A microwave imaging sensor is disclosed for continuously monitoring the condition and status of an in situ reverse osmosis membrane that forms a semi-permeable barrier between a region of water containing a dissolved salt and a region of water which is intended to be substantially free of the dissolved salt. A device according to embodiments of the invention can be used to monitor the condition of the membrane, i.e., the ability of the membrane to filter water by reverse osmosis; and to detect failures in the membrane such as leaking saline water into the output stream. The results of the monitoring can be used to determine in real time whether or not the membrane should be replaced.