June 1, 2009

Focus on Organics in Steam

David G. Daniels

Removing NOM with Pretreatment

NOM has long caused problems with conventional ion exchange demineralizer trains. In addition to shortening run lengths and creating silica in the effluent, organic fouling on the anion beads has been shown to soak up sodium ion during the anion regeneration and bleed it back during operation, increasing the sodium, pH, and conductivity in anion bed effluent. A number of conventional treatments have been examined for their effect on NOM removal (see table).

Pretreatment technologies for removing natural organic matter. Source:M&M Engineering Associates Inc.

Ultrafiltration (UF) followed by reverse osmosis (RO) has been proven the best treatment process for removing natural organic compounds in raw water. Although UF does not eliminate a significant percentage of the small molecular weight organic materials, it does remove a significant portion of the particulate organic particles, including the polysaccharides, microbiologically generated molecules. UF also significantly reduces fouling on the RO membranes. Whatever organic compounds are not removed by UF are taken out by the RO. Testing has shown that nearly 100% of all types of organic molecules are removed by RO.

In cases where there is an existing ion exchange train, organic removal is improved by conventional, and some unconventional, treatments. Conventional lime softening removes 20% to 40% of the dissolved organic compounds in the water. Activated carbon filters also remove organic compounds, but not for the reason that most believe.

Organic compounds are initially absorbed on a fresh carbon bed, but the carbon quickly becomes a biofilter supporting bacteria that use the incoming organic compounds as food. Acting as a "biofilter," activated carbon beds can remove a significant portion of some organic compounds. Likewise, multimedia filters can support bacterial growth. Biofilters, however, are not necessarily stable. A sudden influx of chlorine, an extended stagnant period, or a change in the water supply chemistry may result in a sudden release of organic compounds into the water.

The fact that anion resins are so good at absorbing organic compounds has worked to the plant’s advantage when specialized anion resins are placed ahead of their conventional demineralizer specifically for organic removal. The demineralizer bed then must be regularly regenerated with an alkaline brine solution to remove the organics. The combination of an activated carbon biofilter and specialized organic scavenger bed with anion resins can be a simple and cost-effective organic removal system.

Traditionally, RO membranes have preceded ion exchange capacity, but in some cases RO can be used after a conventional cation and anion vessel and before the mixed bed, specifically to remove any organic compounds.

Care should also be taken that the organic chemicals added to water treatment don’t solve one problem and create another. The wrong type, or high feed rates, of coagulants in the clarifier may improve clarity but may leave the clarifier and contaminate the resin beads of the demineralizer.