Modelling multiple mineral precipitation in anaerobic digester liquor.

Mineral precipitation problems have been experienced with the conveyance and treatment of anaerobically digested primary and waste activated sludge blends. This paper describes an experimental investigation into mineral precipitation in anaerobic digester liquor (ADL) from the Cape Flats (CF) Wastewater Treatment Plant (WWTP) (Cape Town, South Africa), and application of the three-phase (aqueous/solid/gas) physical and chemical processes kinetic model developed by Musvoto et al. (Water Res. 34 (2000) 1857; Water Res. 34 (2000) 1868; Water SA 26(4) (2000) 417) to the experimental data. From the experimental investigation and theoretical modelling, it is concluded inter alia that: (i) there is a close correlation between experimental measured and theoretically predicted data, (ii) the dominating mineral that precipitates is struvite, with small amounts of amorphous calcium phosphate and negligible newberyite, calcite and magnesite, (iii) the precipitation of struvite is governed by the increase in pH when CO2 is lost from the ADL, (iv) the ADL is initially undersaturated with respect to struvite, but becomes supersaturated at pH > 7.3-7.7, (v) the rate and mass of struvite precipitation are controlled by the rate of pH increase and the initial Mg concentration and (vi) the three-phase kinetic model is able to simulate accurately the time dependent precipitation data for multiple minerals competing for the same species and allows determination of specific precipitation rates for a number of minerals simultaneously in an integrated manner from a single batch test. Some operational strategies to minimise struvite precipitation are proposed.

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.