Pilot-scale validation

In activated sludge processes, effluent ammonia or nitrite concentration was mainly impacted by SRT, DO, temperature, and ammonia shock load. From the bench-scale experiment, it was found that, at the 10-day and 20-day SRTs, effluent nitrite initially gave the first feedback on insufficient DO before the effluent ammonia and COD did, and, when DO was further reduced, the effluent ammonia gave the first feedback before the effluent nitrite and COD. At the 40-day SRT, the effluent ammonia gave the first feedback on insufficient DO before the nitrite and COD did. In addition, with all test SRTs, about 1 to 2 hours of peak loading did not lead to noticeable incomplete

nitrification. Therefore, the combination of ammonia and nitrite has great potential to be used as the control parameter for aeration system operation when the SRT is larger than 10 days. However, the effect of low temperature was not tested in the bench-scale reactors. It is still possible that incomplete nitrification could occur at a low temperature, even when the DO is adequate. However, the adverse effect of low temperature on nitrification may be overcome by extending the SRT. A field pilot-scale experiment was conducted to validate the findings obtained from lab bench-scale experiments and to test the hypothesis that complete nitrification can still be achieved with a low temperature when the SRT is long enough.

3.5.1 Reactor performance

Fig. S1 (Supplementary data) shows the changes in temperature, DO, MLSS, influent COD and TN loads, effluent COD, effluent ammonia, and effluent nitrite in the field pilot-scale reactor. The lowest temperature occurred during the period from January to February when it was approximately 8 ˚C. The highest temperature of approximately 27 ˚C occurred in July and August. The pilot-scale reactor was fed with different flow rates over the experimental period. As a result, the influent organic and TN loads varied significantly and in the ranges of 3.5 – 59.3 kg-COD/d and 0.38 to 3.84 kg-N/d,

respectively. During the experiment, constant aeration was provided. As a result of

changes in the inflow rate, strength, and temperature, the DO varied from 0.1 to 8.7 mg/L.

Before the middle of November, the SRT was 30 – 40 days and the MLSS ranged from 1.8 to 6.3 g/L (with an average value of 3.9 ± 1.07 g/L). To overcome the effect of low temperature on nitrification in winter, the SRT was increased to 60 - 80 days, starting from the middle of November, and the average MLSS concentration was increased to 6.5

± 1.28 g/L.

As presented in Fig. S1, there was no significant change in the effluent COD concentration under various loading, temperature, and DO conditions. The average effluent COD was 26 ± 10.8 mg/L. The effluent nitrite concentration during the

experiment varied slightly, from 0.02 to 1.8 mg-N/L. However, the fluctuation of effluent ammonia was significant, from 0.03 to 18.0 mg-N/L. The highest effluent ammonia occurred during the time period when the DO was very low, rather than when there were fluctuation of other conditions, such as low temperature and high loadings. Even though

the temperature in the reactor was generally lower than 10 ˚C in December, complete nitrification was achieved. Therefore, DO was a more sensitive operational parameter than the other parameters considered (such as temperature and influent loading) as long as the SRT was appropriate.

3.5.2 Discussion on pilot-scale experiment

Fig. 5 shows the correlation between the effluent ammonia and nitrite and the operational DO. There was a clear trend that a high effluent ammonia concentration occurred when the DO was low, indicating that the DO was the predominant factor for effluent ammonia concentration. The nitrite concentration also followed the same trend, but this observation was not conclusive due to the low nitrite concentration range. In most cases, when the DO was lower than 1.5 mg/L, or even 0.5 mg/L, the effluent ammonia concentration was still lower than 1 mg-N/L. When aeration is controlled, based on a DO set-point, some energy would be wasted in providing unnecessary aeration if complete nitrification could be achieved at a DO lower than that set-point. As shown in Fig. 5, when the DO was higher than 2 mg/L, the effluent ammonia and nitrite were always lower than 0.5 mg-N/L, indicating that 0.5 mg-N/L could be used as the lower threshold value for aeration control. Once both effluent ammonia and nitrite were lower than 0.5 mg-N/L, aeration could be decreased. For this particular treatment system, the nitrite concentration was always lower than the ammonia concentration due to the extremely long SRT; therefore, nitrite control was unnecessary. However, for systems that have lower SRTs, the nitrite concentration could respond to a low DO first; therefore,

both ammonia and nitrite would be needed to control the aeration system to an ensure appropriate effluent quality.

As shown in Fig. S1 and Fig. 5, only in the first several days was a high effluent nitrite (higher than 1 mg-N/L) detected. However, no nitrite accumulation was detected after July, even when the DO was lower than 0.5 mg/L. In addition, no nitrite had accumulated with a low temperature, as exhibited in Fig. S1. The results from the field pilot-scale experiment fully support the conclusion that nitrite would not accumulate with low DO, when the SRT was long enough. Fig. S1 clearly indicates that there was no significant difference in the effluent COD with various DO concentrations and, when the SRT was long enough, a low temperature did not significantly impact the effluent COD.