Biological rapid sand filters are often used to remove ammonium from groundwater for
drinking water supply. They often operate under dynamic substrate and hydraulic loading
conditions, which can lead to increased levels of ammonium and nitrite in the effluent.
To determine the maximum nitrification rates and safe operating windows of rapid sand
filters, a pilot scale rapid sand filter was used to test short-term increased ammonium
loads, set by varying either influent ammonium concentrations or hydraulic loading
rates. Ammonium and iron (flock) removal were consistent between the pilot and the
full-scale filter. Nitrification rates and ammonia-oxidizing bacteria and archaea
were quantified throughout the depth of the filter. The ammonium removal capacity
of the filter was determined to be 3.4 g NH4-N m(-3) h(-1), which was 5 times greater
than the average ammonium loading rate under reference operating conditions. The ammonium
removal rate of the filter was determined by the ammonium loading rate, but was independent
of both the flow and influent ammonium concentration individually. Ammonia-oxidizing
bacteria and archaea were almost equally abundant in the filter. Both ammonium removal
and ammonia-oxidizing bacteria density were strongly stratified, with the highest
removal and ammonia-oxidizing bacteria densities at the top of the filter. Cell specific
ammonium oxidation rates were on average 0.6 × 10(2) ± 0.2 × 10(2) fg NH4-N h(-1) cell(-1).
Our findings indicate that these rapid sand filters can safely remove both nitrite
and ammonium over a larger range of loading rates than previously assumed.