Development of a sequential continuous stirred tank reactor (CSTR) system for anaerobic digestion of liquid dairy manure
2006
A continuous 5-sequential completely mixed reactors system was developed to
anaerobically digest liquid dairy manure with different organic strength. Within the range of
experimental conditions, i.e., influent organic strength (10 to 30 g/L of COD and 0.8% to
2.7% of total solid) and hydraulic retention time (HRT) (0.33 to 15 day), the total amount of
biogas and the proportion of methane produced from the first reactor were always at the
highest levels, and then decreased with sequential reactors. The liquid pH of all the reactors
was maintained at a neutral range (7.2-7.7) with an alkalinity ranging from 3.5 to 4 g
CaCO3/L. The profiles of biogas production and liquid pH with the sequential reactors
suggested that the two-stage anaerobic digestion process (i.e., acidogenesis reactor
produces less biogas with an acidic pH, while the methanogenesis reactor produces more
biogas with a neutral pH) could not be implemented by the system developed in this work.
The reason was attributed to: (1) the alkalinity of the reactor liquid were at a high level which
maintained a neutral liquid pH; and (2) the manure influent contained certain amount of volatile fatty acids (especially acetate), which supplied to the first reactor for the
methanogens to produce more biogas than the following reactors. The sequential reactors
system was also designed in such a way that the anaerobic digestion performance at
different HRT levels could be investigated at a given influent feeding rate and reactor
working volume. Between HRT ranging from 0.33 day to 15 day, the specific biogas
production rate decreased, while COD and solid removal percentage increased with HRT. It
was found that at a specific HRT level, the anaerobic digestion performances (biogas
production, COD, TS/VS reduction) of a sequential-reactors combination (approximated as a
plug-flow type reactor) were similar to those of a single completely mixed reactor.
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