Non-discharging evapotranspiration bed systems : A thesis presented in fulfilment of the requirement for the degree of Master of Engineering in the Department of Natural Resources Engineering in Lincoln University, Canterbury, New Zealand

Abstract

One of the more attractive options for on-site disposal of wastewater is by evaporation. A type of evaporative disposal system was investigated in this study: the evapotranspiration bed (ET bed). Measurements were made during a period (23 April 1991 to 15 October 1991) of the year when evaporation rates are low and precipitation high.

Evapotranspiration beds can be designed for exclusively evaporative disposal with no discharge to the adjacent soil, or as combination beds utilizing seepage as well as evaporation disposal. The non­discharging type of ET bed was investigated in this study. A small scale field plot ET bed was constructed.

The actual evapotranspiration rate, ETa, was estimated, compared to the meteorologically estimated potential evapotranspiration rate, ETp, and a scaling factor λ = ETₐ /ETₚ equal to 1.21 was obtained.

A water balance model for use in different climatic conditions was developed and validated as follows:

∆W = Q + PPTN - ETₐ

ETₐ = λ * ETₚ

where: ∆W = variation of storage water in the bed; in mm,

Q = influent wastewater; in mm or litre/m²,

PPTN = total precipitation (rain, snow melt water) falling on the bed; in mm,

ETₐ = actual evapotranspiration; in mm,

ETp = meteorologically estimated evapotranspiration;in mm,

λ = ET scaling factor; dimensionless.

The performance of the system under different climatic conditions and its appropriateness were assessed. It was found that, the truly non-discharging ET bed system's feasible application may be highly questionable in the study location (Lincoln area). This is due to the fact that precipitation rate for the location generally exceeds the ETp during critical winter months.