Enrichment of nutrients from anaerobically digested centrate minimizing microplastics content using a combination of membrane processes

Centrifuge effluent, or centrate, is a liquid stream generated in the sludge line of wastewater treatment plants in the sludge dewatering process. Nutrients are solubilized in the anaerobic digestion of the sludge, mainly in the form of ammonium-nitrogen and phosphates. Thus, when the dewatering of the digested sludge is performed (usually by centrifugation), a sludge liquor stream enriched in nutrients is generated. However, it also contains microparticles, including microplastics, since most of the microparticles from the wastewater are transferred to the sludge treatment line in wastewater treatment plants. In this work, microplastics in centrate have been analyzed (counted and identified), and membrane technologies (ultrafiltration and forward osmosis) have been applied to concentrate nutrients and to obtain a microplastic-free stream for further nutrient recovery. Two alternative configurations have been compared, changing the application order of these processes. The results showed that obtaining a stream with a concentration higher than 6000 mg/L of ammonium has been possible by using ammonium sulfate (150 g/L) as a draw solution in the forward osmosis process. On the other hand, as a consequence of the ultrafiltration application, microparticles were concentrated in the reject stream up to 800 microparticles/L. At the same time, the permeate presents a lower concentration of microplastics without reducing the concentration of the nutrients. In this way, this pioneering study enables the production of a nutrient-enriched stream with reduced microplastic concentrations, that could be applied to the agricultural soil as biofertilizer.