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• Fate of nine tri-OPFRs in sludge was studied in a WWTP in Beijing, China. • Sludge contained high concentrations of tri-OPFRs (971.48–1,702.22 ng/g). • Mass flow of tri-OPFRs showed seasonal decreases. • The dehydration unit removed the most tri-OPFRs during sludge treatment. • Ecological risk of OPFRs in treated sludge was reduced. Organophosphate flame retardants (OPFRs) are emerging environmental pollutants that cause endocrine disruption, neurotoxicity, and reproductive toxicity. Sewage sludge is an important source of tri-OPFRs that are released into the environment. The occurrence, distribution, and ecological risk of OPFRs in the full-scale "Cambi® thermal hydrolysis (TH) + advanced anaerobic digestion (AAD) + plate-frame pressure filtration" sludge treatment process is closely related to the application of sewage sludge. We tested sludge samples from a wastewater treatment plant in Beijing, China. Nine tri-OPFRs were detected in the sludge samples collected at different treatment units during four seasons. The ΣOPFRs decreased from 1,742.65–2,579.68 ng/g to 971.48–1,702.22 ng/g. The mass flow of tri-OPFRs in treated sludge decreased by 61.4%, 48.9%, 42.4%, and 63.9% in spring, summer, autumn and winter, respectively, effectively reducing the corresponding ecological risk. The ecological risk of tri-OPFRs in sludge in forestland utilization mainly lies in chlorinated tri-OPFRs, especially TCPP and TCEP. No >42.20 t/hm2 of sludge could be used continuously for one year to prevent tri-OPFRs from exceeding the low ecological risk level, indicating that the current commonly applied proportion of sludge (1.6–30 t/hm2) will likely not raise the ecological risk of tri-OPFRs. [ABSTRACT FROM AUTHOR]