Continuous monitoring of a soil aquifer treatment system's physico-chemical conditions to optimize operational performance

Abstract

Soil aquifer treatment (SAT) is a tertiary process for wastewater treatment, where the wastewater infiltrates through a thick vadose zone for purification and storage in the underneath aquifer. SAT infiltration basins are typically flooded intermittently, while maintaining a fixed ratio between the wetting and the drying stages. However, infiltration basins exhibit different physical and chemical properties, limiting the generalization of SAT operation to attain optimal efficiency. Since frequent sampling of the soil pore water to verify the SAT's biodegradation efficiency can be arduous, continuous monitoring of the SAT vadose zone's physico-chemical conditions is required. In this study, redox potential (Eh) was continuously monitored, together with other variables, such as volumetric water content (θ), soil temperature, and gaseous oxygen (O2), at multiple depths of a SAT vadose zone throughout the year and while the system was constrained to different operational modes. Hydrological models were calibrated and validated to water content observations, and they illustrated the seasonal changes in water infiltration. Furthermore, it was shown that, under long wetting stages during winter, there was a reduction in the SAT's drainage capabilities. The Eh observations, under long wetting stages, demonstrated larger variability and very negative values as ambient temperature increased. Assembling the daily Eh observations illustrated that a wetting stage should cease after about 30 h, once suboxic conditions are established. A drying stage's optimal duration should be 36 h, according to the Eh and O2 observations during summer and winter. Ultimately, the study shows that the length of wetting and drying stages should be defined separately, rather than by adhering to the wetting / drying ratio.