Water reuse: >90% water yield in MBR/RO through concentrate recycling and CO2 addition as scaling control

Over 1.5 years continuous piloting of a municipal wastewater plant upgraded with a double membrane system (ca. 0.6 m³ d⁻¹ of product water produced) have demonstrated the feasibility of achieving high water quality with a water yield of 90% by combining a membrane bioreactor (MBR) with a submerged ultrafiltration membrane followed by a reverse osmosis membrane (RO). The novelty of the proposed treatment scheme consists of the appropriate conditioning of MBR effluent prior to the RO and in recycling the RO concentrates back to the biological unit.All the 15 pharmaceuticals measured in the influent municipal sewage were retained below 100 ng L⁻¹, a proposed quality parameter, and mostly below detection limits of 10 ng L⁻¹. The mass balance of the micropollutants shows that these are either degraded or discharged with the excess concentrate, while only minor quantities were found in the excess sludge. The micropollutant load in the concentrate can be significantly reduced by ozonation. A low treated water salinity (<10 mM inorganic salts; 280 ± 70 μS cm⁻¹) also confirms that the resulting product has a high water quality.Solids precipitation and inorganic scaling are effectively mitigated by lowering the pH in the RO feed water with CO₂ conditioning, while the concentrate from the RO is recycled to the biological unit where CO₂ is stripped by aeration. This causes precipitation to occur in the bioreactor bulk, where it is much less of a process issue. SiO₂ is the sole exception. Equilibrium modeling of precipitation reactions confirms the effectiveness of this scaling-mitigation approach for CaCO₃ precipitation, calcium phosphate and sulfate minerals.