Editorial for the Special Issue “Wastewater and Waste Treatment: Overview, Challenges and Current Trends (Volume II)”

1. Introduction

Liquid and solid waste management is one of the most important challenges of the 21st century, as continued population growth, intensive agriculture, urbanization and industrialization have led to increased pollutant loads on natural ecosystems [1,2,3,4]. Innovative, efficient and environmentally sustainable treatment technologies are imperative [5], as conventional systems often fail to meet new requirements [6], both in terms of the removal of persistent pollutants and the reuse of treated effluents [7,8,9]. In the context of the circular economy, the conversion of waste into resources takes on a central role, offering solutions that combine environmental protection with the production of useful products such as biopolymers [10], fuels [11], adsorption materials [12] and clean water [13,14,15,16].

This Special Issue, entitled “Wastewater and Waste Treatment: Overview, Challenges and Current Trends (Volume II)”, brings together recent scientific papers covering a wide range of thematic areas [17]: from life cycle assessments (LCAs) and technoeconomic analyses of sustainable processes to the applications of biological, nanotechnological and thermochemical solutions for waste decontamination and utilization. These approaches address the environmental impacts of pollution while demonstrating new possibilities for the recovery of energy and raw materials. Examining multidisciplinary approaches and applications in realistic conditions, the published articles capture contemporary trends and prospects for a more sustainable and circularly organized society.

2. Overview of Contributions

The papers published in the second edition (Volume II) of the Special Issue (S.I.) entitled “Wastewater and Waste Treatment: Overview, Challenges and Current Trends” can be separated into four (4) distinct categories: life cycle assessment (LCA) in the service of sustainable technologies for waste management (two papers), biological waste treatment (three papers), circular economies based on waste-to-energy utilization (four papers), innovative technologies for water and air purification (six papers) and industrial processes based on resource recovery from waste (two papers). The distribution of publications (%) is depicted in Figure 1.

The role of LCA is vital for ensuring waste management technologies are sustainable [18], as demonstrated by the two relevant papers published in this S.I. The application of LCA to wastewater treatment (WWT) methods showed that energy consumption produced the greatest environmental burden (ca. 80% of the total footprint), whereas the greenhouse gas emissions corresponded to 14–36% CO₂ and 23–43% N₂O of the total emissions of WWT methods, as demonstrated by Rashid et al. [19]. The use of biogas in resource recovery processes may reduce CO₂ emissions by up to 102 kt annually. Similarly, an LCA conducted for waste glass geopolymerization showed a decrease in CO₂ emissions of up to 26 kg_CO2/ton, while the use of NaOH contributed significantly to the environmental burden of the process in terms of impact to human health (9 × 10⁻⁵ DALY/ton_waste-glass) as presented by Manthos et al. [20]. Overall, the proposed method had an environmental benefit equal to ca. 20 mPt/ton, introducing an alternative to traditional recycling.

Biological treatment methods have a special role in waste management technologies [21,22]. Radice et al. [23] highlighted the role of bioremediation in environmental decontamination. The use of microorganisms (bacteria and microalgae) could be a key strategy for removing oil pollutants. Specifically, the application of the microalga Chlorella vulgaris resulted in an 80% removal of emulsified oil after 5 days, whereas the combination of the abovementioned microalga with bacteria increased this percentage by up to 92%. The bacteria Candidatus Scalindua achieved a 98.9% removal of NH₄⁺ and a 99.6% removal of NO₂⁻ from aquaculture wastewater, as presented by Micolucci et al. [24]. The presence of antibiotics in wastewater has been found to reduce the growth rate of Chlorella sorokiniana by up to 61%, thus negatively affecting biofuel production, as reported by Kim et al. [25].

The utilization of waste for energy production is based on the circular economy [26,27]. Four papers within the present S.I. are related to this intriguing subject, thus underscoring its significance. Gasification was shown to be the most economically sustainable solution for the utilization of construction waste and algae biomass, resulting in an income equal to 0.13 EUR/kg and a treatment cost of 0.09 EUR/kg, as demonstrated by Manthos et al. [28]. The internal rate of return of the gasification process was found to be equal to 9%, thus making it competitive compared to other thermochemical methods. The synthesis of graphene oxide (GO) using recycled Zn-C batteries was investigated by Sperandio et al. [29] with a view to application as a support in Ni/Co nanocatalysts for NaBH₄ hydrolysis in H₂ production. The estimated performance for energy production was 90%, while the as-prepared catalytic system was active after seven consecutive operation cycles. According to Jiménez-García et al. [30], the recycling of PVC using an optimized pyrolysis process allowed the synthesis of CO₂ adsorbents capable of capturing up to 45.6 mg CO₂/g. Activation with the use of KOH at 760 °C, maintaining an activation agent/carbon ratio equal to 2/1, resulted in an enhanced adsorption capacity, whereas the use of NaOH at 840 °C increased the adsorption capacity by 25%. Pozo-Morales and co-workers [31] performed a technoeconomic assessment of the production of polyhydroxyalkanoates (PHAs) by a WWT plant in Spain. The aerobic dynamic feeding (ADF) strategy exhibited the highest yield, producing 0.226 kg of CODPHA/kg COD with a cost equal to 0.11 EUR/kg CODPHA, while reducing sledge production by 6%.

Water and air purification technologies take the lion’s share of published works in this S.I. (six contributions). The potential introduction of proton exchange membrane (PEM) electrolysis process for O₂ supply in activated sludge systems was investigated with a technoeconomic analysis [32]. It was revealed that electrolysis employing a PEM could replace effectively a conventional ventilation system covering 99% of the O₂ needs of a unit serving 80,000 citizens. The estimated cost was 128 EUR/ton. The use of cellulose nanocrystals for per- and poly-fluoroalkyl substance (PFAS) removal from contaminated water was studied by Franco et al. [33]. Cellulose nanocrystals, modified with the Moringa oleifera cationic protein, improved PFAS adsorption, thus increasing the removal capacity of perfluorooctanoic acid (PFOA) from 47.1 to 61.1 mg/g. Equilibrium was achieved within 15 min, thus highlighting the effectiveness of the proposed approach. The removal of amoxicillin (AMX) from aqueous solutions utilizing biochar materials rich in calcium was studied by Jellali et al. [34]. Ca-rich biochar materials derived from the co-pyrolysis of poultry manure, palm oil waste and marble dust were used, and achieved an AMX removal rate equal to 56.2 mg_AMX/g. The highest AMX adsorption was found for the biochar derived after a heat treatment at 900 °C.

Operational parameters such as mesh pore size and air mass load were investigated for the effective operation of a dynamic bioreactor used for municipal WWT by Boulerial et al. [35]. The use of a 20 μm mesh combined with a 5 min aeration after 4 h resulted in a steady flow and low turbidity. Co-modified biochar was employed for metronidazole (MNZ) degradation by Hu and co-workers [36], demonstrating a removal higher than 90% within 60 min. Lastly, as it concerns novel water and air treatment processes, the review of Jurík et al. [37] related to the quaternary treatment of urban WWT discussed various intriguing aspects. In general, quaternary WWT is performed with coagulation, membrane filtration (UF/NF) and UV disinfection, ensuring the effective removal of micropollutants. The European Union (regulation 2020/741) promotes the reuse of treated wastewater due to increasing water demand. By implementing this technology, agriculture can benefit from the safe use of recycled water while addressing scarcity problems.

Finally, industrial processes based on resource recovery from waste are of great significance, since they provide practical solutions to processes that are already utilized [38,39]. Jia et al. [40] studied the production of polyhydroxybutyrate (PHB) from paper mill waste. The fermentation of non-recyclable paper mill fiber with recombinant Escherichia coli produced 6.27 g/L PHB. Waste pretreatment was optimized by increasing the cellulose yield to 83%, allowing high sugar concentrations for fermentation. Subcritical extraction was used to recover phenolics, flavonoids and antioxidants from waste nut shells [41]. Walnut shells treated at 300 °C for 15 min yielded the highest amount of total phenolics (127.08 mg_GA/g), while peanut shells treated at 200 °C for 60 min contained the highest number of flavonoids (10.18 mg_QU/g).

3. Challenges and Perspectives

In conclusion, our analysis of the 17 contributions to this Special Issue highlights the urgent need to develop sustainable technologies for waste management, recycling and environmental remediation. LCA underscores the significance of reducing the environmental impact of industrial processes, emphasizing reducing energy consumption and utilizing environmentally friendly chemicals. Biological and nanotechnological solutions exhibited high efficiency in pollutant degradation, but require further thorough investigation in order to be implemented in real environmental systems. Circular economies and the production of high-value products from waste introduce significant opportunities, but the economic viability and the potential scaling up of the proposed processes remain challenging.

The development and implementation of innovative technologies for wastewater and waste treatment is accompanied by significant challenges. First, despite the environmental benefits of technologies such as geopolymerization and PHA production, their dependence on high-impact reagents such as NaOH increases their environmental and economic costs. Furthermore, the stability and efficiency of biological technologies (e.g., bacteria and microalgae) are affected by the presence of toxic pollutants, such as antibiotics, requiring an enhanced biotechnological approach. Beyond the technical issues, the industrial scaling of many of these solutions is still in its early stages, with a need for cost reduction and better integration into production structures. Finally, their adoption also depends on product safety and on their compliance with environmental requirements.

Future prospects are focused on optimizing innovative technologies to make them more efficient and sustainable. The development of energy-efficient and environmentally friendly catalytic materials can improve the performance of adsorbent and geopolymer materials. The use of synthetic biology offers the potential to enhance the resilience of microorganisms in toxic environments, thus improving the biodegradation of pollutants. Also, combined treatment techniques—mixing physical, chemical and biological methods—are a powerful approach for removing difficult pollutants, such as PFASs. The utilization of waste for biomaterial production may further promote the circular economy, while the integration of digital tools and artificial intelligence promises better prediction, monitoring and optimization of the proposed waste treatments. The successful implementation of these directions requires close cooperation between the scientific community, industry and institutional bodies.