Effect of acetate to biomass ratio on simultaneous polyhydroxybutyrate generation and direct microbial growth in fast growing microbial culture.

The study investigated the effect of variations in the acetate to biomass ratio on substrate storage potential, and the kinetics of substrate utilization. A series of batch experiments were conducted with biomass taken from the fill and draw reactor operated at a sludge age of 2 d. One of the batch reactors duplicated the substrate loading in the main reactor. The others were started with different initial acetate to biomass ratios both in lower and higher ranges. Increasing available acetate did not totally divert excess substrate to storage; the microbial culture adjusted the kinetics of the metabolic reactions to a higher growth rate so that more substrate could be utilized for direct growth at high acetate levels. Conversely, storage rate was increased, utilizing a higher substrate fraction for polyhydroxybutyrate generation when acetate concentration was lowered. The physiological and molecular bases of storage at low substrate levels were discussed.

Effect of storage on the respirometric relationship between substrate utilization and microbial growth.

The paper evaluated the impact of substrate storage on the respirometric assessment of process stoichiometry based on oxygen uptake rate (OUR) measurements. Two parallel sequencing batch reactors were operated with pulse feeding of synthetic substrate mixture at a sludge age of 8 days and 2 days. During the cycle experiments with acetate, 40-45% of acetate was converted to polyhydroxybutyrate, which was partly consumed during each cycle. Respirometric analysis also yielded OUR profiles for the corresponding cyclic operation. A mass balance expression was derived based on oxygen utilization. Oxygen demands calculated on the basis of partial PHB utilization closely matched the experimental values retrieved from OUR profiles within limits of analytical precision. The relative contribution of storage mechanism represented more than 50% of overall oxygen demand. Substrate storage, when totally disregarded or not properly evaluated, was observed to involve an error of around 10% on overall the oxygen demand.