Controlling calcium carbonate scaling in groundwater reverse osmosis systems: Maximizing recovery and minimizing antiscalant consumption
Nasir Mangal (PhD Candidate IHE Delft), Maria Kennedy (promotor IHE Delft), Walter van der Meer (promotor), Sergio Salinas (supervisor IHE Delft), Antoine Kemperman (supervisor)
Funding: Oasen Drinking Water Company (based in Netherlands) and Grundfos (based in Denmark).
Maximizing recovery in brackish water reverse osmosis (BWRO) applications is essential, as at maximum recovery, the production of brine and also the specific energy consumption (kWh/m3) are minimum. However, the main obstacle to achieve high recoveries (80 - 90 %) in BWRO is scaling, particularly CaCO3 scaling. Membrane scaling is caused by the precipitation of sparingly soluble salts exceeding their solubility levels at high recoveries on the membrane surface and feed spacer. Scaling has severe adverse effects on the operation and performance of BWRO applications including but not limited to lowering the quality of RO permeate, reducing the life expectancy of the RO membranes and increasing the operational and maintenance cost of the system. In order to overcome the scaling problem at high recoveries, antiscalants are added to the feed water, which prevents the precipitation of sparingly soluble salts on the membrane surface.
With the use of antiscalants, the main question which arises is: How to determine the optimum dose of antiscalant to prevent scaling in BWRO applications? Antiscalant dose for a given water composition is generally determined using the antiscalant manufacturer’s proprietary programs or simply recommended by the manufacturer. However, the method employed by the antiscalant suppliers is not known and therefore the end users/consultants cannot verify the doses calculated by the projection programs. Besides, as antiscalants are combinations of several organic compounds that delay the precipitation process of the scaling compounds, it is supposed that natural organic matter, especially humic substances present naturally in the RO feed might have positive effect on scaling and would work as antiscalant. The projection programs of the antiscalant manufacturers do not consider the effect of humic substances in calculating the antiscalant dose.
Briefly, the current methods used to calculate antiscalant dose may result in overdosing of the antiscalant which not only increases the operational cost but also, in many cases, may give rise to other types of fouling.
The aim of this study is to develop a method to efficiently determine maximum recovery and optimum antiscalant dose to prevent calcium carbonate scaling in brackish water reverse osmosis.