Piet Lens

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 upward velocity and sulphide concentration on volatile fatty acid degradation in a sulphidogenic granular sludge reactor

During anaerobic treatment of sulphate-containing wastewaters, sulphate-reducing bacteria (SRB) compete with methane-producing bacteria (MPB) for the available electron-donors. In this work, the anaerobic treatment of a synthetic wastewater, consisting of a mixture of acetate, propionate and butyrate and high concentrations of sulphate (COD: sulphate ratio 0·5) was studied in an upflow anaerobic granular sludge bed reactor. The influence of the superficial upward liquid velocity (vup), the influent composition and reactor pH on the competition between SRB and MPB was investigated. At a vup of 2 m h−1 and pH 8, 93–97% of the COD was degraded by SRB. With increasing vup-values, COD removal efficiencies decreased, while at a vup of 6 m h−1 the fraction of COD removed by MPB rose to 23%. Elevation of the influent acetate concentrations, by decreasing the vup (lower recirculation) or by the use of an influent volatile fatty acid mixture with a higher acetate content, resulted in an increase of methanogenesis up to 41% of the total COD removal. In contrast, elevated levels of propionate and butyrate in the influent favoured the sulphate reducing process. A decrease of pH from 8 to 7 resulted in free hydrogen sulphide concentrations higher than 200 mg litre−1. This strongly inhibited methanogenesis while SRB were hardly affected, with a subsequent decrease of the COD removed by MPB from 41 to 7% as a result.

LONG-TERM COMPETITION BETWEEN SULPHATE-REDUCING AND METHANE-PRODUCING BACTERIA DURING FULL-SCALE ANAEROBIC TREATMENT OF CITRIC ACID PRODUCTION WASTEWATER

Abstract The bacterial population structure of sludge present in a full-scale upflow, fully-packed anaerobic digester treating a sulphate-containing wastewater (12xa0g COD/l, 4xa0g SO42−/l) from the citric acid production industry for a period of 5 years was determined. Populations of sulphate-reducing, methanogenic, syntrophic and homoacetogenic bacteria were found to prevail in the reactor sludge. The degradation patterns of several key anaerobic intermediates were studied. Both propionate and H2/CO2 were completely metabolised by sulphate reducing bacteria. Methanogenic bacteria outcompeted sulphate-reducers for acetate, whereas competition took place for butyrate and ethanol. The outcome of competition in the sludge was not affected by the location within the reactor or by the cell residence time.

Direct treatment of domestic wastewater by percolation over peat, bark and woodchips

Abstract Laboratory columns were used to determine the treatment capacity of Sphagnum spp peat and Pinus spp bark and woodchips to treat primary (unsettled) domestic sewage by percolation. Bark and peat were found to be applicable materials, woodchips seemed inadequate because of poor CODtot removal and lack of disinfection. Matured percolator columns containing a layer of 50 cm bark packed at a density of 0.150 g/cm3 and supplied with 10 cm wastewater per day gave the following reductions: SS, 72%; CODtot, 63%; BOD5, 97%; NH+4 -N, 64%; and Ntot-N, 35%. The effluent had a pH of 7.5. The bark percolator had, however, a poor disinfecting capacity: faecal indicator organisms were reduced by only 1 log unit. The peat percolator (operated at 0.075 g/cm3 and 10 cm/d) had a good disinfecting capacity for faecal bacteria, reducing them by 3–4 log units. The effluent had a low pH (pH = 4.5) during the first 3 months of operation. Once matured, the following average reductions were obtained: SS, 91%; CODtot, 50%; BOD5, 99%; NH+4 -N, 93% and Ntot-N, 38%. The treatment capacity was not significantly (α = 0.05) affected by the applied densities of a percolator layer (0.075 and 0.100 g/cm3 for peat and woodchips; 0.150 and 0.175 g/cm3 for bark) and the hydraulic loading rates (2.5 or 10.0 cm/d). Combining two or three materials by mixing (equal volume) or by applying them in layers (equal layer thickness) gave similar effluent qualities, though percolators containing a layer of peat were more effective in the removal of faecal bacteria. The temporary presence of a biocide in the wastewater and a prolonged period of drought (14 days) disturbed NOx-N formation but did not influence SS, CODtot and NH+4-N removal.

Anaerobic bioprocessing of organic wastes.

Anaerobic digestion of dissolved, suspended and solid organics has rapidly evolved in the last decades but nevertheless still faces several scientific unknowns. In this review, some fundamentals of bacterial conversions and adhesion are addressed initially. It is argued in the light of ΔG-values of reactions, and in view of the minimum energy quantum per mol, that anaerobic syntrophs must have special survival strategies in order to support their existence: redistributing the available energy between the partners, reduced end-product fermentation reactions and special cell-to-cell physiological interactions. In terms of kinetics, it appears that both reaction rates and residual substrate thresholds are strongly related to minimum ΔG-values. These new fundamental insights open perspectives for efficient design and operation of anaerobic bioprocesses. Subsequently, an overview is given of the current anaerobic biotechnology. For treating wastewaters, a novel and high performance new system has been introduced during the last decade; the upflow anaerobic sludge blanket system (UASB). This reactor concept requires anaerobic consortia to grow in a dense and eco-physiologically well-organized way. The microbial principles of such granular sludge growth are presented. Using a thermodynamic approach, the formation of different types of aggregates is explained. The application of this bioprocess in worldwide wastewater treatment is indicated. Due to the long retention times of the active biomass, the UASB is also suitable for the development of bacterial consortia capable of degrading xenobiotics. Operating granular sludge reactors at high upflow velocities (5–6 m/h) in expanded granular sludge bed (EGSB) systems enlarges the application field to very low strength wastewaters (chemical oxygen demand < 1 g/l) and psychrophilic temperatures (10°C). For the treatment of organic suspensions, there is currently a tendency to evolve from the conventional mesophilic continuously stirred tank system to the thermophilic configuration, as the latter permits higher conversion rates and easier sanitation. Integration of ultrafiltration in anaerobic slurry digestion facilitates operation at higher volumetric loading rates and at shorter residence times. With respect to organic solids, the recent trend in society towards source separated collection of biowaste has opened a broad range of new application areas for solid state anaerobic fermentation.

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 3.7u2009kg 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 (100u2009mg glucoseu2009gVSS−1h−1) by the sludge from the third reactor, despite not having been exposed to a sugar-containing influent for 563␣days.

Distribution of extracellular polysaccharides and flotation of anaerobic sludge

Abstractu2002Extracellular polysaccharides (EPS) were quantified in dense granules and loose flocs by chemical analysis of the uronic acid content. Their distribution within the aggregates was determined by microscopic staining. Granules contained a higher amount of EPS (1–1.6u2004mg/g volatile suspended solids, VSS) than flocs (0.3u2004mg/g VSS). In granules approximately 50% of the total amount of EPS was present in a 40-μm-thick zone on the surface. The remainder was dispersed in the rest of the aggregate. In flocs the highest concentration was present in the centre and the EPS layer on the surface was not found. Tests showed that flocculent sludge was very sensitive to flotation, while the studied granules did not float. The lower susceptibility to flotation of granules as compared to flocs was attributed to the presence of the hydrophilic EPS coating that prevents attachment of gas bubbles.

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.

Biofilms in Medicine, Industry and Environmental Biotechnology - Characteristics, Analysis and Control

Biofilms are of great practical importance for beneficial technologies such as water and wastewater treatment and bioremediation of groundwater and soil. In other settings biofilms cause severe problems, for example in 65% of bacterial infections currently treated by clinicians (particularly those associated with prosthetics and implants), accelerated corrosion in industrial systems, oil souring and biofouling. Until recently, the structure and function of biofilms could only be inferred from gross measures of biomass and metabolic activity. This limitation meant that investigators involved in biofilm research and application had only a crude understanding of the microbial ecology, physical structure and chemical characteristics of biofilms. Consequently, opportunities for the exploitation and control of biofilms were very limited. The past decade has witnessed the development of several new techniques to elucidate the structure and function of biofilms. Examples include: the use of molecular probes that identify different microbes in complex communities as well as their metabolic functions; the use of microsensors that show concentration gradients of key nutrients and chemicals; the use of confocal laser scanning microscopy to describe the physical structure of biofilms and the development of a new generation of mathematical models that allow for the prediction of biofilm structure and function. However, much progress remains to be made in efforts to understand, control and exploit biofilms. This timely book will introduce its readers to the structure and function of biofilms at a fundamental level as determined during the past decade of research, including: Extracellular polymers as the biofilm matrix; Biofilm phenotype (differential gene expression, interspecies signalling); Biofilm ecology; Biofilm monitoring; Resistance of biofilms to antimicrobial agents and Biofilm abatement. Biofilms in Medicine, Industry and Environmental Technology offers a holistic and multi-disciplinary description of the topic, including biofilm formation and composition, but also biofilm monitoring, disinfection and control. All these aspects are presented from three points of views: medical, industrial and environmental biotechnological in a compact, easy to read format.

Characterization of biomass from a sulfidogenic, volatile fatty acid-degrading granular sludge reactor.

This paper describes the properties of sulfidogenic aggregates developed in a granular sludge bed reactor operating under different upward liquid velocities (1–6 m h−1), influent composition (acetate, propionate, and butyrate ratios of 1:1:1, 5:3:2, or 1:2:2) and pH (8 and 7). The limiting factor for the treatment performance was the acetate removal rate with maximum sulfidogenic degradation rates of 0.38 g C2-COD g−1 VSS d−1. Although the reactor operated with an excess of sulfate (CODSO42− ratio 0.5), methanogenic bacteria still contributed partially to the COD removal. Sludge activity tests were used to quantify the partitioning of methanogenic and sulfidogenic COD utilization. The use of high superficial upward liquid velocities (4 and 6 m h−1) in the reactor increased the methanogenic contribution to the total activity to 40% and 70%, respectively, during the degradation of a volatile fatty acid mixture and with acetate as the sole substrate. An increase in the reactor free hydrogen sulfide concentration from 100 to 240 mg l−1 decreased the methanogenic contribution to less than 10% and 20% for the conversion of a volatile fatty acid mixture and acetate, respectively. 13C-NMR spectroscopy showed that propionate conversion proceeded via the randomizing pathway. In the absence of sulfate, only acetate was converted by methanogenic activity. The poor propionate and butyrate removal in the absence of sulfate suggests a lack of hydrogenotrophic methanogenic activity in the sulfidogenic sludge. Granulation occurred under sulfidogenic conditions as demonstrated by the continuous increase in the average granular size. The obtained sulfidogenic granules had a low granular strength (13–25 kN m−2), high settling velocities (14–37 m h−1 for 95% of the biomass), and a density of 1,022 kg m−3.