STIMULATION OF CONVERSION RATES AND BACTERIAL-ACTIVITY IN A SILAGE-FED 2-PHASE BIOGAS PROCESS BY INITIATING LIQUID RECIRCULATION

The effects of liquid recirculation on a liquefaction-acidogenic react or in an anaerobic two-phase digesting system operating with grass-clo ver silage was studied during 40 days after initiating recirculation o f effluent from the methanogenic reactor to the liquefaction-acidogeni c reactor. An increase in alkalinity and, thus, an increase in pH from 5.2 to 6.0 occurred in the liquefaction-acidogenic reactor. During th e same period, a 10-fold increase (from 0.2 to 1.9 g . l(-1). h(-1)) i n the degradation rate of mannitol and an almost 9-fold increase in th e activity of hydrogenotrophic methanogens was observed. The estimated number of these bacteria increased by one order of magnitude. The ave rage degradation rate of lactate increased 3-fold, probably as a conse quence of the more efficient hydrogen consumption by the hydrogenotrop hic methanogens. An observed increase in net mineralization of organic nitrogen compounds was probably the main reason for an enhanced net p roduction of organic acids (from 0.2 to 0.9 g . l(-1). d(-1)). The liq uefaction of cellulose and hemicellulose was low from the start of rec irculation (3% and 20% reduction, respectively) and did not seem to be affected by the liquid recirculation. This was in accordance with the low number of cellulose degraders (4.0 . 10(2) counts ml(-1)) observe d. The results from this investigation show that the initiation of liq uid recirculation in silage-fed two-phase biogas processes will stimul ate the activity of hydrogenotrophic methanogens in the liquefaction-a cidogenic reactor. This will lead to more thermodynamically favourable conditions for acidification reactions which are dependent upon inter species transfer of reducing equivalents.