Integrated chemical/physical and biological processes modeling Part 2 - Anaerobic digestion of sewage sludges

Abstract

The development and validation of a two phase (aqueous-gas) integrated mixed weak acid/base chemical, physical and biological processes kinetic model for anaerobic digestion (AD) of sewage sludge are described. The biological kinetic processes for AD are integrated into a two phase subset of the three phase mixed weak acid/base chemistry kinetic model of Musvoto et al. (1997, 2000a,b,c). The approach of characterising sewage sludge into carbohydrates, lipids and proteins, as is done in the International Water Association (IWA) AD model No 1 (ADM1, Batstone et al., 2002), requires measurements that are not routinely available on sewage sludges. Instead, the sewage sludge is characterised with the COD, carbon, hydrogen, oxygen and nitrogen (CHON) composition. The model is formulated in mole units, based on conservation of C, N, O, H and COD. The model is calibrated and validated with data from laboratory mesophilic anaerobic digesters operating from 7 to 20 d sludge age and fed a sewage primary and humus sludge mixture. These digesters yielded COD mass balances between 107 and 109% and N mass balances between 91 and 99%, and hence the experimental data is accepted as reasonable. The sewage sludge is found to be 64 to 68% biodegradable (depending on the kinetic formulation selected for the hydrolysis process) and to have a C,sub>3.5H₇O₂N_0.196 composition. For the selected hydrolysis kinetics of surface mediated reaction (Contois), with a single set of kinetic and stoichiometric constants, for all retention times
free and saline ammonia (FSA),
short chain fatty acids (SCFA),
H₂CO₃* alkalinity and pH of the effluent stream, and
CO₂ and CH₄ gases in the gas stream. The measured composition of primary sludge from two local wastewater treatment plants ranged between C_3.38H₇O_1.91N_0.21 and C_3.91H₇O_2.04N_0.16. The predicted composition is therefore within 5% of the average measured composition providing persuasive validation of the model.

Keywords: anaerobic digestion, weak acid/base chemistry, kinetic modelling, sewage sludge

Water SA Vol. 31(4) 2005: 545-568