An integrated membrane bioreactor - nanofiltration concept with concentrate recirculation for wastewater treatment and nutrient recovery

Increasing water shortages drive the need for water reuse. Membranes are a very suitable technology for purification of wastewater. Membrane bioreactor (MBR) permeate can be polished by nanofiltration (NF), allowing the production of high quality reusable water. The NF concentrate potentially is an environmental hazard when discharged to the environment. A decrease of this environmental impact is reached when NF concentrate is returned to the MBR. This concentrated waste streams allows recovery of valuable components (e.g. minerals). NF concentrate recirculation to MBR causes reduced sludge production (21%) and improves the compactness and settleability of the sludge. A main drawback, is the increased fouling of the MBR membranes caused by non-biodegradable organics. The NF permeate quality is not impacted by the recirculation. Fouling of the MBR membranes is caused by the MBR supernatant due to colloidal and dissolved organics. NF membrane fouling mostly consists of inorganics (calcium and phosphate), while elevated concentrations of organics (e.g. humic acids) even enhance the NF flux. Phosphorous recovery from the NF concentrate within such a process potentially decreases NF scaling. P recovery is achieved in an electrochemical cell, divided into anode and cathode compartment separated by a cation exchange membrane. Precipitation of phosphorous in nanofiltration concentrate occurs after creating supersaturated conditions by locally increasing the cathode pH (water electrolysis). 70 - 95% of total orthophosphate is recovered (at pH 8 - 10). Analysis by ICP, XRD and ATR-FTIR confirm amorphous calcium phosphate (ACP) as precipitate, which could be recovered. Continuous phosphorous recovery reached a sufficiently low P level, allowing water discharge or reuse. P recovery improves the filterability of the MBR permeate. Scaling on the NF membrane is reduced by the additional P recovery. An MBR NF process produces high quality, reusable NF permeate, and reduces the amount of concentrated waste, which even allows the recovery of valuable phosphorous. Nevertheless, two subsequent membrane systems can be energy and maintenance intensive. MBR NF is additionally very attractive for micropollutant removal and more research on the behavior of specific micropollutants in the MBR NF process with concentrate recirculation is important.