Qiong Zhang

Effect of low COD/N ratios on stability of single-stage partial nitritation/anammox (SPN/A) process in a long-term operation

This study investigates the effects of varying COD/N ratios on single-stage partial nitritation/anammox (SPN/A) process in a SBR. The operational period was divided into three phases with different influent COD/N ratios (0.4, 0 and 0.5). Stable nitrogen removal was achieved in phase I with a COD/N of 0.4. In phase II COD was absent, effluent nitrite and nitrate increased and nitrogen removal performance gradually deteriorated. In phase III SPN/A failed to recover from nitrate accumulation when COD/N was increased. Microbial activity was measured and microbial community was analyzed by high-throughput sequencing. These results revealed that ordinary heterotrophic organisms (OHO) was suppressed when influent COD was absent, leading to the promotion of nitrification even at a low DO (0.2mgL-1). Therefore, nitrite oxidizing bacteria (NOB) was gradually enriched and anammox bacteria was suppressed. Besides, it was observed that flocs were sensitive to influent COD variations than granules, which requires further investigation.

Enhanced nitrogen and phosphorus removal from municipal wastewater in an anaerobic-aerobic-anoxic sequencing batch reactor with sludge fermentation products as carbon source

An anaerobic-aerobic-anoxic sequencing batch reactor (AOA-SBR) using sludge fermentation products as carbon source was developed to enhance nitrogen and phosphorus removal in municipal wastewater with low C/N ratio (<4) and reduce sludge production. The AOA-SBR achieved simultaneous partial nitrification and denitrification (SND), aerobic phosphorus uptake and anoxic denitrification through the real-time control and the addition of sludge fermentation products. The average removal efficiencies of total nitrogen (TN), phosphorus (PO43--P) and chemical oxygen demand (COD) after 145-day operation were 88.8%, 99.3% and 81.2%, respectively. Nitrite accumulation ratio (NAR) reached 99.1% and sludge reduction rate reached 44.1-52.1%. Specifically, 34.4% of the TN removal was carried out by SND and 57.5% by denitrification. Illumina MiSeq sequencing indicated that ammonium-oxidizing bacteria (Nitrosomonas) were enriched and nitrite-oxidizing bacteria (Nitrospira) did not exist in AOA-SBR. The system demonstrated potential to solve the dual problem of insufficient carbon source and sludge reduction.

Characterization of EPS Compositions and microbial community in an Anammox SBBR system treating landfill leachate

The biofilm system is beneficial for Anammox process designed to treat landfill leachate. In this study, the composition of extracellular polymeric substances (EPS) and the microbial community in an Anammox biofilm system were analyzed to determine the functions driving the biofilms ability to treat landfill leachate. The results demonstrated that increasing influent carbon oxygen demand (COD) could stimulate EPS production. EPS helped enrich Anammox bacteria and supplied them with nutrients and enzymes, facilitating effective nitrogen removal (approximately 95%). The variation in Anammox bacteria was similar to the variation in EPS composition. In the tested Anammox Sequencing Biofilm Batch Reactor (SBBR) system, Candidatus Kuenenia was dominant among known Anammox genus, because of its high substrate affinity and because it adapts better to landfill leachate. The relative abundance of Candidatus Kuenenia in the biofilm rose from 3.26% to 12.38%, illustrating the protection and enrichment offered by the biofilm in carrying out Anammox.

The inhibitory effects of free ammonia on ammonia oxidizing bacteria and nitrite oxidizing bacteria under anaerobic condition

The free ammonia (FA) inhibition on ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) under anaerobic condition was investigated in this study. The results indicated that NOB was more sensitive to the FA anaerobic treatment than AOB. The FA anaerobic inhibition on nitrifier gradually heightened with the increase of FA concentration. Accompanied with FA concentration increase from 0 to 16.82mgNH3-N·L-1 (the highest concentration adopted in this study), the activity of AOB reduced by 15.9%, while NOB decreased by 29.2%. After FA anaerobic treatment, nitrite was accumulated during nitrification. However, the nitrite accumulation disappeared on the sixth cycle of activity recovery tests with excessive aeration. Based on this result, a novel strategy for achieving nitritation is proposed, which involves recirculating a portion of the activated sludge through a side-line sludge treatment unit, where the sludge is subjected to treatment with FA under anaerobic condition.

Achieving partial nitrification by inhibiting the activity of Nitrospira-like bacteria under high-DO conditions in an intermittent aeration reactor

It is generally accepted that a low dissolved oxygen (DO) concentration is more beneficial for achieving partial nitrification than high-DO. In this study, partial nitrification was not established under low-DO conditions in an intermittent aeration reactor for treating domestic wastewater. During the operational period of low-DO conditions (DO: 0.3±0.14mg/L), stable complete nitrification was observed. The abundance of Nitrospira-like bacteria, which were the major nitrite-oxidizing bacteria, increased from 1.03×106 to 2.64×106cells/mL. At the end of the low-DO period, the batch tests showed that high-DO concentration (1.5, 2.0mg/L) could inhibit nitrite oxidation, and enhance ammonia oxidation. After switching to the high-DO period (1.8±0.32mg/L), partial nitrification was gradually achieved. Nitrospira decreased from 2.64×106 to 8.85×105cells/mL. It was found that suddenly switching to a high-DO condition could inhibit the activity and abundance of Nitrospira-like bacteria, resulting in partial nitrification.

Rapid start-up of partial nitritation and simultaneously phosphorus removal (PNSPR) granular sludge reactor treating low-strength domestic sewage

Obtaining desirable partial nitritation (PN) is crucial for successful application of the combined PN and anammox process. In this study, the partial nitritation and simultaneously phosphorus removal (PNSPR)1 granular sludge reactor treating low-strength domestic sewage was rapidly started up in 67days through seeding denitrifying phosphorus removal (DPR)2 sludge. The nitrite/ammonium ratio in effluent was approximately 1 and the nitrite accumulation rate (NAR) was more than 95%, about 93% of orthophosphate was removed. The DPR sludge rich in phosphate accumulating organisms (PAOs) with few nitrifying bacteria could promote the achievement of PNSPR. Quantitative microbial analysis showed that the ammonium oxidizing bacteria (AOB) gene ratio in sludge increased from 0.21% to 3.43%, while nitrite oxidizing bacteria (NOB) gradually decreased to 0.07%. The average particle size of sludge increased from 114 to 421μm, indicating the formation of PNSPR granules. The high phosphorus content in sludge and phosphorus removal facilitated rapid granulation.

Achieving nitritation by treating sludge with free nitrous acid: The effect of starvation

Side-stream sludge treatment using free nitrous acid (FNA) is a novel strategy to achieve nitritation in mainstream of wastewater treatment plants (WWTPs). To optimize nitritation, the effect of starvation on this strategy was investigated in this study. The results showed that pre-starvation, which is the starvation before FNA treatment, enhanced the resistance of sludge to FNA. This led to a decrease in the nitrite accumulation rate (NAR), which dropped from 70% to 27% after aerobic pre-starvation. This was further confirmed in the FNA treatment using the sludge collected from the secondary settling tank (anoxic pre-starvation) and the aerobic zone (without starvation) of an Anaerobic-Anoxic-Oxic system. The post-starvation, which was the starvation after FNA treatment, decreased NAR from 63% to 14%. To obtain a higher NAR, the sludge used for FNA treatment should be collected from aerobic zone, and be returned to aerobic zone after treatment to avoid pre-starvation and post-starvation.

Effects of salinity build-up on the performance and microbial community of partial-denitrification granular sludge with high nitrite accumulation

High inorganic salts inevitably impose a toxic impact on biological treatment processes. In this study, the effect of salinity on the performance and microbial community structures of partial-denitrification (PD) was firstly investigated. Results showed the denitrifying activities of non-domesticated PD sludge were completely inhibited under a temporary high salinity (≥1.5u202fwt%). However, after domestication, denitrifying activities maintained above 50% of the maximum with salinity build-up step-by-step from 0.0u202fwt% to 3.0u202fwt%. High nitrite production was stably achieved during 120 days with nitrate-to-nitrite transformation ratio around 90%. Further investigation showed extracellular polymeric substances content of PD sludge increased from 184.59u202fmg gVSS-1 to 560.64u202fmg gVSS-1, accompanied by the elevation of average particle size. This occurred against high salinity as a protective response of PD bacteria. Moreover, Thauera, the functional bacteria of PD system, was still dominant with the relative abundance increasing to 83.36% (3.0u202fwt%) from 51.33% (0.0u202fwt%).

Stable and efficient partial nitritation granular sludge reactor treating domestic sewage at low temperature

The success of combined partial nitritation (PN) and anammox process treating low-strength domestic wastewater depends on achieving a stable and efficient PN. In this study, desirable PN for domestic sewage with low temperature of 11.8-16.9u202f°C was achieved in a granular sludge reactor operated in anaerobic/aerobic (A/O) mode. Average nitrite accumulation ratio of 97.3% was obtained with an effluent nitrite/ammonium ratio of 1.2 for influent ammonium of 39.3-78.7u202fmg·L-1. Quantitative microbial analysis and activity batch test showed that nitrite oxidizing bacteria (NOB) were effectively suppressed, while ammonium oxidizing bacteria (AOB) were dominant. For the efficient suppression of NOB, A/O mode, aerobic phosphorus uptake and granular sludge could play important roles. Furthermore, high AOB activity was obtained with an average ammonium oxidation rate of 11.6u202fmgu202fN·L-1·h-1, which could be due to the abundant psychrotolerant microorganisms, increased content of extracellular polymeric substances and relatively high dissolved oxygen condition of the reactor.

Achieving advanced nitrogen removal from low C/N wastewater by combining endogenous partial denitrification with anammox in mainstream treatment

Successful application of mainstream anammox would be favorable for energy- and resource-efficient sewage treatment. This study presents a new strategy to achieve mainstream anammox, which combined with endogenous partial denitrification (EPD) for treating sewage wastewater. In this EPD-Anammox system, nitrite was stably produced by EPD with a nitrate-to-nitrite transformation ratio of 80%. Through adjusting the volume exchange ratio of EPD-reactor after anaerobic reaction, a suitable NO2--N/NH4+-N ratio of ∼1.20 for anammox reaction was achieved. Further, results showed a stable, high nitrogen removal efficiency (90%) with an effluent total nitrogen of 5.8u202fmgu202fN/L under low C/N (∼2.9). Anammox contributed 49.8% of the overall nitrogen removal owing to the steady nitrite supply from EPD. Denitrifying glycogen-accumulating organisms (GAOs, 36.6%) having potential for endogenous denitrification and Candidatus Brocadia (34.6%) were respectively dominated in the EPD-SBR and anammox-UASB and responsible for the high nitrite accumulation and anammox reaction.