Recent advances in membrane bio-technologies for sludge reduction and treatment

Significant in situ sludge yield reduction in an acidic activated sludge system

Minimizing sludge generation in activated sludge systems is critical to reducing the operational cost of wastewater treatment plants (WWTPs), particularly for small plants where bioenergy is not recovered. This study introduces a novel acidic activated sludge technology for in situ sludge yield reduction, leveraging acid-tolerant ammonia-oxidizing bacteria (Candidatus Nitrosoglobus). The observed sludge yield (Yobs) was calculated based on the cumulative sludge generation and COD removal during 400 d long-term operation. The acidic process achieved a low Yobs of 0.106 ± 0.004 gMLSS/gCOD at pH 4.6 to 4.8 and in situ free nitrous acid (FNA) of 1 to 3 mg/L, reducing sludge production by 58 % compared to the conventional neutral-pH system (Yobs of 0.250 ± 0.003 gMLSS/gCOD). The acidic system also maintained effective sludge settling and organic matter removal over long-term operation. Mechanism studies revealed that the acidic sludge displayed higher endogenous respiration, sludge hydrolysis rates, and higher soluble microbial products and loosely-bounded extracellular polymer substances, compared to the neutral sludge. It also selectively enriched several hydrolytic genera (e.g., Chryseobacterium, Acidovorax, and Ottowia). Those results indicate that the acidic pH and in situ FNA enhanced sludge disintegration, hydrolysis, and cryptic growth. Besides, a lower intracellular ATP content was observed for acidic sludge than neutral sludge, suggesting potential decoupling of catabolism and anabolism in the acidic sludge. These findings collectively demonstrate that the acidic activated sludge technology could significantly reduce sludge yield, contributing to more cost- and space-effective wastewater management.

Application of a water-energy-carbon coupling index to evaluate the long-term operational stability of the anaerobic-anoxic-oxic-membrane bioreactor (A2/O-MBR) process under the influence of rainstorms

In the context of global climate change, sudden rainstorms and typhoons induce fluctuations in hydraulic shocks to wastewater treatment plants (WWTPs) in coastal areas, causing two challenges of stable effluent quality and low-carbon operation. We established a quantitative evaluation method for resistance of wastewater treatment processes to hydraulic shocks based on the water-energy-carbon nexus using operational data from a WWTP in southeast coastal China from July 2018 to December 2022. The effects of hydraulic shocks on the operational stability of the anaerobic-anoxic-oxic-membrane bioreactor (A2/O-MBR) process were analyzed following five steps. The results showed that the gray water footprint (GWF) of the process was 9.3% lower than that of the A2/O process. The energy footprint (ENF) and carbon footprint (CF) were approximately 2.1 times and 1.7 times higher than those of the A2/O process, respectively. The resistance to hydraulic shocks of the A2/O-MBR process is approximately 5.5 times higher than that of the A2/O process. In conclusion, the A2/O-MBR process exhibits higher process operational stability when subjected to hydraulic shocks, which is more conducive to the efficient and stable operation of WWTPs in rainstorm and typhoon-prone areas. The evaluation methodology provides qualitative technical support for selecting upgrading processes for WWTPs in different regions.

New insights on destruction mechanisms of waste activated sludge during simultaneous thickening and digestion process via forward osmosis membrane

This study delved into the efficacy of sludge digestion and the mechanisms involved in sludge destruction during the implementation of forward osmosis process for sludge thickening and digestion (FO-MSTD). Utilizing a lab-scale FO membrane reactor for the thickening and digestion of waste activated sludge (WAS), the investigation explored the effects of sludge thickening and digestion in FO-MSTD processes using draw solutions of varying concentrations. The findings underscored the significance of hydraulic retention time (HRT) as a pivotal parameter influencing the swift thickening or profound digestion of sludge. Consequently, tailoring the HRT to specific processing objectives emerged as a key strategy for achieving desired treatment outcomes. In the investigation, the use of a 1 M NaCl draw solution in the FO-MSTD process showcased enhanced thickening and digestion capabilities. This specific setup raised the concentration of mixed liquor suspended solids (MLSS) to over 30 g/L and achieved a 42.7% digestion efficiency of mixed liquor volatile suspended solids (MLVSS) within an operational timeframe of 18 days. Furthermore, the research unveiled distinct stages in the sludge digestion process of the FO-MSTD system, characterized by fully aerobic digestion and aerobic-local anaerobic co-existing digestion. In the fully aerobic digestion stage, the sludge digestion rate exhibited a steady increase, leading to the breakdown of sludge floc structures and the release of a substantial amount of nutrients into the sludge supernatant. The predominant microorganisms during this stage were typical functional microorganisms found in wastewater treatment systems. Transitioning into the aerobic-local anaerobic co-existing digestion stage, both MLSS concentration and MLVSS digestion efficiency continued to rise, accompanied by a decreasing dissolved oxygen (DO) concentration. More organic matter was released into the supernatant, and sludge microbial flocs tended to reaggregate. The localized anaerobic environment within the FO-MSTD reactor fostered an increase in the relative abundance of bacteria with nitrogen and phosphorus removal functions, thereby positively impacting the mitigation of total nitrogen (TN) and total phosphorus (TP) concentrations in the sludge supernatant. The results of this research enhance comprehension of the advanced FO-MSTD technology in the treatment of WAS.

Sewage sludge minimisation by OSA-MBR: A pilot plant experiment

This study presents the excess sludge minimisation in a Membrane Bioreactor (MBR) system by an Oxic Settling Anaerobic (OSA) process. The pilot plant was fed with real wastewater and OSA was operated with two different hydraulic retention times (HRT), respectively 4 (Period II) and 6 h (Period III) and compared to an MBR (Period I). Multiple parameters/variables were monitored: sludge minimisation, nitrogen and carbon removal, membrane fouling, and biokinetic behaviour through respirometry. With respect to the current literature, greenhouse gas emissions were also here monitored, often neglected. Results demonstrated that combining MBR and OSA systems can significantly reduce excess sludge production (89.7%, in Period III and 59.7% in Period II, compared to Period I). However, Period III presented better PO4-P removal efficiencies but worse performances in the other parameters (COD, NH4 and Total Nitrogen). No substantial variation in membrane fouling was obtained over the experimental periods. Finally, the HRT increase in the anaerobic reactor promoted a N2O-N increase inside the unaerated reactors, highlighting the need for a trade-off between sludge minimisation and GHG emission.

Interaction between Cr(VI) and Tubificidae in sludge reduction system: effect, reduction, and redistribution of Cr(VI)

The treatment of heavy metals in sewage treatment systems has gained more attention with the increase in heavy metal hazards. Tubificidae in sludge reduction have been widely studied; however, little is known about the effect of Tubificidae in the treatment of Cr-containing wastewater. In this study, the mechanism of Tubificidae in the sludge reduction system with Cr stress was studied. Predation experiments by Tubificidae in a Cr-containing sludge reduction system were conducted to investigate the changes in enzyme activities in the Tubificidae under different concentrations of Cr, and the distribution of Cr in the sludge reduction reactor was analyzed. The kinetic model of uptake and elimination of Cr in Tubificidae was established. The results showed that the maximum activation multiplier factor of superoxide dismutase (SOD) activity was 1.95 under the low concentration of Cr(VI), which indicated that Tubificidae had a certain detoxification. After the effect of Tubificidae on Cr(VI) experiments, the Cr concentrations in Tubificidae, sludge, and feces increased first and then decreased with exposure time, and the proportion of total Cr and Cr(VI) in the sludge decreased from 71.98% and 42.7% to 29.18% and 6.82%, respectively. The detoxification mechanism of the Tubificidae could be activated with Cr stress, and 63.22% of the Cr(VI) was converted to Cr(III). The bioconcentration factor (BCF) for theoretical equilibrium was 446, the maximum bioaccumulation factor (BAF) reached 0.97 on the 15th day. It can be seen that Tubificidae could be considered a good scavenger of environmental Cr(VI). The hyperbolic model fits the process of Cr uptake and elimination well and can be used as a predictive tool for Tubificidae accumulation.

Unravelling roles of the intermediate settler in a microaerobic hydrolysis sludge in situ reduction process

The roles of the intermediate settler in the sludge process reduction activated sludge process (SPRAS), and the influences of its hydraulic retention time (HRT_ST) on pollutant removal and sludge reduction were investigated. Prolonging HRT_ST from 3.0 to 4.5 and 6.0 h resulted in sludge reduction efficiencies increased from 46.8% to 61.5% and 62.7%. The sludge accumulation in the intermediate settler formed an anaerobic zone but inhibited methane production, and the alternating microaerobic and anaerobic environment in the sludge process reduction (SPR) module increased the microbial diversity and enriched the hydrolytic and fermentative bacteria. Prolonging HRT_ST accelerated dissolved organic matter release and elevated the degradation of refractory fraction, and improved the sludge properties of the SPRAS. Metagenomic analysis showed that the SPR module enhanced the glycolysis pathway and decoupling metabolism for sludge reduction. The results revealed that the intermediate settler plays dual roles in solid-liquid separation and sludge reduction metabolism.

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 R_SPR 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.

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.

Microalgae production in human urine: Fundamentals, opportunities, and perspectives

The biological treatment of source-separated human urine to produce biofuel, nutraceutical, and high-value chemicals is getting increasing attention. Especially, photoautotrophic microalgae can use human urine as media to achieve environmentally and economically viable large-scale cultivation. This review presents a comprehensive overview of the up-to-date advancements in microalgae cultivation employing urine in photobioreactors (PBRs). The standard matrices describing algal growth and nutrient removal/recovery have been summarized to provide a platform for fair comparison among different studies. Specific consideration has been given to the critical operating factors to understand how the PBRs should be maintained to achieve high efficiencies. Finally, we discuss the perspectives that emphasize the impacts of co-existing bacteria, contamination by human metabolites, and genetic engineering on the practical microalgal biomass production in urine.

A review on anaerobic membrane bioreactors for enhanced valorization of urban organic wastes: Achievements, limitations, energy balance and future perspectives

Sustainable urban development is threatened by an impending energy crisis and large amounts of organic wastes generated from the municipal sector among others. Conventional waste management methods involve greenhouse gas (GHG) emission and limited resource recovery, thus necessitating advanced techniques to convert such wastes into bioenergy, bio-fertilizers and valuable-added products. Research and application experiences from different scale applications indicate that the anaerobic membrane bioreactor (AnMBR) process is a kind of high-rate anaerobic digester for urban organic wastes valorization including food waste and waste sludge, while the research status is still insufficiently summarized. Through compiling recent achievements and literature, this review will focus on the following aspects, including AnMBR treatment performance and membrane fouling, technical limitations, energy balance and techno-economic assessment as well as future perspectives. AnMBR can enhance organic wastes treatment via complete retention of functional microbes and suspended solids, and timely separation of products and potential inhibitory substances, thus improving digestion efficiency in terms of increased organics degradation rates, biogas production and process robustness at a low footprint. When handling high-solid organic wastes, membrane fouling and mass transfer issues can be the challenges limiting AnMBR applications to a wet-type digestion, thus countermeasures are required to pursue extended implementations. A conceptual framework is proposed by taking various organic wastes disposal and final productions (permeate, biogas and biosolids) utilization into consideration, which will contribute to the development of AnMBR-based waste-to-resource facilities towards sustainable waste management and more economic-environmental benefits output.

A review of sludge production in South Africa municipal wastewater treatment plants, analysis of handling cost and potential minimization methods

The government of South Africa through the department of water and sanitation has installed numerous activated sludge systems in most of the municipal wastewater treatment plants (MWWTPs) to ensure adequate sanitation. However, secondary sludge generation and handling is a major challenge of the AS process. This work reviews the sludge production potential in selected regions in South Africa including Midvaal, Emfuleni and Lesedi municipalities. Further, the sludge handling cost and potential methods of sludge minimization are discussed. This study found that the selected MWWTPs discharge effluent volume of between 3 and 65 ML/day with average COD of about 350 mg/L leading to sludge production of between 5 and 23 tons/day with an estimated handling cost of €57,000 to €320,000 per year. Some of the technologies reviewed for sludge minimization to cut down plant operation cost include chemical oxidation using ozone and potassium ferrate (K2FeO4), application of oxic-settling-anaerobic (OSA) process, anaerobic/anoxic/oxic (AAO) combined with K2FeO4 oxidation side stream reactor (SSR), SANI® technology and use of anaerobic side stream reactor (ASSR) in the conventional activated sludge (AS) line.

Insights into the minimization of excess sludge production in micro-aerobic reactors coupled with a membrane bioreactor: Characteristics of extracellular polymeric substances

The production of excess sludge by the activated sludge system of wastewater treatment plants is a problem. In this study, the EPS characteristics on production and degradation were investigated in the real-scale food processing wastewater treatment system (i.e., a micro-aerobic reactor coupled with a membrane bioreactor (MAR-MBR)) with a treatment capacity of 150 t d^-1, which could cater for the low production of excess sludge (i.e., 9 t·a^-1; 76% moisture content). The total organic carbon concentrations in the different EPS fractions were in the following order: soluble EPS (S-EPS) < loosely bound EPS (LB-EPS) < tightly bound EPS (TB-EPS). Although the components (e.g., protein and humic acid-like substances) of each EPS fraction changed significantly throughout the MAR-MBR process owing to the low production of excess sludge, the degrees of change in S-EPS, LB-EPS, and TB-EPS were significantly different from the corresponding change in their relative molecular weights. Furthermore, the microbial community composition was beneficial for the release and degradation of EPS, and the regulation of gene functions via the MAR-MBR enhanced this process.

A novel pilot-scale IFAS-MBR system with low aeration for municipal wastewater treatment: Linkages between nutrient removal and core functional microbiota

In this study, we proposed a novel IFAS-MBR with low aeration for the treatment of real municipal wastewater. With biocarriers packed in the anoxic tank, the pilot-scale IFAS-MBR operated with average dissolved oxygen concentrations of 0.56 mg/L in the oxic tank. Over 110 days of operation, highly efficient nutrient removal was achieved with the total nitrogen (TN) and phosphorus (TP) removal efficiencies of 78.1 ± 7.2% and 93.7 ± 5.8%, respectively. The average effluent concentrations of TN and TP reached 5.4 and 0.26 mg/L, respectively. Meanwhile, the removal efficiency of COD reached 95.3 ± 1.3% in the system, and the concentrations of COD decreased from 31.9 ± 3.7 (sludge supernatant) to 12.7 ± 1.6 mg/L (permeate) after membrane filtration. Microbial community analysis showed that Nitrosomonas (0.32%) and Nitrospira (1.85%) in activated sludge were the main drivers of the nitrification process, while various denitrifying bacteria in activated sludge and biofilms were responsible for nitrate reduction in the anoxic tank. Candidatus Accumulibacter (0.34%) and Dechloromonas (1.31%) primarily contributed to denitrifying phosphorus uptake in the anoxic tank. Furthermore, these organisms (i.e., core functional microbiota) exhibited stable levels over the entire operation. The highly enriched hydrolytic fermentation bacteria drove community succession, and the remarkable functional robustness of microbial communities in activated sludge and biofilms favored nutrient removal. Overall, the novel IFAS-MBR system provides an energy-efficient MBR alternative owing to its highly efficient performance and low operating costs enabled by low aeration rates and the absence of an external carbon source.

Multilayer calcium alginate beads containing Diatom Biosilica and Bacillus subtilis as microecologics for sewage treatment

Organic matter pollution and heavy metal pollution have become one of the main problems in water recycling, and the strategy to simultaneously remove soluble organic matter and metal ions is crucial for sewage treatment. In this study, multilayer calcium Alginate beads (n-Alg-DBs-Bas) containing Diatom Biosilica (DBs) and Bacillus subtilis (Bas) were designed as microecologics for sewage treatment. The introduction of DBs in beads and the multilayer structure could promote Bas growth, prolong the stability of the beads, and enhance the adsorption of beads, further improve the sewage treatment efficiency. The organic matter degradation of 3 layered Alg-DBs-Bas reached to 68.23 ± 0.95 % of COD and 58.88 ± 0.84 % of NH₄⁺-N, and the metal ion adsorption was up to 119.31 mg/g for Fe³⁺, 110.81 mg/g for Zn²⁺ and 141.34 mg/g for Cu²⁺. The prepared multilayer calcium alginate beads combined organic matter degradation and metal ions absorption, which is significant for environmental applications.

Self-generated electric field to suppress sludge production and fouling development in a membrane bioreactor for wastewater treatment

Compared with conventional sludge reduction techniques, electric field assisted membrane bioreactor (MBR) is a cost-effective technology with low power consumption. In this study, spontaneous electric field without extra power supply was introduced into the MBR for wastewater treatment to complete the in situ sludge reduction and membrane fouling mitigation. A novel spontaneous electric field membrane bioreactor (SEF-MBR) equipped with Cu-nanowires (Cu-NWs) conductive microfiltration membrane as cathode was established by using baffles to form anaerobic and aerobic tanks. SEF-MBR 1 with external resistance of 500 Ω maintained a highest electric field intensity of 1.25 mV/cm. Compared with Control-MBR, the reduction of mixed liquor suspended solids (MLSS) growth rate, extracellular polymeric substances (EPS) growth rate, total cell number and water content of SEF-MBR 1 reached 50.0%, 43.0%, 37.1% and 6.4%, respectively. After 43 days operation, SEF-MBR 1 obtained the minimum MLSS concentration and sludge volume, which were 29.9% and 83.8% lower than that of it in Control-MBR. The total biovolume of the contaminants (i.e., EPS and cells) on the membrane surface of SEF-MBR 1 was 68.8% lower than that of Control-MBR. SEF-MBR 1 exhibited a better performance with a lower membrane fouling rate (0.58 kPa/d) than Control-MBR (1.09 kPa/d). Economic analysis showed that a total of 148.1 kWh/m³ of electric energy was saved in the SEF-MBR 1. This technology reduced the sludge production in the sewage biological treatment process, which realized the sludge reduction had a positive impact on the membrane fouling mitigation.

Dewatering efficiency of sewage sludge during Fe2+-activated persulfate oxidation: Effect of hydrophobic/hydrophilic properties of sludge EPS

To clarify the effect of the hydrophobic/hydrophilic polarity of extracellular polymeric substances (EPS) on sludge filterability improvement during S₂O₈^2-/Fe²⁺ oxidation, waste activated sludge (WAS), glucose-fed hydrophilic sludge (HPI-WAS), and sodium acetate-fed hydrophobic sludge (HPO-WAS) samples were cultivated, and their dewatering behaviors were individually explored. Experimental results showed that S₂O₈^2- oxidation effectively disintegrated the polymeric EPS and led to a more significant reduction in the water content for HPO-WAS than for HPI-WAS (12.87-15.23% vs 9.31-12.12%), especially regarding the bound water (W_b) content. After oxidation, as high as 38.88-42.61% of the W_b within HPO-WAS samples were declined, much higher than the HPI-WAS samples (19.27-29.20%). Specifically, carbohydrates within sludge EPS negatively influenced the dewatering process of S₂O₈^2-/Fe²⁺ oxidation. By contrast, abundant existence of humic acids and polymeric proteinaceous components (especially those hydrophilic proteins and transitional humic acids) within the sludge EPS exhibited a converse trend. FT-IR and EEM spectral, as well as particle sizes variation for the sludge samples before and after S₂O₈^2-/Fe²⁺ oxidation was also evaluated. This study provides new insight into the enhancement of S₂O₈^2-/Fe²⁺ oxidation for sludge dewatering based on polarity analysis of EPS.

Sequential 'acid leaching-anion exchange-aerobic composting' process for recycle of municipal sludge

Municipal sludge disposal and recycle has become a prominent research theme. In this study, a sequential process for integral treatment of municipal sludge was primarily presented, combining acid leaching, anion exchange and aerobic composting. The aim of the process was to remove chromium (Cr) from the sludge and reuse the sludge as manure. Firstly, Cr was removed from municipal sludge via the acid leaching process; the removal rate was up to 57.43%. Then, ion exchange resin was used to remove Cr from leachate; the removal rate reached 95%. Aluminum sheet was used to replace the Cr from eluent; the replacement rate was 63.3%. The aerobic composting process could be successfully warmed up to above 55 °C and lasted for 4 days; the seed germination index reached 68.3%. After the composting process, the residual Cr in sludge mainly existed at a more stable residual state and organic binding state. Overall, this novel sequential process serves as a potential high-efficiency, green, low-energy way for municipal sludge recycle.

Evaluating influence of filling fraction of carriers packed in anaerobic side-stream reactors on membrane fouling and microbial community of the coupled membrane bioreactors

An anoxic/oxic membrane bioreactors (AO-MBR) and three identical anaerobic side-stream reactor coupled with anoxic/oxic membrane bioreactors (ASSR-MBR) were constructed and operated in parallel to investigate the appropriate filling fraction of carriers packed in ASSR, influences on pollutants removal, sludge reduction, membrane fouling and microbial community of ASSR-MBR. Inserting ASSR achieved efficient COD removal and nitrification, and packing carriers in ASSR obtained the highest sludge reduction efficiency of 50.5 % at filling fraction of 25 %. Compared to AO-MBR, inserting ASSR without carriers induced the release of viscous components in extracellular polymeric substances (EPS) and the formation of calcium carbonate-related bacteria on membrane surface, and thus deteriorated membrane fouling. Packing carriers with 25 % filling fraction promoted the hydrolysis of soluble microbial products and EPS, whilst reduced the viscoelasticity of sludge flocs. Higher filling fraction of 50 % increased the shear forces to the biofilm and biomarkers related to membrane fouling, and thus showed little improvement to alleviate membrane fouling. MiSeq sequencing revealed that although it enriched in the bulk sludge of conventional ASSR-MBR and the coupled reactor with filling fraction of 50 %, the floc-forming, hydrolytic and fermentative bacteria were more inclined to attach on the membrane surface and alleviate fouling process.

Simultaneous sludge minimization, biological phosphorous removal and membrane fouling mitigation in a novel plant layout for MBR

The integration of one anaerobic reactor in the mainstream (AMSR) of a pre-denitritication-MBR was evaluated with the aim to achieve simultaneous sludge minimization and phosphorous removal. The excess sludge production was reduced by 64% when the AMSR was operated under 8 h of hydraulic retention time (HRT). The highest nutrients removal performances referred to organic carbon (98%), nitrogen (90%) and phosphorous (97%) were obtained under 8 h of HRT. In contrast, prolonged anaerobic-endogenous conditions were found to be detrimental for all nutrients removal performances. Similarly, the lowest membrane fouling tendency (FR = 0.65∙10¹¹ m^-1 d^-1) was achieved under 8 h of HRT, whereas it significantly increased under higher HRT. The highest polyphosphate accumulating organisms kinetics were achieved under HRT of 8 h, showing very high exogenous P-release (46.67 mgPO₄-P gVSS^-1 h^-1) and P-uptake rates (48.6 mgPO₄-P gVSS^-1 h^-1), as well as a not negligible P-release rate under endogenous conditions at low COD/P ratio (≈1).

Effects of bioporous carriers on the performance and microbial community structure in side-stream anaerobic membrane bioreactors

The aim of this study was to investigate the effects of a volcanic rock porous carrier (VRPC) on sludge reduction, pollutant removal, and microbial community structure in an anaerobic side-stream reactor (ASSR). Three lab-scale membrane bioreactors (MBRs), including an anoxic-oxic MBR, which served as the control (C-MBR), an ASSR-coupled MBR (A-MBR), and an A-MBR filled with VRPC (FA-MBR) were stably and simultaneously operated for 120 days. The effect of the three reactors on the removal of chemical oxygen demand (COD) was almost negligible (all greater than 95%), but the average removal efficiency of ammonium nitrogen, total nitrogen, and total phosphorus was significantly improved by the insertion of an ASSR, especially when the ASSR was filled with VRPC. Finally, A-MBR and FA-MBR achieved 16.2% and 26.4% sludge reduction rates, with observed sludge yields of 0.124 and 0.109 g mixed liquid suspended solids/g COD, respectively. Illumina MiSeq sequencing revealed that microbial diversity and richness were highest in the VRPC, indicating that a large number of microorganisms formed on the carrier surface in the form of a biofilm. Abundant denitrifying bacteria (Azospira, Comamonadaceae_unclassified, and Flavobacterium) were immobilized on the carrier biofilm, which contributed to increased nitrogen removal. The addition of a VRPC to the ASSR successfully immobilized abundant hydrolytic, fermentative, and slow-growing microorganisms, which all contributed to reductions in sludge yield.

Characterization of sludge reduction and bacterial community dynamics in a pilot-scale multi-stage digester system with prolonged sludge retention time

Sludge reduction performance and bacterial community dynamics in a pilot-scale multi-stage digester system with prolonged sludge retention time were characterized. Throughout the operation period of 281 days, the total loading sludge and the total digested sludge were 4700 and 3300 kg-MLSS. After 114 days of operation, the residual MLSS (RMLSS) in the reactors for sludge treatment was maintained at 18-25 kg-RMLSS m^-3, and the sludge reduction efficiency achieved 95% under the F/M ratio (kg-loading MLSS kg-RMLSS^-1) of less than 0.018. Also, among the sludge components, both fixed suspended solids and volatile suspended solids were reduced. Based on the sludge reduction performance and the RNA-based bacterial community characteristics, the combined action of the maintenance metabolism, lysis-cryptic growth, and particulate inorganic matter is proposed as the sludge reduction mechanism in the multi-stage sludge treatment process.

A novel approach for fouling mitigation in anaerobic-anoxic-oxic membrane bioreactor (A2O-MBR) by integrating worm predation

Membrane fouling is one of the biggest challenges in the widespread application of membrane bioreactors. In this study, a combined system of anaerobic-anoxic-oxic membrane bioreactor (A²O-MBR) and worm reactor (WR) was established for fouling control. In A²O-MBR-WR, the membrane filtration cycle was prolonged by 66.7% due to the confluence of microaerobic treatment and worm predation in WR with the interaction between WR and A²O-MBR. Compared with conventional A²O-MBR, membrane rejection of soluble and colloidal foulants (SCF) in the combined system was decreased by 26.0%, which could be attributed to the higher biodegradability of SCF and the higher bacterial activity in A²O-MBR. Although floc size in A²O-MBR was reduced due to sludge disintegration and worm predation in WR, changes of floc surface properties could counteract this negative effect on fouling. Complex effects of sludge flocs on membrane fouling were further analyzed by the interaction energy between sludge flocs and the clean/fouled membrane based on extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The energy barriers indicated that flocs in A²O-MBR-WR were difficult to adhere to the membrane and were more likely to detach. Moreover, high-throughput sequencing analysis revealed that the microbial community of the cake layer in the combined system was more even and had a higher proportion of foulants degradation related bacteria, which was beneficial for fouling mitigation. The combination of A²O-MBR and WR has shown significant advantages in membrane fouling mitigation.

Identify driving forces of MBR applications in China

During the last two decades, MBR applications in China grow exponentially with the first pilot test of 10 m3/d in 1999 and the first application with capacity of 110,000 m3/d commissioned in 2009. It is critical to examine the drivers of MBR applications in China, which can provide sound scientific basis for future development of MBR applications. This study summarized the historical development of MBR applications and analyzed the driving forces by survey, literature review and interviews with MBR suppliers. The results showed that: (1) technical advantages of MBR and public policy related to water resources and environment promoted MBR beyond lab and pilot test into wide commercial applications in China; (2) petrochemical industry needs for wastewater treatment and reuse promoted medium-scale MBRs as public policy and regulation on water resources and environment tightens; (3) when the breakthrough of capacity of a single project above 10 thousand m3/d, the Green Olympic Games and Asian Games and tightening effluent regulations in environmentally sensitive areas incentivized MBR applications; and (4) the emergence of 100,000 m3/d MBR was mainly stimulated by water resources stress. Water resources stress and public policy related on resources and the environment are the primary driving forces in the last several decades. The future drivers of MBR applications in China appear to be decreasing operation cost.

Applications of membrane bioreactors for water reclamation: Micropollutant removal, mechanisms and perspectives

Membrane bioreactors (MBRs) have attracted attention in water reclamation as a result of the recent technical advances and cost reduction in membranes. However, the increasing occurrence of micropollutants in wastewaters has posed new challenges. Therefore, we reviewed the current state of research to identify the outstanding needs in this field. In general, the fate of micropollutants in MBRs relates to sorption, biodegradation and membrane separation processes. Hydrophobic, nonionized micropollutants are favorable in sorption, and the biological degradation shows higher efficiency at relatively long SRTs (30-40 days) and HRTs (20-30 h), as a result of co-metabolism, metabolism and/or ion trapping. Although the membrane rejection rates for micropollutants are generally minor, final water quality can be improved via combination with other technologies. This review highlights the challenges and perspectives that should be addressed to facilitate the extended use of MBRs for the removal of micropollutants in water reclamation.

Impact of dosing order of the coagulant and flocculant on sludge dewatering performance during the conditioning process

The combined use of coagulant and flocculant can achieve excellent dewatering performance. In this study, we investigated the impact of dosing order of the coagulant and flocculant on sludge dewatering performance. The results showed that capillary suction time (CST) values during the coagulation-flocculation process decreased 20-25% compared to those during the flocculation-coagulation process using the same doses of additives. Moisture content of the sludge during the coagulation-flocculation process was lower. The dosing order of coagulants and flocculants during the conditioning process was clearly important for sludge dewatering, and the coagulant should be dosed before the flocculant. Furthermore, a mechanism for the different dewatering performance was proposed: larger agglomerated and destabilized colloid particles formed, and more bound water was released into the sludge bulk solution during the coagulation-flocculation process, compared with the flocculation-coagulation process, which resulted in better dewatering performance, as reflected in the CST value and moisture content of the sludge cake. These results enable a better understanding of combined conditioning with coagulants and flocculants on sludge dewatering.

Effects of hydraulic retention time on process performance of anaerobic side-stream reactor coupled membrane bioreactors: Kinetic model, sludge reduction mechanism and microbial community structures

An anoxic/oxic membrane bioreactor (AO-MBR) and three anaerobic side-stream reactor (ASSR) coupled MBRs (ASSR-MBR) were operated to investigate the effects of hydraulic retention time of ASSR (HRT_A) and to elucidate sludge reduction mechanisms in ASSR-MBRs. Increasing HRT_A from 3.3 to 6.6 h improved nitrogen removal, and enhanced sludge reduction from 8.0% to 40.9% in ASSR-MBR. The sludge decay coefficient was 0.0221 d^-1 in MBRs, and 0.0231-0.0345 d^-1 in ASSRs. The measured lysis rate coefficient of heterotrophic biomass was 0.083-0.112 d^-1 in MBRs and 0.079-0.111 d^-1 in ASSRs. The hydrolysis rate coefficient of inactive particulate organic matters (POMs) in ASSRs significantly exceeded that in the MBR. At HRT_A of 6.6 h, POMs hydrolysis in ASSR (38.6%) is the dominant route of sludge reduction, and cell lysis occurred principally in aerobic tanks. Illumina-MiSeq sequencing showed ASSR-MBRs enriched hydrolytic and fermentative bacteria, and confirmed that anaerobic hydrolysis contributed most to sludge reduction.

Sludge reduction and microbial structures of aerobic, micro-aerobic and anaerobic side-stream reactor coupled membrane bioreactors

An anoxic/oxic membrane bioreactor (MBR) and three side-stream reactor (SSR) coupled membrane bioreactors were operated in parallel to investigate effects of dissolved oxygen (DO) level in SSR on sludge reduction and microbial community structure of SSR-MBRs. The four MBRs were equally efficient in COD and ammonium nitrogen removal. The anaerobic and micro-aerobic SSR favored nitrogen removal through denitrification, simultaneous nitrification and denitrification and autochthonous substrate release as carbon source. The micro-aerobic SSR achieved greatly higher sludge reduction efficiency (61.1%) than anaerobic (37.3%) and aerobic SSR (7.9%). Micro-aerobic SSR obtained the highest endogenous decay constant (0.035 d^-1) compared to anaerobic (0.023 d^-1) and aerobic SSR (0.015 d^-1). High-throughput sequencing results revealed that anaerobic SSR enriched hydrolytic and fermentative bacteria, aerobic environment favored the growth of slow-growing bacteria, and micro-aerobic SSR stimulated biological activities of both anaerobic and aerobic bacteria.

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.

Biological minimization of excess sludge in a membrane bioreactor: Effect of plant configuration on sludge production, nutrient removal efficiency and membrane fouling tendency

Excess sludge minimization was studied in a MBR with pre-denitrification scheme. Sludge minimization, nitrogen removal performance and membrane fouling tendency were investigated in two configurations, characterized by a different position of the sludge retention reactor (SRR). In particular, the SRR was placed: i) in the return activated sludge line (Anaerobic Side-Stream Reactor - ASSR configuration) and ii) in the mainstream between the anoxic and aerobic reactor (Anaerobic Main-Stream Reactor - AMSR configuration). The achieved results demonstrated that the ASSR enabled a higher excess sludge reduction (74% vs 32%), while achieving lower biological nitrogen removal (BNR) (TN = 63% vs 78%) and membrane fouling tendency (FR = 2.1 · 10¹² m^-1 d^-1vs 4.0 · 10¹¹ m^-1 d^-1) than the AMSR. It was found that metabolism uncoupling, destruction of EPS and endogenous decay simultaneously occurred in the ASSR. Conversely, selective enrichment of bacteria population with low biomass yield was found the main mechanism affecting sludge minimization in the AMSR.

Application of membrane bioreactor for sulfamethazine-contained wastewater treatment

The presence of antibiotics in wastewater has been widely confirmed. Membrane bioreactor (MBR), as an efficient wastewater treatment technology, has attracted increasing interest in its ability to remove antibiotics in recent years. However, its long-term operation stability and the underlying mechanisms for antibiotics removal are still poorly understood. In this study, a hollow fiber MBR was used to treat low concentration sulfamethazine (SMZ) contained wastewater. The long-term effects of various SMZ concentrations on nutrients removal, SMZ degradation, and sludge characteristics were investigated. During the 244 days operation, the overall SMZ removal efficiency could reach 95.4 ± 4.5% under various SMZ concentrations and hydraulic retention times. The reactor exhibited high chemical oxygen demand and NH₄⁺-N removal efficiencies, which reached 93.0% and 96.2%, respectively. A sludge concentration of 4.1 ± 0.3 g/L was maintained in the system without excess sludge discharge. The dosage of SMZ had obvious effect on sludge characteristics. The contents of extracellular polymeric substances (EPS) in MBR decreased after a long-term operation of the reactor under SMZ pressure. The low sludge concentration and the reduced EPS content were also beneficial for mitigating membrane fouling. Thus, this study provides a low-cost, efficient and simple approach to treat SMZ-contained wastewater.

Effect of metabolic uncoupler, 3,3′,4′,5-tetrachlorosalicylanilide (TCS) on Bacillus subtilis: biofilm formation, flocculability and surface characteristics

In order to understand the inhibitory mechanism of metabolic uncoupler in biofilm, this study investigated the effect of TCS on B. subtilis biofilm formation, flocculability, surface characteristics and thermodynamic properties. An optimal concentration of TCS, a metabolic uncoupler, was observed to substantially inhibit biofilm formation and the secretion of extracellular polymeric substances (EPS). The effect of TCS on the zeta potential and flocculability of bacterial suspension implied the addition of 100 μg L⁻¹ TCS increased the net negative charge of cell surface which induced the reduction of B. subtilis flocculability. Meanwhile, the effects of TCS on bacterial surfacial thermodynamic properties were analyzed by the Derjaguin–Landau–Verwey–Overbeek (DLVO) and extend DLVO (XDLVO) theories. As DLVO and XDLVO predicted, the primary energy barrier between bacterial cells incubated with 100 μg L⁻¹ TCS were increased compared to that of control, indicating that B. subtilis incubated with 100 μg L⁻¹ TCS must consume more energy to aggregate or form biofilm.

This study aimed to investigate the inhibitory mechanism of metabolic uncoupler on biofilm formation through surface characteristics and thermodynamics analysis.

Resource Recovery from Wastewater by Biological Technologies: Opportunities, Challenges, and Prospects

Limits in resource availability are driving a change in current societal production systems, changing the focus from residues treatment, such as wastewater treatment, toward resource recovery. Biotechnological processes offer an economic and versatile way to concentrate and transform resources from waste/wastewater into valuable products, which is a prerequisite for the technological development of a cradle-to-cradle bio-based economy. This review identifies emerging technologies that enable resource recovery across the wastewater treatment cycle. As such, bioenergy in the form of biohydrogen (by photo and dark fermentation processes) and biogas (during anaerobic digestion processes) have been classic targets, whereby, direct transformation of lipidic biomass into biodiesel also gained attention. This concept is similar to previous biofuel concepts, but more sustainable, as third generation biofuels and other resources can be produced from waste biomass. The production of high value biopolymers (e.g., for bioplastics manufacturing) from organic acids, hydrogen, and methane is another option for carbon recovery. The recovery of carbon and nutrients can be achieved by organic fertilizer production, or single cell protein generation (depending on the source) which may be utilized as feed, feed additives, next generation fertilizers, or even as probiotics. Additionlly, chemical oxidation-reduction and bioelectrochemical systems can recover inorganics or synthesize organic products beyond the natural microbial metabolism. Anticipating the next generation of wastewater treatment plants driven by biological recovery technologies, this review is focused on the generation and re-synthesis of energetic resources and key resources to be recycled as raw materials in a cradle-to-cradle economy concept.

Improving carbohydrate production of Chlorella sorokiniana NIES-2168 through semi-continuous process coupled with mixotrophic cultivation

Biofuels from microalgae is now a hot issue of great potential. However, achieving high starch productivity with photoautotrophic microalgae is still challenging. A feasible approach to enhance the growth and target product of microalgae is to conduct mixotrophic cultivation. The appropriate acetate addition combined with CO2 supply as dual carbon sources (i.e., mixotrophic cultivation) could enhance the cell growth of some microalgae species, but the effect of acetate-mediated mixotrophic culture mode on carbohydrate accumulation in microalgae remains unclear. Moreover, there is still lack of the information concerning how to increase the productivity of carbohydrates from microalgae under acetate-amended mixotrophic cultivation and how to optimize the engineering strategies to achieve the goal. This study was undertaken to develop an optimal acetate-contained mixotrophic cultivation system coupled with effective operation strategies to markedly improve the carbohydrate productivity of Chlorella sorokiniana NIES-2168. The optimal carbohydrate productivity of 695 mg/L/d was obtained, which is the highest value ever reported. The monosaccharide in the accumulated carbohydrates is mainly glucose (i.e., 85-90%), which is very suitable for bio-alcohols fermentation. Hence, by applying the optimal process developed in this study, C. sorokiniana NIES-2168 has a high potential to serve as a feedstock for subsequent biofuels conversion.

Effect of sludge characteristics on membrane fouling during start-up of a tertiary submerged membrane bioreactor

In membrane bioreactors applied to wastewater treatment, fouling is typically a complex function of sludge characteristics. A pilot-scale tertiary submerged membrane bioreactor (tMBR) was continuously operated for over 200 days to assess the effect of biomass physiological state and environmental stress on process performance. Sludge characteristics were evaluated in terms of suspended solid concentration (MLSS and MLVSS), apparent viscosity, bioflocculation state, filterability, bioactivity, biopolymeric clusters (BPCs) and soluble microbial products. During the initial period of the tMBR start-up, when MLSS was below 3000 mg/L, the biomass was found to be very sensitive to environmental stress by sudden oxygen increase or organic shock loading, resulting in temporary biomass deflocculation and BPC release, and consequently, severe induced membrane fouling. However, at higher MLSS values, low stable biomass growth (0.04 ± 0.002 kg MLVSS/kg COD) was measured, regardless of organic overloading shocks or feeding failures. This period was also characterised by low bioactivity, BPC content and membrane fouling. Statistical analysis showed that BPCs have an important role when compared with other sludge properties as indicators of its fouling potential.

Effects of dissolved oxygen on performance and microbial community structure in a micro-aerobic hydrolysis sludge in situ reduction process

A sludge process reduction activated sludge (SPRAS), with a sludge process reduction module composed of a micro-aerobic tank and a settler positioned before conventional activated sludge process, showed good performance of pollutant removal and sludge reduction. Two SPRAS systems were operated to investigate effects of micro-aeration on sludge reduction performance and microbial community structure. When dissolved oxygen (DO) concentration in the micro-aerobic tank decreased from 2.5 (SPH) to 0.5 (SPL) mg/L, the sludge reduction efficiency increased from 42.9% to 68.3%. Compared to SPH, activated sludge in SPL showed higher contents of extracellular polymeric substances and dissolved organic matter. Destabilization of floc structure in the settler, and cell lysis in the sludge process reduction module were two major reasons for sludge reduction. Illumina-MiSeq sequencing showed that microbial diversity decreased under high DO concentration. Proteobacteria, Bacteroidetes and Chloroflexi were the most abundant phyla in the SPRAS. Specific comparisons down to the class and genus level showed that fermentative, predatory and slow-growing bacteria in SPL community were more abundant than in SPH. The results revealed that micro-aeration in the SPRAS improved hydrolysis efficiency and enriched fermentative and predatory bacteria responsible for sludge reduction.

Enhanced waste activated sludge digestion using a submerged anaerobic dynamic membrane bioreactor: performance, sludge characteristics and microbial community

Anaerobic digestion (AD) plays an important role in waste activated sludge (WAS) treatment; however, conventional AD (CAD) process needs substantial improvements, especially for the treatment of WAS with low solids content and poor anaerobic biodegradability. Herein, we propose a submerged anaerobic dynamic membrane bioreactor (AnDMBR) for simultaneous WAS thickening and digestion without any pretreatment. During the long-term operation, the AnDMBR exhibited an enhanced sludge reduction and improved methane production over CAD process. Moreover, the biogas generated in the AnDMBR contained higher methane content than CAD process. Stable carbon isotopic signatures elucidated the occurrence of combined methanogenic pathways in the AnDMBR process, in which hydrogenotrophic methanogenic pathway made a larger contribution to the total methane production. It was also found that organic matter degradation was enhanced in the AnDMBR, thus providing more favorable substrates for microorganisms. Pyrosequencing revealed that Proteobacteria and Bacteroidetes were abundant in bacterial communities and Methanosarcina and Methanosaeta in archaeal communities, which played an important role in the AnDMBR system. This study shed light on the enhanced digestion of WAS using AnDMBR technology.

Biomass based activated carbon obtained from sludge and sugarcane bagasse for removing lead ion from wastewater

Sewage sludge and bagasse were used as raw materials to produce cheap and efficient adsorbent with great adsorption capacity of Pb(2+). By pyrolysis at 800 °C for 0.5 h, the largest surface area (806.57 m(2)/g) of the adsorbent was obtained, enriched with organic functional groups. The optimal conditions for production of the adsorbent and adsorption of Pb(2+) were investigated. The results of adsorb-ability fitted the Langmuir isotherm and pseudo-second-order model well. The highest Pb(2+) (at pH = 4.0) adsorption capacity was achieved by treating with 60% (v/v) HNO3. This is a promising approach for metal removal from wastewater, as well as recycling sewage sludge and bagasse to ease their disposal pressure.

Long-term investigation of a novel electrochemical membrane bioreactor for low-strength municipal wastewater treatment

A novel and cost-effective electrochemical membrane bioreactor (EMBR) was proposed for low-strength wastewater treatment. In this system, influent organic matter could act as electron donor in the anode chamber and then denitrification was driven on the cathodes. During 270 days of operation, a positive and time-lag correlation was found between temperature and exoelectrogenesis process. At lower temperatures (<10∼15 °C), power production in the EMBR was negligible and therefore the integration did not improve the reactor performance. With the rebound of water temperatures over 15∼20 °C, efficient redox reactions were achieved in the EMBR, which subsequently resulted in sludge reduction (27.3% lower than the control MBR) and membrane fouling alleviation. Due to the capture of electrons that were then used at the cathode to drive the denitrification process, the total nitrogen removal efficiency of the EMBR averaged 78.2% at high temperatures, despite the decrease of organic loading rate of the feed for heterotrophic denitrification. Pyrosequencing revealed that the denitrifiers of Denitratisoma, Ottowia, Sulfuritalea and Thiobacillus were notably enriched in the cathode biofilm of the EMBR at high temperatures, and 6.05% of the sequences in the activated sludge were assigned into the denitrification related phylotypes, which was 65% higher compared to that of the control MBR. The enrichment of versatile nitrate/nitrite reducers could be conducive to the autotrophic and heterotrophic denitrification processes. Energy balance analysis also indicated that the total energy consumption of the EMBR was decreased by 20% compared to that of the CMBR (0.386 kWh/m(3) wastewater). Overall, the obtained results clearly demonstrate that this novel EMBR is capable of providing superior effluent quality, alleviating membrane fouling and incorporating into existing treatment facilities for improving the sustainability of wastewater treatment.

A micro-aerobic hydrolysis process for sludge in situ reduction: performance and microbial community structure

A sludge process reduction activated sludge (SPRAS) system by inserting a sludge process reduction (SPR) module, composed of a micro-aerobic tank and a settler, before activated sludge process was operated for sludge in situ reduction. The average removal efficiencies of COD and ammonium nitrogen were 86.6% and 87.9%, respectively. Compared to anoxic/aerobic (AO) process, SPRAS process reduced sludge production by 57.9% with observed sludge yield of 0.076 gVSS/gCOD. Pyrosequencing analyses revealed that the relative abundance and stability of microbial communities in SPRAS system were higher than AO system. Fermentative acidogenic classes Anaerolineae, Actinobacteria, Cytophagia and Caldilineae were enriched in the SPR module and responsible for sludge reduction. Specific comparison down to the genus level identified the enrichment of oxyanion-reducing bacteria (Sulfuritalea; Azospira; Ramlibacter), fermentative acidogenic bacteria (Propionivibrio; Opitutus; Caldilinea), slow growers (Ramlibacter) and predatory bacteria (Myxobacteria) in SPRAS system. Nitrifiers were also more abundant in SPRAS system than AO system.

Start-up of an anaerobic dynamic membrane digester for waste activated sludge digestion: temporal variations in microbial communities

An anaerobic dynamic membrane digester (ADMD) was developed to digest waste sludge, and pyrosequencing was used to analyze the variations of the bacterial and archaeal communities during the start-up. Results showed that bacterial community richness decreased and then increased over time, while bacterial diversity remained almost the same during the start-up. Proteobacteria and Bacteroidetes were the major phyla. At the class level, Betaproteobacteria was the most abundant at the end of start-up, followed by Sphingobacteria. In the archaeal community, richness and diversity peaked at the end of the start-up stage. Principle component and cluster analyses demonstrated that archaeal consortia experienced a distinct shift and became stable after day 38. Methanomicrobiales and Methanosarcinales were the two predominant orders. Further investigations indicated that Methanolinea and Methanosaeta were responsible for methane production in the ADMD system. Hydrogenotrophic pathways might prevail over acetoclastic means for methanogenesis during the start-up, supported by specific methanogenic activity tests.