Stratification dynamics in wastewater stabilization ponds

Abstract

Measurements show that wastewater stabilization ponds although often only 1–2 m deep stratify and destratify intermittently depending primarily on weather. Stratification can be observed in vertical profiles of water temperature, dissolved oxygen, pH and other water quality parameters. In three stabilization ponds of a small Minnesota town, stratification develops primarily by differential heating of the pondwater through its surface and, in the absence of artificial aeration or mixing devices, by insufficient wind mixing. The resulting water temperature stratification affects other parameters in a variety of ways through chemical, microbial and planktonic kinetics and reduced vertical mixing. To gain a better understanding of stabilization pond water quality dynamics, temperature profiles were monitored at 20-min intervals, and a dynamic lake water quality model was modified and applied to simulate the temperature stratification. A 12-h timestep option was incorporated into the program in order to capture the diurnal variation in stratification. Measurements and simulations were made for three wastewater stabilization ponds at Harris, Minnesota. The level of agreement between field measurements and numerical simulations demonstrated that water temperatures and stratification dynamics in a shallow and small pond can be simulated on a diurnal timescale with a standard error from 1.0 to 1.5°C between simulation and measurements. The model includes wastewater inflow in the form of a vertical jet and water transfer between ponds in the form of non-surface inflow and outflow. Three types of stratification were identified and their respective durations were determined. Stratification occurred on about 55% of all days from 1 April to 30 November. Information on pond stratification presented can be used to guide field studies and reactor modeling of ponds which can lead to further improvements in design and operation.