Dewatered digested sludge and compost may act as conduit of microplastics (<5 mm) in terrestrial and subsequently aquatic systems. However, standardized methods for microplastics analyses are lacking. Thus, the aim is to demonstrate the applicability of wet-sieving as a way to quantify large microplastic particles (MPP, 1–5 mm) in dewatered digested sludge and compost. Additionally, we investigated the organic fraction of municipal solid waste, expired drinks and slaughter house waste used as co-substrate for anaerobic digestion at Wastewater treatment plants (WWTP). Therefore, we collected samples from six WWTP and two biogas plants. These were then wet-sieved and potential MPP analysed via Fourier Transform Infrared Spectroscopy (ATR-FTIR). In dewatered digested sludge the amount of microplastics ranged from 0 to 326 MPP/kg TS while compost contained 39–102 MPP/kg TS. Our results show that with 0–36 MPP/kg TS co-substrates are not necessarily a source of microplastics in WWTP. Furthermore, we found film to be the most abundant shape in the biogas plant samples, whereas, in WWTP samples film, fragments and fibers were detected the most. FTIR revealed that polyvinyl chloride (PVC), polyester (PES), polypropylene (PP), and polyethylene (PE) were the most abundant materials found across all samples.