Can digestate recirculation promote biohythane production from two-stage co-digestion of rice straw and pig manure?

Digestate recirculation is often considered an important way to improve system stability (system acidification, ammonia inhibition, hydrolysis limitations, etc.) and gas production performance. However, it is not clear how the promotion of biohythane production works in anaerobic co-digestion with digestate recirculation of rice straw (RS) and pig manure (PM). Two sets of laboratory-scale two-stage continuous stirred tank reactors were operated continuously for 95 d to investigate the performance of biohythane production in the first/second phase under mesophilic (M)/thermophilic (T) and digestate recirculation conditions. Firstly, biohythane was not produced by PM with RS under digestate recirculation. The main reasons were: 1) Digestive recirculation promoted the growth of hydrogenotrophic methanogenic bacteria; and 2) limitations in hydrolysis. Secondly, digestate recirculation has positive effects on the removal rates (removal rates of TS, VS, polysaccharide, protein and TCOD increased by 30.4%, 22.3%, 9.9%, 31.4%, and 11.9%, respectively) and energy yield (up to 68.7%). Finally, there was a higher abundance of hydrogen-producing bacteria (Fervidobacterium [44.9%] and Coprothermobacter [18.8%]) in T2, accounting for >80% of the total, and of which the huge hydrogen production potential cannot be ignored. The results provide new ideas for alleviating the energy crisis and developing green energy in the future.

Enhancing autotrophic nitrogen removal with a novel dissolved oxygen-differentiated airlift internal circulation reactor: Long-term operational performance and microbial characteristics.

Autotrophic nitrogen removal (ANR) processes have not been widely applied in wastewater treatment due to their long start-up time and unstable performance. In this study, a novel dissolved oxygen-differentiated airlift internal circulation reactor was developed to enhance ANR from wastewater. During 200 days of continuous operation, the reactor start-up was achieved within 30 days; a high total nitrogen removal efficiency of 80% was achieved and stably maintained under an aeration rate of 0.90 L/min and hydraulic retention time of 6 h. Additionally, the color of sludge went from a light yellow to dark red, and the amount and size of the micro-granules increased obviously. Medium-sized (1.0-2.5 mm) micro-granules accounted for 72.4% on day 190. The specific anammox activity increased from 0.53 to 1.43 g-N/g-VSS/d, while the SNOA decreased from 0.93 to 0.08 g-N/g-VSS/d. Furthermore, the microbial analysis showed that the Nitrosomonas (4.2%) and Candidatus Brocadia (22.6%) were enriched and formed the micro-granules after the reactor's long-term operation. The results indicate that novel configuration realizes the partitioning of dissolved oxygen (DO), optimizes nitritation and anammox reactions, and accelerates biochemical reactions, thereby enhancing ANR performance. This study provides a practical alternative to enhance ANR performance and a scientific basis for the development and application of novel nitrogen removal reactors.

[Enhanced nitrogen removal by bioelectrochemical coupling anammox and characteristics of microbial communities].

To investigate the bioelectrochemical enhanced anaerobic ammonia oxidation (anammox) nitrogen removal process, a bioelectrochemical system with coupled anammox cathode was constructed using a dual-chamber microbial electrolysis cell (MEC). Specifically, a dark incubation batch experiment was conducted at 30 ℃ with different influent total nitrogen concentrations under an applied voltage of 0.2 V, and the enhanced denitrification mechanism was investigated by combining various characterization methods such as cyclic voltammetry, electrochemical impedance spectroscopy and high-throughput sequencing methods. The results showed that the total nitrogen removal rates of 96.9%±0.3%, 97.3%±0.4% and 99.0%±0.3% were obtained when the initial total nitrogen concentration was 200, 300 and 400 mg/L, respectively. In addition, the cathode electrode biofilm showed good electrochemical activity. High-throughput sequencing results showed that the applied voltage enriched other denitrifying functional groups, including Denitratisoma, Limnobacter, and ammonia oxidizing bacteria SM1A02 and Anaerolineaceae, Nitrosomonas europaea and Nitrospira, besides the anammox bacteria. These electrochemically active microorganisms comprised of ammonium oxidizing exoelectrogens (AOE) and denitrifying electrotrophs (DNE). Together with anammox bacteria Candidatus Brocadia, they constituted the microbial community structure of denitrification system. Enhanced direct interspecies electron transfer between AOE and DNE was the fundamental reason for the further improvement of the total nitrogen removal rate of the system.

Revealing the response of community structure and metabolic pathway to varying organic matter stress in a dissolved oxygen-differentiated airlift internal circulation partial nitritation-anammox system.

In practice, the influent organic matter is often pre-treated to reduce the impact on partial nitritation-anammox (PNA) process. However, the influent organics may also drive the denitrification process and improve total nitrogen removal efficiency of the PNA process. Thus, we designed and operated a novel dissolved oxygen-differentiated airlift internal circulation PNA (PNA-DOAIC) system in this study at various influent C/N ratios of 0-4.0. Nitrogen removal performance, microbial activity and community, and metabolic pathways in response to varying organic matter stress were investigated via the continuous experiment combined with batch test. The results showed that the optimum influent C/N ratio was 2.0 in this system, and the efficient and stable operation was still maintained at the C/N ratios of 0-3. At this time, the TN removal efficiency and removal rate could reach 95.1 % and 0.93 kg-N/m3/d, respectively, while COD efficiency remained at 95.4 %. Efficient removal performance was achieved via the PNA coupled with denitrification. However, the anammox bacteria (AnAOB) activity and abundance declined persistently as the influent C/N ratio was further raised, and heterotrophic bacteria gradually replaced AnAOB as dominate genus. Meanwhile, metabolic functions involving the material exchange and organic degradation were significantly enhanced. Nitrogen removal pathways changed from PNA to the nitrification-denitrification process. This study provides deep insights into effects of organic matter on the PNA process and can expand the application scope of this novel PNA-DOAIC bioreactor.

Swine wastewater treatment using combined up-flow anaerobic sludge blanket and anaerobic membrane bioreactor: Performance and microbial community diversity.

To address the existing economic and environmental issues associated with swine wastewater (SW) treatment, a process combining up-flow anaerobic sludge blanket (UASB) and anaerobic membrane bioreactor (AnMBR) was developed and continuously operated for 137 d. Bioreactor conversion and microbial community dynamics in reactors were analyzed. The UASB-AnMBR process yielded excellent pollutants removal efficiencies of 96% and 63% for chemical oxygen demand (COD) and total phosphorous (TP), respectively. More than 60% of Firmicutes (Terrisporobacter, Turicibacter, and Clostridium sensu stricto 1), which were dominated by Methanosaeta and Methanobacterium with relative abundances of 58.6% and 36.8% in the UASB and 22.5% and 40.3% in the AnMBR, respectively, converted complex compounds into organic acids for methanogenesis. This research presented an analysis of pollutants removal and microbial dynamics of UASB-AnMBR, which significantly affected the large-scale application of UASB-AnMBR process.

A critical review on microbial ecology in the novel biological nitrogen removal process: Dynamic balance of complex functional microbes for nitrogen removal.

The novel biological nitrogen removal process has been extensively studied for its high nitrogen removal efficiency, energy efficiency, and greenness. A successful novel biological nitrogen removal process has a stable microecological equilibrium and benign interactions between the various functional bacteria. However, changes in the external environment can easily disrupt the dynamic balance of the microecology and affect the activity of functional bacteria in the novel biological nitrogen removal process. Therefore, this review focuses on the microecology in existing the novel biological nitrogen removal process, including the growth characteristics of functional microorganisms and their interactions, together with the effects of different influencing factors on the evolution of microbial communities. This provides ideas for achieving a stable dynamic balance of the microecology in a novel biological nitrogen removal process. Furthermore, to investigate deeply the mechanisms of microbial interactions in novel biological nitrogen removal process, this review also focuses on the influence of quorum sensing (QS) systems on nitrogen removal microbes, regulated by which bacteria secrete acyl homoserine lactones (AHLs) as signaling molecules to regulate microbial ecology in the novel biological nitrogen removal process. However, the mechanisms of action of AHLs on the regulation of functional bacteria have not been fully determined and the composition of QS system circuits requires further investigation. Meanwhile, it is necessary to further apply molecular analysis techniques and the theory of systems ecology in the future to enhance the exploration of microbial species and ecological niches, providing a deeper scientific basis for the development of a novel biological nitrogen removal process.

Intensifying single-stage denitrogen by a dissolved oxygen-differentiated airlift internal circulation reactor under organic matter stress: Nitrogen removal pathways and microbial interactions.

Current research focuses on efficient single-stage nitrogen removal from organic matter wastewater using the partial nitritation-anammox (PNA) process. In this study, we constructed a single-stage partial nitritation-anammox and denitrification (SPNAD) system using a dissolved oxygen-differentiated airlift internal circulation reactor. The system was operated continuously for 364 days at 250 mg/L NH4+-N. During the operation, the COD/NH4+-N ratio (C/N) was increased from 0.5 to 4 (0.5, 1, 2, 3, and 4), and the aeration rate (AR) gradually increased. The results showed that the SPNAD system maintained efficient and stable operation at C/N = 1-2 and AR = 1.4-1.6 L/min, with an average total nitrogen removal efficiency of 87.2%. The removal pathways of pollutants in the system and the interactions between microbes were revealed by analyzing the changes in sludge characteristics and microbial community structure at different phases. As the influent C/N increased, the relative abundance of Nitrosomonas and Candidatus Brocadia decreased, and that of denitrifying bacteria, such as Denitratisoma, increased to 44%. The nitrogen removal pathway of the system gradually changed from autotrophic nitrogen removal to nitrification-denitrification. At the optimum C/N, the SPNAD system synergistically removed nitrogen through PNA and nitrification-denitrification. Overall, the unique reactor configuration facilitated the formation of dissolved oxygen compartments, providing a suitable environment for different microbes. An appropriate organic matter concentration maintained the dynamic stability of microbial growth and interactions. These enhance microbial synergy and enable efficient single-stage nitrogen removal.

Revealing the effect of biofilm formation in partial nitritation-anammox systems: Start-up, performance stability, and recovery.

Successful application of partial nitritation-anammox (PNA) processes is currently and primarily associated with biofilm systems. Biofilm characteristics significantly influence start-up, performance stability, and recovery. Here, two PNA systems with and without carriers were implemented simultaneously for treating wastewater containing 50 mg-NH4/L. The performance characteristics of these two PNA systems were compared. Stable nitrogen removal efficiencies of 76.3 ± 2.8% and 72.9 ± 1.6% were obtained for suspended sludge and biofilm systems, respectively. The slow process of biofilm colonization resulted in a long start-up time in the biofilm system. Biofilm enrichment and protection conferred stable performance when exposed to aeration shock. The suspended sludge system displayed good elasticity during performance recovery after shock compared to the slow recovery in the biofilm system. Moreover, suitable control of dissolved oxygen could improve the activity and abundance of the functional microbes. This study provides new insights into the operation and control of PNA systems for treating mainstream wastewater.

Dual inner circulation and multi-partition driving single-stage autotrophic nitrogen removal in a bioreactor.

The single-stage autotrophic nitrogen removal (ANR) process is impeded by a long start-up cycle and unstable operation performance. In this study, an airlift inner-circulation partition bioreactor (AIPBR) was operated continuously for 215 days to explore methods of strengthening the performance and stable operation of the single-stage ANR system. AIPBR start-up period took around 38 days, the total nitrogen removal efficiency was > 85% on day 35. With the decrease of hydraulic retention time and the increase of aeration rate, the nitrogen removal rate increased to 0.85 ± 0.02 kg-N/m3/day. The sludge morphology gradually changed into dark-red floc-coupled granular sludge. Nitrosomonas (9.95%) and Candidatus Brocadia (6.41%) were dominant in the sludge. During long-term operation, AIPBR achieved the dual inner circulation of sewage and sludge and then formed effective dissolved oxygen and sludge partitions to provide a suitable growth environment for various functional bacteria, promote synergy between them, and strengthen the ANR performance.

Insights into the synergy between functional microbes and dissolved oxygen partition in the single-stage partial nitritation-anammox granules system.

The rapid start-up and stable operation of the single-stage partial nitritation-anammox (PNA) process remains a challenge in practical applications. An integrated investigation of nitrogen removal performance, sludge characteristics, activity and abundance, and microbial dynamics was implemented for 360 days via an airlift internal circulation reactor. During long-term operation, the reactor realized a stable dissolved oxygen (DO) partition and cultivated granular sludge. The nitrogen removal rate increased from 0.15 kg-N/m3/d to 1.24 kg-N/m3/d, and a high nitrogen removal efficiency of 82.6% was obtained. A stable DO partition further accelerated the bioreaction rates and enhanced the activity of functional microbes. The activities of ammonia oxidation and anammox reached 1.21 g-N/g-VSS/d and 1.43 g-N/g-VSS/d, respectively. Sludge granulation efficiently enriched the abundances of Candidatus Brocadia (7.4%) and Nitrosomonas (5.2%). These results demonstrated that efficient DO partition and stable culture of granular sludge could enhance the synergy of functional microbes for autotrophic nitrogen removal.

Insights into rapidly recovering the autotrophic nitrogen removal performance of single-stage partial nitritation-anammox systems: Reconstructing granular sludge and its functional microbes synergy.

Partial nitritation-anammox (PNA) deteriorates easily and is difficult to recover. After an airlift inner-circulation partition bioreactor was impacted by low NH4+-N wastewater containing organic matter, Nitrospira and Denitratisoma propagated rapidly, granular sludge disintegrated, and the total nitrogen removal efficiency (TNRE) decreased from 68.27 % to 5.97 %. This study used a unique strategy to recover deteriorated single-stage PNA systems and explored the mechanism of rapid performance recovery. The TNRE of the system recovered up to 61.77 % in 43 days. The high nitrogen loading rate and hydraulic shear force from the airlift caused the sludge in the reactor to granulate again. The microbial community structure recovered, with a decrease in the abundance of Nitrospira (0.05 %) and enrichment of Candidatus Brocadia (8.82 %). A favorable synergy among functional microbes in the reactor was thus re-established, promoting the rapid recovery of the nitrogen removal performance. This study provides a feasible recovery strategy for PNA processes.

Key factors governing the performance and microbial community of one-stage partial nitritation and anammox system with bio-carriers and airlift circulation.

A one-stage airlift internal circulation biofilm reactor was continuously operated for 668 days to treat 50 mg/L of ammonia wastewater to pursue the long-term stability of partial nitritation and anammox (PNA) process. The operational performance and microbial community structure of the biofilm and the flocs were investigated. A nitrogen removal efficiency (NRE) of 70% was obtained successfully at a dissolved oxygen (DO) of 0.05-0.15 mg/L by regulating aeration rate. The microbial analysis indicated Candidatus Brocadia (29.5%) and Nitrosomonas (6.8%) were dominant in both biofilms and flocs. It was found that DO control and aeration rate were the key factors in performance stability, and a stable performance could be recovered and maintained under oxygen-limiting conditions. Further, the achievement of activated ammonia oxidation bacteria (AOB), dominated anammox bacteria (AMX), suppressed NOB, and controlled heterotrophic bacteria (HB) in the biofilms played a major role in the long-term stable operation.

Recent advances in partial denitrification-anaerobic ammonium oxidation process for mainstream municipal wastewater treatment.

To solve the bottleneck of the unstable accumulation of nitrite in the partial nitrification (PN)-anammox (AMX) in municipal wastewater treatment, a novel process called partial denitrification (PD)-AMX has been developed. PD-AMX, which is known for cost-efficiency and environmental friendliness, has currently exhibited a promising potential for the removal of biological nitrogen from municipal wastewater and has attracted much research interest regarding its process mechanisms, as well as its practical applications. Here, we review the recent advances in the PD process and its coupling to the anammox process, including the development, basic principles, main characteristics, and critical process parameters of the stable operation of the PD-AMX process. We also explore the microbial community and its characteristics in the system and summarize the knowledge of the dominant bacteria to clarify the key factors affecting PD-AMX. Then, we introduce the engineering feasibility and economic feasibility as well as the potential challenges of the process. The induction and implementation of partial denitrification and maintenance of mainstream anammox are critical issues to be urgently solved. Meanwhile, the implementation of a full mainstream anammox application remains burdensome, while the mechanism of partial denitrification coupled to anammox needs to be further studied. Additionally, stable operation performance and process control1 methods need to be optimized or developed for the PD-AMX system for better engineering practice. This review can help to accelerate the research and application of the PD-AMX process for municipal wastewater treatment.