Membrane Concentrate Recirculation to Activated Sludge: Balancing Organic Micropollutant Removal and Salt Retention

Abstract

Current wastewater treatment plants have not been designed to remove organic micropollutants (OMPs) that are now prevalent in surface waters. This desktop study investigates the Membrane Concentrate Recirculation to Activated Sludge (MCRAS) process, which enhances the removal of the OMP by combining conventional activated sludge treatment with membrane filtration and recirculation of the concentrate back to the activated sludge. The process limits the release of the OMP to the environment and offers an integrated approach for treating the concentrate. Four model OMPs (diclofenac, carbamazepine, ibuprofen, and triclosan) were studied using a mass balance model and literature data, comparing the performance of five membrane types (XLE, NF90, NF270, TFC-SR2, and dNF40). Four removal scenarios were identified based on biodegradation and membrane retention. Notably, with low biodegradation and high membrane retention, OMP removal can be significantly enhanced: diclofenac removal increased from 29 to 72% with an NF270 membrane and up to 97% with XLE or NF90 membranes. However, membrane use also leads to the accumulation of salts, as salts are not biodegradable. This highlights the need for a balance between the OMP and salt retention. Therefore, future membrane development should focus on improving the retention of the OMP while minimizing salt retention.

The Membrane Concentrate Recirculation to Activated Sludge (MCRAS) process enhances organic micropollutant (OMP) removal from wastewater while salts and OMPs may be accumulated in this process.