Simultaneous sludge minimization and membrane fouling mitigation in membrane bioreactors by using a microaerobic - Settling pretreatment module.

Membrane fouling is the major obstacle for membrane bioreactors operated at a long sludge retention time to reduce sludge production. In this study, a sludge process reduction (SPR) module, consisting of a microaerobic tank and a settler, was inserted before an anoxic/oxic MBR (AO-MBR) to achieve dual objectives of fouling alleviation and sludge reduction. Three SPR-MBRs were operated to investigate influences of sludge recirculation ratios from the SPR settler to the microaerobic tank on process performance. Compared to AO-MBR, the SPR-MBRs reduced sludge production by 43.1-56.4% by maintaining sludge retention times above 175 d, and decreased foulant layer resistance and pore clogging resistance. Inserting SPR reduced the accumulation of dissolved organic matters and extracellular polymeric substances, enlarged sludge flocs, and decreased sludge viscoelasticity. However, increasing RSPR stimulated outward diffusion of extracellular polymeric substances and increased sludge viscosity. SPR-MBRs achieved effective sludge reduction by enriching hydrolytic (Trichococcus and Aeromonas) and fermentative genera (Lactococcus, Paludibacter, Macellibacteroides, and Acinetobacter) in the SPR, and alleviated membrane fouling by prohibiting the growth of extracellular polymeric substance-secreting bacteria and enriching filamentous bacteria to enlarge particle size. The results revealed that the SPR-MBR maximized sludge reduction with a very long sludge retention time, and alleviated membrane fouling synchronously.

Comparison on treatment strategy for chemical cleaning wastewater: Pollutants removal, process design and techno-economic analysis.

Chemical cleaning wastewater (CCW) usually consists of pickling wastewater (PW) and alkaline cleaning wastewater (ACW), and the strategy of separate treatment or combined treatment affects pollutant removal efficiency and cost. In this study, separate and combined treatment of real PW and ACW generated from an on-site cleaning campaign were investigated. A neutralization - fluoride removal - coagulation - oxidation process was constructed for PW and mixed wastewater (MW) treatment, and operational conditions of each process were optimized. The optimal mixing ratio of PW and ACW in the primary neutralization tank was 3:7, which obtained a near neutral pH, efficient chromaticity and turbidity removal and good settling performance. The neutralized MW and PW were both adjusted pH to 9.5 to precipitate metal ions as hydroxides. After fluoride precipitated as CaF2, the fluoride removal rates of MW and PW were both 99.9%, respectively, and polyaluminum chloride was dosed to improve the settleability of CaF2. Then sodium hypochlorite oxidization was employed to remove NH3-N and soluble COD. Techno-economic analysis based on pilot-scale tests showed that separate treatment of PW and ACW obtained better effluent quality than combined treatment. The total cost of combined treatment (37.44 $/m3) was greatly higher than that of separate treatment of PW and ACW (18.20 $/m3). This study proposed a cost-effective strategy for CCW treatment, and suggested that neutralization with acidic or alkaline wastewater should be systematically considered for technical and economic feasibility.

Sulfate removal from wastewater using ettringite precipitation: Magnesium ion inhibition and process optimization.

One of the main challenges in industrial wastewater treatment and recovery is the removal of sulfate, which usually coexists with Ca2+ and Mg2+. The effect of Mg2+ on sulfate removal by ettringite precipitation was investigated, and the process was optimized in the absence and presence of Mg2+. In the absence of Mg2+, the optimum conditions with sulfate removal of 99.7% were obtained at calcium-to-sulfate ratio of 3.20, aluminum-to-sulfate ratio of 1.25 and pH of 11.3 using response surface methodology. In the presence of Mg2+, sulfate removal efficiency decreased with increasing Mg2+ concentration, and the inhibitory effect of Mg2+ matched the competitive inhibition Monod model with half maximum inhibition concentration of 57.4 mmol/L. X-ray diffraction and Fourier transform infrared spectroscopy analyses of precipitates revealed that ettringite was converted to hydrotalcite-type (HT) compound in the presence of Mg2+. The morphology of precipitates was transformed from prismatic crystals to stacked layered crystals, which confirmed that Mg2+ competes with Ca2+ for Al3+ to form HT compound. A two-stage process was designed with Mg2+ removal before ettringite precipitation to eliminate the inhibitory effect, and is potential to realize sludge recovery at the same time of effective removal of sulfate and hardness.

Reusing effluent of flue gas desulfurization wastewater treatment process as an economical calcium source for phosphorus removal.

Flue gas desulfurization (FGD) wastewater treatment by conventional neutralization, chemical precipitation and coagulation process removes most suspended solids and heavy metals, and provides an effluent rich in calcium, alkalinity and chloride, which obstructs its reclamation and reuse but is in favor of phosphorus (P) precipitation. The goals of this study were to investigate feasibility of reusing FGD effluent as a calcium source for P removal from P-rich wastewater. Results revealed that increasing the volumetric ratio between FGD effluent and P-rich wastewater achieved higher pH value and Ca/P ratio, and thus enhanced P removal efficiency to 94.3% at the ratio of 40%. X-ray diffraction and scanning electron microscope analysis of harvested precipitates showed that increasing pH from 8 to 10 induced the conversion of hydroxyapatite to tri-calcium phosphate, and then to whitlockite. This study demonstrated that for reusing FGD effluent for P removal was highly feasible, both technically and economically. This process not only saves the cost of precipitants for P removal, but also provides an economical alternative for current zero liquid discharge technology for FGD wastewater, which requires high energy consumption and capital costs.

Phosphonate removal from discharged circulating cooling water using iron-carbon micro-electrolysis.

Phosphonate is a commonly used corrosion and scale inhibitor for a circulating cooling water (CCW) system. Its discharge could cause eutrophication of receiving waters. The iron-carbon (Fe/C) micro-electrolysis technology was used to degrade and remove phosphonate from discharged CCW. The influences of initial pH, Fe/C ratio (FCR) and temperature on phosphonate removal were investigated in a series of batch tests and optimized by response surface methodology. The quadratic model of phosphonate removal was obtained with satisfactory degrees of fitness. The optimum conditions with total phosphorus removal efficiency of 95% were obtained at pH 7.0, FCR of 1.25, and temperature of 45 °C. The phosphonate removal mechanisms were also studied. Phosphonate removal occurred predominantly via two consecutive reactive phases: the degradation of phosphonate complexes (Ca-phosphonate) and the precipitation of Fe/C micro-electrolysis products (PO4(3-), Ca²âº and Fe³âº).

Carbon footprint analysis of wastewater treatment processes coupled with sludge in situ reduction.

The goal of this study was to assess the impacts or benefits of sludge in situ reduction (SIR) within wastewater treatment processes with relation to global warming potential in wastewater treatment plants, with a comprehensive consideration of wastewater and sludge treatment. The anaerobic side-stream reactor (ASSR) and the sludge process reduction activated sludge (SPRAS), two typical SIR technologies, were used to compare the carbon footprint analysis results with the conventional anaerobic - anoxic - oxic (AAO) process. Compared to the AAO, the ASSR with a typical sludge reduction efficiency (SRE) of 30 % increased greenhouse gas (GHG) emissions by 1.1 - 1.7 %, while the SPRAS with a SRE of 74 % reduced GHG emissions by 12.3 - 17.6 %. Electricity consumption (0.025 - 0.027 kg CO2-eq/m3), CO2 emissions (0.016 - 0.059 kg CO2-eq/m3), and N2O emissions (0.009 - 0.023 kg CO2-eq/m3) for the removal of secondary substrates released from sludge decay in the SIR processes were the major contributor to the increased GHG emissions from the wastewater treatment system. By lowering sludge production and the organic matter content in the sludge, the SIR processes significantly decreased the carbon footprints associated with sludge treatment and disposal. The threshold SREs of the ASSR for GHG reduction were 27.7 % and 34.6 % for the advanced dewatering - sanitary landfill and conventional dewatering - drying-incinerating routes, respectively. Overall, the SPRAS process could be considered as a cost-effective and sustainable low-carbon SIR technology for wastewater treatment.

Role of primary sedimentation on plant-wide energy recovery and carbon footprint.

The goal of this paper is to show the effect of primary sedimentation on the chemical oxygen demand (COD) and solids fractionation and consequently on the carbonaceous and energy footprints of wastewater treatment processes. Using a simple rational procedure for COD and solids fraction quantification, we quantify the effects of varying fractions on CO2 and CO2-equivalent mass flows, process energy demand and energy recovery. Then we analysed two treatment plants with similar biological nutrient removal processes in two different climatic regions and quantified the net benefit of gravity separation before biological treatment. In the cases analysed, primary settling increases the solid fraction of COD that is processed in anaerobic digestion, with an associated increase in biogas production and energy recovery, and a reduction in overall emissions of CO2 and CO2-equivalent from power importation.

[Protective performance and subjective evaluation of N95 filtering-facepiece respirators].

OBJECTIVE: To evaluate the protective performance of N95 filtering-facepiece respirators (FFRs) used widely in China and to investigate participants' subjective evaluation about them. METHODS: Four models (A1, A2, B1, B2) of N95 FFRs from two manufactures were chosen to measure the filter penetration and inhalation resistance. Inward leakage was measured by Condensation Nuclei Counting method (CNC) in 50 participants selected using the Chinese respirator fit test panel. Each participant was asked subjective feelings after wearing a respirator by questionnaire survey. RESULTS: The filter penetration and inhalation resistance of four FFRs complied with national standard (GB 2626-2006). The geometric mean fit factors (GMFFs) for four models were 20.9, 14.6, 74.0, 49.1 and there passing rates were 4%, 4%, 42%, 10%. All of four models had bad seal performance, especially the passing rate of A1 and A2 were lower than 10%. The self-feelings about the resistance for FFRs had no significant difference (P > 0.05). The results indicated that B (B1 and B2) has a better fit than A (A1 and A2) according to participants' subject evaluation (P < 0.05). The folding style respirator (B2) was significant more comfortable than the cup style respirator (B1). The subjective feeling of respirators leakage by participant was poor consistent with objective inward leakage test. The kappa index was 0.067 (95%CI: -0.029∼0.163, P = 0.18) and the consistent rate was 50%. CONCLUSION: The poor seal performance was the biggest problem of N95 FFRs in Chinese market. Respirators should be resigned or improved rely on Chinese facial features. Dust workers should choice a fit respirator according to the result of objective leakage test rather than subjective feeling of leakage.

Under-loaded operation of an anaerobic-anoxic-aerobic system in dry and wet weather dynamics to prevent overflow pollution: Impacts on process performance and microbial community.

Effects of low hydraulic loading rate (HLR) in dry weather and high HLR in wet weather on pollutant removal, microbial community, and sludge properties of a full-scale wastewater treatment plant (WWTP) were extensively studied to explore the risk of under-loaded operation for overflow pollution control. Long-term low HLR operation had an insignificant effect on the pollutant removal performance of the full-scale WWTP, and the system could withstand high-load shocks in wet weather. Low HLR resulted in higher oxygen and nitrate uptake rate due to the storage mechanism under the alternating feast/famine condition, and lower nitrifying rate. Low HLR operation enlarged particle size, deteriorated floc aggregation and sludge settleability, and reduced sludge viscosity due to the overgrowth of filamentous bacteria and inhibition of floc-forming bacteria. The remarkable increase in Thuricola and the contract morphology of Vorticella in microfauna observation confirmed the risk of flocs disintegration in low HLR operation.

Comparison on biological nutrient removal and microbial community between full-scale anaerobic/anoxic/aerobic process and its upgrading processes.

A comparative study was conducted for the anaerobic/anoxic/aerobic (AAO) process and its two upgrading processes, five-stage Bardenpho and AAO coupling moving bed bioreactors (AAO + MBBR), using long-term operation data of six full-scale wastewater treatment plants. The three processes all had good COD and phosphorus removal performance. The reinforcing effects of carriers on nitrification were moderate at full-scale applications, while the Bardenpho was advantageous in nitrogen removal. The AAO + MBBR and Bardenpho processes both had higher microbial richness and diversity than the AAO. The AAO + MBBR favored bacteria to degrade complex organics (Ottowia and Mycobacterium) and to form biofilms (Novosphingobium), and preferentially enriched denitrifying phosphorus-accumulating bacteria (DPB) (norank_o__Run-SP154) with the highest anoxic to aerobic phosphorus uptake rates of 65.3 % - 83.9 %. The Bardenpho enriched bacteria tolerant to varied environments (Norank_f__Blastocatellaceae, norank_o__Saccharimonadales, and norank_o__SBR103), and was more suitable for the upgrading of the AAO because of its excellent pollutant removal performance and flexible operation mode.

Towards more accurate design and specification of aeration systems using on-site column testing.

Fine-pore diffuser systems are selected for their potential energy efficiency, and during design their propensity for fouling and for an increase in pressure drop with time must be considered. Both fouling and pressure-drop increase cause an increase in blower power requirements. This paper presents a new approach to improve this design procedure, without altering the technical structure of the classical approach. While the administrative and bidding milestones are being carried out (i.e., in the first 6 months of the project milestones), an independent aeration team can test candidate diffusers suitable for design in an aeration column in situ. An extended fouling test in the plant's aeration tanks allows the collection of site-specific aeration performance data. These improve the accuracy of the design process, and limit the reliance on safety factors.

[Fit test and improvement of self-inhalation air-purifying dust respirator].

OBJECTIVE: To investigate the fit of self-inhalation air-purifying dust respirator for Chinese workers, to improve the respirators according to Chinese facial features and to evaluate the protective effects of improved respirators. METHODS: Two types of self-inhalation air-purifying dust respirators (cup respirators A1 and folding respirators B1) were tested by Condensation Nuclei Counting method (CNC) in 25 representative subjects (15 males and 10 females). According to the Chinese facial features and fit factors, A1 and B1 respirators were improved. The fit tests were performed for the improved A1 and B1 respirators. RESULTS: The fit factors ≥ 100 served as the qualified standard of self-inhalation air-purifying dust respirators. The qualified rate for cup respirator A1 was 0.0%. Its geometric mean fit factor was 20.7 (6.9 ∼ 46.9). The qualified rate for cup respirator B1 was 4.0%. Its geometric mean fit factor was 26.0 (6.8 ∼ 154.9). After improvement, the qualified rates and fit factors significantly increased. The qualified rate for cup respirator A2 was 72.0% and geometric mean fit factor was 223.5 (2.2 ∼ 5932.7). There were significantly differences between respirator A1 and A2 (χ(2) = 25.09, P < 0.05). The qualified rate for cup respirator B2 was 88.0 % and geometric mean fit factor was 429.8 (41.5 ∼ 3692.9). The significant differences (χ(2) = 32.21, P < 0.05) between B1 and B2 were found. CONCLUSION: To ensure the protective effects, the self-inhalation air-purifying dust respirators were designed according to Chinese facial features. The respirator fit test must be conducted when workers choose respirators.

Sludge decay kinetics and metagenomic analysis uncover discrepant metabolic mechanisms in two different sludge in situ reduction systems.

A comparative study was conducted between an anaerobic side-stream reactor (ASSR) process and a sludge process reduction (SPR) activated sludge (SPRAS) process for uncovering crucial metabolic mechanisms governing sludge reduction. Both of two processes were efficient in removing pollutants, while the SPRAS (62.3 %) obtained much higher sludge reduction than the ASSR (27.9 %). The highest rate coefficients of sludge decay, heterotroph lysis and particles hydrolysis were 0.106, 0.219 and 0.054 d-1 in the SPR module, followed by ASSR with coefficients of 0.060, 0.135 and 0.047 d-1. The SPR module achieved an 81.9 % higher sludge decay mass with a 32.8 % smaller volume than the ASSR module. The SPR module preferentially enriched hydrolytic/fermentative and slow-growing bacteria. Metagenomic analysis revealed that SPR strengthened the key hydrolases and L-lactate dehydrogenase in the glycolysis pathways and weakened the citrate cycle, inducing metabolic uncoupling due to the reduced biosynthesis of ATP. Inserting ASSR only altered the ATP biosynthesis pathway, but maintenance metabolism was dominant for sludge reduction, with a long sludge retention time prolonging the food chain for predation.

Fouling characterization and aeration performance recovery of fine-pore diffusers operated for 10 years in a full-scale wastewater treatment plant.

Fouling characterization and aeration performance recovery of fine-pore diffusers operated for 10 years in a full-scale wastewater treatment plant were investigated to elucidate fouling mechanisms and develop cleaning strategy. The performance decline of diffusers was observed with dynamic wet pressure increased by 3.2 times and standard oxygen transfer efficiency dropped to 73%, which contributed to 15.0% increase in total energy consumption. Oxygen-affinity, filamentous and extracellular polymeric substances secreting bacteria tended to accumulate on the diffuser surface. External (mainly biofilm growth), internal (organic and inorganic matters) and irrecoverable (mainly material aging) foulants accounted for 34.1%, 45.4% and 20.1% of total fouling, respectively. HCl cleaning failed to restore aeration efficiency because it eliminated structural support formed by inorganics, leaving organic foulants broken into smaller fragments and distributed more dispersed. NaClO showed better cleaning efficiency by effectively removing organic foulants. Sequential cleaning by NaClO and HCl, which achieved the best recovery, was recommended.

A two-stage desalination process for zero liquid discharge of flue gas desulfurization wastewater by chloride precipitation.

A two-stage desalination process was developed to achieve zero liquid discharge (ZLD) of flue gas desulfurization (FGD) wastewater by precipitating chloride as Friedel's salt. Influential factors for Friedel's salt precipitation, including dosage, reaction time, concentration of sulfate, were investigate by batch tests. Batch results showed that at calcium to aluminum molar ratio of 3.0, the optimal chloride removal and the highest crystallinity of Friedel's salt were obtained. Sulfate impeded Friedel's salt precipitation by competitive inhibition mechanism, and thus calcium sulfate removal was designed in advance of chloride removal. Batch results and long-term results of bench-scale experiments showed that magnesium and part of sulfate were effectively removed by lime addition in Stage I of the proposed process, and then the remaining sulfate and 48.1 % of chloride were precipitated as ettringite and Friedel's salt in Stage II. The effluent of the two-stage process was alkaline with low turbidity, and had considerable desulfurization capacity. Techno-economic evaluation showed that the two-stage process is technically feasible, economically viable and environmentally friendly technology for ZLD of FGD wastewater.

Understanding mechanisms of sludge in situ reduction in anaerobic side-stream reactor coupled membrane bioreactors packed with carriers at different filling fractions.

An anoxic/oxic membrane bioreactor (AO) and three pilot-scale anaerobic side stream reactors (ASSR) coupled MBRs (ASSR-MBRs), packed with 0%, 25% and 50% carriers in ASSRs, were continuously operated to study the mechanisms for sludge reduction. Four systems showed efficient COD and NH4+-N removal, while packing carriers significantly enhanced nitrogen removal. 25% filling fraction (AP25) achieved the highest sludge reduction efficiency of 50.5% compared to 0% (21.7%) and 50% (39.7%). Compared to ASSR-MBR, carriers enhanced the release of dissolved organic matters, and accelerated the secretion of enzyme for cell lysis and hydrolysis. In AP25, the presence of carriers prompted the formation of environment propitious to sludge reduction in bulk sludge. AP25 tended to enrich hydrolytic, fermentative and denitrifying bacteria to accelerate hydrolysis process, while excessive carriers had negative effect on biomass stability and movement of carriers.

A novel sulfate removal process by ettringite precipitation with aluminum recovery: Kinetics and a pilot-scale study.

A novel sulfate removal process via ettringite precipitation was developed by dissolving ettringite and recycling Al3+ under low pH condition. Effects of solid to liquid ratios, pH and temperature on ettringite dissolution, Al recovery and transformation of precipitates were investigated by batch experiments. The optimum condition for Al recovery is pH =3.0, suspended solid of 9.8 g/L and temperature below 303 K. Ettringite dissolution consists of two stages, (i) rapid but inconsistent dissolution with the fastest release of sulfate, followed by calcium, and then Al(OH)63-; (ii) slow dissolution of Al(OH)63- core and gypsum precipitation. Dissolution of Al(OH)63- core follows the first-order kinetics with activation energy of 41.18 kJ/mol, while gypsum re-precipitation follows the second-order kinetics with activation energy of 26.36 kJ/mol. Long-term results of pilot-scale systems for treatment of real flue gas desulfurization wastewater showed that the process achieved sulfate removal of 98.3%-99.5% and Al recovery above 98.4%, and converted 98.8% sulfate in ettringite to CaSO4, which resulted in 66.0% of sludge reduction and improved sludge dewaterability. Economic evaluation shows that the process with Al recovery reduces cost of ettringite precipitation by 35.1%, and is highly feasible and cost-effective for industrial application of high-sulfate content wastewater treatment.

Insight into the roles of packing carriers and ultrasonication in anaerobic side-stream reactor coupled membrane bioreactors: Sludge reduction performance and mechanism.

The sludge in situ reduction process by inserting an anaerobic side-stream reactor (ASSR) in a sludge return line provides a cost-effective approach to reduce sludge production in activated sludge systems. In this study, four pilot-scale membrane bioreactors (MBRs), including an AO-MBR for control, ASSR coupled MBR (ASSR-MBR), a MBR with ASSR packed with carriers (AP-MBR) and an AP-MBR with part of sludge ultrasonicated before fed into ASSR (AUP-MBR) were operated in parallel to investigate enhancing effects of ultrasonication and packing carriers on sludge reduction and pollutants removal performance under both normal and low temperature. Low temperature showed negligible impact on COD removal, deteriorated NH4+N and TN removal from 98.3% to 69.7% at 21.6 °C to 92.5% and 48.8% at 2.6 °C, and decreased sludge reduction efficiency (SRE) in ASSR-MBR. Packing carriers and ultrasonication both enhanced sludge reduction, especially under low temperature with SRE values increased from 8.2% of ASSR-MBR to 17.1% of AP-MBR and 32.6% of AUP-MBR at 4.5 ±â€¯2.5 °C. Packing carriers and ultrasonication increased cell rupture by 11.1% and 14.5% in aerobic MBR, enhanced protease activity in ASSR by 60.0% and 116.3%, and reduced ATP content for heterotrophic metabolism by 31.4% and 7.3%, respectively. MiSeq sequencing results showed that packing carriers enriched hydrolytic bacteria (Terrimonas, Dechloromonas and Woodsholea), slow growers (Sulfuritalea, Thauera and Azospira) and predatory bacteria (Bdellovibrio and norank_Saprospiraceae), while ultrasonication further enriched hydrolytic bacteria (norank_Saccharibacteria and Ferruginibacter). Packing carriers is more cost-effective than ultrasonication to enhance sludge reduction by partial damage to bacterial cells and promoting better interaction between bacteria, enzymes and substrates to favor particles hydrolysis.

Sludge reduction by a micro-aerobic hydrolysis process: A full-scale application and sludge reduction mechanisms.

The process performance of a full-scale sludge process reduction activated sludge (SPRAS) system in long-term operation were investigated by inserting a micro-aerobic tank and a clarifier before conventional activated sludge process. The full-scale SPRAS for industrial park wastewater treatment achieved efficient pollutants removal and a low observed sludge yield of 0.074 g SS/g COD. Batch tests showed that influent feeding into the micro-aerobic tank favored sludge reduction, and obtained a sludge decay constant of 0.168 d-1. The SPRAS enriched slow growers and hydrolytic bacteria for sludge reduction, showed high simultaneous nitrification and denitrification efficiency in the micro-aerobic tank with abundant denitrifying bacteria, and improved sludge settleability by enriching floc-forming bacteria. Process configuration of the SPRAS was beneficial to enhance maintenance metabolism, cyclic micro-aerobic and anaerobic uncoupling, and lysis-cryptic growth for sludge reduction. Techno-economic analysis showed that the SPRAS greatly reduced sludge production with small footprint and low cost.

Correlation of microbial community structure with pollutants removal, sludge reduction and sludge characteristics in micro-aerobic side-stream reactor coupled membrane bioreactors under different hydraulic retention times.

A micro-aerobic side-stream reactor (MSSR) was inserted in returned sludge line of a membrane bioreactor (MBR) for sludge reduction. Three MSSR coupled MBRs (M-MBRs) and an anoxic/oxic MBR (AO-MBR) were operated in parallel to investigate effects of micro-aerobic condition and hydraulic retention time (HRT) of MSSR (HRTM) on process performance. M-MBRs and AO-MBR were equally effective in COD and NH4-N removal. Rising HRTM from 3.3 to 6.7 h increased sludge reduction from 10.8% to 47.5%, and promoted dissolved organic matters release and utilization by enriching hydrolytic, fermentative and predatory bacteria. M-MBR at HRTM of 3.3 h obtained the highest nitrogen removal, and enriched denitrifiers (Thauera and Hyphomicrobium) in the MSSR. The highest relative abundance of polyphosphate-accumulating organisms was observed in M-MBR at HRTM of 5.0 h, which achieved the highest phosphorus removal. Prolonging HRTM to 6.7 h improved sludge dewaterability by enriching floc-formation bacteria (Nannocystis) and inhibiting filamentous bacteria (Candidatus_Alysiosphaera).