Spyros G. Pavlostathis

Kinetics of anaerobic treatment: A critical review

Abstract The fundamentals of microbial kinetics and continuous culture models are presented. The kinetics of the anaerobic treatment processes are reviewed recognizing that anaerobic degradation of complex, polymeric organic materials is a combination of series and parallel reactions. Such reactions include hydrolysis, fermentation, anaerobic oxidation of fatty acids, and methanogenesis. The intrinsic rates of each step are reviewed and literature data summarized. Whenever possible, available kinetic information is summarized on the basis of substrate composition (such as carbohydrates, proteins, and lipids). The effect of temperature and inhibitors on the intrinsic kinetic rates is discussed. Stoichiometric and bioenergetic considerations are reviewed. Mass transfer limitations (both external and internal) associated with biofilms and microbial agglomerates, in general, and their effect on the intrinsic kinetic rates are presented. Areas requiring further research are identified.

Biological Chromium(VI) Reduction in the Cathode of a Microbial Fuel Cell

The biocathode of a microbial fuel cell (MFC) offers a promising potential for the reductive treatment of oxidized pollutants. In this study, we demonstrated biological Cr(VI) reduction in the cathode of a MFC and identified putative Cr(VI) reducing microorganisms. The MFC was continuously monitored for Cr(VI) reduction and power generation. Acetate was provided to the anode compartment as substrate and bicarbonate was added to the cathode compartment as the sole external carbon source. The contribution of biomass decay and abiotic processes on Cr(VI) reduction was minimal, confirming that most of the Cr(VI) reduction was assisted by microbial activity in the cathode, which utilizes electrons and protons generated from the oxidation of acetate in the anode compartment. Relatively fast Cr(VI) reduction was observed at initial Cr(VI) concentrations below 80 mg/L. However, at 80 mg Cr(VI)/L, Cr(VI) reduction was extremely slow. A maximum Cr(VI) reduction rate of 0.46 mg Cr(VI)/g VSS.h was achieved, which resulted in a current and power density of 123.4 mA/m(2) and 55.5 mW/m(2), respectively. The reduced chromium was nondetectable in the supernatant of the catholyte which indicated complete removal of chromium as Cr(OH)(3) precipitate. Analysis of the 16S rRNA gene based clone library revealed that the cathode biomass was largely dominated by phylotypes closely related to Trichococcus pasteurii and Pseudomonas aeruginosa, the putative Cr(VI) reducers.

Aerobic biodegradation of selected monoterpenes

Abstract Batch experiments were conducted to assess the biotransformation potential of four hydrocarbon monoterpenes (d-limonene, α-pinene, γ-terpinene, and terpinolene) and four alcohols (arbanol, linalool, plinol, and α-terpineol) under aerobic conditions at 23°C. Both forest-soil extract and enriched cultures were used as inocula for the biodegradation experiments conducted first without, then with prior microbial acclimation to the monoterpenes tested. All four hydrocarbons and two alcohols were readily degraded. The increase in biomass and headspace CO2 concentrations paralleled the depletion of monoterpenes, thus confirming that terpene disappearance was the result of biodegradation accompanied by microbial growth and mineralization. Plinol resisted degradation in assays using inocula from diverse sources, while arbanol degraded very slowly. A significant fraction of d-limonene-derived carbon was accounted for as non-extractable, dissolved organic carbon, whereas terpineol exhibited a much higher degree of utilization. The rate and extent of monoterpene biodegradation were not significantly affected by the presence of dissolved natural organic matter.

Biodegradation kinetics of monoterpenes in liquid and soil-slurry systems

Abstract Batch experiments were conducted to evaluate the biodegradation rates of limonene, α-pinene, γ-terpinene, terpinolene and α-terpineol at 23 °C under aerobic conditions. Biodegradation was demonstrated by the depletion of monoterpene mass, CO2 production and a corresponding increase in biomass. Monoterpene degradation in liquid cultures devoid of soil followed Monod kinetics. The maximum specific growth rate (μmax) was 0.02 h−1 and 0.06 h−1 and the half-velocity constant (Ks ) varied from 32 mg/l to 3 mg/l for the limonene and α-terpineol respectively. The recovery of monoterpenes by solvent extraction from autoclaved and azide-amended soil-slurry samples decreased over time and ranged from 69% to 73% for 120 h of incubation period. Although a significant fraction of monoterpene hydrocarbon could not be extracted, mineralization of these compounds in the soil-slurry systems took place, as shown by CO2 production. The soil-normalized degradation rates for the hydrocarbon monoterpenes ranged from 0.6 μg g−1 h−1 to 2.1 μg g−1 h−1. A kinetic model – which combined monoterpene biodegradation in the liquid phase and net desorption – was developed and applied to data obtained from soil-slurry assays.

Methane recovery from the anaerobic codigestion of municipal sludge and FOG

The anaerobic biodegradability of a mix of municipal primary sludge (PS), thickened waste activated sludge (TWAS) and fat, oil, and grease (FOG) was assessed using semi-continuous feed, laboratory-scale anaerobic digesters operated at mesophilic (35 degrees C) and thermophilic (52 degrees C) temperature. Addition of a large FOG fraction (48% of the total VS load) to a PS+TWAS mix, resulted in 2.95 times larger methane yield, 152 vs. 449 mL methane @ STP/g VS added at 35 degrees C and 2.6 times larger methane yield, 197 vs. 512 mL methane @ STP/g VS added at 52 degrees C. The high FOG organic load fraction was not inhibitory to the process. The results of this study demonstrate the benefit of sludge and FOG codigestion.

Quaternary ammonium disinfectants: microbial adaptation, degradation and ecology

Disinfectants play an important role in maintaining acceptable health standards by significantly reducing microbial loads as well as reducing, if not eliminating, pathogens. This review focuses on quaternary ammonium compounds (QACs), a widely used class of organic disinfectants. Specifically, it reviews the occurrence, microbial adaptation, and degradation of QACs, focusing on recent reports on the ecology of QAC-degraders, the pathways and mechanisms of microbial adaptation which lead to resistance to QACs, as well as to antibiotics. With the help of culture-dependent and nonculture-dependent tools, as well as advanced analytical techniques, a better understanding of the fate and effect of QACs and their biotransformation products is emerging. Understanding the underlying mechanisms and conditions that result in QAC resistance and biodegradation will be instrumental in the prudent use of existing QAC formulations and foster the development of safer disinfectants. Development and implementation of (bio)technologies for the elimination of QACs from treated wastewater effluents will lessen adverse impacts to both humans and the environment.

Aerobic biodegradation of thiocyanate

Abstract The biotransformation of thiocyanate (SCN − ) and cyanate (OCN − ) in a predominantly heterotrophic, activated sludge-derived culture was investigated. Acclimation and enrichment of aerobic, thiocyanate-degrading microorganisms led to complete thiocyanate oxidation to ammonia, bicarbonate, and sulfate. Low levels of cyanate were detected, indicating that its hydrolysis is not the rate-limiting step in the complete aerobic oxidation of thiocyanate. Cyanate removal followed the hydrolytic route to ammonia and bicarbonate under aerobic conditions. Abiotic assays confirmed that the observed thiocyanate removal was biologically mediated while cyanate hydrolysis was enhanced enzymatically. Neither thiocyanate nor cyanate inhibited the aerobic removal and assimilation of organic compounds simulating typical municipal wastewaters. However, partial inhibition of nitrification, especially the conversion of nitrite to nitrate, was observed, and was attributed to thiocyanate and/or free ammonia.

Long-Term Exposure to Benzalkonium Chloride Disinfectants Results in Change of Microbial Community Structure and Increased Antimicrobial Resistance

The effect of benzalkonium chlorides (BACs), a widely used class of quaternary ammonium disinfectants, on microbial community structure and antimicrobial resistance was investigated using three aerobic microbial communities: BACs-unexposed (DP, fed a mixture of dextrin/peptone), BACs-exposed (DPB, fed a mixture of dextrin/peptone and BACs), and BACs-enriched (B, fed only BACs). Long-term exposure to BACs reduced community diversity and resulted in the enrichment of BAC-resistant species, predominantly Pseudomonas species. Exposure of the two microbial communities to BACs significantly decreased their susceptibility to BACs as well as three clinically relevant antibiotics (penicillin G, tetracycline, ciprofloxacin). Increased resistance to BACs and penicillin G of the two BACs-exposed communities is predominantly attributed to degradation or transformation of these compounds, whereas resistance to tetracycline and ciprofloxacin is largely due to the activity of efflux pumps. Quantification of several key multidrug resistance genes showed a much higher number of copies of these genes in the DPB and B microbial communities compared to the DP community. Collectively, our findings indicate that exposure of a microbial community to BACs results in increased antibiotic resistance, which has important implications for both human and environmental health.

Sorption of quaternary ammonium compounds to municipal sludge

The sorptive behavior of four quaternary ammonium compounds (QACs) - hexadecyl trimethyl ammonium chloride (C(16)TMA), dodecyl trimethyl ammonium chloride (C(12)TMA), hexadecyl benzyl dimethyl ammonium chloride (C(16)BDMA), and dodecyl benzyl dimethyl ammonium chloride (C(12)BDMA) - to municipal primary, waste activated, mesophilic digested, and thermophilic digested sludges was assessed at 22 degrees C. Batch adsorption of all four separately tested QACs to primary sludge reached equilibrium within 4h. At a nominal, initial QAC concentration of 300mg/L and a sludge volatile solids concentration of 1g/L, the extent of adsorption was 13, 88, 67, and 89% for the C(12)TMA, C(16)TMA, C(12)BDMA, and C(16)BDMA, respectively, and correlated positively to the QAC hydrophobicity and negatively to their critical micelle concentration. Equilibrium partitioning data were described by the Freundlich isotherm model. The adsorption capacity of the four sludges was very similar. In binary QAC mixtures, QACs with relatively high adsorption affinity and at relatively high aqueous concentrations decreased the adsorption of QACs with a low adsorption affinity. At pH 7, about 40% of the sludge-C(12)TMA desorbed, whereas less than 5% of the sludge-C(16)BDMA desorbed in 10 days. The effect of pH was negligible on the desorption extent of C(12)TMA at a pH range 4-10 over 10 days, whereas increasing the solution pH to 10 resulted in more than 50% desorption of C(16)BDMA. Given the fact that approximately 50% of the municipal biosolids are land-applied in the US, the data of this study would help in the assessment of the fate of QACs and their potential effect on human and environmental health.

Microbial community adaptation to quaternary ammonium biocides as revealed by metagenomics.

Quaternary ammonium compounds (QACs) represent widely used cationic biocides that persist in natural environments. Although microbial degradation, sensitivity and resistance to QACs have been extensively documented, a quantitative understanding of how whole communities adapt to QAC exposure remain elusive. To gain insights into these issues, we exposed a microbial community from a contaminated river sediment to varied levels of benzalkonium chlorides (BACs, a family of QACs) for 3 years. Comparative metagenomic analysis showed that the BAC-fed communities were dramatically decreased in phylogenetic diversity compared with the control (no BAC exposure), resulting presumably from BAC toxicity, and dominated by Pseudomonas species (> 50% of the total). Time-course metagenomics revealed that community adaptation occurred primarily via selective enrichment of BAC-degrading Pseudomonas populations, particularly P. nitroreducens, and secondarily via amino acid substitutions and horizontal transfer of a few selected genes in the Pseudomonas populations, including a gene encoding a PAS/PAC sensor protein and ring-hydroxylating dioxygenase genes. P. nitroreducens isolates were reproducibly recoverable from communities after prolonged periods of no-BAC exposure, suggesting that they are robust BAC-degraders. Our study provides new insights into the mechanisms and tempo of microbial community adaptation to QAC exposure and has implications for treating QACs in biological engineered systems.