Demonstration of CAPS pretreatment of surface water for RO

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/M^2-hr bar. This treated water was concentrated ten-fold 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%.