Comparison of different procedures to stabilize biogas formation after process failure in a thermophilic waste digestion system: Influence of aggregate formation on process stability

Abstract

Originally published as:Kleyböcker, A., Liebrich, M., Kasina, M., Kraume, M., Wittmaier, M., Würdemann, H. (2012) ABSTRACTFollowing a process failure in a full-scale biogas reactor, different counter measures were undertaken to stabilize the process of biogas formation, including the reduction of the organic loading rate, the addition of sodium hydroxide (NaOH), and the introduction of calcium oxide (CaO). Corresponding to the results of the process recovery in the full-scale digester, laboratory experiments showed that CaO was more capable of stabilizing the process than NaOH. While both additives were able to raise the pH to a neutral milieu (pH > 7.0), the formation of aggregates was observed particularly when CaO was used as the additive. Scanning electron microscopy investigations revealed calcium phosphate compounds in the core of the aggregates. Phosphate seemed to be released by phosphorus-accumulating organisms, when volatile fatty acids accumulated. The calcium, which was charged by the CaO addition, formed insoluble salts with long chain fatty acids, and caused the precipitation of calcium phosphate compounds. These aggregates were surrounded by a white layer of carbon rich organic matter, probably consisting of volatile fatty acids. Thus, during the process recovery with CaO, the decrease in the amount of accumulated acids in the liquid phase was likely enabled by (1) the formation of insoluble calcium salts with long chain fatty acids, (2) the adsorption of volatile fatty acids by the precipitates, (3) the acid uptake by phosphorus-accumulating organisms and (4) the degradation of volatile fatty acids in the aggregates. Furthermore, this mechanism enabled a stable process performance after re-activation of biogas production. In contrast, during the counter measure with NaOH aggregate formation was only minor resulting in a rapid process failure subsequent the increase of the organic loading rate.2

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

“…Thus, the addition of CaO contributes significantly to the stabilization of the biogas formation process allowing for higher OLRs than without CaO addition. With sewage sludge and rapeseed oil as substrates, the effect of process stabilization by CaO addition was mainly based on the precipitation and adsorption processes, together with the biofilm formation, which resulted in aggregate (with a size between 0.5 and 5 mm) formations as discussed in detail in Kleyböcker et al (2012). Clearly, the microorganisms within these aggregates were protected from conditions such as high hydrogen concentrations, which were observed in the gaseous phase.…”

“…1 in Kleyböcker et al 2012). The reactors were maintained at 50 °C by a thermostat (Thermo Haake B7, Phoenix II) connected to a heating coil that was placed around the reactor.…”

“…In previous experiments, the addition of calcium by the supply of CaO contributed significantly to the recovery of a biogas formation process after an excess accumulation of fatty acids, and this concept was identified as an efficient measure to stabilize the biogas formation processes (Kleyböcker et al 2012). The calcium formed insoluble salts with long chain fatty acids (LCFAs), and the amount of phosphate that was very likely released from the phosphate accumulating organisms (PAOs) during the VFA uptake after their accumulation.…”

Foam formation in full-scale biogas plants processing biogenic waste

“…The anaerobic digestion (AD) of organic matter is the core of the overall biogas production process, the main problems of which are over-acidification [3,4], forming of floating layers [5], and excessive foam formation. While the causes of over-acidification and floating layer formation are relatively well understood, the causes of foam formation have remained unidentified to a large extent in practice.…”

Floating layer formation, foaming, and microbial community structure change in full-scale biogas plant due to disruption of mixing and substrate overloading

“…Feeding with profitable but unsuitable substrates may lead to serious process interruptions. An overacidification event is often caused by substrate overloading and accumulation of volatile fatty acids (VFA) [2]. Kleyböcker et al [3] developed two early warning indicators in terms of overacidification.…”