Riverbank filtration for water quality improvement in a Managed Aquifer Recharge scheme in Berlin, Germany

Riverbank Filtration is a water withdrawal method in which water is pumped from the ground via the banks of a river (or other surface water body, in that case called Bank Filtration). The water abstracted is thus surface water that has received a preliminary treatment by passing a short distance through sediments and soil to where it is abstracted (https://www.emergency-wash.org/water/en/technologies/technology/riverbank-filtration).

Detailed explanation

Riverbank Filtration is a technology that operates by pumping water from wells drilled along the banks of a river. During the pumping process, river water infiltrates into the riverbed sediments being purified along the way (Fig. 5).

RBF is a natural pre-treatment technology (nature-based solution, NBS) that uses sand and gravel aquifers as a natural filter – instead of chemicals – to pre-treat surface water and groundwater supplies, therefore being a cost-effective approach.

Fig. 5 – River Bank Filtration scheme (https://www.youtube.com/watch?v=Oy3mmckWz1k and Hiscock & Grischek, 2002)

The type of sediments between the surface water source and the groundwater intake, as well as the distance between the water source and abstraction point, determine both water quantity and quality. A balance of the two is required: the intake needs to produce sufficient quantity and an acceptable quality needs to be achieved.

Fig. 6 shows the typical flow conditions associated with different types of bank filtration schemes.

1-The majority of river bank filtration schemes are of Type 1. Groundwater flow beneath the river (Types 3, 4 and 6) is typically neglected at most sites. The formation of unsaturated conditions beneath the river occurs if groundwater abstraction rates are not adapted to the hydraulic conductivity of the river bed or if the hydraulic conductivity of the river bed material becomes clogged due to surface water pollution inputs (Type 4). At some sites, the river bed cuts into the confining layer (Type 5). Collector wells are used with laterals at different depths, of different lengths and directions. Type 6 gives only one example with a lateral towards the river.

Concerning water quantity, riverbed clogging can be a massive ground loosening. To overcome this, the construction of artificial ditches and side channels for further infiltration zones has been created. Besides, RBF intakes can also be increased through managed aquifer recharge methods, such as infiltration basins, gully plugs, check dams, leaky dams and groundwater dams in seasonal rivers.

To improve water quality and to achieve better infiltration zones, a specific sand layer is more recently being incorporated in percolation ditches, channels, and ponds. A further stage of development has been the construction of recharge basins.

The water well intakes are located at a short distance away from a surface water source (usually less than 50 meters), which means that the percolation and retention time may vary from 5 to 100 days. The microbiological, chemical and physical water quality of surface water is much improved through RBF due to the combination of natural treatment processes, though a final treatment is usually still needed. Indeed, the increasing chemical pollution, which may result in high concentrations of nutrients, heavy metals, organic compounds, and micropollutants in the river water, lead to the introduction of supplementary pre- and post-treatment steps to build up a multiple-barrier system.

A variety of technologies may be applied to treat bank filtrate, depending on the river water quality (Fig. 7).

Fig. 7 – Process scheme development of river water treatment in Germany (Schmidt et al., 2003)

Typically, aeration or ozone may be used to oxidize iron and manganese from anaerobic aquifers, and activated carbon can be used for adsorption and protection against more persistent contaminants. Granular activated carbon filters, often combined with ozonation and filtration are also used.

Fig. 7 also presents the usual treatment scheme when artificial groundwater recharge is added in infiltration basins to increase the water volume available.

RBF water treatment provides particle removal, pathogen removal, organic and inorganic chemical removal, peak smoothing in spills, temperature equalization, reduction in DBP formation, and production of more biologically stable water.

Future outlook

The majority of Berlin drinking water supply comes from bank filtration and artificial groundwater recharge. This is a process where river water infiltrates into the riverbed sediments being purified along the way and pumped in wells located along the river.

This natural pre-treatment technology, which uses the aquifer sedimentary material as a natural filter to pre-treat surface water and groundwater supplies, is being a cost-effective approach. Though bank filtration diminishes the loads of contaminants present in source water, there is still a need for future research in some aspects, e.g.:

– The influence of scale factors and local environmental conditions on soil-water interactions and biodegradation processes.

– The production of undesirable transformation products into the aquifer and eventually in drinking water wells.

– The seasonal dynamics of temperature and redox conditions temporally establishing separate zones where certain compounds can modify their reactivity, their capacity to produce metabolites, and/or their redox behaviour.

Globally, there are also future challenges derived from climate changes, such as higher temperatures, less summer precipitation, which can also impact in higher concentrations of nutrients and persistent trace organics which can led to a shift to more anoxic (anaerobic conditions) retarding the contaminants degradation.

Management of bank filtration schemes should be incorporated into wider catchment planning in order to limit potentially polluting activities in the groundwater recharge area and also to balance river infiltration losses with the ecological needs of the river.