Partial nitrification and denitrification of mature landfill leachate using a pilot-scale continuous activated sludge process at low dissolved oxygen

Revamping of a Full-Scale Membrane Plant for Landfill Leachate Pretreatment Using Partial Nitritation

This paper describes a case study involving a revamping of a full-scale membrane bioreactor that treats landfill leachate and other liquid wastes. The main change was the introduction of nitritation/denitritation in alternating cycles instead of the classic denitrification/nitrification process, together with the installation of fine bubble diffusers, a reduction in the volume of the biological compartment, and an increase in the equalization volume. The most significant results were obtained for the biological compartment, with a decrease in the specific energy consumption of 46.6%. At the same time, the removal efficiency of COD, BOD, and TN substantially remained the same before and after plant revamping, while the removal efficiency of TP increased over the years, reaching an average value of almost 71%. Regarding the ultrafiltration unit, the specific flux (or permeability) was characterized by an increasing trend. At the same time, the specific energy consumption of this section decreased by 9.4%. These results led to the conclusion that the changes introduced with the revamp led to a more stable process, a reduction in membrane fouling, and important energy savings.

Effective recovery of the nitritation process through hydrogen peroxide

This study successfully achieved stable nitritation by adding hydrogen peroxide (H2O2) to the nitrification sludge whose nitritation stability had been destroyed. The batch experiment demonstrated that, the activity of ammonia-oxidizing bacteria (AOB) was restored more rapidly than that of nitrite oxidizing bacteria (NOB) after the addition of H2O2, thereby selectively promoting AOB enrichment and NOB washout. When the H2O2 concentration was 6.25 mg/L, the NOB activity was significantly reduced and the nitrite accumulation rate (NAR) was more than 95% after 18 cycles of nitrifying sludge restoration. As a result, H2O2 treatment enabled a nitrifying reactor to recover stable nitritation performance via H2O2 treatment, with the NAR and ammonia removal efficiency (ARE) both exceeding 90%. High-throughput sequencing analysis revealed that H2O2 treatment was successful in restoring nitritation, as the relative abundance of Nitrosomonas in the nitrifying reactor increased from 6.43% to 41.97%, and that of Nitrolancea decreased from 17.34% to 2.37%. Recovering nitritation by H2O2 inhibition is a low operational cost, high efficiency, and non-secondary pollution nitritation performance stabilization method. By leveraging the varying inhibition degrees of H2O2 on AOB and NOB, stable nitrification can be efficiently restored at a low cost and without causing secondary pollution.

Effect of phosphorylated polyvinyl alcohol matrix size of cell entrapment on partial nitrification of ammonia in wastewater

Partial nitrification is known as first and critical step for autotrophic nitrogen removal in high strength nitrogenous wastewater. Phosphorylated polyvinyl alcohol gel entrapment was used for suppressing oxygen to nitrite-oxidizing bacteria (NOB) in the gel matrix. The study investigated the effect of the size of gel matrix on partial nitrification. Results show that ammonia-oxidizing bacteria (AOB) proportion in the inoculum rather than the size of gel matrix governed ammonia oxidation. Nitrite oxidation depended on the size of gel matrix not the relative proportions of NOB and AOB in the inoculum. Larger size of gel matrix lead to less in situ oxygen penetration and available for NOB resulting in higher nitrite accumulation. This finding gains a better understanding of using suitable inoculum to control partial nitrification that is beneficial for the preparation of anaerobic ammonium oxidation-suited effluent.

Pilot-scale study of step-feed anaerobic coupled four-stage micro-oxygen gradient aeration process for treating digested swine wastewater with low carbon/nitrogen ratios

This study developed an innovative step-feed anaerobic coupled four-stage micro-oxygen gradient aeration process to treat digested swine wastewater. An anaerobic zone was used for prepositive denitrification; four micro-oxygen reactors (zones O1-O4) were used for simultaneous partial nitrification and denitrification through low-dissolved oxygen gradient control, step-feed, and swine wastewater-digested swine wastewater distribution. The nitrogen-removal efficiency was satisfactory (93 ± 3 %; effluent total nitrogen, 53 ± 19 mg/L). Mass balance coupled with quantitative polymerase chain reaction analysis revealed that simultaneous partial nitrification and denitrification was achieved in four micro-oxygen zones. Zones O1 were the major denitrification zones for nitrogen removal; nitrification was primary happened in zones O2 and O3. Correlation analysis confirmed that low-dissolved oxygen gradient control was the key to achieving efficient nitrogen removal. This study provides a low oxygen energy consumption method to treat digested swine wastewater with a low carbon/nitrogen ratio (<3).

Applying response surface methodology to optimize partial nitrification in sequence batch reactor treating salinity wastewater

In this study, the operation parameters of a partial nitrification process (PN) treating saline wastewater were optimized using the Box-Behnken design via the response surface methodology (BBD-RSM). A novel strategy based on the control of the carbon/nitrogen ratio (C/N), alkalinity/ammonia ratio (K/A), and salinity in three stages was used to achieve PN in a sequence batch reactor. The results demonstrated that a high and stable PN was completed after 50 d with an ammonia removal efficiency (ARE) of 98.37 % and nitrite accumulation rate (NAR) of 85.93 %. Next, BBD-RSM was applied, where ARE and NAR were the responses. The highest responses from the confirmation experiment were 99.9 % ± 0.04 and 95.25 % ± 0.32 when the optimum C/N, K/A, and salinity were identified as 0.84, 2, and 5.5 (g/L), respectively. The results were higher than those for the nonoptimized reactor. The developed regression model adequately forecasts the PN performance under optimal conditions. Therefore, this study provides a promising strategy for controlling the PN process and shows how the BBD-RSM model can improve the PN performance.

Co-treatment of landfill leachate with urban wastewater by chemical, physical and biological processes: Fenton oxidation preserves autochthonous bacterial community in the activated sludge process

The impact of Fenton oxidation (FO) and Air stripping (AS) pre-treatments on the bacterial community of a biological activated sludge (B-AS) process for the co-treatment of mature landfill leachate (MLL) and urban wastewater (UWW) was assessed. In this work high-throughput sequencing was used to identify changes in the composition of the bacterial communities when exposed to different landfill leachate's pre-treatments. The combination of FO and AS to increase biodegradability (BOD₅/COD) and reduce ammonia concentration (NH₃) respectively, allowed to successfully operate the B-AS and effectively treat MLL. In particular, BOD₅/COD resulted to be the key factor for bacterial community shifting. The microbiological community of the B-AS, mainly composed by the phylum Bacteroidota (Saprospiraceae, PHOS-HE51, Chitinophagaceae) after FO pre-treatment, shifted to Pseudomonadota (Caulobacteraceae and Hyphomicrobiaceae) when FO was not used. At the same time a drastic reduction in BOD₅ removal was observed (90%-58%). On the other hand, high NH₃ concentration affected the abundance of the family Saprospiraceae, known to play a key role in the degradation of complex organic compounds in B-AS. The results obtained suggest that a suitable combination of pre-treatments can reduce the negative effect of MLL on the B-AS process, reducing the pressure on autochthonous bacteria and therefore the acclimatization time of the biological process.

High-Density Microarray Analysis of Microbial Community Structures in Membrane Bioreactor at Short Sludge Retention Time

Membrane bioreactors (MBR) have become prevalent in wastewater treatment because of their high effluent quality and low sludge generation. Sludge retention time (SRT) is an important parameter in the operation of MBR, and it has a direct effect on the microbial community. In this study, microarrays were used to analyze the microbial communities of three different MBRs at short SRTs. The results showed that MBR at SRT 5 days (CS5) has the highest operational taxonomic units (OTUs) richness, but the lowest diversity and uniformity compared to SRT 3 days at continuous CS3 and the sequencing batch (SS3). Proteobacteria were the dominant phylum of three reactors. Bacteroidetes were the second dominant phylum in MBRs at the continuous model, instead of Actinobacteria at the sequencing model. At the class level, the dominant group of Proteobacteria exhibited a remarkable difference between the three MBRs. γ-Proteobacteria was the dominant group in CS5 and CS3, while α-Proteobacteria was the main group in SS3. The samples from the three MBRs had similar compositions of α-, β- and δ-Proteobacteria. However, γ-Proteobacteria showed different community compositions at the order level between the three MBRs. Enterobacteriales were the dominant group in CS5 and CS3, while Pseudomonadales were the dominant group in SS3. The bacterial community concentration of SRT 5 days was generally higher than that of the other two MBRs. The community composition of CS5 was significantly different from that of CS3 and SS3, and the phylogenetic relationships of the three MBRs were relatively different.

Effect of aeration modes on nitrogen removal and N2O emission in the partial nitrification and denitrification process for landfill leachate treatment

The anoxic/multi-aerobic process is widely applied for treating landfill leachate with low carbon to nitrogen ratio. In this study, the effect of two aeration modes in the aerobic phase, i.e. decreasing dissolved oxygen (DO) and increasing DO, on nitrogen removal and N₂O emission in the process were systematically compared. The results demonstrate that the aerobic phase with increasing DO mode has a positive effect on improved total nitrogen removal (78 %) under the COD/N ratio as low as 3.45 and minimized N₂O emission. DO concentration higher than 1.5 mg/L in the aerobic phase reduced nitrogen removal and led to a significant high N₂O emission in the process. Complete nitrite denitrification in the anoxic phase correlated with minimized N₂O emission. Under efficient nitrogen removal stage, N₂O emission factor was 2.4 ± 1.0 % of the total incoming nitrogen. Microbial analysis revealed that increasing DO mode increased the abundance of ammonia oxidizing bacteria and denitrifiers.

Mature landfill leachate treatment using granular sludge-based reactor (GSR) via nitritation/denitritation: Process startup and optimization

Mature landfill leachate wastewater (LLW) was characterized by high ammonia, refractory chemical oxygen demand (COD) and heavy metal contents, which limits the nitrogen removal in conventional activated sludge systems. Granular sludge is known to be more resistant to toxic compounds because of its dense structure and diverse microbial community. Here, granular sludge-based reactor (GSR) was applied with nitritation/denitritation (Nit/DNit) process for effective ammonia-rich mature LLW treatment at 20 °C. After a short startup period, the efficiencies of ammonia removal and total inorganic nitrogen removal stabilized at 99 % and 93 %, respectively, under a hydraulic retention time (HRT) of 6 h. High ammonia oxidation rate (~ 0.64 g N/g VSS/d) was achieved, with ~93 % ammonia conversing to nitrite before being reduced to nitrogen gas. Microbial analysis results revealed that Nitrosomonas (ammonia oxidizing bacteria) and Thauera (denitrifiers) were the dominant bacteria with key functional genes involved in the Nit/DNit. With an increase in the LLW loading, increased ammonia oxidation rates and biomass retention were also observed. This study demonstrated that granular sludge-based technology is feasible for mature LLW treatment.

An efficient anoxic/aerobic/aerobic/anoxic process for domestic sewage treatment: From feasibility to application

In this paper, the anoxic/aerobic/aerobic/anoxic (AOOA) process was proposed using fixed biofilms in a continuous plug-flow multi-chamber reactor, and no sludge reflux operation was performed during the 190 days of operation. The reactor volume ratio of 1.5:2:1.5:1 (A/O/O/A) with the dissolved oxygen (DO) concentration of 2 mg L⁻¹ in the aerobic zone was the optimal condition for reactor operation. According to the results obtained from the treatment of real domestic sewage, when the hydraulic retention time (HRT) was 6 h, the effluent of the reactor could meet the discharge standard even in cold conditions (13°C). Specifically, the elemental-sulfur-based autotrophic denitrification (ESAD) process contributed the most to the removal of total inorganic nitrogen (TIN) in the reactor. In addition, the use of vibration method was helpful in removing excess sludge from the biofilms of the reactor. Overall, the AOOA process is an efficient and convenient method for treating domestic sewage.

Landfill leachate biological treatment: perspective for the aerobic granular sludge technology

Landfill leachates are high-strength complex mixtures containing dissolved organic matter, ammonia, heavy metals, and sulfur species, among others. The problem of leachate treatment has subsisted for some time, but an efficient and cost-effective universal solution capable of ensuring environmental resources protection has not been found. Aerobic granular sludge (AGS) has been considered a promising technology for biological wastewater treatment in recent years. Granules' layered structure, with an aerobic outer layer and an anaerobic/anoxic core, enables the presence of diverse microbial populations without the need for support media, allowing simultaneous removal of different pollutants in a single unit. Besides, its strong and compact arrangement provides higher tolerance to toxic pollutants and the ability to withstand large load fluctuations. Furthermore, its good that settling properties allow high biomass retention and better sludge separation. Nevertheless, AGS-related research has focused on carbon-nitrogen-phosphorus removal, mainly from sanitary sewage. This review aims to summarize and analyze the main findings and problems reported in the literature regarding AGS application to landfill leachate treatment and identify the knowledge gaps for future applications.

Surrogate-based follow-up of activated carbon adsorption preceded by ozonation for removal of bulk organics and micropollutants from landfill leachate

Although combined ozonation with activated carbon (AC) adsorption is a promising technique for leachate treatment, little is known about how ozone-induced changes in leachate characteristics affect the organics adsorption, especially in view of emerging micropollutants (MPs) removal. Furthermore, the online monitoring of MPs is challenging but desirable for efficient treatment operation. This study investigates how preceding ozonation impacts the adsorption of bulk organics (expressed as chemical oxygen demand (COD)) and ozone-recalcitrant MPs, i.e., primidone, atrazine and alachlor, in leachate using batch and column adsorption tests. Additionally, a new surrogate-based model was evaluated for predicting MPs breakthrough. Batch tests revealed that ozonation results in a decreasing apparent affinity of COD towards AC, but the non-adsorbable part did not obviously change. The adsorption of MPs in ozonated leachate was (1-41%) higher than that in non-ozonated leachate, especially for the more hydrophobic alachlor and atrazine, due to a reduced sites competition from bulk organics. Column adsorption showed that ozonation delayed COD and MPs breakthrough due to the reduced COD loading and sites competition, respectively. An increased empty bed contact time (EBCT, 10-40 min) led to an increased COD uptake by a factor of 3.0-3.2 for ozonated and non-ozonated leachates, while MPs adsorption also increased, suggesting that pore blockage rather than site competition could be the dominant inhibitory effect. The data from column adsorption demonstrate the applicability of developed surrogate-based model for predicting MPs breakthrough. Particularly, the fitting parameters were not affected by change of leachate characteristics, while they were impacted by change of EBCT.

Treatment of the actual landfill leachate in different constructed wetlands through intermittent and varied aeration mode

This study focused on the removal of organic matter and nitrogen and explored the feasible operation strategies to achieve short-cut nitrification and denitrification in two constructed wetlands (CWs), which were designed to treat the actual landfill leachate from a small county in parallel. The two CWs were horizontal sub-surface flow constructed wetlands (HFCW) with partial-area aeration and vertical sub-surface flow constructed wetlands (VFCW) with full-area aeration. The experimental results showed that both CWs could achieve an excellent organic matter and nitrogen removal performance under the conditions of intermittent aeration with high frequency and medium intensity (2 h of aeration and 4 h of rest). The removal efficiencies of COD and total nitrogen by HFCW were 89.08% and 73.22%, and the corresponding values of VFCW were 84.51% and 71.44%, respectively. Meanwhile, the inhibition kinetics model indicated that HFCW with partial-area aeration could enhance the free ammonium (FA) tolerance of ammonium-oxidizing bacteria (AOB) and reduce the conversion percentage of ammonia nitrogen. In addition, the intermittent aeration mode with high frequency and medium intensity could keep the DO concentration below under 0.60 mg L^-1 in HFCW, which helped to achieve stable short-cut nitrification and ensure the average nitrite accumulation rate (NAR) reach 50.96%. These results suggested that the intermittent aeration in partial-area could achieve successful short-cut nitrification in HFCW, thereby improving the removal efficiency of nitrogen in landfill leachate.

Simultaneous Partial Nitrification and Denitrification Maintained in Membrane Bioreactor for Nitrogen Removal and Hydrogen Autotrophic Denitrification for Further Treatment

Low C/N wastewater results from a wide range of factors that significantly harm the environment. They include insufficient carbon sources, low denitrification efficiency, and NH4+-N concentrations in low C/N wastewater that are too high to be treated. In this research, the membrane biofilm reactor and hydrogen-based membrane biofilm reactor (MBR-MBfR) were optimized and regulated under different operating parameters: the simulated domestic sewage with low C/N was domesticated and the domestic sewage was then denitrified. The results of the MBR-MBfR experiments indicated that a C/N ratio of two was suitable for NH4+-N, NO2−-N, NO3−-N, and chemical oxygen demand (COD) removal in partial nitrification-denitrification (PN-D) and hydrogen autotrophic denitrification for further treatment. The steady state for domestic wastewater was reached when the MBR-MBfR in the experimental conditions of HRT = 15 h, SRT = 20 d, 0.04 Mpa for H₂ pressure in MBfR, 0.4–0.8 mg/L DO in MBR, MLSS = 2500 mg/L(MBR) and 2800 mg/L(MBfR), and effluent concentrations of NH4+-N, NO3−-N, and NO2−-N were 4.3 ± 0.5, 1.95 ± 0.04, and 2.05 ± 0.15 mg/L, respectively. High-throughput sequencing results revealed the following: (1) The genus Nitrosomonas as the ammonia oxidizing bacteria (AOB) and Denitratisoma as potential denitrifiers were simultaneously enriched in the MBR; (2) at the genus level, Meiothermus,Lentimicrobium, Thauera,Hydrogenophaga, and Desulfotomaculum played a dominant role in leading to NO3−-N and NO2−-N removal in the MBfR.

Combining chemical coagulation with activated coke adsorption to remove organic matters and retain nitrogen compounds in mature landfill leachate

Mature landfill leachate usually contains high levels of both recalcitrant organic matters and nitrogen compounds, which are hard to be removed simultaneously. In view of the difficulty, this study explored an innovative alternative that treated organic matters and nitrogen compounds separately by combining ferric trichloride (FTC) and polyacrylamide (PAM) coagulation with activated coke adsorption. Our study results have shown that the combination of chemical coagulation (750 mg/L of ferric trichloride and 2.0 mg/L of anionic polyacrylamide at pH = 5) with activated coke adsorption (5 g per 100 mL) was able to remove total organic carbon (TOC), chemical oxygen demand (COD), and colority by 91%, 57%, 100%, respectively. The removal efficiency (R.E.) of humic- and protein-like matters both exceeded 95%. Meanwhile, nitrogen compounds, such as nitrite and ammonia nitrogen, were mostly retained in the effluent. They could either be recovered as value-added products through technologies such as negative pressure steam-stripping or removed through methods such as air stripping and ion exchange. Overall, the proposed coagulation-adsorption process may provide a feasible alternative for tackling the worldwide concern over the secondary pollution caused by mature landfill leachate and its effective disposal.

The benefits of autotrophic nitrogen removal from high concentration of urea wastewater through a process of urea hydrolysis and partial nitritation in sequencing batch reactor

For the sake of high efficiency and saving operational cost for high-concentration urea wastewater treatment, a novel two-stage partial nitritation (PN)-anammox process containing simultaneous urea hydrolysis and PN in sequencing batch reactor (SBR) was investigated. Although the influent urea concentration increased from 500 to 1200 mg/L, the SBR simultaneously achieved urea removal efficiency higher than 98% and stable PN with effluent NO₂^--N/NH₄⁺-N ratio of 1.0-1.3 without any extra alkalinity addition. The intracellular hydrolysis was the dominant mechanism for urea removal and persistent free ammonia inhibition on nitrite-oxidizing bacteria was the main reason for nitrite accumulation of 97.92% in SBR. The subsequent anammox reactor showed efficient nitrogen removal performance with average ammonium removal efficiency, nitrogen removal efficiency and maximum nitrogen removal loading rate of 98.08%, 81.45% and 1.05 kg N·m^-3·d^-1 respectively. High-throughput sequencing results indicated Gemmatimonadetes became the most abundant bacterial phylum related to potential intracellular urea hydrolysis and displayed obvious ammonium-oxidizing bacteria enrichment and nitrite-oxidizing bacteria inhibition in SBR, and the dominant anammox bacteria (Candidatus_Kuenenia) in anammox reactor. The proposed process was proven to be promising for high-concentration urea wastewater treatment, facilitating the sustainable development of the urea industry in the future.

A novel halophilic Exiguobacterium mexicanum strain removes nitrogen from saline wastewater via heterotrophic nitrification and aerobic denitrification

The utilization of halophilic bioresources is limited due to a lack of isolation and characterization work. A halophilic bacterium strain SND-01 of Exiguobacterium mexicanum was isolated in this study, which is the first report on its novel function in heterotrophic nitrification-aerobic denitrification (HN-AD). The strain SND-01 is slightly halophilic, surviving at 0 up to 9% (w/v) salinity. When utilizing ammonium, nitrate or nitrite as the sole nitrogen source in aerobic conditions, the isolated strain showed the maximum nitrogen removal rate of 2.24 ± 0.14 mg/(L·h), 3.63 ± 0.21 mg/(L·h) and 2.30 ± 0.23 mg/(L·h), respectively. Functional genes and key enzymes involved in heterotrophic-aerobic nitrogen transformations were characterized, establishing the pathway of HN-AD. The nitrogen removal via HN-AD is dependent on the C/N ratio, salinity and temperature. The halophilic Exiguobacterium mexicanum strain SND-01 shows a significant potential in biotreatment of saline wastewater in an easy and cost-effective way.

One-stage partial nitrification and anammox process in a sequencing batch biofilm reactor: Start-up, nitrogen removal performance and bacterial community dynamics in response to temperature

A one-stage partial nitrification and anammox (PN/A) process was started up and operated under varying temperatures in a lab-scale sequencing batch biofilm reactor. The start‑up phase took 110 days with an intermittent aeration strategy, and the removal efficiencies of ammonia‑nitrogen and total nitrogen were found to be 92.22% and 76.07%, respectively. The total nitrogen removal efficiency (NRE) increased by 9.49% when temperature decreased from 30 °C to 25 °C, but declined by 83.84% from 25 °C to 20 °C. The PN process was inhibited and subsequently limited the nitrogen removal performance at 20 °C. When temperature returned to 28 °C, the NRE recovered to 67.27%, but it was still lower than the value before the decrease in temperature (79.40%). Microbial community analysis showed that the predominant ammonia oxidation bacteria and anammox bacteria were Nitrosomonas and Candidatus Kuenenia, respectively. Nitrosomonas grew, while the relative abundance of Candidatus Kuenenia increased as temperature decreased and vice versa.

Probing the dynamics of three freshwater Anammox genera at different salinity levels in a partial nitritation and Anammox sequencing batch reactor treating landfill leachate

Partial nitritation/Anammox was applied to treat NaCl-amended landfill leachate. The reactor established robust nitrogen removal of 85.7 ± 2.4% with incremental salinity from 0.61% to 3.10% and achieved 0.91-1.05 kg N/m³/d at salinity of 2.96%-3.10%. Microbial community analysis revealed Nitrosomonas, Nitrospira, and denitrifiers occupied 4.1%, <0.2% and 10.9%, respectively. Salinity variations impelled the dynamics of Anammox bacteria. Jettenia shifted to Brocadia and Kuenenia at salinity of 0.61%-0.81%. Kuenenia outcompeted Brocadia and occupied 51.5% and 50.9% at salinity of 1.48%-1.54% and 2.96%-3.10%, respectively. High nitrite affinity and fast growth rate were proposed as key factors fostering Brocadia overgrew Jettenia. Functionalities of sodium-motive-force facilitated energy generation and intracellular osmotic pressure equilibrium regulation crucially determined Kuenenia's dominance at elevated salinity. Co-occurrence network further manifested beneficial symbiotic relationships boosted Kuenenia's preponderance. Knowledge gleaned deepen understanding on survival niches of freshwater Anammox genera at saline environments and lead to immediate benefits to its applications treating relevant wastewaters.

Achieving Partial Nitrification via Intermittent Aeration in SBR and Short-Term Effects of Different C/N Ratios on Reactor Performance and Microbial Community Structure

A sequencing batch reactor (SBR) with an intermittent aeration mode was established to achieve partial nitrification (PN) and the short-term effects of C/N ratios were investigated. Stable nitrite accumulation was achieved after 107 cycles, about 56d, with the average ammonia nitrogen removal efficiency (ARE) and nitrite accumulation rate (NAR) of 96.92% and 82.49%, respectively. When the C/N ratios decreased from 4.64 to 3.87 and 2.32, ARE and NAR still kept a stable and high level. However, when the C/N ratio further decreased to 0.77, nitrite accumulation became fluctuation, and ARE, total nitrogen (TN), and chemical oxygen demand (COD) removal performance declined obviously. Except for four common phyla (Proteobacteria, Bacteroidetes, Chloroflexi, and Actinobacteria) in the wastewater treatment system, Patescibacteria, the newly defined superphylum, was found and became the most dominant phylum in the PN sludge for their ultra-small cell size. The only ammonia oxidation bacteria (AOB), Nitrosomonas, and nitrite oxidation bacteria (NOB), Nitrospira, were detected. The relative abundance of NOB was low at different C/N ratios, showing the stable and effective inhibition effects of intermittent aeration on NOB growth.

Performance and mechanism of urea hydrolysis in partial nitritation system based on SBR

Urea hydrolysis in partial nitritation process forming nitrite and ammonia is advantageous to subsequent treatment with ANAMMOX for total nitrogen removal. In this study, stable partial nitritation for urea wastewater with urea increasing from 250 to 2000 mg L^-1 were achieved in an aerobic SBR. Urea removal efficiency and nitrite accumulation percentage both kept above 98%, with nitrite production rate about 0.985 kg N·m^-3·d^-1. Urea hydrolysis mechanism in this aerobic system was described as, (1) massive urea in the bulk was absorbed into cell, (2) urea was hydrolyzed by intracellular urease inside cell, (3) produced ammonia then slowly diffused into the bulk through membrane, which is later converted by ammonia-oxidizing bacteria (AOB) into nitrite. Due to this mechanism, the activity of AOB could not be inhibited by high FA (free ammonia) value under high urea concentration condition while nitrite-oxidizing bacteria (NOB) remained to be inhibited. An uncultured genus belonging to poorly characterized phylum Gemmatimonadetes was found enriched in this process and became dominant genus. This genus was speculated to have same energy pathway like ureaplasma, by absorbing excessive urea from environment and utilize urea hydrolysis to generate energy. So it was believed to be responsible for urea hydrolysis mechanism mentioned above. This SBR showed stable partial nitritation and high urea removal efficiency for treating urea wastewater, which was obviously feasible as the pretreatment process for subsequent ANAMMOX.

Enhancement of nitrogen and phosphorus removal, sludge reduction and microbial community structure in an anaerobic/anoxic/oxic process coupled with composite ferrate solution disintegration

Waste activated sludge (WAS) was disintegrated by composite ferrate solution (CFS) in this work, and the effect of CFS disintegrated sludge supernatant (CDSS), as a supplementary carbon source, on enhancement of nitrogen and phosphorus removal and sludge reduction in an AAO-CFSSDR (Anaerobic/Anoxic/Oxic combined with CFS-Sludge disintegration reactor) process was evaluated. The results showed that CDSS was easily utilizable by the denitrification bacteria due to the high content of readily biodegradable substrates. When compared with the AAO process, the operation results of AAO-CFSSDR suggested that the removal efficiencies of TN, NH₄⁺-N and TP increased from 71.15, 79.23 and 85.52% to 85.05, 87.70 and 90.06%, respectively; and the sludge was reduced by 34.79%. The 16SrRNA high-throughput sequencing results showed that the introduction of CDSS weakened the microbial diversity but enhanced the microbial richness; and the abundance of bacteria related to the removal of nitrogen and phosphorus, increased in the AAO-CFSSDR process.

Unclassified Anammox bacterium responds to robust nitrogen removal in a sequencing batch reactor fed with landfill leachate

Treatment of landfill leachate was conducted in a lab-scale sequencing batch reactor (SBR). The SBR was started through inoculating activated sludge with controlling dissolved oxygen of 0.5-1.0 mg/L. Anammox reaction took place within around three months. The SBR established robust nitrogen removal with incremental NLRs of 0.25-2.17 kg N/m³/d. At the final phase, it achieved elevated nitrogen removals of 1.68-1.91 kg N/m³/d. 16S rRNA gene amplicon sequencing analysis revealed Nitrosomonas, unclassified Anammox bacterium, and diverse denitrifying populations coexisted and accounted for 4.02%, 20.05% and 34.69%, respectively. Phylogenic analysis and average nucleotide identity comparison jointly suggested the unclassified Anammox bacterium potentially pertained to a novel Anammox lineage. The functional profiles' prediction suggested sulfate reduction, arsenate reduction and eliminations of antibiotics and drugs likely occurred in the SBR. The finding from this study suggests contribution of unclassified Anammox bacteria in influencing nitrogen budget in natural and engineering systems is currently being underestimated.

Intermittent rotation as an innovative strategy for achieving nitritation in rotating biological contactors

The nitritation step is essential when the anammox process is focused, and alternative technologies to achieve partial nitritation-anammox are required. Rotating Biological Contactors (RBCs) are a promising and cost-effective technology, allowing the development of aerobic and anoxic zones in the biofilm, coupled to low energy consumption. This study evaluated nitritation in a RBC with two discs rotation strategies: continuous and intermittent. Continuous rotation resulted in high dissolved oxygen (DO) concentrations and was not favorable for achieving stable nitritation. However, intermittent rotation, coupled with a nitrogen load of 1000 g N·m^-3·d^-1 and a HRT of 12 h, decreased DO by 77.8% and resulted in nitritation efficiencies of 45.3%. FISH analyses suggested that simultaneous partial nitritation/anammox (PN/A) could also be favored. These results indicated that intermittent rotation may be a core strategy for producing an anammox-suitable effluent or even to promote PN/A in RBCs, upgrading their applicability for wastewater treatment.

New progress of ammonia recovery during ammonia nitrogen removal from various wastewaters

The recovery of ammonia-nitrogen during wastewater treatment and water purification is increasingly critical in energy and economic development. The concentration of ammonia-nitrogen in wastewater is different depending on the type of wastewater, making it challenging to select ammonia-nitrogen recovery technology. Meanwhile, the conventional nitrogen removal method wastes ammonia-nitrogen resources. Based on the circular economy, this review comprehensively introduces the characteristics of several main ammonia-nitrogen source wastewater plants and their respective challenges in treatment, including municipal wastewater, industrial wastewater, livestock and poultry wastewater and landfill leachate. Furthermore, we introduce the main methods currently adopted in the ammonia-nitrogen removal process of wastewater from physical (air stripping, ion exchange and adsorption, membrane and capacitive deionization), chemical (chlorination, struvite precipitation, electrochemical oxidation and photocatalysis) and biological (classical and typical activated sludge, novel methods based on activated sludge, microalgae and photosynthetic bacteria) classification based on the ammonia recovery concept. We discuss the applicable methods of recovering ammonia nitrogen in several main wastewater plants. Finally, we prospect the research direction of ammonia removal and recovery in wastewater based on sustainable development.

Adsorption - Membrane process for treatment of stabilized municipal landfill leachate

The aim of this study was to investigate the efficiency of removal of difficult-to-biodegrade organic compounds from real stabilized landfill leachate with a membrane process alone and in combination with powdered-activated-carbon (PAC) adsorption. For filtration, ceramic membranes were used. The characteristics of the raw leachate were 788 mg COD/L and color of 0.4458 cm^-1. With all combinations of PAC-adsorption and a membrane process (MF, UF, fine-UF) and with fine-UF alone, leachate treatment was highly efficient. For each membrane, treatment was more efficient when the membrane process was combined with PAC addition. This means that adsorption (PAC dose 3 g/L, adsorption time 30 min) made the largest contribution to leachate treatment (COD and color removal efficiency of 73.1% and 94.4%, respectively). In all cases, organic particles bigger than 100 kDa were removed most efficiently, whereas particles smaller than 3 kDa were removed least efficiently. The efficiency of leachate treatment with PAC + MF, PAC + UF and PAC + fine-UF did not differ significantly (>87% COD and > 97% color). With regard to membrane flux, however, these combinations can be ranked in the following order: PAC + MF > PAC + UF > PAC + fine-UF. Therefore, PAC + MF (0.3 MPa) was selected as the most effective solution (COD and color removal efficiencies of 87.8% and 97.2%, respectively; permeate flux of 167.6 L/(m²∙h)), as it combined efficient pollutant removal with low membrane pressure.

Nitrite accumulation stability evaluation for low-strength ammonium wastewater by adsorption and biological desorption of zeolite under different operational temperature

How to achieve stable nitrite accumulation was still a huge challenge for low-carbon and energy-saving biological nitrogen removal of low-strength ammonium wastewater. This study proposed a new way to solve this problem with zeolite biological fixed bed (ZBFB) by cycle operation of adsorption and biological desorption. In order to evaluate nitritation performance of this reactor, the influence of operational temperature on nitrite accumulation stability was investigated by 126 cycles operation in four parallel ZBFB reactors for low-strength ammonium wastewater (50 mg/L NH₄⁺-N). It was found that higher operational temperature (i.e., 36.0 °C), rather than other temperature (i.e., 27.0 °C, 30.0 °C, 33.0 °C), could maintain stable nitrite accumulation with nitrite production rate of 0.312 kg NO₂^--N·m^-3 zeolite·day^-1 and nitrite accumulation ratio higher than 95.0% after biological desorption. High-throughput sequencing analysis results showed that bacterial structure significantly changed in ZBFB under different operational temperature, and obvious enrichment of genus Nitrosomonas (AOB) and gradually enhanced free ammonia (FA) inhibition on genus Nitrospira and Nitrobacter (NOB) were found by elevation of operational temperature, leading to different nitrite accumulation performance in ZBFB reactors. The mechanism for stable nitrite accumulation performance by ZBFB might be attributed to overwhelming growth rate of AOB than NOB, faster ammonium desorption and enhanced FA inhibition on NOB under operational temperature (i.e., 36.0 °C). All in all, keeping high temperature for biological desorption step should be extremely crucial for stable nitrite accumulation by ZBFB, which could facilitate further low-carbon and energy-saving biological nitrogen removal for low-strength ammonium wastewater treatment.

Ammonia removal and recovery from municipal landfill leachates by heating

In this research, ammonia evaporation capacity under atmospheric and vacuum pressure conditions, as well as distillation capacity of different concentrations of landfill leachates, were evaluated. Simple evaporation and vacuum pressure evaporation tests showed high NH₃-N removal efficiencies, ranging from 95% to 98% for raw landfill leachates, indicating that vacuum pressure would not be necessary during ammonia removal and recovery processes when applying temperature of 300 °C. Distillations tests also showed the promising NH₃-N recovery potential in ultra-concentrated leachates (over 100 gNH₃-N/L) in the order of 91%-94% in few minutes, evaporating a small portion of landfill leachate. The results presented encourages the recovery of ammonia from landfill leachate and its industrial and agricultural, highlighting its feasibility as well as simultaneously preventing the ammonia release to water bodies or the atmosphere.

Pilot study of nitrogen removal from landfill leachate by stable nitritation-denitrification based on zeolite biological aerated filter

A pilot (about 1 m³/d) process consisting of pre-denitrification and zeolite biological aerated filter (ZBAF) was established and run for nitrogen removal of landfill leachate. The results showed that stable nitritation and denitrification was achieved for landfill leachate with removal efficiency of Chemical Oxygen Demand (COD_Cr), ammonium and total nitrogen (TN) of 53.2 ± 3.0%, 93.5 ± 2.4% and 74.7 ± 9.4%, respectively. Based on the ammonium adsorption equilibrium by zeolite, stable free ammonia could be maintained for inhibition of nitrite oxidizing bacteria (NOB) and dominance of ammonia oxidizing bacteria (AOB) in ZBAF, resulting in efficient nitritation with a nitrite accumulation ratio higher than 90.0% and an average nitrite production rate of 1.387 kg NO₂^--N m^-3 day^-1. High-throughput sequencing analysis further revealed enrichment of AOB and elimination of NOB in ZBAF. Compared to two-stage anoxic-oxic process, the pilot-scale process could save approximate 5000 mg/L glucose (about 3.10 US dollar/m³) with almost similar TN removal performance. All results obtained demonstrated the feasibility of the pilot process, which might be highly promising for the nitritation and denitrification of low C/N landfill leachate in the future.

Simultaneous partial nitrification, anammox and denitrification (SNAD) process for nitrogen and refractory organic compounds removal from mature landfill leachate: Performance and metagenome-based microbial ecology

In this study, a simultaneous partial nitrification, Anammox and denitrification (SNAD) bioreactor was constructed for mature landfill leachate treatment, which exhibited favorable NH₄⁺-N (98.9-99.9%), TN (90.7-94.9%) and bio-refractory organic compounds (46.2-67.7%) removal efficiencies. Stoichiometric analysis demonstrated that the synergy of ammonium-oxidizing bacteria and Anammox bacteria dominated TN removal (96.1-97.2%). NO₃^--N produced in Anammox could be further reduced through (partial) denitrification and dissimilatory nitrate reduction to ammonium (DNRA). The results highlighted that humic-like and their intermediates might serve as the electron donor for these (partial) denitrifiers and DNRA bacteria to remove NO₃^--N, and could be effectively removed from mature landfill leachate in SNAD bioreactor. Metagenomic characterization further demonstrated that phyla Chloroflexi, Chlorobi and genera Nitrosomonas, Ignavibacterium and Aminiphilus might be responsible for such humic-like degradation. Overall, this work offers new insights into the metagenome-based bioinformatic roles for the previously understudied microorganisms in SNAD bioreactor for mature landfill leachate treatment.

Dissolved oxygen/free ammonia (DO/FA) ratio manipulation to gain distinct proportions of nitrogen species in effluent of entrapped-cell-based reactors

Oxygen-limiting and/or free ammonia (FA)-accumulating conditions are two common operating strategies for partial nitrification in wastewater. Controlling either bulk dissolved oxygen (DO) or free ammonia (FA) concentration to maintain partial nitrification can be challenging due to the strong interdependency between these two parameters as substrates for ammonia oxidation. In this study, DO/FA ratio is proposed as a controlling parameter for partial nitrification by entrapped-cell-based reactors. At DO/FA >1.5, both ammonia and nitrite oxidation proceeded without inhibition leading to complete oxidation of ammonia to nitrate. An effluent containing nitrate as the main nitrogen species can be produced at these ratios. At a DO/FA ratio range of 0.2-1.5, ammonia oxidation proceeded without efficiency deterioration, while nitrite oxidation decreased with decreasing DO/FA ratio. At the ratios of 0.2-0.6, an effluent containing mainly nitrite can be generated. At DO/FA <0.2, both ammonia oxidation and nitrite oxidation were inhibited and the effluent with nearly equal molar of ammonia and nitrite was obtained. By controlling DO/FA ratio, effluents with different proportions of nitrogen species can be produced allowing the entrapped-cell-based system to be adaptable as an initial reactor for various nitrogen removal approaches.

Identification and nitrogen removal characteristics of Thauera sp. FDN-01 and application in sequencing batch biofilm reactor

A strain FDN-01 was isolated from the sequencing batch biofilm reactor (SBBR) which was seeded with wasted activated sludge from a municipal wastewater treatment plant in Shanghai. Bacterium FDN-01 was identified as Thauera sp., and Genbank Sequence_ID was KY393097. By comparing inorganic total nitrogen (TN) removal efficiency by strain FDN-01 under different conditions, the optimal initial pH, carbon source and the ratio of carbon to nitrogen were 7.5, sodium succinate and 4.0, respectively. Inorganic TN removal efficiency was 93% within 3 d while the concentration of nitrate was 100 mg/L, and the type of substrates affected extracellular polymeric substances (EPS) production and the ratio of protein to polysaccharide in the EPS. Further investigation for the application of strain FDN-01 in the SBBRs showed that anoxic ammonia oxidation occurred at room temperature, and the removal efficiencies of inorganic TN were noticeably enhanced by the augmentation of bacterium FDN-01 back into the SBBR. This study provided a promising method of TN removal requiring less carbon source in the wastewater.

Effect of alkalinity on bio-zeolite regeneration in treating cold low-strength ammonium wastewater via adsorption and enhanced regeneration

Low temperature severely inhibits microbial activity, making biological method inefficient for ammonium removal from wastewater. A zeolite biological fixed-bed (ZBFB) was successfully established for 6.0-8.0 °C low-strength ammonium wastewater treatment via adsorption-regeneration. Ion exchange was a remarkable alternative and zeolite was mostly applied. Nevertheless, insufficient zeolite bio-regeneration rate was the key obstacle for economically sustainable utilization. By adsorption, effluent NH₄⁺-N was around 1.5-2.5 mg/L. About 26% regeneration rate was obtained. With a ceramsite biological aerobic filter (CBAF) operated with ZBFB in series at the regeneration stage, the regeneration rate reached 95%, 3.5 times higher. Studies of alkalinity effects on bio-zeolite regeneration process indicated that Na₂CO₃ worked better than NaHCO₃. Greater amount and one dose mode of alkalinity addition, higher regeneration rate could be obtained. The bio-zeolite regeneration process followed pseudo first-order kinetics with K = 0.0629 h^-1. High-throughput sequencing analysis indicated the enriched nitrifying microorganisms in CBAF fully oxidized NH₄⁺-N in regeneration solution, which accelerated desorption and conversion of NH₄⁺-N by the circulation of regeneration solution between ZBFB and CBAF. The dynamic adsorption experiment proved that ZBFB-CBAF was feasible for cold low-strength ammonium wastewater treatment.

Nitrogen removal from iron oxide red wastewater via partial nitritation-Anammox based on two-stage zeolite biological aerated filter

Partial nitritation-anaerobic ammonium oxidation (PN-Anammox) was successfully applied for high-strength ammonium iron oxide red wastewater (IORW) treatment based on stable PN performance of zeolite-biological aerated filter (ZBAF). By separating Na₂CO₃ dosage avoid the high free ammonia (FA) inhibited nitritation, two-stage ZBAF was applied for achieving efficient PN with Na₂CO₃ as the alkalinity donor and saving about 40.0% of the alkalinity cost compared to NaHCO₃. Moreover, Anammox was used for further nitrogen removal from IORW and stable total nitrogen (TN) removal was obtained at the influent NH₄⁺-N concentration of 567 mg/L and TN removal efficiency kept above 70.0% after 100 days operation. High throughput sequencing-based approaches showed that Nitrosomoadaceae (AOB) and Kuenenia was dominance in two-stage ZBAF and Anammox samples respectively, while Nitrospire and Nitrobacter (NOB) undetected. The combined process should have advantages for similar high-strength ammonium wastewater treatment.

Salt inhibition on partial nitritation performance of ammonium-rich saline wastewater in the zeolite biological aerated filter

Influences of various concentrations of salinity (NaCl 0-25.0 g/L) on partial nitritation performance in a zeolite biological aerated filter (ZBAF) were investigated. The highest nitrite production rate (NPR) of 1.357 kg/(m³·d) and nitrite accumulation rate (NAR) of 99.0% was achieved with salt free wastewater. While after adding extra salt, NPR and ammonium removal efficiency (ARE) presented an obvious decline trend. The lowest NPR (0.599 kg/(m³·d)) and ARE (34.2%) were obtained with salinity of 25.0 g/L NaCl. It has been confirmed that addition of salt resulted in the increase of free ammonia (FA), and then further inhibited ammonium oxidizing bacteria (AOB). High-throughput sequencing analysis results further revealed that the relative abundance of AOB decreased in ZBAF. All results demonstrated that AOB was inhibited not only by high osmotic pressure caused by salt, but also by high FA.

Recent advances in nitrogen removal from landfill leachate using biological treatments - A review

Landfill leachate, generated from the wastes in a landfill, is a type of wastewater with high concentrations of ammonia and organics, causing a serious environmental pollution. Because of its complex and changing characteristics, it is difficult to remove nitrogen from landfill leachate economically and effectively. Hence, nitrogen removal is a significant research priority of landfill leachate treatment in recent years. Biological processes are known to be effective in nitrogen removal. In this work, the biological nitrogen removal treatments were divided into the following processes: conventional nitrification-denitrification process, nitritation-denitritation process, endogenous denitritation process, and anaerobic ammonium oxidation (Anammox) process. This manuscript summarized the theories and applications of these approaches in detail, and concluded that appropriate processes should be selected in accordance with different characteristics of landfill leachate, in order to effectively remove nitrogen from all stages of landfill leachate and reduce disposal costs. Finally, perspective on the challenges and opportunities of biological nitrogen removal from landfill leachate was also presented.

Response of nitritation performance and microbial community structure in sequencing biofilm batch reactors filled with different zeolite and alkalinity ratio

Ammonium (NH₄⁺-N) adsorption capacity of zeolite varies from place to place, a unique attempt to use different zeolite as adsorbent media in sequencing biofilm batch reactor (SBBR) for maintaining appropriate free ammonia (FA) range to achieve partial nitritation. SBR filled with synthetic zeolite (SSBBR) and natural zeolite (NSBBR) were applied to evaluate the NH₄⁺-N adsorption capacity impacts on nitrogen transformation and microbial characteristics. Significant differences in nitrite production rate (NPR) were both observed in two reactors during 4 different alkalinity ratios. The highest NPR in SSBBR and NSBBR were both obtained when the alkalinity ratio was 5:1 with the values of 1.11 and 0.90 kg N/(m³·d), respectively. According to Haldane model with inhibition by FA, the kinetics of the reaction were analyzed. High-throughput sequencing analysis results further presented that SSBBR had higher relative abundance average of nitrosobacteria in genus level, which was in favor of better partial nitritation.

A cascade of a denitrification bioreactor and an aerobic biofilm reactor for heavy oil refinery wastewater treatment

The performance of an efficient denitrification bioreactor–aerobic biofilm reactor cascade for heavy oil refinery wastewater treatment was investigated.

Nitrogen removal via nitritation pathway for low-strength ammonium wastewater by adsorption, biological desorption and denitrification

Stable nitritation for low-strength ammonium wastewater was the key obstacle for cost-effective and low-carbon biological nitrogen removal. A zeolite biological fixed bed (ZBFB) and an anoxic sequencing batch reactor (ASBR) were successfully applied for achieving nitritation-denitrification of low-strength ammonium wastewater by adsorption, biological desorption and denitrification. Based on free ammonia inhibition on biofilm, stable nitrite accumulation could be realized with suitable operational time and aeration in biological desorption. During cycle operation, adsorption effluent NH₄⁺-N kept at 3.0-4.0 mg/L, biological desorption effluent NO₂^--N maintained at 226.8-243.2 mg/L with average nitrite accumulation ratio of 97.18%, and nitrite removal rate was about 0.628-0.672 kg NO₂^--N·m^-3·day^-1, revealing obvious feasibility of ZBFB and ASBR for low-strength ammonium wastewater treatment. High-throughput sequencing analysis results further presented significant microbial community variations happened after cycle operation, with ammonia oxidizing bacteria enrichment and nitrite oxidizing bacteria inhibition in ZBFB and dominance of denitrifiers in ASBR.

Performance and mechanism of a novel algal-bacterial symbiosis system based on sequencing batch suspended biofilm reactor treating domestic wastewater

A novel algal-bacterial symbiosis system based on sequencing batch suspended biofilm reactor (A-SBSBR) was developed for simultaneously enhanced nitrogen (N) and phosphorus (P) removal from domestic wastewater. Results showed that the total N (TN) and P (TP) removal efficiencies in A-SBSBR increased to 69.91% and 94.78%, respectively. The mechanism analysis indicated that TN removal mainly occurred at non-aeration stage, and TP removal happened during the whole cycle in A-SBSBR. Compared to control SBSBR, TN removal by denitrification and anabolism and TP removal by anabolism in A-SBSBR increased by 12.70%, 7.64% and 50.13%, respectively. The Chlorophyll a accumulation in biofilm increased to 4.80 ± 0.08 mg/g. Algae related to Chlorella and Scenedesmus and bacteria related to Flavobacterium, Micropruina and Comamonadaceae were enriched in A-SBSBR and responsible for the enhanced nutrients removal effect. This study may provide a new solution to achieve nutrients removal enhancement from wastewater.

Treatment of Landfill Leachate Using Activated Sludge Technology: A Review

Landfill leachate contains a large amount of organic matter and ammoniacal nitrogen. As such, it has become a complex and difficult issue within the water treatment industry. The activated sludge process has been found to be a good solution with low processing costs and is now therefore the core process for leachate treatment, especially for nitrogen removal. This paper describes the characteristics and treatment of leachate. Treatment of leachate using the activated sludge process includes the removal of organic matter, ammoniacal nitrogen, and total nitrogen (TN). The core method for the removal of organic matter involves anaerobic treatment supplemented with an aerobic process. Ammoniacal nitrogen is commonly removed using a conventional aerobic treatment, and advanced TN removal is achieved using endogenous denitrification or an anaerobic ammonium oxidation (ANAMMOX) process. Since biological processes are the most economical method for TN removal, a key issue is how to tap the full potential of the activated sludge process and improve TN removal from leachate. This complex issue has been identified as the focus of current scholars, as well as an important future direction for leachate research and development.

Rapid start-up and performance of denitrifying granular sludge in an upflow sludge blanket (USB) reactor treating high concentration nitrite wastewater

Denitrifying granular sludge reactor holds better nitrogen removal efficiency than other kinds of denitrifying reactors, while this reactor commonly needs seeding anaerobic granular sludge and longer period for start-up in practice, which restricted the application of denitrifying granular sludge reactor. This study presented a rapid and stable start-up method for denitrifying granular sludge. An upflow sludge blanket (USB) reactor with packings was established with flocculent activated sludge for treatment of high concentration nitrite wastewater. Results showed mature denitrifying granular sludge appeared only after 15 days with highest nitrogen removal rate of 5.844 kg N/(m³ day), which was much higher than that of compared anoxic sequencing batch reactor (ASBR). No significant nitrite inhibition occurred in USB and denitrification performance was mainly influenced by hydraulic retention time, influent C/N ratio and internal reflux ratio. Hydraulic shear force created by upflow fluid, shearing of gaseous products and stable microorganisms adhesion on the packings might be the reasons for rapid achievement of granular sludge. Compared to inoculated sludge and ASBR, remarkable microbial communitiy variations were detected in USB. The dominance of Proteobacteria and Bacteroidetes and enrichment of species Pseudomonas_stutzeri should be responsible for the excellent denitrification performance, which further verified the feasibility of start-up method.

High nitrite concentration accelerates nitrite oxidising organism's death

High nitrite is a known operation parameter to inhibit the biological oxidation of nitrite to nitrate. The phenomenon is traditionally expressed using a Monod-type equation with non-competitive inhibition, in which the reaction associated with the biomass growth is reduced when high nitrite is present. On the other hand, very high nitrite is also known to slay nitrifiers. To clarify the difference between the growth inhibition and the poisoning, cell counting for living microorganisms in the nitrite oxidiser-enriched activated sludge was conducted in batch conditions under various nitrite concentrations together with measurements of biomass chemical oxygen demand (COD) concentration and oxygen uptake rate. The experiments demonstrated that these measureable parameters were all decayed when nitrite concentration exceeded 100-500 mgN/L at pH 7.0 in the system, indicating that nitrite poisoning took place. Biomass growth was recognised in lower range of nitrite which was expressed with growth inhibition only. Based on the response, a kinetic model for the biological nitrite oxidation was developed with a modification of IWA ASM1. The model was further utilised to calculate a possibility to wash out nitrite oxidiser in the aeration tank where a part of the return activated sludge was exposed to high nitrite liquor in a side-stream partial nitritation reactor.

Release behavior of chloride from MSW landfill simulation reactors with different operation modes

The chloride ion (Cl^-), a very common monatomic anion, has high ecological toxicity at high concentrations because of its non-biodegradability, and can easily migrate from landfill site into the surrounding environment. Four lab-scale landfill simulation reactors were established to investigate Cl^- release behavior: the anaerobic landfill mode (R1), the semi-aerobic landfill mode (R2), the anaerobic landfill with leachate re-circulation mode (R3), and the semi-aerobic landfill with leachate re-circulation mode (R4). The landfill operation modes had a great influence on the release of Cl^-. In 256 days, the cumulative release amounts of Cl^- in the four reactors were 64.52, 132.07, 56.10, and 33.1 g for R1-R4, respectively. Once air enters anaerobic landfill, the leachate Cl^- concentration may sharply increase. The highest leachate Cl^- concentrations were 6.6 g L^-1 in anaerobic reactor and 18 g L^-1 in semi-aerobic reactor. However, the leachate re-circulation can maintain the release of Cl^- at dynamic equilibrium state. Theoretically, the Cl^- release behavior from anaerobic landfill with leachate re-circulation (R3) will be continuous. In contrast, under the other conditions, it can be anticipated to occur once the leachate recirculation stops (R1) or when the landfill encounters air incursion (R2 and R4). The semi-aerobic operation modes had significantly lower COD/Cl and NH₄-N/Cl ratios than the anaerobic modes. This indicates that the Cl^- pollution risk from semi-aerobic modes is lower than that from anaerobic modes.

Influence of reflux ratio on two-stage anoxic/oxic with MBR for leachate treatment: Performance and microbial community structure

A lab-scale two-stage Anoxic/Oxic with MBR (AO/AO-MBR) system was operated for 81 days for leachate treatment with different reflux ratio (R). The best system performances were observed with a R value of 150%, and the average removal efficiencies of chemical oxygen demand, ammonia and total nitrogen were 85.6%, 99.1%, and 77.6%, respectively. The microbial community were monitored and evaluated using high-throughput sequencing. Proteobacteria were dominant in all process. Phylogenetic trees were described at species level, genus Thiopseudomonas, Amaricoccus, Nitrosomonas and Nitrobacter played significant roles in nitrogen removal. Co-occurrence analyzing top 20 genera showed that Nitrosomonas-Nitrobacter presented perfect positive relationship, as well as Paracoccus-Brevundimonas and Pusillimonas-Halobacteriovorax.

Partial nitrification performance and mechanism of zeolite biological aerated filter for ammonium wastewater treatment

A zeolite biological aerated filter (ZBAF) with continuous feeding was successfully applied for achieving stable partial nitrification. Excellent nitrite accumulation (higher than 98.0%) and high nitrite/nitrate production rate (NPR) (approximately 0.760kg/m³/d) were obtained with increase influent ammonium concentration from 250 to 550mg/L within a nitrogen loading rate (NLR) of 0.854-1.200kg/m³/d. Owning to the adsorption of zeolite to ammonium, free ammonia (FA) concentration could remain at an appropriate range for inhibition of nitrite oxidizing bacteria (NOB) and dominance of ammonia-oxidizing bacteria (AOB), which should be responsible for the excellent partial nitrification realized in ZBAF. Kinetic study showed that the production of nitrite in ZBAF followed the zero-order kinetics model and high-throughput sequencing analysis further presented the enrichment of AOB and inhibition of NOB in ZBAF. All the results demonstrated that ZBAF hold a great potential in the application of partial nitrification for ammonium wastewater treatment.