Enhancement And Maintaince Of Partial Nitrification By Low-intensity Ultrasound

Achievement of partial nitrification acquires rigorous strategies and NO₂^--N accumulation is unstable when temperature changes,which restrains its application.Low-intensity ultrasound has become a new approach to achieve partial nitrification and could improve ammonia-oxidizing bacteria（AOB）activity and suppress nitrite-oxidizing bacteria（NOB）activity simultaneously with suitable ultrasonic parameters,whose application approach needs further detection.This paper used two kinds of SBRs treating low-strength wastewater,from which condensed sludge was irradiated by ultrasound.Improvement and maintenance of partial nitrification by ultrasound under different ultrasonic treatment and SBR operation condition were studied.（1）Feasibility of partial nitrification achievement by ultrasound depend on suitable ultrasonic condition.In this part,experiment was performed on six SBRs.Effect of ultrasonic parameters,treated sludge proportion and ultrasonic treatment with aeration on partial nitrification were investigated.Results showed that under high dissolve oxygen（DO）concentration（about 3.4mg/L）and ambient temperature（17²1oC）ultrasound with irradiation time 20 min and density 0.05 W/mL could improve partial nitrification significantly and NO₂^--N accumulation rate（NAR）reached 59.1%.With the optimal ultrasonic parameters,AOB activity reached the maximum and increased by 114%compared with the untreated group,concurrently NOB activity decreased by 36.1%.However,when temperature enhanced to 24²8oC above ultrasonic parameters couldn’t achieve partial nitrification.But ultrasonic treatment added with aeration could improve NAR to 60.3%due to increased ultrasonic stable cavitation.With increased treated sludge proportion NOB activity gradually declined but AOB activity changed slightly.Under treated sludge proportion of 100%,NOB activity was only 26.1%of untreated group’s and AOB activity was relatively highest.（2）Multipal strategies favored partial nitrification stability.In this paper,low-intensity ultrasound was combined with alternating anoxic/aerobic conditions to keep partial nitrification.Nitrogen transformation and activity of AOB and NOB were studied in ultrasonic and control SBR under four intermittent aeration conditions.We found that low-intensity ultrasound and alternating anoxic/aerobic operation could inhibit NOB activity synergistically.However,AOB activity was also suppressed by ultrasound with long anoxic period（45 min）.And anoxic/aerobic time of 15 min/30 min was the optimal condition to maintain partial nitrification with low-intensity ultrasound,under which AOB activity in ultrasonic group was increased by 189.9%compared with the control.Meanwhile,NOB activity was decreased by 38.2%.NAR of ultrasonic SBR decreased when room temperature enhanced,under which partial nitrification was destroyed with average DO about 2.2 mg/L.Then partial nitrification in ultrasonic group was recovered within 5 days under irradiation with aeration and improved ultrasonic density（from 0.05 W/mL to 0.75 W/mL）with low DO（<1 mg/L）.After recovery,NAR of ultrasonic group was kept over 94%.During continuous periodical ultrasound treatment,NOB amount declined gradually and amount of NOB and heterotrophic bacteria increased slightly.Sludge community was changed to be partial nitrification sludge after long-time ultrasound irradiation.When stopping ultrasound irradiation,NAR of ultrasonic group was still maintained over 95%with free ammonia concentration over 3.8 mg/L.Hence,low-intensity ultrasound with suitable condition could increase and maintain partial nitrification significantly.Low-intensity ultrasound is good at partial nitrification rapid recovery for its easy control.