SIMULATION OF PROCESS STEADY-STATE IN LIVESTOCK WASTE METHANOGENESIS

A predictive indicator of process steady-state is an invaluable tool i n operating sensitive biological processes such as anaerobic methanoge nesis. Nitrogen ratio (NR) the ratio of total Kjeldahl nitrogen (TKN-N ) entering an anaerobic fermenter to the TKN-N leaving the fermenter ( TKN(i)n/TKNout), has been established as a reliable indicator of ferme nter steady-state (Cobb and Hill, 1990). The methanogenesis model deve loped by Hill (1982) and subsequently modified by Hill and Bolte (1987 ) and Hill and Cobb (1993) has been updated to include nitrogen balanc es for TKN-N and organic nitrogen (ON-N) in order to calculate NR. The previously validated computer model has been revised to include utili zation and conversion of TKN-N, NH4+-N, and ON-N within the fermentati on vessel. Specifically, the model has been modified to compute TKN-N and ON-N levels based on their concentration in the influent wastestre am, the conversion of ON-N to NH4+-N by acetogenic bacteria, and the u ptake of NH4+-N by the various bacterial cultures. Results are present ed for the quantitative verification and validation of the model for N R for four transient operating states. Simulation results are within /- 10% of real NR data. The utility of the model is then demonstrated by simulating various other process states.