Mathematical Model for Meso- and Thermophilic Anaerobic Sewage Sludge Digestion

Abstract

A mathematical model is developed to describe the dynamic behavior of mesophilic (35 +/- 5 degrees C) and thermophilic digestion (55 +/- 5 degrees C). Special emphasis is given to acetotrophic methanogenesis and propionate degradation, as the steps that determine the stability of anaerobic digestion, as well as to hydrolysis rate, which determines the degradation efficiency of particulate degradable organic carbon. Within the range of 6-20 (mesophilic) and 2-8 d (thermophilic) hydraulic retention time (HRT), the observed maximum growth rates for acetotrophic methanogens are 0.33 and 1.3 d(-1), respectively, with a 15% decay rate. Temperature and pH dependence as well as ammonia inhibition of acetate and propionate conversion are determined and included in the model, which allows us to simulate the effect of protein- and nitrogen-rich waste addition and the consequences of temporarily increased free ammonia at high pH. No inhibition of hydrogen conversion was observed in the same free ammonia range. The pH optimum is between 6.6 and 7.3. Acetotrophic methanogenesis is strongly inhibited below pH 6.2, whereas above pH 7.4 it can be inhibited by free ammonia. For digesters fed with ordinary municipal sewage sludge, free ammonia inhibition of acetate conversion leads to an increase in acetate at about 35 and 140 mg of N/L for mesophilic (HRT = 20 d) and thermophilic (HRT = 6 d) conditions, respectively. The hydrolysis rate constant is 0.25 and 0.4 d(-1) respectively for these two conditions. The model is validated with load variation experiments in laboratory and full-scale digesters for step and shock loads.

Kinetic study of biogas production from energy crops and animal waste slurry: Effect of organic loading rate and reactor size

“…Furthermore, high-energetic substrates raise the danger of overloading. Several models and reactor configurations have been developed in recent years which make it possible to predict real process response to specific operation conditions [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. For example, a simple model has been developed to describe the relation between the hydraulic retention time and methane yield in the digestion of animal waste [12,13].…”

Hydrolysis rates of domestic wastewater sludge using biochemical methane potential tests

“…Aldin [16] reported hydrolysis rate constants (kh) for wastewater sludge within the range of 0.0096 to 1.94 day -1 for primary sludge, 0.005 to 0.2 day -1 for sewage sludge, with 0.08 to 2.0 day -1 as the general range for most types of sludge. Eastman and Ferguson [35], Batstone et al [36] and Siegrist et al [37], reported hydrolysis rate constants for primary sludge between 0.2 -0.5 day -1 at mesophilic conditions, while Mahmoud [34] reported 0.23 day -1 for settle-able solids from domestic wastewater at 35°C. Kassab et al [33] reported hydrolysis rate constants based on first order kinetics as 0.006 day -1 for seeded domestic wastewater sludge, and 0.004 day -1 for unseeded domestic wastewater sludge.…”

Post-aerobic digestion of waste sludge: performance analysis and modelling of nitrogen fate under alternating aeration

“…fed to the aerobic reactor (N-NH 4fed ) and produced in the aerobic hydrolysis of removed VS (N-NH 4hyd ). This latter nitrogen input according to Angelidaki et al (1999) and Siegrist et al (2002) has been determined assuming a stoichiometric coefficient of 1 for VS hydrolysis; therefore, we can approximately assume that the released nitrogen is equal to the nitrogen content associated with the hydrolysed sludge. The fed sludge in this case is the anaerobic digested sludge for which, according to European Commission (EU 2002), the nitrogen content varies within the range of 2.5-14 % of the VS depending on the characteristics of the fed sludge and on the operating conditions of the digestion.…”