Acetic acid inhibition on methanogens in a two-phase anaerobic process

The inhibitory effect of acetic acid on methanogens in a two-phase anaerobic process was evaluated. The results in this study showed that some methanogens still existed in the acidogenic phase although their dominance in the total microbial community was only 1% compared to 9.6% in the methanogenic phase. The inhibition threshold of acetic acid on acidogenic phase methanogens was, however, higher than that on methanogenic phase methanogens. At pH 6.00, acetic acid inhibition on methanogenic phase methanogens was observed when acetic acid concentration was higher than 1619.47 mg HAc/L although there was no obvious inhibition on acidogenic phase methanogens in the range of 1646.47–2781.19 mg HAc/L. There was also no acetic acid inhibition on acidogenic phase methanogens at pH 5.50, 6.00 and 6.50 in the range of 565.29–2781.19 mg HAc/L. However, for methanogenic phase methanogens, the inhibition was obvious and a second order substrate inhibition model, qs = qmS/[Ks + S + (S2/Ki)], could be adapted to describe the inhibition kinetics and mechanism of undissociated acetic acid on methanogenic phase methanogens. The results showed substrate saturation constant Ks, substrate inhibition constant Ki, and maximum specific utilization rate of acetic acid qm, were 1.66 mg unHAc/L, 145.17 mg unHAc/L, and 3.53 mg HAc/L g MLVSS h, respectively.Accepted Versio

Monitoring and Control Improvement of Single and Two Stage Thermophilic Sludge Digestion Through Multivariate Analysis

Stable operation of high-rate thermophilic sludge anaerobic reactors is sometime hard to achieve because of the nature of the anaerobic digestion (AD) process itself and the combination of biological and chemical reactions. An interesting and innovative way to handle AD data resides in multivariate statistical approaches since a more accurate analysis can be performed and fault or abnormal schemes detections can be enhanced. In this paper, principal component analysis (PCA) was the basic multivariate tool used to compare single and two stage AD process performances when treating waste activated sludge (WAS), in order to improve their monitoring and control. Two experiments were carried out to perform single and two-stage AD using WAS and fermented WAS as substrates, respectively. Findings from the PCA model agreed with results from the univariate data analysis but additionally showed a higher variability and changes on the stability trend in the AD of WAS. Besides, multivariate statistical process control using Hotelling T2 and Shewhart control charts combined with PCA displayed an out-ofcontrol scheme revealing a transition period, in which the stability pattern of this experiment changed strongly, towards an accumulation of volatile fatty acids