High-rate Contact Stabilization 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.

Deciphering the Interplay of Sludge Age and Aeration on Reactor Performance and Microbial Dynamics in High-Rate Contact Stabilization

Deciphering the Interplay of Sludge Age and Aeration on Reactor Performance and Microbial Dynamics in High-Rate Contact Stabilization

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.

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).

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.

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.

High-rate activated sludge systems combined with dissolved air flotation enable effective organics removal and recovery

High-rate activated sludge (HRAS) systems typically generate diluted sludge which requires further thickening prior to anaerobic digestion (AD), besides the need to add considerable coagulant and flocculant for the solids separation. As an alternative to conventional gravitational settling, a dissolved air flotation (DAF) unit was coupled to a HRAS system or a high-rate contact stabilization (HiCS) system. The HRAS-DAF system allowed up to 78% removal of the influent solids, and the HiCS-DAF 67%. Both were within the range of values typically obtained for HRAS-settler systems, albeit at a lower chemical requirement. The separated sludge had a high concentration of up to 47 g COD L−1, suppressing the need of further thickening before AD. Methanation tests showed a biogas yield of up to 68% on a COD basis. The use of a DAF separation system can thus enable direct organics removal at high sludge concentration and with low chemical needs.

A-stage and high-rate contact-stabilization performance comparison for carbon and nutrient redirection from high-strength municipal wastewater

This study performed a parallel comparison of the A-stage (adsorption) and high-rate contact-stabilization (CS) technology for carbon and nutrient redirection, operating both systems at similar sludge retention time (SRT) of 0.16–0.3 d and treating high-strength raw wastewater. Overall at the average 0.22 d SRT condition, both A-stage and CS had similar carbon capture behavior (42–43%) and thus the similar potential for energy recovery. However, the A-stage had better effluent quality (67 mg VSS/L) through the growth of more heterotrophic biomass leading to increased oxidation (22% vs 18%), and increased fraction of nitrogen (26% vs 19%) and phosphorous (36% vs 30%) redirection compared to the CS. At biomass limited conditions and at lower SRT, CS maintained better performance, potentially through a better extracellular polymeric substance management under feast-famine conditions. Full-scale plant energy calculations based on this study results showed that chemically enhanced primary treatment (CEPT), A-stage, CS, primary treatment + CS and CEPT + CS could all lead to energy neutral plants as enough carbon can be redirected to generate the energy needed to support wastewater treatment. Given the superior performance of CS under lower loading or SRT limitation, the best energy gain (200%) could be potentially reached with a combination of CEPT and CS in series to enhance carbon capture up to 68% in combination with mainstream deammonification.

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.

Capture-Ferment-Upgrade: A Three-Step Approach for the Valorization of Sewage Organics as Commodities.

This critical review outlines a roadmap for the conversion of chemical oxygen demand (COD) contained in sewage to commodities based on three-steps: capture COD as sludge, ferment it to volatile fatty acids (VFA), and upgrade VFA to products. The article analyzes the state-of-the-art of this three-step approach and discusses the bottlenecks and challenges. The potential of this approach is illustrated for the European Union's 28 member states (EU-28) through Monte Carlo simulations. High-rate contact stabilization captures the highest amount of COD (66-86 g COD person equivalent-1 day-1 in 60% of the iterations). Combined with thermal hydrolysis, this would lead to a VFA-yield of 23-44 g COD person equivalent-1 day-1. Upgrading VFA generated by the EU-28 would allow, in 60% of the simulations, for a yearly production of 0.2-2.0 megatonnes of esters, 0.7-1.4 megatonnes of polyhydroxyalkanoates or 0.6-2.2 megatonnes of microbial protein substituting, respectively, 20-273%, 70-140% or 21-72% of their global counterparts (i.e., petrochemical-based esters, bioplastics or fishmeal). From these flows, we conclude that sewage has a strong potential as biorefinery feedstock, although research is needed to enhance capture, fermentation and upgrading efficiencies. These developments need to be supported by economic/environmental analyses and policies that incentivize a more sustainable management of our resources.

Reply for comment on “Bioflocculation management through high-rate contact-stabilization: A promising technology to recover organic carbon from low-strength wastewater by Rahman, A., Meerburg, F. A., Ravadagundhi, S., Wett, B., Jimenez, J., Bott, C., Al-Omari, A., Riffat, R., Murthy, S. and De Clippeleir, H. [Water Research 104 (2016) 485–496

Reply for comment on “Bioflocculation management through high-rate contact-stabilization: A promising technology to recover organic carbon from low-strength wastewater by Rahman, A., Meerburg, F. A., Ravadagundhi, S., Wett, B., Jimenez, J., Bott, C., Al-Omari, A., Riffat, R., Murthy, S. and De Clippeleir, H. [Water Research 104 (2016) 485–496

Comment on “Bioflocculation management through high-rate contact stabilization: A promising technology to recover organic carbon from low-strength wastewater by Rahman, A., Meerburg, F.A., Ravadagundhi, S., Wett, B., Jimenez, J., Bott, C., Al-Omari, A., Riffat, R., Murthy, S. and De Clippeleir, H. [Water Research 104 (2016) 485–496

Comment on “Bioflocculation management through high-rate contact stabilization: A promising technology to recover organic carbon from low-strength wastewater by Rahman, A., Meerburg, F.A., Ravadagundhi, S., Wett, B., Jimenez, J., Bott, C., Al-Omari, A., Riffat, R., Murthy, S. and De Clippeleir, H. [Water Research 104 (2016) 485–496

Impact of aerobic famine and feast condition on extracellular polymeric substance production in high-rate contact stabilization systems

The high-rate contact stabilization (CS) system is gaining back its popularity for recovering carbon from wastewater. This study investigated the role of extracellular polymeric substance (EPS) on bioflocculation improvement and carbon capture from municipal wastewater in pilot-scale and bench-scale CS systems. Results showed that a rapid increase in EPS was established from the famine stabilizer to the aerobic feast contactor, and this mechanism was responsible for improved bioflocculation, carbon capture efficiency (linear function) and effluent quality in CS systems. In contrast, these improvements were not observed under anaerobic contactor conditions, thus favoring application of aerobic contactor. The EPS production was driven by the high organic loading rate for high-strength wastewater and minimum stabilization time required to induce starvation condition for low-strength wastewater systems. Mechanistic understanding of the feast-famine regimes in CS system through this study, will aid future design and operation of CS systems for enhanced carbon recovery.

Potential of high-rate contact-stabilization for maximizing carbon redirection and capture compared to plug flow A-stage systems

Potential of high-rate contact-stabilization for maximizing carbon redirection and capture compared to plug flow A-stage systems

Bioflocculation management through high-rate contact-stabilization: A promising technology to recover organic carbon from low-strength wastewater

A series of pilot-scale studies were performed to compare conventional high-rate activated sludge systems (HRAS) (continuous stirred tank reactor (CSTR) and plug flow (PF) reactor configurations) with high-rate contact-stabilization (CS) technology in terms of carbon recovery potential from chemically enhanced primary treatment effluent at a municipal wastewater treatment plant. This study showed that carbon redirection and recovery could be achieved at short solids retention time (SRT). However, bioflocculation became a limiting factor in the conventional HRAS configurations (total SRT ≤ 1.2 days). At a total SRT ≤1.1 day, the high-rate CS configuration allowed better carbon removal (52–59%), carbon redirection to sludge (0.46–0.55 g COD/g CODadded) and carbon recovery potential (0.33–0.34 gCOD/gCODadded) than the CSTR and PF configurations (28–37% COD removal, carbon redirection of 0.32–0.45 g COD/g CODadded and no carbon harvesting). The presence of a stabilization phase (famine), achieved by aerating the return activated sludge (RAS), followed by low dissolved oxygen contact with the influent (feast) was identified as the main reason for improved biosorption capacity, bioflocculation and settleability in the CS configuration. This study showed that high-rate CS is a promising technology for carbon and energy recovery from low-strength wastewaters.

Live Fast, Die Young: Optimizing Retention Times in High-Rate Contact Stabilization for Maximal Recovery of Organics from Wastewater.

Wastewater is typically treated by the conventional activated sludge process, which suffers from an inefficient overall energy balance. The high-rate contact stabilization (HiCS) has been proposed as a promising primary treatment technology with which to maximize redirection of organics to sludge for subsequent energy recovery. It utilizes a feast-famine cycle to select for bioflocculation, intracellular storage, or both. We optimized the HiCS process for organics recovery and characterized different biological pathways of organics removal and recovery. A total of eight HiCS reactors were operated at 15 °C at short solids retention times (SRT; 0.24-2.8 days), hydraulic contact times (tc; 8 and 15 min), and stabilization times (ts; 15 and 40 min). At an optimal SRT between 0.5 and 1.3 days and tc of 15 min and ts of 40 min, the HiCS system oxidized only 10% of influent chemical oxygen demand (COD) and recovered up to 55% of incoming organic matter into sludge. Storage played a minor role in the overall COD removal, which was likely dominated by aerobic biomass growth, bioflocculation onto extracellular polymeric substances, and settling. The HiCS process recovers enough organics to potentially produce 28 kWh of electricity per population equivalent per year by anaerobic digestion and electricity generation. This inspires new possibilities for energy-neutral wastewater treatment.

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.