Sequential sulfur-based denitrification/denitritation and nanofiltration processes for drinking water treatment

Inhibitory mechanism of Cr(VI) on sulfur-based denitrification: Bio-toxicity, bio-electron characteristics, and microbial evolution

Sulfur-based denitrification is a promising technology for efficient nitrogen removal in low-carbon wastewater, while it is easily affected by toxic substances. This study revealed the inhibitory mechanism of Cr(VI) on thiosulfate-based denitrification, including bio-toxicity and bio-electron characteristics response. The activity of nitrite reductase (NIR) was more sensitive to Cr(VI) than that of nitrate reductase (NAR), and NIR was inhibited by 21.32 % and 19.86 % under 5 and 10 mg/L Cr(VI), resulting in 10.12 and 15.62 mg/L of NO2--N accumulation. The biofilm intercepted 36.57 % of chromium extracellularly by increasing 25.78 % of extracellular polymeric substances, thereby protecting microbes from bio-toxicity under 5 mg/L Cr(VI). However, it was unable to resist 20-30 mg/L of Cr(VI) bio-toxicity as 19.95 and 14.29 mg Cr/(g volatile suspended solids) invaded intracellularly, inducing the accumulation of reactive oxygen species by 165.98 % and 169.12 %, which triggered microbial oxidative-stress and damaged the cells. In terms of electron transfer, S2O32- oxidation was inhibited, and parts of electrons were redirected intracellularly to maintain microbial activity, resulting in insufficient electron donors. Meanwhile, the contents of flavin adenine dinucleotide and cytochrome c decreased under 5-30 mg/L Cr(VI), reducing the electron acquisition rate of denitrification. Thermomonas (the dominant genus) possessed denitrification and Cr(VI) resistance abilities, playing an important role in antioxidant stress and biofilm formation. ENVIRONMENTAL IMPLICATION: Sulfur-based denitrification (SBD) is a promising method for nitrate removal in low-carbon wastewater, while toxic heavy metals such as Cr(VI) negatively impair denitrification. This study elucidated Cr(VI) inhibitory mechanisms on SBD, including bio-toxicity response, bio-electron characteristics, and microbial community structure. Higher concentrations Cr(VI) led to intracellular invasion and oxidative stress, evidenced by ROS accumulation. Moreover, Cr(VI) disrupted electron flow by inhibiting thiosulfate oxidation and affecting electron acquisition by denitrifying enzymes. This study provided valuable insights into Cr(VI) toxicity, which is of great significance for improving wastewater treatment technologies and maintaining efficient and stable operation of SBD in the face of complex environmental challenges.

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.

Performance of sulfur-based autotrophic denitrification process for nitrate removal from permeate of an MBR treating textile wastewater and concentrate of a real scale reverse osmosis process

Textile is one of the industrial sectors generating the highest amount of wastewater with various polluting substances. Lately, water reuse in textile industries, especially, with the reverse osmosis (RO) process following membrane bioreactor (MBR) treatment has been applied more commonly. In this study, an autotrophic sulfur-based denitrifying column performance was evaluated, for the first time, for nitrate reduction from permeate of a lab-scale MBR receiving real textile wastewater and from the concentrate stream of a real scale-RO plant used for recovering water from textile wastewater. Nitrate concentration in the MBR effluent and RO concentrate averaged 35 ± 3 and 12 ± 2 mg-N/L, respectively. With the sulfur-based column bioreactor, quite high (≥90%) denitrification performances were attained both for MBR effluent and RO concentrate up to nitrate loadings of 0.432 and 0.12 g-N/(L.d), respectively. COD present in wastewater was not utilized in the column bioreactor, which illustrates no or minimal contribution of heterotrophic denitrification. Alkalinity concentration in the wastewater was enough to buffer the acid formation during autotrophic denitrification. Sulfate was generated accompanied by nitrate reduction and sulfide was formed at low nitrate loadings. In the batch tests, the denitrification rates for the MBR effluent and RO concentrate were 0.31 and 0.28 g-N/(g-VSS.d), respectively, which were relatively higher than the ones observed for the synthetic nitrate-contaminated groundwater. Autotrophic sulfur-based denitrification is a promising and robust process alternative even for textile RO concentrate with high concentrations of salinity, non-biodegradable COD, and color.

Impact of temperature and biomass augmentation on biosulfur-driven autotrophic denitrification in membrane bioreactors treating real nitrate-contaminated groundwater

Nitrate (NO₃^-) contamination of groundwater is a major health concern worldwide as it can lead to serious illnesses such as methemoglobinemia and cancer. Autotrophic denitrification is a smart approach for treating groundwater, being typically organic-deficient. Lately, biogenic sulfur (S⁰_bio) has emerged as a sustainable, free, and high-efficiency substrate to fuel membrane bioreactors (MBRs) treating contaminated groundwater. However, the effects of moderate temperature and biomass concentration on the performance and fouling of the S⁰_bio-fed MBR were not investigated previously. This study shows that biomass levels of ~1 g MLVSS/L limit membrane fouling but also denitrification efficiency. Biomass augmentation up to 3 g MLVSS/L enhanced denitrification but worsened fouling due to increase of extracellular polymeric substance (EPS) levels in the bulk liquid. Temperature decrease from 30 °C to 20 °C halved denitrification efficiency, which could be partially recovered through bioaugmentation. The mechanisms affected by temperature decrease, practical applications, and future research needs were discussed.

Simultaneous nitrification-denitrification in biofilm systems for wastewater treatment: Key factors, potential routes, and engineered applications

Simultaneous nitrification-denitrification (SND) is an advantageous bioprocess that allows the complete removal of ammonia nitrogen through sequential redox reactions leading to nitrogen gas production. SND can govern nitrogen removal in single-stage biofilm systems, such as the moving bed biofilm reactor and aerobic granular sludge system, as oxygen gradients allow the development of multilayered biofilms including nitrifying and denitrifying bacteria. Environmental and operational conditions can strongly influence SND performance, biofilm development and biochemical pathways. Recent advances have outlined the possibility to reduce the carbon and energy consumption of the process via the "shortcut pathway", and simultaneously remove both N and phosphorus under specific operational conditions, opening new possibilities for wastewater treatment. This work critically reviews the factors influencing SND and its application in biofilm systems from laboratory to full scale. Operational strategies to enhance SND efficiency and hints to reduce nitrous oxide emission and operational costs are provided.

A new inoculation method of sulfur autotrophic denitrification reactor for accelerated start-up and better low-temperature adaption

Elemental sulfur (S⁰) autotrophic denitrification (SAD) has been proved feasible for nitrate removal from aquatic environments. The long start-up period up to weeks of the SAD reactor impedes its industrial application. To accelerate the start-up process, this study employed S⁰ powder packed sequencing batch reactor operated for 10 days to obtain a seed biofilm, which was inoculated into a regular S⁰ flake packed bed reactor afterwards. Merely two days after inoculation, the reactor inoculated with seed biofilm was well started up and outperformed the control reactor, which was inoculated with regular anaerobic sludge and operated for more than 10 days, delivering much increased denitrification rate of 126 ± 0.68 mg N/(L·d) and a high nitrate removal efficiency of 93.0%. Batch tests during the start-up period showed that the seed biofilm developed well on S⁰ flakes and delivered improved nitrate removal performance than the control. Extracellular polymeric substance (EPS) analysis revealed an abundant content of protein in tightly bound EPS in the biofilm developed from the seed biofilm, which was recognized as a major contributor to facilitate the biofilm's attachment and growth onto S⁰ flakes. After operating under moderate temperature, the reactors were tested at a reduced temperature of 15 °C. Results indicated that the reactor inoculated with seed biofilm showed stronger adaptation ability towards low temperature and sustained better denitrification performance than the control, which was attributed to increased protein content in tightly bound EPS produced by the microbes against low-temperature. Determination of the microbial communities in tested reactors when the whole experiment was closing found that sulfur-related genera were dominating in the packed-bed reactor inculcated with seed biofilm, which played an important role in the S⁰-based denitrification process.

Modelling the effect of SMP production and external carbon addition on S-driven autotrophic denitrification

The aim of this study was to develop a mathematical model to assess the effect of soluble microbial products production and external carbon source addition on the performance of a sulfur-driven autotrophic denitrification (SdAD) process. During SdAD, the growth of autotrophic biomass (AUT) was accompanied by the proliferation of heterotrophic biomass mainly consisting of heterotrophic denitrifiers (HD) and sulfate-reducing bacteria (SRB), which are able to grow on both the SMP derived from the microbial activities and on an external carbon source. The process was supposed to occur in a sequencing batch reactor to investigate the effects of the COD injection on both heterotrophic species and to enhance the production and consumption of SMP. The mathematical model was built on mass balance considerations and consists of a system of nonlinear impulsive differential equations, which have been solved numerically. Different simulation scenarios have been investigated by varying the main operational parameters: cycle duration, day of COD injection and quantity of COD injected. For cycle durations of more than 15 days and a COD injection after the half-cycle duration, SdAD represents the prevailing process and the SRB represent the main heterotrophic family. For shorter cycle duration and COD injections earlier than the middle of the cycle, the same performance can be achieved increasing the quantity of COD added, which results in an increased activity of HD. In all the performed simulation even in the case of COD addition, AUT remain the prevailing microbial family in the reactor.

Pyrite-assisted denitrification in recirculated biofilter tolerates pH lower than 5

This study demonstrates the feasibility of pyrite-assisted denitrification to treat mildly acidic wastewaters and proposes a convenient solution for nitrogen removal from acidic mining waters. Pyrite is a highly available and low-cost electron donor for autotrophic denitrification, whereas mining waters represent a source of nitrogen (N) contamination due to the use of N-containing explosives during mining operations and cyanide (CN)-based compounds for ore processing. In this study, the denitrification performance of a recirculated pyrite-packed biofilter (RPPB) treating simulated N-contaminated acidic mining waters is evaluated under decreasing pH conditions. The effect of feed organic carbon on the autotrophic process has been also investigated. N removal efficiencies (NRE) > 60% could be achieved at feed pH ≥ 4.5 and HRT ≥ 5 h. Decrease of influent pH to 3.0 reduced the NRE to 20%. The addition of 10 and 20 mg/L of ethanol to the influent did not significantly impact NRE of the RPPB. PRACTITIONER POINTS: Pyrite-assisted denitrification is proposed for treating acidic wastewaters. Nitrogen removal >60% was maintained at feed pH as low as 4.7. Decrease of feed pH to 3 strongly inhibited denitrification. The presence of organic carbon in the feed did not affect the autotrophic process.

High-Rate Sulfate Removal Coupled to Elemental Sulfur Production in Mining Process Waters Based on Membrane-Biofilm Technology

It is anticipated that copper mining output will significantly increase over the next 20 years because of the more intensive use of copper in electricity-related technologies such as for transport and clean power generation, leading to a significant increase in the impacts on water resources if stricter regulations and as a result cleaner mining and processing technologies are not implemented. A key concern of discarded copper production process water is sulfate. In this study we aim to transform sulfate into sulfur in real mining process water. For that, we operate a sequential 2-step membrane biofilm reactor (MBfR) system. We coupled a hydrogenotrophic MBfR (H₂-MBfR) for sulfate reduction to an oxidizing MBfR (O₂-MBfR) for oxidation of sulfide to elemental sulfur. A key process improvement of the H₂-MBfR was online pH control, which led to stable high-rate sulfate removal not limited by biomass accumulation and with H₂ supply that was on demand. The H₂-MBfR easily adapted to increasing sulfate loads, but the O₂-MBfR was difficult to adjust to the varying H₂-MBfR outputs, requiring better coupling control. The H₂-MBfR achieved high average volumetric sulfate reduction performances of 1.7-3.74 g S/m³-d at 92-97% efficiencies, comparable to current high-rate technologies, but without requiring gas recycling and recompression and by minimizing the H₂ off-gassing risk. On the other hand, the O₂-MBfR reached average volumetric sulfur production rates of 0.7-2.66 g S/m³-d at efficiencies of 48-78%. The O₂-MBfR needs further optimization by automatizing the gas feed, evaluating the controlled removal of excess biomass and S⁰ particles accumulating in the biofilm, and achieving better coupling control between both reactors. Finally, an economic/sustainability evaluation shows that MBfR technology can benefit from the green production of H₂ and O₂ at operating costs which compare favorably with membrane filtration, without generating residual streams, and with the recovery of valuable elemental sulfur.

Exploiting the Nutrient Potential of Anaerobically Digested Sewage Sludge: A Review

The world is currently witnessing a rapid increase in sewage sludge (SS) production, due to the increased demand for wastewater treatment. Therefore, SS management is crucial for the economic and environmental sustainability of wastewater treatment plants. The recovery of nutrients from SS has been identified as a fundamental step to enable the transition from a linear to a circular economy, turning SS into an economic and sustainable source of materials. SS is often treated via anaerobic digestion, to pursue energy recovery via biogas generation. Anaerobically digested sewage sludge (ADS) is a valuable source of organic matter and nutrients, and significant advances have been made in recent years in methods and technologies for nutrient recovery from ADS. The purpose of this study is to provide a comprehensive overview, describing the advantages and drawbacks of the available and emerging technologies for recovery of nitrogen (N), phosphorus (P), and potassium (K) from ADS. This work critically reviews the established and novel technologies, which are classified by their ability to recover a specific nutrient (ammonia stripping) or to allow the simultaneous recovery of multiple elements (struvite precipitation, ion exchange, membrane technologies, and thermal treatments). This study compares the described technologies in terms of nutrient recovery efficiency, capital, and operational costs, as well as their feasibility for full-scale application, revealing the current state of the art and future perspectives on this topic.