Free nitrous acid-based nitrifying sludge treatment in a two-sludge system enhances nutrient removal from low-carbon wastewater

Enhancing synthetic vinasse treatment efficiency using an integrated UASB-Modified Bardenpho Process

Vinasse poses considerable environmental problems due to its complex composition of organic matter, minerals, and toxic compounds. If discharged into the environment without treatment, it can cause adverse impacts on ecosystems. This research investigated the effectiveness of an integrated treatment system involving an upflow anaerobic sludge blanket (UASB) reactor and the modified Bardenpho process (MBP) for purifying synthetic vinasse. The study lasted for 167 days, during which the integrated UASB-MBP system processed untreated synthetic vinasse with organic loading rates (OLR) ranging from 1.6 to 12.5 kgCOD/m3 day. The UASB-MBP system impressively achieved a COD removal efficiency of 99.41%. Removal efficiencies of approximately 98.14, 99.91, and 99.63% were also achieved for total nitrogen (TN), total phosphorus (TP) and total ammonium (NH4+-N), respectively. The final discharge was 51.06 mg/L. The concentrations of NH4+-N and TN in the outflow of the settlement tank were 0.8-1.2 mg/L and 5.1-7.9 mg/L, respectively. Optimal performance was achieved when the HRT and nitrate recycle ratio were 15.5 h and 200%, respectively. The temperature was kept in the mesophilic range (33-35 °C) during the experiments. These results underscores the potential of the integrated UASB reactor and modified Bardenpho process to provide an effective and eco-friendly approach for concurrent removal of COD and nutrients from vinasse treatment, offering broad prospects for implementation in wastewater treatment.

The Impact of Bisphenol A on the Anaerobic Sulfur Transformation: Promoting Sulfur Flow and Toxic H2S Production

Bisphenol A (BPA), as a typical leachable additive from microplastics and one of the most productive bulk chemicals, is widely distributed in sediments, sewers, and wastewater treatment plants, where active sulfur cycling takes place. However, the effect of BPA on sulfur transformation, particularly toxic H2S production, has been previously overlooked. This work found that BPA at environmentally relevant levels (i.e., 50-200 mg/kg total suspended solids, TSS) promoted the release of soluble sulfur compounds and increased H2S gas production by 14.3-31.9%. The tryptophan-like proteins of microbe extracellular polymeric substances (EPSs) can spontaneously adsorb BPA, which is an enthalpy-driven reaction (ΔH = -513.5 kJ mol-1, ΔS = -1.60 kJ mol-1K -1, and ΔG = -19.52 kJ mol-1 at 35 °C). This binding changed the composition and structure of EPSs, which improved the direct electron transfer capacity of EPSs, thereby promoting the bioprocesses of organic sulfur hydrolysis and sulfate reduction. In addition, BPA presence enriched the functional microbes (e.g., Desulfovibrio and Desulfuromonas) responsible for organic sulfur mineralization and inorganic sulfate reduction and increased the abundance of related genes involved in ATP-binding cassette transporters and sulfur metabolism (e.g., Sat and AspB), which promoted anaerobic sulfur transformation. This work deepens our understanding of the interaction between BPA and sulfur transformation occurring in anaerobic environments.

Enhancement of denitrification by sulfur-based carrier in sequencing batch reactor (SBR) for advanced wastewater treatment

This study was to enhance the nitrogen removal efficiency in the sequencing batch reactor (SBR) process by adding sulfur-based carriers. The nitrogen removal efficiency of the control group was compared with that of the experimental group through a two-series operation of SBR1 without carrier and SBR2 with the carrier under the condition of no external carbon source. A total nitrogen (T-N) removal efficiency of 6.6%, 72.6%, and 79.9% was observed in SBR1, SBR2 (5%), and (10%), respectively. The T-N removal efficiency was improved in the system with carriers, which showed an increase in the removal efficiency of approximately 91.7%. The results suggest that the inclusion of the carrier led to an elevation in the sulfur ratio, implying an augmented surface area for sulfur-based denitrifying microorganisms. Additionally, CaCO3 contributed essential alkalinity for sulfur denitrification, thereby preventing a decline in pH. Regardless of the carrier, the efficiency of organic matter removal surpassed 89%, indicating that the sulfur-based carrier did not adversely affect the biological reaction associated with organic matter. Therefore, autotrophic denitrification was successfully performed using a sulfur carrier in the SBR process without an external carbon source, improving the nitrogen removal efficiency.

Response of the mainstream anammox process to the biodegradable carbon sources in the granule-based systems: The difference in self-stratification of the microbial community

The inevitable introduction of biodegradable carbon sources (such as monosaccharides and volatile fatty acids) originating from pretreatment units might affect the performance of the mainstream anaerobic ammonium oxidation (anammox) process. Two model carbon sources (glucose and acetate) were selected to investigate their effects on granule-based anammox systems under mainstream conditions (70 mg total nitrogen (TN) L^-1, 15 °C). At a nitrogen loading rate of 2.87 ± 0.80 kg N m^-3 d^-1, a satisfactory effluent quality (TN < 10 mg L^-1) was achieved in the presence of glucose or acetate at a chemical oxygen demand (COD/N) ratio of 0.5. The contribution of anammox to nitrogen removal decreased with increasing COD/N ratio to 1.0 because the expression of anammox functional genes was inhibited, whereas the expression of denitrifying functional genes was promoted. However, the nitrogen removal efficiency of the two considered reactors was maintained above 80 %. Self-stratification of the microbial community along the reactor height facilitated a functional balance through the retention of anammox bacteria in granules but resulted in washout of denitrifying bacteria in flocs under a high-flow pattern. These findings highlighted the advantages of granule-based systems in the mainstream anammox process due to their inherent biomass self-segregation property and the need for the development of targeted biomass retention strategies.

Insights into rapidly recovering the autotrophic nitrogen removal performance of single-stage partial nitritation-anammox systems: Reconstructing granular sludge and its functional microbes synergy

Partial nitritation-anammox (PNA) deteriorates easily and is difficult to recover. After an airlift inner-circulation partition bioreactor was impacted by low NH₄⁺-N wastewater containing organic matter, Nitrospira and Denitratisoma propagated rapidly, granular sludge disintegrated, and the total nitrogen removal efficiency (TNRE) decreased from 68.27 % to 5.97 %. This study used a unique strategy to recover deteriorated single-stage PNA systems and explored the mechanism of rapid performance recovery. The TNRE of the system recovered up to 61.77 % in 43 days. The high nitrogen loading rate and hydraulic shear force from the airlift caused the sludge in the reactor to granulate again. The microbial community structure recovered, with a decrease in the abundance of Nitrospira (0.05 %) and enrichment of Candidatus Brocadia (8.82 %). A favorable synergy among functional microbes in the reactor was thus re-established, promoting the rapid recovery of the nitrogen removal performance. This study provides a feasible recovery strategy for PNA processes.

Rhamnolipid increases H2S generation from waste activated sludge anaerobic fermentation: An overlooked concern

Rhamnolipid (RL), one representative biosurfactant, is widely regarded as an economically feasible and environmentally beneficial additive to improve fermentation efficiency and resource recovery from waste activated sludge (WAS). However, its potentially detrimental impact on WAS fermentation such as H₂S generation was overlooked previously. This study therefore aims to fill the gap through exploring whether and how the presence of RL affects H₂S generation from WAS anaerobic fermentation. Experimental results showed that when RL increased from 0 to 40 mg/g total suspended solids (TSS), the cumulative H₂S yield enhanced from 323.6 ×  10^-4 to 620.3 ×  10^-4 mg/g volatile suspended solids (VSS). Mechanism analysis showed that RL reduced WAS surface tension, which benefited transformations of organic sulfurs (e.g., aliphatic-S and sulfoxide) and inorganic sulfate from solid to liquid phase. The presence of RL not only reduced the ratio of α-helix/(β-sheet + random coil) and damaged the hydrogen bonding networks of organic sulfurs but also promoted substrate surface charges and cell membrane permeability. These facilitated the contact between hydrolase and organic sulfurs, thereby increasing sulfide production from organic sulfurs hydrolysis. Further investigations showed that RL promoted the expression of key genes (e.g., aprA/B and dsrA/B) involved in the dissimilatory sulfate reduction, which accelerated the reaction of adenosine 5'-phosphosulfate (APS)→ sulfite→ sulfide. Meanwhile, RL inhibited the corresponding key genes such as CysH, and Sir, responsible for assimilatory sulfate reduction (APS→3'-phosphoadenosine-5'phosphosulfate→organosulfur), which reduced substrate competition in favor of H₂S production from dissimilatory sulfate reduction. Besides, RL decreased the fermentation pH, which benefited the transformation of HS^- to H₂S.

Efficient nitrogen removal from mature landfill leachate in a step feed continuous plug-flow system based on one-stage anammox process

A continuous plug-flow multistage anoxic/oxic (A/O) system based on one-stage partial nitrification coupled anammox (PNA) process with integrated fixed-film activated sludge (IFAS) was established and operated over 400 days. A step feed strategy effectively controlled free ammonia concentration and alleviated impacts on ammonia oxidizing bacteria (AOB) and anammox bacteria (AnAOB). During day 301-405, 98.1% of total inorganic nitrogen was removed from mature landfill leachate, whereas chemical oxygen demand (COD) removal efficiency was 52.9%. With the enrichment of AnAOB in oxic biofilm, nitrogen removal via the anammox pathway reached 94.3%-95.0%. During system operation, the dominant anammox genus shifted from Candidatus_Brocadia to Candidatus_Kuenenia. Fluorescent in situ hybridization (FISH) indicated AnAOB encapsulated by AOB colonies were mainly distributed inside of the biofilm, which promoted nitrite utilization by the anammox process. This innovative system and the results are of great value to practical applications.

Subclinical Hypothyroidism in Families Due to Chronic Consumption of Nitrate-Contaminated Water in Rural Areas with Intensive Livestock and Agricultural Practices in Durango, Mexico

Nitrate is a widely disseminated water pollutant and has been linked to health disorders, including hypothyroidism. Here, we evaluated the relationship between thyroid function and chronic exposure to nitrates in rural zone families, in addition to the genetic and autoimmune factors. Exposure and effect biomarkers, thyroid hormones, and autoantibodies of tiroperoxidase were measured, as well the presence of two FOXE1 polymorphisms (rs965513, rs1867277). Pearson’s correlation, principal component analysis, Kruskal–Wallis, and chi-squared tests were used for statistical analysis. A total of 102 individuals were analyzed; 45% presented subclinical hypothyroidism, a negative correlation was observed between methemoglobin and the total T3 (r = −0.43, p = 0.001) and free T3 levels (r = −0.34, p = 0.001), as well as between TSH and the free T4 (r = −0.41, p = 0.0001) and total T4 (r = −0.36, p = 0.0001). A total of 15.7% had positive antithyroid ab-TPO, while the polymorphic genotype (AA) represented only 3% (rs965513) and 4% (rs1867277) among subjects with subclinical hypothyroidism. The high frequency of subclinical hypothyroidism in the population under study could be related, mainly, to chronic exposure through the consumption of nitrate-contaminated water.

Ecological Insights Into Community Interactions, Assembly Processes and Function in the Denitrifying Phosphorus Removal Activated Sludge Driven by Phosphorus Sources

The microbial characteristics in the wastewater treatment plants (WWTPs) strongly affect their optimal performance and functional stability. However, a cognitive gap remains regarding the characteristics of the microbial community driven by phosphorus sources, especially co-occurrence patterns and community assembly based on phylogenetic group. In this study, 59 denitrifying phosphorus removal (DPR) activated sludge samples were cultivated with phosphorus sources. The results suggested that homogeneous selection accounted for the largest proportion that ranged from 35.82 to 64.48%. Deterministic processes dominated in 12 microbial groups (bins): Candidatus_Accumulibacter and Pseudomonas in these bins belonged to phosphate-accumulating organisms (PAOs). Network analysis revealed that species interactions were intensive in cyclic nucleoside phosphate-influenced microbiota. Function prediction indicated that cyclic nucleoside phosphates increased the activity of enzymes related to denitrification and phosphorus metabolism and increased the α-diversity of microorganism but decreased the diversity of metabolic function. Based on these results, it was assumed that cyclic nucleoside phosphates, rather than inorganic phosphates, are the most available phosphorus source for majority microorganisms in DPR activated sludge. The study revealed the important role of phosphorus source in the construction and assembly of microbial communities and provided new insights about pollutant removal from WWTPs.

Effect of sodium dodecylbenzene sulfonate on hydrogen production from dark fermentation of waste activated sludge

Sodium dodecylbenzene sulfonate (SDBS), a typical surfactant being widely used in various applications, was highly accumulated in waste activated sludge. To date, however, its effect on hydrogen production from dark fermentation of sludge has not been documented. The work therefore aimed to explore whether and how SDBS affects hydrogen production. Experimental results showed that with an increase of SDBS from 0 to 30 mg/g TSS, the maximal hydrogen yield increased from 2.47 to 10.73 mL/g VSS (without any treatment) and from 13.05 to 23.51 mL/g VSS (under free ammonia pretreatment). Mechanism exploration showed that SDBS lowered surface tension, facilitated organics transfer from solid to liquid. SDBS also destroyed hydrogen bonding networks of protein, promoted macromolecular organics degradation. Besides, SDBS improved the electric charge in organics, then weakened the mutual repulsion, improved adsorb, interact and promoted the availability of reaction sites between anaerobes and organic substances. Enzyme activity analysis showed that SDBS not only improved the activities of enzymes related to hydrolysis and acidification processes, but also inhibited the activities of homoacetogens and methanogens. SDBS presence lowered sludge ORP and created an environment which was helpful to the growth of butyric-type bacteria, thus enhanced butyric-type fermentation, which contributed hydrogen production largely. Microbial community analysis revealed that SDBS existence affected distributions of microbial populations, and increased the abundances of hydrogen producing microorganisms (e.g., unclassified_f_Synergistaceae). PICRUSt2 analysis showed that SDBS reduced hydrogenotrophic methanogens activity for its inhibitory effect on the biotransformation of 5,10-Methenyl-THMPT to 5-methyl-THMPT.

Stable nitritation of mature landfill leachate via in-situ selective inhibition by free nitrous acid

In-situ free nitrous acid (FNA) and free ammonia (FA) treatments are more feasible than side-stream methods to achieve nitritation. To assess the optimum conditions and long-term performance of in-situ inhibition by FNA, batch tests and a sequencing batch reactor (SBR) treating mature landfill leachate were conducted and established. As a result, the selective inhibition characteristic by FNA was more conspicuous than FA, and FNA (0.175 mg N/L, 6 h) treatment are more biocidal to nitrite oxidizing bacteria (NOB). Moreover, ammonia oxidizing bacteria (AOB) were more sensitive to the FA environment but its activity recovered preferentially compared to NOB. The SBR achieved a sustained nitrite accumulation rate above 90% for 200 days, with a significant decrease of NOB activity and microbial abundance according to qPCR and 16S rRNA gene sequencing results. In-situ selective inhibition by FNA (0.175 mg N/L, 6 h) has been proved to be effective to maintain stable nitritation.

Effect of microplastic particle size to the nutrients removal in activated sludge system

Microplastics as emerging environmental pollutants, its effect to the bioprocess of water and wastewater treatment has aroused concern. This study investigated the effects of microplastic polystyrene (PS) particle size to the activated sludge nutrient removal process. The ammonia, nitrite, nitrate and phosphorus removal under various PS particle size during nitrification and denitrification process was tested. The results indicated that with PS particle size 150-300 μm, the ammonia oxidation during nitrification process was inhibited to 71%, 92%, and 80% as compared with the blank reactor, for PS concentration at 0.01 g/L, 0.05 g/L and 0.10 g/L, respectively. The nitrite accumulation during nitrification process was also high at PS particle size 150-300 μm and concentration no less than 0.05 g/L. The nitrate reduction during the denitrification process was all inhibited to 69%-94% as compared with the blank, except for reactor No.4. The phosphate removal during nitrification process was not affected by the existence of microplastics PS, the average removal rate was over 80% after 2 h and over 95% after 3 h, respectively. The microplastics particle size plays important role in affecting the activated sludge nutrient removal process.

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

An integrated mainstream aeration and sidestream sludge treatment was demonstrated to be effective in overcoming the adaptationof nitrite oxidizing bacteria (NOB) in an anoxic/oxic process. Results showed that by employing the alternating free nitrous acid and free ammonia (FNA/FA) sidestream sludge treatment alone, nitritation was established but varied, which was addressed by integrating alternating aeration with step feeding (ALASF) in reactor. Two critical considerations contributed to stable effluent nitrite accumulation (>83.8 %)and nitrogen removal (>83.0 %): 1) aerobic sludge rather than return sludge should be taken for FNA/FA treatment to avoid anoxic starvation which facilitated NOB recovery; 2) ALASF ensured timely denitritation and created constant anoxic disturbance for NOB inhibition. Nitrospira and Nitrobacter after 540-day operation were 0.38 % of seed sludge.A20 % reduction of operating cost was obtained in this nitritation process. This study moved nitritation one step closer to application in continuous plug-flow process from municipal wastewater.

Improving the treatment of waste activated sludge using calcium peroxide

The treatment and disposal of waste activated sludge (WAS) has become one of the major challenges for the wastewater treatment plants (WWTPs) due to large output, high treatment costs and enriched substantial emerging contaminants (ECs). Therefore, reducing sludge volume, recovering energy and resource from WAS, and removing ECs and decreasing environmental risk have gained increasing attentions. Calcium peroxide (CaO₂), a versatile and safe peroxide, has been widely applied in terms of WAS treatment including sludge dewatering, anaerobic sludge digestion and anaerobic sludge fermentation due to its specific properties such as generating free radicals and alkali, etc., providing supports for sludge reduction, recycling, and risk mitigation. This review outlines comprehensively the recent progresses and breakthroughs of CaO₂ in the fields of sludge treatment. In particular, the relevant mechanisms of CaO₂ enhancing WAS dewaterability, methane production from anaerobic digestion, short-chain fatty acids (SCFA) and hydrogen production from anaerobic fermentation, and the removal of ECs in WAS and role of experiment parameters are systematically elucidated and discussed, respectively. Finally, the knowledge gaps and opportunities in CaO₂-based sludge treatment technologies that need to be focused in the future are prospected. The review presented can supply a theoretical basis and technical reference for the application of CaO₂ for improving the treatment of WAS.

Responses of microbial structures, functions, metabolic pathways and community interactions to different C/N ratios in aerobic nitrification

The responses of microbial structures, functional profiles and metabolic pathways during nitrification to four C/N ratios (0, 5, 10 and 15) were investigated in four parallel SBRs denoted as S₀, S₅, S₁₀, S₁₅. Results indicated that microbial diversities were affected by C/N ratios, while the same dominant taxa were observed, mainly including Proteobacteria, Betaproteobacteria, Rhodocyclales, Rhodocyclaceae, Zoogloea. The unique biomarkers were identified in each sludge sample through LEfSe analysis. Functional genera/enzymes responsible for removing organics and nitrogen coexisted in four SBRs at different abundances, except for that ammonia oxidizing bacteria (AOB) Nitrosomonas (0.33%-0.66%) and ammonia monooxygenase (amo) (9.4 × 10^-7-2.8 × 10^-6) were only detected in S₀. Moreover, PICRUSt analysis indicated similar overall patterns of metabolic pathways in four sludge samples. The network analysis revealed that total nitrogen removal positively correlated with hcp (Spearman's ρ of 0.853), and ammonia oxidizing rate was associated with amo (Spearman's ρ of 0.096).

Role of polyurethane-modified layered double hydroxides on SCFAs extraction from waste activated sludge fermentation liquid for elevating denitrification: Kinetics and mechanism

Extraction of short-chain fatty acids (SCFAs) from fermentation liquid of waste activated sludge (WAS) is the key bottleneck hindering its application as electron donor in denitrification. This study explores the feasibility of polyether-type polyurethane (PU)-modified layered double hydroxides (LDHs, prepared using eggshell waste as calcium source) in SCFAs adsorbing from WAS fermentation liquid (SFL). The adsorption parameters were first optimized by adsorption tests using artificial fermentation liquid (AFL). Then, adsorption kinetics, thermodynamic and isotherms were explored to further understand the adsorption mechanism. It revealed that SCFAs absorption by PU-LDHs from SFL was an endothermic and spontaneous process with positive enthalphy (ΔH^◦) values and negative Gibbs free energy (ΔG^◦) values. In addition, the maximum adsorption capacity of 208.0 mg SCFAs/g PU-LDHs was obtained from the Langmuir isotherm. Noting that both soluble carbohydrates and soluble proteins were simultaneously extracted, with efficiencies of 30.9%, 6.2%, respectively, compared with 62.9% SCFAs. The reuse tests confirmed that the prepared PU-LDHs can be used at least three times with high adsorptive capacity. With PU-LDHs-loaded SFL as external carbon source in the biodenitrification process, a denitrification rate of 0.014 mg NO₃^--N/mg mixed liquid suspended solids (MLSS)·d was recorded. This study provided a sound basis for the preparation of cost-effective biodenitrification carbon source from SFL by a novel adsorbent.

Characteristics of flow regime adjustment enhancing carbon source recovery in activated primary sedimentation tank

In this study, different numbers of baffles were arranged in settling zone of an activated primary sedimentation tank (APST) to adjust the flow regime of sewage, and the characteristics of flow regime adjustment enhancing carbon source recovery in the APST were explored. The results showed that, compared with the APST without baffles, setting one baffle at the front end of settling zone led to the lowest sedimentation loss of soluble chemical oxygen in the settling zone, and the SCOD and volatile fatty acid concentrations in the effluent increased by 52 mg/L and 4.49 mg/L, respectively, furthermore, the SCOD/total nitrogen (TN), and SCOD/total phosphorus (TP) in the effluent also increased by 22.47%, and 11.95%, respectively. To evaluate the mechanism of setting baffles for carbon source recovery, the numerical simulation of flow regime adjustment was utilized. The results showed that, under the condition of setting one baffle, three large recirculation zones were formed which increased the probability of collision and friction between particulates to avoid the dissolved carbon that had been desorbed in the mechanical agitation zone from being re-adsorbed and settling in the settling zone. Moreover, the baffle changed the streamlines of sewage, thereby resuspending more small carbon sources that had already settled and increasing the carbon source content of effluent. Thus, this study provided a new method of adjusting sewage flow regime in APST for enhancing the efficiency of carbon source recovery which could help improving biological nitrogen and phosphorus removal in wastewater treatment plants.

Norfloxacin-induced effect on enhanced biological phosphorus removal from wastewater after long-term exposure

In this study, long-term experiments were performed under synthetic wastewater conditions to evaluated the potential impacts of norfloxacin (NOR) (10, 100 and 500 μg/L) on enhanced biological phosphorus removal (EBPR). Experimental result showed that long-term exposure to 10 μg/L NOR induced negligible effects on phosphorus removal. The presence of 100 μg/L NOR slightly decreased phosphorus removal efficiency to 94.41 ± 1.59 %. However, when NOR level further increased to 500 μg/L, phosphorus removal efficiency was significantly decreased from 97.96 ± 0.8 5% (control) to 82.33 ± 3.07 %. The mechanism study revealed that the presence of 500 μg/L NOR inhibited anaerobic phosphorus release and acetate uptake as well as aerobic phosphorus uptake during long-term exposure. It was also found that 500 μg/L NOR exposure suppressed the activity of key enzymes related to phosphorus removal but promoted the transformations of intracellular polyhydroxyalkanoate and glycogen. Microbial analysis revealed that that the presence of 500 μg/L NOR reduced the abundances of polyphosphate accumulating organisms but increased glycogen accumulating organisms, as compared the control.

Adaptation of neutrophilic Paracoccus denitrificans to denitrification at highly alkaline pH

Bacterial denitrification is widely documented at neutral pH in order to improve the removal of nitrate in wastewater treatment processes. However, certain industrial contexts generate alkaline waste and effluent containing nitrate that must be denitrified. To obtain more information on denitrification at alkaline pH, this study evaluated the possibility of adapting a neutrophilic denitrifying strain, Paracoccus denitrificans, to alkaline pH. Firstly, P. denitrificans' denitrifying activity was evaluated without acclimation in batch bioreactors at pH 7.0, 8.0, 9.0 and 10.0. Then, two acclimation methods using successive batch bioreactors and a continuous bioreactor allowed P. denitrificans to be gradually exposed to alkaline pH: from 8.5 to 11.2 in 26 and 72 days respectively. Results showed that P. denitrificans could grow and catalyse nitrate reduction (i) at pH 9.0 without acclimation, (ii) at pH 10.5 in successive batch cultures with progressively increasing pH and (iii) at pH 10.8 in continuously fed culture with a hydraulic retention time (HRT) of 8 days. It was shown that denitrification affected the pH despite the presence of carbonate buffering of the P. denitrificans growth medium. With acetate as an electron donor, the pH of a carbonate buffered medium tends towards pH 10 during the process of denitrification. Graphical abstract.

Recovering partial nitritation in a PN/A system during mainstream wastewater treatment by reviving AOB activity after thoroughly inhibiting AOB and NOB with free nitrous acid

Starting up or recovering partial nitritation is a major challenge for achieving or maintaining stable partial nitritation/anammox (PN/A) during mainstream wastewater treatment. This study presents a novel strategy for recovering the nitrite pathway by selectively reviving ammonium oxidizing bacteria (AOB) after thoroughly inhibiting AOB and nitrite oxidizing bacteria (NOB) using free nitrous acid (FNA). A sequencing batch reactor was operated for PN/A to treat real domestic wastewater for 423 days, during which twice FNA treatment was temporarily implemented. Results showed that with a single 0.45 mg/L FNA treatment on flocculent sludge, the NO₃^--N concentration during the aerobic period showed an uptrend again and the partial nitritation performance was deteriorated. In contrast, 1.35 mg/L FNA treatment induced the inhibition of both AOB and NOB leading to regressive ammonium oxidation, but a subsequently higher DO (1.5 mg/L) and longer aeration duration recovered partial nitritation. For the relative abundances of the acquired biomass related to nitrogen conversion, Nitrosomonas, Nitrospira and Nitrolancea increased to 9.65%, 10.27% and 4.35%, respectively, at the beginning of the 1.35 mg/L FNA treatment, and Nitrospira and Nitrolancea decreased to 2.80% and 0.03% whereas Nitrosomonas declined to 8.71% after 76 days. Ca. Brocadia showed less resilience after the 1.35 mg/L FNA treatment, with the relative abundance decreasing from 13.38% to 0.62% due to insufficient nitrite. Molecular ecological network analysis indicates that among anammox taxa, Ca. Kuenenia and Ca. Brocadia formed important links with other N cycle processes. Moreover, the proposed strategy shows operational flexibility because it can be easily used to control NOB in mainstream PN/A applications offered by flocculent sludge systems.

Elucidating performance failures in use of granular sludge for nutrient removal from domestic wastewater in a warm coastal climate region

The effects of domestic wastewater and a coastal warm climate on granular sludge and biological nutrient removal were evaluated using a pilot-scale sequencing batch reactor (SBR). The reactor operation employed two different operational strategies (OS) based on up-flow feeding regimes, defined as fast (OS1, flow rate = 18.0 L min^-1 and flow velocity = 22.0 m h^-1) and slow (OS2, flow rate = 3.5 L min^-1 and flow velocity = 4.3 m h^-1). Under slow feeding, larger (OS1: 290 µm; OS2: 450 µm) and faster settling granules were obtained (OS1: 109; OS2: 74 mL g^-1 TSS). The slow feeding regime was also advantageous for the hydrolysis of particulate organic matter (OS1: 1.3; OS2: 3.1 g COD_tot g^-1 VSS d^-1) and for phosphorus removal (OS1: <33%; OS2: >97.5%). Neither strategy resulted in substantial biomass accumulation in the reactor (OS1: 0.7; OS2: 1.5 g VSS L^-1), and high concentrations of nitrite were observed in the effluent (9-27 mg [Formula: see text] L^-1). Ordinary heterotrophic organisms dominated the granular sludge developed under both feeding regimes (OS1: 30% of Thauera; OS2: 56% of Comamonas), while polyphosphate-accumulating organisms (PAOs) were only detected during OS2 (2.3-3.4% of total bacteria). A successful granular sludge process should be able to cope with high fluctuations in wastewater loads caused by rain events (82-182 mm month^-1 in Florianopolis, Brazil). In order to achieve higher water quality, strategies identified for an efficient granular sludge SBR operation included (i) management of an anaerobic phase for PAO selection, and (ii) aeration control for successful nitrification/denitrification.

Aerobic granular sludge for high-strength ammonium wastewater treatment: Effect of COD/N ratios, long-term stability and nitrogen removal pathways

Aerobic granular sludge (AGS) technology is increasingly considered for wastewater treatment. AGS stability particularly under lower COD/N ratios is an impediment for AGS technology. This study evaluated AGS stability and nitrogen removal at different loading rates of 0.03 to 4 kg NH4+-N m-3 d-1 and COD/N ratios of 18.3 to 0.13. Ammoniacal and total nitrogen removals were high at 99.9% and 99.3%, respectively, during 440 days. MiSeq sequencing revealed a reduction in bacterial diversity and enrichment of ammonia oxidizing bacteria (AOB), anammox and denitrifying bacteria. Quantitative PCR showed enrichment of AOB, anammox bacteria, Nitrospira and denitrifiers. Chemical data and bacterial community supported occurrence of nitritation and anammox pathways. AGS had stable granular structure with excellent settling properties at lower COD/N ≤ 1. Removal of high-strength ammonium could be partly explained by the existing nitrogen pathways suggesting novel mechanisms. Nevertheless, results presented here support implementation of AGS process for ammonium wastewaters.

Innovative, compact and energy-efficient biofilm process for nutrient removal from wastewater

Rapid population growth, industrial development and stringent demand for treatment of wastewater require developing and emerging economies to upgrade existing wastewater treatment plants (WWTPs) or planning new WWTPs. In the context of unavailability or unaffordability of land and resources for infrastructure expansion, low cost, small footprint, less energy consumption and product reuse are some of the major factors to be considered when either upgrading or designing new WWTPs in developing and emerging economies. Although the transition from activated sludge to biofilm processes has partly solved these challenges, there are innovations that can make the processes even more compact and more efficient. Newly developed CFIC (continuous flow intermittent cleaning) process is the next generation moving bed biological wastewater treatment system and is an example for addressing these issues. The CFIC pilot studies showed promising performance for biological chemical oxygen demand and nitrogen removal as well as particle separation facilitating wastewater reuse.

Improving wastewater management using free nitrous acid (FNA)

Free nitrous acid (FNA), the protonated form of nitrite, has historically been an unwanted substance in wastewater systems due to its inhibition on a wide range of microorganisms. However, in recent years, advanced understanding of FNA inhibitory and biocidal effects on microorganisms has led to the development of a series of FNA-based applications that improve wastewater management practices. FNA has been used in sewer systems to control sewer corrosion and odor; in wastewater treatment to achieve carbon and energy efficient nitrogen removal; in sludge management to improve the sludge reduction and energy recovery; in membrane systems to address membrane fouling; and in wastewater algae systems to facilitate algae harvesting. This paper aims to comprehensively and critically review the current status of FNA-based applications in improving wastewater management. The underlying mechanisms of FNA inhibitory and biocidal effects are also reviewed and discussed. Knowledge gaps and current limitations of the FNA-based applications are identified; and perspectives on the development of FNA-based applications are discussed. We conclude that the FNA-based technologies have great potential for enhancing the performance of wastewater systems; however, further development and demonstration at larger scales are still required for their wider applications.

New perspectives on microbial communities and biological nitrogen removal processes in wastewater treatment systems

Biological nitrogen removal (BNR) is a critical process in wastewater treatment. Recently, there have new microbial communities been discovered to be capable of performing BNR with novel metabolic pathways. This review presents the up-to-date status on these microorganisms, including ammonia oxidizing archaea (AOA), complete ammonia oxidation (COMAMMOX) bacteria, anaerobic ammonium oxidation coupled to iron reduction (FEAMMOX) bacteria, anaerobic ammonium oxidation (ANAMMOX) bacteria and denitrifying anaerobic methane oxidation (DAMO) microorganism. Their metabolic pathways and enzymatic reactions in nitrogen cycle are demonstrated. Generally, these novel microbial communities have advantages over canonical nitrifiers or denitrifiers, such as higher substrate affinities, better physicochemical tolerances and/or less greenhouse gas emission. Also, their recent development and/or implementation in BNR is discussed and outlook. Finally, the key implications of coupling these microbial communities for BNR are identified. Overall, this review illustrates novel microbial communities that could provide new possibilities for high-performance and energy-saving nitrogen removal from wastewater.

Co-metabolic substrates enhanced biological nitrogen removal from cellulosic ethanol biorefinery wastewater using aerobic granular sludges

The effect of different co-metabolic substrates (glucose, acetic acid and ethanol) on aerobic granular sludge treating cellulose ethanol wastewater was investigated using sequencing batch reactors. The efficiencies of the three substrates in removing chemical oxygen demand were respectively 18.87%, 28.88% and 27.99%, all of which were remarkably higher than without co-metabolic substrates, indicating that co-metabolic substrates can promote the degradation of the refractory substances. With acetic acid as the co-metabolic substrate, the removal amount of ammonium nitrogen and nitrate nitrogen was greater than glucose and ethanol used. The nitrogen removals by the three co-metabolic substrates were 53.18%, 72.15%, 69.36%, respectively, which were 1.4, 1.8, and 1.6 times the removal without co-metabolic substrates. Fluorescence in situ hybridization results showed that the proportion of ammonium-oxidizing bacteria in the granular sludge was greater than that of nitrite-oxidizing bacteria after adding co-metabolic substrates, and the order was acetic acid > ethanol > glucose.

Characterization of aerobic granular sludge of different sizes for nitrogen and phosphorus removal

Granular size plays a key role in the performance of the aerobic granular sludge (AGS). As the diameter of the granule increases, stratification may begin to appear due to the increase in mass transfer resistance. Aerobic granules harvested from a lab-scale anaerobic-aerobic sequencing batch reactor (AO-SBR) were classified into three categories according to their size: (a) 0.15-0.28 mm, (b) 0.28-0.45 mm and (c) larger than 0.45 mm. In this study, the categories were called small-size, medium-size and large-size granules, respectively. A fraction of the different forms of phosphate and denitrification efficiency was investigated in each category. Results show that small-size granules present much more easily mobile phosphorus than other granules. Moreover, the denitrification performance has been tested by using dumping and trickling patterns for COD and NO3--N feeding. The results demonstrated that the large-size granules exhibit poor denitrification rates, as opposed to the medium-size granules. Therefore, medium-size granules, with a size of 0.28-0.45 mm, are regarded as the most suitable granular size for AGS in this experiment from the perspective of denitrification and phosphorus removal.

Nitrate addition improves hydrogen production from acidic fermentation of waste activated sludge

In this work, a low-cost alternative method (i.e., adding nitrate into WAS) to significantly enhance hydrogen production was reported. Experimental results showed that with an increase of nitrate addition from 0 to 362 mg/L, the maximal hydrogen production from acidic (pH 5.5) fermentation of WAS obviously increased from 12.6 ± 0.5 to 19.3 ± 0.9 mL per gram volatile suspended solids (VSS). The mechanism investigations illustrated more substrates were provided for subsequent hydrogen production. Although the nitrate added inhibited all the biological processes, its inhibition to the hydrogen consumption processes was much severer than that to the hydrogen production processes. The enzyme analyses on the long-term semi-continuous fermenters showed that the nitrate addition slightly inhibited the relative activities of protease, butyrate kinase, acetate kinase, CoA-transferase, and [FeFe] hydrogenase but largely suppressed the relative activities of coenzyme F₄₂₀, carbon monoxide dehydrogenase, and adenylyl sulfate reductase.

Achieving efficient nitrogen removal from real sewage via nitrite pathway in a continuous nitrogen removal process by combining free nitrous acid sludge treatment and DO control

The incomplete denitrification due to insufficient carbon resource in the wastewater treatment plants (WWTPs) resulted in low nitrogen removal efficiency, which has become a widespread problem in China and all around the world. Reducing the requirement of carbon source by manipulating the nitrogen removal pathway from conventional nitrification-denitrification to partial nitrification-denitrification is considered as an efficient solution. In this article, the feasibility of combining free nitrous acid (FNA) sludge treatment and DO control to achieve partial nitrification-denitrification in a continuous flow system (aerobic-anoxic-oxic process) using real sewage was assessed. The nitrite pathway was rapidly established in the experimental reactor within 23 days by simultaneously lowering DO concentration in aerobic zone to 0.5 mg/L and treating 30% of the activated sludge per day from the reactor in the FNA sludge treatment unit with FNA concentration of 1.2 mg N/L and exposure time of 18 h. The nitrite oxidizing bacteria (NOB) were efficiently washed out and the partial nitrification process could maintain stable in the experimental reactor even after cease of FNA treatment and increase of DO concentrations in the main stream to 1.5 mg/L, with an average nitrite accumulation rate of above 78%. In contrast, the nitrite accumulation rate was just around 58% during low DO concentrations phase and declined quickly to below 1% after the DO concentrations were increased to 1.5 mg/L in the control reactor which only utilized single strategy of DO control to achieve nitrite pathway. Moreover, a better sludge settleability and nitrogen removal performance could also be realized in the experimental reactor. The results of nitrifying bacteria activities and quantities detection demonstrated that although NOB activities in both reactors were effectively inhibited, a certain amount of NOB (6.26 × 10⁶ copies/g MLSS) were remained in the control reactor and multiplied rapidly as the DO concentration increased, which might break down the partial nitrification. Furthermore, the quantity results of nitrogen cycling related functional genes showed that the increment of the ratio of nitrate reduced bacteria to total bacteria was 0.35% larger than that of nitric oxide bacteria in the control reactor, while those two ratios increased similarly by 1.11% and 1.12% in the experimental reactor, respectively, which might be one potential cause of reduction in N₂O emission of nitrite pathway achieved by FNA-based technologies.

High-efficient nitrogen removal from mature landfill leachate and waste activated sludge (WAS) reduction via partial nitrification and integrated fermentation-denitritation process (PNIFD)

Biological nitrogen removal from mature landfill leachate is ineffective due to the extremely low carbon/nitrogen (C/N) ratio. Moreover, a large amount of waste activated sludge (WAS) is inevitably generated from WWTPs during the municipal sewage treatment process. In this study, an innovative process was developed to enhance nitrogen removal from low C/N (1:1) mature landfill leachate and to reduce the WAS during a 300-day operation. Two sequencing batch reactors (SBRs) were involved in this process. Firstly, the mature landfill leachate was pumped into an aerobic reactor to undergo partial nitrification (PN-SBR). Then, the PN-SBR effluent and WAS were pumped into an anoxic reactor to undergo integrated fermentation and denitritation (IFD-SBR). The pH profile was treated as a real-time parameter to precisely control the duration of the PN and IFD processes. Partial nitrification and integrated fermentation-denitritation (PNIFD) system achieved a total nitrogen removal efficiency of 95.0% and an average nitrogen removal rate (NRR) of 0.63 kg/m³·d during the last operational phase. Due to a variety of refractory contaminants, the effluent COD concentration was 1865.9 mg/L and a 19.7% COD removal efficiency was obtained under an influent concentration of 2324.5 mg/L. Compared with the traditional nitrogen removal process, PNIFD not only decreased requirements for oxygen by 25% and the external organic carbon by 100%, but also achieved simultaneous reduction of external WAS. More than 53.7% of the external sludge was reduced during the IFD-SBR operational cycle, with an average external sludge reduction rate (SRR) of 5.09 kg/m³·d. Fermentation/denitritation related microorganisms, such as Anaerolineaceae, Acidimicrobiaceae and Thauera, accounted for up to 41.5% of the total abundance in the IFD-SBR. Based on the long and stable operation, this study provides a simple and promising approach for synchronous nitrogen removal and WAS reduction.

Influence of aluminium accumulation on biological nitrification and phosphorus removal in an anoxic-oxic membrane bioreactor

Poly-aluminium chloride (PAC) is often used to enhance phosphorus removal and control membrane fouling in membrane bioreactors (MBRs). However, the influence of aluminium accumulation on the biological nitrification and phosphorus removal of MBRs has not been well assessed. In the present study, the effects of accumulated aluminium on sludge activity and morphology were investigated in a lab-scale anoxic-oxic membrane bioreactor. The reasonably high removal efficiencies of NH₄⁺-N, TN, and COD, i.e. 94.9%, 84.8%, and 92.8%, respectively, were achieved in the reactor when the percentage of atomic aluminium on sludge surface increased to 14.2%. However, the decreases in the ammonia oxidation rate, nitrite oxidation rate, and specific oxygen uptake rate of sludge by 82.1%, 79.8%, and 46.4%, respectively, were observed. Meanwhile, the activity of phosphate-accumulating organisms was completely inhibited. Furthermore, the protein content in the extracellular polymeric substances of sludge decreased substantially, and the sludge became more dispersed due to the alum accumulation, compared with that of the initial phase. Therefore, long-term dosing of PAC in the MBR should be managed to avoid excessive aluminium accumulation in the sludge.

Enhanced hydrogen accumulation from waste activated sludge by combining ultrasonic and free nitrous acid pretreatment: Performance, mechanism, and implication

This study presents a novel and effective method, i.e., adding nitrite into acidic fermentations after ultrasonic (US) pretreatment to form free nitrous acid (FNA), to further enhance hydrogen yield. Experimental results showed that when 180 mg/L nitrite was added into the US (2 W/mL, 15 min) pretreated waste activated sludge (WAS), the maximal hydrogen yield of 24.81 ± 1.24 mL/g VSS (volatile suspended solids) was obtained under acidic fermentation (1.0 mg/L FNA was initially formed under this condition), which was 2.21-folds (or 1.36-folds) of that from US pretreatment (or FNA treatment) alone. This combination approach caused a positive synergy on sludge disintegration and enhanced the transformation of the released organics from non-biodegradable to biodegradable. Further study showed that the inhibiting effect of this combination method on hydrogen consuming microorganism was severer. Considering its pollution free, this combination strategy is an attractive technology for hydrogen recovery from WAS.

Treatment of low strength wastewater using compact submerged aerobic fixed film (SAFF) reactor filled with high specific surface area synthetic media

Studies on laboratory-scale submerged aerobic fixed film reactor (SAFF) packed with synthetic media having specific surface area of 165 m²/m³ with a void volume of 89% were carried out to assess its performance under various organic loading rates (OLR) and hydraulic retention times (HRT). Synthetic wastewater having chemical oxygen demand (COD) and biochemical oxygen demand (BOD) of 400 ± 10% and 210 ± 10% mg/L respectively was fed and the reactor was subjected to OLRs ranging from 0.37 to 1.26 kg COD/m³.d. It was observed that steady sloughing of biofilm occurs within the SAFF reactor all the times and average concentration of sloughed biomass in the effluent was 26 mg/L. The COD and BOD removal efficiencies varied between 85 and 89% and 86 to 94%, respectively. The kinetic studies demonstrated that SAFF reactor followed Stover-Kincannon and Grau models, with high correlation coefficients (R²) of 0.9977 and 0.9916, respectively. Thus, the values of kinetic coefficients such as maximum substrate utilization rate, U_max = 64.1 g/(L^.d); saturation value constant, K_B = 72.31 g/(L.d) and Grau second-order substrate removal rate constant, Ks = 2.44 day^-1 can be useful to develop and design large scale SAFF reactors. Finally, the study reveals that the optimum range for OLR can vary within 0.68-0.94 kg COD/m³.d.

Improving Wastewater Nitrogen Removal and Reducing Effluent NOx - -N by an Oxygen-Limited Process Consisting of a Sequencing Batch Reactor and a Sequencing Batch Biofilm Reactor

A series of reactors including a sequencing batch reactor (SBR) and a sequencing batch biofilm reactor (SBBR) were used for nitrogen removal. The aim of this study was simultaneous removal of NH4+-N and NOx–-N from synthetic wastewater. In the novel proposed method, the effluent from SBR was sequentially introduced into SBBR, which contained 0.030 m3 biofilm carriers, so the system operated under a paired sequence of aerobic-anoxic conditions. The effects of different carbon sources and aeration conditions were investigated. A low dissolved oxygen (DO) level in the biofilm depth of the fixed-bed process (SBBR) simulated the anoxic phase conditions. Accordingly, a portion of NH4+-N that was not converted to NO3–-N by the SBR process was converted to NO3–-N in the outer layer of the biofilm in the SBBR process. Further, simultaneous nitrification and denitrification (SND) was achieved in the SBBR where NO2–-N was converted to N2 directly, before NO3–-N conversion (partial nitrification). The level of mixed liquid suspended solids (MLSS) was 2740 mg/l at the start of the experiments. The required carbon source (C: N ratio of 4) was provided by adding an internal carbon source (through step feeding) or ethanol. Firstly, as part of the system (SBR and SBBR), SBR operated at a DO level of 1 mg/l while SBBR operated at a DO concentration of 0.3 mg/l during Run-1. During Run-2, the system operated at the low DO concentration of 0.3 mg/l. When the source of carbon was ethanol, the nitrogen removal rate (RN) was higher than the operation with an internal carbon source. When the reactors were operated at the same DO concentration of 0.3 mg/l, 99.1 % of the ammonium was removed. The NO3–-N produced during the aerobic SBR operation of the novel method was removed in SBBR reactor by 8.3 %. The concentrations of NO3--N and NO2–-N in the SBBR effluent were reduced to 2.5 and 5.5 mg/l, respectively. Also, the total nitrogen (TN) removal efficiency was 97.5 % by adding ethanol at the DO level of 0.3 mg/l.

When C:N adjustment was carried out SND efficiency at C:N ratio of 6.5 reached to 99 %. The increasing nitrogen loading rate (NLR) to 0.554 kg N/m3 d decreased SND efficiency to 80.7 %.

Enhanced short-chain fatty acids production from waste activated sludge by sophorolipid: Performance, mechanism, and implication

It was found in this study that the presence of sophorolipid (SL) enhanced the production of short-chain fatty acid (SCFA) from anaerobic fermentation of waste activated sludge (WAS). Experimental results showed that with an increase of SL addition from 0 to 0.1 g/g TSS, the maximal SCFA yield increased from 50.5 ± 4.9 to 246.2 ± 7.5 mg COD/g VSS. The presence of SL reduced the surface tension between hydrophobic organics and fermentation liquid, which thereby accelerated the disintegration of WAS and improved the biodegradability of the released organics. SL promoted the carbon/nitrogen ratio of the fermentation system, enhancing the conversion of proteins in WAS. Moreover, SL suppressed severely the activities of methanogens, probably due to the drop of pH caused by SL addition. Amplicon sequencing analyses revealed that SL increased the abundance of hydrolytic microbes such as Bacteroides sp. and Macellibacteroides sp., and SCFA producers (e.g., Acinetobacter sp.).

Impact of different running conditions on performance of biofilters treating secondary effluent during start-up

In this study, the impact of different aeration sequences, hydraulic retention time (HRT), aeration time ratios and external carbon types on performance of biofilters treating secondary effluent during start-up was explored. For an intermittently aerated nitrifying filter, the adjustment of aeration sequence with prior nitrification or denitrification had no significant impact on overall performance. Extending HRT promoted sedimentation and denitrification. However, extending HRT to enhance performance is inadvisable due to incompleted denitrification. Similarly, the impact of different aeration time ratios on performance was not as apparent as that of start-up time. External carbon addition to denitrifying filter could enhance performance with glucose more favorable for denitrification and sodium acetate better for P sequestration. When the synthetic over-standard secondary effluent treated by the nitrifying and denitrifying filters in order with the latter added with glucose, all the monitored indices could reach the A standard (GB 18918-2002, China).

Population Structure and Morphotype Analysis of "Candidatus Accumulibacter" Using Fluorescence In Situ Hybridization-Staining-Flow Cytometry

"Candidatus Accumulibacter" is the dominant polyphosphate-accumulating organism (PAO) in denitrifying phosphorus removal (DPR) systems. In order to investigate the community structure and clade morphotypes of "Candidatus Accumulibacter" in DPR systems through flow cytometry (FCM), denitrifying phosphorus removal of almost 100% using nitrite and nitrate as the electron acceptor was achieved in sequencing batch reactors (SBRs). An optimal method of flow cytometry combined with fluorescence in situ hybridization and SYBR green I staining (FISH-staining-flow cytometry) was developed to quantify PAOs in DPR systems. By setting the width value of FCM, bacterial cells in a sludge sample were divided into three groups in different morphotypes, namely, coccus, coccobacillus, and bacillus. Average percentages that the three different PAO populations accounted for among total bacteria from SBR1 (SBR2) were 42% (45%), 14% (13%), and 4% (2%). FCM showed that the ratios of PAOs to total bacteria in the two reactors were 61% and 59%, and the quantitative PCR (qPCR) results indicated that IIC was the dominant "Candidatus Accumulibacter" clade in both denitrifying phosphorus removal systems, reaching 50% of the total "Candidatus Accumulibacter" bacteria. The subdominant clade in the reactor with nitrite as the electron acceptor was IID, accounting for 31% of the total "Candidatus Accumulibacter" bacteria. The FCM and qPCR results suggested that clades IIC and IID were both coccus, clade IIF was coccobacillus, and clade IA was bacillus. FISH analysis also indicated that PAOs were major cocci in the systems. An equivalence test of FCM-based quantification confirmed the accuracy of FISH-staining-flow cytometry, which can meet the quantitative requirements for PAOs in complex activated sludge samples.IMPORTANCE As one group of the most important functional phosphorus removal organisms, "Candidatus Accumulibacter," affiliated with the Rhodocyclus group of the Betaproteobacteria, is a widely recognized and studied PAO in the field of biological wastewater treatment. The morphotypes and population structure of clade-level "Candidatus Accumulibacter" were studied through novel FISH-staining-flow cytometry, which involved denitrifying phosphorus removal (DPR) achieving carbon and energy savings and simultaneous removal of N and P, thus inferring the different denitrifying phosphorus removal abilities of these clades. Additionally, based on this method, in situ quantification for specific polyphosphate-accumulating organisms (PAOs) enables a more efficient process and more accurate result. The establishment of FISH-staining-flow cytometry makes cell sorting of clade-level noncultivated organisms available.

Stable partial nitrification of domestic sewage achieved through activated sludge on exposure to nitrite

Partial nitrification is crucial for application of autotrophic nitrogen removal which is beneficial for treating carbon-limited wastewater. This study presents an alternative strategy for achieving partial nitrification of domestic wastewater treatment, by treating activated sludge of a nitrifying reactor on long-term nitrite exposure. Initially, the nitrifying reactor obtained complete nitrification. After the sludge of the reactor was treated with nitrite at concentration of 5-30 mg·L^-1 without feeding for 32 days, the nitrite accumulation ratio (NAR) above 90% was achieved in 30 days, and then the NAR stabled at 97.9% under low temperature of 16.6 °C. Further analysis showed that the activity decay rate of ammonium-oxidizing bacteria (AOB) (0.020 d^-1) was lower than that of nitrite-oxidizing bacteria (NOB) (0.035 d^-1) under nitrite stress. Meanwhile, the NOB were effectively suppressed while AOB were dominant. These observations supported the feasibility of achieving stable partial nitrification by treating sludge on long-term exposure to nitrite.

Free ammonia aids ultrasound pretreatment to enhance short-chain fatty acids production from waste activated sludge

Ultrasound pretreatment was proved to be effective in accelerating waste activated sludge (WAS) disintegration and promoting anaerobic fermentative short-chain fatty acids (SCFA) production. However, due to the high activities of SCFA consumers, SCFA yield is limited. Herein we reported an efficient strategy, i.e., using free ammonia (FA) to aid ultrasound pretreatment, to enhance SCFA accumulation. Experimental results showed that the greatest SCFA yield of 316.7 ± 14.9 mg COD/g VSS was obtained at 2 w/mL ultrasound pretreatment for 15 min followed by initial 60 mg/L FA pretreatment for 2 d, which was 2.2 times (1.7 times) of that from sole ultrasound (FA) pretreatment. The mechanism study revealed that this combination method accelerated sludge solubilization, enhanced the release of biodegradable organics, and caused severe inhibitions to SCFA consumption. Moreover, this method has inhibitory effects on specific activities of hydrolytic microorganisms, SCFA-producing bacteria and methanogens, but the inhibitions to methanogens were severe.

Achieving completely anaerobic ammonium removal over nitrite (CAARON) in one single UASB reactor: Synchronous and asynchronous feeding regimes of organic carbon make a difference

At least 11% of total nitrogen (TN) remains in the anammox effluent, making it difficult to meet increasingly stringent discharge standards. To overcome this bottleneck, an innovative process to achieve completely anaerobic ammonium removal over nitrite (CAARON) in one single up-flow anaerobic sludge blanket reactor was proposed in this study. The synchronous feeding of acetate at a C/N (nitrite) ratio of 0.6 significantly reduced the nitrogen removal capacity of anammox reactor by limiting the abundance and metabolism of anammox bacteria. In contrast, the asynchronous feeding of acetate optimized the partition of the reactor column into two specific compartments: the lower half favoring anammox and the upper half dominated by DEAMOX (DEnitrifying AMmonium Oxidation). A high TN removal efficiency of 96.2±0.4% and a low effluent TN concentration of 9.3±0.9mgL^-1 were obtained under a high TN loading rate of 9.0kgNm^-3d^-1. The dominant functional microbes in the CAARON process were identified as Candidatus Kuenenia and Thauera, which were responsible for the anammox and denitratation reactions, respectively. Overall, the results in this study provide valuable insight into the coupling of anammox with denitratation, which is a cost-efficient approach for treating ammonium-rich wastewaters.

The roles of free ammonia (FA) in biological wastewater treatment processes: A review

Free ammonia (FA) can pose inhibitory and/or biocidal effects on a variety of microorganisms involved in different biological wastewater treatment process, which is widely presented in wastewater treatment plants (WWTPs) due to the high levels of ammonium in the systems. This review article gives the up-to-date status on several essential roles of FA in biological wastewater treatment processes: the impacts of FA, mechanisms of FA roles, modeling of FA impacts, and implications of FA for wastewater treatment. Specifically, the impacts of FA on both wastewater and sludge treatment lines were firstly summarized, including nitrification, denitrification, anaerobic ammonium oxidation (Anammox), enhanced biological phosphorus removal and anaerobic processes. The involved mechanisms were then analyzed, which indicated FA inhibition can slow specific microbial activities or even reconfigure the microbial community structure, likely due to negative impacts of FA on intracellular pH, specific enzymes and extracellular polymeric substances (EPS), thus causing cell inactivation/lysis. Mathematical models describing the impact of FA on both wastewater and sludge treatment processes were also explored to facilitate process optimization. Finally, the key implications of FA were identified, that is FA can be leveraged to substantially enhance the biodegradability of secondary sludge, which would further improve biological nutrient removal and enhance renewable energy production.

Nitritation of real sewage: start-up and maintenance by the side-stream heat-shock treatment

In this study, the side-stream heat-shock treatment was used to start up and maintain the nitritation of real sewage. Complete nitrification was obtained when the real sewage was treated in a sequencing batch reactor (SBR). Then, about 50% of the mixed sludge was collected from the SBR and treated with the heat-shock treatment at 60 °C for 40 min in another reactor every 2 weeks. After providing the heat-shock treatment for four times, the effluent nitrate in the SBR gradually decreased from 22.5 to 3.2 mg/L, while the nitrite accumulation rate increased from 4.4% to 81.8%, indicating a successful start-up of nitritation. Further, the sewage nitritation was stable with the regular side-steam heat-shock treatment for 91 days, and the ammonium removal efficiency of 80.6% and nitrite accumulation rate of 91.2% were achieved. This study suggests that the side-stream heat-shock treatment could be used to start up sewage nitritation and maintain stability for a long-term operation.

The difference of morphological characteristics and population structure in PAO and DPAOgranular sludges

We examined how long-term operation of anaerobic-oxic and anaerobic-anoxic sequencing batch reactors (SBRs) affects the enhanced biological phosphorus removal (EBPR) performance and sludge characteristics. The microbial characteristics of phosphorus accumulating organism (PAO) and denitrifying PAO (DPAO) sludge were also analyzed through a quantitative analysis of microbial community structure. Compared with the initial stage of operation characterized by unstable EBPR, both PAO and DPAO SBR produced a stable EBPR performance after about 100-day operation. From day 200 days (DPAO SBR) and 250 days (PAO SBR) onward, sludge granulation was observed, and the average granule size of DPAO SBR was approximately 5 times larger than that of PAO SBR. The DPAO granular sludge contained mainly rod-type microbes, whereas the PAO granular sludge contained coccus-type microbes. Fluorescence in situ hybridization analysis revealed that a high ratio of Accumulibacter clade I was found only in DPAO SBR, revealing the important role of this organism in the denitrifying EBPR system. A pyrosequencing analysis showed that Accumulibacter phosphatis was present in PAO sludge at a high proportion of 6%, whereas it rarely observed in DPAO sludge. Dechloromonas was observed in both PAO sludge (3.3%) and DPAO sludge (3.2%), confirming that this organism can use both O₂ and NO₃^- as electron acceptors. Further, Thauera spp. was identified to have a new possibility as denitrifier capable of phosphorous uptake under anoxic condition.

Effects of free nitrous acid treatment conditions on the nitrite pathway performance in mainstream wastewater treatment

Inline sludge treatment using free nitrous acid (FNA) was recently shown to be effective in establishing the nitrite pathway in a biological nitrogen removal system. However, the effects of FNA treatment conditions on the nitrite pathway performance remained to be investigated. In this study, three different FNA treatment frequencies (daily sludge treatment ratios of 0.22, 0.31 and 0.38, respectively), two FNA concentrations (1.35 mgN/L and 4.23 mgN/L, respectively) and two influent feeding regimes (one- and two-step feeding) were investigated in four laboratory-scale sequencing batch reactors. The nitrite accumulation ratio was positively correlated to the FNA treatment frequency. However, when a high treatment frequency was used e.g., daily sludge treatment ratio of 0.38, a significant reduction in ammonia oxidizing bacteria (AOB) activity occurred, leading to poor ammonium oxidation. AOB were able to acclimatise to FNA concentrations up to of 4.23 mgN/L, whereas nitrite oxidizing bacteria (NOB) were limited by an FNA concentration of 1.35 mgN/L over the duration of the study (up to 120 days). This difference in sensitivity to FNA could be used to further enhance nitrite accumulation, with 90% accumulation achieved at an FNA concentration of 4.23 mgN/L and a daily sludge treatment ratio of 0.31 in this study. However, this high level of nitrite accumulation led to increased N₂O emission, with emission factors of up to 3.9% observed. The N₂O emission was mitigated (reduced to 1.3%) by applying two-step feeding resulting in a nitrite accumulation ratio of 45.1%. Economic analysis showed that choosing the optimal FNA treatment conditions depends on a combination of the wastewater characteristics, the nitrogen discharge standards, and the operational costs. This study provides important information for the optimisation and practical application of FNA-based sludge treatment technology for achieving the mainstream stable nitrite pathway.

Free ammonia-based pretreatment enhances phosphorus release and recovery from waste activated sludge

The recovery of phosphorus from waste activated sludge (WAS) was usually at low levels due to low phosphorus release. This study presents a novel, cost-effective and eco-friendly pretreatment method, e.g., using free ammonia (FA) to pretreat WAS, to enhance the phosphorus release from WAS. Experimental results showed that the phosphorus release from WAS was significantly increased after FA pretreatment at up to 189.4 mg NH₃-N L^-1 for 24 h, under which the released PO₄^3--P (i.e. 101.6 ± 6.7 mg L^-1) was higher than that pH 9 (i.e. 62.6 ± 4.54 mg L^-1) and control (without pH and FA pretreatment) (i.e. 15.1 ± 1.86 mg L^-1). More analysis revealed that the FA induced improvement in phosphorus release could be attributed to the disintegration of extracellular polymeric substances (EPS) and cell envelope of sludge cells. Moreover, the released phosphorus recovered as magnesium ammonium phosphate (MAP) was confirmed. The findings reported may guide engineers to develop an economic and practical strategy to enhance resources and energy recovery from WAS.

Free nitrous acid promotes hydrogen production from dark fermentation of waste activated sludge

Simultaneous sludge fermentation and nitrite removal is an effective approach to enhance nutrient removal from low carbon-wastewater. It was found in this work that the presence of nitrite largely promoted hydrogen production from acidic fermentation of waste activated sludge (WAS). The results showed that with an increase of nitrite from 0 to 250 mg/L, the maximal hydrogen yield increased from 8.5 to 15.0 mL/g VSS at pH 5.5 fermentation and 8.1-13.0 mL/g VSS at pH 6 fermentation. However, the maximal hydrogen yield from WAS fermentation at pH 8 remained almost constant (2.9-3.7 mL/g VSS) when nitrite was in the range of 0-250 mg/L. Further analyses revealed that free nitrous acid (FNA) rather than nitrite was the major contributor to the promotion of hydrogen yield. The mechanism investigations showed that FNA not only accelerated the disruption of sludge cells but also promoted the biodegradability of organics released, thereby provided more biodegradable substrates for subsequent hydrogen production. Although FNA inhibited activities of all microbes involved in the anaerobic fermentation, its inhibitions to hydrogen consumers were much severer than those to hydrolytic microorganisms and hydrogen producers. Further investigations with microbial community showed that FNA increased the abundances of hydrogen producers (e.g., Citrobacter sp.) and denitrifiers (e.g., Dechloromonas sp.), but reduced the abundances of hydrogen consumers (e.g., Clostridium_aceticum). This work demonstrated for the first time that FNA in WAS fermentation systems enhanced hydrogen production. The findings obtained expand the application field of FNA and may provide supports for sustainable operation of wastewater treatment plants.

Effects of return sludge alkaline treatment on sludge reduction in laboratory-scale anaerobic-anoxic-oxic process

Three alkaline treatments (pH 10, 11, and 12 for 1 h) were used to treat return sludge alone to reduce sludge production in laboratory-scale anaerobic-anoxic-oxic processes. After 99 days of operation, alkaline treatments at pH 10 and 11 led to accumulative excess sludge production and sludge yield reduction of 18.8%-31.7% and 14.7%-27.8%, respectively. However, alkaline treatment at pH 12 led to system breakdown because of sludge bulking. The alkaline treatment at pH 10 did not affect the chemical oxygen demand and NH₄⁺-N removals of the system and sludge volume index (SVI) of aerobic activated sludge. However, alkaline treatments at pH 11 and 12 obviously deteriorated the wastewater treatment efficiencies and sludge SVI. Although the three treatments increased the effluent pH by 0.08 to 0.38, the effluent pH of three systems were all lower than 9.00. The treatments at pH 10 and 11 increased the specific oxygen uptake rate of activated sludge, whereas the treatment at pH 12 decreased this rate.

Response of algal-bacterial granular system to low carbon wastewater: Focus on granular stability, nutrients removal and accumulation

The effect of influent chemical oxygen demand to nitrogen (COD/N) ratio on the granular stability, nutrients removal and accumulation of the algal-bacterial AGS was investigated. Two sequencing batch reactors were operated under different influent COD/N ratio, i.e., R₁ (control, COD/N = 8) and R₂: (COD/N = 8, 4, 2, and 1 through stepwise decrease of COD). Results showed that the integrity coefficient of the granules in R₂ stabilized at 0.7-5.4% during the whole operation. Significantly enhanced dissolved inorganic carbon (DIC) uptake and the faster growth of algae indicated the great potential for reduction in greenhouse gases (GHGs) emission by using the algal-bacterial AGS system. The algal-bacterial AGS biomass contained high phosphorus (P) and N contents as well as extremely high P bioavailability (up to 98%) which could be easily used for resource recovery. Loosely bound extracellular polymeric substances (LB-EPS) might be the key factor to control the deterioration of granular stability in this system.

Improved methane production from waste activated sludge by combining free ammonia with heat pretreatment: Performance, mechanisms and applications

Anaerobic digestion of waste activated sludge (WAS) is often limited by low hydrolysis efficiencies and poor methane potentials. This work presents a novel pretreatment technology for WAS anaerobic digestion, i.e., combining free ammonia with heat pretreatment (CFHP). Experimental results showed that compared with control, solo free ammonia (135.4 mg NH₃-N/L) and solo heat (70 °C) pretreatment, the combined free ammonia and heat (135.4 mg NH₃-N/L with 70 °C) obtained 52.2%, 25.5% and 30.2% faster in hydrolysis rate and 25.2%, 17.9% and 16.5% higher in biochemical methane potential, respectively. Mechanism investigations showed that the combined pretreatment not only largely facilitated the disintegration of WAS but also increased the proportion of biodegradable organic matters, thereby providing better contract between biodegradable organics and the anaerobic microbes for methane production. Considering its effectiveness and renewability, the combined pretreatment is an attractive technology for the application in real-world situations.