Overcoming floc formation limitations in high-rate activated sludge systems

The effect of fine grits and fine debris concentrations on the MLVSS/MLSS ratio of an activated sludge system

This study embarks on an explorative investigation into the effects of typical concentrations and varying particle sizes of fine grits (FG, the involatile portion of suspended solids) and fine debris (FD, the volatile yet unbiodegradable fraction of suspended solids) within the influent on the mixed liquor volatile suspended solids (MLVSS)/mixed liquor suspended solids (MLSS) ratio of an activated sludge system. Through meticulous experimentation, it was discerned that the addition of FG or FD, the particle size of FG, and the concentration of FD bore no substantial impact on the pollutant removal efficiency (denoted by the removal rate of COD and ammonia nitrogen) under constant operational conditions. However, a notable decrease in the MLVSS/MLSS ratio was observed with a typical FG concentration of 20 mg/L, with smaller FG particle sizes exacerbating this reduction. Additionally, variations in FD concentrations influenced both MLSS and MLVSS/MLSS ratios; a higher FD concentration led to an increased MLSS and a reduced MLVSS/MLSS ratio, indicating FD accumulation in the system. A predictive model for MLVSS/MLSS was constructed based on quality balance calculations, offering a tool for foreseeing the MLVSS/MLSS ratio under stable long-term influent conditions of FG and FD. This model, validated using data from the BXH wastewater treatment plant (WWTP), showcased remarkable accuracy.

Indicative impacts of sludge properties and biological metabolic characteristics on high-rate contact stabilization process performance

Regarding curtailing carbon emissions in wastewater treatment, the high-rate contact stabilization (HiCS) process outperforms others in removing dissolved organic matter (DOM) but struggles with poor settling performance. To boost operation performance and clarify the correlation between process parameters, DOM variations, effluent quality, and microbial metabolism within the HiCS system, the impacts of sludge properties on sludge settlement and organic matter (OM) capture efficiency were explored, and soluble fluorescent components in the DOM and extracellular polymeric substances (EPS) were identified and scrutinized. Results unveil that the feast/famine (F/F) regime in the HiCS process predominantly governs sludge activation in the stabilization phase, influencing sludge properties such as morphology characteristics, biological activity, and EPS secretion. At the same hydraulic retention time, reducing the sludge retention time (SRT) led to looser and smaller activated sludge flocs, increased microbial activity, and higher EPS production, particularly protein content in loosely bound EPS (LB-PN), which adversely impacted settling performance. High-throughput sequencing revealed that richness and diversity of the microbial community decreased with SRT. Acidobacteriota and Patescibacteria, associated with nitrifying and denitrifying bacteria, significantly decreased. EPS-producing Firmicutes increased, enhancing EPS secretion, while filamentous Chloroflexi decreased, aligning with a reduced organic mineralization rate. Settlement and biological activity emerged as key factors affecting OM recovery, peaking at 43.97% with a 4-day SRT. The ratio of the sum of tryptophan-like and tyrosine-like components to fulvic-like components ((C1+C2)/C3) was proposed as a fluorescence indicator, serving as a hub to connect operational parameters, F/F regime, sludge status and process performance. When this ratio falls within the range of 1.04-1.36 during the stabilization phase, HiCS sludge achieves optimal status for OM capture with low aeration energy consumption.

Sludge floc characteristics and microbial community in high-rate activated sludge and high-rate membrane bioreactor for organic recovery

High-rate activated sludge (HRAS) and high-rate membrane bioreactor (HRMBR) are considered as potential processes for organic recovery through bioflocculation and biosorption of particulate COD and colloidal COD with sludge flocs. In this study, bioflocculation and biosorption, in terms of sludge floc characteristics and microbial community, in HRAS and HRMBR was investigated in relation to organic recovery performance for low strength wastewater treatment. HRAS and HRMBR were operated at two different solids retention times (SRTs) of 2 and 0.8 days. Reducing the SRT of HRAS from 2.0 to 0.8 days resulted in failure in total COD (tCOD) removal efficiency (from 79 ± 2 to 34 ± 13 %) and lowering organic recovery (from 40.8 to 15.7 %). This contrasted with HRMBR, which showed high tCOD removal efficiency (84 ± 2 and 84 ± 1 %) and organic recovery (43.4 and 46.3 %) at both SRTs of 2.0 and 0.8 days. Analysis of sludge floc characteristics showed that the lower organic recovery of the HRAS operated at an SRT of 0.8 days could be associated with poor bioflocculation and biosorption, as evidenced by relatively larger floc size, higher extracellular polymeric substance, higher protein/polysaccharide ratio, and higher zeta potential value of the sludge. These characteristics were in contrast to the HRMBR operated at an SRT of 0.8 days, that exhibited the highest organic recovery among the reactors studied. The microbial taxa Bdellovibrio, Clostridium sensu stricto 9, Hyphomicrobium, and Ideonella could play a role in the poor bioflocculation and biosorption in HRAS. Rhodanobacter, Enterobacter, Terrimonas, Nakamurella, and Mizugakiibacter may be associated with bioflocculation and biosorption and organic recovery in HRMBR. The results of this study enhanced our understanding on the relationships between the microbial community, sludge floc characteristics, and organic recovery performance of HRAS and HRMBR for future optimization of the systems.

Biological and physical selectors for mobile biofilms, aerobic granules, and densified-biological flocs in continuously flowing wastewater treatment processes: A state-of-the-art review

There have been significant advances in the use of biological and physical selectors for the intensification of continuously flowing biological wastewater treatment (WWT) processes. Biological selection allows for the development of large biological aggregates (e.g., mobile biofilm, aerobic granules, and densified biological flocs). Physical selection controls the solids residence times of large biological aggregates and ordinary biological flocs, and is usually accomplished using screens or hydrocyclones. Large biological aggregates can facilitate different biological transformations in a single reactor and enhance liquid and solids separation. Continuous-flow WWT processes incorporating biological and physical selectors offer benefits that can include reduced footprint, lower costs, and improved WWT process performance. Thus, it is expected that both interest in and application of these processes will increase significantly in the future. This review provides a comprehensive summary of biological and physical selectors and their design and operation.

Mechanistic assessment reveals the significance of HRT and MLSS concentration in balancing carbon diversion and removal in the A-stage process

Nowadays, the shift toward energy and resource-efficient wastewater treatment plants (WWTPs) has become a necessity rather than a choice. For this purpose, there has been a restored interest in replacing the typical energy and resource-extensive activated sludge process with the two-stage Adsorption/bio-oxidation (A/B) configuration. In the A/B configuration, the role of the A-stage process is to maximize organics diversion to the solids stream and control the following B-stage's influent to allow for the attainment of tangible energy savings. Operating at very short retention times and high loading rates, the influence of the operational conditions on the A-stage process become more tangible than typical activated sludge. Nonetheless, there is very limited understanding of the influence of operational parameters on the A-stage process. Moreover, no studies in the literature have explored the influence of any operational/design parameters on the Alternating Activated Adsorption (AAA) technology which is a novel A-stage variant. Hence, this article mechanistically investigates the independent effect of different operational parameters on the AAA technology. It was inferred that solids retention time (SRT) shall remain below 1 day to allow for energy savings up to 45% and redirecting up to 46% of the influent's COD to the recovery streams. In the meantime, the hydraulic retention time (HRT) can be increased up to 4 h to remove up to 75% of the influent's COD with only 19% decline of the system's COD redirection ability. Moreover, it was observed that the high biomass concentration (above 3000 mg/L) amplified the effect of the sludge poor settleability either due to pin floc settling or high SVI₃₀ which resulted in COD removal below 60%. Meanwhile, the concentration of the extracellular polymeric substances (EPS) was not found to be influenced or to influence process performance. The findings of this study can be employed to formulate an integrative operational approach in which different operational parameters are incorporated to better control the A-stage process and achieve complex objectives.

High-rate activated sludge at very short SRT: Key factors for process stability and performance of COD fractions removal

In high-rate activated sludge (HRAS) processes, reducing the solid retention time (SRT) minimizes COD oxidation and allows to obtain the maximum energy recovery. The aim of this research was to operate a pilot plant with an automatic control strategy to assure the HRAS process stability and high COD fractions removal at very low SRT. This study combines simulation and experimental tools (pilot plant 35 m³·d ^- 1) operating at SRT (0.2 d), HRT (0.6 h) and DO (0.5 mg·L ^- 1) treating high-strength raw wastewater, at 18-26°C, at variable flow. The research includes the effects of temperature, influent concentration and MLSS reactor concentration over the sCOD, cCOD and pCOD removal. The study points out that the best parameter to control the HRAS at a low SRT is not strictly the SRT but rather the reactor MLSS concentration: operating at 2,000±200mg·L ^- 1 assured a stable process despite the large influents variation. Low SVI values of 50-70ml·g ^- 1 indicated the good settling properties of the biomass. With only a 6.9% COD oxidation, a high organic matter removal (57±9% for COD and 56±10% for BOD₅), was reached. The high removal efficiencies for pCOD (74%) compared to the (29%) for sCOD and (12%) for cCOD also confirmed the importance of settling efficiency and stability in the HRAS. The direct correlation between COD influent and COD removal makes advisable to use the HRAS as a replacement of the primary clarifier. The HRAS acted efficiently as a filter for COD and pCOD peak loads and, in a lesser extent, for BOD₅, while sCOD peaks were not buffered. The adopted model presented a good fit for COD fractions except for pCOD when the temperature exceeds 23 °C.

Boosting aerobic microbial protein productivity and quality on brewery wastewater: Impact of anaerobic acidification, high-rate process and biomass age

Consortia of aerobic heterotrophic bacteria (AHB) are appealing as sustainable alternative protein ingredient for aquaculture given their high nutritional qualities, and their production potential on feed-grade industrial wastewater. Today, the impacts of pre-treatment, bioprocess choice and key parameter settings on AHB productivity and nutritional properties are unknown. This study investigated for the first time AHB microbial protein production effects based on (i) raw vs anaerobically fermented brewery wastewater, (ii) high-rate activated sludge (HRAS) without vs with feast-famine conditions, and (iii) three short solid retention time (SRT): 0.25, 0.50 and 1.00 d. High biomass (4.4-8.0 g TSS/L/d) and protein productivities (1.9-3.2 g protein/L/d) were obtained while achieving COD removal efficiencies up to 98 % at SRT 0.50 d. The AHB essential amino acid (EAA) profiles were above rainbow trout requirements, excluding the S-containing EAA, highlighting the AHB biomass replacement potential for unsustainable fishmeal in salmonid diets.

Enhancing bioflocculation in high-rate activated sludge improves effluent quality yet increases sensitivity to surface overflow rate

High-rate activated sludge (HRAS) relies on good bioflocculation and subsequent solid-liquid separation to maximize the capture of organics. However, full-scale applications often suffer from poor and unpredictable effluent suspended solids (ESS). While the biological aspects of bioflocculation are thoroughly investigated, the effects of fines (settling velocity < 0.6 m³/m²/h), shear and surface overflow rate (SOR) are unclear. This work tackled the impact of fines, shear, and SOR on the ESS in absence of settleable influent solids. This was assessed on a full-scale HRAS step-feed (SF) and pilot-scale HRAS contact-stabilization (CS) configuration using batch settling tests, controlled clarifier experiments, and continuous operation of reactors. Fines contributed up to 25% of the ESS in the full-scale SF configuration. ESS decreased up to 30 mg TSS/L when bioflocculation was enhanced with the CS configuration. The feast-famine regime applied in CS promoted the production of high-quality extracellular polymeric substances (EPS). However, this resulted in a narrow and unfavorable settling velocity distribution, with 50% ± 5% of the sludge mass settling between 0.6 and 1.5 m³/m²/h, thus increasing sensitivity towards SOR changes. A low shear environment (20 s^-1) before the clarifier for at least one min was enough to ensure the best possible settling velocity distribution, regardless of prior shear conditions. Overall, this paper provides a more complete view on the drivers of ESS in HRAS systems, creating the foundation for the design of effective HRAS clarifiers. Tangible recommendations are given on how to manage fines and establish the optimal settling velocity of the sludge.

A-Stage process - Challenges and drawbacks from lab to full scale studies: A review

In response to the growing global resource scarcity, wastewater is increasingly seen as a valuable resource to recover and valorise for the benefit of the society rather than another waste that needs treatment before disposal. Conventional wastewater treatment plants (WWTPs) oxidise most of the organic matter present in wastewater, instead of recovering it as a feedstock for biomaterials or to produce energy in the form of biogas. In contrast, an A-Stage is capable of producing a concentrated stream of organic matter ready for valorisation, ideally suited to retrofit existing large plants. This technology is based on the principle of high-rate activated sludge process that favours biosorption and storage over oxidation. In this paper, we summarize peer-reviewed research of both pilot-scale and full-scale studies of A-Stage process under real conditions, highlighting key operational parameters. In the majority of published studies, the sludge retention time (SRT) was identified as a key operational parameter. An optimal SRT of 0.3 days seems to maximize the redirection of influent COD - up to 50% to the sludge flux, while simultaneously keeping mineralization under 25% of total influent COD. Other key optimal parameters are a hydraulic residence time of 30 min and dissolved oxygen levels of 0.5 mg⋅L^-1. In addition, nutrient removal efficiencies of 15-27% for total nitrogen and 13-38% for total phosphorus are observed. Influence of mixing on settling efficiencies remain largely underexplored, as well as impact of wet weather flow and temperature on overall recovery efficiencies, which hinders to provide recommendations on these aspects. Evolution of modelling efforts of A-Stage process are also critically reviewed. The role of extracellular polymeric substances remain unclear and measures differ greatly according to the different studies and protocols. Better understanding the settling processes by adding Limit of Stokesian and Threshold of Flocculation measures to Sludge Volume Index could help to reach a better understanding of the A-Stage process. Reliable modelling can help new unit processes find their place in the whole treatment chain and help the transition from WWTPs towards Wastewater Resource Recovery Facilities.

A review of enhanced municipal wastewater treatment through energy savings and carbon recovery to reduce discharge and CO2 footprint

Municipal wastewater treatment that mainly performed by conventional activated sludge (CAS) process faces the challenge of intensive aeration-associated energy consumption for oxidation of organics and ammonium, contributing to significant directly/indirectly greenhouse gas (GHG) emissions from energy use, which hinders the achievement of carbon neutral, the top priority mission in the coming decades to cope with the global climate change. Therefore, this article aimed to offer a comprehensive analysis of recently developed biological treatment processes with the focus on reducing discharge and CO2 footprint. The biotechnologies including "Zero Carbon", "Low Carbon", "Carbon Capture and Utilization" are discussed, it suggested that, by integrating these processes with energy-saving and carbon recovery, the challenges faced in current wastewater treatment plants can be overcome, and a carbon-neutral even be possible. Future research should investigate the integration of these methods and improve anammox contribution as well as minimize organics lost under different scales.

Introducing bioflocculation boundaries in process control to enhance effluent quality of high-rate contact-stabilization systems

High-rate activated sludge (HRAS) systems suffer from high variability of effluent quality, clarifier performance, and carbon capture. This study proposed a novel control approach using bioflocculation boundaries for wasting control strategy to enhance effluent quality and stability while still meeting carbon capture goals. The bioflocculation boundaries were developed based on the oxygen uptake rate (OUR) ratio between contactor and stabilizer (feast/famine) in a high-rate contact stabilization (CS) system and this OUR ratio was used to manipulate the wasting setpoint. Increased oxidation of carbon or decreased wasting was applied when OUR ratio was <0.52 or >0.95 to overcome bioflocculation limitation and maintain effluent quality. When no bioflocculation limitations (OUR ratio within 0.52-0.95) were detected, carbon capture was maximized. The proposed control concept was shown for a fully automated OUR-based control system as well as for a simplified version based on direct waste flow control. For both cases, significant improvements in effluent suspended solids level and stability (<50-mg TSS/L), solids capture over the clarifier (>90%), and COD capture (median of 32%) were achieved. This study shows how one can overcome the process instability of current HRAS systems and provide a path to achieve more reliable outcomes. PRACTITIONER POINTS: Online bioflocculation boundaries (upper and lower limit) were defined by the OUR ratio between contactor and stabilizer (feast/famine). To maintain effluent quality, carbon oxidation was minimized when bioflocculation was not limited (0.52-0.95 OUR ratio) and increased otherwise. A fully automated control concept was piloted, also a more simplified semiautomated option was proposed. Wasting control strategies with bioflocculation boundaries improved effluent quality while meeting carbon capture goals. Bioflocculation boundaries are easily applied to current wasting control schemes applied to HRAS systems (i.e., MLSS, SRT, and OUR controls).

Up-concentration processes of organics for municipal wastewater treatment: New trends in separation

Carbon neutrality is a pressing goal for the whole society. Over 20% of municipality electrical energy on public utilities was consumed by the operation of wastewater treatment plants (WWTPs). Up-concentration of organic matters and maximum energy recovery is essential for a more sophisticated municipal wastewater management. Chemical coagulation and biological adsorption have been used to achieve efficient carbon capture, while separation is an overlooked step. It may lead to poor effluent quality, as well as consume most of the time and volume. The introduction of new driving forces, such as pressure and magnetism, significantly improved the retention rate and speed, respectively. In this paper, recent works were comprehensively reviewed and a horizontal comparison was conducted from aspects of separation speed, retention rate, concentrate characteristics and economic costs. This review also discussed the selection of technologies under different conditions. Finally, the practical application, fouling mitigation with considering the value of the concentrate, identification of unique concentrate characteristics, and the establishment of an evaluation system was suggested as core issues for future researches. This review will promote the development of an energy-efficient wastewater treatment system with up-concentration processes.

Towards more predictive clarification models via experimental determination of flocculent settling coefficient value

Improved settleability has become an essential feature of new wastewater treatment innovations. To accelerate adoption of such new technologies, improved clarifier models are needed to help with designing and predicting improvement in settleability. In general, the level of mathematics of settling clarifier models has gone far beyond the level of existing experimental methods available to support these models. To date, even for simple one-dimensional (1D) clarifier models, no experimental method has been described for flocculent settling coefficient (r_p). As a consequence, r_p cannot be considered as a sludge characteristic and is used as a calibration parameter to achieve observed effluent quality. In this study, we focused on the development of an empirical function based on a simple and practical experimental approach for the calculation of the r_p value from sludge characteristics. This approach provided a similar approach as currently taken for hindered settling coefficient calculations (Veslind equation) and allowed for the model to predict effluent quality, thus increasing the power of the 1D model. The threshold of flocculation (TOF), which describes the collision efficiency of particles, directly correlated with the effluent quality of the five tested activated sludge systems and was selected as experimental method. The proposed empirical function between TOF and r_p was validated for four years of validating data with five different sludge types operated under different operational conditions and configurations. The good effluent quality prediction with this approach brings us one step closer in making the clarification models more predictive towards effluent quality and clarifier performance.

Increasing oxygen transfer efficiency through sorption enhancing strategies

The link between aeration efficiency and biosorption capacity in water resource recovery facilities was extensively investigated, with special emphasis on wastewater characteristics and the development of strategies to maximize adsorption. Biosorption of oxygen transfer inhibitors (i.e., surfactants, colloidal, and soluble fractions) was examined by a series of pilot batch-scale experiments and full-scale studies. The impact of a sorption-enhancing strategy (i.e., bioaugmentation) deployed at full-scale over a five-year period was evaluated. Bench-scale experiments determined the inhibition coefficient (Ki) to measure the impact of surfactants and COD fractions as inhibitors of oxygen transfer efficiencies (αSOTE) in wastewater systems. The inhibition constant for surfactants Ki was found at 2.4 ± 0.4 mg L^-1 SDS while for colloidal material was at 14 ± 1 mg L^-1 (no inhibition for soluble fraction was found). Two enhancing biosorption configurations (i.e., contact stabilization and anaerobic selector) resulted in significant improvements in both aeration efficiency indicators (αSOTE) and surfactants removals. αSOTE improvements of 46% and 54% in comparison to conventional high rate activated sludge process (HRAS) were reported. Similarly, the removal of surfactants was increased by 27% and 56% using optimized enhancing-sorption strategies. Further analyses helped elucidate the underlying mechanisms of surfactants removal. Findings are expected to help full-scale applications increase their sorption potential as well as the concurrent aeration efficiency, which helps WRRFs to advance toward energy-positive wastewater treatments.

Effects of exogenous N-acyl-homoserine lactones on nutrient removal, sludge properties and microbial community structures during activated sludge process

This study investigated the effects of exogenous N-acyl-homoserine lactone (AHL) signal molecules, N-hexanoyl-_l-homoserine lactone (C6-HSL) and N-octanoyl-_l-homoserine lactone (C8-HSL), on treatment performance, sludge properties and microbial community structures in activated sludge systems. Results showed that the nitrification and denitrification efficiencies were enhanced with the addition of signal molecules. The particle size, irregularity, and internal mass transfer resistance of activated sludge flocs (ASFs) increased, primarily because dosing AHLs led to a content increase and chemical composition variation of extracellular polymeric substances (EPS) in sludge. Microbial analysis indicated an increase in both the bacterial richness and diversity of the systems. The relative abundances of the key functional groups, including bacteria related to C and N removal and EPS production, varied correspondingly. This study presents an insight into the comprehensive understanding of the effects of AHL-based quorum sensing on activated sludge treatment process.

Operational impacts of heavy metals on activated sludge systems: the need for improved monitoring

Biological nutrient removal is highly reliant on maintaining a heterogeneous, balanced, and metabolically active microbial community that can adapt to the fluctuating composition of influent wastewater and encompassing environmental conditions. Maintaining this balance can be challenging in municipal wastewater systems that sporadically receive wastewater from industrial facilities due to the impact of heavy metals and other contaminants on the microbial ecology of the activated sludge. A thorough understanding of the impacts of heavy metals on activated sludge and of practical monitoring options is needed to support decision-making at the wastewater utility level. This paper is divided into two parts. In the first part, the review explains what happens when heavy metals interact with activated sludge systems by highlighting biosorption and bioaccumulation processes, and when an activated sludge system switches from bioaccumulation to toxic shock. Here, it also summarizes the impacts of heavy metal exposure on plant performance. In the second part, the review summarizes practical approaches that can be used at the plant outside the realm of traditional toxicological bioassays testing to determine the possible impacts of influent heavy metal concentrations on the BNR process. These approaches include the following: monitoring operational parameters for major shifts; respirometry; microscopy; ATP; chemical analyses of heavy metals with a focus on synergistic impacts and inhibitory limits; and other novel approaches, such as EPS chemical analyses, molecular techniques, and quorum sensing.

The Impact of Local Hydrodynamics on High-Rate Activated Sludge Flocculation in Laboratory and Full-Scale Reactors

High rate activated sludge (HRAS) processes have a high potential for carbon and energy recovery from sewage, yet they suffer frequently from poor settleability due to flocculation issues. The process of flocculation is generally optimized using jar tests. However, detailed jar hydrodynamics are often unknown, and average quantities are used, which can significantly differ from the local conditions. The presented work combined experimental and numerical data to investigate the impact of local hydrodynamics on HRAS flocculation for two different jar test configurations (i.e., radial vs. axial impellers at different impeller velocities) and compared the hydrodynamics in these jar tests to those in a representative section of a full scale reactor using computational fluid dynamics (CFD). The analysis showed that the flocculation performance was highly influenced by the impeller type and its speed. The axial impeller appeared to be more appropriate for floc formation over a range of impeller speeds as it produced a more homogeneous distribution of local velocity gradients compared to the radial impeller. In contrast, the radial impeller generated larger volumes (%) of high velocity gradients in which floc breakage may occur. Comparison to local velocity gradients in a full scale system showed that also here, high velocity gradients occurred in the region around the impeller, which might significantly hamper the HRAS flocculation process. As such, this study showed that a model based approach was necessary to translate lab scale results to full scale. These new insights can help improve future experimental setups and reactor design for improved HRAS flocculation.

Carbon capture for blackwater: chemical enhanced high-rate activated sludge process

Blackwater has more benefits for carbon recovery than conventional domestic wastewater. Carbon capture and up-concentration are crucial prerequisites for carbon recovery from blackwater, the same as domestic wastewater. Both chemical enhanced primary treatment (CEPT) and high-rate activated sludge (HRAS) processes have enormous potential to capture organics. However, single CEPT is subject to the disruption of influent sulfide, and single HRAS has insufficient flocculation capacity. As a result, their carbon capture efficiencies are low. By combining CEPT and HRAS with chemical enhanced high rate activated sludge (CEHRAS) process, the limitations of single CEPT and single HRAS offset each other. The carbon mineralization efficiency was significantly influenced by SRT rather than iron salt dosage. An iron dosage significantly decreased chemical oxygen demand (COD) lost in effluent. Both SRT and iron dosage had a significant influence on the carbon capture efficiency. However, HRT had no great impact on the organic mass balance. CEHRAS allowed up to 78.2% of carbon capture efficiency under the best conditions. The results of techno-economic analysis show that decreasing the iron salt dosage to 10 mg Fe/L could promise profiting for blackwater treatment. In conclusion, CEHRAS is a more appropriate technology to capture carbon in blackwater.

Gaining deeper insights into the bioflocculation process occurring in a high loaded membrane bioreactor used for the treatment of synthetic greywater

In the present study, a high loaded membrane bioreactor (HL-MBR) operated at a hydraulic retention time (HRT) of 1.5 h, and three different sludge retention times (SRTs) in the range of 0.5-2 days, was used for the treatment of synthetic greywater. The chemical oxygen demand (COD) removal efficiency of the system was in the range 87-89% at all SRTs. Bioflocculation efficiency (defined as the percentage of suspended COD in the concentrate stream), COD bio-oxidation, total extracellular polymeric substances (EPS), tightly bound (TB) EPS and the ratio of EPS protein (EPS_p) to carbohydrate (EPS_c) increased when SRT was increased from 0.5 to 2 days. Sludge supernatant soluble microbial products (SMP) increased with increase in SRT from 0.5 to 2 days, while the effluent SMP was negligible. Particle size distribution analyses revealed a bimodal distribution at an SRT of 0.5 days, and normal distributions at other SRTs. Furthermore, depending on the value of the F/M ratio, different SRTs in the range of 0.5-2 days had either positive or negative effects on the mean particle size. Linear correlation analyses were performed using the data obtained during both transient and steady-state operations of the HL-MBR system. TB-EPS and EPS_p showed strong correlations with the biofloccultaion efficiency, whereas loosely bound (LB) EPS correlated with soluble COD removal. TB-EPS and EPS_c had negative correlations with the energy recovery potential of the system. The trend of change of parameters affecting membrane fouling intensity with SRT suggested that, in the range studied, the lowest rate of membrane fouling would be expected at SRT of 0.5 days.