(404b) REAL Time Monitoring of Membrane Mineral Scaling and Application to RO Plant Optimization
Reverse osmosis (RO) desalination is a relatively mature technology that is widely used in both seawater and inland brackish water desalination, as well as in water reuse application. In contrast with seawater desalination which is typically carried out at recovery below about 50%, inland desalination has to be carried out at a reasonably high recovery in order to reduce the challenge of concentrate management and avoid the loss of valuable water resources. However, RO deployment for treatment of brackish water is often impeded at high recovery operation by membrane mineral scaling due to precipitation of sparingly water soluble salts. At high recovery operation, the concentrations of mineral salts on the feed-side and near the RO membrane surface can increase to levels exceeding their solubility limits, which then leads to mineral scale formation on the membrane surface. Mineral surface scaling leads to permeate flux decline and can ultimately result in membrane damage. In order to effectively combat membrane scaling it is imperative to be able to deploy a capability for early detection and continuous real-time monitoring of mineral scaling. In order to address the above need, a novel direct membrane monitor (MeMo) was developed at UCLA as an ex-situ sensor for early detection and quantification of the evolution of mineral scaling in RO membrane plants. Mineral scale detection and monitoring with MeMo is accomplished via direct membrane surface optical imaging, under RO process flow conditions, along with specially developed real-time image analysis software. Previous work with an early MeMo prototype demonstrated effective detection of the onset of gypsum scaling in RO membrane operation. In the present work, the MeMo technology has been extended to enable, for the first time, detection of the onset and monitoring of calcium carbonate and silica scaling. Using advanced and automated high resolution real-time image analysis techniques, the MeMo quantifies the degree of mineral scaling in terms of surface area covered by scale and the surface number density of mineral growth crystals or deposited scale precipitate particles. Mineral scaling tests at different levels of solution saturation at the membrane surface were then carried out whereby the level of concentration polarization was adjusted via setting of the permeate flux and crossflow velocity. The rate of mineral scale nucleation was then quantified as a function of the level of solution saturation and shown to follow the classical nucleation theory. Finally, it is shown that with MeMo early scale detection RO plant operation can be optimized with respect to both operating conditions (flux and cross flow velocity) and antiscalant dose to ensure mineral scale-free operation. Results from field operation of a spiral-wound RO plant will be presented and discussed demonstrating the advantages of the MeMo approach for RO plant optimization.