Hydrologic modeling of aquatic plant treatment systems polishing dairy lagoon effluents

In this study, a mathematical model of the hydrologic balance of an aquatic plant treatment system (APTS) has been developed. The mass balance approach has been adopted and the major components of the water balance, such as precipitation, evapotranspiration (ET) and percolation have been incorporated into the model. For estimation of ET for duckweed and water hyacinth plants, mathematical relationships were established between ET and pan evaporation using data collected at the site. The observed ET rates of water hyacinths were up to 66% higher than the pan evaporation rates. But for duckweed, the observed ET rates were 10 to 20% lower than the pan evaporation rates. Using the available historic precipitation and pan evaporation data, several computer simulations of the model were run to estimate the HLR and HRT of the ponds under different design requirements. The results indicate that aquatic ponds with water hyacinths can operate at greater HLR's than ponds supporting duckweed. For a zero discharge system, the allowable HLR for a water hyacinth pond was found to be 5 times that of a pond containing duckweed.