Microbial Population Dynamics during Anaerobic Digestion of Guinea Grass ( Panicum maximum )

Abstract

The effect of rumen fluid on microbial population dynamics during anaerobic digestion of Guinea grass (Panicum maximum) at ambient condition with respect to time was investigated. A one stage batch-typemesophilic anaerobic digestion system was configured using rumen fluid (RF) as inoculums (ADRF) and a low solid loading of approximately 7.0% total solid (TS). Physicochemical parameters such as process temperature (PTMRF), process pHRF, chemical oxygen demand (CODRF) and volatile fatty acid (VFARF) were monitored with time. Selected indicator microbial populations were monitored by standard cultural enumerations based on metabolic capacity and oxygen sensitivity with respect to time. Furthermore, their respective growth rates and population proportions were determined. Result showed that the average PTMRF increased from 27.5°C to 35.2°C while average process pHRF ranged from 6.5 to 7.9 with time, respectively. The CODRF decreased from 11,250.60 mg/L to 2,865.20 mg/L, while VFARF ranged from 1,080.00 mg/L to 4,800.33 mg/L with time, respectively. In terms of metabolic capacity, the populations of cellulolytic bacteria (ACBRF), lactose fermenting bacteria (LFBRF) and glucose fermenting bacteria (GFBRF) ranged from 3.6 x 104 MPN/ml to 2.9 x 105 MPN/ml, 3.4 x 104 MPN/ml to 2.9 x 105 MPN/ml and 4.4 x 104 MPN/ml to 4.6 x 105 MPN/ml respectively with time. The populations of propionate oxidizing bacteria (POBRF), ethanol oxidizing bacteria (EOBRF) and acetate oxidizing methanogens (AOMRF) ranged from 2.9 x 104 MPN/ml to 2.4 x 105 MPN/ml, 2.7 x 104 MPN/ml to 2.1 x 105 MPN/ml and 1.4 x 104 MPN/ml to 2.1 x 105 MPN/ml respectively with time. In terms of O2-sensitivity, the populations of obligate anaerobic bacteria (OABRF) and facultative aerobic and anaerobic bacteria (FAABRF) ranged from 2.12 x 105 CFU/ml to 4.53 x 106 CFU/ml and 4.6 x 105 CFU/ml to 4.74 x 106 CFU/ml respectively with time. The population of GFBRF had the highest growth rate of 0.057 day-1 while the population of EOBRF had the lowest growth rate of 0.021 day-1. In terms of O2-sensitivity, the population of FAABRF had the highest growth rate of 0.051 day-1 compared to the population of OABRF with growth rate of 0.040 day-1. The population of GFBRF predominated (26.3%), while the population of AOMRF were the minority (10.44%). In terms of O2-sensitivity, the population of FAABRF predominated (56.73%) compared to the population of OABRF (43.23%). Rumen fluid significantly (p < 0.05) increased the microbial populations inside ADRF with respect to time. Therefore, rumen fluid could be used to boost the microbial population in anaerobic digesters as this could enhance depolymerisation, obtain higher degradation rates of cellulosic (or lignocellulosic) substrates and thus higher energy (biogas/methane) benefits.