High-resolution monitoring of VFA dynamics reveals process failure and exponential decrease of biogas production

Abstract

The future of biogas production will be characterized by on-demand provision to compensate the unpredictability of solar and wind power. Such biogas production through feedstock management is a promising possibility but requires close monitoring. The dynamics of volatile fatty acid (VFA) formation and further degradation to methane production are of special interest when providing high portions of fast degradable carbohydrates. Their fast degradability can impair process stability. The correlation of VFA loading and the biogas process was tested in four anaerobic continuously stirred tank reactors supplied with maize silage hourly and with sugar beet silage twice a day at a 12-h interval. The reactors differed in the amount of sugar beet silage and thus in total organic loading rate from 2.0 to 3.5 kgVS m−3 day−1. The VFA concentrations increased immediately after each input of sugar beet silage but levelled down until the next feeding period. At the highest organic loading rate, successive VFA accumulation escalated after 25 days (50 feeding periods) at 3.5 kgVS m−3 day−1, causing process failure with propionic acid concentrations exceeding 3500 mg L−1. The data revealed a strong negative exponential relationship between VFA concentrations and biogas and methane yields, respectively. High-resolution monitoring showed the instant dynamics of VFA production after intermittent sugar beet silage supply and the cumulative impact during increasing process disturbance.