Adapting the self-cycling fermentor to anoxic conditions

The self-cycling fermentation (SCF) technique for cell cultivation is a useful tool for studying microorganisms. To date, this method has only been applied to aerobic systems, since dissolved oxygen has been used exclusively as the feedback control parameter, which defines this technique. To extend the range of redox environments to which the SCF can be applied, an alternate measurement is required. Transient oxidation reduction potential (ORP) and carbon dioxide evolution were screened as potential feedback control parameters in a denitrifying system. The characteristic change in slope of the transient ORP profile commonly referred to as the "nitrate break point" was found to be a useful indicator of complete removal of oxidized nitrogen only in the presence of significant quantities of nitrate. An inflection point occurring after the break point was found to be a more general indicator of oxidized nitrogen removal. A control strategy based upon real-time detection of the inflection point was found to result in robust operation of the SCF. When variable amounts of nitrate (0-930 mg L-1) and nitrite (0-300 mg L-1) were added to the reactor each cycle, the control strategy automatically adjusted the cycle time, resulting in an effluent containing less than 1 mg L-1 of each of the oxidized nitrogen species.