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Sustainable Wastewater Treatment and the Circular Economy—2nd Edition

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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: 20 May 2026 | Viewed by 23822

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Special Issue Editor

Dr. Tao Zhang

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Guest Editor

College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China
Interests: wastewater treatment; circular economy; nutrients recovery; sustainable waste management; biomass utilization; machine learning
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Dear Colleagues,

Today’s restrictions on resources and the environment are very severe. A circular economy is a promising path to the sustainable development of wastewater treatment, a process in which the quantity of biomass waste has risen sharply, causing serious environmental pollution. In terms of sustainable wastewater treatment, the discharge of biomass waste is a major societal concern; thus, the utilization of biomass waste has practical significance in a circular economy. At present, much research has been conducted on biomass waste utilization and the circular economy, mainly focusing on thermal conversion, as well as aerobic and anaerobic technologies. These technologies can transform the recyclable organic materials from biomass wastes into solid, liquid, and gas fuels, which are of great significance for alleviating energy shortages and environmental pollution. Therefore, this research will focus on the sustainable use of wastewater treatment biomass waste to promote the development of a circular economy. We propose the publication of a Special Issue in Water to highlight recent research carried out in these fields.

This Special Issue intends to present novel, high-quality, original research articles, as well as review articles, short communications, and/or letters focusing on sustainable wastewater treatment and the circular economy. Topics of interest include the following:

Thermal conversion technology/anaerobic technology/aerobic technology for a circular economy and sustainable wastewater treatment.
Resource recovery technologies for sustainable wastewater treatment.
The application of bioenergy and bioresources in a circular economy for wastewater treatment.
Green chemistry technologies in a circular economy and sustainable wastewater treatment.

Dr. Tao Zhang
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 250 words) can be sent to the Editorial Office for assessment.

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.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

wastewater treatment
circular economy
biomass waste
resource recovery
green chemistry
thermal conversion
anaerobic process
aerobic process

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Related Special Issue

Sustainable Wastewater Treatment and the Circular Economy in Water (11 articles)

Published Papers (7 papers)

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Research

Jump to: Review

16 pages, 1504 KB

Open AccessArticle

Feasibility and Local Perceptions About Treated Wastewater Reuse for Irrigation: Insights from the Prato Circular City Framework (Italy)

by Leonardo Borsacchi, Donatella Fibbi, Lorenzo Baronti, Gabriele Feligioni, Tommaso Toccafondi, Leonardo Bogani and Patrizia Pinelli

Water 2026, 18(7), 809; https://doi.org/10.3390/w18070809 - 28 Mar 2026

Viewed by 441

Abstract

The reuse of treated wastewater for agricultural irrigation is increasingly considered a strategic response to water scarcity and climate change, particularly in Mediterranean regions. This study examines the local feasibility and social acceptance of water reuse within the framework of Regulation (EU) 2020/741, focusing on its implementation in Italy. The research combines policy analysis, technical assessment of effluent quality from the GIDA wastewater treatment plant (Prato, Tuscany), GIS-based spatial evaluation, and a mixed-method survey of local agri-food producers. Results show substantial compliance with EU minimum quality requirements, alongside additional constraints arising from national regulatory thresholds. Survey findings reveal cautious but tangible openness among farmers toward reclaimed water use, particularly in response to increasing climate-related pressures. The case of Prato is further analysed within the Prato Circular City and local food policy frameworks, highlighting the role of participatory governance and multi-actor engagement in supporting reuse initiatives. The study contributes empirical evidence on the interaction between EU regulation, national implementation measures, and local socio-institutional conditions shaping peri-urban water reuse systems. Furthermore, it serves as a preliminary framework for future economic feasibility studies and the subsequent regulatory and permitting phases required to operationalize this practice. Full article

(This article belongs to the Special Issue Sustainable Wastewater Treatment and the Circular Economy—2nd Edition)

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22 pages, 3798 KB

Open AccessArticle

Deciphering Phosphorus Recovery from Wastewater via Machine Learning: Comparative Insights Among Al³⁺, Fe³⁺ and Ca²⁺ Systems

by Yanyu Liu and Baichuan Jiang

Water 2026, 18(2), 182; https://doi.org/10.3390/w18020182 - 9 Jan 2026

Viewed by 404

Abstract

Efficient phosphorus recovery is of great significance for sustainable wastewater management and resource recycling. While chemical precipitation is widely used, its effectiveness under complex multi-factor conditions remains challenging to predict and optimize. This study compiled a multidimensional dataset from recent experimental literature, encompassing key operational parameters (reaction time, temperature, pH, stirring speed) and dosages of three metal precipitants (Al³⁺, Ca²⁺, Fe³⁺) to systematically evaluate and benchmark phosphorus recovery performance across these distinct systems, six machine learning algorithms—Random Forest (RF), eXtreme Gradient Boosting (XGBoost), Gaussian Process Regression (GPR), Elastic Net, Artificial Neural Network (ANN), and Partial Least Squares Regression (PLSR)—were developed and cross-validated. Among them, the GPR model exhibited superior predictive accuracy and robustness. (

R^{2}

= 0.69,

R M S E

= 0.54). Beyond achieving high-fidelity predictions, this study advances the field by integrating interpretability analysis with Shapley Additive Explanations (SHAP) and Partial Dependence Plots (PDP). These analyses identified distinct controlling factors across systems: reaction time and pH for aluminum, Ca²⁺ dosage and alkalinity for calcium, and phosphorus loading with stirring speed for iron. The revealed factor-specific mechanisms and synergistic interactions (e.g., among pH, metal dose, and mixing intensity) provide actionable insights that transcend black-box prediction. This work presents an interpretable Machine Learning (ML) framework that offers both theoretical insights and practical guidance for optimizing phosphorus recovery in multi-metal systems and enabling precise control in wastewater treatment operations. Full article

(This article belongs to the Special Issue Sustainable Wastewater Treatment and the Circular Economy—2nd Edition)

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16 pages, 3243 KB

Open AccessArticle

Enhanced Nitrification of High-Ammonium Reject Water in Lab-Scale Sequencing Batch Reactors (SBRs)

by Sandeep Gyawali, Eshetu Janka and Carlos Dinamarca

Water 2025, 17(9), 1344; https://doi.org/10.3390/w17091344 - 30 Apr 2025

Cited by 1 | Viewed by 1611

Abstract

Dewatering anaerobic digested sludge leaves a liquid fraction known as reject water, a liquid organic fertilizer containing high amounts of ammonium nitrogen (NH₄-N). However, its concentration should be enhanced to produce commercial fertilizer. Thus, reject water nitrification for stabilization as well as for nitrate capture in biochar to be used as a slow-release fertilizer is proposed. This study attempted to accomplish enhanced nitrification by tuning the operating parameters in two lab-scale sequential-batch reactors (SBRs), which were fed reject water (containing 520 ± 55 mg NH₄-N/L). Sufficient alkalinity as per stoichiometric value was needed to maintain the pH and free nitrous acid (FNA) within the optimum range. A nitrogen loading rate (NLR) of 0.14 ± 0.01 kg/m³·d and 3.34 days hydraulic retention time (HRT) helped to achieved complete 100% nitrification in reactor 1 (R₁) on day 61 and in reactor 2 (R₂) on day 82. After a well-developed bacterial biomass, increasing the NH₄-N concentration up to 750 ± 85 mg/L and NLR to 0.23 ± 0.03 kg/m³·d did not affect the nitrification process. Moreover, a feeding sequence once a day provided adequate contact time between nitrifying sludge and reject water, resulting in complete nitrification. It can be concluded that enhanced stable nitrification of reject water can be achieved with quick adjustment of loading, alkalinity, and HRT in SBRs. Full article

(This article belongs to the Special Issue Sustainable Wastewater Treatment and the Circular Economy—2nd Edition)

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Review

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29 pages, 1463 KB

Open AccessReview

AI-Enabled Membrane Bioreactors: A Review of Control Architectures and Operating-Parameter Optimization for Nitrogen and Phosphorus Removal

by Mingze Xu and Di Liu

Water 2025, 17(19), 2899; https://doi.org/10.3390/w17192899 - 7 Oct 2025

Cited by 5 | Viewed by 3026

Abstract

Stricter requirements on nutrient removal in wastewater treatment are being imposed by rapid urbanization and tightening water-quality standards. Despite their excellent solid–liquid separation and effective biological treatment, MBRs in conventional operation remain hindered by membrane fouling, limited robustness to influent variability, and elevated energy consumption. In recent years, precise process control and resource-oriented operation have been enabled by the integration of artificial intelligence (AI) with MBRs. Advances in four areas are synthesized in this review: optimization of MBR control architectures, intelligent adaptation to multi-source wastewater, regulation of membrane operating parameters, and enhancement of nitrogen and phosphorus removal. According to reported studies, increases in total nitrogen and total phosphorus removal have been achieved by AI-driven strategies while energy use and operating costs have been reduced; under heterogeneous influent and dynamic operating conditions, stronger generalization and more effective real-time regulation have been demonstrated relative to traditional approaches. For large-scale deployment, key challenges are identified as improvements in model interpretability and applicability, the overcoming of data silos, and the realization of multi-objective collaborative optimization. Addressing these challenges is regarded as central to the realization of robust, scalable, and low-carbon intelligent wastewater treatment. Full article

(This article belongs to the Special Issue Sustainable Wastewater Treatment and the Circular Economy—2nd Edition)

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27 pages, 870 KB

Open AccessReview

Thermochemical Conversion of Sewage Sludge: Progress in Pyrolysis and Gasification

by Yibo Hu and Ziwei Chen

Water 2025, 17(12), 1833; https://doi.org/10.3390/w17121833 - 19 Jun 2025

Cited by 9 | Viewed by 4120

Abstract

Sewage sludge, as a by-product of wastewater treatment, poses severe environmental challenges due to its high moisture, ash, and heavy metal content. Thermochemical conversion technologies, including pyrolysis and gasification, offer promising pathways for transforming sludge into valuable products such as bio-oil, biochar, and syngas. This paper systematically reviews recent advancements in pyrolysis and gasification, focusing on process optimization and catalyst development to enhance product quality and energy recovery. In pyrolysis, factors such as temperature, residence time, and heating rate significantly influence product yields and properties, while catalytic and co-pyrolysis approaches further improve product structure and reduce environmental risks. In gasification, parameters like the equivalence ratio, steam-to-sludge ratio, and catalyst application are key to enhancing syngas yield and quality, with biomass co-gasification offering additional benefits. Despite substantial progress, commercialization remains challenged by high operational costs, catalyst durability, and environmental impacts. Future research should emphasize improving sludge pretreatment, optimizing thermochemical processes, developing efficient and cost-effective catalysts, and addressing critical issues such as bio-oil quality, tar management, and syngas purification to promote the industrial application of these technologies. Full article

(This article belongs to the Special Issue Sustainable Wastewater Treatment and the Circular Economy—2nd Edition)

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22 pages, 2869 KB

Open AccessReview

A Review on Uses of Lemna minor, a Beneficial Plant for Sustainable Water Treatments, in Relation to Bioeconomy Aspects

by Constantina-Bianca Vulpe, Ioana-Maria Toplicean, Bianca-Vanesa Agachi and Adina-Daniela Datcu

Water 2025, 17(9), 1400; https://doi.org/10.3390/w17091400 - 7 May 2025

Cited by 8 | Viewed by 6690

Abstract

This review seeks to highlight the issue of utilizing a widely distributed aquatic species within the broader context of the transition from a linear to a circular economy and the growing emphasis on environmental sustainability. To promote a cleaner aquatic environment and ensure compliance with current regulations, the use of bioindicators and plant bioaccumulators presents a viable alternative. Lemna minor, a small aquatic species, serves as a noteworthy example that warrants greater consideration. A review of specialized literature was conducted to provide a comprehensive overview of these issues, drawing from the most relevant sources. This paper offers a broad discussion on bioeconomy and water management, along with an in-depth examination of L. minor, its characteristics, and its practical applications. The biological characteristics, ecological significance, and useful applications of L. minor in wastewater treatment, bioenergy, and bioproduct production are summarized in this research. The analysis also identifies research gaps for further investigation and looks at how this plant fits into new frameworks for the circular economy. Full article

(This article belongs to the Special Issue Sustainable Wastewater Treatment and the Circular Economy—2nd Edition)

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32 pages, 5862 KB

Open AccessEditor’s ChoiceReview

Advances in Hydrothermal Carbonization for Biomass Wastewater Valorization: Optimizing Nitrogen and Phosphorus Nutrient Management to Enhance Agricultural and Ecological Outcomes

by Guoqing Liu and Tao Zhang

Water 2025, 17(6), 800; https://doi.org/10.3390/w17060800 - 11 Mar 2025

Cited by 15 | Viewed by 6463

Abstract

This study presents a novel approach that integrates hydrothermal carbonization (HTC) technology with circular economy principles to optimize the management of nitrogen and phosphorus in agricultural wastewater. Given the increasing global resource scarcity and continuous ecological degradation, the valorization of biomass wastewater has become a critical pathway for the promotion of sustainable development. Biomass wastewater, which contains crop residues, forestry leftovers, and food processing byproducts, has long been regarded as useless waste. However, this wastewater contains abundant organic matter and possesses significant renewable energy potential. The valorization of biomass wastewater can significantly reduce environmental pollution. Through the optimization of the HTC process parameters, we achieved an improvement in the quality and yield of carbonized products, facilitating the efficient recycling and utilization of resources. This research demonstrates that HTC technology can transform agricultural wastewater into valuable biofertilizers, biomass energy, and organic feed, while simultaneously reducing the reliance on fossil fuels, decreasing greenhouse gas emissions, and mitigating the environmental impact of agricultural activities. This paper provides a comprehensive exploration of the application of HTC technology in agricultural ecosystems, highlighting its beneficial role in nitrogen and phosphorus management, resource utilization efficiency, and environmental pollution reduction. The findings of this study suggest that HTC technology holds significant potential in optimizing agricultural wastewater treatment, promoting resource recycling, and advancing sustainable agricultural development. Furthermore, this research offers theoretical support and practical guidance for the implementation of HTC technology in agricultural ecosystems, which is of paramount importance in fostering circular economic development and achieving sustainable agriculture. Full article

(This article belongs to the Special Issue Sustainable Wastewater Treatment and the Circular Economy—2nd Edition)

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