G. Lettinga

Anaerobic digestion and wastewater treatment systems.

Upflow Anaerobic Sludge Bed (UASB) wastewater (pre-)treatment systems represent a proven sustainable technology for a wide range of very different industrial effluents, including those containing toxic/inhibitory compounds. The process is also feasible for treatment of domestic wastewater with temperatures as low as 14–16° C and likely even lower. Compared to conventional aerobic treatment systems the anaerobic treatment process merely offers advantages. This especially is true for the rate of start-up. The available insight in anaerobic sludge immobilization (i.e. granulation) and growth of granular anaerobic sludge in many respects suffices for practice. In anaerobic treatment the immobilization of balanced microbial communities is essential, because the concentration of intermediates then can be kept sufficiently low.So far ignored factors like the death and decay rate of organisms are of eminent importance for the quality of immobilized anaerobic sludge. Taking these factors into account, it can be shown that there does not exist any need for ‘phase separation’ when treating non- or slightly acidified wastewaters. Phase separation even is detrimental in case the acidogenic organisms are not removed from the effluent of the acidogenic reactor, because they deteriorate the settleability of granular sludge and also negatively affect the formation and growth of granular sludge. The growing insight in the role of factors like nutrients and trace elements, the effect of metabolic intermediates and end products opens excellent prospects for process control, e.g. for the anaerobic treatment of wastewaters containing mainly methanol.Anaerobic wastewater treatment can also profitably be applied in the thermophilic and psychrophilic temperature range. Moreover, thermophilic anaerobic sludge can be used under mesophilic conditions.The Expanded Granular Sludge Bed (EGSB) system particularly offers big practical potentials, e.g. for very low strength wastewaters (COD ≪ 1 g/l) and at temperatures as low as 10° C. In EGSB-systems virtually all the retained sludge is employed, while compared to UASB-systems also a substantially bigger fraction of the immobilized organisms (inside the granules) participates in the process, because an extraordinary high substrate affinity prevails in these systems. It looks necessary to reconsider theories for mass transfer in immobilized anaerobic biomass.Instead of phasing the digestion process, staging of the anaerobic reactors should be applied. In this way mixing up of the sludge can be significantly reduced and a plug flow is promoted. A staged process will provide a higher treatment efficiency and a higher process stability. This especially applies for thermophilic systems.

A review: The anaerobic treatment of sewage in UASB and EGSB reactors

The anaerobic treatment process is increasingly recognized as the core method of an advanced technology for environmental protection and resource preservation and it represents, combined with other proper methods, a sustainable and appropriate wastewater treatment system for developing countries. Anaerobic treatment of sewage is increasingly attracting the attention of sanitary engineers and decision makers. It is being used successfully in tropical countries, and there are some encouraging results from subtropical and temperate regions. In this review paper, the main characteristics of anaerobic sewage treatment are summarized, with special emphasis on the upflow anaerobic sludge blanket (UASB) reactor. The application of the UASB process to the direct treatment of sewage is reviewed, with examples from Europe, Asia and the Americas. The UASB reactor appears today as a robust technology and is by far the most widely used high-rate anaerobic process for sewage treatment.

Azo dye decolourisation by anaerobic granular sludge.

The decolourisation of 20 selected azo dyes by granular sludge from an upward-flow anaerobic sludge bed (UASB) reactor was assayed. Complete reduction was found for all azo dyes tested, generally yielding colourless products. The reactions followed first-order kinetics and reaction rates varied greatly between dyes: half-life times ranged from 1 to about 100 h. The slowest reaction rates were found for reactive dyes with a triazine reactive group. There was no correlation between a dyes half-life time and its molecular weight, indicating that cell penetration was probably not an important factor. Since granular sludge contains sulphide, eight dyes were also monitored for direct chemical decolourisation by sulphide. All these dyes were reduced chemically albeit at slower rates than in the presence of sludge at comparable sulphide levels. Increasing sulphide concentrations, even when present in huge excess, stimulated the azo reduction rate. The results indicate that granular sludge can decolourise a broad spectrum of azo dye structures due to non-specific extracellular reactions. Reducing agents (e.g., sulphide) in sludge play an important role. The presence of anaerobic biomass is probably beneficial for maintaining the pools of these reduced compounds.

Sodium inhibition of acetoclastic methanogens in granular sludge from a UASB reactor

Abstract The effect of sodium on the formation of methane from acetate in granular sludge from a UASB reactor has been determined at various acetate concentrations and pH-levels. At neutral pH sodium concentrations of 5, 10 and 14 g Na+l−1caused 10, 50 and 100% inhibition respectively, relative to the maximum specific acetoclastic methanogenic activity of the granular sludge. These values reflect the sensitivity of Methanothrix sp. towards sodium, as this is the predominant acetoclastic methanogen in the granular sludge used in this study. The pH did not affect the inhibitory action of sodium significantly in the range 6.5–7.2, but at pH-levels near 8 the inhibition was more pronounced. At acetate concentrations below 500 mg l−1diffusion limitation partly masked the influence of sodium on the specific activity of the granular sludge. No adaptation of Methanothrix sp. to high sodium concentrations could be obtained in a period of 12 weeks. Net growth of Methanothrix sp. could be obtained at sodium concentrations of approx. 10 g l−1. At equal sodium concentrations, sodium chloride had a somewhat stronger effect on the specific activity than sodium sulfate, but the difference is unimportant for design purposes. The results obtained with short term activity measurements can reliably be used for predictions of the effect of sodium salts on continuously fed UASB reactors.

Biodegradation of selected azo dyes under methanogenic conditions

Biological treatment of wastewaters discharged by the textile industry could potentially be problematic due to the high toxicity and recalcitrance of the commonly-used azo dye compounds. In the present report, the fate of two azo dyes under methanogenic conditions was studied. Mordant Orange 1 (MO1) and Azodisalicylate (ADS) were completely reduced and decolorised in continuous UASB reactors in the presence of cosubstrates. In the MO1 reactor, both 5-aminosalicylic acid (5-ASA) and 1,4-phenylenediamine were identified as products of azo cleavage. After long adaptation periods, 5-ASA was detected at trace levels, indicating further mineralization. ADS, a pharmaceutical azo dye constructed from two 5-ASA units, was completely mineralized even in the absence of cosubstrate, indicating that the metabolism of 5-ASA could provide the reducing equivalents needed for the azo reduction. Batch experiments confirmed the ADS mineralization. These results demonstrate that some azo dyes could serve as a carbon, energy, and nitrogen source for anaerobic bacteria.

Anaerobic treatment of sulphate-rich wastewaters

Until recently, biological treatment of sulphate-rich wastewater was rather unpopular because of the production of H2S under anaerobic conditions. Gaseous and dissolved sulphides cause physical-chemical (corrosion, odour, increased effluent chemical oxygen demand) or biological (toxicity) constraints, which may lead to process failure. Anaerobic treatment of sulphate-rich wastewater can nevertheless be applied successfully provided a proper treatment strategy is selected. The strategies currently available are discussed in relation to the aim of the treatment: i) removal of organic matter, ii) removal of sulphate or iii) removal of both. Also a whole spectrum of new biotechnological applications (removal of organic chemical oxygen demand, sulphur, nitrogen and heavy metals), recently developed based on a better insight in sulphur transformations, are discussed.

Advanced anaerobic wastewater treatment in the near future

New insights into the anaerobic degradation of very different categories of compounds, and into process and reactor technology will lead to very promising new generations of anaerobic treatment system, such as ‘Expanded Granular Sludge Bed’ (EGSB) and ‘Staged Multi-Phase Anaerobic’ (MPSA) reactor systems. These concepts will provide a higher efficiency at higher loading rates, are applicable for extreme environmental conditions (e.g. low and high temperatures) and to inhibitory compounds. Moreover, by integrating the anaerobic process with other biological methods (sulphate reduction, micro-aerophilic organisms) and with physical-chemical methods, a complete treatment of the wastewater can be accomplished at very low costs, while at the same time valuable components can be recovered for reuse.

High-Rate Anaerobic Waste-Water Treatment Using the UASB Reactor under a Wide Range of Temperature Conditions

(1984). High-Rate Anaerobic Waste-Water Treatment Using the UASB Reactor under a Wide Range of Temperature Conditions. Biotechnology and Genetic Engineering Reviews: Vol. 2, No. 1, pp. 253-284.

The effect of liquid upward velocity and hydraulic retention time on granulation in UASB reactors treating wastewater with a high sulphate content

The effect of hydraulic retention time and liquid upward velocity on the granulation process and the competition between sulphate-reducing and methane-producing bacteria during anaerobic treatment of sulphate-containing wastewater was studied. The results showed that hydrogen, generated during the anaerobic mineralisation process, was completely oxidised by sulphate-reducing bacteria. Acetate was oxidised by both sulphate-reducing and methane-producing bacteria. The fraction of acetate used by sulphate reducers relative to methanogens increased with time, resulting in a predominance of sulphate-reducing bacteria, especially at relative long hydraulic retention time (40 h). The granulation process was favoured by the combination of high upward velocity and short hydraulic retention time. Very thin filaments (possibly sulphate reducers) may serve as primary nuclei for the attachment of Methanothrix, which starts the granulation process. No difference between the attachment capacity of sulphate-reducing and methanogenic bacteria was found.

Anaerobic degradation of volatile fatty acids at different sulphate concentrations

The effect of sulfate on the anaerobic breakdown of mixtures of acetate, propionate and butyrate at three different sulfate to fatty acid ratios was studied in upflow anaerobic sludge blanket reactors. Sludge characteristics were followed with time by means of sludge activity tests and by enumeration of the different physiological bacterial groups. At each sulfate concentration acetate was completely converted into methane and CO2, and acetotrophic sulfate-reducing bacteria were not detected. Hydrogenotrophic methanogenic bacteria and hydrogenotrophic sulfate-reducing bacteria were present in high numbers in the sludge of all reactors. However, a complete conversion of H2 by sulfate reducers was found in the reactor operated with excess sulfate. At higher sulfate concentrations, oxidation of propionate by sulfate-reducing bacteria became more important. Only under sulfate-limiting conditions did syntrophic propionate oxidizers out-compete propionate-degrading sulfate reducers. Remarkably, syntrophic butyrate oxidizers were well able to compete with sulfate reducers for the available butyrate, even with an excess of sulfate.