Modeling the low pH limit of Nitrosomonas eutropha in high-strength nitrogen wastewaters

Comparing the adsorption of micropollutants on activated carbon from anaerobically stored, organics-depleted, and nitrified urine

Separate collection and treatment of urine optimizes nutrient recovery and enhances micropollutant removal from municipal wastewater. One typical urine treatment train includes nutrient recovery in three biological processes: anaerobic storage, followed by aerobic organics degradation concurrently with nitrification. These are usually followed by activated carbon adsorption to remove micropollutants. However, removing micropollutants prior to nitrification would protect nitrifiers from potential inhibition by pharmaceuticals. In addition, combining simplified biological treatment with activated carbon adsorption could offer a cheap and robust process for removing micropollutants where nutrient recovery is not the first priority, as a partial loss of ammonia occurs without nitrification. In this study, we investigated whether activated carbon adsorption could also take place between the three biological treatment steps. We tested the effectiveness of micropollutant removal with activated carbon after each biological treatment step by conducting experiments with anaerobically stored urine, organics-depleted urine, and nitrified urine. The urine solutions were spiked with 19 pharmaceuticals: amisulpride, atenolol, atenolol acid, candesartan, carbamazepine, citalopram, clarithromycin, darunavir, diclofenac, emtricitabine, fexofenadine, hydrochlorothiazide, irbesartan, lidocaine, metoprolol, N4-acetylsulfamethoxazole, sulfamethoxazole, trimethoprim, venlafaxine, and two artificial sweeteners, acesulfame and sucralose. Batch experiments were conducted with powdered activated carbon (PAC) to determine how much activated carbon achieve which degree of micropollutant removal and how organics, pH, and speciation change from ammonium to nitrate influence adsorption. Micropollutant removal was also tested in granular activated carbon (GAC) columns, which is the preferred technology for micropollutant removal from urine. The carbon usage rates (CUR) per person were lower for all urine solutions than for municipal wastewater. The results showed that organics depletion would be needed when micropollutant removal was the sole aim of urine treatment, as the degradation of easily biodegradable organics prevented clogging of GAC columns. However, CUR did hardly improve with organics-depleted urine compared to stored urine. The lowest CUR was achieved with nitrified urine. This resulted from the additional organics removal during nitrification and not the lower pH or the partial conversion of ammonium to nitrate. In addition, we showed that the relative pharmaceutical removal in all solutions was independent of the initial pharmaceutical concentration unless the background organics matrix changed considerably. We conclude that removal of micropollutants in GAC columns from organics-depleted urine can be performed without clogging, but with the drawback of a higher carbon usage compared to removal from nitrified urine.

Complete ammonia oxidation (comammox) at pH 3-4 supports stable production of ammonium nitrate from urine

This study developed a new process that stably produced ammonium nitrate (NH4NO3), an important and commonly used fertilizer, from the source-separated urine by comammox Nitrospira. In the first stage, the complete conversion of ammonium to nitrate was achieved by comammox Nitrospira. In this scenario, the pH was maintained at 6 by adding external alkali, which also provided sufficient alkalinity for full nitrification. In the second stage, the NH4NO3 was produced directly by comammox Nitropsira by converting half of the ammonium in urine into nitrate. In this case, no alkali was added and pH automatically dropped and self-maintained at an extremely acidic level (pH 3-4). In both scenarios, negligible nitrite accumulation was observed, while the final product of the second stage contained ammonium and nitrate at the molar ratio of 1:1. The dominance of comammox Nitrospira over canonical ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) was systematically proved by the combination of 16S rRNA gene amplicon sequencing, quantitative polymerase chain reaction, and metagenomics. Notably, metagenomic sequencing suggested that the relative abundance of comammox Nitrospira was over 20 % under the acidic condition at pH 3-4, while canonical AOB and NOB were undetectable. Batch experiments showed that the optimal pH for the enriched comammox Nitrospira was ∼7, which could sustain their activity in a wider pH range from 4 to 8 surprisingly but lost activity at pH 3 and 9. The findings not only present an application potential of comammox Nitrospira in nitrogen recovery from urine wastewater but also report the survivability of comammox bacteria in acidic environments.

Enhanced ammonia removal in tidal flow constructed wetland by incorporating steel slag: Performance, microbial community, and heavy metal release

Utilizing alkaline solid wastes, such as steel slag, as substrates in tidal flow constructed wetlands (TFCWs) can effectively neutralize the acidity generated by nitrification. However, the impacts of steel slag on microbial communities and the potential risk of heavy metal release remain poorly understood. To address these knowledge gaps, this study compared the performance and microbial community structure of TFCWs filled with a mixture of steel slag and zeolite (TFCW-S) to those filled with zeolite alone (TFCW-Z). TFCW-S exhibited a much higher NH4+-N removal efficiency (98.35 %) than TFCW-Z (55.26 %). Additionally, TFCW-S also achieved better TN and TP removal. The steel slag addition helped maintain the TFCW-S effluent pH at around 7.5, while the TFCW-Z effluent pH varied from 3.74 to 6.25. The nitrification and denitrification intensities in TFCW-S substrates were significantly higher than those in TFCW-Z, consistent with the observed removal performance. Moreover, steel slag did not cause excessive heavy metal release, as the effluent concentrations were below the standard limits. Microbial community analysis revealed that ammonia-oxidizing bacteria, ammonia-oxidizing archaea, and complete ammonia-oxidizing bacteria coexisted in both TFCWs, albeit with different compositions. Furthermore, the enrichment of heterotrophic nitrification-aerobic denitrification bacteria in TFCW-S likely contributed to the high NH4+-N removal. In summary, these findings demonstrate that the combined use of steel slag and zeolite in TFCWs creates favorable pH conditions for ammonia-oxidizing microorganisms, leading to efficient ammonia removal in an environmentally friendly manner.

Dynamic Simulation of Nitrifying Microbial Communities for Establishing Acidic Partial Nitritation in Suspended Activated Sludge

Acidic partial nitritation (PN) is a promising technology to achieve low-cost and energy-efficient shortcut nitrogen removal from wastewater. However, a comprehensive understanding of the acidic PN under dynamic changes of pH in a sequencing batch reactor (SBR) is still lacking. In this study, we successfully established acidic PN (NO2- accumulation ratio >80%) under dynamic pH variation from 7.0 to 4.5 in a lab-scale SBR. By accumulating in situ free nitrous acid (FNA) generation based on the dynamic pH change, acidic PN maintained stability even at a low NH4+ concentration of 100 mg N L-1. The microbial community analysis revealed that two ammonium-oxidizing bacteria (AOB) genera, Nitrosospira and Nitrosomonas, successfully coexisted and cooperated during acidic PN. None of the species of nitrite-oxidizing bacteria (NOB) showed adaptation to intermittent inhibition of in situ FNA even under high DO conditions (>4.0 mg O2 L-1). Furthermore, we innovatively incorporated the classic nitrification model with the growth and decay of different nitrifying bacterial species and their inhibition by pH, FNA, and free ammonia (FA) to predict the nitrifying microbial communities shifting for establishing acidic PN. The extended model was calibrated by using short-term batch experiments and was validated by using long-term dynamic data of the nitrifying microbial community during SBR operation. The validated model was further used to identify feasible influent conditions for the SBR PN process, including influent HCO3- concentration, NH4+ concentration and molar ratio (HCO3/NH4+). Outcomes from this study support the optimal design of acidic PN-based short-cut nitrogen removal processes for future application.

The advantage of a two-stage nitrification method for fertilizer recovery from human urine

Recycling nutrients (nitrogen, phosphorus, and potassium) from human urine can potentially offset more than 13% of global agricultural fertilizer demand. Biological nitrification is a promising method for converting volatile ammonia in high-strength human urine into stable ammonium nitrate (a typical fertilizer), but it is usually terminated in the intermediate production of nitrite due to the inhibition of nitrite-oxidizing bacteria by free nitrous acid (FNA). This study aimed to develop a stable nitrification process in a unique two-stage bioreactor by removing critical barriers associated with FNA inhibition. Experimental results show that half of the ammonium in high-strength urine was successfully converted into nitrate, forming valuable ammonium nitrate (with a nitrogen concentration greater than 1500 mg N/L). The ammonium nitrate solution could retain most phosphorus (75% ± 3%) and potassium (96% ± 1%) in human urine, resulting in nearly full nutrient recovery. Once concentrated, the liquid compound fertilizer of ammonium nitrate was generated. Based on an assessment of economic and environmental impacts at the urban scale, urine diversion for nutrient recovery using a technical combination of nitrification and reverse osmosis could reduce total energy input by 43%, greenhouse gas emission by 40%, and cost by 33% compared to conventional wastewater management. Further research is needed to optimize the two-stage nitrification method on a larger scale.

Comammox Nitrospira and Ammonia-Oxidizing Archaea Are Dominant Ammonia Oxidizers in Sediments of an Acid Mine Lake Containing High Ammonium Concentrations

Exploring nitrifiers in extreme environments is vital to expanding our understanding of nitrogen cycle and microbial diversity. This study presents that complete ammonia oxidation (comammox) Nitrospira, together with acidophilic ammonia-oxidizing archaea (AOA), dominate in the nitrifying guild in sediments of an acid mine lake (AML). The lake water was characterized by acidic pH below 5 with a high ammonium concentration of 175 mg-N/liter, which is rare on the earth. Nitrification was active in sediments with a maximum nitrate production potential of 70.5 μg-N/(g-dry weight [dw] day) for mixed sediments. Quantitative PCR assays determined that in AML sediments, comammox Nitrospira and AOA amoA genes had relative abundances of 52% and 41%, respectively, among the total amoA genes. Further assays with 16S rRNA and amoA gene amplicon sequencing and metagenomics confirmed their dominance and revealed that the comammox Nitrospira found in sediments belonged to comammox Nitrospira clade A.2. Metagenomic binning retrieved a metagenome-assembled genome (MAG) of the comammox Nitrospira from sediments (completeness = 96.76%), and phylogenomic analysis suggested that it was a novel comammox Nitrospira. Comparative genomic investigation revealed that this comammox Nitrospira contained diverse metal resistance genes and an acidophile-affiliated F-type ATPase. Moreover, it had a more diverse genomic characteristic on nitrogen metabolism than the AOA in sediments and canonical AOB did. The results suggest that comammox Nitrospira is a versatile nitrifier that can adapt to acidic environments even with high ammonium concentrations. IMPORTANCE Ammonia-oxidizing archaea (AOA) was previously considered the sole dominant ammonia oxidizer in acidic environments. This study, however, found that complete ammonia oxidation (comammox) Nitrospira was also a dominant ammonia oxidizer in the sediments of an acidic mine lake, which had an acidic pH < 5 and a high ammonium concentration of 175 mg-N/liter. In combination with average nucleotide identity analysis, phylogenomic analysis suggested it is a novel strain of comammox Nitrospira. Moreover, the adaption of comammox Nitrospira to the acidic lake had been comprehensively investigated based on genome-centric metagenomic approaches. The outcomes of this study significantly expand our understanding of the diversity and adaptability of ammonia oxidizers in the acidic environments.

Enhancement of iron-based nitrogen removal with an electric-magnetic field in an upflow microaerobic sludge reactor (UMSR)

Traditional denitrification often produces high operating costs and excessive sludge disposal expenses due to conventional carbon sources. A novel electric-magnetic field (MF) 48 mT with Fe⁰ and C-Fe⁰ powder in an upflow microaerobic sludge reactor (UMSR) improved nitrogen removal from wastewater without organic carbon resources and gave richness to the heterotrophic bacterial community. In the current study, the reactor was operated for 78 ± 2 days, divided into five stages (without Fe⁰, with Fe⁰, coupling with MF, without coupling with MF, and coupling with MF again), at a hydraulic retention time (HRT) of 2.5 h, with an influent loading of ammonium (NH₄⁺-N) 50 ± 2 mg/L, at 25-27 °C, and less than 1.0 mg/L dissolved oxygen (DO). The results demonstrated nitrogen removal efficiency enhanced after coupling with MF on the levels of NO₃^--N by 76% with an effluent concentration of 8.7 mg/L, NH₄⁺-N by 72% with an effluent concentration of 13.6 mg/L, and total nitrogen removal (TN) by 76%, respectively. After coupling the MF with the reactor, the microbial community data analysis showed the dominant abundance of ammonia-oxidizing bacteria, heterotrophic nitrifying bacteria, and denitrifying bacteria on the level of Anaerolineaceae_uncultured 2%, which is capable of denitrification that uses Fe²⁺ as an electron source, Gemmatimonadaceae_uncultured 4%, Hydrogenophaga 4% which is capable of catalyzing hydrogenotrophic denitrification and correlating to nitrate removal, denitrification and desulfurization bacteria SBR1031_norank 18%, anammox-bacteria Saccharimonadales_norank 2%, and (AOM) Limnobacter 3% in the sludge.

Significant production of nitric oxide by aerobic nitrite reduction at acidic pH

The acidic (i.e., pH ∼5) activated sludge process is attracting attention because it enables stable nitrite accumulation and enhances sludge reduction and stabilization, compared to the conventional process at neutral pH. Here, this study examined the production and potential pathways of nitric oxide (NO) and nitrous oxide (N₂O) during acidic sludge digestion. With continuous operation of a laboratory-scale aerobic digester at high dissolved oxygen concentration (DO>4 mg O₂ L^-1) and low pH (4.7±0.6), a significant amount of total nitrogen (TN) loss (i.e., 18.6±1.5% of TN in feed sludge) was detected. Notably, ∼40% of the removed TN was emitted as NO, with ∼8% as N₂O. A series of batch assays were then designed to explain the observed TN loss under aerobic conditions. All assays were conducted with a low concentration of volatile solids (VS), i.e., VS<4.5 g L^-1. This VS concentration is commensurate with the values commonly found in the aeration tanks of full-scale wastewater treatment systems, and thus no significant nitrogen loss should be expected when DO is controlled above 4 mg O₂ L^-1. However, nitrite disappeared at a significant rate (with the chemical decomposition of nitrite excluded), leading to NO production in the batch assays at pH 5. The nitrite reduction could be associated with endogenous microbial activities, e.g., nitrite detoxification. The significant NO production illustrates the importance of aerobic nitrite reduction during acidic aerobic sludge digestion, suggesting this process cannot be neglected in developing acidic activated sludge technology.

Ammonia oxidation by novel "Candidatus Nitrosacidococcus urinae" is sensitive to process disturbances at low pH and to iron limitation at neutral pH

Acid-tolerant ammonia-oxidizing bacteria (AOB) can open the door to new applications, such as partial nitritation at low pH. However, they can also be problematic because chemical nitrite oxidation occurs at low pH, leading to the release of harmful nitrogen oxide gases. In this publication, the role of acid-tolerant AOB in urine treatment was explored. On the one hand, the technical feasibility of ammonia oxidation under acidic conditions for source-separated urine with total nitrogen concentrations up to 3.5 g-N L^-1 was investigated. On the other hand, the abundance and growth of acid-tolerant AOB at more neutral pH was explored. Under acidic conditions (pH of 5), ammonia oxidation rates of 500 mg-N L^-1 d^-1 and 10 g-N g-VSS^-1 d^-1 were observed, despite high concentrations of 15 mg-N L^-1 of the AOB-inhibiting compound nitrous acid and low concentration of 0.04 mg-N L^-1 of the substrate ammonia. However, ammonia oxidation under acidic conditions was very sensitive to process disturbances. Even short periods of less than 12 h without oxygen or without influent resulted in a complete cessation of ammonia oxidation with a recovery time of up to two months, which is a problem for low maintenance applications such as decentralized treatment. Furthermore, undesirable nitrogen losses of about 10% were observed. Under acidic conditions, a novel AOB strain was enriched with a relative abundance of up to 80%, for which the name "Candidatus (Ca.) Nitrosacidococcus urinae" is proposed. While Nitrosacidococcus members were present only to a small extent (0.004%) in urine nitrification reactors operated at pH values between 5.8 and 7, acid-tolerant AOB were always enriched during long periods without influent, resulting in an uncontrolled drop in pH to as low as 2.5. Long-term experiments at different pH values showed that the activity of "Ca. Nitrosacidococcus urinae" decreased strongly at a pH of 7, where they were also outcompeted by the acid-sensitive AOB Nitrosomonas halophila. The experiment results showed that the decreased activity of "Ca. Nitrosacidococcus urinae" correlated with the limited availability of dissolved iron at neutral pH.

Exploring the recovery of potassium-rich struvite after a nitrification-denitrification process in pig slurry treatment

The integration of biological nitrogen (N) removal with struvite-type material recovery, which contained phosphorus (P) and potassium (K), was proved to be technically feasible in pig slurry treatment. Phosphate (PO₄) salts were precipitated by raising the pH-value, using denitrified effluent and waste sludge purged from the bioreactor. When P was limiting, the unbalanced composition of the denitrified effluent resulted in low K-removal efficiency from the liquid phase; 10 % maximum when the initial pH-value was adjusted to 11.5 (93 % PO₄-P recovery). By processing the waste sludge in two steps, using first ethylenediaminetetraacetic acid (EDTA) as an acidifier to release PO₄ while preventing calcium interference, the K-removal efficiency reached 25 % (75 % PO₄-P recovery). When K was limiting, the addition of newberyite particles resulted in the highest K-removal efficiency, up to 90 % (under online pH control to 10.5). Overall, new opportunities are envisaged for producing second-generation fertilizers potentially containing 0-1 % N, 11-17 % P and 6-8 % K.

Melon/cowpea intercropping pattern influenced the N and C soil cycling and the abundance of soil rare bacterial taxa

The high use of pesticides, herbicides, and unsustainable farming practices resulted in losses of soil quality. Sustainable farming practices such as intercropping could be a good alternative to traditional monocrop, especially using legumes such as cowpea (Vigna unguiculata L. Walp). In this study, different melon and cowpea intercropping patterns (melon mixed with cowpea in the same row (MC1); alternating one melon row and one cowpea row (MC2); alternating two melon rows and one cowpea row (MC3)) were assayed to study the intercropping effect on soil bacterial community through 16S rRNA region in a 3-year experiment. The results indicated that intercropping showed high content of total organic carbon, total nitrogen and ammonium, melon yield, and bacterial diversity as well as higher levels of beneficial soil microorganisms such a Pseudomonas, Aeromicrobium, Niastella, or Sphingomonas which can promote plant growth and plant defense against pathogens. Furthermore, intercropping showed a higher rare taxa diversity in two (MC1 and MC2) out of the three intercropping systems. In addition, N-cycling genes such as nirB, nosZ, and amoA were more abundant in MC1 and MC2 whereas the narG predicted gene was far more abundant in the intercropping systems than in the monocrop at the end of the 3-year experiment. This research fills a gap in knowledge about the importance of soil bacteria in an intercropping melon/cowpea pattern, showing the benefits to yield and soil quality with a decrease in N fertilization.

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.

State of the art of urine treatment technologies: A critical review

Over the last 15 years, urine treatment technologies have developed from lab studies of a few pioneers to an interesting innovation, attracting attention from a growing number of process engineers. In this broad review, we present literature from more than a decade on biological, physical-chemical and electrochemical urine treatment processes. Like in the first review on urine treatment from 2006, we categorize the technologies according to the following objectives: stabilization, volume reduction, targeted N-recovery, targeted P-recovery, nutrient removal, sanitization, and handling of organic micropollutants. We add energy recovery as a new objective, because extensive work has been done on electrochemical energy harvesting, especially with bio-electrochemical systems. Our review reveals that biological processes are a good choice for urine stabilization. They have the advantage of little demand for chemicals and energy. Due to instabilities, however, they are not suited for bathroom applications and they cannot provide the desired volume reduction on their own. A number of physical-chemical treatment technologies are applicable at bathroom scale and can provide the necessary volume reduction, but only with a steady supply of chemicals and often with high demand for energy and maintenance. Electrochemical processes is a recent, but rapidly growing field, which could give rise to exciting technologies at bathroom scale, although energy production might only be interesting for niche applications. The review includes a qualitative assessment of all unit processes. A quantitative comparison of treatment performance was not the goal of the study and could anyway only be done for complete treatment trains. An important next step in urine technology research and development will be the combination of unit processes to set up and test robust treatment trains. We hope that the present review will help guide these efforts to accelerate the development towards a mature technology with pilot scale and eventually full-scale implementations.

Effect of carrier particle size on enrichment and shift in nitrifier community behaviors for treating increased strength wastewater

In activated sludge systems, adding carriers can improve nitrifier enrichment. Different attachment area induced by different particle sizes of carriers may result in different nitrifier community. This research investigated the effect of different particle sizes of coal ash on nitrifier enrichment treating increased strength wastewater. Results indicated efficient nitrifying coal ash was obtained with smaller coal ash. The ammonia removal rates reached over 98%, which outclassed that in negative control (63.28%), and no nitrite accumulated in these systems under high nitrogen concentration of 1123.35 mg N/L. The high-throughput sequencing assays indicated carriers changed the microbial community structure significantly, thus facilitated the nitrification capacity. Increase abundance of nitrifier has a negative correlation with particle size of carriers. Nitrosomonas became the biggest beneficiary, which maximum composed 50.29% in fillers system and only 13.69% in negative control, whereas the number of Nitrobacter (less than 3.04%) became much lower than ammonia-oxidizing bacteria (AOB). However, the shift of microbial structures, large number of Dokdonella for instance, may guarantee the complete nitrification in systems with smaller carriers. Batch experiments showed a high dissolved oxygen (DO) concentration (4 mg/L) and slightly alkaline condition (pH 8.0) had a positive effect on nitrifying coal ash. PRACTITIONER POINTS: The increase size of nitrifier has a negative correlation with particle size of coal ash. The smaller coal ash reduces the adverse effect of high nitrogen on nitrification. The ammonia removal rate reached 99.82% with influent of 1123.35 mg NH 4 + - N /L in the smallest carriers system.

Bioleaching of toxic metals from anaerobically digested sludge without external chemical addition

Anaerobically digested (AD) sludge is widely applied to agricultural land as fertilizer. However, heavy metals in AD sludge potentially pose a significant threat to environment. This study reports a novel bioleaching approach, with no need for externally added chemicals. Sludge acidification was achieved using the protons produced from microbial oxidation of the inherent ammonium in AD sludge. An acid-tolerant microbial consortium, dominated by ammonia-oxidizing bacteria from the genus Candidatus Nitrosoglobus (i.e. relative abundance of 72.5 ± 2.3% based on 16S rRNA gene sequencing), was enriched after 120 days incubation in a laboratory sequencing batch reactor. The consortium oxidizes ammonium even at pH 2.5, at approximately 30% of its maximum rate, measured at pH 5.5. Inoculating the consortium at a solid ratio of 1:20, caused the pH of the AD sludge to decrease from 7.5 to 2.0 over five days under aerobic conditions. As a result, metals in the AD sludge were efficiently extracted into the liquid phase. In particular, two of the most abundant toxic metals, Cu and Zn, were solubilized with high efficiencies of 88 ± 4% and 96 ± 3%, respectively. Overall, the results of this study enable the economical and safe reuse of excess sludge generated during biological wastewater treatment.

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.

Insights into Nitrous Oxide Mitigation Strategies in Wastewater Treatment and Challenges for Wider Implementation

Nitrous oxide (N₂O) emissions account for the majority of the carbon footprint of wastewater treatment plants (WWTPs). Many N₂O mitigation strategies have since been developed while a holistic view is still missing. This article reviews the state-of-the-art of N₂O mitigation studies in wastewater treatment. Through analyzing existing studies, this article presents the essential knowledge to guide N₂O mitigations, and the logics behind mitigation strategies. In practice, mitigations are mainly carried out by aeration control, feed scheme optimization, and process optimization. Despite increasingly more studies, real implementation remains rare, which is a combined result of unclear climate change policies/incentives, as well as technical challenges. Five critical technical challenges, as well as opportunities, of N₂O mitigations were identified. It is proposed that (i) quantification methods for overall N₂O emissions and pathway contributions need improvement; (ii) a reliable while straightforward mathematical model is required to quantify benefits and compare mitigation strategies; (iii) tailored risk assessment needs to be conducted for WWTPs, in which more long-term full-scale trials of N₂O mitigation are urgently needed to enable robust assessments of the resulting operational costs and impact on nutrient removal performance; (iv) current mitigation strategies focus on centralized WWTPs, more investigations are warranted for decentralised systems, especially decentralized activated sludge WWTPs; and (v) N₂O may be mitigated by adopting novel strategies promoting N₂O reduction denitrification or microorganisms that emit less N₂O. Overall, we conclude N₂O mitigation research is reaching a maturity while challenges still exist for a wider implementation, especially in relation to the reliability of N₂O mitigation strategies and potential risks to nutrient removal performances of WWTPs.

The Tolerance of Anoxic-Oxic (A/O) Process for the Changing of Refractory Organics in Electroplating Wastewater: Performance, Optimization and Microbial Characteristics

In order to investigate the tolerance of an anoxic-oxic (A/O) process for the changing of refractory organics in electroplating wastewater, optimize the technological parameters, and reveal the microbial characteristics, a pilot-scale A/O process was carried out and the microbial community composition was analyzed by high-throughput sequencing. The results indicated that a better tolerance was achieved for sodium dodecyl benzene sulfonate, and the removal efficiencies of organic matter, ammonia nitrogen (NH4+-N), and total nitrogen (TN) were 82.87%, 66.47%, and 53.28% with the optimum hydraulic retention time (HRT), internal circulation and dissolved oxygen (DO) was 12 h, 200% and 2–3 mg/L, respectively. Additionally, high-throughput sequencing results demonstrated that Proteobacteria and Bacteroidetes were the dominant bacteria phylum, and the diversity of the microbial community in the stable-state period was richer than that in the start-up period.

Stoichiometric and kinetic characterization of an acid-tolerant ammonia oxidizer 'Candidatus Nitrosoglobus'

Recently, acidic (i.e. pH<5) nitrification in activated-sludge is attracting attention because it enables stable nitritation (NH₄⁺ → NO₂^-), and enhances sludge reduction and stabilization. However, the key acid-tolerant ammonia oxidizers involved are poorly understood. In this study, we performed stoichiometric and kinetic characterization of a new acid-tolerant ammonia-oxidizing bacterium (AOB) belonging to gamma-proteobacterium, Candidatus Nitrosoglobus. Ca. Nitrosoglobus was cultivated in activated-sludge in a laboratory membrane bioreactor over 200 days, with a relative abundance of 55.1 ± 0.5% (indicated by 16S rRNA gene amplicon sequencing) at the time of the characterization experiments. Among all known nitrifiers, Ca. Nitrosoglobus bears the highest resistance to nitrite, low pH, and free nitrous acid (FNA). These traits define Ca. Nitrosoglobus as an adversity-strategist that tends to prosper in acidic activated-sludge, where the low pH (< 5.0) and high levels of FNA (at parts per million levels) sustained and inhibited all other nitrifiers. In contrast, in the conventional pH-neutral activated-sludge process, Ca. Nitrosoglobus is less competitive with canonical AOB (e.g. Nitrosomonas) due to the relatively slow specific growth rate and low affinities to both oxygen and total ammonia. These results advance our understanding of acid-tolerant ammonia oxidizers, and support further development of the acidic activated-sludge process in which Ca. Nitrosoglobus can play a critical role.

Acidic aerobic digestion of anaerobically-digested sludge enabled by a novel ammonia-oxidizing bacterium

Anaerobic digestion is a commonly used process for the reduction and stabilization of wasted activated sludge generated in wastewater treatment plants. However, anaerobically-digested (AD) sludge is still a problematic waste stream due to its large volume and often poor quality. In this study, two aerobic digesters were set up to treat anaerobically-digested sludge, with one digester operated in self-generated acidic condition as the experimental reactor, and one at neutral pH as the control reactor. The acidic condition in the experimental reactor was driven by an inoculated special ammonia-oxidizing bacterium, 'Candidatus Nitrosoglobus', which can tolerate low pH. As a result of ammonium oxidation by Ca. Nitrosoglobus, the pH decreased to 4.8 ± 0.2 and nitrite accumulated to and stayed at 200.0 ± 17.2 mg N L^-1, from which free nitrous acid (FNA) at 8.5 ± 1.8 mg HNO₂N L^-1 formed in-situ. As a combined effect of low pH and high concentration of FNA, the experimental reactor reduced the total solids (TS), volatile solids (VS) and non-volatile solids (NVS) in the AD sludge by 25.2 ± 7.0%, 29.8 ± 4.3%, and 22.6 ± 5.5%, respectively. In contrast, the control reactor without Ca. Nitrosoglobus inoculation (operated at a near-neutral pH of 6.8 ± 0.3 and no FNA formation) only reduced VS in the AD sludge by 10.4 ± 4.3%, along with negligible NVS reduction. Additionally, the acidic aerobic digestion in the experimental reactor significantly stabilized AD sludge, decreasing the specific oxygen uptake rate (SOUR) to 0.5 ± 0.1 mg O₂ g^-1VS h^-1 and the most probable number (MPN) of Faecal Coliforms to 2.4 ± 0.1 log(MPN g^-1TS), both of which meet USEPA standards for Class A biosolids. In comparison, the control reactor produced biosolids at Class B level only, with an SOUR of 1.8 ± 0.2 mg O₂ g^-1VS h^-1 and a Faecal Coliforms MPN of 3.6 ± 0.1 log(MPN g^-1TS). By reducing the volume and improving the quality of the AD sludge, the acidic aerobic digestion of AD sludge enabled by Ca. Nitrosoglobus has the potential to significantly save the sludge disposal costs in wastewater treatment.

Robust Nitritation of Anaerobic Digester Centrate Using Dual Stressors and Timed Alkali Additions

Nitrogen is commonly removed from wastewater by nitrification to nitrate followed by nitrate reduction to N₂. Shortcut N removal saves energy by limiting ammonia oxidation to nitrite, but nitrite accumulation can be unstable. We hypothesized that repeated short-term exposures of ammonia-oxidizing communities to free ammonia (FA) and free nitrous acid (FNA) would stabilize nitritation by selecting against nitrite-oxidizing bacteria (NOB). Accordingly, we evaluated ammonium oxidation of anaerobic digester centrate in two bench-scale sequencing batch reactors (SBRs), seeded with the same inoculum and operated identically but with differing pH-control strategies. A single stressor SBR (SS/SBR) using pH set-point control produced HNO₃, while a dual stressor SBR (DS/SBR) using timed alkalinity addition (TAA) produced HNO₂ (ammonium removal efficiency of 97 ± 2%; nitrite accumulation ratio of 98 ± 1%). The TAA protocol was developed during an adaptation period with continuous pH monitoring. After adaptation, automated TAA enabled stable nitritation without set-point control. In the SS/SBR, repeatedly exposing the community to FA (8-10 h/exposure, one exposure/cycle) selected for FA-tolerant ammonia-oxidizing bacteria (Nitrosomonas sp. NM107) and NOB (Nitrobacter sp.). In the DS/SBR, repeatedly exposing the community to FA (2-4 h/exposure, three exposures/cycle) and FNA (4-6 h/exposure, two exposures/cycle) selected for FA- and FNA-resistant AOB (Nitrosomonas IWT514) and against NOB, stabilizing nitritation.

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.

Comparison of performance of two large-scale vertical-flow constructed wetlands treating wastewater treatment plant tail-water: Contaminants removal and associated microbial community

The removal efficiency of contaminants in large-scale integrated vertical-flow constructed wetland (IVCW) and vertical-flow constructed wetland (VCW) for wastewater treatment plant (WWTP) tail-water was evaluated, and the microbial community was also investigated in this study. The results for 14 months study period indicated that 40.05% chemical oxygen demand (COD), 45.47% ammonia nitrogen (NH₄⁺-N), 62.55% total phosphorus (TP), 55.53% total nitrogen (TN) and 57.20% total suspended solids (TSS) average removal efficiencies were achieved in the IVCW. There was a poor performance of TN removal in the VCW, with an average removal efficiency of 38.13%. There was no significant seasonal difference in TP removal, and a strong positive correlation between influent TP load and removed load. The high-throughput sequencing analysis revealed that Proteobacteria, Planctomycetes, Bacteroidetes and Acidobacteria were dominant in nature and wetland systems. The relative abundance of nitrifying bacteria, denitrifying bacteria and anammox bacteria confirmed that nitrification, denitrification and anammox may be the main processes for nitrogen removal in the IVCW.

Full-scale N removal from centrate using a sidestream process with a mainstream carbon source

An innovative approach to treat centrate for rapid nitrogen load discharge reduction was investigated and applied at the Truckee Meadows Water Reclamation Facility (TMWRF) in Reno, NV. This process allowed TMWRF to circumvent an anticipated exceedance of the individual waste load allocation in 2018. Existing infrastructure and equipment were re-purposed in a full-scale suspended growth biological centrate treatment system, attaining simultaneous nitrification, and denitrification with no additional capital investment. Functioning within a few days of start-up, the average ammonia reduction was 81% (1,106 kg/day) and the average total nitrogen reduction in the sidestream was 53% (757 kg/day) using primary effluent as carbon source. Alkalinity and carbon limitations were both anticipated and observed; however, adaptive operations allowed for balancing of nitrification and denitrification processes, providing pH stability and success in meeting treatment goals. Immediately after the sidestream treatment system was placed into service, nitrogen in the mainstream facility was measured at concentrations significantly lower than typical and was sustained at historically low concentrations throughout the operation. This translated into a significant methanol cost savings of $1,500 per day (USD). The system has become a critical supplemental treatment process during upcoming rehabilitation projects to address aging infrastructure of existing nitrogen treatment facilities. PRACTITIONER POINTS: Full scale demonstration of sidestream N removal using a hybrid process. Integration of the sidestream N process to reduce N effluent load without alkalinity or supplementary carbon augmentation. Operational solution to reduce operating costs without new infrastructure.

Operation of pilot-scale nitrification-anammox reactors for the treatment of reject-water produced from the anaerobic digestion of thermal hydrolysis-treated sludge

Two-stage pilot-scale partial nitrification (PN)-anammox reactors were successfully operated for the treatment of reject-water (record as TRW) produced from the anaerobic digestion of thermal hydrolysis-treated sludge (THPAD). The PN reactor was operated stalely and Nitrosomonas was the major ammonia-oxidizing bacteria. In the anammox reactor, anammox activity doubled from day 3 to day 53 demonstrating that anammox adapted to the PN effluent. After acid shock at pH 4 for approximately 1 h, anammox was seriously inhibited and required approximately 60 days for recovery. This provided a reference for handling similar accidents. In the next 166 days, the load reached 0.40-0.51 kg N/(m³·d) in the presence of high concentration COD (798-1313 mg/L), suggesting anammox can be used in high concentration organic wastewater. Under the combined action of anammox and denitrification, 94.7% nitrogen was removed. These results demonstrated TRW can be treated using PN-anammox technology which was conducive to the popularization of THPAD.

Achieving Stable Partial Nitritation in an Acidic Nitrifying Bioreactor

Partial nitritation providing a suitable effluent for subsequent anammox is a critical step in a two-stage autotrophic nitrogen removal system. This study demonstrates an innovative approach for attaining partial nitritation in an acidic bioreactor operating at a slightly low pH (i.e., 5-6). This approach is based on our hypothesis in this study that acid-tolerant ammonia-oxidizing bacteria (AOB) can produce nitrite and protons to self-sustain free nitrous acid (FNA, NO₂^- + H⁺ ↔ HNO₂) at a parts per million level, as an inhibitor of nitrite-oxidizing bacteria (NOB). With influent nitrogen of about 200 mg/L and operating conditions of high dissolved oxygen, long sludge retention time, and moderate temperature, a lab-scale acidic bioreactor with FNA up to 2 mg of HNO₂-N/L successfully established stable nitrite accumulation in the effluent for 200 days, with an average ratio [NO₂^-/(NO₂^- + NO₃^-)] exceeding 95%. A 16S rRNA amplicon sequencing analysis showed that Nitrosospira was the dominant AOB in the biomass of the bioreactor, while Nitrosomonas and Nitrospira, two typical nitrifying genera in neutral wastewater treatment, both disappeared after the startup of partial nitritation. Kinetic characterization revealed that Nitrosospira had a substrate affinity of 11.4-16.5 mg of total ammonia (NH₄⁺ + NH₃)/L. It also revealed that less than 3.5 mg of HNO₂-N/L FNA did not inhibit AOB activity significantly. Acidic operation is economically attractive because it can be achieved via acidophilic ammonia oxidation without adding chemical acid. However, hazardous gas, nitric oxide (NO), should be removed from gas produced by acidic nitrifying bioreactors.

High-rate partial-nitritation and efficient nitrifying bacteria enrichment/out-selection via pH-DO controls: Efficiency, kinetics, and microbial community dynamics

Conventional nitrification/denitrification process is gradually being replaced with partial-nitritation/anammox (PN/A) processes due to its installation and running cost. However, high ammonia-oxidizing bacteria (AOB) and anaerobic ammonia-oxidizing (anammox) bacteria activity as well as optimum out-selection of nitrite-oxidizing bacteria (NOB) are necessary to achieving efficient PN/A process. Consequently, to enhance PN process via nitrifying bacteria enrichment/out-selection within psychrophilic environment, a novel pH-DO (dissolved oxygen) control strategy was proposed and the response of PN, kinetics, AOB enrichment, and NOB out-selection efficiency was investigated during start-up and long-term operation. With DO of 0.7 mg/L and pH of 7.5-7.9, quick start-up of the PN process was established within 34d as NO₂^--N accumulation ratio (NAR) reached 90.08 ± 1.4%. Again, when NLR was elevated to 0.8 kg/m³·d (400mgNH₄⁺-N/L), DO curtailed to 0.2 mg/L, pH maintained at 7.7 and free ammonium at 6.5 mg/L, NAR and NH₄⁺-N removal rate could still reach 97.04 ± 2.4% and 97.84 ± 1.5%, respectively. After optimum control factors had been established, real nitrogen-rich-mine-wastewater was fed (DO, 0.2 mg/L, pH, 8.9, and free ammonia, 6.5 mg/L) and NAR and NH₄⁺-N removal rate reached was 97.33 ± 0.5% and 97.76 ± 1.1%, respectively. Estimated kinetic parameters including maximum degradation rate (Vmax = 1.58/d), half-rate constant (Km = 33.8 mg/L), and inhibition constant (Ki = 201.6 mg/L) suggested that inhibition on NH₄⁺-N oxidation was most feasible at higher concentration of NH₄⁺-N. To elucidate biological mechanisms, 16S rRNA high-throughput revealed that AOB (Nitrosomonas) enrichment had increased from 0.08% to 49% whereas NOB (Nitrospira) abundance reduced from 1% to 0.034%, indicating pH-DO control efficiently enriched AOB and out-selected NOB. Conversely, when influent NH₄⁺-N was curtailed to about 200 mg/L and free ammonia concentration maintained at 6.5 mg/L, the population of AOB was observably reduced by 6% within a period of 14 days, indicating control strategies including pH-DO control and substrate availability were the key factors which substantially influenced and promoted the activities and growth of AOBs in the present SBR.

Nitrogen removal from wastewater via simultaneous nitrification and denitrification using a biological folded non-aerated filter

A conventional biological filter has been shown to be a viable method for removing nitrogenous compounds from wastewater, but it still has many disadvantages. In this study, a biological folded non-aerated filter (BFNAF) was designed, and its feasibility for nitrogen-loaded wastewater treatment has been confirmed. Effects of the HRT and the COD/N ratio on the performance of BFNAF were investigated. Through response surface method, when the COD/N ratio and the HRT were 5.39 and 10.83 h, removal efficiencies of NH₄⁺, COD and TN reached maximum values of 88.62 ± 0.81%, 76.12 ± 0.57%, and 50.48 ± 1.02%, respectively. In addition, it was found that several denitrifying bacteria, such as Azoarcus, Arcobacter, Flavobacterium, along with many ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, co-existed in the community of the biofilm. All the results showed that the BFNAF could realize the simultaneous nitrification and denitrification (SND) process effectively.

Refinery and concentration of nutrients from urine with electrodialysis enabled by upstream precipitation and nitrification

Human urine is a valuable resource for nutrient recovery, given its high levels of nitrogen, phosphorus and potassium, but the compositional complexity of urine presents a challenge for an energy-efficient concentration and refinery of nutrients. In this study, a pilot installation combining precipitation, nitrification and electrodialysis (ED), designed for one person equivalent (1.2 L_urine d^-1), was continuously operated for ∼7 months. First, NaOH addition yielded calcium and magnesium precipitation, preventing scaling in ED. Second, a moving bed biofilm reactor oxidized organics, preventing downstream biofouling, and yielded complete nitrification on diluted urine (20-40%, i.e. dilution factors 5 and 2.5) at an average loading rate of 215 mg N L^-1 d^-1. Batch tests demonstrated the halotolerance of the nitrifying community, with nitrification rates not affected up to an electrical conductivity of 40 mS cm^-1 and gradually decreasing, yet ongoing, activity up to 96 mS cm^-1 at 18% of the maximum rate. Next-generation 16S rRNA gene amplicon sequencing revealed that switching from a synthetic influent to real urine induced a profound shift in microbial community and that the AOB community was dominated by halophilic species closely related to Nitrosomonas aestuarii and Nitrosomonas marina. Third, nitrate, phosphate and potassium in the filtered (0.1 μm) bioreactor effluent were concentrated by factors 4.3, 2.6 and 4.6, respectively, with ED. Doubling the urine concentration from 20% to 40% further increased the ED recovery efficiency by ∼10%. Batch experiments at pH 6, 7 and 8 indicated a more efficient phosphate transport to the concentrate at pH 7. The newly proposed three-stage strategy opens up opportunities for energy- and chemical-efficient nutrient recovery from urine. Precipitation and nitrification enabled the long-term continuous operation of ED on fresh urine requiring minimal maintenance, which has, to the best of our knowledge, never been achieved before.

Nitrogen cycle microorganisms can be reactivated after Space exposure

Long-term human Space missions depend on regenerative life support systems (RLSS) to produce food, water and oxygen from waste and metabolic products. Microbial biotechnology is efficient for nitrogen conversion, with nitrate or nitrogen gas as desirable products. A prerequisite to bioreactor operation in Space is the feasibility to reactivate cells exposed to microgravity and radiation. In this study, microorganisms capable of essential nitrogen cycle conversions were sent on a 44-days FOTON-M4 flight to Low Earth Orbit (LEO) and exposed to 10^-3-10^-4 g (gravitational constant) and 687 ± 170 µGy (Gray) d^-1 (20 ± 4 °C), about the double of the radiation prevailing in the International Space Station (ISS). After return to Earth, axenic cultures, defined and reactor communities of ureolytic bacteria, ammonia oxidizing archaea and bacteria, nitrite oxidizing bacteria, denitrifiers and anammox bacteria could all be reactivated. Space exposure generally yielded similar or even higher nitrogen conversion rates as terrestrial preservation at a similar temperature, while terrestrial storage at 4 °C mostly resulted in the highest rates. Refrigerated Space exposure is proposed as a strategy to maximize the reactivation potential. For the first time, the combined potential of ureolysis, nitritation, nitratation, denitrification (nitrate reducing activity) and anammox is demonstrated as key enabler for resource recovery in human Space exploration.

The pH-based fed-batch for lipid production from Trichosporon oleaginosus with crude glycerol

In this study, it was found that the optimal pH for the growth of Trichosporon oleaginosus was related to the fermentation medium. A neutral or weak acid pH condition was optimal for the growth of Trichosporon oleaginosus in the extract-peptone-dextrose and wastewater sludge medium. Significant inhibition was observed at neutral pH in the wastewater sludge + crude glycerol medium due to the high soap content of the crude glycerol. By converting the soap to free fatty acid (FFA) at pH 5, the soap inhibition could be prevented. Fed-batch fermentation was employed to produce lipid from Trichosporon oleaginosus at pH 5 controlled by feeding crude glycerol. A remarkably high biomass (65.63 g/L) and lipid (35.79 g/L) concentration were achieved from the pH-based fed-batch fermentation in this study.

Post carbon removal nitrifying MBBR operation at high loading and exposure to starvation conditions

This study investigates the performance of MBBR nitrifying biofilm post carbon removal at high loading and starvation conditions. The nitrifying MBBR, treating carbon removal lagoon effluent, achieved a maximum SARR of 2.13gN/m²d with complete conversion of ammonia to nitrate. The results also show the MBBR technology is capable of maintaining a stable biofilm under starvation conditions in systems that nitrify intermittently. The biomass exhibited a higher live fraction of total cells in the high loaded reactors (73-100%) as compared to the reactors operated in starvation condition (26-82%). For both the high loaded and starvation condition, the microbial communities significantly changed with time of operation. The nitrifying community, however, remained steady with the family Nitrosomonadacea as the primary AOBs and Nitrospira as the primary NOB. During starvation conditions, the relative abundance of AOBs decreased and Nitrospira increased corresponding to an NOB/AOB ratio of 5.2-12.1.

Growth of Nitrosococcus-Related Ammonia Oxidizing Bacteria Coincides with Extremely Low pH Values in Wastewater with High Ammonia Content

Ammonia oxidation decreases the pH in wastewaters where alkalinity is limited relative to total ammonia. The activity of ammonia oxidizing bacteria (AOB), however, typically decreases with pH and often ceases completely in slightly acidic wastewaters. Nevertheless, nitrification at low pH has been reported in reactors treating human urine, but it has been unclear which organisms are involved. In this study, we followed the population dynamics of ammonia oxidizing organisms and reactor performance in synthetic fully hydrolyzed urine as the pH decreased over time in response to a decrease in the loading rate. Populations of the β-proteobacterial Nitrosomonas europaea lineage were abundant at the initial pH close to 6, but the growth of a possibly novel Nitrosococcus-related AOB genus decreased the pH to the new level of 2.2, challenging the perception that nitrification is inhibited entirely at low pH values, or governed exclusively by β-proteobacterial AOB or archaea. With the pH shift, nitrite oxidizing bacteria were not further detected, but nitrous acid (HNO₂) was still removed through chemical decomposition to nitric oxide (NO) and nitrate. The growth of acid-tolerant γ-proteobacterial AOB should be prevented, by keeping the pH above 5.4, which is a typical pH limit for the N. europaea lineage. Otherwise, the microbial community responsible for high-rate nitrification can be lost, and strong emissions of hazardous volatile nitrogen compounds such as NO are likely.

Intermittent Aeration Suppresses Nitrite-Oxidizing Bacteria in Membrane-Aerated Biofilms: A Model-Based Explanation

Autotrophic ammonium oxidation in membrane-aerated biofilm reactors (MABRs) can make treatment of ammonium-rich wastewaters more energy-efficient, especially within the context of short-cut ammonium removal. The challenge is to exclusively enrich ammonium-oxidizing bacteria (AOB). To achieve nitritation, strategies to suppress nitrite-oxidizing bacteria (NOB) are needed, which are ideally grounded on an understanding of underlying mechanisms. In this study, a nitrifying MABR was operated under intermittent aeration. During eight months of operation, AOB dominated, while NOB were suppressed. On the basis of dissolved oxygen (DO), ammonium, nitrite, and nitrate profiles within the biofilm and in the bulk, a 1-dimensional nitrifying biofilm model was developed and calibrated. The model was utilized to explore the potential mechanisms of NOB suppression associated with intermittent aeration, considering DO limitation, direct pH effects on enzymatic activities, and indirect pH effects on activity via substrate speciation. The model predicted strong periodic shifts in the spatial gradients of DO, pH, free ammonia, and free nitrous acid, associated with aerated and nonaerated phases. NOB suppression during intermittent aeration was mostly explained by periodic inhibition caused by free ammonia due to periodic transient pH upshifts. Dissolved oxygen limitation did not govern NOB suppression. Different intermittent aeration strategies were then evaluated for nitritation success in intermittently aerated MABRs: both aeration intermittency and duration were effective control parameters.

Operating a pilot-scale nitrification/distillation plant for complete nutrient recovery from urine

Source-separated urine contains most of the excreted nutrients, which can be recovered by using nitrification to stabilize the urine before concentrating the nutrient solution with distillation. The aim of this study was to test this process combination at pilot scale. The nitrification process was efficient in a moving bed biofilm reactor with maximal rates of 930 mg N L(-1) d(-1). Rates decreased to 120 mg N L(-1) d(-1) after switching to more concentrated urine. At high nitrification rates (640 mg N L(-1) d(-1)) and low total ammonia concentrations (1,790 mg NH4-N L(-1) in influent) distillation caused the main primary energy demand of 71 W cap(-1) (nitrification: 13 W cap(-1)) assuming a nitrogen production of 8.8 g N cap(-1) d(-1). Possible process failures include the accumulation of the nitrification intermediate nitrite and the selection of acid-tolerant ammonia-oxidizing bacteria. Especially during reactor start-up, the process must therefore be carefully supervised. The concentrate produced by the nitrification/distillation process is low in heavy metals, but high in nutrients, suggesting a good suitability as an integral fertilizer.