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

Post-treatment of high-rate activated sludge effluent via zeolite adsorption and recovery of ammonium-nitrogen as alternative fertilising products

This study investigates the potential to connect nutrient flows between wastewater treatment and agriculture through a two-stage nitrogen (N) recovery system composed of high-rate activated sludge treatment in contact stabilisation mode (HRAS/CS) and column adsorption with zeolite. The HRAS/CS process removes organic matter and suspended solids in wastewater, leaving N behind in the effluent. The N was successfully recovered with the zeolite column under different scenarios, generating N and K-rich by-products. The regeneration effluent from the zeolite column with KCl contained 60-845 mg NH4+-N/L and 1.6-14.3 g K/L, having potential for use as fertigation water. The N-saturated zeolite contained 1.5-8.4 mg N/g and 14.3-19.3 mg K/g of the product fresh weight and low contaminant content, making it potentially eligible as various fertilising products. Adsorption can thus concentrate N from HRAS/CS effluent and produce by-products with potential agricultural value while meeting chemical oxygen demand and total nitrogen discharge standards.

Optimizing hydraulic retention time of high-rate activated sludge designed for potential integration with partial nitritation/anammox in municipal wastewater treatment

The integration of high-rate activated sludge (HRAS), an effective carbon redirection technology, with partial nitritation/anammox (PN/A) is a novel AB treatment process for municipal wastewater. In this study, an airlift HRAS reactor was operated in the continuous inflow mode for 200 d at a wastewater treatment plant. The balance between potential PN/A system stability and peak HRAS performance under decreasing hydraulic retention time (HRT) was optimized. Energy consumption and recovery and CO2 emissions were calculated. The results showed that the optimal HRT suitable with the PN/A process was 3 h, achieving 2-3 g/L mixed liquor volatile suspended solid, 67.8 % chemical oxygen demand (COD) recovery, 81 % total COD removal efficiency, 2.27 ± 1.03 g COD/L/d organic loading rate, 62 % aeration reduction, and 0.24 kWh/m3 power recovery potential. Such findings hold practical value and contribute to the development of the optimal AB process capable of achieving energy autonomy and carbon neutrality.

Degradation of COD in antibiotic wastewater by a combination process of electrochemistry, hydroxyl-functionalized ball-milled zero-valent iron/Fe3O4 and Oxone

In this study, the significant iron-based material, hydroxyl-functionalized ball-milled zero-valent iron/Fe3O4 (HFB-ZVI/Fe3O4) was employed for the experiments. The performance of the Electro + HFB-ZVI/Fe3O4 + Oxone system for the degradation of chemical oxygen demand (COD) in antibiotic wastewater was investigated. A direct current was applied between a graphite plate anode and two iron plate cathodes, and a series of operational parameters, such as applied electric current, the dosage of HFB-ZVI/Fe3O4 composite, the dosage of Oxone, and initial solution pH, were explored to evaluate the oxidation process. The application of electric current enhanced the gradual degradation of COD and the increase of current intensity accelerated COD degradation. The neutral condition was favourable for the rapid degradation of COD in a short reaction time by the Electro + HFB-ZVI/Fe3O4 + Oxone process and promoted the degradation efficiency of COD. An increase of electric current gradually decreased the reaction solution pH, the larger the electric current applied in the reaction process, the lower the final pH of the reaction solution. Under the optimal experimental conditions (1 g/L HFB-ZVI/Fe3O4 composite, 0.3 g/L Oxone, current intensity = 500 mA, initial solution pH = 7.85), Electro + HFB-ZVI/Fe3O4 + Oxone achieved 99% COD degradation in antibiotic wastewater. Radicals quenching experiments indicated the contribution to COD degradation by hydroxyl radicals (HO•), sulphate radicals (SO4•-) and other oxidants were 66.03%, 24.014% and 9.756%, respectively. The possible mechanism of COD degradation in the Electro + HFB-ZVI/Fe3O4 + Oxone system was also discussed in this study. The findings in this work provided useful information for the treatment of wastewater.

Insights on pretreatment technologies for partial nitrification/anammox processes: A critical review and future perspectives

For almost 20 years, partial nitrification-anammox (PN/A) has been the subject of intensive study and development. Pretreatment of wastewater for PN/A is crucial because the inhibitory substances in the influent may reduce the performance of PN/A. In this review, the current PN/A pretreatment technologies are comprehensively summarized. The selection of pretreatment technology for PN/A depending on the source of the wastewater and its main characteristics (high-strength wastewater or municipal wastewater, organic matters, suspended solids). Comparison of pretreatment technologies through multiple perspectives including wastewater characteristics, the objectives of the wastewater treatment (treating requirement, energy and resource recovery demand), reactor configuration of PN/A. Based on the discussion, two integrated processes, HRAS + one-stage PN/A and advanced AD + two-stage PN/A, are recommended as the preferred processes for treating municipal wastewater and wastewater with a high-strength ammonium, respectively. This review aims to provide guidance for future research and development of PN/A.

Preparation of cationic aggregates derived from sewage sludge for efficient capture of organic matter

Capturing the abundant organic matter residing in wastewater can not only reduce the emission of CO₂ from the source, but the enriched organics can also be used for anaerobic fermentation to generate and offset energy consumption in wastewater treatment processes. The key is to find or develop low-cost materials that can capture organic matter. Herein, sewage sludge-derived cationic aggregates (SBC-g-DMC) were successfully prepared via a hydrothermal carbonization process coupled with a graft copolymerization reaction for recovering organic matter from wastewater. Based upon preliminary screening of synthesized SBC-g-DMC aggregates regarding grafting rate, cationic degree, and flocculation performance, SBC-g-DMC_2.5 aggregate prepared with 60 mg of initiator, DMC-to-SBC mass ratio of 2.5:1, 70 °C, and 2 h of reaction time was selected for further characterization and evaluation. Results showed that SBC-g-DMC_2.5 aggregate has a positively-charged surface over a wide pH range of 3-11 and a hierarchical micro-/nano-structure, endowing it with an excellent organic matter capture efficiency (97.2% of pCOD, 68.8% of cCOD, and 71.2% of tCOD). Meanwhile, SBC-g-DMC_2.5 exhibits inappreciable trapping ability for the dissolved COD, NH₃-N, and PO₄^3-, guaranteeing the regular running of subsequent biological treatment units. Electronic neutralization, adsorption bridging, and sweep coagulation between cationic aggregates surface and organic matter were identified as the primary mechanisms for SBC-g-DMC_2.5 to capture organics. This development is expected to provide a theoretical reference for sewage sludge disposal, carbon reduction, and energy recovery during municipal wastewater treatment.

A two-stage dissolved air flotation saturator configuration for significant microbubble improvement

The presence of suspended contaminants in water and wastewater, such as algae, colloids, fats and oil, necessitates the use of systems such as dissolved air flotation (DAF) for their removal. In the current study, a novel setup has been proposed for bubble enhancement. An industrial scale (pilot) DAF system was tested at saturator pressures of 3-7 atm and flow rates of 5-20 L/min in three different configurations, namely, empty, packed, and the innovative two-stage (TS) configuration. In the TS system, after the nucleation of micro bubbles, the water is returned to the saturator to undergo pressurization for a second time before it is passed through the nozzle once more and is released. The results show that the highest volume of released air as well as the smallest microbubbles are seen in the TS configuration, followed by packed mode, with the empty configuration showing the least favourable results. Moreover, the bubbles produced at the lowest residence time and pressure (3 atm) with the novel setup are better than the bubbles produced by the standard configuration, even with pressures as high as 7 atm. Thus, the novel TS setup can allow for significantly lower energy requirements and lower capital costs. For real-world application of the TS system, the feed for the saturator could be extracted from within or near the contact zone, i.e. where the bubbles are released in the DAF tank.

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 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.

Treatment of Wastewater from Thermal Desorption for Remediation of Oil-Contaminated Soil by the Combination of Multiple Processes

Thermal desorption (TD) is one of the methods commonly used to remediate contaminated soil. However, as water is the liquid adsorbent of the off-gas treatment system in the TD stage, the wastewater generated after multiple cycles in the TD stage has low biodegradability and contains complex organic pollutants. In addition to petroleum hydrocarbon, there are also a lot of ammonia, emulsified oil, phenols, aldehydes, and ketones. In this study, effective removal of contaminants was achieved using a combined process of demulsification and flocculation (DF), ammonia stripping (AS), Fenton oxidation (FO), and reverse osmosis (RO). The combined process was optimized, and the maximum chemical oxygen demand (COD), NH3-N, turbidity, and extractable petroleum hydrocarbons (EPH) removal efficiencies reached 93.3%, 79.8%, 97.6%, and 99.9%, respectively. The FO was the key process for the efficient removal of contaminants. Ultraviolet-visible (UV/Vis), excitation-emission matrix (EEM), fluorescence spectroscopy, and gas chromatography-mass spectroscopy (GC-MS) showed that refractory macromolecular organic pollutants in water were removed, especially aromatics, phenols, and conjugated aldehydes or conjugated ketones, and further ring cleavage of benzene rings and carbocycles with carbon double bonds was observed. The cost-benefit analysis of the combined process was also carried out. The operating cost was 8.73 US$/m3, indicating that the combined process involved moderate costs for recalcitrant wastewater treatment. No studies have been published on combined processes for the treatment of wastewater from TD for the remediation of oil-contaminated soils. Therefore, this study could provide fundamental information based on experimental results and guidelines for wastewater treatment in engineering applications.

Determining Factors for Nitrite Accumulation in an Acidic Nitrifying System: Influent Ammonium Concentration, Operational pH, and Ammonia-Oxidizing Community

Acidic nitrification is attracting wide attention because it can enable robust suppression of nitrite-oxidizing bacteria (NOB) in wastewater treatment. This study reports a comprehensive assessment of the novel acidic nitrification process to identify the key factors that govern stable nitrite accumulation. A laboratory-scale moving-bed biofilm reactor receiving low-alkalinity wastewater was continuously operated under acidic conditions (pH < 6) for around two years, including nine stages varying influent and operational conditions. The results revealed that nitrite accumulation was related to three factors, i.e., influent ammonium concentration, operating pH, and ammonia-oxidizing microbial community. These three factors impact nitrite accumulation by altering the in situ concentration of free nitrous acid (FNA), which is a potent inhibitor of NOB. The critical FNA concentration is approximately one part per million (ppm, ∼1 mg HNO₂-N/L), above which nitrite accumulation is stably maintained in an acidic nitrifying system. The findings of this study suggest that stable nitrite accumulation via acidic ammonia oxidation can be maintained under a range of influent and operational conditions, as long as a ppm-level of FNA is established. Taking low-strength mainstream wastewater (40-50 mg NH₄⁺-N/L) with limited alkalinity as an example, stable nitrite accumulation was experimentally demonstrated at a pH of 4.35, under which an in situ FNA of 2.3 ± 0.6 mg HNO₂-N/L was attained. Under these conditions, Candidatus Nitrosoglobus became the only ammonia oxidizer detectable by 16S rRNA gene sequencing. The results of this study deepen our understanding of acidic nitrifying systems, informing further development of novel wastewater treatment technologies.

Enhancement of sewage sludge thickening and energy self-sufficiency with advanced process control tools in a full-scale wastewater treatment plant

On their path to becoming sustainable facilities, it is required that wastewater treatment plants reduce their energy demand, sludge production, and chemical consumption, as well as increase on-site power generation. This study describes the results obtained from upgrading the sludge line of a full-scale wastewater treatment plant over 6 years (2015-2021) using three advanced process control strategies. The advanced process control tools were designed with the aim of (i) enhancing primary and secondary sludge thickening, (ii) improving anaerobic digestion performance, and (iii) reducing chemical consumption in the sludge line. The results obtained show that the use of advanced process control tools allows for optimising sludge thickening (increasing solids content by 9.5%) and anaerobic digestion (increasing both the removal of volatile solids and specific methane yield by 10%, respectively), while reducing iron chloride and antifoam consumption (by 75% and 53%, respectively). With the strategies implemented, the plant increased its potential energy self-sufficiency from 43% to 51% and reduced de-watered sludge production by 11%. Furthermore, the upgrade required a low investment, with a return of capital expense (CAPEX) in 1.98 years, which presents a promising and affordable alternative for upgrading existing wastewater treatment plants.

Recent progress in flocculation, dewatering, and drying technologies for microalgae utilization: Scalable and low-cost harvesting process development

Microalgal research has made significant progress in terms of the high-value-added industrial application of microalgal biomass and its derivatives. However, cost-effective techniques for producing, harvesting, and processing microalgal biomass on a large scale still need to be fully explored in order to optimize their performance and achieve commercial robustness. In particular, technologies for harvesting microalgae are critical in the practical process as they require excessive energy and equipment costs. This review focuses on microalgal flocculation, dewatering, and drying techniques and specifically covers the traditional approaches and recent technological progress in harvesting microalgal biomass. Several aspects, including the characteristics of the target microalgae and the type of final value-added products, must be considered when selecting the appropriate harvesting technique. Furthermore, considerable aspects and possible future directions in flocculation, dewatering, and drying steps are proposed to develop scalable and low-cost microalgal harvesting systems.

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.

Controlling biofilm retention time in an A-stage high-rate moving bed biofilm reactor for organic carbon redirection

The A-stage of the AB process can minimize carbon oxidation by redirecting carbon to side-stream processes for harvesting carbon as energy and/or bioproduct. The redirection/harvesting of carbon has been studied in systems which utilize suspended biomass cultures. The potential of high-rate moving bed biofilm reactors, however, has not been explored. This study sought to control the biofilm solids retention time in a high-rate moving bed biofilm reactor operated at 17 ± 4 g-bCOD m^-2d^-1. Biofilm solids retention time was controlled by one of two strategies (i.e., 100% and 60% effective biofilm removal) that targeted several nominal biofilm solids retention times (i.e., 8, 6, 4, and 2 days) by employing different biocarrier replacement times. The results demonstrated that the suspended solids activity could be reduced by decreasing the nominal biofilm solids retention time. Using the 60% biofilm removal strategy, the actual biofilm solids retention time with a nominal biofilm solids retention time of 2 days was 12 h. When utilizing the 100% biofilm removal strategy, an actual biofilm solids retention time of less than 3 h was achieved with a nominal biofilm solids retention time of 2 days. The control reactor, which was a conventional moving bed biofilm reactor with no biocarrier replacement, was estimated to have a biofilm solids retention time of 2 days. Overall, the biofilm removal strategies favored carbon redirection and maximized the biomass yield at 1.1 ± 0.3 g-TSS g-COD^-1 removed.

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.