Monitoring the membrane scaling propensity of retentate in reverse osmosis desalination plants

Abstract

Scaling appears first in the tail spiral wound membrane (SWM) elements of desalination plants due to increased salt concentration in the retentate; in particular, membrane scaling starts locally when supersaturation of sparingly soluble salts (commonly CaCO₃ and CaSO₄) is attained. Reliable determination of the onset of membrane scaling in the tail elements (as a function of the local retentate physico-chemical properties) provides a threshold condition serving a dual purpose; i.e., it imposes essentially an upper limit to the degree of permeate recovery, and guides the selection of an appropriate anti-scaling program involving additives. Judicious use of the latter is important for reduction of desalination cost and environmental impact. The current practices for estimating incipient scaling within the spacer filled reverse osmosis (RO) membrane elements involve a great deal of uncertainty. In this study, slightly supersaturated in CaCO₃ brackish waters were desalinated in a test section comprising a narrow gap spacer filled channel simulating local (constant flux) conditions in SWM modules. Detailed data on incipient membrane scaling were obtained leading to the following main conclusions:

(a) minute quantities of very small particles, ever present in RO plants, apparently promote heterogeneous nucleation; moreover, under the conditions tested, induction period for membrane scaling is practically nonexistent.

(b) the initial scale deposit rates measured by special techniques (but undetectable with common bulk flow measurements) are substantial, and if not mitigated would be detrimental to SWM performance over a relatively short operating time period.

(c) accurate determination of supersaturation ratio at the membrane surface S_w can be a reliable criterion for the onset of scaling, instead of currently used indices (e.g. Langelier Saturation Index) or practical type recommendations; a reliable method has been employed to determine S_w under local retentate conditions. Based on these results, a novel system was developed for monitoring the scaling propensity of retentate, which involves combination of a limited number of measurements in key locations of a RO plant, reliable algorithms for local equilibria and supersaturation (S_w) calculations, and a modern wireless electronic system for plant wide monitoring.

Keywords:

• Membrane desalination
• Incipient CaCO₃ scaling
• Spacer filled channels
• Monitoring scaling propensity

Acknowledgement

Grateful acknowledgment is made of Saudi Aramco for funding a project related to this paper.