Short HRT Research Articles

Optimization of two CSTRs using granular sludges dominated by Ca. Jettenia asiatica and Ca. Brocadia sinica

Two CSTRs using granules dominated by Ca. Jetteni asiatica (R1) and Ca. Brocadia sinica (R2) were operated depending on influent T-N concentrations and HRT. R1 achieved the highest NRR of 1.64 kgN/m3/day at HRT 8 h and T-N 600 mg/L, while R2 obtained the highest NRR of 2.05 kgN/m3/day at HRT 4 h and T-N 400 mg/L. The both reactors achieved high performance in high T-N concentration with the increasing of HRT in relatively short time. The sufficient T-N concentration and favorable short HRT were giving significant effects in promoting biomass increment and maintaining high removal in R1 and R2. In R1, granules larger than 2 mm accounted for over 60 %, while in R2 granules in the range of 1-2 mm comprised approximately 60 %. Granules in R1 were relatively harder and denser, and the surface of granules in R1 appeared rough, whereas the surface of granules in R2 seemed smoother. The abundance of anammox bacteria were much improved in both R1 and R2, operating under optimal operational conditions, showed significant improvement. The distribution of Ca. Brocadia sinica increased from 2.6 % to 42.0 % for R1 and from 9.1 %, to 90.5 % for R2, while there was little change in Ca. Jettenia asiatica. In this study, while the optimal HRT and nitrogen concentration as influent varied depending on the dominant anammox bacteria and granule characteristics in the inoculum, Ca. Brocadia sinica appears to play a significant role in nitrogen removal.

Optimization of membrane filtration and cleaning strategy in a high solid thermophilic AnMBR treating food waste

Anaerobic membrane bioreactor is advantageous over traditional processes for food waste treatment, i.e. short retention time, high loading rate, and particulate clean permeate. However, establishing a sustainable membrane filtration is a long-standing challenge because of its high viscosity and solids concentration characteristics. Therefore, this study investigated the changes in the membrane permeability before and after the cleaning during a 130-day thermophilic anaerobic experiment. Results show that the AnMBR system could maintain high stability even under a short HRT of 10 days and OLR of 9.0 kg-COD/(m3·d) with low volatile fatty acid of 50 mg/L. The membrane filtration deteriorates with the concurrence of a sharp increase of viscosity when the volatile solids reached 23 g/L. A critical flux was achieved at 5.5 L/(m2·h) under optimized operation conditions, membrane filtration/relaxing ratio with less than 4:1 at a hydraulic retention time of 15 d. Membrane fouling can be removed by soaking the membrane in NaClO (1 g/L, 15 h) and citric acid (2 g/L, 2 h). Conclusively, this work provides insight to establish the operation strategy for a thermophilic AnMBR treating food waste.

Biological degradation and mineralization of tetracycline antibiotic using SBR equipped with a vertical axially rotating biological bed (SBR-VARB)

Tetracycline (TC) is a widely used antibiotic with a complex aromatic chemical structure and is highly resistant to biodegradation. In this study, an SBR equipped with a vertical axially rotating biological bed (SBR-VARB) was used for the biodegradation and mineralization of TC. SBR-VARB showed high efficiency in removing TC (97%), total phenolic compounds (TP) (95%), and COD (85%) under optimal operating conditions (TC = 50 mg/L, HRT = 1.75 d, and OLR = 36 g COD/m3 d). The SBR-VARB was able to treat higher concentrations of TC in shorter HRT than reported in previous studies. The contribution of VARB to improve SBR efficiency in removing TC, TP, and COD was 16, 36, and 48%, respectively. Intermediate compounds formed during the biodegradation of TC were identified using GC–MS under the optimal operating conditions of the bioreactor. These are mainly organic compounds with linear chemical structures. Based on the complete biodegradation of TC under the optimal operating conditions of the bioreactor, 93% and 36% of the chlorine and nitrogen atoms in the chemical structure of TC appeared in the wastewater, respectively. According to the sequence analysis of 16SrDNA, Pseudomonas sp., Kocuria Polaris, and Staphylococcus sp. were identified in the biofilm of VARB and the suspended biomass of the bioreactor. Therefore, SBR-VARB showed high efficiency in the biodegradation and mineralization of TC and can be used as a suitable option for treating wastewater containing antibiotics and other toxic compounds.

Inhibition and recovery of UASB-Anammox process: Performance characteristics and microbial community dynamics in response to substrate concentration and dissolved oxygen exposure

Anammox process is a high-efficiency and low-consumption biological nitrogen removal technology that has recently attracted a great deal of attention. Many studies have focused on the changes in the nitrogen removal performance of the anammox process under inhibition. However, little mention has been made of microbial community, metabolic pathways, and the characteristics and differences among different inhibitors. In this study, a UASB-Anammox process was operated for 470 days during which the process underwent substrate inhibition and dissolved oxygen (DO) exposure. Characteristics and differences in performance inhibition and recovery were investigated in terms of process performance, sludge composition, and community structure. A high nitrogen removal rate of 3.72 kg-N/m3/d was achieved at an HRT of 2 h. The results showed that the suppression caused by the two shocks was reversible. DO exposure had a higher effect on anammox activity compared to substrate inhibition. Additionally, the dominant anammox genus changed from Candidatus Kuenenia to Candidatus Brocadia after DO exposure. A low-concentration influent and a short HRT to increase the nitrogen loading rate could effectively avoid the substrate inhibition and enhance nitrogen removal performance. The results provide a deep insight into the inhibition and recovery of anammox process, and also optimize the regulation of process performance.

Penerapan Waktu Siklus Singkat pada Granular Activated Carbon Sequencing Batch Reactor (GAC-SBR) untuk Pengolahan Limbah Cair Rumah Makan

Has resulted in the generation of restaurant wastewater also increasing. Restaurant wastewater contains pollutants BOD, COD, TSS, Total-N, PO4, oil and grease with high enough levels so that it might pollute the environment if discharged into river bodies without going through a processing process. In this study, an experiment was conducted by applying granular activated carbon - sequencing batch reactor to treat restaurant wastewater. In this study was operated the reactor with short HRT of 6, 9, and 12 hours and compared with a comparison HRT of 36 hours. In the reaction phase, a GAC mass of 0.4 and 2.5 grams was added. The highest allowance for BOD is 91.23%, COD is 93.35%, TSS is 98.94%, PO4 is76.60 %, Total-N is 89.78%, and oils and grease are 99,99%. GAC was affixed during the reaction phase with a mass variation of 0.4 grams and 2.5 grams, the optimal GAC mass was obtained at a GAC mass of 0.4 grams. The aeration rate was operated at a rate of 20 and 40 L/min, the optimal aeration rate was obtained at a rate of 20 L/min.

Assessment of a novel microalgae-cork based technology for removing antibiotics, pesticides and nitrates from groundwater

Groundwater pollution has increased in recent years due to the intensification of agricultural and livestock activities. This results in a significant reduction in available freshwater resources. Here, we have studied the long term assessment of a green technology (1-4L/day) based on a photobioreactor (PBR) containing immobilised microalgae-bacteria in polyurethane foam (PF) followed by a cork filter (CF) for removing nitrates, pesticides (atrazine and bromacil), and antibiotics (sulfamethoxazole and sulfacetamide) from groundwater. The prototype was moderately effective for removing nitrates (58%) at an HRT of 8 days, while its efficiency decreased at a HRT of 4 and 2 days (<20% removal). The combined use of PBR-CF enabled antibiotics and pesticides to be attenuated by up to 95% at an HRT of 8 days, but their attenuation decreased with shorter HRT, with pesticides being the compounds most affected (reducing from 97 to 98% at an HRT of 8 days to 23-45% at an HRT of 2 days). Pesticide transformation products were identified after the CF, supporting biodegradation as the main attenuation process. A gene-based metataxonomic assessment linked the attenuation of micropollutants to the presence of specific pesticide biodegradation species (e.g. genus Phenylobacterium, Sphingomonadaceae, and Caulobacteraceae). Therefore, the results highlighted the potential use of microalgae and cork to treat polluted groundwater.

Efficient direct membrane filtration (DMF) of municipal wastewater for carbon recovery: Application of a simple pretreatment and selection of an appropriate membrane pore size

Considerable attention has been paid in recent years to the recovery and effective utilization of organic matter in municipal wastewater for the establishment of a circular economy. Direct membrane filtration (DMF) of municipal wastewater using microfiltration (MF) or ultrafiltration (UF) membranes to retain and concentrate the organic matter in municipal wastewater could be a practical option for this purpose. However, severe membrane fouling and high concentrations of organic matter remaining in the DMF permeate are concerns to be addressed. Application of a simple pretreatment using fixed biofilms was investigated to address these issues. In this study, experiments were carried out at an existing municipal wastewater treatment plant. A moving bed biofilm reactor (MBBR) process operated under a very short HRT of 1 h and DO concentration of 0.5 mg/L selectively degraded low-molecular-weight dissolved organic matter in municipal wastewater without degradation of membrane-recoverable suspended and colloidal organic matter. Application of the pretreatment did not reduce the amount of organic carbon recovered by DMF using an MF membrane (approximately 70% of the influent COD being recovered), while it dramatically mitigated the membrane fouling probably due to the alteration of characteristics of dissolved organic matter in wastewater. The pretreatment also reduced the concentration of organic matter in the DMF permeate by 41%: COD concentration in the DMF permeate was as low as 40 mg/L. With the established MBBR pretreatment, performances of MF (0.1 µm) and UF (molecular weight cut-off: 150,000) membranes for DMF were compared in parallel. It was found that the increase of the recoverable amount of organic matter by using UF was marginal (about 5%), whereas fouling in UF was much more severe than that in MF. The severe fouling in UF was caused by inorganic colloids such as FeS that could pass through MF membranes but be retained by UF membranes. Based on the results obtained in this study, it is concluded that MF is more suitable than UF for efficient DMF.

고율침전여과를 이용한 공단비점유출수의 재이용

The objectives of this research are to evaluate the performances of inclined-plate sedimentation and high-rate filtration processes using polyethylene+activated carbon as filter media and to assess the potential for reclamation and reuse of industrial complex runoff rainwater (ICRR). Characteristics of ICRR within the rainfall duration time of 1hr showed there was a close correlation between SS and organic and nutrient as correlation coefficients of 0.87-0.95. Inclined-plate settler at HRT 10 min showed the removal efficiency of approximately less than 30% for organic, nutrient and heavy metals. Compared to existing urban rainwater runoff treatment systems using centrifugal separation or filtration techniques, the IPS/HRF system with much shorter HRT < 2 min(LV500 m/d) showed high removal efficiencies of >80% for nonsoluble fractions such as SS, heavymetals, and particulate COD as well as approximately >50% for organic and nutrient through high-rate filtration. The results indicate that the IPS/HRF system was considerably cost-effective for the ICRR treatment, achieving an effluent quality able to be used for industrial or multipurpose water as well as recharge water for water bodies.

Denitrification performance and characteristics of untreated corncob for enhanced nitrogen removal of municipal sewage with low C/N ratio

Unpretreated corncob was applied in denitrification bio-filter (DNBF) and anoxic tank of AAO system, respectively, to treat sewage with low C/N ratio, and both two approaches achieved good denitrification performance. Although shorter HRT could effectively decrease effluent chroma and COD of corncob-DNBF, nitrogen removal efficiency declined unexpectedly. Higher internal reflux ratio was beneficial for corncob-AAO without damage to anoxic environment for denitrification, while there was no risk of effluent chroma and excessive COD. Different supplement modes could realize same denitrification effect with distinct advantages, which were higher specific denitrification rate and biomass amount, respectively. The latter mode, applying corncob at secondary treatment, was preferable for its operational stability and convenience. Stoichiometry analysis indicated the unit COD demand of AAO decreased from 5.70 to 5.04 g COD/g N after adding corncob, and the oxygen demand (or energy consumption of aeration) decreased as well. The dominant substrates decomposer in corncob-AAO altered to Kouleothrix (affiliated to phylum Chloroflexi), and the main denitrifying bacteria were unclassified_f__Methylophilaceae and Azospira. Accordingly, functional enzymes for degrading glucan, xylan and lignin and processing denitrification showed satisfying abundance in the integrated system, especially in the newly formed biofilm.

Cascade systems to recover resources from sludge by the integration of pretreatments to fermentation-based anaerobic bioleaching process

Future sustainable technological solutions need to balance the development of sludge as resource against the paramount necessity to protect human health and environment. Nutrients, metals, embedded energy, and the avoidance of sludge handling costs are important examples of recoverable assets. The virtuous shift to an efficient resource recovery platform should integrate pretreatments with biological conversion processes to extract values from complex waste streams as waste activated sludge. The aims of this study were: i. to assess the feasibility of sludge pretreatments as ultrasounds to promote the release of metals and organics; ii. to explore the potential of anaerobic microorganisms to leach out embedded resources from sludge through an enhanced hydrolysis/acidification step. The study proved that the acid addition to the sonicated feedstocks promoted a significant solubilization and acidification of the system during the feeding days. Crucial strategy was the inhibition of methanogens by thermal treatment of the start-up inoculum that assured pH stability, high VFAs production, and elements (As, Co and Se) solubilization. High throughput analysis of 16 S rRNA gene sequencing performed on this system revealed the presence of a high abundance of reads affiliated with hydrolytic and fermentative bacteria, in particular belonging to the phyla Bacteroidetes, Firmicutes, and Actinobacteria. The produced VFAs, fed into a semi-pilot digester with a short HRT of 15 days, were successfully converted into methane thanks to the dominant acetoclastic methanogenic population. Mass and energy balance for a full-scale scenario of this innovative biorefinery platform is reported and compared to a conventional sludge treatment scenario.

Efficacies of Nitrogen Removal and Comparisons of Microbial Communities in Full-Scale (Pre-Anoxic Systems) Municipal Water Resource Recovery Facilities at Low and High COD:TN Ratios

At a low COD:TN ratio (≤5) in influent, maintaining a longer HRT (≥9 h) and longer SRT (≥30 d) are suggested to improve higher N removal efficiency in case of operation at low DO (Dissolved oxygen) level (0.9 ± 0.2 mg-O2/L). However, in case of operation at high DO level (4.0 ± 0.5 mg-O2/L), short HRT (1 h) and typical SRT (17 d) make it possible to achieve nitrogen removal. On the other hand, at a high COD:TN ratio (≥8.4), a typical HRT (9–15 h), SRT (12–19 d), and DO level (1.3–2.6 mg-O2/L) would be applied. Microbial distribution analysis showed an abundance of AOA (Ammonia-oxidizing archaea) under conditions of low DO (≤0.9 mg-O2/L). Nitrosomonas sp. are mostly found in the all investigated water resource recovery facilities (WRRFs). Nitrosospira sp. are only found under operating conditions of longer SRT for WRRFs with a low COD:TN ratio. In comparison between abundances of Nitrobacter sp. and Nitrospira sp., abundances of Nitrobacter sp. are proportional to low DO concentration rather than abundance of Nitrospira sp. A predominance of nosZ-type denitrifiers were found at low DO level. Abundance of denitrifiers by using nirS genes showed an over-abundance of denitrifiers by using nirK genes at low and high COD:TN ratios.

Investigating the effects of physical properties of sulphur-based carriers on autotrophic denitrification

ABSTRACT In this study, four sulphur-based carriers (C1–C4) which have different mass ratio of sodium silicate to carrier from 30% to 50% (C1–C3) and the existence of water (C4) were prepared in order to evaluate the effect of the different physical properties on denitrification in sulphur-based autotrophic processes. While the apparent density and the compressive strength decreased as the proportion of sodium silicate increased and water was added in the carriers, the average pore size and the porosity increased from 0.43 to 3.13 µm and from 38% to 67%, respectively. The treatment system using the carrier C4 with the highest surface area was stabilized most rapidly and achieved the highest nitrogen removal efficiency of 85.6 ± 5.0% during a relatively short HRT of 3 h. The efficiency of nitrate removal was enhanced by 36.9% due to the increase of the ratio of sodium silicate in the carriers from C1 to C3, and more 4.8% point of removal rate increased in the carrier C4 by adding water to the carrier C3. The sum of Thiobacillus and Sulfurimonas was obtained up to 65.90% among the microbial community in the carrier C4 which has the highest distribution (38.35%) of pore size above 20 µm considered to be favourable for retaining autotrophic denitrifiers. From the above results, it is obvious that the physical properties of the sulphur-based carrier and its ability of denitrification can be influenced significantly by the composition of the carrier.

Rapidly achieving partial nitrification of municipal wastewater in enhanced biological phosphorus removal (EBPR) reactor: Effect of heterotrophs proliferation and microbial interactions

Stable nitritation is the major challenge for short-cut nitrogen removal from municipal wastewater. This paper demonstrated a rapid achievement of partial nitrification (PN) in an enhanced biological phosphorus removal (EBPR) reactor treating domestic wastewater. Polyphosphate accumulating organisms (PAOs) were enriched operated at a short aerobic HRT (2.0 h) and SRT (10 d), with satisfactory phosphorus removal efficiency (95.9%). Both of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were elutriated simultaneously. Interestingly, AOB recovered much faster than NOB by a subsequent extension of aerobic HRT and SRT, resulting in a rapid development of PN within 15 days. Ammonia oxidation rates of AOB significantly increased by 44.2%, facilitating a high nitrite accumulation rate (NAR) of 95.8%. Genus Tetrasphaera, Halomonas, Paracoccus and Candidatus_Accumulibacter belonging to PAOs accounted for 4.6%. The proliferation of heterotrophs, typically as PAOs, maximized the microbial competition against NOB by favoring AOB activity and synergy with functional bacteria.

Pre-anaerobic treatment enhanced partial nitrification start-up coupled with anammox for advanced nitrogen removal from low C/N domestic wastewater

A modified two-stage partial nitrification/anammox (PN/A) process with short aerobic HRT of 5.7 h was established and realized advanced nitrogen removal from domestic wastewater. The first process was partial nitrification in the PN-SBR, the initiation and stable maintenance of partial nitrification was achieved by pre-anaerobic treatment without inoculation or addition of inhibitor, nitrite accumulation rate was over 90% and maintained over 200 days, meanwhile pre-anaerobic improved the storage of endogenous carbon sources to enhance the nitrogen removal efficiency. The second process was anammox in the AMX-SBR, which was fed with the effluent of PN-SBR, the effluent total inorganic nitrogen was below 5 mgN/L, nitrogen removal efficiency reached 92.06%. Furthermore, Candidatus-Brocadia was enriched after treating domestic sewage with low ammonia from 0.02% to 0.23%.This study demonstrated the feasibility of starting and maintaining partial nitrification by pre-anaerobic treatment and the feasibility of nitrogen removal by coupling simultaneous-nitrification–denitrification with anammox to treat actual municipal sewage.

Metabolic patterns reveal enhanced anammox activity at low nitrogen conditions in the integrated I-ABR.

Substrate concentrations greatly influence bacterial growth and metabolism. However, optimal nitrogen concentrations for anammox bacteria in nitrogen-limited environments remain unclear. Here, we observed enhanced nitrogen metabolism and anabolism of anammox bacteria at low nitrogen conditions. Efficient nitrogen removal was achieved at ammonium and nitrite influent concentration of 30mg/L under HRT of 1hr, with an average nitrogen removal rate (NRR) of 0.73kgN/(m3 ·day) in I-ABR composed of four compartments. The highest anammox activity of 6.25mmolN/ (gVSS·hr) was observed in the fourth compartment (C4) with the lowest substrate levels (ammonium and nitrite of 11.6mg/L and 7mg/L). This could be resulted from the highest expression level of genes involved in nitrogen metabolism in C4, which was 1.49-1.67 times higher than that in other compartments. Besides, the second compartment (C2) exhibited the most active anabolism at ammonium and nitrite of 17mg/L and 13mg/L, respectively, which contributed to the most active amino acid synthesis and thus the highest EPS (1.35 times higher) in C2. This enhanced amino acid auxotrophy between anammox bacteria with heterotrophs, and consequently, heterotrophs thrived and competed for nitrite. These results hint at the potential application of anammox process in micro-polluted water. PRACTITIONER POINTS: High nitrogen removal and efficient biomass retention at low nitrogen concentrations under short HRT was achieved in I-ABR. Optimal concentrations for anammox nitrogen removal and anabolism were discussed under low nitrogen concentrations. More active anabolism contributed to enhanced amino acid synthesis and thus higher EPS contents. Low substrate levels led to enhanced expression of genes involved in nitrogen metabolism and thus high anammox activity.

Future perspectives of energy saving down-flow hanging sponge (DHS) technology for wastewater valorization—a review

The down-flow hanging sponge (DHS) process has been rigorously studied as a significant post-treatment of anaerobically pretreated sewage and later for broader applications in industrial wastewater and pre-settled sewage treatment. The DHS has several advantages over conventional wastewater treatment technologies, i.e., simple process control, less workforce requirement, less footprint, low construction, operation, and maintenance costs due to simple design and less mechanical parts. DHS technology requires no external aeration, which saves energy use and minimizes the associated costs. Low excess sludge yield in the DHS process adds to economic efficiency. DHS achieved high organics removal efficiency up to 99% with considerable nutrient and pathogen removals at extremely short HRT. Up-to-date, six DHS configurations have been developed and extensively tested at pilot scale applications under different operational conditions, i.e., HRTs, OLRs, and SRTs. Accordingly, the DHS module has successfully been translated into full-scale demonstration plants in Japan, India, Egypt, and Thailand. This paper comprehensively reviews the mechanism and development history of the DHS system. The process performance for municipal and industrial wastewaters treatment covers a broad range of parameters, i.e., organics, nutrients, pathogens (bacteria, viruses, parasites) has been summarized. The unique feature of sludge reduction efficiency of DHS is comprehensively discussed. Moreover, the process microbiology, energy efficiency, emerging application of DHS for direct sewage treatment, pilot and full-scale experiences in the last 20 years are also reviewed. DHS technology has not yet being fully explored, mainly for nutrients removal, process modelling, optimization of wastewater and air flow configurations in DHS reactor, and biomass growth, which could help for further advancement in the process for better performance and wider applications.

Role of porous polymer carriers and iron-carbon bioreactor combined micro-electrolysis and biological denitrification in efficient removal of nitrate from wastewater under low carbon to nitrogen ratio

In the current research, a novel bioreactor composed of porous polymer carriers and iron-carbon (PPC@FeC) was established through bacterial immobilized technology. The influence of key factors was studied on the nitrate removal performance of the PPC@FeC bioreactor. The experimental results showed that the highest removal rate of nitrate (7.33 mg L−1 h−1) can be obtained with short hydraulic retention times (HRT = 2.0 h) and low carbon-to-nitrogen ratio (C/N = 2.0). The results of high-throughput sequencing revealed that Zoogloea sp. L2 was the dominant strain in bioreactor responsible for nitrate removal. Moreover, the SEM and XRD analyses elucidated that Fe2O3 was the final product produced by the interaction of FeC and strain L2. These findings showed that the PPC@FeC bioreactor successfully combined micro-electrolysis and biological denitrification, which exhibited great potential in removing nitrate effectively from wastewater under low C/N ratio and short HRT conditions.

Simultaneous enhanced biological phosphorus removal and semi-nitritation (EBPR-SN) followed by anammox process treating municipal wastewater at seasonal temperatures: From summer to winter

This work investigated the feasibility of a novel simultaneous enhanced biological phosphorus removal and semi-nitritation (EBPR-SN) plus anammox process treating real municipal wastewater from summer to winter (28.1– 15.3 °C). Two lab-scale sequential reactors were used in this study, namely EBPR-SN and Anammox sequencing batch reactors (SBRs). Long-term operation suggested that ammonium oxidizing bacteria abundance decreased from 1.67% to 0.89% whereas nitrite oxidizing bacteria decreased to nearly undetected in the EBPR-SN SBR, maintaining the stable nitritation (nitrite accumulation ratio: 98.3 ± 1.0%). Lowering airflow rate was effective to retain nitritation with temperature decrease. Reliable nutrient removal was still maintained in winter (16.4 ± 0.7 °C), i.e. the removal efficiencies for nitrogen and phosphorus were 80.0 ± 3.5% and 95.4 ± 5.2%, respectively, with short aerobic HRT (6.4 h) and low dissolved oxygen (0.2–1.5 mg/L). The percentage of anammox contribution to nitrogen-removal increased with temperature decrease, although Candidatus Brocadia abundance decreased. Additionally, the protection of extracellular polymeric substances was important to the successful performance.

Characteristics and formation mechanism of hollow Anammox granular sludge in low-strength ammonia sewage treatment

• Hollow Anammox granular sludge formed in low-strength ammonia wastewater. • TN removal efficiency was 80% with NRR 0.93 kg N m −3 d −1 under a short HRT (1.8 h). • Tryptophan or protein-like was the active component of PN as the main part of EPS. • Candidatus Kuenenia lived and coexisted with other bacteria under EPS shield. • The formation of bubble cavitation was one of the key steps of regranulation. Hollow anaerobic ammonium oxidation (Anammox) granular sludge formed and became the main part in the treatment of low-strength ammonia wastewater. The characteristics and formation mechanism of hollow Anammox granular sludge were investigated in a continuous stirred tank reactor (CSTR) at room temperature. Granular sludge was found to be cavitated, and Candidatus Kuenenia was the dominant bacteria that lived and coexisted with other bacteria in walls under an extracellular polymeric substance (EPS) shield. The EEM-PARAFAC analysis was applied to characterize the EPS, while EPS secretion was the expression of specific Anammox activity. Tryptophan or protein-like was the active component of proteins (PN) as the main part of EPS in granules. These results confirm that hollow Anammox granular sludge is suitable for low-strength ammonia sewage. Finally, a conceptual model of hollow Anammox granules growth and breakage was developed, and cavitation was a key step of regranulation.

The role of ballast specific gravity and velocity gradient in ballasted flocculation

This study investigated the concealed interaction between applied velocity gradient (G value) and ballast specific gravity (SG) in ballasted flocculation (BF). The objective was to unravel the participation of applied surface concentration (SC: 0.005 m2L−1–0.02 m2L−1) of high specific gravity ballasts (SG: 2.9–5.57) in BF aggregation phenomenon at varied velocity gradients (G value: 750s−1–1250s−1). Static mixer was used to perform the BF experiments, and aggregated flocs were characterized using charge coupled device (CCD) camera. The results revealed that conventionally adopted velocity gradient (G value: 150s-1 - 300s-1) in BF studies was insufficient for efficient floc development due to inadequate suspension of denser ballasts during mixing. This resulted in poor turbidity removal (< 40 %) and immature slow settling flocs (< 25 mh−1) despite higher ballast consumption. However, appropriate optimization of G value (1250s−1) corresponding to high specific gravity ballast (SG: 5.57) resulted in 99.5 % turbidity removal (residual turbidity: 1NTU) achieved in a shorter settling interval of 30 s consuming significantly less ballast concentration. This expeditious settling phenomenon was also evident in CCD camera observations of the ballasted flocs achieving superficial settling velocity (105mh-1). Therefore, it was concluded that appropriate optimization of the G value corresponding to the pertinent concentration of denser ballasts can exhibit rapid elimination of micropollutants, and superficial sedimentation with efficient material and energy use. This can lead to efficient BF design with a short HRT, compact footprint, and ability to handle highly turbid influent.