Converging Strategies for Novel Wastewater Treatment and Reuse

1. Introduction

Wastewater treatment has long been described as a matter of engineering infrastructure, ensuring that what leaves our cities, industries, or households does not harm the receiving environment. Conventional framing, however, is no longer sufficient. As populations grow and consumption intensifies, and regulatory standards become stricter, the field has begun to view wastewater not merely as something to dispose of, but as a complex mixture of potential resources. Nutrients, organic matter, and even energy are embedded in these effluents, alongside a growing array of contaminants that are far more difficult to remove [1]. The result is a dual imperative: to safeguard environmental and public health by meeting higher treatment standards, while simultaneously developing processes that recover value and contribute to a circular economy [2].

This Special Issue on “Novel Technologies for Wastewater Treatment and Reuse” captures the diversity of approaches being explored to meet this challenge. Its contributions include laboratory work on algal cultivation systems equipped with smart monitoring [2], carefully designing anaerobic co-digestion studies that probe how different waste streams interact [3,4], and the development of low-cost sorbent materials for pharmaceutical contaminants [5]. Alongside these experimental studies, two reviews remind us of the importance of biological and ecological strategies: one focuses on fungal biodegradation of industrial effluents [1], while the other synthesizes lessons from decades of constructed wetland projects in Italy [6]. Taken together, these papers reveal a field in motion, experimenting with very different tools, but united by the search for solutions that combine efficiency, resilience, and sustainability.

2. Overview of the Published Articles

One of the central themes emerging from this collection is the attempt to redesign biological processes so that they not only treat wastewater effectively but also generate products of value. The study on Galdieria phlegrea, an extremophilic microalga, is exemplary in this respect. By cultivating the organism in a twin-layer photobioreactor fed with municipal wastewater, the authors demonstrate how nutrient-rich effluents can become substrates for biomass production [2]. What makes this work distinctive is not simply the choice of organism but the attention to system architecture and monitoring. By attaching biomass to a porous substrate, harvesting becomes far less energy-intensive than in conventional suspended cultures, while embedded sensors allow for the continuous adjustment of conditions. In effect, the reactor is both a treatment unit and a controlled production platform, pointing toward the integration of wastewater management with biorefinery concepts. Yet, as with many algal technologies, questions of scale remain. Laboratory runs can be optimized with precision, but real effluents fluctuate daily and seasonally, and the robustness of the system under such conditions needs to be properly proven.

A related concern with scale is evident in the papers devoted to anaerobic digestion. The principle that co-digestion can stabilize processes and increase methane yields is well known, but the novelty in these contributions lies in the careful selection of substrates and the quantitative analysis of kinetics. One study turns to the fibrous residues of Posidonia oceanica, a material that accumulates in coastal areas and is often treated as a nuisance. When digested alone, it offers little energy potential, but when mixed with nitrogen-rich substrates such as peptone or casein the process is transformed: methane yields rise substantially and the kinetics accelerate [4]. Another paper pursues a similar logic with a different set of wastes, combining sewage sludge with residues from the orange juice and dairy industries. Here too, the complementarity of substrates enhances performance, underscoring that what matters in practice is not the absolute availability of a single waste stream but the ability to balance inputs [3]. This is a subtle but important point. It implies that operators of digestion facilities should think less in terms of fixed recipes and more in terms of dynamic blending, adjusting feedstock mixes as availability and composition shift. Such adaptability is not yet common practice, but the evidence suggests it will be necessary if anaerobic digestion is to deliver stable energy recovery on a larger scale.

While biological processes address nutrients and organic matter, they are less effective against the trace contaminants now recognized as pollutants of concern. Pharmaceuticals, personal care products, and other micropollutants persist through conventional treatment trains and demand additional steps. One of the papers in this issue responds directly to this challenge by developing a modified adsorbent derived from coconut shells [5]. By impregnating the activated carbon with aluminum, the authors create a material that captures ibuprofen with high efficiency. The study is rigorous in its characterization, testing adsorption isotherms, kinetics, and thermodynamics. Such bench-scale work provides essential parameters for future process design. But it also raises the broader question of context: in real wastewater with its complex mixture of dissolved organic matter and competing solutes, will such materials perform as well? And what of regeneration and disposal? These are not trivial questions, and they remind us that promising laboratory findings must be considered within the messier realities of full-scale operation.

The two reviews included in this issue serve as useful counterpoints to these laboratory-driven innovations. One surveys the potential of white rot fungi, organisms long known for their lignin-degrading enzymes, to tackle industrial effluents laden with dyes, phenolics, and other recalcitrant compounds. The enzymatic machinery of these fungi is remarkably versatile, and case studies show their capacity to degrade pollutants that resist conventional treatment [1]. Yet, practical deployment faces hurdles. Maintaining fungal activity in continuous systems, preventing clogging and biofouling, and ensuring that degradation by-products are non-toxic are significant challenges. The other review turns to constructed wetlands, focusing on the Italian experience over several decades [6]. These systems, by contrast, are firmly established in practice. They are valued for their low energy demand, their ability to integrate into landscapes, and their effectiveness in removing nutrients and particulates. However, their limitations are also clear: they require substantial land, their performance varies seasonally, and they might not be sufficient to meet stringent standards for micropollutants. Nonetheless, the review demonstrates how, in the right contexts, nature-based solutions remain a robust component of wastewater strategies.

If one steps back from the details, what unites these contributions is the recognition that no single technology suffices. Instead, the future lies in hybrid systems that draw on the strengths of each approach. A next-generation wastewater treatment plant might feature an anaerobic digestion hub optimized with flexible feedstock blending, an algal bioreactor capturing nutrients and producing biomass, a sorbent unit polishing effluent of residual pharmaceuticals, and a constructed wetland providing both final treatment and ecological co-benefits. Such a modular vision does not imply that every plant will look the same; on the contrary, it emphasizes adaptation. Small rural communities may favor wetlands and simple adsorption units, while industrial clusters may opt for tightly controlled bioreactors and advanced digestion schemes. What matters is the ability to combine processes coherently, guided by life-cycle assessments, local resource availability, and regulatory frameworks.

3. Conclusions

The studies presented in this Special Issue underline how wastewater treatment is shifting from a purely protective activity toward a more integrated strategy that also recovers energy, nutrients, and valuable resources. The experimental works on algal cultivation, co-digestion, and advanced adsorbents demonstrate the technical feasibility of innovative solutions, while the reviews on fungi and constructed wetlands remind us of the importance of ecological approaches and long-term operational experience. What unites these contributions is the recognition that no single technology can meet the multiple and increasingly stringent demands placed on wastewater treatment.

Future progress will depend on combining methods into modular systems that can adapt to local conditions. To move in this direction, three priorities are essential: scaling up from controlled laboratory experiments to continuous real-world operation, integrating life-cycle and economic assessments into technical research, and embedding robust monitoring and adaptive control into biological and hybrid systems. By advancing along these lines, wastewater management can evolve into a field that is not only environmentally protective but also resource-productive, offering concrete contributions to sustainability and the circular economy.