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Advanced Biological Wastewater Treatment and Nutrient Removal

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A special issue of Water (ISSN 2073-4441). This special issue belongs to the section "Wastewater Treatment and Reuse".

Deadline for manuscript submissions: closed (15 March 2026) | Viewed by 18442

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Special Issue Editors

Dr. Hong Wang

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Guest Editor

State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China
Interests: biological nitrogen removal; anammox; biofilm; denitrifying phosphorus removal; activated sludge
Special Issues, Collections and Topics in MDPI journals

Dr. Qiulai He

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Guest Editor

College of Civil Engineering, Hunan University, Changsha 410082, China
Interests: new theories and technologies for low-carbon water treatment; biological wastewater treatment and resource recovery
Special Issues, Collections and Topics in MDPI journals

Dr. Yingmu Wang

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Guest Editor

College of Civil Engineering, Fuzhou University, Fuzhou 350116, China
Interests: biological nitrogen removal; bioelectrochemical technology; anammox; autotrophic denitrification; microalgae; constructed wetland; MABR; GHGs reduction

Special Issue Information

Dear Colleagues,

The traditional activated sludge process, developed over more than 100 years ago, has become the most widely adopted biological treatment technology in wastewater treatment plants worldwide. However, with the continuous increase in wastewater production and the tightening of discharge standards, the traditional biological treatment process faces several challenges, including low treatment efficiency, high energy consumption, and low volumetric load. These limitations make it difficult to meet the new demands for efficient and low-energy wastewater treatment under carbon-neutral targets. In recent years, innovations in wastewater biological treatment theories have driven technological advancements and attracted widespread attention from researchers.

This Special Issue mainly focuses on cutting-edge research and technological applications related to advanced wastewater biological treatment and nutrient removal. For this Special Issue, we invite the submission of original research papers or review papers. The topics include, but are not limited to, the following:

New principles and metabolic pathways for biological nitrogen and phosphorus removal from wastewater;
Stable operation and optimization strategies for the anammox process;
Applications and regulation of aerobic granular sludge technology;
New biofilm technologies based on functional carriers;
Engineering applications of new technologies for nitrogen and phosphorus removal from wastewater.

Dr. Hong Wang
Dr. Qiulai He
Dr. Yingmu Wang
Guest Editors

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Water is an international peer-reviewed open access semimonthly journal published by MDPI.

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Keywords

wastewater biological treatment
nitrogen and phosphorus removal
anammox
aerobic granular sludge technology
biofilm technology
wastewater treatment plant upgrade

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Published Papers (8 papers)

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Research

21 pages, 3217 KB

Open AccessArticle

Transitioning Deammonification from Sidestream to Main-Stream Treatment: Long-Term Comparison of Integrated Fixed Film Activated Sludge and Moving Bed Biofilm Reactors with Polyurethane Foam Carriers at Lab-Scale

by Hanna Jagenteufel, Vanessa Parravicini, Norbert Kreuzinger, Ernis Saracevic, Karl Svardal and Jörg Krampe

Water 2026, 18(9), 1021; https://doi.org/10.3390/w18091021 - 24 Apr 2026

Viewed by 868

Abstract

Deammonification, which is based on partial nitritation and anammox (PN/A), is a well-established sidestream treatment for nitrogen removal. However, transferring deammonification to mainstream wastewater treatment remains challenging due to low temperatures, the need to retain slow-growing anammox bacteria (AnAOB), and their competition for nitrite with nitrite-oxidizing bacteria (NOB) and heterotrophic denitrifiers. This work investigates cubic polyurethane foam carriers to promote growth and retention of AnAOB. A moving bed biofilm reactor (MBBR) and an integrated fixed-film activated sludge (IFAS) reactor were compared over a three-year experimental period at lab-scale. The feasibility of the biofilm carriers for deammonification was first evaluated under sidestream conditions, followed by a stepwise transition to mainstream operational conditions. The impact of operational parameters, including dissolved oxygen concentration, pH value, and aeration strategy, was evaluated with respect to the activity of aerobic ammonium-oxidizing bacteria (AOB), NOB, and AnAOB, as well as nitrogen removal rates. Deammonification reached nitrogen removal rates of 0.04–0.12 kg N m⁻³ d⁻¹ (IFAS reactor) and 0.02–0.28 kg N m⁻³ d⁻¹ (MBBR) at subphases with reactor bulk concentrations above 60 mg NH₄-N L⁻¹. Highest nitrogen removal degrees of 77 ± 6% (IFAS) and 76 ± 5% (MBBR) were achieved at reactor bulk concentrations of 96 mg NH₄ L⁻¹ and 97 mg NH₄ L⁻¹, respectively. Lower concentrations triggered NOB activity in both reactors, leading to an increase in nitrate concentration up to 22 mg NO₃-N L⁻¹. AOB and AnAOB activities were on average 6-fold higher on the carriers compared to suspended biomass throughout all experimental phases, demonstrating the feasibility of using cubic polyurethane foam carriers for deammonification. This was also confirmed by fluorescence in-situ hybridization (FISH) measurements. Median nitrogen removal rates over all experimental phases of 0.07 kg N m⁻³ d⁻¹ for the IFAS reactor and 0.05 kg N m⁻³ d⁻¹ for the MBBR were achieved, which are comparable to conventional activated sludge systems performing nitrogen removal via nitrification–denitrification. While at lower nitrogen concentrations, the IFAS reactor yielded superior nitrogen removal rates, peak nitrogen removal rates of 0.28 kg N m⁻³ d⁻¹ were measured in the MBBR configuration. However, controlling NOB activity at lower temperatures and concentrations remains a challenge in MBBR and IFAS configurations. In our study, in the IFAS reactor NOB activities were visible on fewer days than in MBBR. At mainstream-like conditions, higher nitrogen removal rates of IFAS (0.09–0.12 kg N m⁻³ d⁻¹) were achieved compared to the MBBR (0.06–0.09 kg N m⁻³ d⁻¹). This demonstrates the advantage of the IFAS reactor in treating mainstream wastewater via deammonification. As an autotrophic nitrogen removal process, the implementation of deammonification in the mainstream of municipal wastewater treatment plants enables enhanced recovery of biogas from sewage organic matter. The latter would otherwise be consumed during the conventional nitrification-denitrification pathway. Consequently, the overall energy balance for wastewater treatment can be improved, contributing to a more environmentally sustainable process. Full article

(This article belongs to the Special Issue Advanced Biological Wastewater Treatment and Nutrient Removal)

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15 pages, 1516 KB

Open AccessArticle

Enhancing Stable Electricity Generation and Assimilative Ammonium-N Removal in Photosynthetic Algae–Microbial Fuel Cells Using a Chlorella Biofilm-Loaded ZnO-NiO@rGO Carbon-Fiber Composite Cathode

by Haiquan Zhan, Hong Wang, Yanzeng Li, Shiyu Liu, Shijie Yuan and Xiaohu Dai

Water 2026, 18(6), 733; https://doi.org/10.3390/w18060733 - 20 Mar 2026

Viewed by 584

Abstract

Photosynthetic algae–microbial fuel cells (PAMFCs) are attractive for energy-positive wastewater treatment and carbon mitigation. However, PAMFC performance under continuous flow is often constrained by limited cathodic electron-acceptor supply and unstable photosynthetic biofilms, while the extent to which cathode interfacial engineering can stabilize diurnal power output and assimilative NH₄⁺–N removal remains unclear. In this study, the sponge-like and petal-like ZnO_0.2-NiO@rGO-modified carbon fibers (ZnO_0.2-NiO@rGO-pCFs and ZnO_0.2-NiO@rGO-pCFp) and pre-fabricated carbon felt (pCF) were used as cathode materials to construct three sets of PAMFC systems. Under light–dark cycling, the engineered cathodes reached steady operation within about 6.5 d and increased the steady-state voltage to approximately 0.35 V, compared with approximately 0.08 V for pCF. Under continuous-flow conditions, cathodic NH₄⁺–N removal exhibited a stable diurnal rhythm, with higher removal during illumination at about 43–51% than in the dark at about 29–30%, consistent with algal assimilation as the primary nitrogen sink, while cathode modification mainly improved the cathodic microenvironment and response stability. Compared with pCF, the ZnO_0.2–NiO@rGO cathode enriched a more even, Chlorophyta-dominated algal biofilm with an approximate relative abundance of 80%, indicating that its selective interfacial environment favors biofilm stabilization and sustains in situ oxygen production and cathodic electron-acceptor supply. Consequently, the composite cathode enhanced voltage output and stabilized light-enhanced, assimilative NH₄⁺–N removal under aeration-free operation, while establishing an interpretable link between electrochemical performance and 18S rDNA-derived community assembly features, thereby providing a low-cost cathode design basis for nitrogen removal in wastewater treatment. Full article

(This article belongs to the Special Issue Advanced Biological Wastewater Treatment and Nutrient Removal)

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14 pages, 2433 KB

Open AccessArticle

Enhanced Nitrogen Removal by Anammox in Iron-Based Autotrophic Denitrification Filters

by Benzhou Gong, Kui Zhang, Yanjie Huang, Wenhao Yang and Yingmu Wang

Water 2026, 18(4), 451; https://doi.org/10.3390/w18040451 - 9 Feb 2026

Viewed by 570

Abstract

Nitrogen pollution poses significant risks to both environmental systems and human health. Iron-based autotrophic denitrification offers a green and cost-effective strategy for nitrogen removal, but is often accompanied by the accumulation of undesirable byproducts. A nitrogen removal system combining anammox with iron-based autotrophic denitrification was constructed in this study to investigate the enhancement effect of anaerobic ammonium-oxidizing bacteria (AnAOB). The results showed that during the stable operation phase, nitrate removal efficiencies reached 91.45% and 84.29% for groups A (0.5 g/L AnAOB) and B (0.1 g/L AnAOB), respectively, significantly higher than the 62.87% observed in the control group. Furthermore, the experimental groups exhibited markedly reduced accumulation of ammonium byproducts. Microbial community analysis revealed that AnAOB addition increased microbial richness and diversity, and promoted community shifts that favored nitrogen removal. Notably, even low dosages of AnAOB yielded strong performance enhancements, underscoring the economic viability of this integrated approach. Structural characterization using SEM, XRD, and XPS indicated that system performance deterioration in the later stages was primarily due to cell encrustation and iron passivation. Electrochemical analyses further demonstrated that iron passivation impaired electron transfer on the filler surface, thereby reducing denitrification efficiency, whereas extracellular polymeric substances (EPS) did not exhibit such inhibitory effects. These findings provide both mechanistic insight and practical guidance for the design and optimization of anammox-enhanced iron-based denitrification systems. Full article

(This article belongs to the Special Issue Advanced Biological Wastewater Treatment and Nutrient Removal)

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17 pages, 1497 KB

Open AccessArticle

Synergistic Nitrogen and Phosphorus Elimination via Iron–Carbon Micro-Electrolysis in Constructed Wetlands Treating Low-Pollution Water

by Shanshan Sun, Xiaojiao Ren, Jian Shen, Xuejin Zhou, Di Wu and Shengbing He

Water 2025, 17(21), 3139; https://doi.org/10.3390/w17213139 - 1 Nov 2025

Cited by 1 | Viewed by 3575

Abstract

To address the issues of zero-valent iron Fe(0) passivation and limited nitrogen and phosphorus removal in constructed wetlands (CWs), this study investigated the enhancement effect of two carbon materials—activated carbon (AC) obtained through high-temperature pyrolysis and biochar (BC) obtained through low-temperature pyrolysis—when coupled with Fe(0). Four systems were set up: control (CW-C), Fe(0) alone (CW-Fe), Fe(0) with AC (CW-FeAC), and Fe(0) with BC (CW-FeBC). Evaluations covered wastewater treatment performance, microbial community structure, and functional gene abundance. Results showed that iron–carbon coupling significantly improved nitrogen and phosphorus removal, with the CW-FeAC system performing best, achieving 58% total nitrogen (TN) and 90% total phosphorus (TP) removal. This enhancement was attributed to AC’s high conductivity, which strengthened iron–carbon micro-electrolysis, accelerated Fe(0) corrosion, and enabled continuous Fe²⁺/Fe³⁺ release, supplying electrons for denitrification and phosphorus precipitation. Microbial analysis indicated that iron–carbon coupling markedly reshaped community structure, enriching key genera such as Thiobacillus (33.8%) and Geobacter (12.5%) in CW-FeAC. Functional gene analysis further confirmed higher abundances of denitrification (napA/narG→nirS→nosZ) and iron metabolism genes (feoA/feoB), suggesting enhanced nitrogen-iron cycling. This study clarifies the mechanisms by which iron–carbon coupling improves nitrogen and phosphorus performance in CWs and highlights the superiority of AC over BC in facilitating electron transfer and functional microorganism enrichment, providing a basis for the design of enhanced CW systems treating low-carbon-nitrogen-ratio wastewater, such as secondary effluent or lightly polluted surface water. Full article

(This article belongs to the Special Issue Advanced Biological Wastewater Treatment and Nutrient Removal)

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13 pages, 3203 KB

Open AccessArticle

Evaluation and Verification of Starch Decomposition by Microbial Hydrolytic Enzymes

by Makoto Takaya, Manzo Uchigasaki, Koji Itonaga and Koichi Ara

Water 2025, 17(15), 2354; https://doi.org/10.3390/w17152354 - 7 Aug 2025

Viewed by 2177

Abstract

This study investigates the Enzyme Biofilm Method (EBM), a biological wastewater treatment technology previously developed by the authors. EBM employs microbial-derived hydrolytic enzyme groups in the initial treatment stage to break down high-molecular-weight organic matter—such as starch, proteins, and fats—into low-molecular-weight compounds. These compounds enhance the growth of native microorganisms, promoting biofilm formation on carriers and improving treatment efficiency. Over the past decade, EBM has been practically applied in food factory wastewater facilities handling high organic loads. The enzyme groups used in EBM are derived from cultures of Bacillus mojavensis, Saccharomyces cariocanus, and Lacticaseibacillus paracasei. To clarify the system’s mechanism and ensure its practical viability, this study focused on starch—a prevalent and recalcitrant component of food wastewater—using two evaluation approaches. Verification 1: Field testing at a starch factory showed that adding enzyme groups to the equalization tank effectively reduced biological oxygen demand (BOD) through starch degradation. Verification 2: Laboratory experiments confirmed that the enzyme groups possess both amylase and maltase activities, sequentially breaking down starch into glucose. The resulting glucose supports microbial growth, facilitating biofilm formation and BOD reduction. These findings confirm EBM’s potential as a sustainable and effective solution for treating high-strength food industry wastewater. Full article

(This article belongs to the Special Issue Advanced Biological Wastewater Treatment and Nutrient Removal)

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17 pages, 4856 KB

Open AccessArticle

Worldwide Research Progress and Trends in Application of Machine Learning to Wastewater Treatment: A Bibliometric Analysis

by Kun Zhou, Boran Wu and Xin Zhang

Water 2025, 17(9), 1314; https://doi.org/10.3390/w17091314 - 28 Apr 2025

Cited by 4 | Viewed by 2682

Abstract

Efficient wastewater treatment with high-quality effluent and minimal operational costs and carbon emissions is vital for safeguarding the ecological environment and promoting human health. However, the wastewater treatment process is extremely complicated due to the characteristics of multiple treatment mechanisms, high disturbance variability and nonlinear behaviors; therefore, optimizing the wastewater treatment process through intelligent control is a long-standing challenge for researchers and operators. Machine learning models are regarded as effective tools for wastewater treatment with better simulating and controlling complex nonlinear behaviors. With the aid of bibliometric analysis, this paper aimed to summarize worldwide research progress and trends in the application of machine learning to wastewater treatment among 1226 related publications. The findings indicate that China and the United States are the two leading countries, with publications of 342 and 209, respectively, while the United States is an outstanding global collaboration leader in this field. Research institutions and authors are mainly from developing countries, and China accounts for the largest proportion of these. The analysis of journal and cited journal contributions report that almost all of the top 10 journals in publications belong to the Q1 quartile (9/10). Overall, future research will likely focus on developing systematic, strong and multi-objective models for wastewater treatment. A hybrid model could take advantage of two or more machine learning models or mechanistic models, which have been verified as excellent models for tackling limited data. Thus, predicting the pollutants in the effluent rather than the influent using hybrid models is attracting increasing attention because effective prediction contributes to reducing the loading shock of influent sharp fluctuation to wastewater treatment effluent quality. Also, the development of advanced data acquirement devices and the AI model prediction with partially default data should also be another focus of future research. Full article

(This article belongs to the Special Issue Advanced Biological Wastewater Treatment and Nutrient Removal)

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27 pages, 6742 KB

Open AccessArticle

Removal of Nitrogen and Phosphorus from Municipal Wastewater Through Cultivation of Microalgae Chlorella sp. in Consortium

by Flor Maria Ortega-Blas, José C. Ramos-Saravia and Pablo Luis Cossío-Rodríguez

Water 2025, 17(8), 1160; https://doi.org/10.3390/w17081160 - 13 Apr 2025

Cited by 16 | Viewed by 5734

Abstract

Demographic growth in developing countries has increased domestic wastewater generation, posing environmental and health risks due to nitrogen and phosphorus accumulation, the main contributors to eutrophication. This study explores microalgae–bacteria consortia for nutrient removal, using Chlorella sp. for its high pollutant assimilation efficiency and biomass production. A lab-scale experiment was designed using response surface methodology to optimize key variables, revealing that lighting and the culture medium significantly influenced biomass production and nutrient removal, with lighting having the strongest statistical impact (p = 0.0002). The optimal conditions (18 μmolm⁻² s⁻¹ light, municipal wastewater) achieved nitrogen and phosphorus removal efficiencies of 87.16% and 94.43%, respectively. A mathematical model was developed with two independent systems: (1) the first describes biomass generation via photosynthesis, considering CO₂ as a limiting substrate, while (2) the second models nitrogen and phosphorus consumption, assuming nitrogen as limiting substrate and introducing an intermediate (I) that couples phosphorus and nitrogen removal. This coupling is regulated by factor k, which represents a percentage of the total consortium consumption rate. Model predictions showed high accuracy for biomass (SE = 0.07186) and phosphorus (SE = 0.63065), but nitrogen exhibited greater deviation (SE = 3.40285). These findings highlight the system’s potential as a sustainable and cost-effective wastewater treatment alternative. Full article

(This article belongs to the Special Issue Advanced Biological Wastewater Treatment and Nutrient Removal)

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9 pages, 1847 KB

Open AccessArticle

Effect of Co-Dewatering for Aerobic Granular Sludge and Alum Sludge

by Yongfei Chen, Kangmei Tu, Dongsheng Qian, Ningyu Li and Ailan Yan

Water 2025, 17(5), 705; https://doi.org/10.3390/w17050705 - 28 Feb 2025

Cited by 1 | Viewed by 1080

Abstract

Sludge dewatering plays a crucial and indispensable role in the sludge treatment and disposal process, directly affecting the subsequent disposal costs and environmental risks. Aerobic granular sludge (AGS) and alum sludge (AS) from water treatment plants are two common types of sludge with distinct dewatering properties. In this experiment, we thoroughly investigated the effects of co-dewatering by mixing AGS and AS in different proportions. The results showed that the addition of the AS effectively altered the composition and characteristics of the AGS, which significantly improved its settlement performance. When the AGS and AS were mixed in a specific proportion, the water content of the dewatered AGS was reduced from 81.2% to 70.9%, which fully demonstrated a significant improvement in the AGS dewatering performance achieved through the mixed treatment. It is recommended to be widely promoted and applied in practical engineering. Full article

(This article belongs to the Special Issue Advanced Biological Wastewater Treatment and Nutrient Removal)

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