Strategies of nitrogen removal from urban wastewater in a sequencing batch reactor
Wastewater treatment by a Sequencing Batch Reactor (SBR) provides an alternative solution to activated sludge treatment, by carrying out carbon treatment, liquid-solid separation and nutrient removal in a single tank, thanks to the appropriate management of the temporal reaction cycles. Alternating the aeration and anoxic phases, followed by a decantation period, leads, in theory, to the almost total removal of nitrate ions formed during the aerobic nitrification phase. However, depending on the applied load, the carbon that is totally degraded during the preliminary aeration phase, cannot be used as a source of carbon for exogenic denitrification. Several solutions are possible in order to accelerate denitrification: one consists of lengthening sufficiently the second anoxic phase to treat the residual nitrate ions during the endogenous denitrification process; another strategy involves reducing the aerobic reaction time, while increasing the frequency of aerobic/anoxic sequences in order to preserve residual carbon during denitrification. A third solution lies in the addition of a source of exogenic carbon after the nitrification stage, to allow a quicker and more efficient assimilation of the nitrate ions that are formed (exogenic denitrification). This article compares the results of reducing carbon and nitrogen in wastewater, using three types of operation. The cycle of reference has been established starting from previous bibliographical results (WUN JERN and DROSTE, 1989) and simulations using the model ASM1 (HENZE et al., 1986). It consists of an anoxic feeding, followed by an anoxic phase, then an aerobic phase and another anoxic phase. The cycle ends by the settling and decanting phases. The lengths of these different phases are: 1 h, 0.5 h, 4.5 h, 3.25 h, 1 h, 1 h. This reference cycle, carried out at the laboratory, leads to the elimination of 90% of the Chemical Oxygen Demand (COD), 95% of the Biological Oxygen Demand (BOD5) and more than 80% of total nitrogen, i.e. with residual concentrations of 60 mg×L-1 for the Dissolved Organic Carbon (DOC), 5 mg×L-1 for the Biological Oxygen Demand, and 10 mg×L-1 for total nitrogen. These results are comparable with those in the literature (IRVINE et al., 1987, MELCER et al., 1987, YANG et al., 1999). The results obtained during the reference cycle enabled us to reach a rate of total nitrogen removal of 85% and a global nitrogen concentration in the effluent of 11 mg×L-1. Nitrification and denitrification rates yielded values of 0.8 mg N-NH4+×gMVS-1 ×h-1 and 0.8 mg N-NO3 -×gMVS-1×h-1 respectively. Total nitrogen removal was not completely achieved because of the lack of available carbon. This lack of carbon favours endogenic denitrification, characterized by a slow denitrification rate 0.8 mg N-NO3 -×mgMVS-1×h-1, compared to exogenic denitrification characterized by a higher nitrogen reduction rate (about 2 mg N-NO3 -×mgMVS-1 ×h-1). This fact was experimentally confirmed with carbon addition in the form of acetate at the beginning of the second anoxic phase. In this case, carbon addition significantly improves the denitrification rate compared to the same experiment without exogenic carbon addition: 2 mg N-NO3 -×mgMVS-1 ×h-1 versus 0.8 mg N-NO3 -×mgMVS-1 ×h-1. However, this method raises operating costs for the process. In order to reach complete nitrogen removal without the addition of synthetic carbon, it is possible to increase the anoxic phase time scale from 3 hours to 15 hours. Although the results in term of carbon and nitrogen removal are satisfactory when the anoxia phase is lengthened, the concentration in the discharged effluent is 0.33 mg total N×L-1, and thus this technique decreases the productivity of the SBR. The feeding cycles of a biological reactor being variable, a regulation based on the use of the evolution of the pH, or the redox potential, can be considered (PAVELJ et al., 2001; ANDREOTTOLA et al., 2001). This regulation would make it possible to adapt the duration of the phases of anoxia to the necessary treatment. To overcome this drawback, a possible approach consists in replacing the aerobic / anoxic phase in the reference cycle by five aerobic / anoxic phases during the same time. Unfortunately, this method leads to a decrease in nitrogen removal and in the nitrification rate, compared to classical cycle (65% versus 85%, and 0.4 mg N-NO3 - ×mgMVS-1 ×h-1 versus 0.8 mg N-NO3 -×mgMVS-1 ×h-1, respectively). The nitrification rate is, in this case, half that obtained in the reference cycle, probably due to delays related to the induction of nitrification and denitrification. This strategy, consisting of increasing the aeration / no aeration frequency, has to be optimized in term of nitrification and denitrification ratios. A better solution from the economic and productivity points of view is the addition of wastewater at the beginning of anoxic phase. This strategy implies the modification of the cycle. First, after the anoxic feeding, an aerobic phase allows carbon and nitrogen oxidation. In order to supply an available carbon source for exogenic denitrifcation, a second feeding is introduced at the beginning of the second anoxic phase. This addition also contains ammonium ions and implies new nitrification and denitrification steps. This last denitrification phase is then endogenic. Carbon addition in the form of wastewater leads to an improvement in nitrogen removal. The exogenic denitrification rate is twice the value for endogenous denitrification for the same cycle of operation (1.6 mg N-NO3 -×gMVS-1 ×h-1 versus 0.9 mg N-NO3 -×gMVS-1 ×h-1 respectively). This strategy yields a final concentration of 3 mg N×L-1 and the nitrification and denitrification rates are similar to those of the traditional processes. In conclusion, the addition of synthetic carbon in the form of acetate must be preserved as a means of acting quickly in the event of dysfunction (that can be detected by monitoring the redox potential or the pH), although this technique significantly increases the cost of operation. Although the best economic solution to improve denitrification is carbon addition in the form of wastewater, other strategies can be undertaken according to the goals of the treatment process. When the wastewater load is sufficiently weak (night period), the endogenous phase of denitrification can be lengthened. In the event of an important load, carbon addition (in synthetic form or as waste water) makes it possible to eliminate the nitrate ions exogenically.