Effects of hydraulic retention time and organic loading rate on performance and stability of anaerobic digestion of Spirulina platensis

The effects of hydraulic retention time (HRT) and organic loading rate (OLR) were investigated during a continuous anaerobic digestion of Spirulina platensis. The continuous process operated with varying HRT (10-25 d) and OLR (1.0-4.0 [g VS] l^-1 d^-1) resulted in the biogas and methane yields in the range of 0.276-0.502 l [g VS]^-1 and 0.163-0.342 l [g VS]^-1, respectively. The methane yield decreased with the increase in OLR and the reduction in HRT. Maximum methane yield of 0.342 l [g VS]^-1 was produced when the digester was operated at OLR of 1.0 [g VS] l^-1 d^-1 and 25 d HRT. A mathematical model was developed from a kinetic study for predicting methane yield as the function of HRT during a continuous digestion. The model performed well at OLR lower than 2.0 [g VS] l^-1 d^-1, at which the digestion was not limited by high ammonia levels or volatile fatty acid accumulation.