The Effect of the Ammonium Ion on Activated-Sludge Settling Properties

Abstract

High levels of sodium in wastewater have been found to be detrimental to activated-sludge settling and dewatering. Another potentially troublesome monovalent cation in activated-sludge systems is the ammonium ion. This study was conducted to determine if the ammonium ion could alter activated-sludge settling and dewatering properties. Side-by-side laboratory activated-sludge reactors were fed equal concentrations of an organic substrate (bactopeptone), and the same divalent cations were provided in the feed. However, the monovalent cations were varied either by inhibiting nitrification so that the ammonium concentration would increase or by adding sodium. These studies showed that an increase in either sodium or ammonium would cause activated-sludge settling properties to deteriorate. When the monovalent-to-divalent cation ratio on a charge-equivalent basis was increased from 2.4 to 4.7 either by addition of sodium or ammonium, the interfacial settling velocity decreased, although the greatest drop was in the reactor containing ammonium. When addition of the nitrification inhibitor was stopped, rapid recovery of nitrification occurred but the settling properties improved more slowly. It seemed that recovery was due to replacement of the activated sludge with new flocs rather than alteration of the existing biomass.