Green Technologies for Wastewater Treatment: A Critical Narrative Review of Biological, Bio-derived and Hybrid Approaches and Their Translation to Practice

Abstract

Conventional activated-sludge treatment remains energy-intensive, chemically demanding and poorly suited to the recovery of the water, nutrients and energy that wastewater contains. Interest has consequently shifted towards green technologies that treat wastewater through biological, bio-derived or low-input physicochemical processes while lowering carbon and chemical footprints. This critical narrative review examines the principal families of such technologies, namely constructed wetlands and related nature-based systems, phytoremediation with aquatic macrophytes, microalgal and algal–bacterial processes, bioelectrochemical systems, bio-derived adsorbents such as biochar and biosorbents, green-synthesised nanomaterials and solar-driven photocatalysis, and green-oriented bioreactor and electrochemical configurations, before considering how these approaches support resource recovery within a circular-economy framing. Evidence was drawn from scholarly indexes and citation searching, appraised for methodological adequacy, and synthesised thematically rather than catalogued study by study. The available evidence indicates that several green technologies achieve treatment performance approaching or matching conventional processes for specific pollutants, particularly organic matter, nutrients and selected dyes and metals, while offering co-benefits in energy demand, sludge production and resource recovery. Confidence in these conclusions is nonetheless constrained by a predominance of laboratory and short-term studies, uneven geographical representation, inconsistent reporting of operating conditions, and limited long-term field validation. Reported performance varies widely with feedstock, climate and reactor configuration, and mechanistic plausibility has frequently outpaced demonstrated field effectiveness. The synthesis identifies scale-up reliability, standardised performance reporting, life-cycle and techno-economic assessment, and integration into hybrid treatment trains as the most pressing priorities. Green technologies are best understood not as uniform replacements for established infrastructure but as context-dependent components whose value depends on matching process to wastewater type, local conditions and recovery objectives.