J. G. van Andel

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.

Microbial degradation of polycyclic aromatic hydrocarbons : effect of substrate availability on bacterial growth kinetics

SummaryIt is demonstrated that bacterial growth on crystalline or adsorbed polycyclic aromatic hydrocarbons can result in a linear increase in biomass concentration. A simple mathematical approach is presented, showing that under these circumstances mass transfer from the solid phase to the liquid phase is rate-limiting for growth.

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.

Glucose fermentation byClostridium butyricum grown under a self generated gas atmosphere in chemostat culture

SummaryClostridium butyricum was grown anae-robically under glucose-limited conditions in che-mostat cultures under self generated gas atmo-sphere. It is shown that the quantitative composi-tion of the fermentation products is dependent on the pH value, the growth rate, the concentration of glucose in the growth medium and the compo-sition of the gas atmosphere developed in the reactor. The ratio qacetate/qbutyrate increases from 0.06 to 0.66 in parallel with an increase in growth rate from 0.02 h−1 to 0.29 h−1 (at pH = 6.0). De-creasing the partial pressure of H2 results in an in-crease of the qacetate/qbutyrate ratio. The partial pressure of CO2 in the reactor does not influence the fermentation products whatsoever. Increasing pH values (>6.8) and concentrations of glucose in the growth medium also result in increasing qacetate/qbutyrate ratios. The maximal YATP is con-stant from pH 4.8–6.0. The functioning of NADH2-ferredoxin oxi-doreductase is discussed.

Hydrolysis and acidogenic fermentation of a protein, gelatin, in an anaerobic continuous culture

SummaryModel studies of anaerobic protein digestion were performed using gelatin dissolved in a mineral medium, which was fed to a mixed population of bacteria in a carbon-substrate limited chemostat culture. The dilution rate and culture pH value were varied progressively in order to determine the optimal conditions for hydrolysis and acidification (i.e., fatty acids formation). The optimum pH value appeared to be in the neutral region (pH>6.3), and the maximal dilution rate allowing steady state growth was 0.23 h-1. At this dilution rate and at pH 7 hydrolysis of gelatin was 78% complete, and 79% of the protein hydrolysed was fermented to identifiable products. At submaximal dilution rates both these values were higher. The main fermentation products were acetate, propionate, and valerate, and minor amounts of other volatile fatty acids. The product composition was relatively independent of the dilution rate, but varied substantially with the pH value.

Persistence of the pBR 322 plasmid in Escherichia coli K 12 grown in chemostat cultures

Populations of a Escherichia coli K 12 strain, containing the vector plasmid p BR 322, were grown in chemostat culture under glucose- and phosphatelimited conditions. Resistance to tetracycline and ampicillin were lost after prolonged cultivation, resulting in the production of apparent plasmid-free populations which were more competitive than the original population. This competitiveness between plasmid-free and plasmid-containing populations was greatest in environments where the nutrient restriction was severe. Also during sequential subcultivation in batch cultures loss of plasmid was observed.

Effect of micro-organisms on the bioavailability and biodegradation of crystalline naphthalene

Bacterial growth on crystalline naphthalene was measured and found to be related to the dissolution rate of the substrate. No evidence for enhancement of substrate availability due to bacterial influence could be detected. Using a model based on dissolution kinetics for substrate availability and Monod kinetics for bacterial growth it was possible to simulate bacterial growth on naphthalene. This model is widely applicable for growth of micro-organisms on poorly water-soluble substrates and can easily be adapted to a more complex system such as microbial growth on substrates adsorbed to matrices.

Protein degradation in anaerobic digestion: influence of volatile fatty acids and carbohydrates on hydrolysis and acidogenic fermentation of gelatin

SummaryThe hydrolysis and fermentation of gelatin in the presence of a carbohydrate by gelatin-adapted mixed anaerobic bacterial populations in putatively carbon-limited chemostat cultures is investigated. It was shown that the degradation of the protein is progressively retarded with increasing dilution rates, as well as with increased concentrations of carbohydrates present in the feed as a second substrate. That this is not due to high concentrations of fermentation products in the reactor was established. Moreover, the carbohydrate is totally fermented at all dilution rates. It is concluded that for optimal performance of an anaerobic digestion system purifying waste waters containing carbohydrate/protein mixtures, fermentation of carbohydrates should be spatially separated from hydrolysis and fermentation of the protein.

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.