Small Wastewater Treatment Works DPW Design Guidelines

ABSTRACT: Information for the design of evaporation ponds in Wyoming has been developed. The suitability of various models for estimating evaporation and its variability was investigated while the spatial and temporal variabilities of net evaporation at seven locations were described. A routing procedure was developed to analyze the effects of uncertainty in net evaporation estimates on the probability of pond failure. Comparison of equations which estimate evaporation using climatological data showed that the equations vary greatly in their ability to define the variability of evaporation. The Kohler-Nordenson-Fox equation provided monthly and annual evaporation estimates having statistics resembling those of measured pan data closer than any of seven other equations tested. The equation requires temperature, radiation, wind, and humidity data as inputs. The Kohler-Nordenson-Fox equation using climatic data extrapolated from nearby stations provided better definition of the variability of evaporation than did equations requiring only on-site temperature data. However, results indicate that extreme care must be taken in selecting the stations from which data will be extrapolated. Monthly and annual means, standard deviations, and highest and lowest evaporation and net evaporation values have been calculated for seven Wyoming stations. The year-to-year and spatial variation of evaporation and/or net evaporation in Wyoming was shown to be great enough to cause serious problems in defining rates for evaporation pond designs. Several factors were shown to exist which might produce uncertainties in any estimate of evaporation. The routing procedure was applied to analyze the effects of these uncertainties and variations. Results indicate that the liquid depth of an evaporation pond depends greatly on evaporation rates and maintenance of minimum liquid depths without pond overflow is very difficult.

There has been an increasing interest in reuse of wastewater in agriculture over the last few decades due to increased demand for freshwater. Population growth, increased per capita use of water, the demands of industry and of the agricultural sector all put pressure on water resources. Treatment of wastewater provides an effluent of sufficient quality that it should be put to beneficial use and not wasted (Asano, 1998). The reuse of wastewater has been successful for irrigation of a wide array of crops, and increases in crop yields from 10-30% have been reported (cited in Asano, 1998). In addition, the reuse of treated wastewater for irrigation and industrial purposes can be used as strategy to release freshwater for domestic use, and to improve the quality of river waters used for abstraction of drinking water (by reducing disposal of effluent into rivers).