Preliminary evaluation of sludge minimization by a lab-scale OSA (oxic-settling-anaerobic) system

The biological wastewater treatment process involves a significant production of sludge that must be treated before its ultimate disposal. The treatment and disposal of the excess sludge are expensive processes and the minimization of sludge production is a topic of great interest. The oxic – settling – anaerobic (OSA) process is an adaptation of the conventional activated sludge (CAS) treatment, set up by inserting a sludge holding tank (SHT) in the sludge return line. The OSA technology demonstrated to reduce the excess sludge production and several studies have confirmed that the enhanced sludge decay is the decisive cause in OSA process. The present paper reports the preliminary results obtained by an OSA lab-scale plant working with real influent wastewater. The plant operations were monitored by calculations of removal efficiencies and by microfauna composition analysis. Two runs of one month were performed: the average removal efficiencies were 76% for soluble COD and 82% for ammonia nitrogen. Biological denitrification was also observed with an efficiency of 34% for the first run and of 19% for the second one. The microfauna observations showed that the activated sludge remained efficient during the two runs with a high density of microfauna (always greater than 106 organisms per liter) and a highly diversified community.