Non-discharging evapotranspiration bed system for wastewater disposal at Lincoln

Abstract

A non-discharging evapotranspiration bed system installed on a dairy farm at Lincoln University was studied to evaluate actual evapotranspiration rate, ET a , to develop and validate a water balance model for the system, and to assess the feasible application of the vaporative concept for wastewater disposal in the immediate geographic area. Observations, measurements, and calculations indicate that ET a was slightly higher than the meteorologically estimated Penman potential ET (ET p ) for the study period, and a ratio λ=ET a /ET p =1.21 was obtained. A water-balance equation of the system which can be used in different climatic conditions was obtained and can be written as ΔW=Q+PPTN-Et a ; where ΔW is the variation of storage water in the bed (in mm), Q is the effluent wastewater (in mm), and PPTN is the total precipitation. It was also observed that a truly non-discharging ET bed system's feasible application in the study location may be highly questionable. This is due to the fact that the precipitation rate for the location generally exceeds the ET p during critical Winter months. However, two possibilities of improving the performance of the system were found and suggested for further studies. The suggestions were (1) to design the system with its surface adequately crowned to shed large amounts of rain water falling on it, and (2) to design a winter-sheltered-non-discharging ET bed which would consist of an ET bed with a removable transparent plastic roof to shed the total amount of rain water and act as a 'green house'