Full-Scale Application of the Alternating Oxic/Anoxic Process: An Overview

This article describes the results obtained in terms of nitrogen removal, energy savings, and waste sludge production upon application of an alternate cycle process and a remote control system in real wastewater treatment plants (WWTPs). The experimentation, which lasted for eight months, was performed in three extended oxidation plants (denoted WWTP1, WWTP2, and WWTP3) characterized by different structural designs and sewerage influent macropollutant concentrations. The alternate cycle (AC) process provided good total nitrogen biological removal. The results obtained were due to the excellent reduction of nitrates. In fact, comparison with the pre-AC period showed an average decrease of NO3-N of about 64% in WWTP1, 62% in WWTP2, and 33% in WWTP3. The behavior of all WWTPs can be rationalized according to a nitrogen loading rate (NLR) approach, showing that its increment determined a reduction of the length of the anoxic phase to allow for ammonia oxidation. All of these conditions assured the satisfaction of the limit on total nitrogen in effluent established in directive EC 91/271. The energy savings (13-26%) was always observed to be related to the time spent in the anoxic phase during which the blowers were switched off and the mixers were used for mixed liquor suspension and nitrates denitrification. Moreover, it was demonstrated, as a new aspect, that application of the AC process was able to reduce the waste activated sludge by a biomass stressing during the anoxic conditions. In addition, in WWTP3 where the influent mass loading and effluent suspended solid removal mainly were controlled, a consistent specific sludge reduction (up to 47%) was observed.