Model based optimization of biogas production at SNJ plant

Abstract

The main purpose of this thesis is the acquisition of knowledge and familiarization with the SNJ biogas plant and effects of codigestion. Plant operation and performance was monitored in order to understand and evaluate the factors affecting the efficiency of the sludge treatment process. The thesis also presents an overview of anaerobic digestion process, modelling of anaerobic codigestion process, and a general presentation of the Regional Wastewater Treatment Plant of Nord‐Jæren (SNJ). More precisely, the study provides experimental data of several parameters at SNJ plant. Methane production and Chemical Oxygen Demand (COD) removals were quantified under steady state conditions for a wide range of operating parameters. Mathematical model (ADM1) was successfully adapted to reactor system. Non‐steady state analysis was performed to assess the effect of food waste loading on biogas production by codigestion. Calculations reveal that 0.6142 m3 of methane is produced per kg COD removal, based on theoretically estimated biogas production by using the measured data. There is big discrepancy of 29.7% in comparison between the theoretical estimated methane production and the actual methane production measured at SNJ plant. The Volatile Suspended Solids (VSS) reduction during the digestion of raw sludge along with septic sludge and food waste was 58.69%. Likewise, Total Suspended Solids (TSS) reduction was calculated as 47.57%. Results gained by simulation showed differences between the steady state and measured data. The simulated biogas production showed lower values then the measured at SNJ. The result of non‐steady state analysis shows that the biogas production had increased by 62 % in reactor 1 in comparison with reactor 2. The biogas production obtained here, by the addition of food waste, could be attributed to the higher biodegradability of food waste. On the other hand, the high contents of polysaccharides and assembly the carbohydrates in the food waste have been recognized as a main reason for the increased biogas production. Therefore, food waste codigestion with raw sludge is a sustainable and environmentally attractive method to treat and simultaneously convert such a waste mixture to a useful energy source.