On the role of operational dynamics in biogeochemical efficiency of a soil aquifer treatment system

Abstract

Abstract. Sustainable irrigation with treated wastewater (TWW) is a promising solution for water scarcity in arid and semi-arid regions. Soil aquifer treatment (SAT) provides a solution for both the need for tertiary treatment and seasonal storage of wastewater. Stresses over land use and the need to control the obtained water quality makes the optimization of SAT of great importance. This study looks into the influence of SAT systems' operational dynamics (i.e. flooding and drying periods) as well as some aspects of the inflow biochemical composition on their bio-geo-chemical state and the ultimate outflow quality. A series of four long-column experiments was conducted, aiming to examine the effect of different flooding/drying period ratios on dissolved oxygen (DO) concentrations, oxidation-reduction potential (ORP) and outflow composition. Flooding periods were kept constant at 60 minutes for all experiments while drying periods (DP) were 2.5 and 4 times the duration of the flooding periods. Our results show that the longer DP had a significant advantage over the shorter periods in terms of DO concentrations and ORP in the upper parts of the column as well as in the deeper parts, which indicates that larger volumes of the profile were able to maintain aerobic conditions. This advantage was evident also in outflow composition analyses that showed significantly lower concentrations of DOC, TKN and ammonium in the outflow for the longer DP. Comparing experimental ORP values in response to different DP to field measurements obtained in one of the SAT ponds of the SHAFDAN, Israel, we found that despite the major scale differences between the experimental 1D system and the field 3D conditions, ORP trends in response to changes in DP, qualitatively match. We conclude that longer DP not only ensure oxidizing conditions close to the surface, but also enlarge the active (oxidizing) region of the SAT. While those results still need to be verified in full scale, they suggest that SAT can be treated as a pseudo-reactor that to a great extent could be manipulated hydraulically to achieve the desired water quality while increasing the recharge volumes.

Optimization of soil aquifer treatment (SAT) operation using a reactive transport model

“…This observation indicates that as suggested by Ben Moshe et al. (2020), longer DPs increased the active volume of the profile compared with the shorter DPs. For DPs of >200 min, no further advantage was observed (not shown), suggesting that for the current setup, DPs longer than 200 min did not result in further increase in oxidation reaction rates.…”

“…For example, in our system, DPs of >240 min did not pose a significant advantage in terms of nitrification rates or DOC degradation, mainly since in the longer DPs, most of the DOC and were consumed in the upper few centimeters of the profile. However, one of the major benefits of implementing long DPs has been shown to be related to improved aeration of the deeper parts of the vadose zone and extension of the aerobically active part of the soil profile (Ben Moshe et al., 2020). Hence, operating the same SAT system with higher organic load inflow solution at different DPs should theoretically result in a notable difference in degradation efficiency between shorter and longer DPs.…”

“…The experimental setup, sensing devices and chemical analysis of the laboratory investigation are described in Ben Moshe et al. (2020). In short, a 6‐m‐long column equipped with WC, DO, and oxidation‐reduction potential (ORP) sensors was used.…”

“…Measured data recorded during the column experiments described above were used. Calibration was based on longer DP experiment with synthetic WW (FP of 60 min and DP of 240 min; Ben Moshe et al., 2020). Validation of the obtained calibrated model was based on the shorter DP experiment with synthetic WW (FPs of 60 min and DPs of 150 min).…”

“…Since SAT systems are usually operated in cycles of flooding and drying, dissolved oxygen (DO) concentrations and redox conditions depend on the system's hydraulic operation. In a medium‐scale laboratory setup implementing flooding and drying cycles, the advantage of long drying periods (DPs) for deep aeration was demonstrated (Ben Moshe et al., 2020). The objective of this study is to further investigate this idea by the development of a model that will enable to simulate and assess different scenarios.…”