Removal of organic matter and nitrogen in a single reactor with extremely high sludge age and cyclic aeration.

The removal of organic matter and total nitrogen (TN) in domestic wastewater was evaluated in a full-scale activated sludge (AS) system, operating with Extremely High Sludge Ages (EHSA) and cyclic aeration under continuous flow. A single bioreactor was used, applying intermittent aeration with a 45':15' on/off cycle. The results were obtained through six composite sampling periods of 24 h each. The sludge age was approximately 180 days, with average removals of chemical oxygen demand (COD) and TN of 88.6 and 71.7%, respectively. The obtained results save 6 h of energy per day and enable the conversion of conventional AS reactors into tertiary treatment by removing nitrogen, involving minimal investment in the same secondary treatment without constructing additional tanks. Similarly, the very high sludge ages lead to a reduction in construction costs, as the volume can be decreased due to shorter hydraulic detention times. Additionally, the minimal generation of purge sludge results in reduced costs for its final disposal and fewer environmental issues associated with its management.