RECENT PROGRESS IN THE NUMERICAL MODELING OF WASTE-WATER SEDIMENTATION TANKS

The recent progress in numerical modelling techniques applied to sedim entation tanks in wastewater treatment is reviewed to determine their usefulness. The models reviewed range from empirical models to sophist icated computational fluid dynamics (CFD) models and the conclusions a re as follows. Empirical models are still widely used today to predict mainly the characteristics of the effluent and return sludge from sed imentation tanks but cannot model the flow pattern or solids distribut ion within the tank. The solids mass flux model is used to perform a m ass balance on secondary sedimentation tanks for new designs or for th e audit of a poor process and to determine the return sludge flow. It can sometimes be used to monitor the height of the sludge blanket. The lumped parameter model is usually used to determine the characteristi cs of the effluent from primary sedimentation. CFD models are used to predict the flow pattern and suspended solids distribution within the tank and are normally applied in research to find the relationship bet ween the tank hydraulics and process performance. Modelling using CFD in the water industry has not been widespread because of the associate d costs and the unfamiliarity with the mathematical models. The paper suggests that the important CFD modelling criteria for the settling of suspended solids in sedimentation tanks are the velocity distribution , settling velocity distribution of suspended solids, turbulent mass d iffusion of suspended solids, resuspension of settled solids from tank base, temperature effects, flow variation, effect of flow on hoc grow th or breakup, wind effects on the water surface and movement of scrap ers. Gaps in the literature on CFD modelling can be identified for the particle density, particle size, particle flocculation, the turbulent mass diffusion of solids, the effect of the flow on particle growth o r breakup, denitrification, wind on the water surface, baffle designs, inlet pipe geometries, the side wall depth, the slope of the tank, th e hopper design, peripheral weirs and in-board launders. Where possibl e experimental data should be entered in these numerical models to giv e realistic parameters and be used to validate these models.