Integrated chemical-physical processes modelling - II. Simulating aerationtreatment of anaerobic digester supernatants

A three phase (aqueous/solid/gas) mixed weak acid/base kinetic model is dev eloped to simulate the physical and chemical processes that occur on aerati on of anaerobic digester supernatant. Included in the model are the kinetic reactions for (i) weak acid/base dissociations (water, carbonate, ammonium , phosphate, and short-chain fatty acids), (ii) precipitation of struvite, newberyite. amorphous calcium phosphate, calcium and magnesium carbonate an d (iii) stripping of CO2 and NH3 gasses. A preliminary validation of the mo del is done using data available in the literature. The model was then appl ied to simulate batch aeration tests on two anaerobic digester supernatants from (i) a spent wine upflow anaerobic sludge bed digester and (ii) a sewa ge sludge anaerobic digester. In the batch tests pH, Ca, Mg, PO4-P, free an d saline ammonia (FSA) and H2CO3* Alkalinity (from which inorganic carbon i s calculated) were measured. After establishing (i) the minerals most likel y to precipitate viz. struvite (MgNH4PO4), newberyite (MgHPO4), amorphous c alcium phosphate (ACP), CaCO3 and MgCO3 and trial and error determination o f (ii) the solubility products, (iii) specific precipitation rate constants and (iv) the specific gas stripping rates, a good correlation was obtained for all the parameters for both supernatants. The solubility product value s for the minerals that precipitated were the same in both supernatants, an d fall in the range of values quoted in the literature but the specific pre cipitation rate constants of the minerals were different in the two superna tants. Compared to the equilibrium chemistry approach to modelling three ph ase mixed weak acid/base systems, the kinetic approach facilitates (i) inte gration of the weak acid/base model with biological kinetic models and (II) determination of solubility products and precipitation rates in an integra ted manner for a number of minerals simultaneously from a single batch test . (C) 2000 Elsevier Science Ltd. All rights reserved.