Is Suspended Ion Exchange (SIX®) Ready for Market?

In 2010 a new water treatment technology called Suspended Ion Exchange (SIX®) signaled its entrance into the North American water treatment market with presentations at several water industry conferences. SIX® was developed by PWN Technologies, a subsidiary of Netherlands based PWN Water Supply Company North-Holland, primarily as a pretreatment process for the removal of natural organic matter (NOM) prior to membranes.

PWN states (1) that SIX® overcomes limitations of traditional fixed bed ion exchange contactors, such as fouling and blinding of resin pores and surfaces with organic and colloidal matter, and is able to tolerate fluctuations in raw water characteristics. PWN also claims SIX® has advantages over the MIEX® Process, another suspended ion exchange process developed by Orica Watercare Inc., through improved efficiency of the ion exchange contactor and regeneration process while enabling the use of a range of commercially available ion exchange resins, unlike the MIEX Process which uses a proprietary resin.

Before I go any further, I will let you know that I did previously work for Orica Watercare and was involved in the development of the MIEX Process. As I no longer have any allegiances to my previous company, I will be making as impartial a review of SIX as possible. I believe my experience in the commercialization of the MIEX process also helps to provide a unique insight into the readiness of SIX for market.

How SIX® Works

Figure 1: Flow Diagram of the SIX® Process (Reference 2)

The SIX ion exchange vessel consists of vertical cylindrical contact chambers containing baffles and mixing paddles to distribute flow and ensure effective mixing. These continuous mixing chambers are used in series with the object of approaching the ideal contactor kinetics of a plug-flow reactor1. An anion exchange resin is injected into the raw water feed, suspended in the contact tanks and then removed after the second contact tank where it is regenerated in a separate regeneration process. The regeneration process consists of a number of mixed regeneration vessels where a brine solution is contacted with the resin. The brine solution is recycled a number of times to reduce waste volumes. Regenerated resin is then transferred to fresh resin tanks where it is then added back to the raw water feed. PWN has also investigated the use of Nanofiltration to separate NOM from the spent brine regenerant and further reduce waste volumes but the reported pilot results have only seen up to 67% removal of the NOM which would not allow the permeate to be suitable for resin regeneration (2).

The potential advantage of suspended ion exchange is that turbidity can pass through the contactors without impacting process operation, thus allowing SIX to be used as a pretreatment process. In addition, the efficient contacting of the resin in a plug-flow reactor configuration and subsequent ‘exsitu’ regeneration can potentially result in significantly lower brine waste volumes compared to conventional fixed bed ion exchange systems.

This background information has been gleaned from papers and presentations I attended at the 2010 IWA Leading Edge Technologies Conference and the 2010 AWWA WQTC Conference. No additional information is readily available on the process, with little detail on PWN’s website. It is therefore difficult to make a proper quantitative assessment of the true benefits of SIX and the readiness of the technology for market. The kinetic modeling that has been performed is excellent and demonstrates that the plug-flow reactor configuration is a more efficient means of facilitating ion exchange – but most chemical engineers could have told you that without the testing. Reported percent removals of NOM and nitrate are all based on testing performed on one water source. I would like to see more testing conducted on different water sources with a range of characteristics, including different NOM characteristics and ionic compositions, for a better indication of the robustness of the process.

Show me a Mass Balance!

My major beef with the available data is the absence of any mass balance data to quantify claims of regeneration efficiency, low waste volumes and low salt use. The data presented at the IWA Conference just didn’t add up. Dissolved organic carbon (DOC) concentrations in the waste brine were reported only as ‘greater than 300 mg/L’. How much greater… 350 or 500 mg/L?? There was no quantification of how much waste is produced and when I asked the presenter, a PWN representative stood up and said it was about 1 m3 per 5000 m3 treated. That is 200 gallons of waste per 1000 gallons of water treated - pretty impressive, but let’s look again at the waste TOC concentration to see if that makes sense. Assuming the waste TOC concentration was 400 mg/L in 1 m3 of waste; by mass balance the process is removing 400 grams of TOC per 5000 m3 of water treated, or 0.08 mg/L – not very impressive considering the raw water TOC was reported at about 6.0 mg/L. The IWA paper shows a DOC reduction by SIX of about 3.0 mg/L, which from my experience with anion exchange is reasonable. By mass balance, if the waste DOC concentration is 400 mg/L, the waste volume should therefore be more of the order of 37 m3 per 5000 m3 (7,500 gallons per million gallons of water treated) – unless the SIX process is also eating DOC…..


Figure 2: SIX® Pilot Plant (Reference 2)

My final concern is the regeneration process. It looks complex to me (Figure 2). Claims that this is simpler than the MIEX process are outdated by about 6 years. Significant enhancements have been made to the ‘exsitu’ regeneration process used by the MIEX technology since the first full-scale system started up in 2001 resulting in substantial reductions in waste volumes and simplification of operation. SIX still has to go through this learning curve and the sooner PWN can get a continuous demonstration plant operating the sooner it can start to iron out the operability bugs that will inevitably be found.

Despite my above reservations, I think SIX has great potential but it will be a few years yet before the technology is ready for widespread adoption. It would be great for the water industry if the MIEX technology had a truly competitive suspended ion exchange process to drive down costs to customers, speed up process improvements and allow ion exchange pretreatment to be competitively bid. This would be similar to when Memcor developed its submerged membrane process to counter its erosion of market share by Zenon’s Zeeweed process in the late 1990’s. To successfully launch SIX in North American, PWN should take a look at how the MIEX process was brought to market, including initiating pilot studies on a number of different water sources with the participation of locally respected academics and consulting firms and establishing a full-scale demonstration facility ASAP!

1. Friend-Gray, O.P.; Malley, J.P. Jr; “Suspended Ion eXchange (SIX®) for Pre-treatment in Advanced Oxidation and Membrane Water Treatment Systems”, Proceedings of IWA Leading Edge Technologies Conference, Phoenix AZ, June 2010.

2. Friend-Gray, O.P.;”Optimization of the Suspended Ion eXchange (SIX®) for Pre-treatment”, Proceedings of the AWWA Water Quality Technology Conference, Savannah GA, November 2010.