Microplastics　in　wastewater　inevitably　accumulate　in　waste　activated　sludge　(WAS)　via　wastewater　biological　treatment,　potentially　affecting　the　subsequent　sludge　treatment　unit.　Nevertheless,　all　previous　research　studies　focused　on　the　impacts　of　the　direct　addition　of　one　type　of　model　microplastics　on　the　sludge　anaerobic　treatment　process.　This　approach　actually　cannot　reflect　the　real　situation　where　multiple　different　microplastics　simultaneously　get　into　the　wastewater　treatment　unit　prior　to　the　sludge　treatment　unit.　Herein,　this　work　innovatively　proposed　a　more　realistic　method　to　assess　the　real　toxic　influences　of　microplastics　on　anaerobic　WAS　fermentation　for　short-chain　fatty　acid　(SCFA)　production　by　initially　adding　four　typical　microplastics　(i.e.,　polyethylene　terephthalate,　polystyrene,　and　polypropylene)　to　the　biological　wastewater　treatment　system.　Results　showed　that　four　microplastics　initially　entering　the　biological　wastewater　treatment　reactor　had　little　influence　on　the　subsequent　anaerobic　SCFA　production　since　WAS　solubilization　increased　but　hydrolysis　and　acidification　decreased.　In　contrast,　when　the　four　microplastics　were　directly　dosed　in　a　WAS　anaerobic　fermenter,　although　there　was　no　effect　on　WAS　solubilization,　the　bioprocess　of　hydrolysis-acidification　was　clearly　suppressed,　ultimately　significantly　(P　=　1.86　×　10-7)　inhibiting　the　maximal　SCFA　production　from　WAS　by　21.5　±　0.1%　compared　to　the　control　without　microplastic　addition.　The　excessive　oxidative　stress　and　toxic　leachates　from　these　typical　microplastics　reduced　the　relative　abundances　of　key　anaerobes　(e.g.,　Longilinea　sp.)　involved　in　the　anaerobic　fermentation.　This　work　revealed　that　the　different　pathways　of　microplastics　entering　the　sludge　treatment　system　had　different　impacts　on　anaerobic　sludge　fermentation　processes　and　selecting　a　more　realistic　and　accurate　approach　was　important　to　evaluate　the　true　toxicity　of　microplastics　on　the　sludge　anaerobic　treatment　system.　©　2021　American　Chemical　Society.