An Automatically Controlled Alternate Oxic-Anoxic Process for Small Municipal Wastewater Treatment Plants P. Battistoni,* ,† A. De Angelis, ‡ R. Boccadoro, † and D. Bolzonella § Institute of Hydraulics, University of Ancona, Via Brecce Bianche, 60131 Ancona, Italy, Gorgovivo, Water and Wastewater Treatment Company, Ancona, Italy, and Department of Science and Technology, University of Verona, Strada Le Grazie 15, 37134 Verona, Italy A patented automatic control device was applied to management of an alternate oxic-anoxic process in a small wastewater treatment plant (700 PE). The control system enabled the optimal time-length of the aerobic and anoxic phases to be determined by analyzing the dissolved oxygen and the oxidation-reduction potential data. Moreover, also a time set point was introduced to establish the maximum length for the two phases. Results showed high performances in biological nitrogen removal (0.7-5.2 mg of NO 3 -NL -1 in the effluent) and a reliable control of the treatment process also during wet weather events. In comparison with extended aeration plants of similar size, lower energetic consumption was observed, generally <200 Wh PE -1 day -1 . The automatic control device was a reliable system that gave a good performance in a small wastewater treatment plant with low investment and managing costs. Introduction A number of small (<10 000 PE) municipal wastewa- ter treatment plants operate in Italy: over 6000 plants. To achieve good process control, they require constant maintenance and specialized personnel, with conse- quent high management costs. The present Italian legal standards for nitrogen discharge in nonsensitive areas (NH 4 -N ) 11.7 mg L -1 and NO 3 -N ) 20 mg L -1 ) can be easily achieved, ensuring a minimum performance efficiency, generally 30% in nitrogen removal, but it is well-known that low energy costs for process manage- ment can be achieved only when the nitrogen removal reaches 80-90%. In fact, only nearly complete recovery of oxygen bound in nitrates can contribute to energy savings. Obviously, a high efficiency in nitrogen removal is linked to a reliable control of the process (besides the presence of adequate structures and infrastructures of the treatment plant). Therefore, only when a constant control of the process is ensured can the ammonia oxidation and nitrates denitrification be achieved de- spite daily mass loading fluctuations (during dry weather) and hydraulic overloading during wet weather. Gener- ally, extended aeration or an alternate oxic-anoxic process is adopted: the goal is to obtain biological nitrogen removal by spatial or temporal separation of nitrification (aerobic) and denitrification (anoxic) steps. Alternate steps (aerobic-anoxic) have been achieved using Carousel, oxidation ditches, Bio-Denitro, and alternate oxic-anoxic processes. 1,2 The use of continu- ously fed alternate processes allows an effective nitrogen removal alternating two different cycles: the first, aerobic, is a cycle long enough to achieve complete ammonia oxidation, whereas the second, anoxic, achieves nearly complete nitrate denitrification. Furthermore, the anoxic phase enables one to achieve low energy consumption because of the anoxic oxidation of organic compounds where nitrates rather than oxygen are the final electron acceptors. To perform automatic control of the nitrification-denitrification process, typical pro- files of dissolved oxygen (DO) and oxidation-reduction potential (ORP) within the process cycles can be used. Characteristic changes in the DO and ORP profiles with time have been observed in sequential batch reactor processes. These profiles relate to physical-chemical phenomena or biological events and can be successfully used in process control. In particular, the ammonia disappearing (break point) during the aerobic cycle, identified by a flex point in the DO profile with time, at the end of ammonia nitrification, and the nitrates disappearing (break point) in the anoxic cycle, identified by a flex point in the ORP profile with time, at the end of the nitrate denitrification process, are the main evident observations. 3-6 These evidences are generally observed in pilot-plant experiments, where constant flow rates and nutrient loading are used. On the other hand, in full-scale continuously fed plants, the detection of those changes can be difficult because of a number of different factors. Paul et al. 7 showed, as the pollutants, that mass over- and underloading, the over- and un- deraeration, the nitrification inhibition, and the insuf- ficient carbon availability are the main factors involved in hindering the flex points of the OD and ORP profiles with time. Small municipal wastewater treatment plants, oper- ating the alternate oxic-anoxic process, are often automatically controlled. Here, the blowers are switched on and off by means of timers or of the ORP signal elaboration. In a recent study, 8 according to a prelimi- nary work of calibration, where the high and low ORP set points were determined, the best performances in nitrogen nitrification and denitrification were sought. Moreover, the set points were used to control the time- length phases. However, the control protocol was con- sidered complex and not reliable when the hydraulic loading changed, both within the day or during wet weather events. In a second study, 9 an automatic control device, called OGAR, was used to stop the aeration on * To whom correspondence should be addressed. Phone: +39 071 2204530. Fax: +39 071 2204528. E-mail: idrotre@ popcsi.unian.it. † University of Ancona. ‡ Gorgovivo, Water and Wastewater Treatment Co. § University of Verona. 509 Ind. Eng. Chem. Res. 2003, 42, 509-515 10.1021/ie020376g CCC: $25.00 © 2003 American Chemical Society Published on Web 01/09/2003