Processing comprises subjecting the material to nitrification in an aerated reactor contg. active sludge rich in nitrifying bacteria used in the nitrification step. Acid neutralising chemicals are added to the reactor as necessary and the material is subjected to denitrification in a high rate denitrification reactor contg. a very compact biomass capable of converting nitrate to N2 gas. An organic substrate obtd. from the nitrification step is added.    Loading of the nitrification reactor is controlled to obtain an optimum nitrification and denitrification on the basis of: the incoming N2 load; the information from a specified respiration meter; the O2 concn. in the nitrification reactor; the pH value in the nitrification reactor to be 6-8.5; the amt. of air required; the residence time; the temp. in the nitrification and denitrification reactors to be kept below 40 deg.C; the concn. of oxidising N2 in the influent for the denitrification reactor to be 0-4g N/l; the concn. of oxidising N2 in the nitrification reactor in the sludge/liq. mixt. to be 0-4g N/l; the concn. of the C source in the effluent from the denitrification reactor; the gas prodn. in the denitrification reactor; the effluent from the nitrification reactor partially being passed to the denitrification reactor, adding a source of C to the effluent to be passed to the denitrification reactor and passing another portion of the effluent stream to a sepn. step to separate a sludge. The effluent from the sepn. step is passed to a denitrification reactor under the addn. of a C source as necessary. The denitrification reactor is provided with an effluent discharge and a N2 discharge.    USE/ADVANTAGE -   By placing the sepn. step upstream of the effluent denitrification reactor, the organic matter will be removed before passing through the denitrification reactor, so that no ammonium or soluble organic substances will be formed there. By using two types of denitrification reactors, it is possible to add the feed at the influent of the recirculation denitrification reactor and so it is possible to use the organic material in the feed as a C source for the denitrification process. The process and appts. overcome problems inherent in prior art process