INFLUENCE OF RETENTION TIME ON REACTOR PERFORMANCE AND BACTERIAL TROPHIC POPULATIONS IN ANAEROBIC-DIGESTION PROCESSES

The influence of retention time (RT) on reactor performance and bacter ial trophic populations has been investigated by using a CSTR anaerobi c digester fed with starch as the major organic carbon source. RT does not have much effect on the population levels of hydrolytic bacteria and H-2-producing acetogens. The population of hydrolytic bacteria rem ained at 10(8)-10(9)MPN/ml for RTs between 244.8 and 1.5 h, and the po pulation of H-2-producing acetogens was 10(8)-10(11) MPN/ml when RT wa s longer than 3.12 h. However, RT has a considerable effect on the pop ulation levels of methanogens, homoacetogens as well as sulfate-reduci ng bacteria and on the composition of fermentative products. The popul ations of H-2-utilizing methanogens and homoacetogens increased when R T decreased from 244.8 to 6 h, and the maximum levels of these bacteri a were 7.9 x 10(9) MPN/ml at a RT of 6 h and 3.5 x 10(10) MPN/ml at a RT of 12 h, respectively. In contrast, acetate-utilizing methanogens a nd CH3OH-utilizing methanogens were washed out at RTs of 12 and 4.2 h, respectively. Even at a RT of 1. 5 h, methane was produced by H-2-uti lizing methanogens in the acidogenic reactor. The best RT for phase se paration is at a RT of approx. 12 h.