Early warning indicators for process failure due to organic overloading by rapeseed oil in one-stage continuously stirred tank reactor, sewage sludge and waste digesters

Abstract

Originally published as:Kleyböcker, A., Liebrich, M., Verstraete, W., Kraume, M., Würdemann, H. (2012) ABSTRACTEarly warning indicators for process failures were investigated to develop a reliable method to increase the production efficiency of biogas plants. Organic overloads by the excessive addition of rapeseed oil were used to provoke the decrease in the gas production rate. Besides typical monitoring parameters, as pH, methane and hydrogen contents, biogas production rate and concentrations of fatty acids; carbon dioxide content, concentrations of calcium and phosphate were monitored. The concentration ratio of volatile fatty acids to calcium acted as an early warning indicator (EWI-VFA/Ca). The EWI-VFA/Ca always clearly and reliably indicated a process imbalance by exhibiting a two-to threefold increase 3 to 7 days before the process failure occurred. At this time, it was still possible to take countermeasures successfully. Furthermore, increases in phosphate concentration and in the concentration ratio of phosphate to calcium also indicated a process failure, in some cases, even earlier than the EWI-VFA/Ca.

Process upsets in a full-scale anaerobic digestion bioreactor: over-acidification and foam formation during biogas production

“…Kleyböcker et al [1] introduced three EWIs for overacidification in AD processes: EWI-VFA/Ca, EWI-PO 4 / Ca and EWI-PO 4 . The authors developed these indicators and demonstrated their predictive ability in an AD system with sewage sludge and rapeseed oil as substrates [1,26].…”

“…The authors developed these indicators and demonstrated their predictive ability in an AD system with sewage sludge and rapeseed oil as substrates [1,26]. As other substrates rich in carbohydrates and low in grease are used as energy crop fed to AD plants, the verification of these indicators is important for these AD systems.…”

“…However, there are issues still remaining unclear. The most common upsets of the biogas process are over-acidification [1] as well as the formation of foam [2] and floating layers [3].…”

“…Over-acidification occurs mainly as a result of an overload of the biogas system due to either high organic loading rates [1,4] or the occurrence of inhibitors in the digester such as ammonia, sulfide, heavy metals, or substituted phenolic compounds [5][6][7].…”

“…With regard to foam formation, a test set for the estimation of the foaming tendency of substrates [15] has been developed recently. Several abiotic factors were identified to be able to serve as early warning indicators (EWIs) for over-acidification: the total or individual concentration of volatile fatty acids (VFA) [16], the propionic/acetic acid ratio [4], the VFA/ alkalinity ratio (also known as VFA/TIC) [17,18], the VFA/Ca ratio, the phosphate concentration, the PO 4 /Ca ratio [1], phenylacetic acid [19], as well as biogas production [16] and biogas composition, especially the CH 4 yield [20] and hydrogen concentration [21].…”