Integrated chemical, physical and biological processes modelling of anaerobic digestion of sewage sludge

The biological kinetic processes for anaerobic digestion (AD) are integrated into a two phase subset of a three phase mixed weak acid/base chemistry kinetic model. 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), 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 and 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 107–109% and N mass balances between 91–99%, and hence the experimental data is accepted as reasonable. The sewage sludge COD is found to be 32–36% unbiodegradable (depending on the kinetic formulation selected for the hydrolysis process) and to have a C_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 good correlation is obtained between predicted and measured results for: (i) COD; (ii) free and saline ammonia (FSA); (iii) short chain fatty acids (SCFA); (iv) H₂CO₃* alkalinity; (v) pH of the effluent stream; (vi) CO₂; and (vii) 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 based on mass balances is therefore within 5% of the average measured composition providing persuasive validation of the model.