Modification of nitrifying biofilm into nitritating one by combination of increased free ammonia concentrations, lowered HRT and dissolved oxygen concentrationZekker, Ivar; Rikmann, Ergo; Tenno, Toomas; Menert, Anne; Lemmiksoo, Vallo; Saluste, Anne; Tenno, Taavo; Tomingas, MartinJournal of environmental sciences2011 / p. 1113-1121 : ill https://www.sciencedirect.com/science/article/pii/S1001074210605232