In the wake of the success of aerobic granulation in sequential batch reactors (SBRs) for treating wastewater, attention is beginning to turn to continuous flow applications. This is a necessary step ...given the advantages of continuous flow treatment processes and the fact that the majority of full-scale wastewater treatment plants across the world are operated with aeration tanks and clarifiers in a continuous flow mode. As in SBRs, applying a selection pressure, based on differences in either settling velocity or the size of the biomass, is essential for successful granulation in continuous flow reactors (CFRs). CFRs employed for aerobic granulation come in multiple configurations, each with their own means of achieving such a selection pressure. Other factors, such as bioaugmentation and hydraulic shear force, also contribute to aerobic granulation to some extent. Besides the formation of aerobic granules, long-term stability of aerobic granules is also a critical issue to be addressed. Inorganic precipitation, special inocula, and various operational optimization strategies have been used to improve granule long-term structural integrity. Accumulated studies reviewed in this work demonstrate that aerobic granulation in CFRs is capable of removing a wide spectrum of contaminants and achieving properties generally comparable to those in SBRs. Despite the notable research progress made toward successful aerobic granulation in lab-scale CFRs, to the best of our knowledge, there are only three full-scale tests of the technique, two being seeded with anammox-supported aerobic granules and the other with conventional aerobic granules; two other process alternatives are currently in development. Application of settling- or size-based selection pressures and feast/famine conditions are especially difficult to implement to these and similar mainstream systems. Future research efforts needs to be focused on the optimization of the granule-to-floc ratio, enhancement of granule activity, improvement of long-term granule stability, and a better understanding of aerobic granulation mechanisms in CFRs, especially in full-scale applications.
•The status of continuous flow aerobic granulation process development is reviewed for the first time.•Recent years have seen breakthroughs in understanding and applying selection pressure in continuous flow reactors.•Properties and formation mechanism of aerobic granules in continuous flow and sequential batch reactors are compared.•Applying selection pressure and feast/famine conditions are major obstacles to full-scale, mainstream aerobic granulation.
This work describes the development of an intermittently aerated pilot-scale process (V = 0.34 m(3)) operated without oxidized nitrogen recycle and supplemental carbon addition optimized for nitrogen ...removal via nitritation/denitritation. The aeration pattern was controlled using a novel aeration strategy based on set-points for reactor ammonia, nitrite and nitrate concentrations with the aim of maintaining equal effluent ammonia and nitrate + nitrite (NOx) concentrations. Further, unique operational and process control strategies were developed to facilitate the out-selection of nitrite oxidizing bacteria (NOB) based on optimizing the chemical oxygen demand (COD) input, imposing transient anoxia, aggressive solids retention time (SRT) operation towards ammonia oxidizing bacteria (AOB) washout and high dissolved oxygen (DO) (>1.5 mg/L). Sustained nitrite accumulation (NO2-N/NOx-N = 0.36 ± 0.27) was observed while AOB activity was greater than NOB activity (AOB: 391 ± 124 mgN/L/d, NOB: 233 ± 151 mgN/L/d, p < 0.001) during the entire study. The reactor demonstrated total inorganic nitrogen (TIN) removal rate of 151 ± 74 mgN/L/d at an influent COD/ Formula: see text -N ratio of 10.4 ± 1.9 at 25 °C. The TIN removal efficiency was 57 ± 25% within the hydraulic retention time (HRT) of 3 h and within an SRT of 4-8 days. Therefore, this pilot-scale study demonstrates that application of the proposed online aeration control is able to out-select NOB in mainstream conditions providing relatively high nitrogen removal without supplemental carbon and alkalinity at a low HRT.
The high-rate activated sludge (HRAS) process is a technology suitable for the removal and redirection of organics from wastewater to energy generating processes in an efficient manner. A HRAS pilot ...plant was operated under controlled conditions resulting in concentrating the influent particulate, colloidal, and soluble COD to a waste solids stream with minimal energy input by maximizing sludge production, bacterial storage, and bioflocculation. The impact of important process parameters such as solids retention time (SRT), hydraulic residence time (HRT) and dissolved oxygen (DO) levels on the performance of a HRAS system was demonstrated in a pilot study. The results showed that maximum removal efficiencies of soluble COD were reached at a DO > 0.3 mg O2/L, SRT > 0.5 days and HRT > 15 min which indicates that minimizing the oxidation of the soluble COD in the high-rate activated sludge process is difficult. The study of DO, SRT and HRT exhibited high degree of impact on the colloidal and particulate COD removal. Thus, more attention should be focused on controlling the removal of these COD fractions. Colloidal COD removal plateaued at a DO > 0.7 mg O2/L, SRT > 1.5 days and HRT > 30 min, similar to particulate COD removal. Concurrent increase in extracellular polymers (EPS) production in the reactor and the association of particulate and colloidal material into sludge flocs (bioflocculation) indicated carbon capture by biomass. The SRT impacted the overall mass and energy balance of the high-rate process indicating that at low SRT conditions, lower COD mineralization or loss of COD content occurred. In addition, the lower SRT conditions resulted in higher sludge yields and higher COD content in the WAS.
•A HRAS pilot was operated under controlled conditions to evaluate the impact of SRT, DO and HRT.•The removal of the sCOD fraction is difficult to control and minimize completely.•SRT, DO and HRT can be used to control the removal of particulate matter in the HRAS.•The SRT impacted the overall mass and energy balance of the HRAS.•At low SRT, lower COD mineralization and higher sludge yields and COD in the WAS were observed.
The United States Microbead-Free Waters Act was signed into law in December 2015. It is a bipartisan agreement that will eliminate one preventable source of microplastic pollution in the United ...States. Still, the bill is criticized for being too limited in scope, and also for discouraging the development of biodegradable alternatives that ultimately are needed to solve the bigger issue of plastics in the environment. Due to a lack of an acknowledged, appropriate standard for environmentally safe microplastics, the bill banned all plastic microbeads in selected cosmetic products. Here, we review the history of the legislation and how it relates to the issue of microplastic pollution in general, and we suggest a framework for a standard (which we call “Ecocyclable”) that includes relative requirements related to toxicity, bioaccumulation, and degradation/assimilation into the natural carbon cycle. We suggest that such a standard will facilitate future regulation and legislation to reduce pollution while also encouraging innovation of sustainable technologies.
Display omitted
•PD-anammox was described as a viable solution for sustainable N removal.•Advances of performance and mathematical models in PD-anammox were presented.•PD-anammox alleviating N2O ...emissions and energy savings was explored.•Start-up and recovery strategies of PD-anammox were classified and conceptualized.
Anammox is a widely adopted process for energy-efficient removal of nitrogen from wastewater, but challenges with NOB suppression and NO3− accumulation have led to a deeper investigation of this process. To address these issues, the synergy of partial denitrification and anammox (PD-anammox) has emerged as a promising solution for sustainable nitrogen removal in wastewater. This paper presents a comprehensive review of recent developments in the PD-anammox system, including stable performance outcomes, operational parameters, and mathematical models. The review categorizes start-up and recovery strategies for PD-anammox and examines its contributions to sustainable development goals, such as reducing N2O emissions and saving energy. Furthermore, it suggests future trends and perspectives for improving the efficiency and integration of PD-anammox into full-scale wastewater treatment system. Overall, this review provides valuable insights into optimizing PD-anammox in wastewater treatment, highlighting the potential of simultaneous processes and the importance of improving efficiency and integration into full-scale systems.
A partial nitritation continuous flow reactor (CFR) was operated for eight months demonstrating that partial nitritation granular sludge can remain stable under continuous flow conditions. The ...ammonia oxidizing bacteria (AOB)-to-nitrite oxidizing bacteria (NOB) activity ratios were determined for a series of granule sizes to understand the impact of mass diffusion limitation on the free ammonia (FA) inhibition of NOB. When dissolved oxygen (DO) limitation is the only mechanism for NOB suppression, the AOB:NOB ratio was usually found to increase with the granule size. However, the trend is reversed when FA has an inhibitory effect on NOB, as was observed in this study. The decrease in AOB:NOB ratio indicates that smaller granules, e.g. diameter <150 μm, are preferred for nitrite accumulation when high FA concentration is present, as in the partial nitritation process. The trend was further verified by observing the increase in the apparent inhibition coefficient as granule size increased. Indeed, this study for the first time quantified the effect of diffusion limitation on the apparent inhibition coefficient of NOB in aerobic granules. A mathematical model was then utilized to interpret the observed suppression of NOB and predicted that NOB suppression was only complete at the granule surface. The NOB that did survive in larger granules was forced to dwell within the granule interior, where the AOB growth declines due to DO diffusion limitation. This means FA inhibition can be taken advantage of as an effective means for NOB suppression in small granules or thin biofilms. Further, both FA inhibition and DO limitation were found to be required for the suppression of NOB in mainstream aerobic granules.
•Free ammonia (FA) inhibition of NOB is achieved for wastewater of 50–60 mg N L−1.•Small granules are preferred for effective FA inhibition.•Diffusional resistance leads to a stratified distribution of AOB and NOB.•Both FA inhibition and low dissolved oxygen are needed to suppress NOB in granules.•The FA inhibition coefficient of NOB increases with the granule size.
•Domestic wastewater was treated in an A-stage system with varying SRT, HRT and DO.•The effect of EPS production on bioflocculation and settling was measured.•Highest TSS, tCOD, pCOD and cCOD removal ...was at 0.56day SRT and 1.0mg/L DO.•EPS fractions and components did not influence COD capture and redirection.
This paper quantifies the effect of varying solids retention time (SRT), hydraulic retention time (HRT) and dissolved oxygen (DO) concentrations on extracellular polymeric substances (EPS) production and subsequently effluent quality, carbon capture (bioflocculation) and carbon redirection (settling) in a high rate activated sludge A-stage system treating domestic wastewater. Two pilot-scale A-stage reactors were set-up with HRTs of 30 and 60min. Cascade DO control was used to maintain 3 DO set-points of 0.5, 1.0 and 1.5mg/L. A mixed liquor suspended solids (MLSS) concentration of 3000mg/L was maintained and the waste activated sludge (WAS) flow was varied to achieve SRTs of 0.28 and 0.56day. EPS fractions and the protein and polysaccharide concentrations of the mixed liquor were measured. Operation at the 0.56day SRT and 1.0mg/L DO resulted in the highest total suspended solids (TSS), total COD (tCOD), particulate COD (pCOD), and colloidal (cCOD) removal. The best overall performance in terms of bioflocculation (cCOD removal) and carbon capture (percent COD in the WAS) occurred at the 0.56day SRT and coincided with decreasing total EPS concentrations but the settling characteristics of the sludge were better at the 0.28day SRT. Overall, low correlations were found between EPS production and system performance. It is likely that at the high loading rate of the A-stage system, EPS production did not play a major role compared to the influence of operating parameters on effluent quality, carbon capture and redirection.
The integration of biological phosphorus removal (bio‐P) and shortcut nitrogen removal (SNR) processes is challenging because of the conflicting demands on influent carbon: SNR allows for upstream ...carbon diversion, but this reduction of influent carbon (especially volatile fatty acids VFAs) prevents or limits bio‐P. The objective of this study was to achieve SNR, either via partial nitritation/anammox (PNA) or partial denitrification/anammox (PdNA), simultaneously with biological phosphorus removal in a process with upstream carbon capture. This study took place in a pilot scale A/B process with a sidestream bio‐P reactor and tertiary anammox polishing. Despite low influent rbCOD concentrations from the A‐stage effluent, bio‐P occurred in the B‐stage thanks to the addition of A‐stage WAS fermentate to the sidestream reactor. Nitrite accumulation occurred in the B‐stage via partial denitrification and partial nitritation (NOB out‐selection), depending on operational conditions, and was removed along with ammonia by the tertiary anammox MBBR, with the ability to achieve effluent TIN less than 2 mg/L.
Practitioner Points
A sidestream reactor with sufficient fermentate addition enables biological phosphorus removal in a B‐stage system with little‐to‐no influent VFA.
Enhanced biological phosphorus removal is not inhibited by intermittent aeration and is stable at a wide range of process SRTs.
Partial nitritation and partial denitrification are viable routes to produce nitrite within an A/B process with sidestream bio‐P, for downstream anammox in a polishing MBBR.
Carbon capture and shortcut nitrogen removal are compatible with biological phosphorus removal if a portion of the captured carbon is fermented into volatile fatty acids and returned to the system in a sidestream reactor. Consistent low‐effluent phosphorus and nitrogen are possible with no external carbon addition.
Aeration control at wastewater treatment plants based on ammonia as the controlled variable is applied for one of two reasons: (1) to reduce aeration costs, or (2) to reduce peaks in effluent ...ammonia. Aeration limitation has proven to result in significant energy savings, may reduce external carbon addition, and can improve denitrification and biological phosphorus (bio-P) performance. Ammonia control for limiting aeration has been based mainly on feedback control to constrain complete nitrification by maintaining approximately one to two milligrams of nitrogen per liter of ammonia in the effluent. Increased attention has been given to feedforward ammonia control, where aeration control is based on monitoring influent ammonia load. Typically, the intent is to anticipate the impact of sudden load changes, and thereby reduce effluent ammonia peaks. This paper evaluates the fundamentals of ammonia control with a primary focus on feedforward control concepts. A case study discussion is presented that reviews different ammonia-based control approaches. In most instances, feedback control meets the objectives for both aeration limitation and containment of effluent ammonia peaks. Feedforward control, applied specifically for switching aeration on or off in swing zones, can be beneficial when the plant encounters particularly unusual influent disturbances.
A pilot scale process was operated with A‐stage effluent (ASE) and primary clarifier effluent (PCE) in MLE, all tanks aerated, A/O, and A2O configurations. Continuous DO control at high DO (2 mg/L), ...low DO (0.1–0.3 mg/L), ammonia‐based aeration control (ABAC), and ammonia versus NOx (AvN) control (both continuous and intermittent operation) were compared on the basis of total inorganic nitrogen (TIN) removal, and simultaneous nitrification‐denitrification (SND). The highly loaded adsorption/bio‐oxidation (A/B) process configuration (4 hr HRT) with intermittent aeration was capable of achieving a maximum TIN removal of 80%, while the A2O process with PCE feed, an 11 hr HRT, and 0.2–0.3 mg/L DO continuous aeration achieved a maximum of 88% TIN removal. ABAC and AvN control did not always result in DO setpoints low enough to achieve SND, and even if setpoints were low enough to achieve SND that did not always result in increased overall TIN removal over continuous DO control of 2 mg/L. While there are other benefits to transitioning to sensor driven aeration control strategies such as ABAC and AvN, increased TIN removal during continuous aeration is not guaranteed. Results suggest that although low DO is a prerequisite for SND, carbon availability for denitrification in the aerobic zone is more likely to be the limiting factor once low DO conditions are met.
Practitioner points
Intermittent aeration control results in higher TIN removal than continuous aeration at the same total SRT
Continuous aeration AvN control is not likely to result in more TIN removal than continuous aeration ABAC for a given COD and nitrogen load
Configurations that are designed to maximize predenitrification (e.g., MLE and A2O) are less likely to achieve increased SND in the aerobic zone from low DO operation than configurations that are not (e.g., A/O).
Representative examples of sensor feedback for each aeration control strategy.
PDF
