nominal relationship between the ph value and the redox potential value is schematically represented in fig2 of the drawings in the ph range shown here the characteristic value is a straight line with an inclination which had previously been determined by tests in the example approximately 88 mv per ph at 37 u00b0 c a tolerance field respectively bordered by dash dotted lines 41 and 42 is shown on both sides of the characteristic curve 40 in which the microorganism population provides a high decomposition output without harmful side effects with a too negative redox potential unsatisfactory decomposition conditions and anaerobic states result as shown in fig3 on the other side when the limit 42 is passed increasing flocculi formation occurs it is important that it is provided with the aid of the process computer 30 that the redox potential and with it the air supply follow the fluctuating ph values even in case of strongly fluctuating ph values in the described example the correct characteristic curve 40 and if desired also the limits 41 and 42 have been entered into the memory 33 before the start of the operation the memory 33 is programmable it is known that with the partially aerobic method the temperature should lie in the range between 20 to 40 u00b0 c optimally between 35 and 39 u00b0 c with a high cod content of for example 10 000 mg l the waste water temperature is close to 42 u00b0 c the microorganisms are no longer viable at this temperature in the exemplary embodiment represented in fig1 the waste water can be cooled to a suitable nominal temperature in the bypass 11.