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.
Highlights
Water shortages in arid and semi-arid regions lead to great difficulties sustaining local agriculture which have many economic, social and environmental implications (García-Tejero et al, 2014)
Water front progression through the profile can be retraced by timing the first water content (WC) increase following the first flooding periods (FPs) and as reflected by Fig. 2; it indicates that flow rate decreased during the first few minutes of the experiment but stayed relatively constant as the experiment progressed
WC patterns were significantly different between the various depths – while at a depth of 25 cm WC values ranged between 20.5 % and 31.2 %, at a depth of 275 cm below the surface, the maximal WC was 18.4 % and it dropped below 12 % following each drying periods (DPs)
Summary
Water shortages in arid and semi-arid regions lead to great difficulties sustaining local agriculture which have many economic, social and environmental implications (García-Tejero et al, 2014). Soil aquifer treatment (SAT) may be the supplementary treatment component for conventional (secondary) WW treatment, needed to meet regulations and sustainability. These systems involve clusters of infiltration ponds through which TWW is infiltrated through the vadose zone, into the aquifer in cycles of flooding and drying. In the ponds and subsequently in the unsaturated zone, residual dissolved organic carbon (DOC) and other nutrients (like organic and inorganic nitrogen species) are involved in biogeochemical processes (such as adsorption to the soil minerals, consumption by bacteria, etc.) which result in a further decrease in the TWW’s organic load and overall improved chemical composition (Bouwer, 1991; Amy and Drewes, 2007)
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