Integrated chemical–physical processes modelling—II. simulating aeration treatment of anaerobic digester supernatants

Abstract

A three phase (aqueous/solid/gas) mixed weak acid/base kinetic model is developed to simulate the physical and chemical processes that occur on aeration 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 and (iii) stripping of CO 2 and NH 3 gasses. A preliminary validation of the model is done using data available in the literature. The model was then applied to simulate batch aeration tests on two anaerobic digester supernatants from (i) a spent wine upflow anaerobic sludge bed digester and (ii) a sewage sludge anaerobic digester. In the batch tests pH, Ca, Mg, PO 4-P, free and saline ammonia (FSA) and H 2CO 3 ∗ Alkalinity (from which inorganic carbon is calculated) were measured. After establishing (i) the minerals most likely to precipitate viz. struvite (MgNH 4PO 4), newberyite (MgHPO 4), amorphous calcium phosphate (ACP), CaCO 3 and MgCO 3 and trial and error determination of (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 values for the minerals that precipitated were the same in both supernatants, and fall in the range of values quoted in the literature but the specific precipitation rate constants of the minerals were different in the two supernatants. Compared to the equilibrium chemistry approach to modelling three phase mixed weak acid/base systems, the kinetic approach facilitates (i) integration of the weak acid/base model with biological kinetic models and (ii) determination of solubility products and precipitation rates in an integrated manner for a number of minerals simultaneously from a single batch test.