Jack Gilron

Analysis of RO flux decline due to membrane surface blockage

Recent work has shown that, flux decline due to precipitation of a scaling salt such as CaSO4 on RO membranes, is governed by a surface blockage rather than a cake filtration mechanism. Crystal growth sites, scattered on the membrane, spread out laterally, increasingly blocking the membrane free flow area. Flux decline, resulting from gradual blockage of the membrane flow area, is analyzed for both laminar and turbulent flow conditions. Predicted results are shown to conform to experimental observations.

Demonstration of CAPS pretreatment of surface water for RO

Abstract A partial softening process, CAPS (compact accelerated precipitation softening), has previously been shown to have potential to remove various contaminants that can interfere with membrane desalination processes. This has now been demonstrated in a continuous process treating surface water from the Nahal Taninim brook taken from fish ponds and the river itself. This water has an extremely high turbidity (20–25 NTU) when it comes out of gravel and rapid sand filters. On treating 350 L of this water with the CAPS process, the calcium hardness was reduced to 20% of its initial value, and its turbidity was reduced from 12 NTU in the feed to 0.08 NTU in the product. The SDI was reduced to a value of 4–5. On the other hand, the raw feedwater was too dirty to measure (SDI value off the scale). On a second batch the MFI was an order of magnitude smaller for the treated as opposed to the untreated water. Bacteria counts were reduced by two orders of magnitude from 65,000 to 500. Organic content as measured by DOC was reduced by 50%. Specific fluxes for CAPS have been found as high as 2000–5000 L/M2−hr bar. This treated water was concentrated tenfold in an RO unit with a brackish water RO spiral-wound membrane element. When water from Nahal Taninim was not treated by CAPS but only acidified and passed through a 30 μm microfilter, the membrane fouled severely during RO and the water recovery could not be continued beyond 80%.

Analysis of Laminar flow precipitation fouling on reverse osmosis membranes

Abstract Precipitation fouling on reverse osmosis membranes was analyzed by the boundary layer integral method for laminar flow in a parallel plate passage. Flux decline and concentration polarization levels are governed by a complex coupling between precipitation kinetics, mass transfer, and the deposit structure Precipitation at the membrane relieves the concentration polarization of the precipitating salt. Concentration polarization is also relieved by the flux decline accompanying the increased hydraulic resistance of the growing deposit. As precipitation rate increases with growing concentration salt convection into the boundary layer is balanced by the rate of precipitation. In the usual case where an inert salt is also present, the concentration polarization of the precipitating salt may pass through a maximum. Permeate flux is reduced by the increasing osmotic pressure of the inert salt, thereby reducing the concentration polarization of the precipitating salt. The analysis stresses the importance of distinguishing between those effects mainly attributable to precipitation kinetics and those which are primarily a function of the deposit structure. Flux decline data alone are inadequate to elucidate kinetic aspects of precipitation fouling phenomena.