLRs) (1.524 g COD/l/d) by varying the hydraulic retention time (HRT) (1.25, 2.5, and 5d) and the feed wastewater's COD (3750-30,000 mg/l). With an F:C ratio greater than or equal to 0.42, the fed-batch mode operation showed higher system efficiency in COD removal, volumetric methane production rate (VMPR), and specific methane production rate (SMPR) as compared to those in the batch mode with identical VOLR and HRT. In the fed-batch mode, the COD removals reached 86-95% with VOLR up to 12 g COD/l/d. The maximums for VMPR of 3.17 l CH4/l/d and for SMPR of 1.63 g CH4-COD/g VSS/d were achieved with a VOLR of 12 g COD/l/d at HRTs of 2.5 and 1.25 d, respectively. The fed-batch operation presented a lower concentration of volatile fatty acids (VFAs) than those in the batch operation. A lower concentration of VFAs confirmed the stability and efficiency of the fed-batch mode operation. The specific methanogenic activity (SMA) analysis showed that the VFA-degrading activity of the biomass in the fed-batch mode was higher for acetate and butyrate, and lower for propionate. Determined biomass yield and bacterial decay coefficients in the fed-batch operational mode were 0.05 g VSS/g COD rem and 0.001 d(-1), respectively.

Production of bio-hydrogen by mesophilic anaerobic fermentation in an acid-phase sequencing batch reactor.

The pH and hydraulic retention time (HRT) of an anaerobic sequencing batch reactor (ASBR) were varied to optimize the conversion of carbohydrate-rich synthetic wastewater into bio-hydrogen. A full factorial design using evolutionary operation (EVOP) was used to determine the effect of the factors and to find the optimum condition of each factor required for high hydrogen production rate. Experimental results from 20 runs indicate that a maximum hydrogen production rate of 4,460-5,540 mL/L/day under the volumetric organic loading rate (VOLR) of 75 g-COD/L/day obtained at an observed design point of HRT = 8 h and pH = 5.7. The hydrogen production rate was strongly dependent on the HRT, and the effect was statistically significant (P < 0.05). However, no significant effect (P > 0.05) was found for the pH on the hydrogen production rate. When the ASBR conditions were set for a maximum hydrogen production rate, the hydrogen production yield and specific hydrogen production rate were 60-74 mL/g-COD and 330-360 mL/g-VSS/day, respectively. The hydrogen composition was 43-51%, and no methanogenesis was observed. Acetate, propionate, butyrate, valerate, caproate, and ethanol were major liquid intermediate metabolites during runs of this ASBR. The dominant fermentative types were butyrate-acetate or ethanol-acetate, representing the typical anaerobic pathway of Clostridium species. This hydrogen-producing ASBR had a higher hydrogen production rate, compared with that produced using continuous-flow stirred tank reactors (CSTRs). This study suggests that the hydrogen-producing ASBR is a promising bio-system for prolonged and stable hydrogen production.

Feasibility of hydrogen production in thermophilic mixed fermentation by natural anaerobes.

The biological sludge from an animal wastewater treatment plant was treated to enrich hydrogen-producing mixed bacteria, and effects on hydrogen yield were investigated during anaerobic fermentation at 55 degrees C. Enrichment of hydrogen-producing bacteria was conducted at pH adjustment of inocula to 3 and 5 with and without additional heat treatment (NHT and HT). The enriched mixed bacteria were cultivated at initial pHs of 5, 6, and 7 with synthetic organic wastewater containing different levels of nitrogen (2.0 and 0.8 g/l as total nitrogen) under static batch conditions. The main effects of heat treatment and enrichment pH were significant on hydrogen production. There was no significant effect of different nitrogen concentrations on hydrogen production. The methane-free biogas contained hydrogen levels of up to 64% for a fermentative condition that showed maximum hydrogen evolution (at culture pH 5 after enrichment at pH 5 with HT). The dominating intermediate metabolites were acetate, n-butyrate, and ethanol. Yields of produced hydrogen were significantly dependent upon levels of n-butyrate.

Bacterial stress enrichment enhances anaerobic hydrogen production in cattle manure sludge.

Methodology was evaluated to selectively enrich hydrogen-producing species present in biological sludge produced during organic wastewater treatment. The influence of bacterial stress enrichment on anaerobic hydrogen-producing microorganisms was investigated in batch tests using serum bottles. Enrichment conditions investigated included application of acute physical and chemical stresses: wet heat, dry heat and desiccation, use of a methanogen inhibitor, freezing and thawing, and chemical acidification with and without preacidification of the sludge at pH 3. For each enrichment sample, cultivation pH value was set at an initial value of 7. After application of selective enrichment (by bacterial stress), hydrogen production was significantly higher than that of untreated original sludge. Hydrogen production from the inocula with bacterial stress enrichment was 1.9-9.8 times greater when compared with control sludge. Chemical acidification using perchloric acid showed the best hydrogen production potential, irrespective of preacidification. Enhancement is due to the selective capture of hydrogen-producing sporeformers, which induces altered anaerobic fermentative metabolism.

Biokinetics in acidogenesis of highly suspended organic wastewater by adenosine 5' triphosphate analysis.

In this paper, we pointed out the problems of using conventional volatile suspended solids (VSS) and chemical oxygen demand (COD) to evaluate biokinetic coefficients, especially for the treatment of highly suspended organic wastewater. We also introduced a novel approach to evaluate biokinetic coefficients by measurement of adenosine 5'-triphosphate (ATP) of microorganisms. The concept of using ATP analysis in biokinetic evaluations with highly suspended wastewater was shown to be effective. This study also showed that the conventional VSS and COD methods were strongly affected by incoming suspended organics in the wastewater and by biokinetics of microorganisms. A cheese-processing wastewater was used in evaluating the biokinetics of mesophilic acidogens. The concentration of COD and total suspended solids in the wastewater was 63.3 g/L and 12.4 g/L, respectively. The TSS was 23.6% of total solids concentration. A high ratio of VSS to total suspended solids of 96.7% indicated that most of the suspended particles were organic materials. Lactose and protein were the major organic components contributing COD in the wastewater, and a total of 94.2% of the COD in the wastewater was due to the presence of lactose and protein. Two different physiological conditions where the maximum rates of acetate and butyrate production occurred were tested. These were pH 7 (condition A for acetate production) and pH 7.3 (condition B for butyrate production) at 36.2C, respectively. Based on the molecular structures of the major organic substances and microbial ATP analysis, the residual substrate and microbial concentrations were stoichiometrically converted to substrate COD (SuCOD) and microbial VSS (MVSS), respectively, using correlation coefficients reported previously. These SuCOD and MVSS were simultaneously used to evaluate the biokinetic coefficients using Monod-based mathematical equations. The nonlinear least squares method with 95% confidence interval was used to evaluate biokinetic coefficients. The maximum microbial growth rate, mu(max) and half saturation coefficient, K(s), for conditions A and B were determined to be 9.9 +/- 0.3 and 9.3 +/- 1.0 day(-1) and 134.0 +/- 58.3 and 482.5 +/- 156.5 mg SuCOD/L, respectively. The microbial yield coefficient, Y, and microbial decay rate coefficient, k(d) for conditions A and B were determined to be 0.29 +/- 0.03 and 0.20 +/- 0.05 mg MVSS/mg SuCOD, and 0.14 +/- 0.05 and 0.25 +/- 0.05 day(-1), respectively. Specific substrate utilization rate at condition B was 43.8 +/- 20.6 mg SuCOD/mg MVSS/day, which was 31% higher than that at condition A.