Depth investigation of rapid sand filters for drinking water production reveals strong stratification in nitrification biokinetic behavior

- Consistent stratification of NH4+ removal capacity and AOB density was observed in a filter.
- Maximum cell specific NH4+ removal rates were stratified in a filter.
- Top and middle layers had an up-shift capacity not observed in the bottom layer.
- Stratification of both AOB density and biokinetic behavior limit filter capacity.
- Predicted filter capacity is 4 times higher than the operating loading rate.

The biokinetic behavior of NH4+ removal was investigated at different depths of a rapid sand filter treating groundwater for drinking water preparation. Filter materials from the top, middle and bottom layers of a full-scale filter were exposed to various controlled NH4+ loadings in a continuous-flow lab-scale assay. NH4+ removal capacity, estimated from short term loading up-shifts, was at least 10 times higher in the top than in the middle and bottom filter layers, consistent with the stratification of Ammonium Oxidizing Bacteria (AOB). AOB density increased consistently with the NH4+ removal rate, indicating their primarily role in nitrification under the imposed experimental conditions. The maximum AOB cell specific NH4+ removal rate observed at the bottom was at least 3 times lower compared to the top and middle layers. Additionally, a significant up-shift capacity (4.6 and 3.5 times) was displayed from the top and middle layers, but not from the bottom layer at increased loading conditions. Hence, AOB with different physiological responses were active at the different depths. The biokinetic analysis predicted that despite the low NH4+ removal capacity at the bottom layer, the entire filter is able to cope with a 4-fold instantaneous loading increase without compromising the effluent NH4+. Ultimately, this filter up-shift capacity was limited by the density of AOB and their biokinetic behavior, both of which were strongly stratified.

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