What is riverbank filtration ? What is artificial groundwater recharge?

Both techniques filter surface water through the underground, i.e. the sediment, and are sometimes subsumed as “sediment passage”, before it reaches the treatment plant. These techniques take advantage of the – often excellent – filtering, adsorption and degradation properties of the underground. Both apply the hydraulic gradient in a water-permeable underground, with surface water flowing to abstraction wells following gravity and the groundwater depression cone caused by well operation.

The most important characteristics for eliminating hazardous substances are i) microbial degradation and ii) long contact times with the soil. As artificial recharge usually applies artificially designed substrate beds and regular removal of the clogging layer on the surface, contact times are often shorter, and time needed for microbial degradation may be more critical.

Prerequisites for riverbank filtration

In order to utilize riverbank filtration for drinking water treatment the sediments surrounding the surface water source need to feature aquifer characteristics. These sediments are usually fluviatile (transported by rivers) or glacial (transported by glaciers) sandy or gravel deposits. For safe microcystin removal, the distance between abstraction well and surface water body should be selected so that travel times are at least 9 days (see below).

Prerequisites for artificial groundwater recharge

With this technique surface water (from a river or lake or recycled water) with or without pre-treatment (eg. filtration) is conveyed into artificially constructed basins with a sand or gravel bed and direct contact to an underlying aquifer. As for riverbank filtration an appropriate aquifer is needed, however not in direct contact with the surface water source. Drinking water is then pumped from wells in a certain distance from the infiltration basins.

Optimal operation

The current state of knowledge allows some definition of scenarios which appear to be critical for safe elimination of microcystins. Data for other cyanotoxins are scarcely available, but conditions for microcystin elimination may be used as default values:

· Absence of the clogging layer (e.g. after artificial removal, or at river banks with strong erosion through wave action),

· Course-grained material (e.g. gravel)

· Anaerobic conditions,

· Low temperatures.

Optimal conditions for microcystin degradation are:

· Presence of a clogging layer, preferably with a previous history of contact with microcystin (to establish degrading bacteria),

· Sandy or even finer-grained material,

· Aerobic conditons,

· Moderate to warm temperatures (> 10 °C).

Under such optimal conditions, degradation rates reach at least 1.5 d^-1. If a low target value of 0.1 µg/L in drinking-water is set and a degradation rate of 1.5 d^-1 is assumed, the minimal contact times in the underground can be taken from the figure below. This disregards a potential impact of sediments accumulated and lysing on the sediment surface.

Figure 1: Minimal travel time in the underground to attain 0,1 µg/L total microcystins under optimal conditions
in relation to the concentration of dissolved microcystin in the surface water (degradation rate of 1.5 d^-1)

Using the maximal microcystin concentrations reported so far, i.e. 25 mg/L as cell-bound microcystin, the minimal retention time needed will be 8.2 days. Thus, if the system design ensures a contact time (i.e. travel time) of 9 days in the underground, under optimal conditions even at worst-case scenarios nearly total elimination of microcystins may be expected.

In settings such as slow sand filters and artificial groundwater recharge, in which cells accumulated on the substrate are regularly removed, lower travel times (even < 1 day) may be safe, provided optimal operating conditions can be maintained. At high cell density and correspondingly high concentrations of total dissolved carbon, anaerobic conditions may readily occur, and this would lead to much slower microbial degradation.

This critical travel time, together with the hydraulic gradient and the hydraulic conductivity of the sediment, may be used to calculate the distance needed between abstraction well and surface waterbody. For sediments with a higher share of fine-grained material (> 0,1 % clay and silt) a retardation factor may be included in this assessment (e.g. with a retardation factor of 2, the distance needed between well and water-body will be halved).

For further reading, see Grützmacher et al. 2002