R. Moletta

Effects of lignocellulose degradation products on ethanol fermentations of glucose and xylose by Saccharomyces cerevisiae, Zymomonas mobilis, Pichia stipitis, and Candida shehatae

The inhibitory effects of six lignocellulose degradation products on glucose fermentation by Saccharomyces cerevisiae and Zymomonas mobilis on xylose fermentation by Pichia stipitis and Candida shehatae were studied in batch cultures. Toxic compounds were added in varying concentrations and subsequent inhibitions on growth and ethanol production were quantified. Vanillin was shown to be a strong inhibitor of both growth and ethanol production by xylose fermenting yeasts and S. cerevisiae when it was added to the culture media at a concentration of 1 g l−1. Fermentative activities of Z. mobilis were greatly sensitive to the presence of hydroxybenzaldehyde (0.5 g l−1). Analysis of culture media extracts showed that some of the inhibitors, particularly vanillin and furaldehyde, could be assimilated by the tested microbial strains which resulted in the partial recovery in both growth and ethanol production processes on prolonged incubation.

Nitrate and nitrite reductions with anaerobic sludge using various carbon sources: Glucose, glycerol, acetic acid, lactic acid and methanol

Abstract Batch-tests were used to determine the potentials of digested sludge to reduce nitrate and nitrite in the presence of five different carbon sources: glucose, glycerol, acetic acid, lactic acid and methanol. Ammonium accumulation was found in glucose and glycerol media. Dissimilatory reduction to ammonium accounted for up to 50% of reduced nitrate and nitrite. The rest were denitrified. In the media containing these carbon substrates volatile fatty acids, particularly acetic acid, were produced and ammonification was higher than denitrification activities only when glucose and glycerol were still present in the media. Ammonium production was higher in nitrite cultures than in nitrate cultures. In the culture media with acetic and lactic acids and methanol, ammonium was not detected. Nitrate/nitrite reduction in acetic and lactic acids media was essentially denitrification activity. Up to 100% of reduced nitrate and nitrite in the culture media with these acids were denitrified at average rates between 27 and 23 mg N-NOx/g MLVSSh, nitrite reduction rate being about 14% lower than total nitrate reduction rate. COD requirements for nitrate and nitrite reductions were generally lower in cultures with acetic and lactic acids than in glucose and glycerol cultures. Methanol culture media showed a very small reduction rate for the N-NOx indicating the absence (or presence in very small quantity) of the bacteria capable of denitrifying with this substrate.

Bacterial and archaeal 16S rDNA and 16S rRNA dynamics during an acetate crisis in an anaerobic digestor ecosystem

The dynamics of bacterial and archaeal populations of a laboratory-scale anaerobic digestor were investigated during a crisis period of the process reflected by an accumulation of acetate. A culture-independent approach based on single strand conformation polymorphism (SSCP) analysis of total 16S rDNA and 16S rRNA amplification products was used. A spirochete and a Synergistes sp. showed high and changing activity levels during the study. A Clostridium sp. showed a transient increase in presence and activity concomitant with the highest acetate concentrations. A major shift in the most active archaeal populations from hydrogenotrophic to acetoclastic methanogens preceded the recovery of the reactor.

Thermo-chemical pretreatment of a microbial biomass: influence of sodium hydroxide addition on solubilization and anaerobic biodegradability

The influence of sodium hydroxide addition during the thermo-chemical pretreatment of a microbial biomass has been evaluated in terms of particulate chemical oxygen demand (COD) solubilization and anaerobic biodegradability. Both COD solubilization and total solid elimination rates increased with the dose of NaOH added: COD solubilization reached 63%, and total solid elimination was 33% when 5 g Na/l were added. Additional sodium hydroxide addition did not increase COD solubilization further. The observed COD solubilization was due mainly to protein hydrolysis that was directly linked to pH variations. Heating emphasized these pH effects. The higher sodium hydroxide addition (26.1 g/l) led to 85% COD solubilization when heated to 140°C for 30 min instead of 53.2% which was at ambient temperature. Biodegradability and biotoxicity tests were run with pretreated samples. Both methane production and acetate degradation in the presence of pretreated samples were affected as 5 g NaOH/l or more were added. Sodium cation was first suspected to account for the limitations observed. Tests run at pH = 12 with other alkali agents (KOH, Mg(OH)2, or Ca(OH)2), led to the same conclusion: COD solubilization was enhanced but the biodegradability performances were limited. Additional tests run with acetate and increasing NaCl concentrations asserted that sodium cation was not at the origin of the limitations that were attributed to OH− additions: refractory compounds were formed as a consequence of pH modifications.

Denitrification in anaerobic digesters: Possibilities and influence of wastewater COD/N‐NOX ratio

Abstract Laboratory‐scale completely‐stirred anaerobic digesters were fed with synthetic wastewaters containing nitrate and nitrite and with glucose as the only source of organic carbon in order to investigate and compare the denitrification potentials of anaerobic digesters in the presence of nitrate and nitrite. Varying the input nitrate and nitrite concentration at fixed COD and HRT, methane production without denitrification occurred at COD/N‐NOX > 53; denitrification and methane production at 8.86 ≤ COD/N‐NOX ≤ 53 and only denitrification at COD/N‐NOX 53, ammonification appeared to be the main nitrate and nitrite reduction pathway. The successful competition of ammonia formers over the true denitrifiers at high ratios was attributed to the low initial nitrate and nitrite concentrations.

Xylitol production from D-xylose byCandida guillermondii: Fermentation behaviour

SummaryThe ability ofCandida guillermondii to produce xylitol from xylose and to ferment individual non xylose hemicellulosic derived sugars was investigated in microaerobic conditions. Xylose was converted into xylitol with a yield of 0,63 g/g and ethanol was produced in negligible amounts. The strain did not convert glucose, mannose and galactose into their corresponding polyols but only into ethanol and cell mass. By contrast, fermentation of arabinose lead to the formation of arabitol. On D-xylose medium,Candida guillermondii exhibited high yield and rate of xylitol production when the initial sugar concentration exceeded 110 g/l. A final xylitol concentration of 221 g/l was obtained from 300 g/l D-xylose with a yield of 82,6% of theoretical and an average specific rate of 0,19 g/g.h.

Anaerobic digestion of wine distillery wastewater in down-flow fluidized bed

In down-flow fluidization, particles with a specific density smaller than the liquid are flui- dized downward by a concurrent flow of liquid. This paper describes the application of the down-flow (or inverse) fluidization technology for the anaerobic digestion of red wine distillery wastewater. The carrier employed was ground perlite, an expanded volcanic rock. Before starting-up the reactor, physi- cal and fluidization properties of the carrier material were determined. 0.968 mm perlite particles were found to have a specific density of 280 kg m ˇ3 and a minimum fluidization velocity of 2.3 m h ˇ1 . Once the down-flow anaerobic fluidized bed system reached the steady-state, organic load was increased step- wise by reducing HRT, from 3.3-1.3 days, while maintaining constant the feed TOC concentration. The system achieved 85% TOC removal, at an organic loading rate of 4.5 kg TOC m 3 d ˇ1 . It was found that the main advantages of this system are: low energy requirement, because of the low fluidization velocities required; there is no need of a settling device, because solids accumulate at the bottom of the reactor so they can be easily drawn out, and particles with high-biomass content, whose specific density have become larger than 1000 kg m ˇ3 can be easily recovered. # 1998 Elsevier Science Ltd. All rights

Methane yield as a monitoring parameter for the start-up of anaerobic fixed film reactors.

This paper describes the variation of the methane yield during the start-up period of an anaerobic fluidized bed reactor. After a lag phase, with acclimatized sludge, the methane yield increased with time during biofilm development up to the theoretical steady yield value, reported to be around 0.351 CH4/g CODdeg. The establishment of the biofilm required a high consumption of organic material through the microbial synthesis (anabolism), thereby reducing the proportion of substrate converted to methane. As a result, this yield could be an indirect metabolic parameter for evaluating a start-up operation. It could provide vital information about bacterial fixation processes and is easy to be applied to any biofilm reactor, such as anaerobic filters, where biomass sampling is impracticable. Monitoring this parameter could also give useful dynamic information about the different steps of colonization and biomass attachment, which could be used to improve the start-up performance.

Two-phase anaerobic digestion of solid wastes by a multiple liquefaction reactors process

A new solid-waste treatment process was applied to different vegetable substrates: potato peelings, green salad leaves, green peas mixed with carrots, apple pomace. It involved, at a laboratory scale, several liquefaction digesters, each of them treating one type of waste, coupled to a central methane fixed-film reactor. The influence of pH, load and hydraulic retention time on the process performances was studied at 35°C. On average, except for apple pomace, hydrolysis yields were high (up to 80%) during the liquefaction step. Likewise, the mixture of the acidogenic effluents was degraded in a methanation reactor to an extent of 80%. In a final run with average values near 4 g COD l−1 day−1 for loading rate, and 17 days for hydraulic retention time, overall organic matter removal reached 87%.

Advanced control of anaerobic digestion processes through disturbances monitoring

This paper presents a new control strategy for highly loaded anaerobic digestion processes. This strategy is based on the analysis of disturbances added on purpose to the influent flow rate. The control system then carefully overlooks the response of only two parameters, the biogas output flow rate and the pH, in order to determine whether or not the load can be increased. The main advantages of this approach rely on its capability to automatically adapt the input flow rate to changes in the influent concentration, without any information on its concentration, and to optimize the biological treatment in relation to the influent degradability. Experiments have been carried out using two fluidized bed reactors of different sizes and they have demonstrated very good capability of this control strategy. In addition, since only very simple sensors are needed, industrial applications of this strategy can be expected.