Water Loss by Evapotranspiration and Salination of Municipal Effluent Treated in Contructed Wetlands Under Mediterranean-Climate,Coastal Plain,Israel

Student: Michal Ashkenazi
Advisors:  Prof. Avital Gasith and Prof. Hadas Saaroni

 

In regions of water scarcity such as Israel reclaimed wastewater is an important resource for different reuses including stream rehabilitation. The latter requires higher than secondary effluent quality. In this study we examined constructed wetland (CW) technology with respect to water loss. It is an environmental friendly, low-tech approach for upgrading wastewater of diverse qualities. This technology involves flow of effluent through a porous substrate (e.g., gravel) planted with hydrophytes. As effluent flows through the system physical, chemical and biological processes remove pollutants from the water.
 
Hydrophytes are usually wasteful of water losing it in transpiration that is added to the water lost by evaporation from the system surface. Evapotranspiration (ET) rate is expected to be species specific and influenced by meteorological conditions. Water loss during late spring and summer and the change in electric conductance (EC) was tested in two sub-surface vertical flow systems (VSSF) with selected hydrophytes at the Shafdan Water Treatment site (central coastal plain). Our finding support those reported by others that ET is significantly influenced by weather conditions. The factors influencing ET significantly are solar irradiance, temperature, relative humidity and wind velocity. We have shown for the first time that shading reduced ET up to 50% in microcosms.
 
Unexpectedly reducing wind velocity by plastic fencing of the microcosms had not reduced ET. This may be explained by the temperature increase in the enclosed area. Little has been reported on water loss in subsurface CW systems.  ET measured in the microcosms system exhibited diurnal dynamics. In the present study as well as others a negative water loss (i.e water gain) was recorded. We attribute this result to contribution of water by dew.   Highest ET was found for Scirpus holoschoenus, Canna zankri, Cyperus papyrus hasphen and Phragmites australis. Lowest ET was found in Tamarix aphylla (small plants - cuttings), in Halimione portulacoides and Juncus fontanesii. In a given hydrophyte lowering of the above-ground biomass (by cutting) reduced ET. Our findings indicate that ET in vertical sub-surface flow CWs is relatively small and should not limit the use of this technology in Mediterranean-climate regions. Moreover, we showed that shading can reduce water loss significantly. 
 
Salination of the treated effluent as a result of ET is another factor of concern in CW technology, especially when reclaimed wastewater is aimed for irrigation. The limited change in salinity should not affect the decision of using the effluent for irrigation, but will increase the rate of salt accumulation in the soil. In the near future when the fraction of desalinated water in the water budget will become more, the salinity of the effluent will decline and so will the significance of the increase in salinity by CWs.
 
In summary, our findings suggest that ET and salination in CWs in the central coast plain of Israel should not limit their use for effluent upgrade. Reduction in water loss and salination can be obtained by shading as well as by using plant-free CWs. The evaluation of ET and salination in CWs should be extended to more arid regions of the country. 
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