Treatments Of Ammonia Wastewater By A Single-stage Autotrophic Nitrogen Removal Process With Low Strength Ultrasound Application

The single-stage autotrophic nitrogen removal process is recognized as a promising wastewater treatment technique, which offers many advantages, such as short process flows, simple operation and cost-effective. However, drawbacks of the low growth rate of autotrophic bacteria and the seed sludge wash-out lead to reactor instabilities for the nitrogen removal. In order to overcome these difficulties, this study investigated effects of low strength ultrasound on the single-stage autotrophic nitrogen removal process. Three reactors (biofilm reactor, co-immobilizing reactor and granule sludge reactor) were further established to explore the long-term effects of low intensity ultrasound on the nitrogen removal performance and find optimal reactor configuration.This study firstly investigated the effects of low strength ultrasound on the activities of Nitrosomonas. The results showed that the increased ultrasonic energy (UE) led to the elevated biomass activity while maintaining biomass concentration constantly. The highest nitrogen removal efficiency was achieved at UE=43.20 kJ, which was 25.44% higher than that of the control. Under this condition, the ammonia removal rate increased with the concentration of sludge in 0.34 gVSS/L-1.03 gVSS/L corresponding to the per unit biomass of UE (PUE) decreasing from 127.06 kJ/gVSS to 41.94 kJ/gVSS. However, above a certain value of 41.94 kJ/gVSS, the activity of Nitrosomonas showed a decreasing trend. It thus can be concluded that the PUE of 41.94 kJ/gVSS was the optimal condition to obtain the highest activity of Nitrosomonas. Further studies on the mechanism of ultrasound on biomass activity indicated that the carbohydrate, protein and total extracellular polymeric substances increased 18.32%,26.54% and 22.05%, respectively. Besides, the activity of the ammonia monooxygenase, the key enzyme involved in partial nitrification, was also enhanced by 19.82% compared with that of the control.Subsequently, experiments aiming at the effects of low strength ultrasound on Anammox bacteria were carried out. The concentration of sludge was maintained at 1.19 gVSS/L with the UE within 0-72 kJ. The total nitrogen (TN) removal rate increased significantly with UE in 0-43 kJ was observed. At UE=43.20 kJ, the maximum activity of Anammox bacteria was achieved. Under this condition, the TN removal rate increased with the biomass concentration decreasing from 3.75 gVSS/L to 1.19 gVSS/L (corresponding to the PUE in 11.52 kJ/gVSS-34.56 kJ/gVSS), and peaked at PUE=34.56 kJ/gVSS. In addition, either increase or decrease of biomass and UE on a sliding scale to keep the PUE at 34.56 kJ/gVSS, the TN removal rate could be maintained at a stable level which was 14.06% higher than that of the control group. The hydrazine dehydrogenase (HDH) of Anammox bacteria was also enhanced by 15.46% compared with that of the control. Furthermore, direct exposure of HDH to low strength ultrasound of 28.80 kJ could promote its activity. These results suggested that the enhancement of TN removal rate of Anammox process was closley associated with the increase in enzymatic reactions of Anammox bacteria.Finally, three different types of single-stage autotrophic nitrogen removal reactors (biofilm reactor, co-immobilizing reactor and granule sludge reactor) were established to investigate the long-term effects of low strength ultrasound on the TN removal. All reactors were operated for 337 days. Phase Ⅰ (1-177 d) was operated for the cultivation of nitrosomonas and anammox bacteria. After 92 days’steady operation (Phase Ⅱ,178-270 d), the highest TN removal rate of 58.94% was observed in the biofilm reactor while those of co-immobilizing reactor and granule sludge reactor were 55.90% and 45.05%, respectively. During Phase Ⅲ (271-337 d), PUE was applied on these reactors. The nitrogen removal efficiencies of biofilm reactor and granule sludge reactor were accelerated by 33.88% and 22.18%, respectively, whereas the TN removal rate of co-immobilizing reactor did not improve. The present study suggested that low strength ultrasound could enhance the performance of single-stage nitrogen removal process. The biofilm reactor should be highly suggested as the optimal reactor configuration for a stable and efficient operation.