Sequential managed aquifer recharge technology (SMART) for enhancing biodegradation of trace organic chemicals in a heterogeneous aquifer

Managed aquifer recharge (MAR) can improve water availability by enhancing storage, and water quality through biodegradation and filtration processes. Impaired water sources such as WWTP effluent often contain trace organic chemicals (TOrCs) which may have adverse effects on the environment and human health. Removing TOrCs using activated carbon or ozonation is costly and energy intensive. Instead, biodegradation of TOrCs in the aquifer can be enhanced by changing the environmental conditions e.g. via sequential managed aquifer recharge technology (SMART). SMART consists of an initial infiltration step (e.g., bank filtration) followed by an aeration step and subsequent infiltration step under oxic and carbon limited conditions. This study aims to implement SMART in a heterogeneous aquifer and demonstrate the attenuation of TOrCs.  

SMART was implemented on a demonstration scale at a former waterworks site in Berlin to produce raw water which could potentially be used for later drinking water production. Impaired bank filtrate is aerated and iron and manganese are removed. Then, the water is infiltrated into a 25 m long, 1 m wide, 7 m deep infiltration trench filled with gravel. Two production wells located 25 m away from the trench establish a controlled, homogeneous flow field. Another production well 60 m away hydraulically shields the system. The pumping regime extracts more water (approx. 24 m³/h) than is infiltrated (10 m³/h) to comply with permitting requirements. Hydraulic retention time confirmed by tracer tests is approximately seven days from the trench to the first two wells. Groundwater monitoring wells provide online monitoring data at different depths (groundwater level, electrical conductivity, temperature, and dissolved oxygen (DO)). Due to operational constraints, drinking water was infiltrated for approximately one year, facilitating the establishment of plug-flow conditions and an oxic zone (>1 mg/L of DO) in the subsurface. Pre-treated bank filtrate has been infiltrated since February 2024. Additionally, hydraulic and reactive transport models of the site were created to understand and confirm subsurface processes.

Trace organic contaminants were removed in the flow field and can be categorized into different groups. Persistent compounds such as candesartan showed limited biodegradation potential (removal of less than 30 %). A group of easily degradable and volatile substances was already removed during pre-treatment under oxic conditions. The third group of redox-sensitive compounds such as diclofenac were better removed under the oxic and carbon-limited conditions of SMART with removal between 30 % and up to 80 % from the influent to the monitoring well located 20 m downstream of the trench.

This strong attenuation demonstrates the potential of enhancing biodegradation in a heterogeneous aquifer by actively managing the in-situ redox regime to achieve sustainable TOrCs biodegradation. The demo-scale system is currently in operation and open research questions are being investigated, such as how the microbiome adapts and what is needed for a transfer to new locations. Depending on the local conditions, SMART needs to be combined with further advanced drinking water treatment steps to ensure sufficient TOrCs removal. Overall, this approach is a promising solution to implement water reuse locally.