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.

The phosphorus harvest from low-temperature mainstream wastewater through iron phosphate crystallization in a pilot-scale partial nitritation/anammox reactor.

The phosphorus harvest along nitrogen removal in the partial nitritation/anammox (PNA) reactor is promising for saving space and simplifying the management of mainstream wastewater treatment facilities. In this study, the phosphorus recovery from the low-temperature mainstream wastewater was explored through iron phosphate crystallization in a pilot-scale PNA reactor. With the COD-alleviated municipal wastewater as the influent, the ammonium concentration of about 50 mg/L and the phosphorus concentration ranged from 5.4 to 7.1 mg/L, under the temperature of 15 °C and the addition of external ferrous iron of 14 mg/L, the achieved nitrogen removal efficiency and the phosphorus removal efficiency were 37.6 % and 62.7 %, respectively. The good settleability of sludge indicated that the formed iron phosphate was well combined with the biomass. The quantitative analysis confirmed that the main iron phosphate in dry sludge was graftonite, and qualitative analysis confirmed that the equivalent of P2O5 content in the sludge was 5.8 %, which was suitable as fertilizer on agricultural land to realize the direct recycle of discharged phosphorus. In all, this study proposed a pioneering scheme to realize the nitrogen removal and phosphorus cycle in human society and given a meaningful reference for further research and application.

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.

The performance of simultaneous partial nitritation, anammox, denitrification, and COD oxidation (SNADCO) method in the treatment of digested effluent of fish processing wastewater.

The simultaneous partial nitritation, anammox, denitrification, and COD oxidation (SNADCO) method was successfully carried out in an air-lift moving bed biofilm reactor (AL-MBBR) with cylinders carriers for the treatment of digested fish processing wastewater (FPW). Synthetic wastewater was used as substrate at stage 1. It changed into the digested FPW with dilution variation in order to increase the nitrogen and COD loading rates. With influent concentration of NH4+-N of 909 ± 101 mg-N/L and COD of 731 ± 26 mg/L, the nitrogen removal efficiency was 86.8% (nitrogen loading rate of 1.21 g-TN/L/d) and the COD removal efficiency was 50.5% (COD loading rate at 0.98 g-COD/L/d). This study showed that the process has the advantages in treating the real high ammonia concentration of digested wastewater containing organic compounds. The nitritation and anammox route was predominant in nitrogen removal, while COD oxidation and microbe proliferation played the main role in COD removal.

Treating the filtrate of mainstream anaerobic membrane bioreactor with the pilot-scale sludge-type one-stage partial nitritation/anammox process operated from 25 to 15 °C.

At ambient temperature condition, the one-stage partial nitritation/anammox (PNA) process has been successfully adopted to treat the filtrate from the mainstream anaerobic membrane bioreactor (AnMBR). However, there is no investigation of the performance of this process at low-temperature condition. In this study, the nitrogen removal performance of a pilot-scale PNA reactor at the temperature of 15 °C for treating the filtrate of a mainstream AnMBR was investigated. The nitrogen removal rate of 0.09 kg/m3/d and the nitrogen removal efficiency of 37.6% were achieved. The anammox reaction was the rate-limiting step of the nitrogen removal. Nitrogen removal was attributed in part to denitrification activity. The microbial community analysis confirmed that the main functional bacteria comprised of genus Nitrosomonas and genus Kuenenia. In sum, this research demonstrated the applicability of PNA process for mainstream AnMBR filtrate treatment to some extent and enriched the related knowledge.

High nitrogen removal performance of anaerobically treated fish processing wastewater by one-stage partial nitritation and anammox process with hydroxyapatite (HAP)-based syntrophic granules and granule structure.

The one-stage partial nitritation and anammox process with the hydroxyapatite (HAP)-based syntrophic granules was studied for the ammonium nitrogen removal from the effluents of a self-agitated anaerobic baffled reactor treating the fish processing wastewater. When the ammonium in the influent was 1140 mg N·L-1, a high nitrogen removal rate and nitrogen removal efficiency of 1.51 ± 0.10 kg N·m-3·d-1 and 88.2% were obtained, respectively. Anammox bacteria of Candidatus Kuenenia stuttgartiensis and ammonium oxidizing bacteria of Nitrosomonas were the two most predominant bacteria, while nitrite oxidizing bacteria activity was low and could be neglected during the treatment. The inorganic element properties of the sludge were analyzed by several methods to confirm the existence of HAP granules. Optical microscopic observation and scanning electron microscopy analysis revealed the structure of the granular sludge.This study supports the feasibility and potential of this process for high-efficiency nitrogen removal from fish processing wastewater.

The impact of calcium supplementation on methane fermentation and ammonia inhibition of fish processing wastewater.

The effect of trace metal supplementation on the methane fermentation of fish processing wastewater (FPW) was studied in both batch and continuous experiments using a self-agitated anaerobic baffled reactor (SA-ABR). In the batch experiments, a single supplementation of Ca2+, Co2+ and Fe2+ was show to have a significant positive impact on the performance of methane fermentation. The continuous experiment results showed that supplementation with 1.5 g-Ca2+/L-substrate remarkably enhanced the performance of methane fermentation of the SA-ABR in treating FPW with the optimal organic loading rate achieved at 7.62 g-COD/L/d. During the steady states (stages 2 to 5), the average removal efficiencies of COD, protein, carbohydrate and lipid were 89, 85, 80 and 91%, respectively. The biogas conversion rates were in the range of 0.39 to 0.45 L-biogas/g-COD with a high methane content of 74%. Besides, Ca2+ supplementation also improved the resistance of the methane fermentation system to ammonia inhibition.

Mesophilic methane fermentation performance and ammonia inhibition of fish processing wastewater treatment using a self-agitated anaerobic baffled reactor.

The performance of the self-agitated anaerobic baffled reactor (SA-ABR) was investigated by increasing the organic loading rates (OLRs) from 0.46 to 9.50 g-COD/L/d. A good performance was achieved by the SA-ABR for the treatment of fish processing wastewater (FPW). The maximum OLR was 6.77 g-COD/L/d and the biogas production rate reached 2.16 L/L-reactor/d with a methane content of 69% at this OLR. The COD, carbohydrate, protein, lipid and VS removal efficiencies were as high as 64, 65, 68, 78 and 79%, respectively. Ammonia inhibition was assumed with inhibition concentrations of 10% (IC10) and 20% (IC20) at 4140 and 5780 mg/L. However, it was found that the reactor could tolerate ammonia at a high concentration range of 4500-6373 mg/L after a long-term continuous experiment. Ammonia inhibition was addressed by diluting the substrate and the sludge in the reactor with tap water.