MATHEMATICAL MODELLING OF SECONDARY SETTLING TANK (ASP)

Abstract

The design exercise of secondary settling tanks is wrought with a complicated scenario. Either the design uses empirical parameters or the data, i.e solid flux, can be made available only after the settler starts functioning. There is therefore a need of an analytical approach to sizing the unit. To effectively settle, the microbes in aeration tanks must flocculate, than aggregate into units large enough and dense enough to settle out of solution. There are different parameters which affect sludge settling, depend on characteristics of flocs like permeability, density, floc size etc. These characteristics are mainly depends on local flocculation conditions including shear forces, dissolved oxygen concentration, loading rate, shearing time, sludge concentration and ionic strength, which vary throughout the secondary settling tank. Formation of flocs in aeration tank is the basic task in the process. Flocs can be studied on the basis of their physical, chemical and morphological properties. Floc size distribution is the main morphological property which has been discussed in detail in this dissertation. Shearing is done by aeration, so the flocs will form in the aeration tank. Taking shear and time as parameters for growth of flocs in aeration tank, a relation of these parameters has been established with mean diameter of flocs. We have developed a relation between diameter of activated sludge floe and its settling velocity. Using Vesilind's expression for settling velocity, which is often practiced, the reduction in settling velocity varies only from 2% for 3000 rng/L MLSS in the upper zone to about 5% for 10000 mg/L MLSS at the bottom zone of the settler, which is negligible for all practical purposes ..A case study has been done for verification of the relationships obtained and calculations of settling efficiency of secondary settling tank for treating it as a type - I settler and its surface area. Effects of flocculation due to shear, which is negligible due to very small settling velocity, and sweep coagulation during settling operation have been neglected.