Integrated upflow anaerobic fixed-bed and single-stage step-feed process for mainstream deammonification: A step further towards sustainable municipal wastewater reclamation

Recurrent multi-stressor floc treatments with sulphide and free ammonia enabled mainstream partial nitritation/anammox

Selective suppression of nitrite-oxidising bacteria (NOB) over aerobic and anoxic ammonium-oxidising bacteria (AerAOB and AnAOB) remains a major challenge for mainstream partial nitritation/anammox implementation, a resource-efficient nitrogen removal pathway. A unique multi-stressor floc treatment was therefore designed and validated for the first time under lab-scale conditions while staying true to full-scale design principles. Two hybrid (suspended + biofilm growth) reactors were operated continuously at 20.2 ± 0.6 °C. Recurrent multi-stressor floc treatments were applied, consisting of a sulphide-spiked deoxygenated starvation followed by a free ammonia shock. A good microbial activity balance with high AnAOB (71 ± 21 mg N L-1 d-1) and low NOB (4 ± 17 % of AerAOB) activity was achieved by combining multiple operational strategies: recurrent multi-stressor floc treatments, hybrid sludge (flocs & biofilm), short floc age control, intermittent aeration, and residual ammonium control. The multi-stressor treatment was shown to be the most important control tool and should be continuously applied to maintain this balance. Excessive NOB growth on the biofilm was avoided despite only treating the flocs to safeguard the AnAOB activity on the biofilm. Additionally, no signs of NOB adaptation were observed over 142 days. Elevated effluent ammonium concentrations (25 ± 6 mg N L-1) limited the TN removal efficiency to 39 ± 9 %, complicating a future full-scale implementation. Operating at higher sludge concentrations or reducing the volumetric loading rate could overcome this issue. The obtained results ease the implementation of mainstream PN/A by providing and additional control tool to steer the microbial activity with the multi-stressor treatment, thus advancing the concept of energy neutrality in sewage treatment plants.

Recent advances of partial anammox by controlling nitrite supply in mainstream wastewater treatment through step-feed mode

At present, the step-feed process is a very active branch in practical application of mainstream wastewater treatment, and the anammox technology empowers the sustainable development and in-depth research of step-feed process. This review provides a systematically inspection of the realization and application of partial anammox process through step-feed mode, with a particular focus on controlling nitrite supply for anammox. The characteristics and advantages of step-feed mode in traditional management are reviewed. The unique organics utilization strategy by step-feed and indispensable intermittent aeration mode creates advantages for achieving nitritation (NH4+ → NO2-) and denitratation (NO3- → NO2-), providing flexible combination possibility with anammox. Additionally, the lab- or pilot-scale control strategies with different forms of anammox, including nitritation/anammox, denitratation/anammox, and double-anammox (combined nitritation/anammox and denitratation/anammox), are summarized. Finally, future directions and application perspectives on leveraging the relationship between flocs and biofilm, nitritation and denitratation, and different strains to maximize the anammox proportion in N-removal are proposed.

Multi-parameter control-based operation strategy for mainstream deammonification in an integrated anaerobic biofilm reactor-step feed MBR

The mainstream deammonification of municipal wastewater has been recognized as one of the greatest challenges in wastewater engineering. The conventional activated sludge process has disadvantages of high energy input and sludge production. To tackle this situation, an innovative A-B process, where an anaerobic biofilm reactor (AnBR) was functioned as the A stage for energy recovery, and a step-feed membrane bioreactor (MBR) was functioned as the B stage for mainstream deammonification, was constructed for carbon-neutral wastewater treatment. For addressing the challenge associated with selective retention of ammonia-oxidizing bacteria (AOB) over nitrite oxidizing bacteria (NOB), a multi-parameter control-based operation strategy was developed with synergistic control of influent COD redistribution, dissolved oxygen (DO) concentration and sludge retention time (SRT) in the innovative AnBR - step-feed MBR system. Results showed that more than 85% of wastewater COD could be removed with the direct production of methane gas in the AnBR. A relatively stable partial nitritation, which is a prerequisite of anammox, was achieved with the successful suppression of NOB, leading to 98% of ammonium-N and 73% of total nitrogen removed. Anammox bacteria could well survive and enrich in the integrated system, and the contribution of anammox to the total nitrogen removal was more than 70% at optimal conditions. Reactions network involved in the nitrogen transformation in the integrated system was further constructed through the mass balance and microbial community structure analyses. Consequently, this study demonstrated a practically feasible process configuration with high operation and control flexibility towards stable mainstream deammonification of municipal wastewater.

Deciphering the concurrence of comammox, partial denitrification and anammox in a single low-oxygen mainstream nitrogen removal reactor

One-stage anammox-based autotrophic nitrogen removal technology has attracted increasing interest to sustainable biological nitrogen removal for future wastewater treatment. However, its application in mainstream municipal wastewater treatment is still challenging due to low nitrogen and high organics of raw wastewater. Herein, a novel Simultaneous Carbon Oxidation, partial Comammox, Denitratation and Anammox (SCOCDA) was firstly developed in a single sequencing batch biofilm reactor operated at a dissolved oxygen concentration of ∼0.5 mg/L for treating synthetic municipal wastewater (50 mg/L NH₄⁺-N and 100-250 mg/L COD). The long-term operation showed that almost complete COD and nitrogen removal performance could be achieved at a carbon/nitrogen ratio (COD/NH₄⁺-N) of 3-5 with the corresponding effluent total nitrogen (TN)＜5 mg/L. Microbial community and amoA-targeting amplicon sequencing analysis further verified that comammox Nitrospira spp., denitrifier Thauera and other aerobic/facultative heterotrophs could work synergistically with anammox bacteria, Candidatus Kuenenia. Moreover, nitrogen metabolic and inorganic carbon fixation pathways through the interaction between comammox and anammox were also revealed with the aid of Kyoto Encyclopedia of Genes and Genomes (KEGG). Lastly, potential application of proposed SCOCDA process was illustrated. This research sheds new light on advanced nitrogen removal towards limit of technology via the synergy of comammox and anammox.

Simulation and experimental verification of nitrite-oxidizing bacteria inhibition by alternating aerobic/anoxic strategy

Anaerobic ammonium oxidation (ANAMMOX) is a promising technology for sewage treatment. Alternating aerobic/anoxic conditions have been widely adopted to achieve partial nitrification (PN), so as to provide substrates for ANAMMOX. In this study, the feasibility of PN with the strategy of intermittent aeration was investigated under mainstream conditions. At a low dissolved oxygen (DO) concentration, the nitrogen conversion characteristic under different intermittent aeration modes was evaluated by mathematical simulation and experimental method with (1) ordinary activated sludge, (2) mixed sludge with anaerobic ammonia-oxidizing bacteria (AnAOB), and (3) PN sludge, as seed sludge. The existence of functional microorganisms, such as AnAOB and denitrifying bacteria, which can utilize nitrites, was the prerequisite for NOB activity inhibition in the alternating aerobic/anoxic condition. Therefore, low nitrite may be an important factor in NOB activity inhibition under alternating aerobic/anoxic conditions. This study demonstrated a key controlling factor for NOB activity inhibition with alternating aerobic/anoxic condition.

Challenges, solutions and prospects of mainstream anammox-based process for municipal wastewater treatment

Anaerobic ammonia oxidation (anammox) process has a promising application prospect for the mainstream deammonification of municipal wastewater due to its high efficiency and low energy consumption. In this paper, challenges and solutions of mainstream anammox-based process are summarized by analyzing the literature of recent ten years. Slow growth rate of anammox bacteria is a main challenge for mainstream anammox-based process, and enhancement of bacteria retention has been recognized to be necessary. Compared with directly increasing sludge retention time (SRT) with membrane bioreactors or sequencing batch reactors, culturing anammox bacteria in the form of biofilm or granule sludge is more promising for its feasibility of eliminating nitrite oxidizing bacteria (NOB). Besides, adding external electron donors or conductive materials and enriching the concentration of ammonia with absorption materials have also been proved helpful to improve the activity of anammox bacteria. Other challenges include the elimination of NOB and achieving ideal ratio of NH₄⁺ and NO₂^-. To solve these problems and achieve stable partial nitrification, composite control strategies based on low SRT and limited aeration are needed based on the special characteristics of ammonia oxidizing bacteria (AOB) and NOB. When treating actual wastewater, interference of low temperature and components in the influent is another problem. Relatively high activity of anammox bacteria has been realized after artificial acclimation at low temperature and the mechanism was also preliminary explored. Different pre-treatment sections have been designed to reduce the concentration of COD and S^2- from the influent. As for the nitrate produced by the anammox reaction, coupling processes are useful to reduce the concentration of nitrate in the effluent. In brief, suitable reactor and coupling process should be selected according to the temperature, influent quality and discharge targets of different regions. The future prospects of the mainstream anammox-based process are also put forward.

Comparison of recovery characteristics between AnAOB and AOB-AnAOB granular sludge after long-term storage

The recovery characteristics of long-term stored sludge are still elusive. Here, an AnAOB granular sludge reactor (R1) (15 d) was found to recover faster than AOB-AnAOB granular sludge reactor (R2) (21 d) after 240 d 4 °C storage. Higher nitrogen removal performance was also achieved in R1 (5.96 ± 0.14 kg N/(m³·d)) than that of R2 (0.33 ± 0.02 kg N/(m³·d)). It was indicated that more c-di-GMP synthetase was predicted in R1 triggered more amino acid metabolic function genes (Pyruvate kinase and 6-phosphofructokinase) which can secrete more extracellular proteins. Correspondingly, the higher abundance of functional genes related to exopolysaccharide secretion (Glucokinase and UDP-glucose 4-epimerase) trigger by GP6, GP10 and GP16 was found in R2. In addition, some heterotrophic bacteria cooperating with AnAOB (Comamonas and Simplicispira) were found more active in R1 than that of R2 due to the higher relative abundance of functional genes related to folic acid metabolic (Dihydrofolate synthase and Dihydrofolate reductase). However, AOB-AnAOB granular sludge was observed more likely to protect cells through NAD(P)-dependent dehydrogenase. It was indicated that AnAOB granular sludge has better application potential, more active characteristics of aggregation metabolism and collaboration with auxiliary bacteria than that of R2.

Nutrients removal from low C/N actual municipal wastewater by partial nitritation/anammox (PN/A) coupling with a step-feed anaerobic-anoxic-oxic (A/A/O) system

In this study, a novel combined strategy was successfully established by partial nitritation/anammox (PN/A) within a step-feed A/A/O process integrated with fixed-biocarriers to treat municipal sewage for 200 days. The excellent nutrients removal performance of this system compared with national level of discharging standard were achieved: low total inorganic nitrogen (7.1 mg/L) and phosphorus (0.3 mg/L) in the effluent with the influent (51.1 and 4.2 mg/L) at C/N ratios of 3.4 ± 0.5, mainly attributed to the stable PN (oxic zone) and subsequently anammox effect (anoxic zone). Nitrogen mass balance indicated that anammox contribution in anoxic zones to nitrogen loss could be up to 42% at stable phase. Therefore, aeration and carbon cost could be greatly reduced under low DO, low C/N and aerobic hydraulic retention time (HRT) of 7.4 h condition. The low DO and anammox bacteria retention in anoxic chambers promoted the washout of NOB and combination of anammox and partial nitritation process. During long-term operation, the activity of AOB effectively maintained while that of NOB drastically reduced to 0.1 mg N / g MLSS / h resulting in high and stable nitrite accumulation ratios (about 90%). The achievement of partial nitritation was mainly due to low DO (0.4-0.5 mg/L) and effective retention of anammox bacteria even with a low temperature (14.5 °C). Notedly, anammox activity gradually increased both on the biocarriers and in the flocs while a higher anammox abundance was observed on the biocarriers (2.48%) than that in suspend flocs (0.03%). As above, this study indicated that the novel combined strategies could be applicable to mainstream anammox, and a pilot-scale reactor will be established to verify and promote the industrial application of mainstream anammox.

Inhibition of nitrite-oxidizing bacteria in automatic recycling PN/ANAMMOX under mainstream conditions

At present, sustainable and stable partial nitrification has not been widely achieved in the mainstream PN/ANAMMOX process. Here, the feasibility of sustainable and stable partial nitrification was demonstrated in automatic recycling PN/ANAMMOX reactor under mainstream conditions using both simulation and experimental methods. Stable nitrite accumulation in the aerobic zone could be achieved via regulating dissolved oxygen (DO) concentrations and sludge retention time (SRT). The DO concentrations required for the repression of nitrite-oxidizing bacteria (NOB) were lower at longer SRTs. The DO concentrations and SRTs required for NOB repression were lower at lower temperatures. However, NOB repression was diminished by a persistent low DO and short SRT under mainstream conditions. With the introduction of automatic recycling, sustainable and stable partial nitrification was achieved. Effluent recycling could limit the nitrite-nitrogen required for NOB growth. Collectively, effluent recycling may serve as a feasible and useful strategy for NOB inhibition during the PN/ANAMMOX process.

Combustion characteristics and typical pollutant emissions of corn stalk blending with municipal sewage sludge

The co-combustion of sewage sludge and biomass is a key problem in coal-fired power plants. The combustion characteristics and pollutant emissions of municipal sewage sludge and biomass could result in unpredictable operation and environmental problems. In this study, the combustion experiments of corn stalk (CS), municipal sewage sludge (SS), and their blends were conducted in the thermogravimetric analyzer and muffle furnace, focusing on the combustion characteristics and the pollutants (SO₂/NO) emissions with different temperature, proportion, and heating rate. It was found that the combustion characteristics of the mixture are affected by the mix ratio of SS. Compared with those of SS, the SO₂ emission amount and S-SO₂ conversion rate of CS are lower. The content of N in SS is higher than CS, but the conversion rate of N-NO is lower. Although the emissions of SO₂ and NO from CS blending with SS are higher than those from CS combustion alone, the conversion rate of SO₂ and NO decreases. This means that the co-combustion of CS and SS will reduce the content of pollutants released by CS combustion alone and effectively solve the environmental problems associated with CS incineration.

Process performance and anammox community diversity in a deammonification reactor under progressive nitrogen loading rates for swine wastewater treatment

The performance of a deammonification reactor fed with increasing nitrogen loading rates (NLR) was evaluated. The digestate from a continuous stirred tank reactor (CSTR) treating sludge from a swine production unit was diluted to provide different ammonia concentrations. The biomass samples from the end of each experimental phase were analyzed for microorganism community evaluation. The results proved that deammonification system supported a NLR up to 3.27 ± 0.13 g N L^-1 d^-1 with nitrogen removal efficiency of 83%. The specific ammonia consumption rate (µ_NH3-N) did not decrease up to this NLR proving the stability of reactor performance. Anammox bacteria genus shifted along the experiment and at the end the predominant anammox bacteria found in the reactor was candidatus Brocadia. Finally, it was proved that a deammonification reactor for nitrogen removal from CSTR digestate could be easily controlled only by monitoring pH and dissolved oxygen.

A review on mainstream deammonification of municipal wastewater: Novel dual step process

The conventional biological nitrogen removal process is receiving increasing pressure partially due to its energy-negative operation. To address this challenge, various mainstream deammonification processes have been explored for energy-neutral municipal wastewater treatment, whereas these processes appear challenging to be sustainably and stably achieved in conventional process configurations. Therefore, this review aimed to provide a comprehensive analysis of the state-of-the-art of mainstream deammonification, while highlighting the major technical challenges. It appeared that recently developed novel dual step process, i.e. A-B processes, could provide a feasible engineering option for mainstream deammonification, where A-stage is designed for COD capture with the aim to enhance energy recovery, and B-stage is tailored for nutrient removal/recovery. This indeed may lead to a promising integrated mainstream deammonification process towards energy-efficient and environmentally sustainable nitrogen removal. Meanwhile, this review also offered an opinion on future municipal wastewater treatment, aiming for concurrent water reclamation and energy recovery.