Numerical Simulation of Stratification Dynamics and Mixing in Wastewater Stabilization Ponds

Abstract

Stabilization ponds are an economical and efficient method of waste water treatment in small communities. This form of wastewater treatment relies upon the natural ability of a body of water to achieve self-purification. Self-purification means reduction of fecal coliform bacteria, biochemical oxygen demand, and organic content, and returning the dissolved oxygen level to desirable levels. Overall efficiency of waste stabilization ponds is a function of many interacting processes. To gain a better understanding of the important factors and for insight into methods of stabilization pond management, a study of the physical, chemical and biological processes has been initiated. In an earlier report (Luck and Stefan, 1990) field observations on water temperature and dissolved oxygen stratification in the Harris, Minnesota, wastewater stabilization ponds were reported. The alternating mixing and restratification of these ponds over periods of hours or days were documented. These physical processes which are in response to time-variable weather underlie the water chemistry and phytoplankton growth. The ultimate goal of the study is to develop a water quality model for wastewater stabilization ponds, in order to predict the effluent water quality at the time of release. As a first step a water temperature model, including stratification is developed herein. This report describes that model and its ability to predict observed temperature distributions and stratification periods in the ponds. Intermittent stratification, sometimes lasting several days, has a strong effect on sediment/water interaction and vertical transport of dissolved oxygen, nutrients and phytoplankton in the pond. It is therefore considered necessary to first develop a model to simulate stratification and mixing as a basis for a dynamic water quality model.