Sludge Modeling for Simulations in Biological Reactors

Nowadays, the settling of sludge in biological reactors for the water and wastewater treatment is usually analyzed by means of the classic laws of sedimentation formed by Newton and Stokes for discrete, non-flocculating spherical particles

Forces acting on the discrete settlings of particles are:

The gravitational force acting on the particles depends basically on the volume of the particles and the difference of densities between water and solid matter. The drag force is a force created due to viscous friction once motion has been initialized. The drag force depend on many variables such as shape of particle, the projected surface of the particle in direction of flow, Reynolds number and viscosity of liquid media. For the most of the applications, spherical particles are assumed.

In addition to this, also the fact that the concentration of dispersed solid matter can change the rheology of the mixture in a suspension has to be considered. The local concentration of the dispersed particles affects the viscosity of the mixture and sometimes the mixture can show a non-Newtonian behavior when the concentration reach a specific level.

Summarizing these facts, process parameters under consideration to model the settlement of sludge particles are: density of media, diameter of particles, viscosity of liquid medium and effect of local particle concentration on water-particle mixture.

As different kind of sludges show different settling characteristic, it is necessary to numerically model every sludge individually. For each reactor SVI data is used to numerically calibrate the interaction of water-sludge. The Sludge Volume Index is used to describe the settling characteristic of sludge in biological reactors. Using experimental data available our engineers are able to reproduce numerically the correct settling behavior of any sludge for any reactor. This can easily be demonstrated by carrying out a virtual SVI test, which is a numerical reproduction of the same test that usually is conducted in laboratories.