Exploration and verification of the feasibility of the sulfur-based autotrophic denitrification integrated biomass-based heterotrophic denitrification systems for wastewater treatment: From feasibility to application

Using rice straw-augmented ecological floating beds to enhance nitrogen removal in carbon-limited wastewater

Agricultural biomass used as solid carbon substrates in ecological floating beds (EFBs) has been proven to be applicable in nitrogen removal for carbon-limited wastewater treatment. However, the subtle interactions among plants, rhizosphere microorganisms, and supplementary carbon sources have not been thoroughly studied. This study combined rice straw mats with different aquatic macrophytes in EFBs to investigate denitrification efficiency in carbon-limited eutrophic waters. Results showed that rice straw significantly enhanced the nitrogen removal efficiency of EFBs, while enriching nitrogen-fixing and denitrifying bacteria (such as Rhizobium, Rubrivivax, and Rhodobacter, etc.). Additionally, during the denitrification process in EFBs, rice straw can release humic acid-like fraction as electron donors to support the metabolic activities of microorganisms, while aquatic macrophytes provide a more diverse range of dissolved organic matters, facilitating a sustainable denitrification process. These findings help to understand the synergistic effect of denitrification processes within wetland ecosystems using agricultural biomass.

Salinity stress results in ammonium and nitrite accumulation during the elemental sulfur-driven autotrophic denitrification process

In this study, we investigated the effects of salinity on elemental sulfur-driven autotrophic denitrification (SAD) efficiency, and microbial communities. The results revealed that when the salinity was ≤6 g/L, the nitrate removal efficiency in SAD increased with the increasing salinity reaching 95.53% at 6 g/L salinity. Above this salt concentration, the performance of SAD gradually decreased, and the nitrate removal efficiency decreased to 33.63% at 25 g/L salinity. Approximately 5 mg/L of the hazardous nitrite was detectable at 15 g/L salinity, but decreased at 25 g/L salinity, accompanied by the generation of ammonium. When the salinity was ≥15 g/L, the abundance of the salt-tolerant microorganisms, Thiobacillus and Sulfurimonas, increased, while that of other microbial species decreased. This study provides support for the practical application of elemental sulfur-driven autotrophic denitrification in saline nitrate wastewater.

Intensified denitrification in a fluidized-bed reactor with suspended sulfur autotrophic microbial fillers

Sulfur-based autotrophic denitrification (SAD) is a promising low-carbon approach to tackle nitrate pollution. However, practical SAD reactor implementation faces challenges of slow denitrification rates and prolonged start-up periods. In this work, a fluidized-bed denitrification reactor with suspended composite fillers immobilized with elemental sulfur and SAD bacteria was constructed. The reactor reaches a steady state within the first day of operation. A denitrification rate of 0.61 g N L-1 d-1 was realized, which is 2.4-fold higher than that in the packed-bed reactor. Mixotrophic denitrification prevailed during the start-up period, while the SAD process became the predominant pathway (>70%) after several days of operation. The prevailing bacteria in the fillers, notably Thiobacillus, are enriched during denitrification operations. Overall, this study highlights the impressive denitrification capabilities of the fluidized SAD reactor with microbial fillers, providing valuable insights for enhancing denitrification techniques.

Advance in the sulfur-based electron donor autotrophic denitrification for nitrate nitrogen removal from wastewater

In the field of wastewater treatment, nitrate nitrogen (NO3--N) is one of the significant contaminants of concern. Sulfur autotrophic denitrification technology, which uses a variety of sulfur-based electron donors to reduce NO3--N to nitrogen (N2) through sulfur autotrophic denitrification bacteria, has emerged as a novel nitrogen removal technology to replace heterotrophic denitrification in the field of wastewater treatment due to its low cost, environmental friendliness, and high nitrogen removal efficiency. This paper reviews the advance of reduced sulfur compounds (such as elemental sulfur, sulfide, and thiosulfate) and iron sulfides (such as ferrous sulfide, pyrrhotite, and pyrite) electron donors for treating NO3--N in wastewater by sulfur autotrophic denitrification technology, including the dominant bacteria types and the sulfur autotrophic denitrification process based on various electron donors are introduced in detail, and their operating costs, nitrogen removal performance and impacts on the ecological environment are analyzed and compared. Moreover, the engineering applications of sulfur-based electron donor autotrophic denitrification technology were comprehensively summarized. According to the literature review, the focus of future industry research were discussed from several aspects as well, which would provide ideas for the application and optimization of the sulfur autotrophic denitrification process for deep and efficient removal of NO3--N in wastewater.

Response of nitrite accumulation, sludge characteristic and microbial transition to carbon source during the partial denitrification (PD) process

Partial denitrification (PD, nitrate (NO3--N) → nitrite (NO2--N)) as a novel pathway for NO2--N production has been widely concerned, but the specific conditions for highly efficient and stable nitrite maintenance are not yet fully understood. In this study, the effects of carbon sources (acetate, R1; propionate, R2; glucose, R3) on NO2--N accumulation was discussed without seeding PD sludge and the mechanism analysis related to sludge characteristic and microbial evolution were elucidated. The optimal NO2--N, nitrate-to-nitrite transformation ratio (NTR) and nitrite removal efficiency (NRE) reached up to 32.10 mg/L, 98.01 %, and 86.95 % in R1. However, due to the complex metabolic pathway of glucose, the peak time of NO2--N production delayed from 30 min to 60 min. The sludge particle size decreased from 154.2 μm (R1), 130.8 μm (R2) to 112.6 μm (R3) with the increasing extracellular polymeric substances (EPS) from 80.75-85.44 mg/gVSS, 82.68-92.75 mg/gVSS to 106.31-110.25 mg/gVSS, where the ratio of proteins/polysaccharides (PN/PS) was proved to be closely associated with NO2--N generation. For the microbial evolution, Saccharimonadales (70.42 %) dominated the glucose system, while Bacillus (7.42-21.63 %) and Terrimonas (4.24-5.71 %) were the main contributors for NO2--N accumulation in the acetate and propionate systems. The achievement of PD showed many advantages of lower carbon demand, minimal sludge production, lesser greenhouse gas emission and prominent nutrient removal, offering an economically and technically attractive alternative for NO3--N containing wastewater treatment.

Effects of salinity on nitrite and elemental sulfur accumulation in a double short-cut sulfur autotrophic denitrification process

Double short-cut sulfur autotrophic denitrification (DSSADN) coupled with Anammox is of great significance in the low-carbon treatment of nitrogen-containing wastewater. In order to achieve high salinity autotrophic nitrogen removal, the effects of different salinities on the accumulation characteristics of NO₂^--N and S⁰ and microorganisms in DSSADN process were studied. The results showed that the effect of salinity on the DSSADN process could be categorized into the stimulation, stable, and inhibition. When the salinity gradually increased to 2.5 %, the highest NO₂^--N production rate (NiPR) and S⁰ production rate (S⁰PR) of DSSADN were 0.54 kg/(m³·d) and 1.1 kg/(m³·d) respectively. NiPR and S⁰PR gradually decreased as the salinity increased to more than 3 %. However, salinity had a relatively low impact on nitrite accumulation efficiency and S⁰ accumulation efficiency, which were 80 % and 81.5 %, respectively, when the salinity reached 5 %. Salinity has a great influence on the structure and abundance of microbial communities in the system.

Sediment diffusion is feasible to simultaneously reduce nitrate discharge from recirculating aquaculture system and ammonium release from sediments in receiving intensive aquaculture pond

Nitrogen accumulation has become one of the greatest unresolved challenges restricting the development of aquaculture worldwide. In recirculating aquaculture system (RAS), lack of organic matter (OM) and sensitive organisms makes it difficult to apply efficient denitrifying technology, thus leading to a high nitrate‑nitrogen (NO₃^--N) accumulation. In contrast, excess OM accumulation in intensive aquaculture pond sediments is associated with dissolved oxygen depletion and ammonium‑nitrogen (NH₄⁺-N) accumulation in the sediments. Based on the opposing effects of OM on the nitrogen accumulation in RAS and intensive aquaculture ponds, this study assessed the feasibility of simultaneously reducing NO₃^--N discharge from RAS and controlling NH₄⁺-N accumulation in intensive aquaculture ponds by in situ diffusing RAS tailwater containing NO₃^--N into intensive aquaculture pond sediments. The results showed that NO₃^--N diffusion strategy improved the native sediment denitrification capacity, thus increasing NO₃^--N removal efficiency from RAS tailwater and significantly decreasing the NH₄⁺-N concentration in interstitial water and the total organic carbon content in intensive aquaculture pond sediments. High-throughput sequencing and quantitative real-time polymerase chain reaction (qPCR) results revealed that NO₃^--N addition significantly increased both nitrifying bacteria and denitrifying bacteria abundance. These results implied that NO₃^--N diffusion strategy could effectively stimulate microbial decomposition of OM, thus relieving the hypoxia limitation of sediment nitrification. Overall, this study offers a feasible method for simultaneous reduction of NO₃^--N from RAS tailwater and NH₄⁺-N in intensive aquaculture ponds with low cost and high efficiency.

The research progress, hotspots, challenges and outlooks of solid-phase denitrification process

Nitrogen pollution is one of the main reasons for water eutrophication. The difficulty of nitrogen removal in low-carbon wastewater poses a huge potential threat to the ecological environment and human health. As a clean biological nitrogen removal process, solid-phase denitrification (SPD) was proposed for long-term operation of low-carbon wastewater. In this paper, the progress, hotspots, and challenges of the SPD process based on different solid carbon sources (SCSs) are reviewed. Compared with synthetic SCS and natural SCS, blended SCSs have more application potential and have achieved pilot-scale application. Differences in SCSs will lead to changes in the enrichment of hydrolytic microorganisms and hydrolytic genes, which indirectly affect denitrification performance. Moreover, the denitrification performance of the SPD process is also affected by the physical and chemical properties of SCSs, pH of wastewater, hydraulic retention time, filling ratio, and temperature. In addition, the strengthening of the SPD process is an inevitable trend. The strengthening measures including SCSs modification and coupled electrochemical technology are regarded as the current research hotspots. It is worth noting that the outbreak of the COVID-19 epidemic has led to the increase of disinfection by-products and antibiotics in wastewater, which makes the SPD process face challenges. Finally, this review proposes prospects to provide a theoretical basis for promoting the efficient application of the SPD process and coping with the challenge of the COVID-19 epidemic.

The mixed/mixotrophic nitrogen removal for the effective and sustainable treatment of wastewater: From treatment process to microbial mechanism

Biological nitrogen removal (BNR) is one of the most important environmental concerns in the field of wastewater treatment. The conventional BNR process based on heterotrophic nitrogen removal (HeNR) is suffering from several limitations, including external carbon source dependence, excessive sludge production, and greenhouse gas emissions. Through the mediation of autotrophic nitrogen removal (AuNR), mixed/mixotrophic nitrogen removal (MixNR) offers a viable solution to the optimization of the BNR process. Here, the recent advance and characteristics of MixNR process guided by sulfur-driven autotrophic denitrification (SDAD) and anammox are summarized in this review. Additionally, we discuss the functional microorganisms in different MixNR systems, shedding light on metabolic mechanisms and microbial interactions. The significance of MixNR for carbon reduction in the BNR process has also been noted. The knowledge gaps and the future research directions that may facilitate the practical application of the MixNR process are highlighted. Overall, the prospect of the MixNR process is attractive, and this review will provide guidance for the future implementation of MixNR process as well as deciphering the microbially metabolic mechanisms.

An efficient anoxic/aerobic/aerobic/anoxic process for domestic sewage treatment: From feasibility to application

In this paper, the anoxic/aerobic/aerobic/anoxic (AOOA) process was proposed using fixed biofilms in a continuous plug-flow multi-chamber reactor, and no sludge reflux operation was performed during the 190 days of operation. The reactor volume ratio of 1.5:2:1.5:1 (A/O/O/A) with the dissolved oxygen (DO) concentration of 2 mg L⁻¹ in the aerobic zone was the optimal condition for reactor operation. According to the results obtained from the treatment of real domestic sewage, when the hydraulic retention time (HRT) was 6 h, the effluent of the reactor could meet the discharge standard even in cold conditions (13°C). Specifically, the elemental-sulfur-based autotrophic denitrification (ESAD) process contributed the most to the removal of total inorganic nitrogen (TIN) in the reactor. In addition, the use of vibration method was helpful in removing excess sludge from the biofilms of the reactor. Overall, the AOOA process is an efficient and convenient method for treating domestic sewage.

Denitrification Performance in Packed-Bed Reactors Using Novel Carbon-Sulfur-Based Composite Filters for Treatment of Synthetic Wastewater and Anaerobic Ammonia Oxidation Effluent

To avoid nitrate pollution in water bodies, two low-cost and abundant natural organic carbon sources were added to make up the solid-phase denitrification filters. This study compared four novel solid-phase carbon-sulfur-based composite filters, and their denitrification abilities were investigated in laboratory-scale bioreactors. The filter F₄ (mixture of elemental sulfur powder, shell powder, and peanut hull powder with a mass ratio of 6:2.5:1.5) achieved the highest denitrification ability, with an optimal nitrate removal rate (NRR) of 723 ± 14.2 mg NO₃^–-N⋅L^–1⋅d^–1 when the hydraulic retention time (HRT) was 1 h. The HRT considerably impacted effluent quality after coupling of anaerobic ammonium oxidation (ANAMMOX) and solid-phase-based mixotrophic denitrification process (SMDP). The concentration of suspended solids (SS) of the ANAMMOX effluent may affect the performance of the coupled system. Autotrophs and heterotrophs were abundant and co-existed in all reactors; over time, the abundance of heterotrophs decreased while that of autotrophs increased. Overall, the SMDP process showed good denitrification performance and reduced the sulfate productivity in effluent compared to the sulfur-based autotrophic denitrification (SAD) process.

Simultaneously pollutant removal and S0 recovery from composite wastewater containing Cr(VI)-S2- based on biofilm enhancement

Bioaugmentation of extracellular polymeric substances-producing bacteria was applied in pollutant removal and S⁰ recovery from composite wastewater in a mixotrophic denitrification system. In the presence of 200 mg·L^-1 S^2- and 50 mg·L^-1 Cr(VI), the removal efficiencies of chemical oxygen demand, NO₃^-, S^2- and Cr(VI) were 86.38%, 91.82%, 95.75%, and 100.00% respectively, while S⁰ recovery efficiency reached 79.17%. Increased contents of protein and polysaccharide, especially the high ratio of protein/polysaccharide verified the structural stability of biofilm was promoted by biofilm enhancement. The widespread distribution of bacteria/extracellular polymeric substance (EPS) revealed the more obvious biofilms formation in biofilm-enhanced group. High-throughput sequencing analysis showed that EPS-producing bacteria (Flavobacterium, Thauera, Thiobacillus and Simplicispira) were dominant bacteria in the biofilm-enhanced group. Moreover, by comprehensive considering of redundancy analysis, the colonization of selected bacteria improved the robustness of the reactor and treatment performance to wastewater contained toxic pollutions.

Advances in elemental sulfur-driven bioprocesses for wastewater treatment: From metabolic study to application

Elemental sulfur (S0) is known to be an abundant, non-toxic material with a wide range of redox states (-2 to +6) and may serve as an excellent electron carrier in wastewater treatment. In turn, S0-driven bioprocesses, which employ S0 as electron donor or acceptor, have recently established themselves as cost-effective therefore attractive solutions for wastewater treatment. Numerous related processes have, to date, been developed from laboratory experiments into full-scale applications, including S0-driven autotrophic denitrification for nitrate removal and S0-reducing organic removal. Compared to the conventional activated sludge process, these bioprocesses require only a small amount of organic matter and produce very little sludge. There have been great efforts to characterize chemical and biogenic S0 and related functional microorganisms in order to identify the biochemical pathways, upgrade the bioprocesses, and assess the impact of the operating factors on process performance, ultimately aiming to better understand and to optimize the processes. This paper is therefore a comprehensive overview of emerging S0-driven biotechnologies, including the development of S0-driven autotrophic denitrification and S0-based sulfidogenesis, as well as the associated microbiology and biochemistry. Also reviewed here are the physicochemical characteristics of S0 and the effects that environmental factors such as pH, influent sulfur/nitrate ratio, temperature, S0 particle size and reactor configurations have on the process. Research gaps, challenges of process applications and potential areas for future research are further proposed and discussed.