Treatment of supernatant from dewatering of digested sludge was performed in a pilot plant scale at Himmerfjärden wastewater treatment plant in the Stockholm region. A moving bed biofilm reactor was used with Kaldnes rings as support material. Deammonification is a two-step process technology in which the first step involves oxidation of part of the ammonium to nitrite followed by oxidation of remaining ammonium with the formed nitrite into nitrogen gas (anammox). The two processes can be performed in two separate reactors or in one-stage biofilm reactor there the outer biofilm layer performs nitritation and an inner layer the anammox reaction. One-stage technology was started by seeding anammox bacteria into a step with partial nitritation and deammonification was rapidly established with an average nitrogen removal between 55 and 88% with influent ammonium concentrations between 350 and 720 g N m-3. A removal rate of about 15 g N m-3 d-1 could be reached. The process could be monitored by pH and conductivity measurements. Nitrititation was the rate limiting step.