Effect of shear on performance and microbial ecology of continuously stirred anaerobic digesters treating animal manure

Abstract

We determined the effect of different mixing intensities on the performance, methanogenic population dynamics, and juxtaposition of syntrophic microbes in anaerobic digesters treating cow manure from a dairy farm. Computer automated radioactive particle tracking in conjunction with computational fluid dynamics was performed to quantify the shear levels locally. Four continuously stirred anaerobic digesters were operated at different mixing intensities of 1,500, 500, 250, and 50 revolutions per min (RPM) over a 260-day period at a temperature of 34 +/- 1 degrees C. Animal manure at a volatile solids (VS) concentration of 50 g/L was fed into the digesters daily at five different organic loading rates between 0.6 and 3.5 g VS/L day. The different mixing intensities had no effect on the biogas production rates and yields at steady-state conditions. A methane yield of 0.241 +/- 0.007 L CH(4)/g VS fed was obtained by pooling the data of all four digesters during steady-state periods. However, digester performance was affected negatively by mixing intensity during startup of the digesters, with lower biogas production rates and higher volatile fatty acids concentrations observed for the 1,500-RPM digester. Despite similar methane production yields and rates, the acetoclastic methanogenic populations were different for the high- and low-intensity mixed digesters with Methanosarcina spp. and Methanosaeta concilii as the predominant methanogens, respectively. For all four digesters, epifluorescence microscopy revealed decreasing microbial floc sizes beginning at week 4 and continuing through week 26 after which no microbial flocs remained. This decrease in size, and subsequent loss of microbial flocs did not, however, produce any long-term upsets in digester performance.

Anaerobic co-digestion of dairy manure with mulched switchgrass for improvement of the methane yield

“…The manure was blended to facilitate the feeding of the digesters, although this assay showed that the blending consisted in a form of pretreatment that increased the methane production of the manure by 46%. Nevertheless, the methane produced from the manure only assays were in the same range as what is generally presented in the literature for manure anaerobic digestion [27,28]. The replacement of 20% of the manure by switchgrass (wet weight) in the assay yielded 32% more methane, although the gain was 92% if compared with the raw manure.…”

Rapid start-up of thermophilic anaerobic digestion with the turf fraction of MSW as inoculum

“…However, it has also been recommended by some researchers to avoid intense stirring during the start-up phase Stroot et al, 2001;Hoffmann et al, 2008). To clarify these arguments, this experiment compared vigorously stirred and unstirred reactors during the start-up phase.…”

Effect of Mixing Driven by Siphon Flow: Parallel Experiments Using the Anaerobic Reactors with Different Mixing Modes

“…There was a tendency that the abundance of the Methanosaeta showed a marked increase when mixing speed was low (50-60 rpm) or minimal rather than high [29,30]. In this study, the CMR was stirred at 300 rpm and equipped with the impellor twice the diameter of the other researches [29,30]. The strong mixing in the CMR may have negatively affected the growth of the Methanosaeta.…”

“…Another possible factor governing the methanogenic community is the mixing condition [29,30]. There was a tendency that the abundance of the Methanosaeta showed a marked increase when mixing speed was low (50-60 rpm) or minimal rather than high [29,30]. In this study, the CMR was stirred at 300 rpm and equipped with the impellor twice the diameter of the other researches [29,30].…”

“…As such, the reactors were operated under similar conditions in terms of the NH 4 + and VFA concentrations. Another possible factor governing the methanogenic community is the mixing condition [29,30]. There was a tendency that the abundance of the Methanosaeta showed a marked increase when mixing speed was low (50-60 rpm) or minimal rather than high [29,30].…”