Semi-continuous Stirred Tank Reactor Research Articles

The Acidogenic Digestion of the Organic Fraction of Municipal Solid Waste for the Production of Liquid Fuels

Several reactors were tested for the anaerobic conversion of the organic fraction of municipal solid waste into volatile fatty acids. The reactors examined included Batch, Semi-Continuous Stirred Tank Reactor and Multistage-Counter Flow Reactor. After reasonable retention times (12 days), Batch systems operating at 15% w/v of dry matter and under pH controlled at 6.5, gave better performances both in terms of total Volatile Fatty Acids (VFA) concentration (23-24 g/l) and conversion yield (215 g VFA/kg volatile solids). However, these conditions of reaction caused a high level of liquefaction of the feedstock which resulted in a difficult down-stream processing. On the contrary, the acidogenesis under uncontrolled pH, allowed the production of leachates directly processable for acids recovery. Based on this reason, the Multistage-Counter Flow Reactor, operating under uncontrolled pH, appeared to be the most promising system. The leachate obtained from such a system was processed to obtain mixtures of C2-C7 methyl esters, potentially usable as octane enhancers. The digested solid was intended to be dried and refined to obtain soil amendment. A process for the treatment of municipal solid waste based on the previous steps and including conventional sorting facilities and units for the incineration of all residual solids, was evaluated to be economically comparable with landfilling. The revenues from the sales of esters and fertilizers were highly sensitive to the yield of the acidogenic step and to the value and the amount of the derived fertilizer. Further research is still in progress to improve the economics of the process. The activities are addressed to study proper procedures for the improvement of the down-stream processing of the mixtures originating from acidogenesis under almost neutral pH.

Effects of addition of soluble oxidants on the thermophilic anaerobic digestion of biomass to methane

Bioconversion of polymeric substrates in anaerobic digesters is slow. Exploratory research was conducted on the effects of the addition of soluble oxidants to a thermophilic, anaerobic, semicontinuous stirred tank reactor (CSTR) fed a biomass feedstock. After adaptation, added nitrate was quantitatively reduced to ammonia and isotopelabeling experiments confirmed that denitrification reactions did not occur. Addition of sulfate to a continuous nitrate amended digester resulted in sulfate accumulation, whereas sulfate addition to nonnitrate-amended digesters resulted in sulfate reduction. These results support the hypothesis that nitrate is preferentially reduced in the presence of sulfate and nitrate.

Fermentation of aqueous extract of peanuts with Lactobacillus bulgaricus in a semi-continuous stirred tank reactor

Lactobacillus bulgaricus was used to ferment an aqueous extract of peanuts ( Arachis hypogea) (peanut milk) unsupplemented and supplemented with 2% glucose in a semi-continuous stirred tank reactor system. Glucose-supplemented peanut milk supported increased cell populations and resulted in a greater decrease in pH and higher titratable acidity compared to unsupplemented peanut milk. Changes in viscosity and the dissolved oxygen content during fermentation of the unsupplemented and supplemented peanut milks were similar. Total carbohydrate and reducing sugar content in supplemented peanut milk was decreased substantially during fermentation.