An integrated mainstream and sidestream strategy for overcoming nitrite oxidizing bacteria adaptation in a continuous plug-flow nutrient removal process

Start-up and maintenance of indigenous microalgae-bacteria consortium treating toilet wastewater through partial nitrification and nitrite-type denitrification

Microalgae-bacteria consortium (MBC) provides an alternative to sustainable treatment of human toilet wastewater (TWW) and resource recovery. This study compared the conventional activated sludge system and wastewater indigenous MBC system (IMBC) for nitrogen removal in TWW through the coupled partial nitrification (PN) and nitrite-type denitrification process. PN was firstly established by alternating FA and FNA. Subsequently, the successful PN maintenance with the nitrite accumulation rate ranging between 90.1-95.3% was achieved using two strategies: light irradiation with the appropriate specific light energy density at 0.0188-0.0598 kJ/mg VSS and the timely nitrite-type denitrification with the algae-secreted organics as the carbon source, eventually resulting in the nitrite accumulation rate ranging between 90.1-95.3%. In the IMBC-PN system, bacterial metabolism contributed to 91.5% of nitrogen removal and the rest was through microalgal assimilation. This study offers a sustainable hybrid IMBC-PN process for high NH₄⁺-N strength wastewater treatment (e.g., TWW), which theoretically saves 23.5% aeration and 34.2% carbon source as well as reduces 17.0% sludge production.

The mitigation effect of free ammonia and free nitrous acid on nitrous oxide production from the full-nitrification and partial-nitritation systems

The potentials of using endogenous free ammonia (FA) and free nitrous acid (FNA) as nitrous oxide (N₂O) mitigators were investigated in treatment of both mainstream and sidestream wastewater. Although the N₂O emission factor of a sidestream partial-nitritation (PN) reactor (averaged 1.70 % ± 0.39 %, n = 30) was about 2.4 times higher than a mainstream full-nitrification (FN) reactor (averaged 0.72 % ± 0.24 %, n = 30) (P < 0.01), one-hour exposure of PN sludge to 1.5 mg HNO₂-N/L FNA could virtually abolish N₂O emission. As for FN sludge, both 45 mg NH₃-N/L FA and 0.015 mg HNO₂-N/L FNA successfully mitigated N₂O production at varying dissolved oxygen (DO) levels (50 % vs 61 %), while 1.5 mg HNO₂-N/L FNA not only reduced more N₂O (92 %) but also altered the N₂O dependency on DO. Both FNA and FA sludge treatment were effective N₂O mitigation strategies with FNA toward the end of carbon neutrality and FA being more economically appealing (2 % cost saving).

Understanding the influence of free nitrous acid on microalgal-bacterial consortium in wastewater treatment: A critical review

Microalgal-bacterial consortium (MBC) constitutes a sustainable and efficient alternative to the conventional activated sludge process for wastewater treatment (WWT). Recently, integrating the MBC process with nitritation (i.e., shortcut MBC) has been proposed to achieve added benefits of reduced carbon and aeration requirements. In the shortcut MBC system, nitrite or free nitrous acid (FNA) accumulation exerts antimicrobial influences that disrupt the stable process performance. In this review, the formation and interactions that influence the performance of the MBC were firstly summarized. Then the influence of FNA on microalgal and bacterial monocultures and related mechanisms together with the knowledge gaps of FNA influence on the shortcut MBC were highlighted. Other challenges and future perspectives that impact the scale-up of the shortcut MBC for WWT were illustrated. A potential roadmap is proposed on how to maximize the stable operation of the shortcut MBC system for sustainable WWT and high-value biomass production.

Stable nitrogen removal in the novel continuous flow anammox system under deteriorated partial nitrification: Significance and superiority of the anaerobic-oxic-anoxic-oxic operation mode

The collapse of mainstream anammox system caused by deterioration of partial nitrification (PN) is easy to occur and it is vital to quickly restore the stable nitrogen elimination performance. Herein, a novel continuous push-flow anaerobic-oxic-anoxic-oxic (AOAO) process treating sewage was used to restore the nitrogen elimination performance rapidly under deteriorated PN. The increased abundances of Nitrospira and Candidatus Nitrotoga was responsible for the deterioration of PN. Effluent total inorganic nitrogen of 8.7 mg N/L and a stable nitrogen removal rate of 0.083 kg N/m³·d were obtained with the aerobic hydraulic retention time (HRT) of 3.75 h even PN deteriorated. Endogenous partial denitrification coupled anammox in the anoxic zone was essential to maintain stable nitrogen removal under the deterioration of PN and the anammox contribution increased from 17.2 % to 23.6 %. The AOAO system shows robustness on nitrogen removal even PN deteriorated under the decrease of HRT from 16 to 12 h.

A novel process for anammox pretreatment of municipal wastewater: semi-partial nitrification, biological phosphorus removal and recovery

Achieving simultaneous semi-partial nitrification and deep phosphorus removal is a preferred process technology for Anammox pretreatment. In this study, semi-partial nitrification combined with in-situ phosphorus recovery (PNPR) was used to treat municipal wastewater. The SRT conflict between the nitrification and phosphorus removal was resolved by in-situ phosphorus recovery every 20 cycles of Anaerobic/Oxid, and a supernatant with more than 10 times the influent phosphorus concentration was obtained, thus achieving bio-enhanced phosphorus removal and recovery with satisfactory semi-partial-nitrification effluent. Interestingly, the results showed that phosphorus removal and recovery process could improve the activity of AOB. The PNPR system's nitrite accumulation rate (NAR) and phosphorus removal rate (PRR) were more than 90% each, whereas the relative abundance of AOB and PAOs increased from 0.04% to 0.74% and from 0.25% to 0.70%, respectively (P < 0.01). Furthermore, on average, the NO₂^--N_eff/NH₄⁺-N_eff value was 1.96, which laid the foundation for the subsequent anammox treatment.

An Eco-Friendly Conversion of Aquaculture Suspended Solid Wastes Into High-Quality Fish Food by Improving Poly-β-Hydroxybutyrate Production

The aquaculture industry is vital in providing a valuable protein food source for humans, but generates a huge amount of solid and dissolved wastes that pose great risks to the environment and aquaculture sustainability. Suspended solids (in short SS), one of the aquaculture wastes, are very difficult to be treated due to their high organic contents. The bioconversion from wastewater, food effluents, and activated sludge into poly-β-hydroxybutyrate (PHB) is a sustainable alternative to generate an additional income and could be highly attractive to the agricultural and environmental management firms. However, little is known about its potential application in aquaculture wastes. In the present study, we first determined that 7.2% of SS was PHB. Then, the production of PHB was increased two-fold by the optimal fermentation conditions of wheat bran and microbial cocktails at a C/N ratio of 12. Also, the PHB-enriched SS showed a higher total ammonia nitrogen removal rate. Importantly, we further demonstrated that the PHB-enriched SS as a feed could promote fish growth and up-regulate the expression of the immune-related genes. Our study developed an eco-friendly and simple approach to transforming problematic SS wastes into PHB-enriched high-quality food for omnivorous fish, which will increase the usage efficiency of SS and provide a cheaper diet for aquatic animals.

Unravelling multiple removal pathways of oseltamivir in wastewater by microalgae through experimentation and computation

Increased worldwide consumption of antiviral drugs (AVDs) amid COVID-19 has induced enormous burdens to the existing wastewater treatment systems. Microalgae-based bioremediation is a competitive alternative technology due to its simultaneous nutrient recovery and sustainable biomass production. However, knowledge about the fate, distribution, and interaction of AVDs with microalgae is yet to be determined. In this study, a concentration-determined influence of AVD oseltamivir (OT) was observed on the biochemical pathway of Chlorella sorkiniana (C.S-N1) in synthetic municipal wastewater. The results showed that high OT concentration inhibited biomass growth through increased oxidative stress and restrained photosynthesis. Nevertheless, complete OT removal was achieved at its optimized concentration of 10 mg/L by various biotic (82%) and abiotic processes (18.0%). The chemical alterations in three subtypes of extracellular polymeric substances (EPS) were primarily investigated by electrostatic (OT +8.22 mV vs. C.S-N1 -18.31 mV) and hydrophobic interactions between EPS-OT complexes supported by secondary structure protein analysis. Besides, six biodegradation-catalyzed transformation products were identified by quadrupole-time-of-flight mass spectrometer and by density functional theory. Moreover, all the TPs exhibited log Kow ≤ 5 and bioconcentration factor values of < 5000 L/kg, meeting the practical demands of environmental sustainability. This study broadens our understanding of microalgal bioadsorption and biodegradation, promoting microalgae bioremediation for nutrient recovery and AVDs removal.

Insights into the multi-targeted effects of free nitrous acid on the microalgae Chlorella sorokiniana in wastewater

Microalgal-bacterial consortium process (MBCP) proposed as an alternative to the activated sludge process contains free nitrous acid (FNA). FNA antimicrobial influences on nitrifiers have been demonstrated. However, its influence on microalgae is largely unknown, limiting the system stability of MBCP. This study revealed the multi-targeted responses of a model wastewater microalgae, Chlorella sorokiniana, to FNA exposure through physiological and transcriptomic analyses. Results showed a concentration-dependent FNA-influence as both microalgal growth and photosynthesis (Fv/Fm, rETR, Y(II), NPQ) inversely correlated with FNA doses. Increased ROS, MDA content (5.0-fold), SOD (2.7-fold), and LDH (12.0-fold) activities in the treatments revealed FNA-induced oxidative pressure. Moreover, RNA-sequencing results revealed significantly downregulated genes related to photosynthesis, respiration, nitrogen metabolism, and tricarboxylic acid cycle. Comparatively, peroxisome, chlorophyll, and carotenoid genes were upregulated. These findings elucidate the inhibitory mechanisms of FNA on microalgae and contribute towards the prospective practical application of the MBCP system for sustainable wastewater treatment.

A novel stable nitritation process: Treating sludge by alternating free nitrous acid/heat shock

The feasibility of stable nitritation of sludge alternately treated by free nitrous acid (FNA) and heat shock in a sequencing batch reactor (SBR) was investigated in this study. The linear regression method was used to determine the optimal treatment conditions. Results revealed that an FNA concentration of 2.20 mg HNO₂-N/L, exposure time of 24 h, and treatment ratio of 20% could inhibit nitrite oxidizing bacteria (NOB) activity to the greatest extent while maintaining the maximum ammonium oxidizing bacteria (AOB) activity; after heat shock at 60 °C for 20 min, NOB were inhibited while AOB still had certain activity. In the long-term continuous-flow experiment, the single FNA or heat shock treatments easily allowed adapt NOB to affect the stability of nitritation. The alternating FNA/heat shock treatment can achieve long-term stability of nitritation. Microbial community analysis revealed that the alternating FNA/heat shock treatment could inhibit NOB while maintaining high AOB abundance.

Realization of partial nitrification and in-situ anammox in continuous-flow anaerobic/aerobic/anoxic process with side-stream sludge fermentation for real sewage

A continuous-flow anaerobic/aerobic/anoxic reactor with complete suspended activated sludge using sludge alkaline fermentation products as carbon source was utilized to strengthen nitrogen removal performance for low C/N ratio (<4) wastewater. Long-time performance indicated that the nitrite accumulation rate reached 60.40%, which strengthened the contribution of anammox. The average total inorganic nitrogen removal efficiency improved 19.40%. The abundance of ammonia oxidizing bacteria has not changed, but the abundance of nitrite oxidizing bacteria reduced from 5.79% to 0.69%. Quantitative PCR results demonstrated that the abundance of anammox bacteria has raised by 80.5 times. These results indicated that side-stream sludge alkaline fermentation promoted the mainstream partial nitrification, consequently accelerating the in-situ enrichment of anammox bacteria. No external carbon source dosing and short oxic hydraulic retention time (5.3 h) save energy and reduce consumption significantly in this system.

Enhanced nitrogen removal from low COD/TIN mainstream wastewater in a continuous plug-flow reactor via partial nitrification, simultaneous anammox and endogenous denitrification (PN-SAED) process

A continuous plug-flow reactor with anaerobic/front-aerobic/anoxic/post-aerobic zones, where partial nitrification occurred in the front-aerobic zone, followed by simultaneous anammox and endogenous denitrification in the anoxic zone (PN-SAED), was built up to treat municipal wastewater. Alternating anoxic/aerobic conditions and longer anoxic duration facilitated stable partial nitrification. The nitrite accumulation ratio (NAR) was maintained at 97.4 ± 1.2%, with temperatures between 13.3℃ to 19.8℃. Candidatus Brocadia were naturally enriched in-situ from the anoxic zone with relative abundances of 31.93% and 6.67% on the agitator blade and carriers, respectively. High removal efficiencies of total inorganic nitrogen (TIN) (95.1 ± 1.9%) and effluent TIN (2.6 ± 1.1 mg N/L) were acquired from low COD/TIN (3.4 ± 0.4) municipal wastewater with anammox contribution of 13.5%±5.8% to TIN removal. The PN-SAED process is a promising mainstream nitrogen removal method.

Microalgal Activity and Nutrient Uptake from Wastewater Enhanced by Nanoscale Zerovalent Iron: Performance and Molecular Mechanism

Microalgae-based bioremediation presents an alternative to traditional biological wastewater treatment. However, its efficiency is still challenging due to low microalgal activities and growth rate in wastewater. Iron plays an important role in microbial metabolism and is effective to stimulate microbial growth. In this study, a novel approach was proposed to simultaneously promote microalgal activity and nutrient uptake from wastewater using nanoscale zerovalent iron (nZVI), and the underlying molecular mechanism was explored. Compared to the control, 0.05 mg/L of nZVI significantly enhanced biomass production by 113.3% as well as NH₄⁺-N and PO₄^3--P uptake rates by 32.2% and 75.0%, respectively. These observations were attributed to the enhanced metabolic pathways and intracellular regulations. Specifically, nZVI alleviated the cellular oxidative stress via decreased peroxisome biogenesis as indicated by reduced reactive oxygen species, enzymes, and genes involved. nZVI promoted ammonium assimilation, phosphate metabolism, carbon fixation, and energy generation. Moreover, nZVI regulated the biosynthesis and conversions of intracellular biocomposition, leading to increased carotenoid, carbohydrate, and lipid productions and decreased protein and fatty acid yields. The above metabolisms were supported by the regulations of differentially expressed genes involved. This study provided an nZVI-based approach and molecular mechanism for enhancing microalgal activities and nutrient uptake from wastewater.

A successful start-up of an anaerobic membrane bioreactor (AnMBR) coupled mainstream partial nitritation-anammox (PN/A) system: A pilot-scale study on in-situ NOB elimination, AnAOB growth kinetics, and mainstream treatment performance

In this pilot-scale study, an innovative mainstream treatment process that couples the anaerobic membrane reactor (AnMBR) with a one-stage PN/A system was proposed for advancing the concept of carbon neutrality in the municipal wastewater treatment plant. This work demonstrates the start-up procedure of a pilot-scale one-stage PN/A system for mainstream treatment. The 255-day start-up of the one-stage PN/A system involved the cultivation of ammonium-oxidizing bacteria (AOB) from the activated sludge, suppression of nitrite-oxidizing bacteria (NOB), investigation of in-situ growth kinetics of anammox bacteria (AnAOB), and the 50-day operation of the pilot-scale AnMBR-PN/A process for natural mainstream treatment. It is verified in the pilot-scale system for the first time that the in-situ free ammonia (FA) and free nitrous acid (FNA) exposure could effectively eliminate the Nitrospira (the NOB genus) while retaining the Nitosonomas (the AOB genus) community in the suspended sludge. NOB community rebounding was not detected even at the mainstream conditions with low nitrogen concentrations (Influent ammonium concentration=38±6 mg-NH₄⁺-N/L) by intermittent aeration to control the system dissolved oxygen (DO) below 0.5 mg/L. The results of the mainstream treatment showed that the average effluent total nitrogen (TN) in the coupled process was generally lower than 10 mg-N/L, which meets the discharge limits of most prefectures in Japan. The investigated results of the in-situ anammox bacteria (AnAOB) growth kinetics suggested that the promoted start-up strategy of taking advantage of the warm months with higher mainstream temperature to achieve the rapid in-situ growth of the AnAOB is applicable in the investigated regions. From the perspective of the removal performance of the TN and organic substance, the AnMBR-PN/A process has great potential as the layouts of the carbon-neutral mainstream wastewater treatment plants.

Light Irradiation Enables Rapid Start-Up of Nitritation through Suppressing nxrB Gene Expression and Stimulating Ammonia-Oxidizing Bacteria

Nitritation facilitates the application of anaerobic ammonium oxidation (Anammox)-based processes for cost-efficient nitrogen removal from wastewater. This study proposed light irradiation as a novel strategy to rapidly start up nitritation by stimulating both the activities and growth of ammonia-oxidizing bacteria (AOB) while suppressing that of nitrite-oxidizing bacteria (NOB). Batch assays and kinetic model jointly suggested that AOB activity presented an initial increase followed by a decline while NOB decreased continuously throughout the light energy densities applied. Under optimal light energy densities (0.03-0.08 kJ/mg VSS), the highest nitrite accumulation ratio of 70.0% was achieved in sequencing batch reactors with both mainstream online and sidestream offline light treatments when treating real or synthetic municipal wastewater. Light irradiation induced different responses of AOB and NOB, leading to microbial structure optimization. Specifically, the expression of nxrB was downregulated, while the expression of amoA was upregulated under appropriate light irradiation. Moreover, although Nitrosomonas as typical AOB disappeared, the family Nitrosomonadaceae was doubled with enrichment of Ellin6067 and another four Nitrosomonadaceae genera that were only identified in light-treated reactors, thus ensuring AOB predominance and stable nitritation. These findings offer a new approach to rapidly establishing nitritation using light irradiation in municipal wastewater, especially for nitritation/microalgae system.

Comparisons of nitrite accumulation, microbial behavior and nitrification kinetic in continuous stirred tank (ST) and plug flow (PF) moving bed biofilm reactors

Two types of continuous stirred tank moving bed biofilm reactors (ST-MBBR) and plug flow MBBR (PF-MBBR) were compared for nitrification. PF-MBBR showed strong shock resistance to temperature, and ammonium oxidation ratio (AOR) was 9.63% higher than that in the ST-MBBR, although the average biomass and biofilm thickness of ST-MBBR were 7.32-18.59%, 9.44-14.06% higher than those in the PF-MBBR. Meanwhile, a lower nitrite accumulation ratio (NAR) was observed (54.88%) in the PF-MBBR than the ST-MBBR (78.92%) due to different operation modes, and the divergence was demonstrated by the microbial quantitative analysis. Nitrification kinetics revealed that the temperature coefficient (θ) in the ST-MBBR (1.068) was much higher than that in the PF-MBBR (1.006-1.015), proving the contrasting nitrification performances caused by temperature shock. According to the Monod equation, the half-saturation coefficient (K_N) in the ST-MBBR was 0.19 mg/L while it varied around 0.12-0.24 mg/L in the PF-MBBR, revealing various NH₄⁺ affinity owing to different biofilm thickness and microbial composition. Finally, MBBR optimization related to operation mode, temperature, and free ammonium (FA) inhibition for nitrite accumulation was discussed.

Culturing sludge fermentation liquid-driven partial denitrification in two-stage Anammox process to realize advanced nitrogen removal from mature landfill leachate

The two-stage partial nitrification (PN)-Anammox process, during long term treatment of high-ammonia nitrogen leachate, faces challenges such as the adaptation of nitrite oxidation bacteria (NOB) and failure of real-time control of pH. Resultant instabilities including NH₄⁺-N and NO₃^--N accumulation were overcome by culturing sludge fermentation liquid (SFL)-driven partial denitrification (PD) in situ in the Anammox process. Biodegradation of slowly biodegradable organics (SBO) in SFL created organics restriction condition, which limited the activity of denitrification bacteria and achieved its balance with Anammox bacteria. Produced NO₃^--N is reduced to NO₂^--N through PD, which further improved the removal of NH₄⁺-N through Anammox. NO₂^--N was utilized timely by Anammox bacteria, which avoid further reduction of NO₂^--N to N₂, and result in a high nitrate to nitrite transformation ratio (NTR) of 93.3%. Satisfactory nitrogen removal efficiency (NRE) and nitrogen removal rate (NRR) of 99.6% and 822.0 ± 9.0 g N/(m³∙d) were obtained, respectively. Key genera related to degradation of SBO, PD and Anammox were enriched. The value of narG/(nirK+nirS) increased from 0.05 on day 1-0.15 on day 250. Combining SFL-driven PD with two-stage Anammox process provided a novel insight for applying this process to realize advanced nitrogen removal in practical engineering.

Application of Anammox-Based Processes in Urban WWTPs: Are We on the Right Track?

The application of partial nitritation and anammox processes (PN/A) to remove nitrogen can improve the energy efficiency of wastewater treatment plants (WWTPs) as well as diminish their operational costs. However, there are still several limitations that are preventing the widespread application of PN/A processes in urban WWTPs such as: (a) the loss of performance stability of the PN/A units operated at the sludge line, when the sludge is thermally pretreated to increase biogas production; (b) the proliferation of nitrite-oxidizing bacteria (NOB) in the mainstream; and (c) the maintenance of a suitable effluent quality in the mainstream. In this work, different operational strategies to overcome these limitations were modelled and analyzed. In WWTPs whose sludge is thermically hydrolyzed, the implementation of an anerobic treatment before the PN/A unit is the best alternative, from an economic point of view, to maintain the stable performance of this unit. In order to apply the PN/A process in the mainstream, the growth of ammonia-oxidizing bacteria (AOB) should be promoted in the sludge line by supplying extra sludge to the anaerobic digesters. The AOB generated would be applied to the water line to partially oxidize ammonia, and the anammox process would then be carried out. Excess nitrate generated by anammox bacteria and/or NOB can be removed by recycling a fraction of the WWTP effluent to the biological reactor to promote its denitrification.

An evolved native microalgal consortium-snow system for the bioremediation of biogas and centrate wastewater: Start-up, optimization and stabilization

It is necessary to develop sustainable technologies for centrate wastewater (CW) and biogas treatment from sludge anaerobic digestion (AD) systems in an environmentally friendly and economical manner. The microalgae-based bioremediation approach presents a competitive alternative due to its capacity for nutrient recovery and carbon sequestration. However, process instabilities and operating challenges limit its development and implementation largely due to the complexities in the CW and biogas. In this study, the evolved native microalgal consortium (ENMC) was firstly developed using the gradual stress increase method to enhance their adaptation in high ammonium condition. The supplementation of local snow (with Ca²⁺ and Mg²⁺) and biogas into CW significantly enhanced ENMC growth through batch tests. Subsequently, an integrated ENMC-snow (ENMCS) system was proposed consisting of a hydrolysis-acidification reactor (HAR), biogas upgrade reactor, and photobioreactor (PBR). The ENMCS system was systematically investigated under both batch and semi-continuous operations, by adjusting primary process parameters including the fill ratio, feeding time, hydraulic retention time (HRT), wastewater pretreatment, and PBR type. It was eventually optimized as a 24 h, 70% fermented CW diluted with 30% snow water, semi-continuous feeding system with a fill ratio of 50% and HRT of 6 d in an open-PBR. Long-term operation (310 days) showed superior biomass yield (0.3059 ± 0.0039 g/(L•d)) and nutrient removal efficiencies (95.6 ± 0.13% and 90.8 ± 0.44% for NH₄⁺-N and PO₄^3--P removal). Meanwhile, biogas was upgraded with an 82.2% CO₂ reduction. The economic and environmental analysis further demonstrated the ENMCS system as an effective alternative for the bioremediation of AD effluents while simultaneously producing value-added biomass, especially applicable to snowy regions.

A review of biopolymer (Poly-β-hydroxybutyrate) synthesis in microbes cultivated on wastewater

The large quantities of non-degradable single use plastics, production and disposal, in addition to increasing amounts of municipal and industrial wastewaters are among the major global issues known today. Biodegradable plastics from biopolymers such as Poly-β-hydroxybutyrates (PHB) produced by microorganisms are potential substitutes for non-degradable petroleum-based plastics. This paper reviews the current status of wastewater-cultivated microbes utilized in PHB production, including the various types of wastewaters suitable for either pure or mixed culture PHB production. PHB-producing strains that have the potential for commercialization are also highlighted with proposed selection criteria for choosing the appropriate PHB microbe for optimization of processes. The biosynthetic pathways involved in producing microbial PHB are also discussed to highlight the advancements in genetic engineering techniques. Additionally, the paper outlines the factors influencing PHB production while exploring other metabolic pathways and metabolites simultaneously produced along with PHB in a bio-refinery context. Furthermore, the paper explores the effects of extraction methods on PHB yield and quality to ultimately facilitate the commercial production of biodegradable plastics. This review uniquely discusses the developments in research on microbial biopolymers, specifically PHB and also gives an overview of current commercial PHB companies making strides in cutting down plastic pollution and greenhouse gases.