[Studies on factors limiting self-purification of water. I. Experimental studies on the effects of degradable and of toxic substances on the ways of microbial self-purification of water (author's transl)].

The results of the present studies indicate that the inhibition of bacterial growth caused by discharge of toxic substances into water receiving wastewater of directly into wastewater cannot serve as a mechanism inhibiting degradation below the toxic threshold of the depression of oxygen consumption i.e. as determined by BOD. Therefore, this discharge-caused inhibition also does not explain the residues of degradable substances normally found. Observations were made on degradation in the laboratory using a model with creatinine as substrate and P. putida as microorganism. It was found that even "physiologic" and therefore, easily degradable substances as well as subtoxic concentrations of toxic substances (measured against a determined minimal concentration causing inhibition) if discharged into water receiving wastewater or into wastewater will interfere with the degradation of specific wastes (creatinine). The delayed and/or incomplete degradations in our model led to a formation of persisting residual concentrations of the waste substances introduced into the system. It is rather unlikely that such residuals will actually emerge in running water under natural conditions, because here the specific (creatinine) degrading organisms will invariably revert from the stationary phase to a renewed active phase (of exponential growth) whenever the system receives other discharges or even is diluted by other discharges. In their active phase the mixed populations of bacteria formed in nature will almost certainly completely metabolize whatever may left of degradable wastes (creatinine). The mere fact that the time needed for degradation can be lengthened so considerably by interfering "physiological" yet degradable substances as well as by subtoxic concentrations of toxins, offers an explanation for the degradability measured immanentally in defined time-intervals within the BODx taken as a measure of degradable "residual wastes" i.e. substances which are biologically degradable but are not degraded.