The origin of environmental and ecotoxicological impacts of microplastics is wastewater treatment plants (WWTPs), and such plants are a ubiquitous, persistent, and incomprehensible pollutant in modern water bodies. Treated effluents remain one of the most significant and persistent sources of MPs that facilitate the long-term degradation of rivers, lakes, and marine ecosystems due to their impressive resistance towards natural degradation despite the treatment processes in WWTPs. The stresses are chemical and physical in nature. Consuming primary producers and invertebrates (bivalves, crustaceans) to fish models (zebrafish, salmonids), consumption at any trophic level causes physical effects of false satiation, intestinal blockage, reduced feeding efficiency, and reproductive damage. This biological stress disrupts the foundation of the food web and threatens the ecosystem’s integrity by amplifying adverse effects throughout the system. Notably, MPs are transporters of pollutants, or vectors of pollution. The co-contaminants in wastewater, including heavy metals, plastic additives (such as phthalates and bisphenols), pharmaceuticals and personal care products (PPCPs), perfluoroalkyl substances (PFAS), and persistent organic pollutants (POPs) like PCBs and PAHs, are easily absorbed and concentrated on their hydrophobic surfaces. The unique physical and chemical conditions in the stomach of an organism may accelerate the desorption of these concentrated toxicants following ingestion, facilitating trophic transfer and bioaccumulation, and causing a problem with human food security and ecological health. Additionally, microbial colonisation of MPs poses a severe microbiological threat, as it can concentrate Antibiotic Resistance Genes (ARGs) and preferentially harbour pathogens. The chapter highlights the fact that better management of wastewater is required, as well as effective monitoring regimes and innovative treatment processes, to reduce the systemic, persistent, and heterogeneous threat of MPs and co-pollutants of the MPs on the integrity of the aquatic ecosystem and health of all organisms living in the marine ecosystem.