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>3.5H7O2N0.196 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 C3.38H7O1.91N0.21 and C3.91H7O2.04N0.16. 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
Highlights
Anaerobic digestion (AD) is one of the oldest biological waste treatment processes, dating back more than a century
It has been shown that anaerobic digestion (AD) is an effective process for the treatment of a number of types of organic sludges, ranging from municipal waste activated (WAS) and primary sludges (Kayhanian and Tchobanoglous, 1992; Cout et al 1994) to industrial organic sludges and agricultural slurries (Hill and Barth, 1977)
Calculation of pH externally via the charge balance cannot deal with multiple weak acid/base systems in three phases, where several minerals competing for the same species may precipitate simultaneously or sequentially (Musvoto et al, 2000a,c): In some anaerobic digestion systems precipitation of minerals is significant, either within the digester itself or in pipework leading from the digester so that the relevant chemical precipitation processes would require inclusion
Summary
Anaerobic digestion (AD) is one of the oldest biological waste treatment processes, dating back more than a century. Calculation of pH externally via the charge balance cannot deal with multiple weak acid/base systems in three phases (aqueous/gas/solid), where several minerals competing for the same species may precipitate simultaneously or sequentially (Musvoto et al, 2000a,c): In some anaerobic digestion systems precipitation of minerals is significant, either within the digester itself or in pipework leading from the digester so that the relevant chemical precipitation processes would require inclusion. For such situations, the biological processes and multiple weak acid/base systems in three phases should be modelled in an integrated way within the same kinetic model structure.
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