Mechanical pretreatment is an indispensable process in biological treatment plants that remove plastics and other impurities from household biogenic waste (HBW). However, the imperfect separation of plastics in these pretreatment methods has raised concerns that they pose a secondary formation risk for microplastics (MPs). To validate this presumption, herein, quantities and properties of plastic debris and MPs larger than 50 μm were examined in the full chain of three different pretreatment methods in six plants. These facilities received HBW with or without prior depackaging at the source. The key points in the secondary formation of MPs were identified. Moreover, flux estimates of MPs were released, and an analysis of MPs sources was provided to develop an overview of their fate in HBW pretreatment. Pretreated output can contain a maximum of (1673 ± 279) to (3198 ± 263) MP particles per kilogram of wet weight (particles·kg⁻¹ ww) for those undepackaged at source, and secondary MPs formation is primarily attributed to biomass crushers, biohydrolysis reactors, and rough shredders. Comparatively, HBW depackaged at the source can greatly reduce MPs by 8%–72%, regardless of pretreatment processes. Before pretreatment, 4.6–205.6 million MP particles were present in 100 tonnes of HBW. MPs are produced at a rate of 741.11–33 124.22 billion MP particles annually in anaerobic digester feedstock (ADF). This study demonstrated that HBW pretreatment is a competitive source of MPs and emphasized the importance of implementing municipal solid waste segregation at the source. Furthermore, depackaging biogenic waste at the source is recommended to substantially alleviate the negative effect of pretreatment on MPs formation.