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 biogeochemical 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 min for all experiments while drying periods (DPs) were 2.5 and 4 times the duration of the flooding periods. Our results show that the longer DPs 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. DO concentrations in the deeper parts of the column stabilized at ∼3–4 mg L−1 for the longer DPs compared to ∼1–2 mg L−1 for the shorter DPs. This advantage was also evident in outflow composition that showed significantly lower concentrations of NH4+-N, dissolved organic carbon (DOC) and total Kjeldahl nitrogen (TKN) for the longer DPs (∼0.03, ∼1.65 and ∼0.62 mg L−1 respectively) compared to the shorter DPs (∼0.5, ∼4.4 and ∼3.8 mg L−1, respectively). Comparing experimental ORP values in response to different DPs to field measurements obtained in one of the SAT ponds of the SHAFDAN, Israel, we found that despite the large-scale differences between the experimental 1-D system and the field 3-D 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 at 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

“…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 NH + 4 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.…”

“…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.…”

Revisiting Soil Aquifer Treatment: Improving Biodegradation and Filtration Efficiency Using a Highly Porous Material

“…In the Shafdan SAT, the secondary WW effluent infiltrates into the coastal aquifer through a 25-35-m sandy unsaturated system [21]. However, increasing WW volumes and escalating real estate prices around urban areas highlight the need to re-engineer the SAT process and maximize the recharge capacity without sacrificing the effluent quality [22]. Therefore, efforts are being made to maximize the percolation flux by: (1) Studying the spatiotemporal infiltration dynamics across the ponds using hydro-geophysical tools [23]; (2) releasing air which is entrapped below the wetting front [24]; (3) improving tillage technologies to effectively break the biocrust and maintain high infiltration rates in the ponds [25]; and (4) improving the SAT management by optimizing the length of the wetting and drying cycles.…”

Enhanced Removal of Contaminants of Emerging Concern through Hydraulic Adjustments in Soil Aquifer Treatment

“…The optimal ratio of HLCs during SAT is very site-specific, though it was shown that the wet-dry ratio should be smaller than one to keep the system efficient in pollutant removal [29]. A higher HLC frequency with shorter flooding cycles is advantageous for organic compound removal during SAT [30,31]. Compared to continuous infiltration, cyclic infiltration shows better degradation rates for compounds such as diclofenac [32].…”

“…Compared to continuous infiltration, cyclic infiltration shows better degradation rates for compounds such as diclofenac [32]. Nevertheless, SAT design is still advised with flooding cycles of 1-3 days [33], even though a higher frequency of HLCs with shorter cycle phases has shown better performance for water quality improvement due to increased topsoil aerating [30,31]. The drying phase leads to an aeration of the vadose zone.…”