Solids Retention Time Research Articles

Proactive Monitoring of Changes in the Microbial Community Structure in Wastewater Treatment Bioreactors using Phospholipid Fatty Acid Analysis

Diverse microbial community structures (MCS) in wastewater treatment plants (WWTPs) are vital for effectively removing nutrients and chemicals from wastewater. However, the regular monitoring of MCS in WWTP bioreactors remains unattractive owing to the skill and cost required for deploying modern microbial molecular techniques in the routine assessment of engineered systems. In contrast, low-resolution methods for assessing broad changes in the MCS, such as phospholipid fatty acid (PLFA) analysis, have been used effectively in soil studies for decades. Despite using PLFA analysis in soil remediation studies to capture the long-term effects of environmental changes on MCS, its application in WWTPs, where the microbial mass is dynamic and operational conditions are more fluid, remains limited. In this study, microbial communities in a controlled pilot plant and 12 full-scale activated sludge plants (ASPs) were surveyed over a two-year period using PLFA analysis. This study revealed that changes in the MCS in wastewater bioreactors could be detected using PLFA analysis. The MCS comprised 59% Gram-negative and 9% Gram-positive bacteria, 31% fungi, and 1% actinomycetes. The abundances of Gram-negative bacteria and fungi were strongly inversely correlated, with an R2 = 0.93, while the fatty acids cy17:0 and 16:1ω7c positively correlated (R2 = 0.869). Variations in temperature, solid retention time, and WWTP configuration significantly influenced the MCS in activated sludge reactors. This study showed that WWTP bioreactors can be routinely monitored using PLFA analysis, and changes in the bioreactor profile that may indicate imminent bioreactor failure can be identified.

Biodegradation of glucose and paracetamol and process optimisation with open mixed microbial cultures in lab-scale sequencing batch reactors

In the context of biological wastewater treatment, this study investigated xenobiotics (man-made chemicals) biodegradation and process optimisation to maximise sustainability. Lab-scale sequencing batch reactors (SBRs) fed with paracetamol (model xenobiotic) and glucose (model biogenic substrate) each as only carbon source (feed concentration 1 g L−1) were run in a range of HRTs (hydraulic retention time, range 0.5–2 d) and SRTs (solids retention time, range 1–28 d). For both substrates, the removal of COD (chemical oxygen demand) increased as the SRT increased, reaching 97 % for glucose and 86 % for paracetamol. At the lowest HRT, the high solid losses with the effluent caused a reduction in the SRT with worse substrate removal. The fraction of the removed COD which was converted into biomass increased for lower SRT and overall the best performance for sustainability (compromise between substrate removal, energy consumption and plant footprint) was observed with HRT 1 d and SRT 11–12 d. The experimental data were modelled using two different approaches: linearisation of a continuous-flow model and non-linear parameter fitting with a periodic steady-state SBR model. The results indicated faster growth rate and slightly higher growth yield for glucose than for paracetamol.

Solids retention time (SRT) control in the co-treatment of leachate with domestic sewage in aerobic granular sludge systems: Impacts on system performance, operational stability, and bioresource production

This study investigates the co-treatment of leachate and domestic sewage in municipal wastewater treatment plants using aerobic granular sludge (AGS) systems, focusing on granule formation, system stability, and resource production in two units (R1 and R2). In R2, solids retention time (SRT) was controlled between 10 and 25 days, while R1 maintained approximately 9 days. The results show that low leachate proportions (5 %) did not affect system performance or stability. However, increasing the leachate to 10 % reduced the structural stability of extracellular polymeric substances (EPS), leading to a significant decrease in alginate-like exopolysaccharides (ALE) production in R1 (216 mgALE/gVSS) and R2 (125 mgALE/gVSS). Principal component analysis revealed that SRT was crucial for optimizing biopolymer synthesis. Furthermore, SRT control in R2 improved filamentous control, biomass retention, and total nitrogen removal. Thus, selective biomass discharge is essential for maintaining granule stability, enhancing treatment efficiency, and supporting resource production.

Predicting aeration time and nitrite accumulation rate variations for Partial Nitritation: A model incorporating nitrogen oxidation rate dynamics

This study aimed to develop a two-step nitrification model to predict variations in aeration time and nitrite accumulation rate (NAR) under fluctuating operational conditions in mainstream partial nitritation (PN) processes. Lab-scale sequencing batch reactors (SBRs) were used to evaluate the ammonia oxidation rate (AOR) and nitrite oxidation rate (NOR) under different solids retention times (SRT) (10, 15, 20, 30, and 50 days) and total volumetric nitrogen loadings (TVNL) (20–60 mg N/L per cycle). A static model was developed to predict consistent AOR and NOR values in the steady state, whereas a dynamic model was established to capture the growth dynamics of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) under unsteady-state conditions. The static model accurately predicted the AOR, NOR, and aeration time at steady state. The dynamic model quantified the relationship between specific growth rates (μ) and food-to-microorganism ratios (F/M) through exponential fitting, successfully capturing AOB and NOB growth dynamics. Validation experiments (SRT = 10 d, TVNL = 60 mg/L per cycle) demonstrated the ability of the dynamic model to predict trends in NAR and aeration time accurately. This study emphasizes the importance of accurately modeling AOR and NOR variations to predict aeration time and NAR, thereby providing valuable insights for aeration control and precise management of AOB and NOB populations in mainstream PN processes.

Polyhydroxyalkanoates production from cheese whey under near-seawater salinity conditions

Treating saline streams presents considerable challenges due to their adverse effects on conventional biological processes, thereby leading to increased expenses in managing those side streams. With this in consideration, this study explores into the potential for valorizing fermented cheese whey (CW), a by-product of the dairy industry, into polyhydroxyalkanoates (PHA) using mixed microbial cultures (MMC) under conditions of near-seawater salinity (30 gNaCl/L). The selection of a PHA-accumulating MMC was successfully achieved using a sequential batch reactor operated under a feast and famine regime, with a hydraulic retention time of 14.5 h, a variable solids retention time of 3 and 4.5 days, and an organic loading rate (OLR) of 60 Cmmol/(L d). The selected culture demonstrated efficient PHA production rates and yields, maintaining robust performance even under high salinity conditions. During PHA accumulation, a maximum PHA content in biomass of 56.4 % wt. was achieved for a copolymer P(3HB-co-3HHx) with a 3HHx content of 7 %. Additionally, to asses the capacity of the culture to produce polymers with different compositions, valeric acid was supplemented to the real fermented feedstock which resulted in the production of terpolymers P(3HB-co-3HV-co-3HHx) with varied monomeric content and a higher maximum PHA content of 62 % wt. Additionally, this study highlights the potential utilization of seawater as alternative to freshwater for PHA production, thereby enhancing circular economy principles and promoting environmental sustainability.

Microbial ecology of nitrate-, selenate-, selenite-, and sulfate-reducing bacteria in a H2-driven bioprocess.

A hydrogen (H2)-based membrane biofilm reactor (H2-MBfR) can reduce electron acceptors nitrate (NO3-), selenate (SeO42-), selenite (HSeO3-), and sulfate (SO42-), which are in wastewaters from coal mining and combustion. This work presents a model to describe a H2-driven microbial community comprised of hydrogenotrophic and heterotrophic bacteria that respire NO3-, SeO42-, HSeO3-, and SO42-. The model provides mechanistic insights into the interactions between autotrophic and heterotrophic bacteria in a microbial community that is founded on H2-based autotrophy. Simulations were carried out for a range of relevant solids retention times (SRT; 0.1-20 days) and with adequate H2-delivery capacity to reduce all electron acceptors. Bacterial activity began at an ∼0.6-day SRT, when hydrogenotrophic denitrifiers began to accumulate. Selenate-reducing and selenite-reducing hydrogenotrophs became established next, at SRTs of ∼1.2 and 2 days, respectively. Full NO3-, SeO42-, and HSeO3- reductions were complete by an SRT of ∼5 days. SO42- reduction began at an SRT of ∼10 days and was complete by ∼15 days. The desired goal of reducing NO3-, SeO42-, and HSeO3-, but not SO42-, was achievable within an SRT window of 5-10 days. Autotrophic hydrogenotrophs dominated the active biomass, but nonactive solids were a major portion of the solids, especially for an SRT ≥ 5 days.

Bioaugmentation with Clostridium pasteurianum for high-yield continuous bio-hydrogen production in a dynamic membrane bioreactor

This study demonstrated the feasibility of achieving high-yield continuous bio-H2 production through bioaugmentation with Clostridium pasteurianum. C. pasteurianum was introduced during the initial granulation stage in a dynamic membrane bioreactor (DMBR) inoculated with heat-treated digester sludge. Following bioaugmentation, the average hydrogen yield (HY) reached 3.07 ± 0.13 mol H2/mol hexoseconsumed, with a hydraulic retention time (HRT) of 2 h. This high-yield and high-rate hydrogen production performance was achieved alongside a high concentration of suspended and granular biomass as 21.02 ± 1.01 g VSS/L. The DM played a crucial role in prolonging the solid retention time of the suspended biomass to 12.15 h. C. pasteurianum emerged as the predominant species, constituting 92.3 % of the total bacterial population. This dominance significantly contributed to achieving the high HY, reaching 77 % of the theoretical maximum HY (4 mol H2/mol hexoseconsumed) by shifting the metabolic pathway from non-H2 production pathways to hydrogen-producing acetate. However, during further continuous operation, the hydrogen production pathway was gradually shifted towards the butyrate pathway, resulting in a decreased HY of 2.26 mol H2/mol hexoseconsumed along with the decreased population of C. pasteurianum. Additional bioaugmentation of C. pasteurianum during the maturational granulation stage did not impact the metabolic pathways or microbiome significantly, as the added strain was supposed to be washed out. This study provides insights into achieving high-yield continuous bio-H2 production through bioaugmentation with C. pasteurianum during the initial granulation stage. Further studies are necessary to sustain C. pasteurianum-dominated high HY with the acetate pathway during prolonged operation.

Operation parameters and temperature affected sludge microbial metabolisms: An integrated perspective considering extracellular polymeric substances, soluble microbial products, biomass quantities, and community shifts

Effluent soluble microbial products (SMP) and extracellular polymeric substances (EPS) are significant organics that pose challenges to advanced treatment processes. However, their production, transformation, and decomposition remain unclear due to their heterogeneity and the combined effects of environmental and operational factors. In this work, we investigated the impact of solids retention time (SRT), hydraulic retention time (HRT), and temperature on the changes in effluent SMP, with the consideration of the co-variation of EPS, sludge biomass, and community structures. Results show that longer SRT increased the biomass and relative abundance of functional microorganisms such as Myxococcota, Actinobacteria, and Terrimonas, which hindered EPS-to-SMP turnover and/or facilitated SMP consumption. This resulted in the accumulation of EPS and lower SMP concentrations at the beginning of the SRT adjustment. Both longer and shorter HRT (12 h and 8 h) led to increased SMP concentration, with the shorter HRT nearly doubling it (from approximately 6 to 12 mg/L), especially in terms of its protein and polysaccharide contents. Lower temperatures increased the SMP concentration and the relative abundance of Proteobacteria (including Zoogloea, the most dominant phylum and genus, relative abundance from 15.7 % to 61.1 %) while decreasing fluorescent EPS components, indicating the key role of Proteobacteria in SMP production and fluorescent EPS-to-SMP transformation. The results provided key insights into how changes in operational/environmental parameters impact sludge-EPS-SMP interactions, which could benefit the model development and operational optimization of activated sludge systems. This study also highlighted the important role of the sludge community in the EPS/SMP dynamics.

Little alginates synthesized in EPS: Evidences from high-throughput community and metagenes

As a significant structure in activated sludge, extracellular polymeric substances (EPS) hold considerable value regarding resource recovery and applications. The present study aimed to elucidate the relationship between the microbial community and the composition and properties of EPS. A biological nutrient removal (BNR) reactor was set up in the laboratory and controlled under different solid retention times (SRT), altering microbial species within the system. Then EPS was extracted from activated and analyzed by chemical and spectroscopic methods. High-throughput sequencing and metagenomic approaches were employed to investigate bacterial community and metabolic pathways. The results showed that lower SRT with a higher abundance of the family-level Proteobacteria (27.7%-53.5%) favored EPS synthesis, while another dominant group Bacteroidetes (20.0%-32.6%) may not significantly affect EPS synthesis. Furthermore, the abundance of alginates-producing bacteria including Pseudomonas spp. and Azotobacter vinelandii was only 2.53%-6.76% and 1.98%-6.34%, respectively. The alginate synthesis pathway genes Alg8 and Alg44 were also present at very low levels (0.05‱-0.11‱, 0.01‱-0.02‱, respectively). Another important gene related to alginates operons, AlgK, was absent across all the SRT-operated reactors. These findings suggest an impossible and incomplete alginate synthesis pathway within sludge. In light of these results, it can be concluded that EPS does not necessarily contain alginate components.

Using machine learning to identify environmental factors that collectively determine microbial community structure of activated sludge

Activated sludge (AS) microbial communities are influenced by various environmental variables. However, a comprehensive analysis of how these variables jointly and nonlinearly shape the AS microbial community remains challenging. In this study, we employed advanced machine learning techniques to elucidate the collective effects of environmental variables on the structure and function of AS microbial communities. Applying Dirichlet multinomial mixtures analysis to 311 global AS samples, we identified four distinct microbial community types (AS-types), each characterized by unique microbial compositions and metabolic profiles. We used 14 classical linear and nonlinear machine learning methods to select a baseline model. The extremely randomized trees demonstrated optimal performance in learning the relationship between environmental factors and AS types (with an accuracy of 71.43%). Feature selection identified critical environmental factors and their importance rankings, including latitude (Lat), longitude (Long), precipitation during sampling (Precip), solids retention time (SRT), effluent total nitrogen (Effluent TN), average temperature during sampling month (Avg Temp), mixed liquor temperature (Mixed Temp), influent biochemical oxygen demand (Influent BOD), and annual precipitation (Annual Precip). Significantly, Lat, Long, Precip, Avg Temp, and Annual Precip, influenced metabolic variations among AS types. These findings emphasize the pivotal role of environmental variables in shaping microbial community structures and enhancing metabolic pathways within activated sludge. Our study encourages the application of machine learning techniques to design artificial activated sludge microbial communities for specific environmental purposes.

Unravelling the impact of oxic-settling-anaerobic cycle implementation and solid retention time on sludge generation, membrane operation, and contaminant removal in membrane bioreactors

Sewage sludge production represents a critical issue nowadays. The oxic-settling-anaerobic (OSA) system is a promising strategy for reducing sludge production during biological wastewater treatment. However, the distinct effect of OSA process and solid retention time (SRT) on sludge production has not been clarified yet. Therefore, during this study, the impacts of OSA system implementation at different SRT on sludge generation, membrane operation, and contaminant removal were elucidated in two lab-scale membrane bioreactors working at 20 d (MBR20) and 60 d (MBR60). After OSA implementation, the MBR-OSA20 and MBR-OSA60 were operated at a hydraulic retention time in the sludge holding tank (HRTSHT) of 6–12 h and of 6–24 h, respectively. The estimated observed sludge yield (Yobs) showed a reduction of 38–48 % for the MBR-OSA20, but no beneficial effect was observed for MBR-OSA60 at 6 h of HRTSHT. At the HRTSHT of 24 h, Yobs decreased by 17 % due to hydrolysis and acidogenic biomass fermentation. OSA implementation and long SRT had no negative effects on organic carbon and ammonium removal (96(±3) % and 100 %, respectively) and improved total nitrogen (TN) reduction by 19–41 %. Membrane fouling and sludge filterability were not significantly affected, whereas sludge settleability worsened after the implementation of the OSA cycle. This activity provided additional insights about the distinct impact of the OSA process and SRT on sludge production and characteristics, confirming the OSA process as a valid solution for reducing sludge production under a moderate SRT, but revealing a negligible effect on Yobs under a prolonged SRT and a short HRTSHT.

Modification of Anaerobic Digestion Model No. 1 for modeling anaerobic digestion of cattle manure with changing solids retention time

In this study, Anaerobic Digestion Model No.1 (ADM1) was modified to incorporate changes in biochemical parameters due to solids retention time (SRT) variations. Cattle manure (CM) and thermally hydrolyzed CM were selected for testing. Continuous anaerobic digestion reactors were operated under different SRT conditions ranging from 6.6 to 36.0 days for both samples. The biochemical parameters (kch, kli, kpr, um,ac, um,bu, um,pro, um,va, Kac, Kbu, Kpro, and Kva) for each SRT condition were determined. To modify ADM1, the equations obtained through linear regression were substituted into biochemical parameters as a function of SRT. The modified ADM1 demonstrated superior accuracy compared with conventional ADM1. This study implies the feasibility of optimizing biochemical parameters for modeling in response to changes in environmental variables.

Enhancement of Biodegradability of Chicken Manure via the Addition of Zeolite in a Two-Stage Dry Anaerobic Digestion Configuration.

Leach bed reactors (LBRs) are dry anaerobic systems that can handle feedstocks with high solid content, like chicken manure, with minimal water addition. In this study, the chicken manure was mixed with zeolite, a novel addition, and packed in the LBR to improve biogas production. The resulting leachate was then processed in a continuous stirred tank reactor (CSTR), where most of the methane was produced. The supernatant of the CSTR was returned to the LBR. The batch mode operation of the LBR led to a varying methane production rate (MPR) with a peak in the beginning of each batch cycle when the leachate was rich in organic matter. Comparing the MPR in both systems, the peaks in the zeolite system were higher and more acute than in the control system, which was under stress, as indicated by the acetate accumulation at 2328 mg L-1. Moreover, the presence of zeolite in the LBR played a crucial role, increasing the overall methane yield from 0.142 (control experiment) to 0.171 NL CH4 per g of volatile solids of chicken manure entering the system at a solid retention time of 14 d. Zeolite also improved the stability of the system. The ammonia concentration increased gradually due to the little water entering the system and reached 3220 mg L-1 (control system) and 2730 mg L-1 (zeolite system) at the end of the experiment. It seems that zeolite favored the accumulation of the ammonia at a lower rate (14.0 mg L-1 d-1) compared to the control experiment (17.3 mg L-1 d-1). The microbial analysis of the CSTR fed on the leachate from the LBR amended with zeolite showed a higher relative abundance of Methanosaeta (83.6%) compared to the control experiment (69.1%). Both CSTRs established significantly different bacterial profiles from the inoculum after 120 days of operation (p < 0.05). Regarding the archaeal communities, there were no significant statistical differences between the CSTRs and the inoculum (p > 0.05).

Aerobic granulation and resource production under continuous and intermittent saline stress

Three sequential batch reactors (SBR) were operated to evaluate salt addition's impact on granulation, performance, and biopolymer production in aerobic granular sludge (AGS) systems. System R1 was fed without adding salt (control); system R2 operated with saline pulses, i.e., one cycle with salt (2.5 g NaCl/L) addition followed by another without salt; and R3 received continuous supplementation of 2.5 g NaCl/L. The results indicated that the reactors supplemented with salt presented higher concentrations of mixed liquor volatile suspended solids (MLVSS) and better settleability than R1, showing that osmotic pressure contributed to biomass growth, accelerated granulation, and improved physical characteristics. The faster granulation occurred in R2, thus proving the beneficial effects of intermittent salt addition through alternating pulses. Salt addition did not impair the simultaneous removal of carbon, nitrogen, and phosphorus. In fact, R2 showed better carbon removals. In conclusion, continuous or intermittent (pulsed) supplementation of 2.5 g NaCl/L did not lead to increased production of extracellular polymeric substances (EPS) and alginate-like exopolymers (ALE). This outcome could be attributed to the low saline concentration employed, a higher food-to-microorganism (F/M) ratio observed in R1, and possibly greater endogenous consumption of biopolymers in the famine period in R2 and R3 due to the greater solids retention time (SRT). Therefore, this study brings important results that contribute to a better understanding of the effect of salt in continuous dosing or in pulses as a selection pressure strategy to accelerate granulation, as well as the behavior of the AGS systems for saline effluents.

Performance and microbial community composition of full-scale high-rate cascade sludge digestion system via pie-shaped reactor configuration

A full-scale high-rate cascade anaerobic digestion (CAD) system was evaluated for its ability to enhance enzymatic sludge hydrolysis. The system included a newly built digester, innovatively divided into three pie-shaped compartments (500 m3 each), followed by an existing, larger digester (1500 m3). The system treated a mixture of waste activated sludge and primary sludge, achieving a stable total chemical oxygen demand reduction efficiency (56.1 ± 6.8 %), and enhanced sludge hydrolytic enzyme activities at a 14.5-day total solids retention time (SRT). High-throughput sequencing data revealed a consistent microbial community across reactors, dominated by consortia that govern hydrolysis and acidogenesis. Despite relatively short SRTs in the initial reactors of the CAD system, acetoclastic methanogens belonging to Methanosaeta became the most abundant archaea. ‬‬‬‬‬‬‬‬‬‬‬‬‬ This study proves that the CAD system achieves stable sludge reduction, accelerates enzymatic hydrolysis at full-scale, and paves the way for its industrialization in municipal waste sewage sludge treatment.

Global occurrence of organic micropollutants in surface and ground water: Highlighting the importance of wastewater sanitation to tackle organic micropollutants

The occurrence, fate and distribution of organic micropollutants (OMPs) in different water matrices is of increasing concern due to the negative effects towards flora, fauna and human health. This metadata study provides an updated picture of the global occurrence of OMPs in freshwater and wastewater in five geographical regions, and illustrates how socioeconomic factors are key elements that influence the concentration of these OMPs in the different water matrices. The study also aims to provide a collated database of OMPs removal efficiencies achieved by the different biological treatment technologies. Our findings demonstrated that OMPs like acetaminophen, ibuprofen, and triclosan are present globally in freshwater (> 500 ng/L) and wastewater (> 10, 000 ng/L), and at relatively higher concentrations than the other evaluated OMPs. Significant correlations between OMPs concentration in wastewater versus freshwater demonstrated wastewater to be a potential source of certain OMPs to the freshwater environment. This was further confirmed through correlation between OMPs concentration and socioeconomic factors like infrastructure index and governance index in both wastewater and freshwater. These results highlighted the importance of wastewater sanitation to control and reduce OMPs dissemination in the environment. Thus, various types of biological wastewater treatment processes including activated sludge processes, biofilm processes, wetland and lagoon, and membrane bioreactor were analyzed for their OMP’s removal efficiencies. Findings demonstrated that advanced processes like membrane bioreactors (MBR) may not always outperform other low-costs biotechnologies in the treatment of OMPs (e.g., bisphenol A removal by activated sludge with anoxic and oxic zone, AS-AAO was 93% versus MBR at 89%). But rather, the removal efficiency will depend on pollutant properties and certain operational parameters such as hydraulic retention time, treatment capacity, and removal efficiency of total suspended solids.

Mechanistic understanding of acclimation and energy metabolism of acetoclastic methanogens under different substrate to microorganism ratios

Mechanistic understanding of acetoclastic methanogenesis is pivotal for optimizing anaerobic digestion for efficient methane production. In this study, two different operational modes, continuous flow reactor (CFR) and sequencing batch reactor (SBR), accompanied with solids retention times (SRT) of 10 days (SBR10d and CFR10d) and 25 days (SBR25d and CFR25d) were implemented to elucidate their impacts on microbial communities and energy metabolism of methanogens in acetate-fed systems. Microbial community analysis revealed that the relative abundance of Methanosarcina (16.0%–46.0%) surpassed Methanothrix (3.7%–22.9%) in each reactor. SBRs had the potential to enrich both Methanothrix and Methanosarcina. Compared to SBRs, CFRs had lower total relative abundance of methanogens. Methanosarcina exhibited a superior enrichment in reactors with 10-day SRT, while Methanothrix preferred to be acclimated in reactors with 25-day SRT. The operational mode and SRT were also observed to affect the distribution of acetate-utilizing bacteria, including Pseudomonas, Desulfocurvus, Mesotoga, and Thauera. Regarding enzymes involved in energy metabolism, Ech and Vho/Vht demonstrated higher relative abundances at 10-day SRT compared to 25-day SRT, whereas Fpo and MtrA-H showed higher relative abundances in SBRs than those in CFRs. The relative abundance of genes encoding ATPase harbored by Methanothrix was higher than Methanosarcina at 25-day SRT. Additionally, the relative abundance of V/A-type ATPase (typically for methanogens) was observed higher in SBRs compared to CFRs, while the F-type ATPase (typically for bacteria) exhibited higher relative abundance in CFRs than that in SBRs.

Compact and water flushing resistant mesh biofilms forming at short SRT disappeared naturally under extended SRT in dynamic membrane bioreactor

Extending the solids retention time (SRT) has been demonstrated to mitigate membrane biofouling. Nevertheless, it remains an intriguing question whether the compact and water flushing resistant mesh biofilms developed at short SRT can undergo biodegradation and be removed with extended SRT. In present study, the bio-fouled mesh filter in the 10d-SRT dynamic membrane bioreactor (DMBR), with mesh surfaces and pores covered by compact and water flushing resistant biofilms exhibiting low water permeability, was reused in the 40d-SRT DMBR without any cleanings. After being reused at 40d-SRT, its flux driven by gravity occurred from the 10th day and recovered to a regular level of 36.7 L m−2·h−1 on the 27th day. Both scanning electron microscope (SEM) and confocal laser scanning microscopy (CLSM) analyses indicated that the compact mesh biofilms formed at10d-SRT biodegraded and were removed at 40d-SRT, with the residual biofilms becoming removable by water flushing. As a result, the hydraulic resistance of the bio-fouled mesh filter decreased from 4.36 × 108 to 6.97 × 107 m−1, and its flux fully recovered. The protein and polysaccharides densities in mesh-biofilms decreased from 24.4 to 9.7 mg/cm2 and from 10.7 to 0.10 mg/cm2, respectively, which probably have contributed to the disappearance of compact biofilms and the decrease in adhesion. Furthermore, the sludge and mesh-biofilms in the 40d-SRT reactor contained a higher relative abundance of dominant quorum quenching bacteria, such as Rhizobium (3.52% and 1.35%), compared to those in the 10d-SRT sludge (0.096%) and mesh biofilms (0.79%), which might have been linked to a decline in extracellular polymeric substances and, consequently, the biodegradation and disappearance of compact biofilms.

Effects of contact/stabilization time ratio on organic carbon-methane conversion in high-rate contact stabilization system

The high-rate contact stabilization (HRCS) process is a promising technique for recovering carbon from wastewater. However, its practical application faces challenges due to its low chemical oxygen demand (COD) removal efficiency, which requires further optimization. The objective of this study was to investigate the optimal process conditions for enhancing COD removal efficiency and carbon recovery rate (CRR) in the HRCS process. To achieve this, a sequencing batch reactor (SBR) was utilized to maximize the feast-famine regime. Although the HRCS process exhibited a lower COD removal efficiency compared to the high-rate activated sludge (HRAS) process in SBR mode, it demonstrated a 16.3 % higher CRR. The influence of the contact time to stabilization time (tc/ts ratio) on both COD removal efficiency and CRR was investigated. The optimum condition was achieved at a tc/ts ratio of 0.18, which resulted in a COD removal efficiency of 60.2 ± 3.0 % and a CRR of 0.417 ± 0.041 g-CODCH4/g-CODinf (CODCH4: produced methane as COD, CODinf: influent COD). The microbial communities in HRAS and HRCS exhibited simplification compared to conventional activated sludge. Distinct shifts were observed in response to changes in solids retention time and tc/ts ratio. Under harsher operating conditions, Lactococcus spp. became the predominant species, achieving the greatest dominance at a tc/ts ratio of 0.18. This is because they produce more extracellular polymeric substances and polyhydroxyalkanoates, which improve organic capture. The operating conditions identified in this study are anticipated to make a significant contribution to practical applications aimed at enhancing carbon recovery.

Algal-bacterial shortcut nitrogen removal model with seasonal light variations.

The algal-bacterial shortcut nitrogen removal (ABSNR) process can be used to treat high ammonia strength wastewaters without external aeration. However, prior algal-bacterial SNR studies have been conducted under fixed light/dark periods that were not representative of natural light conditions. In this study, laboratory-scale photo-sequencing batch reactors (PSBRs) were used to treat anaerobic digester sidestream under varying light intensities that mimicked summer and winter conditions in Tampa, FL, USA. A dynamic mathematical model was developed for the ABSNR process, which was calibrated and validated using data sets from the laboratory PSBRs. The model elucidated the dynamics of algal and bacterial biomass growth under natural illumination conditions as well as transformation processes for nitrogen species, oxygen, organic and inorganic carbon. A full-scale PSBR with a 1.2 m depth, a 6-day hydraulic retention time (HRT) and a 10-day solids retention time (SRT) was simulated for treatment of anaerobic digester sidestream. The full-scale PSBR could achieve >90% ammonia removal, significantly reducing the nitrogen load to the mainstream wastewater treatment plant (WWTP). The dynamic simulation showed that ABSNR process can help wastewater treatment facilities meet stringent nitrogen removal standards with low energy inputs.