Process parameters within a 750,000 litre anaerobic digester during a year of disturbed fermenter performance
Introduction
For both ecological and economical reasons anaerobic digestion has gained an increasing importance as a method for treating the organic fraction of wastes of different origin (Gunaseelan, 1997). However, despite the increasing employment of this technique, the majority of the digestion plants are controlled on an empiric basis and the evidence of many of the basic process parameters are still uncertain (Scherer, 2001). Various process parameters have been suggested for characterizing the fermenter performance: pH, concentration of , concentration of several volatile fatty acids (VFA), especially acetatic and propionic acids, quantity and quality of biogas etc., and a couple of respective threshold values have also been proposed (Buhr and Andrews, 1977, Chen et al., 2008, Illmer and Gstraunthaler, 2009). Nevertheless all of these values were also shown to exceed the respective threshold values without negative effects (Pullammanappallil et al., 2001) and on the other hand a bad fermenter performance was shown to occur without alarming process parameters (Illmer and Gstraunthaler, 2009). Both situations contribute to the often contradictory discussions in connection with the controlling of biowaste treatment plants.
Acetate represents one of the most important fatty acid in the food chain of an anaerobic digester. It is an intermediate between higher molecular substances and the end-products CO2 and CH4. Acetogenesis usually means the metabolic process of special microorganisms, called acetogens, to generate acetate (Drake, 1992), although it is often limited to the use of the reductive Acetyl-coenzyme-A pathway (Drake, 1992, Müller, 2003). The ability of acetogenic O-demethylation was used previously to enrich acetogenic bacteria from anoxic environments (Schink, 1994). However, only few data are available on the succession of acetogenic microorganisms in anaerobic digesters so far.
Frequently occurring shortcomings of many studies on biowaste digestions are the brevity of the investigations, limited analytical spectra together with an almost complete omission of microbiological aspects, and difficulties of statistical analysis when investigating constant reactor performance with small variances in process parameters. Within the present study we tried a comprehensive evaluation of the fermentation of a 750,000 litre fermenter, and we investigated a couple of physical, chemical and microbiological properties over 1 year. About half the time was characterized by an extremely and unusually bad fermenter performance. Thus we hoped that causal interrelationships between these parameters and the fermenter performance should be more easily detected.
Section snippets
Fermenter and biowaste
A 750,000 litre thermophilic plug flow digester which is located in Roppen in the province of Tyrol/Austria and which follows the KOMPOGAS-dry fermentation principle was used for our investigation in the present study. Total amounts of organic wastes treated in Roppen are in the range of 2500 tons a−1 of green waste and about 6200 tons a−1 of biowaste. The fermenter itself and basic properties of it were described elsewhere (Wagner et al., 2009, Illmer and Gstraunthaler, 2009). Some properties of
Biogas production and basic properties
As mentioned above biogas production was partially very low throughout the investigated period. This was abnormal for the fermenter, which usually works very well and which is under detailed investigation since several years (Illmer and Gstraunthaler, 2009). However, this bad fermenter performance was an advantage for the present investigation, as extreme values in gas production might have also been connected to extreme values in process parameters, and should make connections between them
Acknowledgments
The study (Project No. 1405) was supported by the Austrian Federal Ministry of Agriculture, Forestry, Environment and Water Management (BMLFUW) and the Counties of Tyrol and Vorarlberg.
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