Ozonation Ozonation is generally considered to improve biodegradation of water pollutants and this improvement in biodegradation is believed to bring about increased organic removal during activated carbon contacting. A review of the literature indicated that the effect of ozonation on biodegradation has not yet been studied in a thorough scientific manner and, therefore, our investigation was undertaken with this purpose in mind. Part I of the study consisted of ozonation followed by respirometric biodegradation studies. Ozonation was accomplished in a 3 m bubble column system capable of accurate O dosage measurements. Respirometric studies were done in an electrolytic respirometer. Total Organic Carbon, Chemical Oxygen Demand, Total Kjeldohe Nitrogen and nitrite plus nitrate were measured before and after each step. The following three substrates were used: (i) natural plus synthetic organic polluted river water, (ii) river water containing high level of natural organic substances, (iii) synthetic nitroaniline solutions. The results indicate that oxygen uptake and organic uptake during biodegradation are usually improved by ozonation, except in the case of (para nitroaniline). A new parameter indicative of the fraction of organics made biodegradable by ozonation is defined.