Limited water resources are one of the main constraints to socio-economic development and even a source of insecurity at national and regional levels. The increasing demand for water in the agricultural sector stimulated the use of reclaimed wastewater for irrigation. Comprehending health and environment risks resulting from contaminants that may often be associated with reclaimed wastewater will lead to the sustainable use of this resource, as well as the conservation of other water resources.
In the present study the fate and transport of one important wastewater constituent, namely E. Coli bacteria, in three Egyptian soils of typical agricultural use was investigated. A series of laboratory experiments and model simulations using a material balance equation were conducted to determine the impacts of varying soil type, soil organic matter content, and HLR on E. Coli transport. The experiments included a series of laboratory soil columns, and CXTFIT package was used to solve the mass transport equation that included advection, dispersion, absorption, bacterial growth, and straining/filtration.
Soil type, texture and organic matter content are significant factors in determining processes affecting E. Coli transport. Absorption and straining/filtration effects increased in soils of finer particles, greater clay and mineral composition, and higher organic matter content. Bacteria growth rate also increased in soils of greater clay and mineral composition, and higher organic matter content. The findings suggested that variations in the organic content of the same soil result in some measurable impact on the magnitude of the dispersion coefficient.
Irrigation hydraulic loading rate (HLR) exerts considerable influence on the impact of transport processes on E. Coli breakthrough in soil columns. At low HLR, absorption, straining/filtration and bacteria growth rate are significant processes in addition to advection and dispersion. However, at high HLRs approaching flood irrigation, E. Coli is essentially unaffected by any reaction processes, with breakthrough a function of advection and dispersion only. The findings suggest that a single absorption partitioning coefficient could be estimated to model E. Coli transport through a soil for a defined range of HLRs. Straining/filtration coefficient decreased in a consistent pattern for increasing HLR. Another important influence of HLR indicated that threshold HLRs exist below which proportional relationships between the dispersion coefficient and pore velocity, and dispersion coefficient and soil grain size are not exhibited.
Independent measurement of absorption partitioning coefficients using batch isotherms did not provide a suitable description of absorption of E. Coli in soil columns as indicated by efforts to simulate breakthrough. It was not concluded whether or not growth coefficients estimated from independent soil column survival experiments are suitable for dynamic modeling in soil columns.
To date, wastewater use biological quality guidelines have been established based on the type of crop and intended irrigation method. They do not incorporate the significant impacts of either soil properties or seasonal variations on fate and transport of the contaminant. To ensure sustainable development when reusing reclaimed wastewater for irrigation, a reevaluation of guidelines taking into account local/site-specific conditions such as soil physical and chemical properties and meteorological conditions is required.
The research presented here constitutes a step in a larger program with the eventual goal of arriving at a set of site specific criteria for safe and sustainable use of treated wastewater in irrigation in Egypt. In continuation of this work, the impacts of varying HLR, soil organic matter content and meteorology (e.g. temperature and sunlight exposure) on E. Coli fate and transport in soil should be further assessed.
Master of Science in Engineering
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(2006).Transport of E. Coli in Egyptian agricultural soils as a result of reclaimed wastewater use for irrigation [Thesis, the American University in Cairo]. AUC Knowledge Fountain.
Badawy, Aimen. Transport of E. Coli in Egyptian agricultural soils as a result of reclaimed wastewater use for irrigation. 2006. American University in Cairo, Thesis. AUC Knowledge Fountain.