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Open research questions in Wastewater Treatment and Nitrogen Removal

103 gap statements mined from Wastewater Treatment and Nitrogen Removal papers in our 4.5M-paper local library, which holds 886 papers on the topic — drawn mostly from each paper's own stated research gap, future-work, challenge and limitation notes, and its abstract. The ones listed below are a selection still marked open; each names the study that raised it, with a DOI link where the paper has one.

What the literature leaves open

  • further research on the climate impacts of Nr mitigation pathways, - evaluation of the potential ecosystem and economic co-benefits of Nr mitigation, - investigation of the effects of Nr mitigation on water quality and fertilizer inputs

    Efficient nitrogen management as a win-win strategy for air quality, health, and climate · 2026 · DOI

  • incorporating molecular monitoring (e.g., qPCR or amplicon sequencing) to elucidate population dynamics - elucidate strain-specific responses to exogenous AHLs within the consortium

    Screening and Identification of High-Efficiency Aerobic Salt-Tolerant Strains with Quorum Sensing Capability for Comprehensive Nitrogen Removal: Development of Bacterial Consortium · 2026 · DOI

  • The need for efficient salt-tolerant nitrifying and denitrifying strains for comprehensive nitrogen removal. The lack of a bacterial consortium that can achieve excellent synergistic activity in removing mixed nitrogen pollutants and total organic carbon.

    Screening and Identification of High-Efficiency Aerobic Salt-Tolerant Strains with Quorum Sensing Capability for Comprehensive Nitrogen Removal: Development of Bacterial Consortium · 2026 · DOI

  • The bioremoval of toxic metals is a complex process that requires the evaluation of multiple experimental conditions. The paper identifies challenges in the development of efficient bioremoval methods, including the need for accurate models of bioremoval efficiency. The study highlights the challenge of interpreting the results of machine learning models for bioremoval.

    Efficient removal of Ni, Al, and As from water using Micrococcus yunnanensis: batch experiments and explainable machine learning · 2026 · DOI

  • Investigating the effects of other environmental factors on the bioremoval process. Evaluating the scalability of the bioremoval process using M. yunnanensis. Comparing the bioremoval efficiency of M. yunnanensis with other microorganisms.

    Efficient removal of Ni, Al, and As from water using Micrococcus yunnanensis: batch experiments and explainable machine learning · 2026 · DOI

  • The dataset is observational, - The study provides limited power for long-term temporal dynamics, - The study has limited ability for causal inference

    Ecological assembly of activated sludge microbiomes under contrasting process configurations and influent regimes · 2026 · DOI

  • Further studies should investigate the long-term temporal dynamics of microbial communities, - Research should focus on causal inference and the relationships between microbial communities and treatment efficiency

    Ecological assembly of activated sludge microbiomes under contrasting process configurations and influent regimes · 2026 · DOI

  • The energy yield from oxidizing sulfite to sulfate is relatively low compared to that from other reduced sulfur compounds. Sulfite exhibits certain biological toxicity, which may inhibit microbial activity. The inoculated sludge may lack microorganisms capable of catalyzing sulfite-based denitrification.

    Evaluating the Feasibility of Two Reduced Sulfur Compounds as Energy Sources and Electron Donors for Partial Autotrophic Denitrification: Thiocyanate and Sulfite · 2026 · DOI

  • the study only evaluated two reduced sulfur compounds, - the operational conditions may not have been optimal for sulfite-based denitrification, - the inoculated sludge may have lacked microorganisms capable of catalyzing sulfite-based denitrification

    Evaluating the Feasibility of Two Reduced Sulfur Compounds as Energy Sources and Electron Donors for Partial Autotrophic Denitrification: Thiocyanate and Sulfite · 2026 · DOI

  • One of the challenges is to understand the mechanisms underlying the effects of phosphorus-depleted biochar on soil phosphorus transformation. Another challenge is to determine the impact of phosphorus-depleted biochar on phosphorus-solubilizing bacterial abundance and diversity. A further challenge is to develop effective strategies for enhancing soil phosphorus fertility in agricultural systems using phosphorus-depleted biochar.

    Effects of biochar on phosphorus fraction, phosphatase activity, and phosphorus-solubilizing bacterial abundance: a phosphorus-depleted biochar design · 2025 · DOI

  • further studies are needed to understand the effect of pH on enzymatic activity, - the development of more efficient and reliable analytical methods for measuring RNI is necessary, - research on the impact of reactive nitrogen on the interplay of microorganisms and microbial processes is required

    Formation and Fate of Reactive Nitrogen during Biological Nitrogen Removal from Water: Important Yet Often Ignored Chemical Aspects of the Nitrogen Cycle · 2024 · DOI

  • Little understanding of the occurrence and significance of reactive nitrogen during biological nitrogen conversions in engineered water systems. Limited knowledge of the pH effect on enzymatic activity and the involvement of different enzymes.

    Formation and Fate of Reactive Nitrogen during Biological Nitrogen Removal from Water: Important Yet Often Ignored Chemical Aspects of the Nitrogen Cycle · 2024 · DOI

  • One challenge is the potential toxicity of the post-hydrothermal liquefaction wastewater. Another challenge is the need to optimize the conversion process and analyze interactions between the fungus and bacteria. The study also faces the challenge of scaling up the treatment process for industrial applications.

    Wastewater Nutrient Recovery via Fungal and Nitrifying Bacteria Treatment · 2024 · DOI

  • There is a lack of research on the treatment of post-hydrothermal liquefaction wastewater. The current study addresses the gap by investigating the integration of fungal and bacterial treatments to convert organic nitrogen compounds into inorganic nitrogen.

    Wastewater Nutrient Recovery via Fungal and Nitrifying Bacteria Treatment · 2024 · DOI

  • The role of phosphorus-solubilizing bacteria in phosphorus transformation during biochar amendment is poorly understood. The effects of phosphorus-depleted biochar on soil phosphorus fractions and phosphatase activity are not well studied. The mechanisms underlying the effects of phosphorus-depleted biochar on soil phosphorus transformation are not fully understood.

    Effects of biochar on phosphorus fraction, phosphatase activity, and phosphorus-solubilizing bacterial abundance: a phosphorus-depleted biochar design · 2025 · DOI

  • Many Patescibacteria lineages are abundant in wastewater treatment plants, yet their interaction partners and ecological roles remain poorly understood.

    A Saccharimonadia epibiont interacts with the polyphosphate-accumulating bacterium Candidatus Phosphoribacter in wastewater treatment plants · 2026 · DOI

  • IMPORTANCE Nitrobacter species play a key role in nitrogen cycling; however, their physiological adaptations and genomic diversity remain largely underexplored.

    Comparative physiological and genomic characterization of a novel Nitrobacter vulgaris strain from a nitrate-contaminated subsurface · 2026 · DOI

  • However, the degree of aggregate dispersion and the efficacy of extraction have rarely been examined, and thus, adequate cell extraction methods from soil remain unclear.

    Single-cell genomics of single soil aggregates: methodological assessment and potential implications with a focus on nitrogen metabolism · 2025 · DOI

  • However, whether AnAOB directly participate in Feammox within complex wastewater treatment systems remains unclear.

    Genome-Resolved Metagenomic and Metatranscriptomics Reveal Feammox Metabolism of Anaerobic Ammonia Oxidation Bacteria in Microaerobic Granular Sludge · 2025 · DOI

  • While most studies focused on the selective suppression of nitrite-oxidizing bacteria, maintaining stable ammonia oxidation as the primary source of nitrite remains equally critical but largely underexplored.

    Niche Adaptation of Ammonia-Oxidizing Bacteria Nitrosomonas in Partial Nitrification System: Keystone Species and Unique Survival Strategies · 2025 · DOI

  • While prevalent in high-temperature ecosystems, the mechanisms regulating temperature-dependent denitrification remain unclear.

    Temperature-Dependent Regulation of Denitrification Intermediates in High-Temperature Ecosystems · 2025 · DOI

  • EPS composition may depend on microbial community composition and the soil physical and chemical environment, nevertheless, not much is known about the EPS constituents' specific roles nor how they interact to alter biofilm's functions.

    Constituent of extracellular polymeric substances (EPS) produced by a range of soil bacteria and fungi · 2025 · DOI

  • • Lack of standardized EPS extraction and analysis methods hinders consistent research findings.

    Extracellular polymeric substances (EPS) in sewage sludge management: A call for methodological standardization · 2025 · DOI

  • Despite their importance, the lack of standardized methods for EPS extraction and analysis has led to inconsistent research findings, hindering a thorough understanding of EPS's role in sludge treatment.

    Extracellular polymeric substances (EPS) in sewage sludge management: A call for methodological standardization · 2025 · DOI

  • To fill this gap in knowledge, we compiled a global dataset on anammox and denitrification, encompassing river, lake, wetland, and estuary ecosystems and comprising 2539 observations from 136 peer-reviewed papers.

    Global patterns and drivers of coupling between anammox and denitrification processes across inland aquatic ecosystems · 2025 · DOI

Most-cited papers in Wastewater Treatment and Nitrogen Removal

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103 gap statements have been mined from Wastewater Treatment and Nitrogen Removal papers in our 4.5M-paper local library, which holds 886 papers on the topic; the gaps come from whichever of those papers state one. They are mostly the research gaps the authors state and the papers' abstracts, plus future-work, limitations and challenges passages. The ones listed below are a selection still marked open; each names the study that raised it, with a DOI link where the paper has one, so you can read the original claim in context.

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