Emer Colleran

Anaerobic treatment of sulphate-containing waste streams

Sulphate-containing wastewaters from the paper and board industry, molasses-based fermentation industries and edible oil refineries present difficulties during anaerobic treatment, leading to problems of toxicity, reduction in methane yield, odour and corrosion. The microbiology and biochemistry of dissimilatory sulphate reduction are reviewed in order to illustrate the potential competition between sulphate reducers and other anaerobes involved in the sequential anaerobic mineralisation process. The theoretical considerations which influence the outcome of competition between sulphate reducers and fermentative, syntrophic, homoacetogenic and methanogenic bacteria are discussed. The actual outcome, under the varying influent organic composition and strength and sulfate concentrations which prevail during digestion of industrial wastewaters, may be quite different to that predicted by thermodynamic or kinetic considerations. The factors governing competitive interactions between SRB and other anaerobes involved in methanogenesis is discussed in the context of literature data on sulphate wastewater treatment and with particular reference to laboratory and full-scale digestion of citric acid production wastewater.

Effect of pH on growth kinetics and sulphide toxicity thresholds of a range of methanogenic, syntrophic and sulphate-reducing bacteria

Abstract The effect of pH on growth rates and sulphide toxicity thresholds of a variety of key bacterial groups involved in anaerobic digestion is described. At pH 7·0–7·5, the growth rates of methane-producing bacteria (MPB) and sulphate-reducing bacteria (SRB) were similar. Above and below this pH range, MPB and SRB, respectively, have more favourable growth properties. Inhibition of all the bacterial groups studied was related to the total sulphide concentration in the pH range 7·2–8·5. Propionate-utilising SRB were the most sensitive of the bacterial groups, both in pure culture and in sludge samples, to high concentrations of total sulphide. At lower pH levels (6·8–7·2), the levels of sulphide which resulted in 50% inhibition of the growth of the bacterial groups were similar, although values obtained were higher for sludge samples than for pure cultures.

Linezolid Compared with Eperezolid, Vancomycin, and Gentamicin in an In Vitro Model of Antimicrobial Lock Therapy for Staphylococcus epidermidis Central Venous Catheter-Related Biofilm Infections

ABSTRACT Central venous catheter (CVC)-related infection (CVC-RI) is a common complication of CVC use. The most common etiological agents of CVC-RI are gram-positive organisms, in particular, staphylococci. An in vitro model for the formation of biofilms by Staphylococcus epidermidis ATCC 35984 on polyurethane coupons in a modified Robbins device was established. Biofilm formation was confirmed by electron microscopy and was quantified by determination of viable counts. Mueller-Hinton broth was replaced with sterile physiological saline (control) or a solution of vancomycin (10 mg/ml), gentamicin (10 mg/ml), linezolid (2 mg/ml), or eperezolid (4 mg/ml). Viable counts were performed with the coupons after exposure to antimicrobials for periods of 24, 72, 168, and 240 h. The mean viable count per coupon following establishment of the biofilm was 4.6 × 108 CFU/coupon, and that after 14 days of exposure to physiological saline was 2.5 × 107 CFU/coupon. On exposure to vancomycin (10 mg/ml), the mean counts were 2.5 × 107 CFU/coupon at 24 h, 4.3 × 106 CFU/coupon at 72 h, 1.4 × 105 CFU/coupon at 168 h, and undetectable at 240 h. With gentamicin (10 mg/ml) the mean counts were 2.7 × 107 CFU/coupon at 24 h, 3.7 × 106 CFU/coupon at 72 h, 8.4 × 106 CFU/coupon at 168 h, and 6.5 × 106 CFU/coupon at 240 h. With linezolid at 2 mg/ml the mean counts were 7.1 × 105 CFU/coupon at 24 h and not detectable at 72, 168, and 240 h. With eperezolid (4 mg/ml) no viable cells were recovered after 168 h. These data suggest that linezolid (2 mg/ml) and eperezolid (4 mg/ml) achieve eradication of S. epidermidis biofilms more rapidly than vancomycin (10 mg/ml) and gentamicin (10 mg/ml).

Simple method for the measurement of the hydrogenotrophic methanogenic activity of anaerobic sludges

The specific hydrogenotrophic activity of anaerobic sludges is usually assayed by gas chromatographic analysis for methane in the headspace of sealed test vials. Gas is sampled with a pressure lock syringe which allows quantification independent of the pressure prevailing in the vials. An alternative method was developed using pressure transducer monitoring of the decrease in headspace gas pressure as the H2/CO2 substrate is converted to CH4. Application of a simple formula related the decrease at each sample point to millilitres of CH4 produced and gave values for the specific hydrogenotrophic activity of granular anaerobic sludge which were in good agreement with the values obtained by the more labor-intensive gas chromatographic method. The simplicity of the method facilitates multiple replicate analyses and allows more accurate determination of initial rates than is achievable by the gas chromatographic method which is prone to analytical error at the very low concentrations of CH4 present in the headspace during the early stages of the assay. Mass transfer of H2 from headspace to liquid was found to be rate-limiting and to result in significant under-estimation of the specific hydrogenotrophic activity of the granular sludge. A test protocol, which used a vial volatile suspended solids concentration between 1.7 and 8 g l−1; a 1:5 ratio between liquid and headspace; incubation of the vials horizontally with vigorous shaking (180 rev./min) and an initial H2/CO2 (8020) gas pressure of 100–150 kPa was found to give reproducible and maximal values for the specific hydrogenotrophic activity of the test sludge.

Anaerobic Granular Sludge Bioreactor Technology

Anaerobic digestion is a mature wastewater treatment technology, with worldwide application. The predominantly applied bioreactor designs, such as the upflow anaerobic sludge blanket and expanded granular sludge bed, are based on the spontaneous formation of granular sludge. Despite the exploitation of granular reactors at full-scale for more than two decades, the mechanisms of granulation are not completely understood and numerous theories have been put forward to describe the process from a biological, ecological and engineering point of view. New technological opportunities are emerging for anaerobic digestion, aided by an improved understanding of microbiological and environmental factors affecting the formation and activity of anaerobic granular sludge.

Effect of sulphate addition on volatile fatty acid and ethanol degradation in an anaerobic hybrid reactor. I: process disturbance and remediation

Abstract The addition of sulphate (4 g/l) to the influent of a mesophilic (35 ± 2°C) laboratory-scale anaerobic hybrid reactor (volumetric loading rate 6 kg/m3/day) treating a propionate-, butyrate- and ethanol-containing (1:1:1 on a COD basis; total COD of 12 g/l) wastewater resulted in severe process disturbance, with a complete inhibition of the propionate-degrading ability of the sludge. Severe inhibition of acetate removal was also observed, with concentrations of propionate and acetate in the reactor effluent of 4000 and 1000 mg/l, respectively. An oscillating pattern of sulphate and sulphide concentrations was observed in the reactor effluent. A control reactor, which had no influent sulphate, maintained a COD removal efficiency of > 95% throughout the trial period. A number of remediation strategies were undertaken with the sulphate-fed reactor. Initially, the levels of total sulphide in the digester were reduced to 1000 mg/l by inclusion of a nitrogen gas sparging system at 4.12 l/l/day (5.36 l/m2/day). Although the levels of total sulphide were reduced in the reactor, little improvement in COD removal was observed. Re-inoculation of 21 of non-sulphate-adapted seed sludge (25 g VSS/l;6.0% inoculum) into the digester did not improve the reactor performance either, suggesting that no improvement could take place in the absence of appropriate sulphate-reducing bacteria or sulphate-adapted syntrophic and methanogenic bacteria. Successful bioaugmentation of the reactor was achieved by the inoculation of 21 sulphate-adapted sludge (25 g VSS/l;6.0% inoculum) from a full-scale digester. Improvement in reactor performance was observed after one retention time (48 h) and the COD removal efficiency exceeded 95% by the conclusion of the trial.

Effect of feed composition and upflow velocity on aggregate characteristics in anaerobic upflow reactors

Abstract Two upflow anaerobic hybrid reactors treated lactose and a mixture of ethanol, propionate and butyrate, respectively, at a volumetric loading rate of 3.7 kg chemical oxygen demand (COD) m−3day−1, a hydraulic retention time of 5 days and a liquid upflow velocity of 0.01 m/h. Under steady-state conditions, the lactose-fed sludge had much higher (20%–100%) specific methanogenic conversion rates than the volatile-fatty acid␣(VFA)/ethanol-fed sludge for all substrates tested, including VFA. In both reactors, a flocculant sludge developed, although a much higher content of extracellular polysaccharide was measured in the lactose-fed sludge [1900 μg compared to 305 μg uronic acid/g volatile suspended solids (VSS)]. When the liquid upflow velocity of a third, VFA/ethanol-fed reactor was increased to 0.5 m/h, granulation of the sludge occurred, accompanied by a large increase (200%–500%) in the specific methanogenic conversion rates for the syntrophic and methanogenic substrates studied. Granulation reduced the susceptibility of the sludge to flotation. Glucose was degraded at a high rate (100 mg glucose gVSS−1h−1) by the sludge from the third reactor, despite not having been exposed to a sugar-containing influent for 563␣days.

Full-scale and laboratory-scale anaerobic treatment of citric acid production wastewater

This paper reviews the operation of a full-scale, fixed-bed digester treating a citric acid production wastewater with a COD: sulphate ratio of 3–4 : 1. Support matrix pieces were removed from the digester at intervals during the first 5 years of operation in order to quantify the vertical distribution of biomass within the digester. Detailed analysis of the digester biomass after 5 years of operation indicated that H2 and propionate-utilising SRB had outcompeted hydrogenophilic methanogens and propionate syntrophs. Acetoclastic methanogens were shown to play the dominant role in acetate conversion. Butyrate and ethanol-degrading syntrophs also remained active in the digester after 5 years of operation.Laboratory-scale hybrid reactor treatment at 55 °C of a diluted molasses influent, with and without sulphate supplementation, showed that the reactors could be operated with high stability at volumetric loading rates of 24 kgCOD.m-3.d-1 (12 h HRT). In the presence of sulphate (2 g/l-1; COD/sulphate ratio of 6 : 1), acetate conversion was severely inhibited, resulting in effluent acetate concentrations of up to 4000 mg.l-1.

Effect of sulphate addition on volatile fatty acid and ethanol degradation in an anaerobic hybrid reactor. II : microbial interactions and toxic effects

The microbial interactions occurring in an anaerobic hybrid reactor which was seeded with non-sulphate-adapted sludge and subsequently exposed to sulphate over a prolonged period of 423 days were studied. The population changes upon inoculation of the reactor with sulphate-adapted sludge from a full-scale treatment plant were also analysed. Addition of sulphate (4000 mg/l) to the reactor influent resulted in a complete inhibition of syntrophic propionate degradation and severe inhibition of acetoclastic methanogenesis. No propionate or acetate-utilising sulphate-reducing bacteria (SRB) could be detected in the sulphate-fed reactor sludge. Toxicity tests carried out on sludge samples from a control reactor showed that acetoclastic methanogenesis was the most sensitive step in sulphide inhibition, with 50% inhibition observed at concentrations of 220–980 mg total sulphide/l (69–150 mg free H2S/l) over the pH range 6.5–8.0. The propionate-utilising syntrophs present in the sludge appeared to be irreversibly inhibited by sulphide and were also inhibited by acetate concentrations as low as 300 mg/l. The inoculation of full-scale sulphate-adapted sludge resulted in the establishment of propionate and hydrogen-utilising SRB in the biofilm section, but not in the granular sludge bed section, of the anaerobic hybrid reactor.

Use of anaerobic hybrid reactors for treatment of synthetic pharmaceutical wastewaters containing organic solvents

The performance of anaerobic hybrid reactors treating an organic solvent-containing synthetic pharmaceutical wastewater was evaluated under various wastewater volumetric loading rates and influent compositional changes. The biodegradation, toxicity and treatability of the target C 3 and C 4 solvents, tert-butanol, isopropanol, isobutanol, sec-butanol and ethyl acetate, were examined. At a hydraulic retention time (HRT) of 2 days and volumetric loading rates ranging from 3.5 to 4.5 kg COD m -3 day -1 , the reactors achieved total and soluble COD removal efficiencies of 97-99% in less than five times the HRT. These removal rates were achieved following the introduction of target solvents not previously supplied to the reactors. However, inadequate removal of tert-butanol resulted in a decrease in the soluble COD removal efficiency to 58%. Bacterial enrichments from the reactor biomass using tert-butanol as the sole substrate proved unsuccessful, confirming that tert-butanol is poorly degradable anaerobically. Inclusion of a trace metal cocktail in the feed did not affect steady-state reactor performance, but was beneficial during changes in the influent composition. After 405 days of operation, the matrix-associated biomass contributed only a minor fraction (2-4%) of the total biomass present in both reactors. On takedown, the retained biomass present in the matrix-free section of both reactors was found to be granular in nature, despite the omission of trace elements from the influent to one of the AHRs. The specific methanogenic activity profile of the granular sludge from the trace element limited AHR was, however, significantly lower (α = 0.05) than that of the reference AHR.