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Effect of mixed liquor suspended solids (MLSS) on simultaneous nitrification and denitrification in a sequencing batch reactor

This study investigated the effect of MLSS on simultaneous nitrification and denitrification in a sequencing batch reactor treating domestic wastewater. The average removal efficiencies of COD and TN increased from 93 and 68.21% to 97 and 74.20% with the increase in MLSS from 3.5-4.0 to 7.5-8.0 g/L, respectively. However, the average removal efficiency of TN decreased to 39.45% when MLSS increased to 9.5-10.0 g/L. Simultaneous nitrification and denitrification (SND) was carried out in high MLSS (7.5-8.0 and 9.5-10.0 g/L), and SND efficiencies were 43.4-52.0% and 72.1-80.7%, respectively. Batch tests suggested that there was a strong potential to apply high MLSS for treating wastewater containing high-strength ammonia nitrogen.


Introduction
In recent years, the requirements for wastewater discharge have become stricter due to the problem of wastewater eutrophication. It is essential and compulsory for nitrogen removal to an appropriate concentration before wastewaters are being discharged into the environment. Biological treatment has been proven to be an economical and feasible process for nitrogen removal. Biological nitrogen removal is usually achieved through traditional nitrification-denitrification process, by which nitrification and denitrification are operated in the separation of aerobic and anoxic zones.
Simultaneous nitrification and denitrification (SND), as one of the economically effective and space-saving technologies, has attracted the interests of many researchers. Compared with traditional biological nitrogen removal processes, the SND system represents a significant advantage. First, SND eliminates the need for a separate anoxic zone, inducing simplified operating procedures and a smaller footprint. It is estimated that the SND system reduces 40% of COD demand during denitrification and saves 25% of aeration energy [1]. Simultaneously, the SND system can complete the nitrification and denitrification process under the neutral pH with less demand for alkalinity. Because of these advantages, increasing attentions have been paid to the research of SND. Therefore, it was found that there are a lot of factors which influence SND, including C/N ratio [2], temperature [3], MLSS [4], etc.
In the present study, the performance of three SBRs was investigated under three kinds of sludge concentrations of 3.5-4.0, 7.5-8.0 and 9.5-10.0 g/L feeding with domestic wastewater. objective of this study was to investigate the effect of MLSS on SND in a sequencing batch reactor, and provided useful guidance for its operation.

Reactors setup and operating conditions
The three SBRs correspond to the sludge concentrations of 3.5-4.0, 7.5-8.0 and 9.5-10.0 g/L, which were called them SBR1, SBR2 and SBR3. Three identical reactors were made of Plexiglas, with an effective volume of 36 L. The exchange volume ratio of three reactors was 12 L/cycle, and the SBR had three cycles a day. Each cycle (8 h) consisted of the following steps: feeding (0.5 h), aeration (5 h), settling (1.5 h), and decanting (0.5 h) and idling (0.5 h). Aeration rate was controlled at 2 L/min for each reactor. The operation of the three reactors was controlled by a programmable logic controller (PLC), and their temperature varied with ambient temperature (25 ± 2 °C).

Batch tests
Three 500 mL measuring cylinders were operated in batch tests to study the utilization characteristics of different high-strength ammonia nitrogen under different MLSS, called RUN1, RUN2 and RUN3. The activated sludge samples were taken from the three reactors (SBR1, SBR2 and SBR3) at the end of the aeration phase on day 100 and settled for 30 min, and the supernatant was drained, washed three times with deionized water.
Test1: The initial NH + 4 -N concentrations was 64 mg/L in RUN1, RUN2 and RUN3, which were aerated for 8 h and had the same aeration rate of 0.1 m 3 /h. Test2: The initial NH + 4 -N concentrations was 122 mg/L in RUN1, RUN2 and RUN3, which were aerated for 10 h and had the same aeration rate of 0.1 m 3 /h. Test3: The initial NH + 4 -N concentrations was 284 mg/L in RUN1, RUN2 and RUN3, which were aerated for 10 h and had the same aeration rate of 0.1 m 3 /h. Ammonia nitrogen utilization rate (R) can be calculated using the following formula: where C1 and C2 was the influent and effluent concentration of ammonia nitrogen. V is the volume of influent and t is the aeration time.

Organic carbon and Nitrogen removal
The time courses of COD concentrations and removal efficiencies under different MLSS in three reactors are shown in Figure 1. The average COD removal efficiencies slightly increased from 93 to 97% with the increase in MLSS from 3.

Simultaneous nitrification and denitrification (SND) in reactors
The occurrence of SND were generally judged by the loss of N under aerobic conditions in the reactor. In this study, three methods were used to analyze the occurrence of SND in the first stable stage. The first method, SND efficiency is calculated using equation (1) which does not take into account sludge assimilation. Where NOxproduced represents the total amount of NO − 2 -N and NO − 3 -N produced during the aerobic period, NH + 4removal represents the total amount of NH + 4 -N removed in the nitrification reaction.  Table 1 provides an example of the nitrogen composition balances related to the tracking research on the 25th day of the stabilization phase. The initial content of various forms of nitrogen was the value at which the aeration began. Since the sludge content and dilution of the three reactors were different, the initial content was also varied. Figure 3 shows the performance of SND efficiency under different MLSS on the 25th, 55th, 85th, 115th, 130th day of the stabilization phase. The SND efficiency of SBR2 varied from 46.2 to 51.2%, while the SND efficiency of SBR3 was above 90%, suggesting that the higher the sludge concentration, the higher the SND efficiency. Compared to the high sludge concentrations, the average SND efficiency of SBR1 with conventional sludge concentration was only 12.4%.
The second method, SND efficiency is calculated using equation (1) which takes into account sludge assimilation. where TN assimilation represents the total amount of TN removed by the assimilation of the sludge, TN removal represents the total amount of TN removed.

100%
(2) In the stable state of activated sludge, the mass of discharged sludge represented the mass of microorganism growth and formation. Nitrogen consumption due to cell assimilation was calculated with the cell formula (C 5 H 7 NO 2 ), in which N accounted for 12%. The comparison of SND efficiency which considered cell assimilation under different MLSS is shown in Table 2. TN removal efficiencies in the SBR2 and SBR3 were larger than cell assimilation, illustrating that SND occurred in the reactors. However, in the SBR1, the amount of cell assimilation was substantially equal to the amount of TN removal, indicating that the removal of nitrogen was achieved by cell assimilation.
On the basis of these results, we concluded that the high sludge concentrations had a lower DO concentration as a result of decreasing oxygen transfer efficiency, which led to the formation of the anoxic zone and the occurrence of SND in SBR2 and SBR3. Additionally, the SND efficiency increased with increasing MLSS. These results were consistent with the results obtained by Baeza et al. (2002) [5] that the aerobic reactors achieve the highest SND efficiency under the lowest dissolved oxygen, however, the concentration of ammonia in the effluent increased as low dissolved oxygen that hinders nitrification.

Effect of high ammonia nitrogen concentrations under different MLSS in batch experiments
Three experiments were operated at influent ammonia nitrogen concentrations of 64, 123 and 284 mg/L, respectively. Figure 4 shows the removal efficiency and utilization rate of ammonia nitrogen at three high concentrations of ammonia nitrogen. In test 1, when the influent NH + 4 -N concentration was 64 mg/L, from RUN1 to RUN3, effluent NH