Integrated chemical/physical and biological processes modeling Part 2 - Anaerobic digestion of sewage sludges
This paper describes the development of a 2 phase (liquid-gas) integrated mixed weak/acid base chemistry and biological processes AD model, by integrating the biological kinetic processes of AD into the 2 phase kinetic mixed weak acid/base model of Musvoto et al. (1997 Musvoto et al. ( , 2000a . In AD pH plays a very important role in regulating the processes, especially as many of the processes have been shown to be extremely sensitive to pH changes. Since short chain fatty acids (SCFA) are
... acids (SCFA) are produced as the main intermediates in AD, the assumption of a constant pH regime clearly becomes invalid. Furthermore, as alkalinity is routinely used as a measure of predicting the state of health of ADs by many operators, compounds not part of the anaerobic processes, but influencing alkalinity become important. Additionally, in some AD systems precipitation of minerals is significant, either within the AD itself, or in pipework leading from the AD. Therefore, a complete kinetic model for ADs should include the biological processes, aqueous phase weak acid/base chemistry with pH explicitly included, aqueous/gas phase gas exchange (in particular CO 2 ) and solid/aqueous phase mineral precipitation. The model was validated by application to the laboratory AD data set of Izzet et al. (1992) ranging from 20 to 7 days sludge age. With a single set of kinetic and stoichiometric constants, close correlation could be achieved between predicted and measured results for effluent (i) COD (ii) free and saline ammonia (FSA), (iii) short chain fatty acids (SCFA), (iv) H 2 CO 3 * alkalinity and (v) pH, as well as (vi) CO 2 and (vii) CH 4 gas production. In developing and validating the model it was found that the approach of characterising sewage sludge into carbohydrates, lipids and proteins was unduly complicated requiring measurements that are not routinely available. As alternative the sewage sludge was characterised with the COD. With this approach, to achieve a COD, C and N balance, and predict the observed gas composition and digester pH, the stoichiometric formula for the sewage sludge feed needs to be known. For the Izzett et al. data, this was calculated to be C 3.5 H 7 O 2 N 0.196 . Future research will focus on measuring sewage sludge COD, VSS, C, N, P, O and H contents.