Oxic-settling-anaerobic System Research Articles

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.

Operational strategies enhancing sewage sludge minimization in a combined integrated fixed-film activated sludge – oxic settling anaerobic system

A lab-scale integrated fixed-film activated sludge (IFAS) reactor was mplemented with the oxic-settling anaerobic (OSA) cycle for reducing sewage sludge production through the addition of an anoxic/anaerobic sludge holding tank (SHT) along the sludge recycle line. The IFAS-OSA system was operated under the different hydraulic retention time (HRT) in the SHT (HRTSHT) of 12 h and 6 h, at an oxidation-reduction potential (ORP) < −91 mV and solid retention time (SRT) between 39 and 126 d. Furthermore, the effect of temperature increase in the SHT (TSHT) from ambient (19.8–25.6 °C) to mesophilic (35 °C) conditions was investigated. The system performances were monitored in terms of sludge minimization and dewaterability efficiencies as well as carbon and nutrients reduction. The observed sludge yield (Yobs) for the IFAS system was 0.37(±0.06) mg VSS/mg COD. After OSA implementation Yobs decreased by 32% and 46–65% at HRTSHT of 12 h and 6 h, respectively, indicating that prolonged exposure to anoxic/anaerobic conditions was not beneficial for sludge reduction. The lowest Yobs of 0.09(±0.05) mg VSS/mg COD (76% lower than that in the IFAS system) was obtained at an HRTSHT of 6 h and when TSHT was set at 35 °C. OSA implementation did not affect COD and NH4+ oxidation of the IFAS system (90–96% and 99%, respectively) and improved total nitrogen (TN) reduction (31–53%) due to improved denitrification in the SHT. On the contrary, sludge dewaterability worsened following OSA implementation, which was linked to the increased levels of exopolymeric substances in the suspended biomass.

Insights on mechanisms of excess sludge minimization in an oxic-settling-anaerobic process under different operating conditions and plant configurations

In the present research, insights about the mechanisms of excess sludge minimization occurring in an oxic-settling-anaerobic (OSA) were provided. The investigation involved two systems operating in parallel. In particular, a conventional activated sludge (CAS) system as control and a system implementing the OSA process both having a pre-denitrification scheme were considered. Five periods (P1–P5) were studied, during which several operating conditions and configurations were tested. Specifically, the hydraulic retention time (HRT) in the anaerobic reactor of the OSA system (P1 8 h, P2–P3 12 h, P4 8 h, P5 12 h) and the return sludge from the anaerobic to the anoxic (scheme A) (P1–P2) or aerobic (scheme B) mainstream reactors (P3–P5) were investigated. The results highlighted that the excess sludge production in the OSA was lower in all the configurations (12–41%). In more detail, the observed yield (Yobs) was reduced from 0.50-0.89 gTSS gCOD−1 (control) to 0.22 -0.34 gTSS gCOD−1 in the OSA process. The highest excess sludge reduction (40%) was achieved when the OSA was operated according to scheme B and HRT of 12 h in the anaerobic reactor (P3). Generally, scheme A enabled the establishment of cell lysis and extracellular polymeric substances (EPS) destructuration, leading to a worsening of process performances when high anaerobic HRT (>8 h) was imposed. In contrast, scheme B enabled the establishment of maintenance metabolism in addition to the uncoupling metabolism, while cell lysis and EPS destruction were minimized. This allowed obtaining higher sludge reduction yield without compromising the effluent quality.

Assessment of sludge reduction and microbial dynamics in an OSA process with short anaerobic retention time

Abstract This study operated for 160 days and evaluated sludge minimization using an oxic-settling-anaerobic (OSA) system fed with real wastewater. At first, a pilot-scale conventional activated sludge (CAS) system was used to perform conventional wastewater treatment over a course of 60 days. Next, the CAS system was coupled with a sludge holding tank (SHT) in the sludge recirculation line to employ the OSA system, using a short 12h-hydraulic retention time (HRT) over a course of 100 days. The observed sludge yield ( Y obs ) dropped from 0.33 (CAS) to 0.19 (OSA) kg TSS kg−1 COD, ensuring a reduction of 43% of sludge in the OSA system. Better wastewater treatment performance was achieved using the OSA system (BOD5: 87%, TKN: 92%, NH 4 + -N: 94%) when compared with the CAS system (BOD5: 76%, TKN: 74%, NH 4 + -N: 78%), considering the organic matter and nitrogen removal rates. Moreover, the enrichment of anaerobic and hydrolyzing communities in the SHT (mainly Pedobacter and Acidovorax) enhanced the COD and nitrogen hydrolysis, which therefore improved the autotrophic metabolism in the aeration tank.

Combination of the OSA process with thermal treatment at moderate temperature for excess sludge minimization

This study investigated the chance to couple the conventional Oxic Settling Anaerobic (OSA) process with a thermic treatment at moderate temperature (35 °C). The maximum excess sludge reduction rate (80%) was achieved when the plant was operated under 3 h of hydraulic retention time (HRT). Compared with the conventional OSA system, the thermic treatment enabled a further improvement in excess sludge minimization of 35%. The observed yield coefficient decreased from 0.25 gTSS gCOD−1 to 0.10 gTSS gCOD−1 when the temperature in the anaerobic reactor was increased to 35 °C, despite the lower HRT (3 h vs 6 h). Moreover, the thermic treatment enabled the decrease of filamentous bacteria, thereby improving the sludge settling properties. The thermic treatment enhanced the destruction of extracellular polymeric substances and the increase of endogenous decay rate (from 0.64 d−1 to 1.16 d−1) that reduced the biomass active fraction (from 22% to 4%).

Potential for simultaneous nitrogen removal and sludge reduction of the oxic-settling-anaerobic process operated as a dual fed sequencing batch reactor

The objective of the study was to explore the impact of different operation strategies on the performance of the Oxic-Settling-Anaerobic (OSA) process. A sequencing batch reactor was selected as the aerobic (oxic) unit of the OSA system and it was operated as a dual mixing phase with step feeding in order to optimize simultaneous nitrogen removal and sludge minimization. For this purpose, the effect of COD/N ratio, filling pattern and the fraction of anaerobic period was investigated on the performance of a laboratory-scale OSA system fed with synthetic wastewater (peptone mixture) approximating the characteristics of sewage. In nine consecutive experimental stages, each sustaining different metabolic processes and biochemical reactions, the sludge reduction potential of the OSA system was not impaired, maintaining an average observed yield value of 0.18 g VSS/g COD, which was approximately half the level observed in the reference reactor. Similarly, the OSA scheme of operation did not affect or alter the fundamental mechanisms of biological nitrogen removal, where system performance could be modified and optimized the same way as in a conventional activated sludge configuration. Furthermore, the OSA system maintained a higher level of biomass concentration compared with the reference reactor. This observation confirmed the previous analyses that the continuous biomass inflow from the side-stream anaerobic bioreactor was the reason to establish the microbial mechanism that resulted in a much lower sludge generation, regardless of the biochemical processes taking place in the reactor.

Incorporating the oxic‐settling‐anaerobic (OSA) process into an anoxic–oxic system for sewage sludge reduction and nutrient removal

The production of sludge in biological wastewater treatments is a growing problem worldwide as it entails economic, environmental, and health problems. The oxic‐settling‐anaerobic (OSA) process is one of the techniques that have been developed to reduce the sludge produced in the activated sludge process (ASP). This article reports the feasibility of the application of the OSA process in a nutrient removal system, specifically, in an anoxic–oxic (A) process. The results reveal TN removal in the A + OSA system of 68.84% and 69.91% (for 7 and 11 h of sludge anaerobic exposure time, SAET, respectively) versus 38.02% and 32.47% in the OSA system with similar SAET. The percentage sludge reductions obtained were, respectively, 22.41% and 31.93% in the OSA system and 23.53% and 32.49% in the A + OSA system compared to the sludge produced in the conventional ASP. The study confirms the suitability of the application of the OSA process in a nutrient removal system, as nutrient removal performances are not affected, while at the same time achieving higher sludge reduction. © 2017 American Institute of Chemical Engineers Environ Prog, 37: 1068–1074, 2018

Behavior of activated sludge systems with an active heterotrophic biomass inflow – a novel perspective for sludge minimization

AbstractBACKGROUNDThis study evaluated the microbial behavior of activated sludge systems operated with an active heterotrophic biomass inflow. The evaluation focused on the possibility of sludge minimization. Thus, related process stoichiometry was derived on the basis of mass balance for essential parameters.RESULTSStoichiometric relationships showed that the reactor holds a larger amount of biomass without the controlling impact of the sludge age, which decreases the magnitude of microbial growth compared with endogenous decay. Consequently, the observed yield, YOBS becomes much lower than what may be observed in conventional activated sludge systems. This stoichiometry exactly defines the microbial basis of the OSA (oxic/settling/anaerobic) system and identifies the interchange stream connecting the anaerobic reactor to the aerobic main reactor as the source of the active biomass inflow. Therefore, biomass inflow, largely overlooked in related studies, offers a novel microbial perspective for the merit of the OSA process in minimizing sludge generation.CONCLUSIONThe impact of lower YOBS, while explaining lower active biomass production, cannot be extrapolated to cover inert particulate COD fractions, namely a similar reduction in the initial inert COD and the residual particulate microbial products. Future studies should be directed to explore the fate of residual particulate compounds. © 2017 Society of Chemical Industry

Preliminary evaluation of sludge minimization by a lab-scale OSA (oxic-settling-anaerobic) system

Preliminary evaluation of sludge minimization by a lab-scale OSA (oxic-settling-anaerobic) system

Simultaneous in-situ sludge reduction and nutrient removal in an A2MO-M system: Performances, mechanisms, and modeling with an extended ASM2d model

Among the existing in-situ sludge reduction processes, the oxic-settling-anaerobic (OSA) process is of particular interest because it has shown significant sludge reduction with several advantages. However, an ideal process for practical application must simultaneously incorporate effluent quality with sludge reduction. In this study, an improved OSA system, the stage-aerated anaerobic, anoxic, micro-aerobic, and oxic system combining a micro-aerobic starvation tank (abbreviated as A2MO-M system) was developed. Compared with OSA3# (hydraulic retention time (HRT) of 12 h), the A2MO-M2# system with optimized HRT of 9 h yielded almost 16.3% less sludge. The average total nitrogen (87.3%) and total phosphorus (91.9%) removal efficiencies in A2MO-M2# were 20.6 and 42.2% higher than those in OSA3#. Investigation of the mechanisms of sludge reduction revealed that, except for the main factors of energy uncoupling metabolism (16.7%) and sludge decay (21.2%), enrichment of slow-growing bacteria and lysis-cryptic growth metabolism analyzed by high-throughput 454 pyrosequencing were shown to contribute to sludge reduction in the A2MO-M system. On the basis of effluent organic matters (EfOM) measurements, soluble microbial products (SMP) were the major components in EfOM; and different reduction-oxidation (redox) potentials controlled in the OSA and A2MO-M systems led to different SMP formation mechanisms. To explore the mechanism and kinetics of SMP formation under different redox potentials, three new components (SUAP, SBAP, and XEPS) were integrated in an extended ASM2d model. Experimental and modeling results revealed that biomass-associated products (BAP) supported a substantial population of SMP that were quite sensitive to different redox potentials. The extended ASM2d model further illustrated that more BAP produced in the alternating anaerobic and aerobic conditions in the OSA system adversely affected its effluent quality.

Effect of sludge reduction in oxic-settling-anaerobic (OSA) systems with different anaerobic hydraulic retention times (HRTs)

AbstractThe performance of an oxic-settling-anaerobic (OSA) system in terms of sludge reduction was investigated for various anaerobic hydraulic retention times (HRTs) (7 d, 5 d, 3 d, 1 d, 12 h, and 6 h) for a sequencing batch reactor (SBR)-OSA system. Compared to the traditional SBR system, the rates of sludge reduction in the 7d, 5d-1, 5d-2, 3d, and 1d OSA systems were observed to be 66.6, 57.7, 52.5, 50.0, and 48.5%, respectively. In terms of the apparent yield and actual yield of sludge, the OSA systems with longer HRTs exhibited lower sludge yields and better sludge reduction performance. The systems with an HRT > 1 d achieved a considerable increase in sludge reduction, whereas those with an HRT < 1 d did not. The anaerobic HRT was observed to be an important factor in determining the sludge reduction of OSA systems.

Microbial community structure of anoxic–oxic-settling-anaerobic sludge reduction process revealed by 454-pyrosequencing

To understand process performance and microbial community structure in oxic-settling-anaerobic (OSA) sludge reduction process, an anoxic–OSA (A+OSA) system and a conventional anoxic–oxic (AO) system were operated for 200days. Both systems were almost equally effective in the removal of COD and ammonium nitrogen, but A+OSA system showed higher nitrogen removal efficiency owing to denitrification and the release of soluble organic matters in the sludge holding tank (SHT). A+OSA process reduced sludge production by 30.4% with observed sludge yield of 0.179gSS/gCOD. Pyrosequencing results revealed that although core populations were shared with highly functional organization, the relative abundance and stability of microbial communities in A+OSA system were higher than AO system. Classes Anaerolineae and Actinobacteria responsible for hydrolysis and fermentation of organic matters were enriched in SHT and played an important role in sludge reduction of A+OSA process. Specific comparison down to the genus level revealed the enrichment of anaerobic fermentative bacteria (Azospira, Propionivibrio and Sulfuritalea) and slow growers (Trichococcus) in A+OSA system.

Performance of biological phosphorus removal and characteristics of microbial community in the oxic-settling-anaerobic process by FISH analysis

Performance of biological phosphorus removal in the oxic-settling-anaerobic (OSA) process was investigated. Cell staining and fluorescent in situ hybridization (FISH) were used to analyze characteristics and microbial community of sludge. Experimental results showed that phosphorus removal efficiency was near 60% and the amount of biological phosphorus accumulation in aerobic sludge of the OSA system was up to 26.9 mg/g. Biological phosphorus removal efficiency was partially inhibited by carbon sources in the continuous OSA system. Contrasted to the OSA system, biological phosphorus removal efficiency was enhanced by 14% and the average total phosphorus (TP) contents of aerobic sludge were increased by 0.36 mg/g when sufficient carbon sources were supplied in batch experiments. Staining methods indicated that about 35% of microorganisms had typical characteristics of phosphorus accumulating organisms (PAOs). FISH analysis demonstrated that PAOMIX-binding bacteria were predominant microbial communities in the OSA system, which accounted for around 28% of total bacteria.

Mechanism on minimization of excess sludge in oxic-settling-anaerobic (OSA) process

The oxic-settling-anaerobic (OSA) process is a promising wastewater treatment technique for efficiently reducing sludge production and improving the stability of process operation. In this paper, the possible factors of sludge reduction such as sludge decay, uncoupled metabolism, and anaerobic oxidation with low sludge production were discussed in the OSA process. It has been confirmed that sludge decay is the decisive cause in the OSA process, accounting for 66.7% of sludge production reduction. Sludge decay includes hydrolysis and acidogenesis of dead microorganisms and particle organic carbon adsorbed in sludge floc and endogenous metabolism. By batch experiments, it has been proven that there is energetic uncoupling in the OSA system since microorganisms were exposed to alternative anaerobic and aerobic environment. It accounts for about 7.5% of sludge production reduction. Soluble chemical oxygen demand (SCOD) released from the anaerobic sludge tank in the OSA process was used as the substrate for cryptic growth. The substrate was used for anoxic denitrifying, anaerobic phosphorus release, sulfate reduction, and methane production. These anaerobic reactions in the sludge anaerobic tank have lower sludge production than in the aerobic oxidation when equivalent SCOD is consumed, which may lead to approximately 23% of sludge reduction in the OSA process. It has been concluded that multiple causes resulted in the minimization of excess sludge in the OSA system. The microbial community structure and diversity of sludge samples from the CAS (conventional activated sludge) and OSA systems were investigated by 16 SrDNA PCR-DG-DGGE (polymerase chain reaction-double gradient-denaturing gradient gel electrophoresis). DGGE profile and cluster analysis showed more abundant species in the OSA system contrasting to microbial communities in the CAS system.

The Aspect of Energetic Uncoupling of Microbial Growth in the Activated Sludge Process - OSA System

A practical application of the concept of uncoupling between catabolism and anabolism during microbial metabolism has been studied in the case of a modified activated sludge system, called OSA (Oxic-Settling-Anaerobic). The OSA system consisted of an oxic completely mixed tank, followed by a settling tank and an anaerobic tank, situated in the returned sludge circuit of the OSA system. The periodic passageway of facultative aerobic activated sludge microorganisms through the anaerobic zone created conditions of uncoupled growth, indicated by ATP stock depletion and resulted in a consecutive reduction of activated sludge production.

The case of both energetic uncoupling and metabolic selection of microorganisms in the OSA activated sludge system

Abstract A modified activated sludge system with anaerobic treatment of returned sludge, named OSA (Oxic‐Settling‐Anaerobic) was studied. The main objective of the research was to create conditions of a physiological stress on microbial growth, resulting in uncoupling of catabolic processes from anabolic ones. This phenomenon, largely described in the literature, is accompanied by energy wastage resulting in biomass yield reduction. The experimental results obtained with a mixed culture (activated sludge) fed on synthetic substrate showed that the anaerobic conditions exerted a kind of selection pressure on microbial metabolism of the activated sludge microorganisms, resulting in metabolic selection of the species being able to favour catabolic pathways. These bacteria, capable of storing polyphosphates under oxic conditions and using them as energy reserves under anaerobic conditions are called poly‐P bacteria. The OSA mixed culture contained about 50 to 60% of these poly‐P bacteria. It has been concluded that the insertion of an anaerobic zone into the returned sludge circuit changed both composition and behaviour of activated sludge microorganisms. The typical phenomena observed in the OSA system were enhanced phosphate removal, reduction of the excess activated sludge production and the suppression of activated sludge filamentous bulking.