Contact Stabilization Process Research Articles

Improvement of organic carbon recovery from wastewater by changing water temperature in high-rate contact stabilization process

Improvement of organic carbon recovery from wastewater by changing water temperature in high-rate contact stabilization process

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.

Performance and Mechanism of Improved Soluble Organic Carbon Recovery from Municipal Wastewater through the Coagulation-Enhanced High-Rate Contact Stabilization Process

High-rate contact stabilization is promising to capture organic carbon from municipal wastewater; however, its engineering application is restricted due to its poor capability of removing soluble organic matter (particle size < 0.1 μm). In this work, the carbon capture efficiency and robustness of the coagulation-enhanced high-rate contact-stabilized process for treating municipal wastewater with a varied content of soluble organic matter was comprehensively evaluated. Results showed that the high-rate contact stabilization process did not perform well in the removal and capture of influent organic substances with a high proportion of soluble components. Adding coagulants into the above process favored organic carbon capture, whose recovery efficiency could reach up to 60% even under the 90% influent soluble chemical oxygen demand condition. Mechanism analysis revealed that the hydrolyzed product of the coagulant would adsorb dissolved components to form colloid and subcolloid particles, which was favored for further capture by sludge. The fitting result of the neutral community model indicated that both deterministic environmental factors and random processes influence the succession of sludge community. This work implied that the coagulation-enhanced high-rate contact stabilization process is feasible to recover organic carbon from municipal wastewater, which may shed a promising light for sustainable carbon control and management.

Efficient energy recovery from wastewater with high rate activated sludge process: oversights and misguidances

AbstractBackgroundThe major aim of the study was to define a scientific framework to evaluate energy recovery from domestic wastewater, based on model assessment of process stoichiometry and kinetics using relevant experimental data. The evaluation was tailored to reveal and explain all the oversights and misconceptions on the subject. A new approach was adopted to focus specifically on a paper by Liang et al. (2022) to better visualize and indicate scientific ways to remedy the key misguidances. The alternative appraisal utilized the latest available support in terms of the database for chemical oxygen demand (COD) fractionation and process kinetics.ResultsThe evaluation first demonstrated the inherent deficiencies of high rate activated sludge (HRAS) configurations such as the sequencing batch reactor and contact stabilization; then, it presented a new way to define excess sludge components in terms of fractions of the influent COD, indicating the major handicap for energy recovery and biomass loss in the effluent due to gravity settling. Effective energy conservation was computed to remain between 17% and 37% when the sludge age was reduced from 2.0 d down to 0.5 d.ConclusionsThe study offered conclusive proof that the contact stabilization process with gravity settling was, in fact, inherently unsuitable for energy conservation. Instead, the emerging scientific approach proposed a single‐volume HRAS system with membrane filtration replacing gravity settling. An equally valid suggestion was to replace anaerobic technology for energy recovery with thermochemical technologies, such as high‐rate pyrolysis, with a much higher energy recovery potential. © 2022 Society of Chemical Industry (SCI).

Sludge retention time effects on recovery of soluble organic matters via polyhydroxyalkanoate storage in contact stabilization process under feast/famine regime

AbstractBACKGROUNDContact stabilization (CS) can be used to capture organic matters from wastewater. However, CS is less efficient in capturing soluble organic matters (SOMs). Recovery of SOMs is required to minimize carbon emission and to maximize organic recovery from wastewater.RESULTSSOMs were recovered by applying a feast/famine (F/F) regime in a CS process. Sludge retention time (SRT) was found to regulate the recovery efficiency of SOMs. At a SRT of 4 d, 44.7% of the removed soluble chemical oxygen demand (SCOD) in wastewater was recovered and stored into intracellular polyhydroxyalkanoates (PHA), leading to 12.8% decrease of CO2 emission in comparison with the conventional activated sludge process. The storage of PHA in waste sludge harvested at a SRT of 4 d significantly increased the methane yield (187.3 ± 5.9 mL (g VSS)−1). The dominant bacteria performing SOM recovery in CS belonged to the genera Tessaracoccus, norank_f__Caldilineaceacea, unclassified_f__Microbacteriaceae and Flavobacterium.CONCLUSIONSThe study demonstrated that applying F/F regime in a CS process can recover SOMs via PHA storage. The regulation of SRT can enhance SCOD recovery. The study provides a solution to enhance SOM recovery from wastewater instead of being oxidized into CO2. © 2022 Society of Chemical Industry (SCI).Highlights SCOD can be recovered via PHA by applying F/F regime in CS mode. The optimum SRT of 4 d resulted 44.7% SCOD recovery via PHA storage. PHA‐accumulating bacteria belonged to four genera. Anaerobic digestion of sludge resulted in a 17.6% higher CH4 yield. CO2 emission decreased 12.8% compared with conventional activated sludge process.

Coagulation enhanced high-rate contact-stabilization process for pretreatment of municipal wastewater: Simultaneous organic capture and phosphorus removal

High-rate contact-stabilization is criticized as its unsatisfactory organic elimination performance and failure for the phosphorus removal, which greatly restricts its engineering implementation. In this work, coagulation enhanced high-rate contact-stabilization process was established to realize simultaneous carbon capture and phosphorus removal in a single treatment unit. Results showed that under a molar ratio of 2:1 (metal ion: P), effluent chemical oxygen demand and total phosphorus less than 50 and 0.50 mg·L-1 could be stably achieved, respectively. Under the ∼ 8% mineralization efficiency, approximate 80% of influent organic carbon was captured as sludge. Coagulation precipitation and bioflocculation both contributed to carbon removal. Over-dosage of coagulants would induce reactive oxygen species accumulation and sludge activity decline. Microbial abundance and redundancy analysis revealed that the dosed coagulants would cause a selective pressure and lead to a reduced microbial richness. For a practical application, an optimized dosing of mixed coagulants was suggested to maximize the carbon capture and phosphorus removal. This work may provide guidance on sustainable carbon management for municipal wastewater sector.

Contact stabilization process for hospital wastewater treatment: effects of colloidal organic matter

Abstract In this study, the treatability of hospital wastewater was investigated using a contact stabilization process on a laboratory scale. A detention time of one hour was selected for sludge settling and separation of treated effluent, and removal efficiency was measured at contact times of 30, 60, and 90 min, and stabilization times of 4.5 and 5.5 h. Based on the different detention times, 6 series of experiments were designed. Results showed that after an initial rapid COD removal in the first 30 min, COD values fluctuate in the time range of 30–90 min. However, in the case where COD values reduce in the second stage, this recovery is negligible; thus, the time of 30 min is considered as the optimal detention time for the contact reactor. Sludge volume index (SVI) values of 119.20 and 109.17 mL/g were obtained for stabilization times of 4.5 and 5.5 h, respectively. Therefore, the longer the stabilization time, the closer the SVI is to 100 mL/g. Moreover, lower settled sludge volume (SSV) value at 5.5 h of stabilization shows better characteristics compared to 4.5 h of stabilization. Furthermore, COD removal efficiency at the optimum contact time is higher when 5.5 h is selected for stabilization.

Maximizing energy recovery from wastewater via bioflocculation-enhanced primary treatment: a pilot scale study

ABSTRACT Anaerobic digestion of municipal sewage sludge is widely used for harvesting energy from wastewater organic content. The more organic carbon we can redirect into the primary sludge, the less energy is needed for aeration in secondary treatment and the more methane is produced in anaerobic digesters. Bioflocculation has been proposed as a promising separation technology to maximize carbon capture in primary sludge. Thus far, only limited data on bioflocculation are available under real conditions, i.e. from pilot-scale reactors treating raw sewage. Moreover, no study has discussed yet the influence of bioflocculation on denitrification potential of sewage. Therefore, we performed bioflocculation of raw sewage in high-rate contact stabilization process in pilot-scale to investigate maximal primary treatment efficiency. During 100 days of operation at sludge retention time of only 2 days, the average removal efficiencies of chemical oxygen demand (COD), suspended solids and total phosphorus were 75%, 87% and 51%, respectively, using no chemicals for precipitation. Up to 76% of incoming COD was captured in primary sludge and 46% for subsequent anaerobic digestion, where energy recovery potential achieved 0.33–0.37 g COD as CH4 per g COD of influent. This study showed in real conditions that this newly adapted separation process has significant benefits over chemically enhanced primary treatment, enabling sewage treatment process to overcome energy self-sufficiency.

Which activated sludge configurations qualify for maximizing energy conservation – Why?

AbstractBACKGROUNDThis paper aimed to provide a critical appraisal on maximizing sludge generation and energy conservation in high‐rate activated sludge (AS) configurations. The role of gravity settling and the positive attributes of high rate membrane bioreactors were emphasized. The appraisal covered data reported in the literature on 40 different experiments testing high‐rate AS configurations for sludge generation and energy conservation.RESULTSIn systems with gravity settling, effluent chemical oxygen demand (COD) was higher than 125 mg L−1 in 60% of the experiments, with similarly high effluent soluble COD values. In high‐rate MBR systems, permeate COD fluctuated around 12–18 mgCOD L−1; in gravity systems, COD loss in the effluent was higher than 200 mg L−1 in 17 runs (65%) and 300 mg L−1 in 12 runs (46%). Corresponding observed yields were only 9–43%. Membrane systems consistently yielded much higher observed yields of 50–60%.CONCLUSIONFor energy recovery, AS systems require a major transition, reducing the sludge age to 1.0–2.0 d; only high‐rate systems are capable of maximizing energy conservation. This argument does not apply to the contact stabilization process, which is inherently unsuitable for energy recovery. A transition is also needed to replace gravity settling by membrane systems for effective control of particulate and soluble COD components. © 2018 Society of Chemical Industry

High-Rate Contact Stabilization Process-Coupled Membrane Bioreactor for Maximal Recovery of Organics from Municipal Wastewater

The high-rate activated sludge (HRAS) process is being studied for the removal and recovery of organics with short solids retention time (SRT) from wastewater, facilitating energy recovery by the subsequent anaerobic digestion process. In the present study, the feasibility of a novel high-rate contact stabilization (HRCS) process coupled with a membrane bioreactor (MBR) was investigated as a HRAS technique to harvest organics compared to a high-loaded MBR (HL-MBR) process treating the same sewage. Results showed that higher chemical oxygen demand (COD) removal efficiency and better bioflocculation performance were obtained using HRCS-MBR compared with HL-MBR with SRTs from 0.5 to 1.8 days. The increased bound extracellular polymeric substances content in the contactor was responsible for the improved biosorption and bioflocculation performance in the HRCS-MBR configuration. At an optimal SRT of 1.2 days, incoming organics of 47.5% and 40.5% were harvested in concentrate for HRCS-MBR and HL-MBR. These harvested organics from the concentrate per liter from HRCS-MBR and HL-MBR produced 4.28 × 10−3 and 3.72 × 10−3 kWh of electricity, respectively. The clear advantage of fouling control for HRCS-MBR was determined because of significantly lower concentrations of colloidal materials and soluble microbial products in the concentrate compared with HL-MBR. Therefore, HRCS-MBR holds promise for organics recovery and sustainable wastewater treatment.

A novel modeling approach for evaluating microbial mechanism and design of contact stabilization process

AbstractBACKGROUNDThe paper offers a novel interpretation of microbial mechanisms and evaluates the assets of contact stabilization (CS) using multi‐component modelling. A model structure re‐defining microbial processes is adopted, where the function of the contact tank was limited to the utilization of soluble substrate fractions and that of the stabilization tank essentially involved endogenous decay and removal of the adsorbed particulate substrate.RESULTSThe model was used to simulate the microbial behaviour and performance of CS in comparison with a conventional activated sludge system (CAS), at an SRT range between 6 and 15 d. The results were confirmed by a stoichiometric description of the process using relevant mass balance relationships, which identified the role of major parameters in system behaviour and performance. A rational process design procedure was proposed based on modelling and process stoichiometry.CONCLUSIONThe CS process achieved effective carbon removal with a much smaller footprint and/or aeration volume with respect to CAS, due to its ability to work at significantly higher volumetric organic loadings; its flexibility to be operated at much higher SRT compared with CAS with the same HRT, allowed minimizing sludge production and sustaining a microbial composition also supporting nitrification. However, process stoichiometry reflected that the CS process, despite its significant advantages, should not be considered a suitable candidate for maximizing sludge harvest and energy recovery. © 2017 Society of Chemical Industry

An optimized biological approach for treatment of petroleum refinery wastewater

Excessive sludge production is cost prohibitive and a major concern in biological treatment of petroleum refinery wastewater. Thus, it is essential to identify operational conditions for treatment systems that result in low sludge production of the system, while maintaining its high removal performance. This study assesses the feasibility of using contact stabilization process for secondary treatment of refinery wastewater through a step by step analysis. The mixed liquor dissolved oxygen (DO) and the rate of activated return sludge (RS) were selected as operational parameters governing the optimum performance of the system. A total number of 32 individual experiments were conducted on a pilot plant under four different aeration phases (DO) and eight RS percentages. The analyses investigated the biokinetic coefficients, observed removal efficiencies, and the amount of produced sludge to identify suitable operational conditions. The results indicated that the system had an optimum performance under applied aeration of 3.7mg oxygen per liter of mixed liquor and 46% return sludge. This operational combination resulted in COD removal efficiency of 78% with daily biomass production of 1.42kg/day.

Toward energy-neutral wastewater treatment: A high-rate contact stabilization process to maximally recover sewage organics

The conventional activated sludge process is widely used for wastewater treatment, but to progress toward energy self-sufficiency, the wastewater treatment scheme needs to radically improve energy balances. We developed a high-rate contact stabilization (HiCS) reactor system at high sludge-specific loading rates (>2kgbCODkg−1TSSd−1) and low sludge retention times (<1.2d) and demonstrate that it is able to recover more chemical energy from wastewater organics than high-rate conventional activated sludge (HiCAS) and the low-rate variants of HiCS and HiCAS. The best HiCS system recovered 36% of the influent chemical energy as methane, due to the combined effects of low production of CO2, high sludge yield, and high methane yield of the produced sludge. The HiCS system imposed a feast-famine cycle and a putative selection pressure on the sludge micro-organisms toward substrate adsorption and storage. Given further optimization, it is a promising process for energy recovery from wastewater.

Optimization method for the treatment of Tehran petroleum refinery wastewater using activated sludge contact stabilization process

ABSTRACTThis paper describes the results of side by side large scale testing of contact stabilization and extended aeration activated sludge processes on the petroleum refinery wastewater at Tehran (Iran) oil refinery by using continuous large scale experiments during the three month investigation period. The process optimization for performance evaluation was studied. Sampling results indicated that the contact stabilization process shows more effective performance than extended aeration process in removing chemical oxygen demand (COD) and biological oxygen demand (BOD), and also the mixed liquor volatile suspended solids were less in contact stabilization as compared to those in the extended aeration process. The excess sludge production was affected by the amount of sludge loading rate and aeration. The applied aeration to the mixed liquor and the sludge recycle rate were found to be critical parameters in the successful optimization of the contact stabilization process. The appropriate optimized opera...

Modeling and Predicting Biological Performance of Contact Stabilization Process Using Artificial Neural Networks

In this paper, the microfauna distribution data of a contact stabilization process were used in a neural network system to model and predict the biological activity of the effluent. Five uncorrelated components of the microfauna were used as the artificial neural network model input to predict the dehydrogenase activity of the effluent (DAE) using back-propagation and general regression algorithms. The models’ optimum architectures were determined for the back-propagation neural network (BPNN) model by varying the number of hidden layers, hidden transfer functions, test set size percentages, and initial weights. Comparison of the two model prediction results showed that the genetic general regression neural network model demonstrated the ability to calibrate the multicomponent microfauna, and yielded reliable DAE close to that resulting from direct experimentation, and thus was judged superior to BPNN models.

Performance-Based Characterization of a Contact Stabilization Process for Slaughterhouse Wastewater

This paper evaluates the effluent treatment plant of a slaughterhouse in Hawalli City, Kuwait processing 1100 heads of livestock a day. Results indicated that the proposed process effectively reduces pollution potential of slaughterhouse wastewater. Influent Chemical Oxygen Demand (COD) ranged from 3335 to 7580 mg L−1, of which approximately 30% were in the form of suspended solids (SS). Removal efficiency was 77% for soluble COD and 82% for insoluble COD, at a volumetric load of 1.8 kg COD m−3 d−1. Values obtained for the biokinetic coefficients, floc uptake (FU), substrate removal efficiency (SRE), specific reaction rate (R R ), maximum reaction rate (R m ) and Yield (Y) of the contact tank were higher than the range of values reported for other continuously fed activated sludge systems. In contrast to the contact tank, the aeration basin biokinetic coefficients were within the range of values reported. Contact process testing demonstrated that controlling solid recirculation to maintain a contact loading of about 120–150 mg COD g−1 VSS in the contact tank generally resulted in high SRE, R R , R m , and also in good settleability as indicated by SVIs being consistently below 150 mL g−1. In the other hand, higher contact loadings of more than 150 mg COD g−1 VSS, resulted in a significant deterioration in SRE, RR , Rm , and SVIs.

Full-Scale Demonstration of Improvement in Aeration Efficiency

This paper describes the results of side-by-side full-scale aeration testing of a plug-flow process and a modified contact stabilization process incorporating an anaerobic selector at the wastewater treatment facility in Fredonia, N.Y. Over 40 tests were completed utilizing the off-gas technique during the 2-month investigation period (summer of 1995). Compared to the plug-flow process, the modified contact stabilization process with internal sludge recycle was shown to have higher α values and to require less blower energy consumption when the selector operation was properly controlled. Dissolved oxygen concentration, selector COD concentration, and internal recycle sludge levels were found to be critical parameters in the successful operation of the modified process. Higher internal recycle sludge levels allowed the plant to run at more stable operating conditions in terms of the oxygen transfer efficiency, α, and sludge volume index.

Use of contact tank to enhance denitrification in oxidation ditches

Poor denitrification in a Pasveeer oxidation ditch is attributed to a lack of carbon sources available in the anoxic zone as it is essential to maintain a high C/N ratio for denitrification. Influent of sewage directly into the anoxic zone is not useful to maintain a high C/N ratio. The adsorptive capacity of activated sludge can rapidly increase the C/N ratio. Similar to a contact-stabilization process, a contact tank can be combined with the Pasveer ditch; it provides contact time (zone) between raw sewage and return sludge before entering the ditch. In principle, insoluble organic substrate can be easily adsorbed onto the floc surfaces and enmeshed in the floc structure at a short retention time. After the contact, mixed influent is introduced into the anoxic zone. As a result, a high C/N ratio is obtained which enhances denitrification. Using this set up, the Pasveer ditch was operated. The experimental results show that the efficiency of denitrification has been enhanced from 45 to 83% for NO−3-N removal. The corresponding denitrification capacity of the sludge is increased by 240%. The contact tank has also the same principle as a ‘selector’ to control bulking sludge caused by filamentous bacteria. The SVI data and microscopic examination indicated improved settleability of the sludge. Further enhancement of denitrification needs an exact control of the dissolved oxygen level in the ditch and/or a concentration increase of denitrifying microorganisms.

Use of contact tank to enhance denitrification in oxidation ditches

Poor denitrification in a Pasveeer oxidation ditch is attributed to a lack of carbon sources available in the anoxic zone as it is essential to maintain a high C/N ratio for denitrification. Influent of sewage directly into the anoxic zone is not useful to maintain a high ON ratio. The adsorptive capacity of activated sludge can rapidly increase the C/N ratio. Similar to a contact-stabilization process, a contact tank can be combined with the Pasveer ditch; it provides contact time (zone) between raw sewage and return sludge before entering the ditch. In principle, insoluble organic substrate can be easily adsorbed onto the floc surfaces and enmeshed in the floc structure at a short retention time. After the contact, mixed influent is introduced into the anoxic zone. As a result, a high C/N ratio is obtained which enhances denitrification. Using this set up, the Pasveer ditch was operated. The experimental results show that the efficiency of denitrification has been enhanced from 45 to 83% for NO−3−N removal. The corresponding denitrification capacity of the sludge is increased by 240%. The contact tank has also the same principle as a ‘selector‘ to control bulking sludge caused by filamentous bacteria. The SVI data and microscopic examination indicated improved settleability of the sludge. Further enhancement of denitrification needs an exact control of the dissolved oxygen level in the ditch and/or a concentration increase of denitrifying microorganisms.

Regeneration of activated sludge under oxic and anoxic conditions

Regeneration of biomass cultivated by synthetic soluble organic substrate under oxic and anoxic conditions was studied by following the maximum substrate removal rates r x, m during the endogenous phase. Two semicontinuously operated units simulating contact stabilization process were operated. In one of the units endogenous anoxic period was established after nitrification was completed. The regeneration process was connected with an increase in r x, m values. The increase was very similar under both oxic and anoxic conditions. The comparison of the contact stabilization process with oxic and combined oxic-anoxic endogenous phase has shown the following: rates of exogenous substrate removal and nitrification were the same, in ox-anox system nitrogen was completely eliminated and oxygen consumption was reduced, the rate of regeneration was the same in both systems. Combined oxic-anoxic regeneration phase can be used as a modification of the contact stabilization process for soluble organic substrate. In addition, during the endogenous phase kinetic constants r x, m , saturation coefficient K 5, and biomass yield coefficient Y have significantly increased. The problem of actual Y value determination is discussed.