Retooling the ethanol industry : thermophilic, anaerobic digestion of thin stillage for methane production and pollution prevention

Abstract

Anaerobic digestion of thin stillage from a corn ethanol plant was tested at thermophilic temperature with a completely stirred tank reactor. Loading at 30, 20, 15, and 12 day hydraulic retention times (HRT) was tested. Ultrasonic pretreatment was used for one digester with another as a control. The influent thin stillage was a concentrated waste stream with 100 g/L total chemical oxygen demand and 60 g/L volatile solids (VS) typical. Significant reduction of VS was achieved with a maximum reduction (89.8%) at the 20 day HRT. Methane yield was also high with a typical yield of 0.6-0.7 L-CH₄/g-VS[subscript removed] during steady state operation. Effluent VFAs were low for a thermophilic anaerobic digester with less than 200 mg/L as acetic acid for the 20 and 30 day HRTs. The influent thin stillage had a low pH (~4) and zero alkalinity, but biological regulation of alkalinity allowed for operation without alkalinity addition. Steady state operation was achieved at 30, 20, and 15 day HRTs, and digester failure occurred at a 12 day HRT. At the 20 day HRT, a sustained shock load with a 20% organic increase was easily handled by the system. Ultrasonic pretreatment did not significantly improve the operation of the system and is not recommended for future use with anaerobic digestion of thin stillage. The high VS reduction could improve water recycling within the ethanol production process. Substantial energy is produced from the system in the form of methane gas, and natural gas displacement is estimated at 43-59% for a dry grind ethanol plant. Energy production value is estimated a