Nitrification reactions in treatment of supernatant from dewatering of digested sludge

Separate nitrification of the discarded water from dewatering of digested sludge has been studied at the Bromma wastewater treatment plant in Stockholm. Experimental studies were carried out in a sequencing batch reactor pilot plant. It was shown that a high nitrification rare could be obtained in the reactor, approximately 30 to 40 g NH4-N/kg mixed liquor volatile suspended solids h. During the nitrification process, nitrite was accumulated until ammonium nitrogen was depleted in the reactor. When all ammonium had been oxidized, all nitrite could thereafter be completely oxidized to nitrate. By use of multivariate methodology, it was shown that nitrite accumulation was primarily caused by the initial concentration of ammonium and secondly by the oxygen concentration. The nitrification rate strongly depended on the oxygen concentration. The strong dependence of nitrification rates on oxygen concentration makes the process easy to control by using measurements of oxygen concentration, air flow, and pH.