Application of Integrated Chemical - Physical Processes Modelling to Aeration Treatment of Anaerobic Digester Liquors

A three phase (aqueous/solid/gas) mixed weak acid/base kinetic model developed by Musvoto et al. is applied to simulate the physical and chemical processes that occur on aeration of anaerobic digester liquors. Included in the model are the kinetic reactions for (i) weak acid/base dissocations (water, carbonate, ammonium, phosphate, and short-chain fatty acids), (ii) precipitation of struvite, newberyite, amorphous calcium phosphate, calcium and magnesium carbonate, (iii) ion pair formation and (iv) stripping of CO2 and NH3 gases. To generate data for model application, batch aeration tests were conducted on two anaerobic digester liquors from (i) a spent wine upflow anaerobic sludge bed (UASB) digester and (a) a sewage sludge anaerobic digester. In the batch tests pH, Ca, Mg, PO4-P, free and saline ammonia (FSA)H2CO3* alkalinity (from which inorganic carbon is calculated) were measured. After establishing from the lIterature values for (i) weak acid/base equilbrium constants (pKa), (ii) weak add/base kinetic rate constants (K(ra)), and (iii) ion pair stability constants (pK(ST)), and trial and error determination of (iv) mineral solubility products (pK(SP)) (within the range reported in the literature), (v) ion pair kinetic rate constants (K(rIP)), (vi) mineral precipitation rate constants (Kppt) and (vii) gas stripping rates (K(rG)), a good correlation between predicted and measured data was obtained for all the parameters for both liquors. The solubility product values for the minerals that precipitated were the same for both liquors and fall in the range of values quoted in the literature, but the specific precipitation rate constants of the minerals differed for the two liquors.