doc ( dissolved organic carbon ) in the organic fraction of the treated excess sludge which is recirculated to one of the denitrification stages serves as control parameter for the running parameters in the treatment of the excess sludge . at the optimum ( setpoint ) doc value , the organic fraction of the excess sludge which is recirculated to the denitrification stage contains the optimum amounts of carbon compounds , n and p for the denitrification , these optimum amounts being dependent on the nitrogen load . a defined optimum doc value can be assigned to each nitrogen load . the running parameters which can be varied during treatment of the excess sludge when the actual value of doc differs from the setpoint value are the ph of the excess sludge , the switching on or switching off of mechanical disintegration and the switching on or switching off of ozone addition and the amount of ozone added . the ph is preferably set in a range between 7 and 13 . the amount of ozone added is preferably no greater than 100 g / kg of dry matter . to each doc value , at least one optimum combination of the three abovementioned running parameters can be assigned . when there is a plurality of optimum combinations , preferably that which means the lowest expenditure or resources is selected . this generally means that the running parameter combinations to be preferred are those in which as far as possible no mechanical disintegration takes place and no or only small amounts of ozone are added . the solution of the problem of the invention is furthermore an apparatus for continuously controlling denitrification in wastewater purification in a sewage treatment plant having at least one denitrification stage under varying nitrogen loads comprising at least one denitrification stage , means for providing excess sludge , a treatment stage for treating the excess sludge according to running parameters determined in advance , means for separating the treated excess sludge into an organic fraction and an inorganic fraction , means for recirculating the organic fraction of the treated excess sludge to one of the denitrification stages , means for measuring dissolved organic carbon ( doc ) in the organic fraction of the treated excess sludge , means for comparing measured actual values for doc with predetermined setpoint values for doc , means for determining altered running parameters for treating the excess sludge if a difference is found between actual values and setpoint values for doc . the means for providing excess sludge from the denitrification stage or from another part of the sewage treatment plant customarily comprises a secondary clarification tank in which , after the sedimentation method , the sludge sinks under the effect of gravity . the excess sludge is taken off at the bottom of the secondary clarification tank . the treatment stage for the excess sludge can comprise a vessel having a feed device for ph - influencing means , a mechanical disintegrator and / or an ozonizer having injectors for introducing the ozone into the excess sludge to be treated . the ozone is customarily added at superatmospheric pressure . mechanical disintegrators which can serve are , for example , a shear - gap homogenizer , a high - pressure homogenizer , an ultrasonic homogenizer or a stirred ball mill , as disclosed by j . kopp et al ., anearober abbau mechanisch aufgeschlossener klärschlämme [ anaerobic degradation of mechanically disintegrated sewage sludges ], 12th . zaf seminar , brunswick 18 - 19 . 9 . 1997 , dfg - forschergruppe zu biologische prozesse mit dispersen feststoffen [ dfg researcher group on biological processes with disperse solids ], chapter 3 : sewage sludge digestion . the means for separating the treated excess sludge into an organic fraction and an inorganic fraction can be a secondary clarification tank or a filter unit for filtering off the inorganic fraction . the means for recirculating the organic fraction of the treated excess sludge into one of the denitrification stages are customarily the corresponding lines which if appropriate are equipped with pumps . the means for measuring dissolved organic carbon ( doc ) in the organic fraction of the treated excess sludge can be a continuous - flow process analytical instrument for online measurement . however , individual samples can also be taken and analyzed in the laboratory by conventional methods . the means for comparison of measured actual values for doc with predetermined setpoint values for doc is usually an electronic data processing unit . the means for determining altered running parameters can be an electronic data processing unit combined with a database . the advantage of the method of the invention and of the apparatus of the invention is that even under varying nitrogen loads , the denitrification process runs continuously at the optimum , or close to the optimum , so that a denitrification as complete as possible takes place with simultaneous conservation of resources . if the denitrification process runs at the optimum , at the same time the excess sludge is minimized . fig1 diagrammatic structure of the wastewater treatment plant in wuppertal fig2 effect of running parameters on doc and excess sludge fig3 a , 3 b effect of running parameters on doc and excess sludge in comparison fig4 dependence of doc in the clear phase on ph under different nitrogen loads fig5 dependence of doc in the clear phase on the mechanical dispersion under different nitrogen loads fig6 dependence of doc in the clear phase on the amount of ozone under different nitrogen loads fig7 dependence of doc in the clear phase on ph and dispersion fig8 dependence of doc in the clear phase on ph and amount of ozone fig9 dependence of doc in the clear phase on dispersion and amount of ozone fig1 dependence of degradation rate of excess sludge on ph under different nitrogen loads fig1 dependence of degradation rate of excess sludge on mechanical dispersion under different nitrogen loads fig1 dependence of the degradation rate of excess sludge on the amount of ozone under different nitrogen loads fig1 dependence of the degradation rate of excess sludge on ph and dispersion fig1 dependence of the degradation rate of excess sludge on ph and amount of ozone fig1 dependence of the degradation rate of excess sludge on dispersion and amount of ozone . fig1 shows diagrammatically the structure of the wastewater treatment plant in wuppertal 10 in which the method of the invention is used . the works wastewater 11 of the bayer works in wuppertal is fed to a first biological clarification stage 12 in which nitrification of the nitrogen compounds such as ammonium , urea or organic nitrogen compounds takes place . via an intermediate clarifier 13 , the wastewater then passes into the second biological clarification stage , the denitrification stage 14 . in a secondary clarifier 15 connected downstream of the denitrification stage 14 , the clear phase 16 and the sludge 21 are separated . the clear phase 16 is removed as purified wastewater . about 90 % of the sludge 21 from the secondary clarifier 15 is recirculated to the denitrification stage 14 . about 10 % of the sludge 21 is subjected to further treatment as excess sludge 17 . in the treatment stage 18 a ph adjustment and / or mechanical disintegration and / or ozone addition takes place . the treated excess sludge 22 is fed to a second secondary clarifier 19 in which the clear phase 23 containing the organic fractions of the treated excess sludge 22 and the inorganic fractions 24 of the treated excess sludge 22 are separated . the inorganic fractions 24 of the treated excess sludge 22 are removed for further disposal . the clear phase 23 is recirculated via the intermediate clarifier 13 to the denitrification stage 14 . measurement 25 of the actual value for the doc proceeds in the clear phase 23 . measurement 25 of the actual value for the doc proceeds online using a continuous - flow process analytical instrument . the setpoint doc is determined via the nitrogen load in the wastewater . the nitrogen load of the wastewater is measured online using a continuous - flow process analytical instrument . the measured nitrogen load is assigned to setpoint doc empirically and according to stoichiometric conditions . if the measured actual value deviates from the setpoint value for doc , an altered set of running parameters ( ph , md , ozone ) is determined . the running parameters here are ph , mechanical disintegration yes / no ( md ) and amount of ozone added ( ozone [ g / kg of dry matter ]). in experimental series on a pilot plant scale , the doc value and the degradation rate of the excess sludge under various nitrogen loads were measured for various combinations of running parameters ( ph , md , ozone ). the effect of the running parameters on the doc value and the degradation rate of excess sludge were analyzed under different nitrogen loads via linear and multiple regression of the measured values as a function of individual running parameters or combinations of running parameters . the experiments showed that the ph has the greatest effect on the doc value and the degradation rate of excess sludge , followed by dispersion and amount of ozone ( fig2 , fig3 a , 3 b ). in the figures , the tested ranges for the running parameters are normalized . the ph of − 1 corresponds to 7 . 6 and + 1 corresponds to 12 . 7 . a dispersion of − 1 means that no mechanical disintegration was used and a dispersion of + 1 means that mechanical disintegration was used . an ozone amount of − 1 corresponds to an ozone amount of 16 g / kg of dry matter and an ozone amount of − 1 corresponds to an ozone amount of 84 g / kg of dry matter . in addition , the nitrogen load was varied in the range from 6 kg / h (=− 1 ) and 12 g / h (=+ 1 ). the degradation rate for the excess sludge is indicated in percent of dry matter and the doc value in the clear phase in milligram per liter , in fig4 to 6 , the dependence of doc in the clear phase on ph , mechanical dispersion and amount of ozone under different nitrogen loads is shown . in fig7 to 9 , the dependence of doc in the clear phase on in each case two of the three running parameters ( ph , md , ozone ) is shown . in fig1 to 12 , the dependence of degradation rate of excess sludge on ph , mechanical dispersion and the amount of ozone under different nitrogen loads is shown . in fig1 to 15 , the dependence of degradation rate of excess sludge on in each case two of the three running parameters ( ph , md , ozone ) is shown . for the altered set of running parameters , it is fundamentally of importance that first the ph is varied , since it has the greatest effect on the doc value and low energy consumption is necessary for the change in setting . the fine adjustment then proceeds via switching on or switching off the mechanical disintegration and varying the amount of added ozone which is kept as low as possible . the set of running parameters assigned to each setpoint value for doc is determined reproducibly on an industrial scale by experimental series , the abovementioned criteria being taken into account and stored in a database . in the plant according to the invention , the denitrification performance can be maximized and the excess sludge reduced by up to 90 %.