A. Cohen

pH influence on acidogenic dissimilation of glucose in an anaerobic digestor

In a CFSTR, inoculated with activated sludge, the acidogenic dissimilation at 30°C of a 1% glucose containing medium (C-limited), the maximum specific growth rate and product distribution, was studied over the pH range from 4.5 to 7.9. An exact description in terms of carbon mass- and O/R-balances is presented. The gas phase consisted of carbon dioxide and hydrogen. Recovery of dissolved products, mainly short-chain fatty acids, gas and biomass, calculated as total carbon, amounted to 85–95%; yield values ranged from 12 to 14%. An optimal growth rate of 0.33 h−1 was achieved at pH 6.0. Product distribution at 90% of the maximum growth rate is fairly constant up to the pH value of 6.0, after which dramatic changes in the main product were found to occur, from butyric acid to lactic acid and subsequently to acetic acid, formic acid and ethanol.

Anaerobic digestion of glucose with separated acid production and methane formation

Abstract In a two-phase anaerobic digestion system, with separate reactors for the acidification phase and the methane fermentation phase, the glucose of a 1% glucose solution was almost completely converted into biomass and gases. The acid reactor was operated at 30°C and pH 6.0, with a retention time of 10 h. Main products of the acid-forming phase were hydrogen, carbon dioxide, butyrate and acetate. On a molar base, these products represented over 96% of all products formed. On average, 12% of the COD content of the influent was evolved as hydrogen. The effluent of the first reactor was pumped to the methane reactor after passing through a storage vessel. The methane reactor was operated at 30°C, pH 7.8, and a retention time of 100 h was given. Approximately 98% of the organic substances fed to this reactor were converted to methane, carbon dioxide and biomass. About 11% of the glucose fed to the digesting system was converted to bacterial mass.

Influence of temperature on the anaerobic acidification of glucose in a mixed culture forming part of a two-stage digestion process

Abstract Using a Continuous Flow Stirred Tank Reactor inoculated with activated sludge, the anaerobic acidification of a 1% glucose monohydrate containing medium, allowing carbon limited growth of microorganisms, was studied with respect to maximum dilution rate and product distribution, over the temperature range from 20 to 60°C. A mesophilic and a thermophilic region could be distinguished, with optimum temperatures of 37°C (Dmax = 0.51 h−1) and 52°C (Dmax = 0.71 h−1) respectively. Optimal sludge loadings were, respectively, 77 and 112 kg COD (kg dw)−1 day−1. The gas phase consisted of carbon dioxide and hydrogen, and yield values ranged between 13.5–18.0% as the temperature was varied from 25 to 55°C. Deviations were obtained at lower and higher temperatures. In the liquid phase, mainly acetate, propionate, butyrate, lactate and ethanol were found. Precise description, in terms of carbon mass, C1 and oxidation/reduction balances, is presented. Product distribution at the maximum loading rates was fairly constant in the mesophilic range but is markedly variable in the thermophilic range. Product distribution also was highly dependent on sludge loading except at high loadings. Separate acidification should be designed at mesophilic temperature below optimal temperature and at high, almost maximal, sludge loadings.

Influence of phase separation on the anaerobic digestion of glucose—I maximum COD-turnover rate during continuous operation

Abstract A mineral medium containing 1% of glucose as the main carbon source was subjected to one-phase and to two-phase anaerobic digestion processes under comparable conditions. The one-phase system consisted of an anaerobic up-flow reactor containing both acidogenic as well as methanogenic populations allowing a complete conversion of the carbon source into gaseous end products and biomass. The two-phase system consisted of an acid reactor and a methane reactor connected in series allowing sequential acidogenesis and methanogenesis of the glucose. Performance of the one-phase system and of the methane reactor of the two-phase system is presented by carbon mass balances. By gradually increasing the feed supply to both systems, maximum turnover of COD was determined. Maximum specific sludge loadings of the methanogenic phase of the two-phase system was over 3 times higher than that of the one-phase system. In a second experiment both systems were subjected to overloading, resulting in the accumulation of volatile fatty acids (VFA). In the one-phase system propionate and acetate were formed in considerable amounts. Although acetate disappeared rapidly after cessation of the feed supply, no turnover of propionate was observed within one week. On overloading the methane reactor of the two-phase system accumulation of several fatty acids within the reactor was observed. Rapid conversion of all fatty acids took place immediately after interruption of feed supply. The eco-physiological significance of phase separation is discussed briefly.

INFLUENCE OF PHASE SEPARATION ON THE ANAEROBIC DIGESTION OF GLUCOSE-~-II

A mineral medium, containing 1% (w/v) glucose as the main carbon source, was subjected to one-phase and to two-phase anaerobic digestion processes under comparable conditions. The one-phase system consisted of an anaerobic up-flow reactor containing both acidogenic as well as methanogenic populations. The two-phase system consisted of an acid reactor and a methane reactor connected in series allowing sequential acidogenesis and methanogenesis of the glucose. After maximum turnover rates of glucose had been attained in both systems, by gradually increasing feed supply rate, both systems were switched to the batch mode and subjected to shock loadings with glucose or fatty acids. Maximum specific turnover rates of fatty acids in the one-phase process averaged 0.39 g COD · g biomass−1 d−1 and 2.23 g g−1 d−1 for the methane reactor of the two-phase system. Charging the one-phase system with doses of glucose resulted mainly in an accumulation of propionate which was degraded relatively slowly. It was concluded that interspecies hydrogen transfer may become rate limiting at high loading rates, stimulating formation of propionate. Therefore a two-phase system, as compared with a one-phase digestion process for easily hydrolyzable carbohydrates, was characterized as being essentially the more stable.

Product inhibition in the acid forming stage of the anaerobic digestion process

The acidification of glucose was studied in a continuous flow stirred tank reactor at a pH of 6.0 and a temperature of 30°C. Together with the glucose, a complete mineral medium was pumped into the reactor vessel, to ensure a carbon limitation, throughout. In various experiments product inhibition was studied with respect to specific growth rate, yield and product distribution. The total amount of inorganic nutrients was varied, in its entirety, at constant glucose input (10g 1−1). No inhibition took place if the inorganic nutrient concentration was gradually increased, however a sudden large change caused reactor failure. Sulphate reduction to sulfide occurred if sulphate was higher than 140 ppm but this caused no inhibition of the process. Product inhibition by the acids formed, was measured by changing the glucose input concentration. After changing the glucose concentration from 2.5 to 75 g l−1, the maximum dilution rate was lowered from 0.54 to 0.044 h−1. Biomass yield decreased from 16 to 8%. Product distribution however was fairly constant. A semi logarithmic relationship was found between the maximum dilution rate and the acids produced. This relationship is briefly discussed.

Role of anaerobic spore-forming bacteria in the acidogenesis of glucose: changes induced by discontinuous or low-rate feed supply

A mineral salts medium containing 1% (w/v) glucose providing carbon-limited growth conditions was subjected to anaerobic acidogenesis by mixed populations of bacteria in chemostat cultures. The formation of butyrate was shown to be dependent on the presence of saccharolytic anaerobic sporeformers in the acid-forming population. By the use of pasteurized activated sludge as an inoculum a culture was obtained consisting solely of anaerobic sporeformers that gave rise to the formation of butyrate, acetate, hydrogen and carbon dioxide as the main fermentation products. No formation of propionate could be detected. In this culture, the role of sporulation was investigated by applying periods of starvation and a single-step lowering of dilution rate (shift-down). In an experiment using a mineral salts medium supplemented with 1% (w/v) glucose and 0.5% (w/v) casein hydrolysate formation of refractile forespores as well as cell lysis could be demonstrated after 6 h starvation.In mixed cultures, initially inoculated with non-pasteurized activated sludge, a regular interruption of feed supply for 1 h per day resulted in selection of non-sporulatiog anaerobes. The fermentation pattern changed to a production of propionate and acetate, with a concomitant reduction of gas production. Similar results were obtained with shift-down in dilution rate.

Anaerobic acidification of glucose in an upflow reactor

Abstract A medium containing either 10 or 50 kg m−3 of glucose was inculated with sewage sludge and fermented in an upflow reactor at pH 5·9 and 30°C. The acid products were mainly butyrate, lactate and acetate. The gas produced consisted of hydrogen and carbon dioxide. The minimum residence times required for complete acidification of the substrate were 26 and 82 min respectively. All measurements were carried out under steady state conditions. The residence time of the biomass was in all cases longer than that of the liquid. At a certain residence time granulation occurred and the biomass retention substantially increased. This residence time amounted to 1 h in the experiment with 10 kg m−3 glucose and 6 h in the experiment with 50 kg m−3 glucose. The maximum glucose conversion rates were 450 and 630 kg m−3 d−1. The results of the fermentation in the upflow reactor were compared with those obtained under similar conditions in a well-stirred tank reactor. The relevance of the results to actual practice is discussed.

Degradation patterns and intermediates in the anaerobic digestion of glucose: Experiments with 14C-labeled substrates

A mineral salts medium containing 1% (w/v) glucose was subjected to anaerobic digestion in an upflow reactor. Performance with respect to utilization of glucose was monitored by collection of fermentation gases and calculation of carbon mass balances. Sub-samples of bacterial supennsions from the upflow reactor were incubated with (U-14C)-glucose, (U-14C)-acetate, (2-14C)-propionate, (1-14C)-butyrate or 14C-carbonate. Individual radioactive products in samples from incubation mixtures were analysed by radio gas chromatography.Quantitatively, acetate and propionate were the only important intermediates in glucose degradation by glucose-adapted sludge, with acetate accounting for the largest part of intermediary fatty acid flux.

Significance of partial pre-acidification of glucose for methanogenesis in an anaerobic digestion process

SummaryThe effect of partial pre-acidification of carbohydrate containing wastewaters on anaerobic digester performance was investigated. The influent was a 1% (w/v) glucose solution in a mineral salts medium imposing carbon-limited growth conditions. Up to 13% of the Chemical Oxygen Demand (COD) was added as volatile fatty acids (VFA).In all cases, addition of VFA to the glucose medium resulted in significant increases in the maximum specific COD-conversion rates of the sludge (both with respect to continuous feeding and following a shock loading), as compared with values found on digestion of glucose media alone.