Study on nitrogen removal performance of sequencing batch reactor enhanced by low intensity ultrasound

New insights into the improved contaminants removal in SBR by intermittently weak ultrasound

The combination of intermittently weak ultrasound and sequencing batch reactor was thoroughly investigated to elucidate the relationship between enhanced contaminants removal and activated sludge characteristics, microbial composition, and regulation of differentially expressed genes (DEGs). At 12 °C, irradiation with an ultrasound intensity of 9.68 W/L, an irradiation time of 10 min, and an interval time of 24 h led to significant increases in COD, NH4+-N, and TP removals with the rates of 93.10 ± 1.51%, 95.75 ± 0.76%, and 92.52 ± 0.95%, respectively. The intermittently weak ultrasound enhanced contaminants removal was primarily attributed to the stimulated microbial metabolism, in which the mechanical oscillation rather than free radical oxidation facilitated the loosening of activated sludge flocs and promoted microorganism proliferation. Elevating the ultrasound intensity or irradiation time could weaken the effect of enhancing ammonia-oxidizing bacteria activity and suppressing nitrite-oxidizing bacteria activity. The results revealed that intermittently weak ultrasound primarily affected the extracellular polymeric substances (EPS), with protein nitrogen playing a more significant role than polysaccharide within EPS against ultrasound-induced stress. Furthermore, ultrasound irradiation elevated the energy barrier in total-binding EPS interaction energy curves, thereby inhibiting activated sludge aggregation. Over prolonged operation, the relative abundance of the prevalent denitrifying genus Thauera increased by 90.3%, whereas that of the fully aerobic denitrifier and nitrite producer Dokdonella increased by 68.7%. The intermittently weak ultrasound induced enhancement of microbial metabolism-related DEGs pathways, which served as the main contributor to the improved contaminants removal. These findings provide novel insights into the mechanisms by which intermittently weak ultrasound enhances the effectiveness of biological wastewater treatment.

Review on ultrasonic technology enhanced biological treatment of wastewater

As a clean, sustainable and efficient technology of wastewater treatment, ultrasonic irradiation has gained special attention in wastewater treatment. It has been widely studied for degrading pollutants and enhancing biological treatment processes for wastewater treatment. This review focuses on the mechanism and updated information of ultrasonic technology to enhance biological treatment of wastewater. The mechanism involved in improving biological treatment by ultrasonic includes: 1) degradation of refractory substances and release carbon from sludges, 2) promotion of mass transfer and change of cell permeability, 3) facilitation of enzyme-catalyzed reactions and 4) influence of cell growth. Based on the above discussion, the effects of ultrasound on the enhancement of wastewater biological treatment processes can be categorized into indirect and direct ways. The indirect effect of ultrasonic waves in enhancing biological treatment is mainly achieved through the use of high-intensity ultrasonic waves. These waves can be used as a pretreatment to improve biodegradability of the wastewater. Moreover, the ultrasonic-treated sludge or its supernatant can serve as a carbon source for the treatment system. Low-intensity ultrasound is often employed to directly enhance the biological treatment of wastewater. The propose of this process is to improve activated sludge, domesticate polyphosphate-accumulating organisms, ammonia-oxidizing bacteria, and anammox bacteria, and achieve speedy start-up of partial nitrification and anammox. It has shown remarkable effects on maintaining stable operation, tolerating adverse conditions (i.e., low temperature, low C/N, etc.), resisting shock load (i.e., organic load, toxic load, etc.), and collapse recovery. These results indicate a promising future for biological wastewater treatment. Furthermore, virous ultrasonic reactor designs were presented, and their potential for engineering application was discussed.

Feasibility of low-intensity ultrasound treatment with hydroxylamine to accelerate the initiation of partial nitrification and allow operation under intermittent aeration

Partial nitrification is a key aspect of efficient nitrogen removal, although practically it suffers from long start-up cycles and unstable long-term operational performance. To address these drawbacks, this study investigated the effect of low intensity ultrasound treatment combined with hydroxylamine (NH2OH) on the performance of partial nitrification. Results show that compared with the control group, low-intensity ultrasound treatment (0.10 W/mL, 15 min) combined with NH2OH (5 mg/L) reduced the time required for partial nitrification initiation by 6 days, increasing the nitrite accumulation rate (NAR) and ammonia nitrogen removal rate (NRR) by 20.4% and 6.7%, respectively, achieving 96.48% NRR. Mechanistic analysis showed that NH2OH enhanced ammonia oxidation, inhibited nitrite-oxidizing bacteria (NOB) activity and shortened the time required for partial nitrification initiation. Furthermore, ultrasonication combined with NH2OH dosing stimulated EPS (extracellular polymeric substances) secretion, increased carbonyl, hydroxyl and amine functional group abundances and enhanced mass transfer. In addition, 16S rRNA gene sequencing results showed that ultrasonication-sensitive Nitrospira disappeared from the ultrasound + NH2OH system, while Nitrosomonas gradually became the dominant group. Collectively, the results of this study provide valuable insight into the enhancement of partial nitrification start-up during the process of wastewater nitrogen removal.

Accelerating anammox nitrogen removal in low intensity ultrasound-assisted ASBBR: Performance optimization, EPS characterization and microbial community analysis

Efficient enrichment of slow-growing anammox species is essential for rapid start-up and stable operation of high-rate anammox reactors. Herein, a low intensity ultrasound (LIU) was introduced into anaerobic sequencing batch biofilm reactors (ASBBRs) to enhance anammox nitrogen removal from nitrogen-rich wastewater. Operation results demonstrated that the maximum total nitrogen (TN) removal efficiency of 91.5% were achieved under the optimal ultrasonic parameters (32.7 °C water temperature, 0.18 W/cm² ultrasonic intensity and 25.7 min ultrasonication time). Moreover, significant increases of extracellular polymeric substances (EPS) components and contents were observed via the ultrasonication stimulation. A close correlation between nitrogen removal and shifts in transformation and intensity of spectrum peaks was also verified by three-dimensional excitation-emission matrix spectroscopy (3D-EEM) analysis. High-throughput sequencing revealed that the relative abundance of Candidatus Kuenenia as the key anammox consortium significantly increased after applying optimal ultrasonication condition. Furthermore, enhancement mechanisms and future prospect of the LIU-assisted anammox process was elucidated and discussed. This research provides a viable and promising acceleration strategy for anammox-based process in practice.

Enhancement of Anammox performances in an ABR at normal temperature by the low-intensity ultrasonic irradiation

A lab-scale ultrasound enhancing Anammox reactor (ABR_U) was established and irradiated once a week by ultrasound with the optimal parameter (frequency of 25.0 kHz, intensity of 1.00 W cm^-2 and exposure time of 36.0 s) obtained by response surface methodology (RSM). ABR_U and the controlled Anammox reactor (ABR_C) without ultrasonic treatment were operated in parallel. The start-up time of Anammox process in ABR_U (59 d) was shorter than that in ABR_C (69 d). At the end of the nitrogen load-enhancing period, NLR (0.500 kg N m^-3 d^-1) and NRR (0.430 kg N m^-3 d^-1) in ABR_U were both higher than NLR (0.400 kg N m^-3 d^-1) and NRR (0.333 kg N m^-3 d^-1) in ABR_C. The results of RTQ-PCR demonstrated that the specific low-intensity ultrasound irradiation improved the enrichment levels of AnAOB in mature sludge. SEM images and the observation of the macroscopic morphology of mature sludge showed that the ultrasound irradiation strengthened the formation of Anammox granular sludge, thereby improved the interception capacity and impact load resistance of the reactor, and enhanced the nitrogen removal performance in ABR_U. The ultrasonic enhanced Anammox reactor based on an ABR with the optimal parameters can promote the rapid start-up and efficient and stable operation of the Anammox process at normal temperature (around 25.0 °C).

Effect of low-intensity ultrasound on partial nitrification: Performance, sludge characteristics, and properties of extracellular polymeric substances

Ultrasound technology, which is environment-friendly and economical, has emerged as a novel strategy that can be used to enhance the partial nitrification process. However, its effect on this process remains unclear. Therefore, in this study, partial nitrification sludge was subjected to low-intensity (0.15 W/mL) ultrasound treatment for 10 min, and the effect of ultrasonic treatment on the partial nitrification process was evaluated based on changes in reactor performance, sludge characteristics, and the properties of extracellular polymeric substances (EPS). The results obtained showed that the ultrasonic treatment enhanced nitrite accumulation performance as well as the activity of ammonia-oxidizing bacteria from 3.3 to 16.6 mg O₂/g VSS,⋅while inhibiting the activity of nitrite-oxidizing bacteria. Further analysis showed that owing to the ultrasonic treatment, there was an increase in EPS contents. Particularly, there was a significant increase in loosely bound polysaccharide (PS) contents, indicating the occurrence of intracellular PS anabolics as well as PS secretion. Additionally, ultrasonic treatment induced a significant increase in carbonyl, hydroxyl, and amine functional group contents, and EPS analysis results revealed that it had a positive effect on mass transfer efficiency; thus, it enhanced the partial nitrification process. Overall, this study describes the effect of intermittent low-intensity ultrasound on the partial nitrification process as well as the associated enhancement mechanism.

Sequencing batch reactor technology for landfill leachate treatment: A state-of-the-art review

Landfill has become an underlying source of surface and groundwater pollution if not efficiently managed, due to the risk of leachate infiltration into to land and aquifers. The generated leachate is considered a serious environmental threat for the public health, because of the toxic and recalcitrant nature of its constituents. Thus, it must be collected and appropriately treated before being discharged into the environment. At present, there is no single unit process available for proper leachate treatment as conventional wastewater treatment processes cannot achieve a satisfactory level for degrading toxic substances present. Therefore, there is a growing interest in examination of different leachate treatment processes for maximum operational flexibility. Based on leachate characteristics, discharge requirements, technical possibilities, regulatory requirements and financial considerations, several techniques have been applied for its degradation, presenting varying degrees of efficiency. Therefore, this article presents a comprehensive review of existing research articles on the pros and cons of various leachate degradation methods. In line with environmental sustainability, the article stressed on the application and efficiency of sequencing batch reactor (SBR) system treating landfill leachate due to its operational flexibility, resistance to shock loads and high biomass retention. Contributions of integrated leachate treatment technologies with SBR were also discussed. The article further analyzed the effect of different adopted materials, processes, strategies and configurations on leachate treatment. Environmental and operational parameters that affect SBR system were critically discussed. It is believed that information contained in this review will increase readers fundamental knowledge, guide future researchers and be incorporated into future works on experimentally-based SBR studies for leachate treatment.

Effects of low-intensity ultrasound on nitrite accumulation and microbial characteristics during partial nitrification

Ultrasound technology has attracted increasing attention in the field of sewage sludge treatment. This study investigated the nitrite accumulation ratio (NAR) and microbial characteristics of the partial nitrification (PN) process in a sequencing batch reactor employing ultrasonic treatment (ultrasound density = 0.25 W/mL, irradiation time = 10 min). PN was achieved over 73 days with a NAR above 85% under ambient temperatures. A low dissolved oxygen (DO) environment was generated in the reactor by enhancing the oxygen utilization rate of ammonia-oxidizing bacteria (AOB). Additionally, the application of long-term ultrasonic treatment led to the enhancement of the dominance of the Nitrosomonas genus of AOB, while populations of the Nitrospira genus of nitrite-oxidizing bacteria (NOB) were eradicated. At the same time, the activities of the aerobic denitrifying bacteria Thauera, Terrimonas, Defluviimonas, and Thermomonas were enhanced and their relative abundance was increased. Overall, the results suggest that ultrasonic treatment can enhance AOB activity and generate a low DO environment that facilitates effective PN.

Effects of ultrasonic treatment on the ammonia-oxidizing bacterial (AOB) growth kinetics

Ultrasound has in the past few decades found applications in a variety of disciplines including chemistry, medicine, physics, and to a much less extent microbiology. Our previous studies found that ultrasonic treatment increases the activity of ammonia-oxidizing bacteria (AOB) while suppressing nitrite-oxidizing bacteria (NOB), resulting in beneficial effects in wastewater treatment. In this study, the kinetic and microbiological features of nitrifying microorganisms in activated sludge intermittently treated with ultrasound were investigated to gain an improved understanding of the mechanism involved in ultrasound-induced stimulation of AOB kinetics. The nitrifying microorganisms were initially enriched over 100 days in a laboratory sequential batch reactor (SBR). Ultrasonic treatment of the sludge was then applied with the treatment time in each 12 h SBR cycle progressively increased from 4 to 24 min. Application of the treatment for 21 days led to a doubled maximum specific ammonia oxidation rate, and also the enhanced dominance of known AOB Nitrosomonas genus in the biomass. This stimulatory effect is well described by a modified enzyme catalyzed reaction model, showing a good linear relationship between the natural logarithm value of μ_max,AOB and the applied ultrasonic energy density. This result suggests that ultrasonic treatment likely reduced the activation energy of key enzymes involved in ammonium oxidation.

Achieving mainstream nitrogen removal via the nitrite pathway from real municipal wastewater using intermittent ultrasonic treatment

Achieving mainstream nitrogen removal via the nitrite pathway (NH₄⁺ → NO₂^- → N₂) is highly beneficial for energy neutral/positive wastewater treatment. Our previous batch assays revealed that ultrasonic treatment can suppress nitrite-oxidizing bacteria (NOB) while enhancing the activity of ammonia-oxidizing bacteria (AOB). Based on this concept, this study investigated the feasibility of applying ultrasonication to achieve the nitrite pathway in mainstream wastewater treatment. Two lab-scale sequencing batch reactors were set-up in parallel and fed with real municipal wastewater. With 100% of the sludge treated every 12 h at a treatment energy input of 0.066 kJ per mg mixed liquor suspended solids, the nitrite pathway was rapidly (within two weeks) established in the experimental reactor with stable effluent nitrite accumulation ratio (NO₂^-/(NO₂^- + NO₃^-)) of above 80% and significantly decreased NOB population. In comparison, the control reactor always possessed the conventional nitrification and denitrification pathway. Economic analysis indicated that energy consumption is too high for practical applications. However, this technology may be used in conjunction with other technologies, whereby this ultrasonic treatment can be used infrequently (e.g. once every few months) when the nitrite pathway becomes unstable.

Evaluating the excess sludge reduction in activated sludge system with ultrasonic treatment

Ultrasonic treatment for enhancing biological processes has recently attracted considerable attention in wastewater treatment. In this study, we systematically investigated the mixed liquor properties of activated sludge under ultrasonic treatment. The sludge samples were collected from the aerobic tank of a full-scale membrane bioreactor (MBR) treating municipal wastewater, and the volatile suspended solids (VSS) concentration was approximately 6.0 g/L. The results showed that ultrasonic treatment induced floc disintegration, organics release, temperature increase, microbial activity and pH variation. The maximum mg soluble chemical oxygen demand (COD) per mg VSS released was estimated to be 0.147 using the Monod equation. The exponential increase in the concentration of dissolved organic matter is related to the loss of relative heterotrophic bacterial activity. A sonolysis-cryptic growth model was demonstrated to be capable of describing ultrasonic sludge reduction, which would support the further development of ultrasonic treatment technology in activated sludge systems.

Toxicity evaluation of wastewater collected at different treatment stages from a pharmaceutical industrial park wastewater treatment plant

The toxicity of water-receiving bodies, the effluent and other treatment stages in wastewater treatment plants has recently been of interest to the public due to the lack of a regulated toxicity-based index for wastewater discharge in China. This study aimed to evaluate the conventional pollution parameters and toxicities of wastewaters collected at different treatment stages from a pharmaceutical industrial park wastewater treatment plant through dehydrogenase activity (DHA) and bioluminescent bacteria (Vibrio qinghaiensis) tests. The results of an analysis of conventional parameters indicated that the total suspended solids (TSS), chemical oxygen demand (COD), total nitrogen (TN), ammonia nitrogen (NH3N), and total phosphorus (TP) were largely removed after various treatments. However, the TN, NH3N and COD still exceeded the regulated standards. The tested pharmaceutical park effluents were mainly polluted with organic pollutants and nitrogenous. The toxicity test results indicated that the toxicities could be markedly reduced after treatment, with the toxicities of two out of the six effluent samples at different treatment stages being greater than the influent toxicity. Spearman's rank correlation coefficients indicated a significantly positive correlation between the toxicity values obtained using the DHA and Vibrio qinghaiensis tests. Compared with the DHA measurement, the Vibrio qinghaiensis test was faster and more sensitive. Meanwhile, the toxicity indicators were significantly and positively correlated with the TSS, TN, TP and COD concentrations. These results may aid the understanding of the toxicity of pharmaceutical industrial park wastewaters and toxicity removal using the treatment techniques that are currently utilized in China.

Ultrasonic Treatment Enhanced Ammonia-Oxidizing Bacterial (AOB) Activity for Nitritation Process

Oxidation of ammonia to nitrite rather than nitrate is critical for nitritation process for wastewater treatment. We proposed a promising approach by using controlled ultrasonic treatment to enhance the activity of ammonia-oxidizing bacteria (AOB) and suppress that of nitrite-oxidizing bacteria (NOB). Batch activity assays indicated that when ultrasound was applied, AOB activity reached a peak level and then declined but NOB activity deteriorated continuously as the power intensity of ultrasound increased. Kinetic analysis of relative microbial activity versus ultrasonic energy density was performed to investigate the effect of operational factors (power, sludge concentration, and aeration) on AOB and NOB activities and the test parameters were selected for reactor tests. Laboratory sequential batch reactor (SBR) was further used to test the ultrasonic stimulus with 8 h per day operational cycle and synthetic waste urine as influent. With specific ultrasonic energy density of 0.09 kJ/mg VSS and continuously fed influent containing above 200 mg NH3-N/L, high AOB reproductive activity was achieved and nearly complete conversion of ammonia-N to nitrite was maintained. Microbial structure analysis confirmed that the treatment changed community of AOB, NOB, and heterotrophs. Known AOB Nitrosomonas genus remained at similar level in the biomass while typical NOB Nitrospira genus disappeared in the SBR under ultrasonic treatment and after the treatment was off for 30 days.

Detection of nitrifiers and evaluation of partial nitrification for wastewater treatment: A review

Partial nitrification has gained broad interests in the biological nitrogen removal (BNR) from wastewater, since it alleviates carbon limitation issues and acts as a shortcut nitrogen removal system combined with anaerobic ammonium oxidation (Anammox) process. The occurrence and maintenance of partial nitrification relies on various conditions, which favor ammonium oxidizing bacteria (AOB) but inhibit or limit nitrite oxidizing bacteria (NOB). The studies of the AOB and NOB activities have been conducted by state-of-the-art molecular techniques, such as Polymerase Chain Reaction (PCR), Quantitative PCR, denaturing gradient gel electrophoresis (DGGE), Fluorescence in situ hybridization (FISH) technique, Terminal Restriction Fragment Length Polymorphism (T-RFLP), Live/Dead BacLight, and quinone profile. Furthermore, control strategies for obtaining partial nitrification are mainly focused on the pH, temperature, dissolved oxygen concentration, real-time aeration control, sludge retention time, substrate concentration, alternating anoxic and aerobic operation, inhibitor and ultrasonic treatment. Existing problems and further perspectives for the scale-up of partial nitrification are also proposed and suggested.

Cold Temperature Effects on Long-Term Nitrogen Transformation Pathway in a Tidal Flow Constructed Wetland

The present study investigated long-term treatment performance and nitrogen transformation mechanisms in tidal flow constructed wetlands (TFCWs) under 4, 8, and 12 °C temperature regimes. High and stable ammonium (NH4(+)-N) removal efficiency (93-96%) was achieved in our TFCWs, whereas nitrate (NO3(-)-N) was accumulated at different levels under different temperatures. Quantitative response relationships showed anammox/amoA, (narG+napA)/amoA, and (narG+napA)/bacteria were the respective key functional gene groups determining 4, 8, and 12 °C NO3(-)-N reduction. Pathway analysis revealed the contribution of these functional gene groups along a depth gradient. In addition, denitrification process increased, while anammox process decreased consistent with a rise in temperature from 4 to 12 °C. Furthermore, cold temperatures exhibited different effects on anammox and denitrification and their long-term acclimatization capacities changed with temperature.

High rate CNP removal from a milk processing wastewater in a single ultrasound augmented up-flow anaerobic/aerobic/anoxic bioreactor

Simultaneous removal of carbon, nitrogen and phosphorus (CNP) in a single bioreactor is of high significance in terms of reactor volume and energy consumption. Therefore, in this study, an innovative up-flow anaerobic/aerobic/anoxic bioreactor (UAAASB) augmented by ultrasound was developed as a high rate single bioreactor for the simultaneous removal of nutrients from a milk processing wastewater. The ultrasonic irradiation used in this work was in the range of high frequency (1.7 MHz). The central composite design (CCD) and response surface methodology (RSM) were applied to design the experimental conditions, model obtained data, and optimize the process. The effects of three independent variables, i.e. hydraulic retention time (HRT), aeration mode and mixed liquor suspended solid (MLSS) concentration on 10 process responses were investigated. The results prove that the ultrasonic irradiation has a positive effect on the sludge settling velocity and effluent turbidity. The optimum conditions were determined as 12-15 h, 4000-5000 mg/l and 1.5-2 for HRT, MLSS concentration and aeration mode, respectively, based on removal efficiency of sCOD ⩾ 90%, TN and TP ⩾ 50%.

Effects of carbon-nitrogen ratio on nitrogen removal in a sequencing batch reactor enhanced with low-intensity ultrasound

A sequencing batch reactor (SBR) enhanced with low-intensity ultrasound was designed to study the removal of nitrogen under different carbon-nitrogen (C/N) ratios. The results showed that the removal efficiencies of CODCr and nitrogen were inversely proportional to C/N ratios. The CODCr of the effluent in the control reactor (CR) and the low-intensity ultrasound-enhanced reactor (UER) were lower than 40 mg L(-1). With a decrease in C/N ratio, the NH4(+)-N removal load of the CR showed little change, while the NH4(+)-N removal load of UER increased from 21.2 to 56.1mg NH4(+)-N/g mixed liquid suspended solids per day. To further understand effects of low-intensity ultrasound, the denaturing gel gradient electrophoresis (DGGE) analysis showed that the similar coefficients of the community structures in the UER and CR were 86%, 52% and 29% when the C/N ratios were 15:1, 10:1, 5:1, respectively.

Use of low frequency and density ultrasound to stimulate partial nitrification and simultaneous nitrification and denitrification

Low frequency and density ultrasound has attracted considerable attention in enhancing wastewater treatment performance, particularly in the removal of nitrogen. In the present study, two sequencing batch reactors were operated to confirm the effects of ultrasound at the frequency of 40 kHz and density of 0.027 W/mL on partial nitrification and simultaneous nitrification and denitrification (SND). At the optimal irradiation time of 2.0 h, the obtained nitrite accumulation ratio and SND efficiency at full aerobic were 73.9% and 72.8%, respectively. Nitrite accumulation was the result of increased NH4(+)-N removal and improved ammonia oxidizing bacteria (AOB) activity with simultaneous inhibition of nitrite oxidizing bacteria (NOB) activity. Ultrasonic treatment could provide suitable conditions in temperature and pH for AOB growth, and destroy the NOB community structure. Moreover, organic matters were released and offered an additional carbon source for denitrification apart from the negative effects on sludge properties.