Bank filtration

Physical filtration or straining takes places when suspended particulates are too large to pass through interstitial spaces between alluvial soil particles.

Biological filtration occurs when soil microorganisms remove and digest dissolved or suspended organic material and chemical nutrients.

[2] Research has also shown that the removal efficiency depends not only on the contaminant, but also on the "hydraulic and chemical characteristics of the bottom sediment and the aquifer, the local recharge-discharge conditions, and biochemical processes".

[1] There have been indications that some pharmaceutical compounds (medical drug traces from human use) may not always be sufficiently removed by bank filtration, and that in areas with substantial contamination of this type, additional treatment may be needed.

Treated wastewater is typically discharged into a percolation pond on the alluvial flood plain rather than flowing directly into the river.