Hybrid Managed Aquifer Recharge – Effects of pretreatment on biodegradation of trace organic contaminants

Pressure on freshwater resources is increasing due to growing water demand for agricultural, industrial and domestic use. The effects of climate change, such as longer periods of drought, further increase water demand. In addition to pressure on water quantity, water quality is affected by pollution, including groundwater resources. Therefore, sustainable and resilient approaches for groundwater use are needed.

Managed aquifer recharge (MAR) can secure water supplies by expanding the amount of water storage available. At the same time, MAR can improve water quality through the filtering effect of soil and groundwater. However, trace organic contaminants (TOrCs) are not sufficiently attenuated by biodegradation processes during subsurface travel. Thus, an adequate pretreatment may be necessary to ensure the best possible use of the treatment effects of MAR.

As a first step, this study investigates the effects of different pretreatments on the removal of TOrCs during MAR. Columns mimicking the groundwater passage receiving differently treated wastewater qualities containing TOrCs allowed for a detailed monitoring of water quality parameters such as dissolved organic carbon (DOC) and dissolved oxygen (DO) along the depth of the columns. The microbial community that adapts to the different substrate conditions is expected to influence the potential for biodegradation of TOrCs.

The experimental setup consists of three saturated columns (1.6 m long, 0.15 m diameter) filled with technical sand. The sand has been exposed to ozonated wastewater treatment plant effluent for 2 years prior to the experiment and therefore has an existing biofilm. The columns receive three different wastewater qualities: (1) secondary effluent + cloth media filter, (2) secondary effluent + coagulation + ultrafiltration, (3) secondary effluent + cloth media filtration + ozonation + biologically activated carbon (BAC) filtration. Feed waters are continuously infiltrated at a flow rate of 9 ml/min. Water samples were taken along the columns at different depths (0.1 m, 0.3 m, 0.6 m below the sand surface). Samples were analyzed for water quality parameters such as DOC, UV absorption, 3D-excitation-emission spectra (3D-EEM), and 32 indicator TOrCs. In-situ DO measurements (DP-PSt3, PreSens GmbH, Germany) were conducted at depths of 0.1 m, 0.2 m, 0.3 m, 0.4 m, 0.6 m and 0.9 m below the sand surface.

As expected, water qualities differ as a function of pre-treatment, e.g. DOC concentrations are highest for the column receiving cloth media filtered water and lowest for the column receiving ozonated water. First results of TOrCs measurement show lowest concentrations in the influent for the column receiving ozonated water, which is expected given the high reactivity of ozone with TOrCs. TOrCs in the other two columns show differences, for example, benzotriazole and venlafaxine are more efficiently removed in the column fed with cloth media filtered water (64 % and 61 %) compared to the UF treated water (37 % and 15 %). A better understanding of biodegradation of TOrCs can help to implement customized pretreatments of MAR at larger scale.