Mitigation of Mineral Scaling and Fouling of RO Desalination via Self-adaptive Operation
Dwindling fresh water supplies from traditional sources, such as surface and ground water, coupled with rapid population growth in developing countries and frequent drought conditions across the globe, have intensified the need for developing alternative and sustainable potable water supplies. In recent years, seawater and brackish water desalination and water reuse technologies have been implemented in various regions of the U.S. and around the globe as part of the movement to diversify the portfolio of available water resources. Generation of the above non-traditional water resources often involves reverse osmosis and nanofiltration membrane technology for desalination and as a barrier against multiple contaminants. However, membrane fouling and scaling are major impediments for robust and effective operation of membrane desalination. Membrane scaling is the result of surface crystallization of sparingly soluble mineral salts and/or the deposition of bulk-formed mineral salt crystals onto the membrane surface. Membrane surface scaling leads to water permeate flux decline and potential membrane damage, thereby limiting recovery and increasing water production cost. Membrane fouling by particulate, colloidal matter, organic, and biofoulant results in reduced membrane permeability and thus result in increased applied pressure requirement for a given target flux, decreased permeate quality, increased frequency of required chemical cleaning and consequently shortening of membrane longevity and as a consequence increased water treatment cost.
In order to alleviate the adverse impact of membrane fouling and mineral scaling, the present work focused on developing a novel approach of self-adaptive operation of reverse osmosis (RO) membrane desalination (including pretreatment). The goal of the approach is to enable effective self-adaptive RO desalination and feed pretreatment operation even when confronted with temporal variability of source feed water quality. In this approach fouling and scale indicators are quantified in real time and the desalination plant autonomously adjust its operating conditions (e.g., triggering of cleaning cycles, coagulant dose setting, feed pretreatment operational strategy).
In order to mitigate mineral scaling, a self-adaptive operation of spiral-wound RO desalting in a cyclic mode of feed-flow reversal (FFR) was evaluated for desalting of brackish water of high mineral scaling potential. Self-adaptive operation was enabled by triggering of FFR once the onset of mineral scaling was detected via a novel ex-situ membrane scale monitoring system. Subsequently, in order to ensure effective RO feed pretreatment the use of ultrafiltration (UF) was explored in a uniquely integrated UF-RO system whereby the RO concentrate was utilized for enhanced UF backwash. The RO concentrate stream was utilized (as both a continuous stream and a high flux pulse achieved using hydraulic accumulators) for UF backwash, with backwash triggering based on by thresholds levels of fouling indicators. The applicability of different fouling indicators for real time performance assessment of UF feed pretreatment and optimization of UF operating conditions (e.g., backwash duration and coagulant dosing strategy) in RO seawater desalination was explored in an extensive field study using an integrated seawater UF-RO desalination pilot plant. Fouling indictors were evaluated with respect to quantification of UF backwashability, unbackwashed fouling resistance and UF fouling rate.