(604f) Use of Oxygen Uptake Rate Data to Measure the Inhibition Effect of Volatile Fatty Acids on Activated Sludge Dedicated for Lipid Production for Fuels

Authors: 
Hernandez, R., University of Louisiana at Lafayette
Fortela, D. L., University of Louisiana at Lafayette

This study focused on the evaluation of the inhibitory effect of volatile fatty acids (VFAs) such as acetic acid, propionic acid, and butyric acid on the survival of activated sludge microorganisms. The issue arises from the development of an integrated anaerobic-aerobic microbial system for the production of microbial lipids for fuels via the volatile fatty acids platform. The objective of the system is to deconstruct lignocellulosic biomass into carbon sources via anaerobic digestion and feed the intermediate carbon sources to activated sludge to trigger the accumulation of lipids that can then be extracted and converted to biodiesel.

It was also pointed out, however, that the VFAs have inhibitory effects on pure cultures of oleaginous microorganisms. The kinetic growth of activated microorganisms was found similar to that of oleaginous microorganisms, but, as of the moment, there is no literature data on the comprehensive view of VFAs inhibition effect on activated sludge microorganisms. Knowing the mixture compositions of VFAs that favor less inhibition of activated sludge is important in engineering an economical microbial lipid accumulation process by activated sludge.

This study shows the use of dissolved oxygen uptake rate (OUR) as a measure of the inhibition effect of VFAs on activated sludge. Activated sludge microorganisms are sensitive to dissolved oxygen (DO) level and the time-change of DO level can be an indicator of microbial activity level. Therefore, the inhibition of VFAs on activated sludge can be measured by the rate of consumption of oxygen and can be expressed as Respiration Inhibition (R.I in %) = 100 – [100 x (OUR of treated/OUR of control)]. Experiments were run using mixture design Simplex Lattice [3,3] which captures the three acids as factors – acetic acid, propionic acid, butyric acid – and three levels for each. It was found that there is high level of inhibition by the VFAs on activated sludge.