The Influence of Long and Short Cycle Schemes of Self-Cycling Fermentation on the Growth of E. Coli and S. Cerevisiae

Self-cycling fermentation (SCF), a cyclic process in which cells divide once per cycle, has been shown to lead to improvements in productivity during bioconversion and, often, whole-culture synchronization. Previous studies have found that in some cases, the completion of synchronized cell replication occurred simultaneously with depletion of a limiting nutrient. However, exceptions were also observed when the end of cell doubling occurred before the exhaustion of the limiting nutrient. In order to better understand the underlying mechanisms and impacts of these growth patterns on bioprocessing, we investigated the growth of Escherichia coli and Saccharomyces cerevisiae in long- and short-cycle SCF strategies. Three characteristic events linked to SCF cycles were identified: (1) the completion of synchronized cell replication, (2) the depletion or a plateau of the limiting nutrient, and (3) characteristic points of control parameters (e.g., the minimum of dissolved oxygen and the maximum of carbon dioxide evolution rate). Three major trends stemming from this study and SCF literature were observed: (A) co-occurrence of the three key events in SCF cycles, (B) cycles for which cell replication ended prior to the co-occurrence of the other two events, and (C) cycles for which the time of depletion or a plateau of the limiting nutrient occurred later than the concurrence of the other two events. Based on these observations, a novel definition for SCF is proposed.

Processing dewatered sewage sludge using electrokinetic technology

High content of heavy metals and presence of pathogens in the dewatered sewage sludge have been the main obstacles for land application of sewage sludge-made fertilizer. The aim of this study was to xamine the effects of the innovative electrokinetic (EK) technology on removal of heavy metals from sewage sludge, on the reduction of pathogens, and on sludge chemical characteristics. The results showed that the removal efficiencies for Zn, Cu, Ni, Cr, As and Pb were 94.9%, 95.4%, 89.7%, 67.8%, 31.2% and 18.7%, respectively. Acidification pretreatment of the dewatered sludge for 29 h decreased the content of heterotrophic bacteria from 1.5 × 108 c.f.u./g of wet sludge to 1.1 × 104 c.f.u./g of wet sludge. Although the initial content of total coliforms and fecal coliforms in sewage sludge were 5.8 × 105 c.f.u./g of wet sludge and 4.0 × 105 c.f.u./g of wet sludge, respectively, no viable cells were detected. Minor losses of K and N were detected, but the loss of P was found to be significant in EK treated sewage sludge. The treated sludge has technically considered as very stable based on the carbon dioxide evolution rate.

Coupling pH-stat and DO-stat titration to monitor degradation of organic substrates

A combined titration methodology at constant pH and dissolved oxygen concentration to monitor the degradation of organics as carbon dioxide evolution rate (CER) and oxygen uptake rate (OUR) is presented. Equations necessary to assess CER from alkali titration rates and the instrument used to test the technique are described. Experiments were performed on samples of activated sludge from a domestic wastewater treatment plant using glucose and ethanol as substrates. OUR and CER were calculated for both substrates, as well as the respiratory quotient RQ = CER/OUR and biomass to substrate yields coefficients. RQ data for ethanol compared better with corresponding stoichiometric values than those for glucose.