Toward Energy Neutrality: Novel Wastewater Treatment Incorporating Acidophilic Ammonia Oxidation.

Chemically enhanced primary treatment (CEPT) followed by partial nitritation and anammox (PN/A) and anaerobic digestion (AD) is a promising roadmap to achieve energy-neutral wastewater treatment. However, the acidification of wastewater caused by ferric hydrolysis in CEPT and how to achieve stable suppression of nitrite-oxidizing bacteria (NOB) in PN/A challenge this paradigm in practice. This study proposes a novel wastewater treatment scheme to overcome these challenges. Results showed that, by dosing FeCl3 at 50 mg Fe/L, the CEPT process removed 61.8% of COD and 90.1% of phosphate and reduced the alkalinity as well. Feeding by low alkalinity wastewater, stable nitrite accumulation was achieved in an aerobic reactor operated at pH 4.35 aided by a novel acid-tolerant ammonium-oxidizing bacteria (AOB), namely, Candidatus Nitrosoglobus. After polishing in a following anoxic reactor (anammox), a satisfactory effluent, containing COD at 41.9 ± 11.2 mg/L, total nitrogen at 5.1 ± 1.8 mg N/L, and phosphate at 0.3 ± 0.2 mg P/L, was achieved. Moreover, the stable performances of this integration were well maintained at an operating temperature of 12 °C, and 10 investigated micropollutants were removed from the wastewater. An energy balance assessment indicated that the integrated system could achieve energy self-sufficiency in domestic wastewater treatment.

Novel Use of a Ferric Salt to Enhance Mainstream Nitrogen Removal from Anaerobically Pretreated Wastewater.

This study aims to demonstrate a new technology roadmap to support the ongoing paradigm shift in wastewater management from pollutant removal to resource recovery. This is achieved by developing a novel use of an iron salt (i.e., FeCl3) in an integrated anaerobic wastewater treatment and mainstream anammox process. FeCl3 was chosen to be dosed in a proposed sidestream unit rather than in a primary settler or a mainstream reactor. This causes acidification of returned activated sludge and enables stable suppression of nitrite-oxidizing bacterial activity and excess sludge reduction. A laboratory-scale system, which comprised an anaerobic baffled reactor, a continuous-flow anoxic-aerobic (A/O) reactor, and a secondary settler, was designed to treat real domestic wastewater, with the performance of the system comprehensively monitored under a steady-state condition. The experimental assessments showed that the system had good effluent quality, with total nitrogen and phosphorus concentrations of 12.6 ± 1.3 mg N/L and 0.34 ± 0.05 mg P/L, respectively. It efficiently retained phosphorus in excess sludge (0.18 ± 0.03 g P/g dry sludge), suggesting its potential for further recovery. About half of influent organic carbon was recovered in the form of bioenergy (i.e., methane). This together with low energy consumption revealed that the system could produce a net energy of about 0.11 kWh/m3-wastewater, assessed by an energy balance analysis.

Determining Factors for Nitrite Accumulation in an Acidic Nitrifying System: Influent Ammonium Concentration, Operational pH, and Ammonia-Oxidizing Community.

Acidic nitrification is attracting wide attention because it can enable robust suppression of nitrite-oxidizing bacteria (NOB) in wastewater treatment. This study reports a comprehensive assessment of the novel acidic nitrification process to identify the key factors that govern stable nitrite accumulation. A laboratory-scale moving-bed biofilm reactor receiving low-alkalinity wastewater was continuously operated under acidic conditions (pH < 6) for around two years, including nine stages varying influent and operational conditions. The results revealed that nitrite accumulation was related to three factors, i.e., influent ammonium concentration, operating pH, and ammonia-oxidizing microbial community. These three factors impact nitrite accumulation by altering the in situ concentration of free nitrous acid (FNA), which is a potent inhibitor of NOB. The critical FNA concentration is approximately one part per million (ppm, ∼1 mg HNO2-N/L), above which nitrite accumulation is stably maintained in an acidic nitrifying system. The findings of this study suggest that stable nitrite accumulation via acidic ammonia oxidation can be maintained under a range of influent and operational conditions, as long as a ppm-level of FNA is established. Taking low-strength mainstream wastewater (40-50 mg NH4+-N/L) with limited alkalinity as an example, stable nitrite accumulation was experimentally demonstrated at a pH of 4.35, under which an in situ FNA of 2.3 ± 0.6 mg HNO2-N/L was attained. Under these conditions, Candidatus Nitrosoglobus became the only ammonia oxidizer detectable by 16S rRNA gene sequencing. The results of this study deepen our understanding of acidic nitrifying systems, informing further development of novel wastewater treatment technologies.

Characterizing and comparing microbial community and biofilm structure in three nitrifying moving bed biofilm reactors.

This study investigated biofilm establishment, biofilm structure, and microbial community composition of biofilms in three laboratory-scale moving bed biofilm reactors. These reactors were filled with three types of plastic carriers with varied depths of living space for microbial growth. The reactors were operated under the same influent and operational conditions. Along with the operation, the results showed that carriers with grids of 50 µm in height delayed the biofilm development and formed the thinnest biofilm and a carpet-like structure with the lowest α-diversity. In comparison, another two carriers with grids of 200 and 400 µm in height formed thick biofilms and large colonies with more voids and channels. Quantified properties of biofilm thickness, biomass, heterogeneity, portion of the biofilm exposed to the nutrient, and maximum diffusion distance were examined, and the results demonstrated that they almost (except for heterogeneity) strongly correlated to the α-diversity of microbial community. These illustrate that depth of living space, as an important parameter for carrier, could drive the formation of biofilm structure and community composition. It improves understanding of influencing factors on biofilm establishment, structure and its microbial community, and would be helpful for the design of biofilm processes.

Robust Nitritation Sustained by Acid-Tolerant Ammonia-Oxidizing Bacteria.

Oxidation of ammonium to nitrite rather than nitrate, i.e., nitritation, is critical for autotrophic nitrogen removal. This study demonstrates a robust nitritation process in treating low-strength wastewater, obtained from a mixture of real mainstream sewage with sidestream anaerobic digestion liquor. This is achieved through cultivating acid-tolerant ammonia-oxidizing bacteria (AOB) in a laboratory nitrifying bioreactor at pH 4.5-5.0. It was shown that nitrite accumulation with a high NO2-/(NO2- + NO3-) ratio of 95 ± 5% was stably maintained for more than 300 days, and the obtained volumetric NH4+ removal rate (i.e., 188 ± 14 mg N L-1 d-1) was practically useful. 16S rRNA gene sequencing analyses indicated the dominance of new AOB, "Candidatus Nitrosoglobus," in the nitrifying guild (i.e., 1.90 ± 0.08% in the total community), with the disappearance of typical activated sludge nitrifying microorganisms, including Nitrosomonas, Nitrospira, and Nitrobacter. This is the first identification of Ca. Nitrosoglobus as key ammonia oxidizers in a wastewater treatment system. It was found that Ca. Nitrosoglobus can tolerate low pH (<5.0), and free nitrous acid (FNA) at levels that inhibit AOB and nitrite-oxidizing bacteria (NOB) commonly found in wastewater treatment processes. The in situ inhibition of NOB leads to accumulation of nitrite (NO2-), which along with protons (H+) also produced in ammonium oxidation generates and sustains FNA at 3.0 ± 1.4 mg HNO2-N L-1. As such, robust PN was achieved under acidic conditions, with a complete absence of NOB. Compared to previous nitritation systems, this acidic nitritation process is featured by a higher nitric oxide (NO) but a lower nitrous oxide (N2O) emission level, with the emission factors estimated at 1.57 ± 0.08 and 0.57 ± 0.03%, respectively, of influent ammonium nitrogen load.

An overlooked nanofluids effect from Fe3O4 nanoparticles enhances mass transfer in anammox granular sludge.

Magnetite (Fe3O4) particles have been widely reported to enhance the anammox's activity in anammox granular sludge (AnGS), yet the underlying mechanisms remain unclear. This study demonstrates that both Fe3O4 microparticles (MPs) and nanoparticles (NPs) at a dosage of 200 mg Fe3O4/L significantly increased the specific anammox activity (SAA) of AnGS. Additionally, the transcriptional activities of the hzs and hdh genes involved in the anammox process, as well as the heme c content in AnGS, were also notably enhanced. Notably, Fe3O4 NPs were more effective than MPs in boosting anammox activity within AnGS. Mechanistically, Fe3O4 MPs released free iron, which anammox bacteria utilized to promote the synthesis of key enzymes, thereby enhancing their activity. Compared to MPs, Fe3O4 NPs not only elevated the synthesis of these key enzymes to a higher level but also induced a nanofluids effect on the surface of AnGS, improving substrate permeability and accessibility to intragranular anammox bacteria. Moreover, the nanofluids effect was identified as the primary mechanism through which Fe3O4 NPs enhanced anammox activity within AnGS. These findings provide new insights into the effects of nanoparticles on granular sludge systems, extending beyond AnGS.

Quadrupling the capacity of post aerobic digestion treating anaerobically digested sludge using a moving-bed biofilm (MBBR) configuration.

Wastewater treatment plants produce large amounts of sludge requiring stabilization before safe disposal. Traditional biological stabilization approaches are cost-effective but generally require either an extended retention time (10-40 days), or elevated temperatures (40-80 °C) for effective pathogens inactivation. This study overcomes these limitations via a novel acidic aerobic digestion process, leveraging an acid-tolerant ammonia-oxidizing bacterium (AOB) Candidatus Nitrosoglobus. To retain this novel but slowly growing AOB, we proposed the first-ever application of a classical wastewater configuration-moving bed biofilm reactor (MBBR)-for sludge treatment. The AOB in biofilm maintains acidic pH and high nitrite levels in sludge, generating free nitrous acid in situ to expedite sludge stabilization. This process was tested in two laboratory-scale aerobic digesters processing full-scale anaerobically digested sludge. At an ambient temperature of 20 °C, pathogens were reduced to levels well below the threshold specified for the highest stabilization level (Class A), within a retention time of 3.5 days. A high volatile solids reduction of 27.4 ± 5.2% was achieved. Through drastically accelerating stabilization and enhancing reduction, this process substantially saves capital and operational costs for sludge disposal.

An integrated approach to vivianite recovery from waste activated sludge.

The waste activated sludge (WAS) of wastewater treatment system is often rich in phosphorus (P), which is a basic element of human life and could use up in the near future. This study proposed an integrated approach to efficiently recover P as vivianite from WAS and simultaneously enhance the sludge dewaterability. The raw WAS was first acidified using FeCl3, which was then fed to anaerobic fermenter for Fe3+ reduction. After fermentation, a technology named acid-elutriation was introduced to convert Fe and P from solid phase to liquid phase and concomitantly enhance the liquor-solid separation. Finally, vivianite was obtained via sludge eluate neutralization. The enhanced sludge dewaterability not only increases the recovery efficiency of Fe and P but also decreases the cost of sludge disposal.

Adaptation of anammox process for nitrogen removal from acidic nitritation effluent in a low pH moving bed biofilm reactor.

Acidic partial nitritation (PN) has emerged to be a promisingly stable process in wastewater treatment, which can simultaneously achieve nitrite accumulation and about half of ammonium reduction. However, directly applying anaerobic ammonium oxidation (anammox) process to treat the acidic PN effluent (pH 4-5) is susceptible to the inhibition of anammox bacteria. Here, this study demonstrated the adaptation of anammox process to acidic pH in a moving bed biofilm reactor (MBBR). By feeding the laboratory-scale MBBR with acidic PN effluent (pH = 4.6 ± 0.2), the pH of an anammox reactor was self-sustained in the range of pH 5 - 6. Yet, a high total nitrogen removal efficiency of over 80% at a practical loading rate of up to 149.7 ± 3.9 mg N/L/d was achieved. Comprehensive microbial assessment, including amplicon sequencing, metagenomics, cryosection-FISH, and qPCR, identified that Candidatus Brocadia, close to known neutrophilic members, was the dominant anammox bacteria. Anammox bacteria were found present in the inner layer of thick biofilms but barely present in the surface layer of thick biofilms and in thin biofilms. Results from batch tests also showed that the activity of anammox biofilms could be maintained when subjected to pH 5 at a nitrite concentration of 10 mg N/L, whereas the activity was completely inhibited after disturbing the biofilm structure. These results collectively indicate that the anammox bacteria enriched in the present acidic MBBR could not be inherently acid-tolerant. Instead, the achieved stable anammox performance under the acidic condition is likely due to biofilm stratification and protection. This result highlights the biofilm configuration as a useful solution to address nitrogen removal from acidic PN effluent, and also suggests that biofilm may play a critical role in protecting anammox bacteria found in many acidic nature environments.

Multifaceted benefits of magnesium hydroxide dosing in sewer systems: Impacts on downstream wastewater treatment processes.

Magnesium hydroxide [Mg(OH)2] is a non-hazardous chemical widely applied in sewer systems for managing odour and corrosion. Despite its proven effectiveness in mitigating these issues, the impacts of dosing Mg(OH)2 in sewers on downstream wastewater treatment plants have not been comprehensively investigated. Through a one-year operation of laboratory-scale urban wastewater systems, including sewer reactors, sequencing batch reactors, and anaerobic sludge digesters, the findings indicated that Mg(OH)2 dosing in sewer systems had multifaceted benefits on downstream treatment processes. Compared to the control, the Mg(OH)2-dosed experimental system displayed elevated sewage pH (8.8±0.1vs 7.1±0.1), reduced sulfide concentration by 35.1%±4.9% (6.7±0.9mgSL-1), and lower methane concentration by 58.0%±4.9% (19.1±3.6mgCODL-1). Additionally, it increased alkalinity by 16.3%±2.2% (51.9±5.4mgCaCO3L-1), and volatile fatty acids concentration by 207.4%±22.2% (56.6±9.0mgCODL-1) in sewer effluent. While these changes offered limited advantages for downstream nitrogen removal in systems with sufficient alkalinity and carbon sources, significant improvements in ammonium oxidation rate and NOx reduction rate were observed in cases with limited alkalinity and carbon sources availability. Moreover, Mg(OH)2 dosing in upstream did not have any detrimental effects on anaerobic sludge digesters. Magnesium-phosphate precipitation led to a 31.7%±4.1% reduction in phosphate concertation in anaerobic digester sludge supernatant (56.1±10.4mgPL-1). The retention of magnesium in sludge increased settleability by 13.9%±1.6% and improved digested sludge dewaterability by 10.7%±5.3%. Consequently, the use of Mg(OH)2 dosing in sewers could potentially reduce downstream chemical demand and costs for carbon sources (e.g., acetate), pH adjustment and sludge dewatering.

Integrated urban water management by coupling iron salt production and application with biogas upgrading.

Integrated urban water management is a well-accepted concept for managing urban water. It requires efficient and integrated technological solutions that enable system-wide gains via a whole-of-system approach. Here, we create a solid link between the manufacturing of an iron salt, its application in an urban water system, and high-quality bioenergy recovery from wastewater. An iron-oxidising electrochemical cell is used to remove CO2 (also H2S and NH3) from biogas, thus achieving biogas upgrading, and simultaneously producing FeCO3. The subsequent dose of the electrochemically produced FeCO3 to wastewater and sludge removes sulfide and phosphate, and enhances sludge settleability and dewaterability, with comparable or superior performance compared to the imported and hazardous iron salts it substitutes (FeCl2, and FeCl3). The process enables water utilities to establish a self-reliant and more secure supply chain to meet its demand for iron salts, at lower economic and environmental costs, and simultaneously achieve recovery of high-quality bioenergy.

Impact of electrochemically generated iron on the performance of an anaerobic wastewater treatment process.

Anaerobic treatment of domestic wastewater has the advantages of lower biomass yield, lower energy demand and higher energy recover over the conventional aerobic treatment process. However, the anaerobic process has the inherent issues of excessive phosphate and sulfide in effluent and superfluous H2S and CO2 in biogas. An electrochemical method allowing for in-situ generation of Fe2+ in the anode and hydroxide ion (OH-) and H2 in the cathode was proposed to overcome the challenges simultaneously. The effect of electrochemically generated iron (e­iron) on the performance of anaerobic wastewater treatment process was explored with four different dosages in this work. The results showed that compared to control, the experimental system displayed an increase of 13.4-28.4 % in COD removal efficiency, 12.0-21.3 % in CH4 production rate, 79.8-98.5 % in dissolved sulfide reduction, 26.0-96.0 % in phosphate removal efficiency, depending on the e­iron dosage between 40 and 200 mg Fe/L. Dosing of the e­iron significantly upgraded the quality of produced biogas, showing a much lower CO2 and H2S contents in biogas in experimental reactor than that in control reactor. The results thus demonstrated that e­iron can significantly improve the performance of anaerobic wastewater treatment process, bringing multiple benefits with the increase of its dosage regarding effluent and biogas quality.

One-year stable pilot-scale operation demonstrates high flexibility of mainstream anammox application.

Mainstream nitrogen removal via anammox is widely recognized as a promising wastewater treatment process. However, its application is challenging at large scale due to unstable suppression of nitrite-oxidizing bacteria (NOB). In this study, a pilot-scale mainstream anammox process was implemented in an Integrated Fixed-film Activated Sludge (IFAS) configuration. Stable operation with robust NOB suppression was maintained for over one year. This was achieved through integration of three key control strategies: i) low dissolved oxygen (DO = 0.4 ± 0.2 mg O2/L), ii) regular free nitrous acid (FNA)-based sludge treatment, and iii) residual ammonium concentration control (NH4 + with a setpoint of ∼8 mg N/L). Activity tests and FISH demonstrated that NOB barely survived in sludge flocs and were inhibited in biofilms. Despite receiving organic-deficient wastewater from a pilot-scale High-Rate Activated Sludge (HRAS) system as the feed, the system maintained a stable effluent total nitrogen concentration mostly below 10 mg N/L, which was attributed to the successful retention of anammox bacteria. This study successfully demonstrated large-scale long-term mainstream anammox application and generated new practical knowledge for NOB control and anammox retention.

Electrochemical iron production to enhance anaerobic membrane treatment of wastewater.

Although iron salts such as iron(III) chloride (FeCl3) have widespread application in wastewater treatment, safety concerns limit their use, due to the corrosive nature of concentrated solutions. This study demonstrates that local, electrochemical generation of iron is a viable alternative to the use of iron salts. Three laboratory systems with anaerobic membrane processes were set up to treat real wastewater; two systems used the production of either in-situ or ex-situ electrochemical iron (as Fe2+ and Fe2+(Fe3+)2O4, respectively), while the other system served as a control. These systems were operated for over one year to assess the impact of electrochemically produced iron on system performance. The results showed that dosing of electrochemical iron significantly reduced sulfide concentration in effluent and hydrogen sulfide content in biogas, and mitigated organics-based membrane fouling, all of which are critical issues inherently related to sustainability of anaerobic wastewater treatment. The electrochemical iron strategy can generate multiple benefits for wastewater management including increased removal efficiencies for total and volatile suspended solids, chemical oxygen demand and phosphorus. The rate of methane production also increased with electrochemically produced iron. Economic analysis revealed the viability of electrochemical iron with total cost reduced by one quarter to a third compared with using FeCl3. These benefits indicate that electrochemical iron dosing can greatly enhance the overall operation and performance of anaerobic membrane processes, and this particularly facilitates wastewater management in a decentralized scenario.

Feasibility of methane bioconversion to methanol by acid-tolerant ammonia-oxidizing bacteria.

Bioconversion of biogas to value-added liquids has received increasing attention over the years. However, many biological processes are restricted under acidic conditions owing to the excessive carbon dioxide (CO2, 30-40% v/v) in biogas. Here, using an enriched culture dominated by acid-tolerant ammonia-oxidizing bacteria (AOB) 'Candidatus Nitrosoglobus', this study examined the feasibility of producing methanol from methane in the CO2-acidified environment (i.e. pH of 5.0). Within the tested dissolved methane range (0.1-0.9 mM), methane oxidation by the acid-tolerant AOB culture followed first-order kinetics, with the same rate constant (i.e. 0.43 (L/(g VSS‧h)) between pH 7.0 and 5.0. The acidic methane oxidation showed robustness against high dissolved concentrations of CO2 (up to 4.06 mM) and hydrogen sulfide (H2S up to 0.11 mM), which led to a high methanol yield of about 30-40%. As such, the raw biogas containing toxic CO2 and H2S can directly serve for methanol production by this acid-tolerant AOB culture, economizing a conventionally costly biogas upgradation process. Afterwards, two batch reactors fed with methane and oxygen intermittently both obtained a final concentration of 1.5 mM CH3OH (equal to 72 mg chemical oxygen demand/L) in the liquid, suggesting it is a useful carbon source to enhance denitrification in wastewater treatment systems. In addition, ammonia availability was identified to be critical for a higher rate of this AOB-mediated methanol production. Overall, our results for the first time demonstrated the capability of a novel acid-tolerant AOB culture to oxidize methane, and also illustrated the technical feasibility to utilize raw biogas for methanol production at acidic conditions.

Inactivation kinetics of nitrite-oxidizing bacteria by free nitrous acid.

Recent studies have shown that free nitrous acid (FNA, i.e., HNO2) is biocidal to many microorganisms, promoting the development of FNA-based technology in biological wastewater treatment. Suppression of nitrite-oxidizing bacteria (NOB) is a critical step for autotrophic nitrogen removal via anammox. In this study, the biocidal effect of FNA on NOB was determined by developing a model methodology combined with NOB incubation. Sixteen groups of FNA exposure tests were conducted at five different FNA concentrations from 0 to 4 mg HNO2-N/L, obtained from three pH values (5.0, 5.5 and 6.0) with nitrite ranged from 21 to 1680 mg NO2--N/L, with one as a control. Nitrate production curves were tracked during incubations of the FNA-exposed sludge, and then used to estimate active NOB concentrations by the kinetic model-based fitting. The results showed that with 24-hour exposure to FNA at a level of over 1 mg HNO2-N/L, the active NOB decreased around two orders of magnitude compared with that in the primordial sludge. The Weibull model can well describe the biocidal effect, which would be useful for the optimization of FNA conditions. The maximum NOB growth rate was increased after FNA exposure. This result suggests that long-term implementation of FNA-based technology can select fast-growing NOB in activated sludge, causing a 'NOB adaptation' issue.

Unravelling adaptation of nitrite-oxidizing bacteria in mainstream PN/A process: Mechanisms and counter-strategies.

Stable suppression of nitrite-oxidizing bacteria (NOB) is still a major challenge for the implementation of partial nitritation and anammox (PN/A) in mainstream treatment. Despite numerous suppression strategies demonstrated, it is increasingly recognized that NOB could develop resistance to these strategies, threatening the long-term stability of the mainstream PN/A process. This study aims to understand adaption mechanisms and develop counter-strategies to overcome the adaptation. To this end, three previously-demonstrated suppression strategies, including NOB inactivation via side stream sludge treatment with free ammonia (FA), the use of low dissolved oxygen (DO), and the use of anammox to scavenge nitrite, were stepwise applied, over a period of 800 days, to a laboratory-scale reactor treating effluent from a high-rate activated sludge (HRAS) plant. FA sludge treatment alone sustained nitrite accumulation for about two months, after which NOB adaptation occurred causing PN to fail. The FA adaptation was induced by a shift in the NOB community from Nitrospira to Ca. Nitrotoga. The latter was found to have higher resistance to FA and also a higher maximum specific growth rate. Low DO at 0.2-0.4 mg O2 L-1 was then applied, in conjunction with FA treatment, which successfully eliminated Ca. Nitrotoga and re-established PN. However, new and unidentified NOB with a higher apparent oxygen affinity emerged in three months, again leading to PN failure. Lastly, as the third strategy for NOB suppression, anammox was introduced as an in-situ nitrite-scavenger. The combo-strategy delivered reliable NOB suppression with no further adaptation in the remaining experimental period (eight months). The resulted one-stage PN/A reactor achieved a nitrogen removal efficiency of 84.2 ± 5.37%. A control reactor, operated in parallel under the same conditions but without FA treatment, only achieved 10.4 ± 4.6% nitrogen removal, with anammox completely outcompeted by NOB in the last phase.

Centralized iron-dosing into returned sludge brings multifaceted benefits to wastewater management.

Iron salts (i.e. FeCl3) are the most used chemicals in the urban wastewater system. Iron is commonly dosed into sewage or the mainstream system, which provides multiple benefits such as enhanced phosphorus removal and improved sludge settleability/dewaterability. This study reported and demonstrated a new approach that dosed FeCl3 into returned sludge in order to bring two more benefits to wastewater management: short-cut nitrogen removal via the nitrite pathway and less biomass production. This approach is achieved based on our findings that with similar amount of FeCl3, centralized iron dosing into a sidestream sludge unit generated iron concentration two orders of magnitude higher than the common mainstream dosing (e.g. 10-40 mg Fe/L-wastewater), leading to sludge acidification (pH = 2.1) with Fe (III) hydrolysis. Together with accumulated nitrite in the supernatant of the sludge, ppm-level of free nitrous acid was generated and thus enabled sludge disintegration, cell lysis, and selective elimination of nitrite-oxidizing bacteria (NOB). Long-term effects on nitrifying bacteria and overall reactor performance were evaluated using two laboratory reactor experiments for over one year. The experimental reactor showed stable nitrite accumulation with an average NO2-/(NO2- + NO3-) ratio above 80% and ∼30% observed biomass yield reduction compared to those in control reactors. In addition, the centralized sludge dosing strategy still provided benefits such as improved settleability and dewaterability of sludge and enhanced phosphorus removal.

An integrated strategy to enhance performance of anaerobic digestion of waste activated sludge.

Anaerobic digestion (AD) is an essential process in wastewater treatment plants as it can reduce the amount of waste activated sludge (WAS) for disposal, and also enables the recovery of bioenergy (i.e. methane). Here, a new pretreatment method to enhance anaerobic digestion was achieved by treating thickened WAS (TWAS) with ferric (as FeCl3) and nitrite simultaneously for 24-hour at room temperature. Biochemical methane potential tests showed markedly improved degradability in the pretreated TWAS, with a relative increase in hydrolysis rate by 30%. A comparative experiment with the operation of two continuous-flow anaerobic digesters further demonstrated the improvement in biogas quantity and quality, digestate disposal, and phosphorus recovery in the experimental digester. The dosed FeCl3 (i.e. ~6 mM) decreased the pH of TWAS to ~5, which led to the formation of free nitrous acid (FNA, HNO2) at parts per million levels (i.e. ~6 mg N/L), after dosing nitrite at 250 mg NO2--N/L. This FNA treatment caused a 26% increase in methane yield and volatile solids destruction, 55% reduction in the viscosity of sludge in digester, and 24% less polymer required in further digestate dewatering. In addition, the dosed Fe(III) was reduced to Fe(II) which precipitated sulfide and phosphorus, leading to decreased hydrogen sulfide concentration in biogas, and increased percentage of vivianite in the total crystalline iron species in digested sludge. Our study experimentally demonstrated that combined dosing of FeCl3 and nitrite is a useful pretreatment strategy for improving anaerobic digestion of WAS.

The acute effects of erythromycin and oxytetracycline on enhanced biological phosphorus removal system: shift in bacterial community structure.

Since extensive application, an increasing amount of antibiotics has been released into wastewater treatment plants. In this study, the enhanced biological phosphorus removal (EBPR) system was fed with synthetic wastewater containing erythromycin (ERY) and oxytetracycline (OTC) for 7 days to evaluate the variations of solution ortho-P (SOP), volatile fatty acid (VFA), poly-bhydroxyalkanoates (PHAs), specific oxygen uptake rater (SOUR), and microbial community in the EBPR system. The obtained results showed that the P-removal efficiency decreased to 0.0%, and at the end of the experiment, only less than 20% of the VFA could be consumed. Besides, the variable processes of P and PHAs were destroyed. Moreover, to better grasp the inhibitory mechanism of antibiotics, microbial community compositions of activated sludge sampled in all reactors were investigated by high-throughput sequencing techniques. Results of comparative and evolutionary analysis revealed that high concentrations (5 and 10 mg/L) of ERY and OTC could seriously shift microbial communities, while combined antibiotics could induce more. Additionally, Accumulibacter and Competibacter were two primary microorganisms at the genus level in the EBPR system. Accumulibacter decreased seriously for exposure to antibiotics, while Competibacter increased in all experimental reactors especially in combined antibiotics reactor.