This work studied the occurrence of microplastics in primary and secondary effluents and mixed sludge of a WWTP as well as in processed heat-dried sludge marketed as soil amendment. Sampled microparticles were divided into fragments and fibres, the latter defined as those with cylindrical shape and length to diameter ratio >3. We showed the presence of 12 different anthropogenic polymers or groups of polymers with a predominance of polyethylene, polypropylene, polyester and acrylic fibres together with an important amount of manufactured natural fibres. The smaller sampled fraction, in the 25–104 μm range, was the largest in both primary and secondary effluents. Fibres displayed lower sizes than fragments and represented less than one third of the anthropogenic particles sampled in effluents but up to 84% of heat-dried sludge. The plant showed a high efficiency (>90%) in removing microplastics from wastewater. However, the amount of anthropogenic plastics debris in the 25 μm - 50 mm range still released with the effluent amounted to 12.8 ± 6.3 particles/L, representing 300 million plastic debris per day and an approximate load of microplastics of 350 particles/m3 in the receiving Henares River. WWTP mixed sludge contained 183 ± 84 particles/g while heat-dried sludge bore 165 ± 37 particles/g. The sludge of the WWTP sampled in this work, would disseminate 8 × 1011 plastic particles per year if improperly managed. The agricultural use of sludge as soil amendment in the area of Madrid could spread up to 1013 microplastic particles in agricultural soils per year. Display omitted •We identified 12 different polymers in microplastics form wastewater and sludge.•High efficiency of secondary A2O treatments in removing microplastics.•Fibres displayed lower sizes than fragments and predominated in samples from sludge.•The plant releases 300 million plastic debris per day to its receiving river.•Sludge agricultural use is a major contributor in disseminating microplastic particles. Wastewater treatment plants release a huge number of particles through effluent and sludge contaminating effluents and agricultural soils.