Ceramic Membranes to Separate Anthropogenic Microparticles from Wastewater Sludge Centrate

Abstract The liquid stream produced during the dewatering of digested sludge in wastewater treatment plants contains significant concentrations of anthropogenic microparticles. Generally, this stream is recirculated into the wastewater line, reintroducing these microparticles into the treatment system. This study focuses on separating these microparticles using ceramic membranes of microfiltration and ultrafiltration. An in-depth research on cross-contamination was also conducted, including tests in the laboratory plant without and with membranes using deionized water (non-doped and doped with polypropylene microplastics). Both multi-channel and single-channel ceramic membranes configurations, composed of $${\text{ZrO}}_{2}$$ ZrO 2 - $${\text{TiO}}_{2}$$ TiO 2 with a $${\text{TiO}}_{2}$$ TiO 2 support structure with different molecular weight cut-offs (1 kDa, 50 kDa, 150 kDa, and 1.4 µm pore size) were compared to determine which provided better microparticle retention. All membranes achieved 100% rejection of the doped microplastics, which were larger than 50 µm. However, other unknown microparticles, not intentionally added but naturally present in the samples, consistently appeared in the permeates. Specifically for ultrafiltration membranes, when using non-doped MP centrate as feed, the results indicate that multi-channel membranes achieved up to 95% separation efficiency for fragment-shaped microparticles and around 70% for fiber-shaped microparticles. In contrast, single-channel membranes with the same cut-off as the multi-channel membranes showed approximately 50% efficiency for fragments without separating fibers. In conclusion, this study has demonstrated that ultrafiltration of centrate does not guarantee a microparticle-free permeate. Several factors contributed to this, microparticle accumulation in the system, the characteristics of the microfibers and microfragments in the centrate, and even cross-contamination.