Ventajas del modelo de Investigación Acción

The most reported response to shock loading is the decrease of degradation degree due to accumulation of VFAs and concomitant decrease in specic methane yield (Cohen et al., 1982; Grobicki and Stuckey, 1991; Borja and Banks, 1995; Chua et al., 1997; Angenent et al., 2002; Masse and Masse, 2005; Ketheesan and Stuckey, 2015). The accumulation is the consequence of kinetically saturated acetoclastic methanogenesis, the rate limit- ing step when liquid substrates are subjected to AD (Pavlostathis and Giraldo-Gomez, 1991) and thus controlling the maximum OLR that can be safely applied (Ketheesan and Stuckey, 2015). Acidogens on the other end seem to exhibit the most rapid conversion step and thus precursors to acetogenesis like propionate and butyrate accumulate. Acet- ogenetic organisms degrade the VFAs at a rate somewhere in between, while obligatory forming H2. In order for these microorganisms to gain energy from H2 forming reac-

tions, a low H2 partial pressure is mandatory and therefore live in syntrophic relation

with H2-consuming organisms like methanogens. Ultimately, kinetic dierences between

acidogenic, acetogenic and methanogenic microorganisms result in the accumulation of intermediates such as VFAs, H2 and CO2 (Ketheesan and Stuckey, 2015). Further accu-

mulation leads to a drop in pH, depending on alkalinity of the system. The proportion of undissociated VFAs increases at lower pH and the uptake of undissociated VFAs is ac- celerated (Fukuzaki et al., 1990). In order to maintain intracellular pH and a functional gradient the excess protons need to be channeled out of the cell, amplifying the decrease in pH and the toxicity of undissociated VFAs (Fukuzaki et al., 1990). The accumulation of intermediary products such as formate, acetic acid and H2 nally lead to the acetogenic

pathway becoming thermodynamically unfavorable, completely blocking the degradation of precursors to acetogenesis with system failure as the result.

VFA accumulation likely occurred in the trials of chapter 5 after applying high OLRs. The euent composition was analyzed only once per day and VFA dynamics could there- fore not be pictured in detail. Nevertheless the lowest degradation degree accompanied by slightly higher VFA concentrations was reached in the OLR-mode 'demand', which repres- ented the higher mean OLR of the two modes of operation. However, the measured VFA concentrations in the euent never came close to any problematic values and renders evidence of a highly stable process, even under the challenging operational conditions applied in the trials.

When operating anaerobic lters for exible biogas production they need to operate within the boundaries of the discussed self inhibitory VFA accumulation. The results presented in the study at hand suggest that anaerobic lters are well suited to handle variations in OLR throughout the day with quick responses in gas production. As demon- strated the gas production follows the applied OLR, with a distinctive expression of each change in the OLR. That marks the process as highly predictable and dened boundaries

within safe operation of AD in terms of VFA accumulation can possibly be satised by process control. The inclusion of three reactors in the analysis emphasize the repeatability and therefore predictability of such an approach of operation.

6.2.2 Variability of Gas Concentrations

The methane concentrations of the produced biogas changed in response to the variations of the OLR in the study presented here and has been reported throughout the literature for shock loading (Kennedy and van den Berg, 1982; Chua et al., 1997; Masse and Masse, 2005; Senturk et al., 2013). In all cases the CO2 concentration increased in response to

increased OLR, with the CH4 concentration reacting conversely. This eect can be at-

tributed to dierent mechanisms. First the accumulation of VFAs prevents the formation of their respective product gases. Secondly, depending on alkalinity, the accumulation of VFAs may cause a drop in pH and subsequently releasing CO2 from the process liquid.

Ultimately, high VFA concentrations or low pH would inhibit the methanogenic process. In the experiment at hand substrate 'A' caused the gas quality to change by a max- imum of >10 % during day one of the experiment and >7 % thereafter, presumably due to the release of large quantities of CO2 after increasing the OLR to its maximum value.

For substrate 'B' however, the gas concentrations only changed by ≈ 3 %, although the intra-daily pH variations were similar for both substrates.

The main buer acting in anaerobic digestion is the carbonate buer. This system could be labeled as open buer system since a decreasing pH causes CO2 to leave the

system via the gas stream, as could be shown in all the experiments conducted in this work. Thus it is not available anymore for buering.

In contrast the NH3 / NH4+ buer could be termed as closed buer, since both

species will remain in the liquid phase of the system. Taking a look at the substrate composition, Substrate 'B' featured higher nitrogen contents, whether it is in the form of proteins or ammonia. In the pH range where the experiments were conducted all nitrogen contributing to the NH3 / NH4+ buer would be in the state of NH4+. By

introducing nitrogenous compounds through the feed stream, the accumulated VFAs were immediately neutralized, reducing the extent of action by the carbonate buer, and thus smaller variations in gas composition were observed. Therefore the NH3 / NH4+ buer

could be valued higher as it prevents extreme variations in gas quality.

The converse reactions, i.e. higher methane and lower carbon dioxide concentrations can be observed during recovery from organic shock loading. Excess VFAs will be de- graded and pH reaches pre-shock levels. The free capacity to dissolve CO2 leads to an

increased methane content since newly produced carbon dioxide partly remains in the liquid.

in this study, more extreme cases are imaginable in practice. If the gas quality dropped below a methane content of 40 %, the shutdown of the CHP would be initiated, destroying the revenue of demand orientated power supply. Hence the observation and control of alkalinity presents itself as crucial when demand orientated biogas/power production is targeted. The minimum observed CH4 concentration during the whole experiment was

51.76 %, thus remaining in an area of safe operation. Again the AF proved itself to be well suited to handle these challenging operational conditions and its suitability for exible biogas production.