Extension and application of the three-phase weak acid/base kinetic model to the aeration treatment of anaerobic digester liquors

The kinetic model for the carbonate (inorganic carbon) system in three phases and the phosphate, short chain fatty acid and amm onia mixed weak acid/base systems in single phase, based on modelling the kinetics of the forward and reverse dissociation processes of the weak acid/base species, is extended to model the three-phase mixed weak acid/base chemical reactions in anaerobic digest er liquor aeration. The kinetic reactions for ion pairing, precipitation of struvite, newberyite, amorphous calcium phosphate, cal cium and magnesium carbonate and stripping of NH 3 gas are added to the model. A preliminary validation of the model was done by comparing model predictions with an equilibrium based struvite precipitation model; and equilibrium experimental results from the literature on simultaneous precipitation of calcium and magnesium carbonate and phosphate minerals. To collect data to vali date the non-steady state behaviour of the model, aeration batch tests were conducted on liquor from a spent wine upflow anaerobic sludge bed (UASB) digester and a sewage sludge (SS) anaerobic digester. In the batch tests pH, Ca, Mg, Total PO 4 -P (P T ), free and saline ammonia (FSA, N T ) and H 2 CO 3 * alkalinity (from which inorganic carbon C T is calculated) with time were measured over 24 to 56 h. To simulate the batch test results, solubility product and ion pairing constants were obtained from the literature and the mineral precipitation and CO 2 and NH 3 gas stripping rates calibrated by minimising the error between the predicted and measured results. A very good correlation was obtained for all the measured parameters in six out of the seven batch tests, four on SS and three on UASB anaerobic digester liquors (ADL). From the model results the masses of the different calcium and magnesium carbonate and phosphate minerals that precipitated and the CO2 and NH3 stripped via the gas phase could be calculated. Comparing the results from the two liquors it was found that: • the minerals that precipitated were very similar in both, viz. in decreasing proportion of precipitate mass formed, struvite (MgNH4PO4) (82 to 89%), amorphous calcium phosphate (ACP) (5 to 15%), calcium carbonate (CaCO 3) (0-6%), magnesium carbonate (MgCO 3 ) (0 to 5%) and newberyite (MgHPO 4 ) (0.1 to 0.3%); • the solubility product values for these minerals were the same in both liquors and within the range of literature values, but • the specific precipitation rates were different; • the rates of struvite and ACP precipitation were 9 and 2 times faster in the UASBDL than in the SSADL respectively; • in contrast, the rate of CaCO 3 precipitation was 140 times faster in the SSADL than in the UASBDL; • the rates of MgCO 3 and MgHPO 4 precipitation were approximately the same in both liquors; and • the stripping/dissolution rates of oxygen (O 2 ), carbon dioxide (CO 2 ) and ammonia (NH 3 ) increased with increased aeration rate in the batch tests. The kinetic modelling approach allows the determination of the specific precipitation rates for a number of minerals simultaneo usly in an integrated manner from a single batch test.