William J. Jewell

Biotransformation of tetrachloroethylene by anaerobic attached-films at low temperatures

Abstract This study was conducted to examine the feasibility of using an anaerobic attached-film expanded-bed (AAFEB) process for the treatment of tetrachloroethylene (PCE) at 15°C. A laboratory-scale continuous-flow reactor, with an expanded-bed volume of 900 ml, was operated at hydraulic retention times of 1.8-4 h and influent PCE concentrations of 8–12 mg/l. Small samples (50 ml) of attached film media were used for batch testing 3–7 mg/l of PCE in a separate 300 ml AAFEB reactor. The attached films were a mixed anaerobic consortium grown on diatomaceous earth support particles under methanogenic conditions. Sucrose was used as an external electron donor and growth substrate. Reductive dechlorination of PCE to trichloroethylene (TCE), cis-1,2-dichloroethylene (DCE), vinyl chloride (VC) and ethylene (ETH) was observed. The conversion efficiency of PCE and TCE to lesser chlorinated compounds and ETH was above 98% during continuous-flow testing. VC accumulated as the major dechlorination product and ETH was produced at very low rates. The maximum PCE dechlorination rate, qmax, was 5.33 mg PCE/g volatile solids-day (32.1 μmol/g VS-d) and the one-half velocity coefficient, Ks, was 0.009 mg PCE/l (0.054 μM) under continuous-flow conditions. Since the AAFEB carried more than 20 g volatile solids per liter of bed, low temperature conversion rates would be expected to exceed 60 mg PCE/lbed-day. This indicates removal efficiencies greater than 99% could be obtained at hydraulic retention times of less than 1 h at ambient groundwater temperatures with this process.

METHODS FOR KINETIC ANALYSIS OF METHANE FERMENTATION IN HIGH SOLIDS BIOMASS DIGESTERS

Methods are presented for kinetic analysis of anaerobic biomass reactors. In some cases, assumptions implicit in kinetic analysis techniques developed for conventional dilute digestion modes are not applicable to systems operating at high rates and/or high solids concentrations. As a result, modified definitions are presented for CST digester retention times and first order kinetic coefficients. Procedures are presented for converting biogas data to standard conditions. Two novel methods for quantifying mass removals, based on direct measurement of reactor mass losses and on biogas production, allow rapid determination of mass removal rates and detection of gas leakage. The use of a per unit mass basis for reporting concentrations and kinetics is recommended.

High solids anaerobic methane fermentation of sorghum and cellulose

Abstract Sorghum and sorghum/cellulose mixtures were digested at 55°C at effluent solids contents over 25% total solids (TS). Sorghum alone as a feedstock led to excess NH 3 accumulation, while a sorghum/alpha-cellulose mix controlled NH 3 at acceptable levels. Trace nutrient supplementation was required for stable digestion. Stable performance at organic loading rates (OLR) of 18 and 24 gVS kg −1 day −1 resulted in steady methane production rates of 5.7 and 7.51 CH 4 kg −1 day −1 , respectively. The efficiency of volatile solids (VS) conversion to biogas was 74.8% and 72.1% at organic loading rates (OLR) of 18 and 24 gVS kg −1 day −1 , respectively.

In situ methane enrichment in methanogenic energy crop digesters.

Abstract A simple in situ technique to enrich digester offgas, which normally contains 30–50% carbon dioxide (CO 2 ) by volume, was developed to take advantage of the differing solubilities of CO 2 and methane (CH 4 ). Dissolved CO 2 was removed from the digester in a recycled leachate stream and gas stripped in an external stripper. Bench-scale systems easily enriched the remaining digester offgas to over 95% CH 4 , and CH 4 purities in excess of 98% were achieved. Quantitative evaluation of system variables defined the effects of leachate recycle rates and alkalinity on the resulting offgas methane contents. Offgas CH 4 contents correlated well with the ratio of CO 2 transport capacity to CO 2 production. This ratio was termed the alkalinity/CO 2 ratio, a variable representing the cumulative effects of leachate recycle, leachate alkalinity and digester gas production. Operation at alkalinities of 2 and 4g l −1 (as CaCO 3 ) was more effective than 8 g l −1 for the specific mode of operation used in this study, as elevated alkalinities required extensive lag times between feeding and initiation of stripping, resulting in blow-by of CO 2 into the digester offgas, lowering the offgas CH 4 content. Excessively high recycle rates led to digester pH levels above 8.1, resulting in volatile fatty acid accumulation and lowered CH 4 production rates.

High rate low solids methane fermentation of sorghum, corn and cellulose

Sorghum, sorghum/alpha-cellulose mixture, and corn were anaerobically digested at 55°C at effluent solids contents of 8–12% total solids (TS), using trace nutrient supplementation. Volatile solids (VS) loading rates at much higher levels than conventional maxima were maintained without volatile fatty acid (VFA) accumulation. Semi-continuously fed digesters with organic loading rates (OLR) up to 12 gVS kg−1 d−1 produced methane at rates up to 3.3 L kg−1d−1. Continuous feeding of corn at an OLR of 18 gVS kg−1 d−1 resulted in a methane production rate of 5.4 L kg−1d−1. VS removal efficiencies at maximum OLRs were 60% (sorghum) and 67% (corn). At an OLR of 4 gVS kg−1 d−1 sorghum alone as a feedstock led to excess ammonia-N accumulation. Excess ammonia did not accumulate at sorghum loading rates of 8 and 12 gVS kg−1 d−1 nor with a sorghum/alpha-cellulose mix loaded at 8 gVS kg−1 d−1. Instantaneous gas production rates were directly related to feedstock cell soluble content, with peak instantaneous biogas production rates from corn (OLR of 8 gVS kg−1 d−1 approaching 25 L kg−1 d−1 following a three-day feeding.

Disturbance, starvation, and overfeeding stresses detected by microbial lipid biomarkers in high-solids high-yield methanogenic reactors

SummaryMicrobial biomass and community structure of methanogenic anaerobic biomass reactors can be quantitatively monitored by signature, lipid analysis. The eubacterial and eukaryotic polar lipid fatty acids and the methanogen polar lipid ethers are reliable measures of their respective biomasses. The pattern of polar lipid fatty acids yields information on the community structure and metabolic state of the eubacteria and eukaryotes. These biomarker methods were applied over a 2-day feeding cycle of a highly productive batch-fed high-solids anaerobic biomass reactor. It was sampled before feeding, 6 h after feeding (disturbed)., at maximum gas production (healthy, 24 h), and after feedstock utilization (starved, 48h). Relative to the healthy condition, the disturbance of feeding significantly decreased eubacterial biomass and the proportion of unsaturated fatty acids, and increased branched fatty acids and the eubacterial stress biomarker,trans/cis 16: 1ω7. The starved condition was not significantly different from the healthy in biomass or proportions of fatty acids, but did show a significant increase in the proportion of the eubacterial stress biomarkertrans/cis 18: 1ω7. This reactor was compared to a second of the same design which had been overfed and showed significantly less productivity. The overfed reactor had a significantly lower methanogenic biomass,iso-branched fatty acids, and higher eubacterial stress markers Cy17:0 andtrans/cis 18: 1ω7 than the highly productive reactor.

Fate of pathogens in thermophilic aerobic sludge digestion

Abstract The effect of autoheated aerobic thermophilic digestion on the pathogen content of sewage sludges was studied and compared to that of conventional mesophilic anaerobic digestion. Both systems were full scale, continuously-fed facilities operated in parallel and utilized a feed sludge of thickened primary and waste-activated sludge. The relative populations of viruses, Salmonella sp., total and fecal coliforms, fecal streptococci and parasites found before and after digestion were compared. The full scale mesophilic anaerobic digesters were operated at relatively constant conditions, i.e. digester temperature constant at 35°C, and loading rates constant, etc., while the full scale autoheated aerobic digester was operated under a wide range of loading conditions. At all of the conditions studied, the autoheated digester temperature exceeded 45 C. Virus and Salmonella sp. concentrations in the effluent from the aerobic unit were below detectable limits in 10 of 11 samples and 6 of 6 samples, respectively, whereas the anaerobic digester effluent contained detectable numbers of viruses and Salmonella sp. Bacterial indicator counts and parasite concentrations were less in the autoheated digester effluent than in the effluent from the anaerobic digester. It was concluded that the simple autoheated aerobic digestion process could be used to produce a virtually pathogen-free sludge at a cost comparable to that of conventional, mesophilic anaerobic digestion.

In situ control of sulfide emissions during the thermophilic (55°C) anaerobic digestion process

Abstract Sulfide volatilization was found to be sensitive to the pH variations expected during normal anaerobic digester operation. As digester pH levels increased from 6.7 to 8.2, gaseous sulfide concentrations decreased from 2900 to 100 ppm H 2 S(g). Although gaseous sulfide control through pH adjustment was technically feasible, its practical use was limited by the resulting increase in soluble sulfide concentration. pH adjustment for biogas sulfide control was recommended only under conditions in which the influent sulfur level was well below sulfide inhibitory concentrations. Control of gaseous sulfide levels through insoluble iron (3+) phosphate addition was an efficient gaseous sulfide control process with no adverse effects on digester performance. By varying the influent FePO 4 -Fe:SO 4 2− -S input ratio from 0.0 to 3.5, gaseous sulfide levels decreased from 2400 to 100 ppm. The availability of iron under anaerobic conditions from an aerobically insoluble compound has been termed reductive solubilization. Using results from this investigation, a unique anaerobic digestion system is outlined to treat sulfur rich wastes in which sulfide inhibition is minimized while maximizing energy recovery.

Microbial biomass and community structure of a phase-separated methanogenic reactor determined by lipid analysis

SummaryAn anaerobic phase-separation biomass reactor was established on cellulose with the hydrolysis and fermentation steps occurring in the first stage, and acetogenesis and methanogenesis in the second stage. Based upon lipid biomarker analysis, eubacterial and eukaryotic cells accounted for approximately 6% of the volatile solids of the first stage and 17% of the second, while methanogens were approximately 1% of the volatile solids in the first stage and 9% of the second. Clustering the polar lipid fatty acids into groups based upon their distributions between the two stages of the reactor clarified the differences in community structure caused by phase-separated operation. Although inoculated from the same source, the two stages maintained very different microbial communities. Signature fatty acids known as indicators of unbalanced growth in eubacteria were significantly higher in the first stage of the reactor.

Starvation and overfeeding stress on microbial activities in high-solids high-yield methanogenic digesters

Abstract Methane production from a high-solids digester was significantly decreased by both feedstock starvation and overfeeding. Both starvation and overfeeding also induced significant decreases in acetate turnover, acetate incorporation into methane, and in the ratio of triglyceride to polar lipid. The physiological status ratio of 14 C-acetate incorporated into poly-β-hydroxybutyrate to polar lipid was unchanged by either treatment. Digester pH, and acetate and ammonia concentrations were unchanged by starvation. Overfeeding decreased the pH, and greatly increased ammonia and acetate, creating a toxic environment. The rate of 14 C-acetate incorporation into total lipid significantly increased with starvation but significantly decreased with overfeeding. The ratio of 14 C-acetate incorporated into eubacterial and eukaryotic fatty acids to methanogen ether lipids significantly increased with starvation and significantly decreased under overfeeding. This study provides one of the most thorough descriptions currently available for starvation and overfeeding stress effects on methanogenic digester performance, including specific eubacterial and archaebacterial activities. These results provide important insight into methanogenic digester operating parameters for stable biomass-to-methane yields.