Case study II: application of the divalent cation bridging theory to improve biofloc properties and industrial activated sludge system performance-using alternatives to sodium-based chemicals.

The objective of this study was to investigate the application of the divalent cation bridging theory (DCBT) as a tool in the chemical selection process at an activated sludge plant to improve settling, dewatering, and effluent quality. According to the DCBT, to achieve improvements, the goal of chemical selection should be to reduce the ratio of monovalent-to-divalent (M/D) cations. A study was conducted to determine the effect of using magnesium hydroxide [Mg(OH)2] as an alternative to sodium hydroxide (NaOH) at a full-scale industrial wastewater treatment plant. Floc properties and treatment plant performance were measured for approximately one year during two periods of NaOH addition and Mg(OH)2 addition. A cost analysis of plant operation during NaOH and Mg(OH)2 use was also performed. During NaOH addition, the M/D ratio was 48, while, during Mg(OH)2 addition, this ratio was reduced to an average of approximately 0.1. During the Mg(OH)2 addition period, the sludge volume index, effluent total suspended solids, and effluent chemical oxygen demand were reduced by approximately 63, 31, and 50%, respectively, compared to the NaOH addition period. The alum and polymer dose used for clarification was reduced by approximately 50 and 60%, respectively, during Mg(OH)2 addition. The dewatering properties of the activated sludge improved dewatering as measured by decreased capillary suction time and specific resistance to filtration (SRF), along with an increase in cake solids from the SRF test. This corresponded to a reduction in the volume of solids thickened by centrifuges at the treatment plant, which reduced the disposal costs of solids. Considering the costs for chemicals and solids disposal, the annual cost of using Mg(OH)2 was approximately 30,000 dollars to 115,000 dollars less than using NaOH, depending on the pricing of NaOH. The results of this study confirm that the DCBT is a useful tool for assessing chemical-addition strategies and their potential effect on activated sludge performance.     

Release dynamic process identification for a cement based material in various leaching conditions. Part II. Modelling the release dynamics for different leaching conditions

This paper deals with process identification and model development for the case of a porous reference material leaching under certain hydrodynamic conditions. Four different dynamic leaching tests have been applied in order to take into account different types of solid/liquid contact conditions corresponding to various real leaching scenarios: monolithic and granular material with sequential eluate renewal, and granular material and continuously renewed eluate with different hydrodynamic conditions (dispersion, residence time). A coupled chemical-mass transfer model has been developed to describe the leaching behaviour under all experimental conditions. Diffusion has been considered as the mass transport mechanism inside the saturated porous material and dispersive convection as that in the leachate. Two specific phenomena have been identified and considered in the model: (i) the early surface dissolution of the material which results in high Ca concentration and (ii) the late weak dissolution of Na and K giving rise to a long-term residual release. The intrinsic material parameters such as the initial concentrations in the pore water and solid phases were determined by applying equilibrium leaching tests and geochemical modelling. Diffusion coefficients for different elements and the late solubility of alkalines have been found to reach the same values in the four tests. The estimated values of the surface dissolution kinetic constant have shown a dependence on leachate hydrodynamics when the thickness of the degraded layer is nearly the same in the four tests (intrinsic parameter of the material). The competition between the four main dynamic processes, i.e. diffusion, convection, late dissolution, and surface dissolution, has been emphasized and compared in the four leaching tests: the hydrodynamic dispersion and the residence time had no effect on the leaching behaviour of alkalines, which is controlled by diffusion, whereas the behaviour of calcium (a major element of the material) was strongly influenced. This has significant effects on eluate pH values and on the concentration of Pb (the monitored pollutant). The model was then applied to simulate a landfill scenario in the case of a stabilized/solidified incinerator residue containing heavy metals and chloride. A high rain infiltration level and the use of small blocs are favourable conditions for enhanced pollutant release.     

A MATHEMATICAL MODEL FOR TRANSIENT FREE SURFACE FLOW IN NONHOMOGENEOUS OR ANISOTROPIC POROUS MEDIA

ABSTRACT A mathematical model was developed to solve a steady free surface flow problem and a rapid drawdown problem in a two-dimensional porous medium. The same problem was also solved by an analogue device and excellent agreement was found to exist between the two solutions. This paper contains the formulation of the numerical problem from first principles and a discussion of measures that had to be taken in order to assure numerical stability and proper convergence of the solution. Although the scope of this study was limited to a two-dimensional flow case, the elements of simulation discussed are general in nature and applicable to three-dimensional problems. It was demonstrated that numerical solution can be obtained for the position of the free surface at given time intervals, for the piezometric head distribution within the flow field and for flow quantities across given boundaries. In addition, the mathematical model will permit consideration of nonhomogeneous or anisotropic characteristics of the porous medium, without difficulty. It is concluded that mathematical models, incorporating some or all of the techniques discussed in this paper, in conjunction with some analogue control device, can be very efficient and reliable tools for solving complex porous flow problems, including those which, so far, have eluded comprehensive analysis, due to physical and/or cost limitation.