Adsorption of ammonium to activated sludge

The adsorption of ammonium (NH4+) to the sludge floc matrix has been investigated in two activated sludge treatment plants. When activated sludge was extracted in 1 N KCl the extractable amounts were between 0.5 and 1.3 mg NH4+-N/l higher than the dissolved ammonium concentration at dissolved ammonium concentrations between 1 and 6 mg NH4+-N/l. The difference between the extractable and the dissolved ammonium, the exchangeable ammonium, thus represented 18–30% of the dissolved ammonium concentration. A part of this, around 0.5–0.6 mg NH4+-N/l was not exchangeable under the actual operational conditions in the treatment plants due to a slow desorption kinetics, but seemed to be accessible for nitrification under prolonged aeration. The highest observed amount of adsorbed ammonium corresponded to 0.4–0.5 mg NH4+-N/gSS. Adsorption of ammonium should be investigated in more details and included in models for nitrogen mass balances for activated sludge.