Synthetic biology (SynBio) offers powerful tools to engineer microorganisms with tailored functions, offering opportunities to address environmental challenges, including removal of microplastics from drinking water. Most of the literature to-date focuses on modifying a single organism and conducting bench-scale experiments in rich or minimal media. More recently, SynBio has been applied to synthetic communities (SynComs), where defined sets of strains are assembled to work together. However, how engineered strains and SynComs establish, persist and remain active within highly adapted mixed microbial communities remains poorly understood. Through the Environmental Biotechnology Innovation Centre (EBIC), this project will address these questions using a combination of classic chassis strains and drinking water biofilm associated species. Engineered variants of each carrying modules for enhanced biofilm formation and/or plastic degradation will be tested in combinations of modifications and organisms, at scale. Controlled experiments will test their establishment, stability and gene retention across conditions, from rich and minimal laboratory media, to drinking water and drinking water biofilms. Persistence will be quantified under varying propagule-pressures, and the influence of engineered function, nutrient limitation and native community structure on survival will be assessed. We hypothesise that increasing genetic load will reduce fitness and persistence of single engineered strains and mixed communities, and that division of labour in SynComs may offset these costs. This work will provide an experimental framework linking genetic load, division of labour and environmental context to the performance of synthetic strains and SynComs, helping define realistic expectations for their persistence in engineered water treatment systems.