What the Flux!

Polymeric vs Ceramic Membranes

My sales rep in the Southeast came to me recently concerned that ceramic membrane companies are promoting fluxes to engineers and water utilities in the region in excess of 200 gfd, asking me how polymeric membranes can compete? My response was WTF! Flux is just a number and just because the flux may seem a lot higher than polymeric membranes it does not mean a ceramic system has a smaller footprint or lower cost. I’ve seen a lot of presentations from ceramic membrane companies trumpeting all the reasons ceramic is better than polymeric but I haven’t yet seen a counter argument from a polymeric membrane company or system supplier. So maybe this is the first counter to some of the ceramic membrane company’s claims. I know I’ll get a push back from the ceramic membrane companies because they are all trying to get established in the market, but I can’t just sit back and let the latest polymeric membranes be unjustly grouped with systems of the past. Note that I am not criticizing the integrity or performance of ceramic membranes at all, and there are situations where these are a great fit, but rather I am just trying to provide a balanced and up to date comparison with polymeric membranes.

WTF!*

Let’s start with the high flux claims. I have seen papers on ceramic pilot studies where fluxes up to 200 gfd have been tested but I don’t yet know of a full-scale system in the US that has been put in service with a design flux this high. The largest ceramic membrane system in the US at Butte MT has a design flux of 69 gfd. A ceramic system that was awarded at Mandaree ND a few years ago had a design flux of 120 gfd for summer. These are pressure ceramic systems where there are feed pumps supplying pressurized modules. 

Figure 1: Membrane System Configurations

The other ceramic configuration is submerged flat sheet operating in the vacuum configuration where a pump draws through the membranes (see Fig 1). Companies such as Cerafiltec and Ovivo are providing this submerged technology and are particularly active in the Southeast. I don’t think a flat sheet submerged ceramic system is installed in the US on a full-scale drinking water system yet, but I have seen several pilot study papers. What strikes me about these recent pilot studies is they are not that impressive. I won’t call out any specific studies, but go search the proceedings from recent AMTA/AWWA Membrane Technology Conferences and you will find them (there may be better pilot studies but I can't find any published). They all spend a lot of time optimizing coagulation ahead of the membranes to reduce rapid TMP buildup and then ramp up the flux in steps over 1 to 2 week periods to get to 200 gfd.  I haven’t seen more than a few weeks operation at anything near 200 gfd in the published papers. Whenever I’ve been involved in a pilot study with polymeric membranes it has been necessary to run at stable operating conditions for at least 30 days. Why is the bar lowered when ceramic membranes get evaluated? Note the Mandaree ND ceramic pilot study did have stable operating periods of at least 30 days at 120 gfd.

 Another criteria for setting design conditions for polymeric membranes is to be a little conservative on the design flux compared to what the manufacturers or pilot studies claim is possible, so if a pilot study shows a flux of 60 gfd is possible based on the feed water quality, the engineer will allow 50 gfd for the full-scale system. While the ceramic membrane companies may claim 200 gfd is possible, when it comes to the design, I haven’t seen more than 120 gfd allowed. Even so, polymeric membranes can be disadvantaged from years of full-scale experience and require a more conservative design flux while ceramic membranes can claim aggressive fluxes without past full-scale experience to suggest otherwise.

 Does Ceramic have a Smaller Footprint?

The claim is often made or implied that due to higher fluxes, ceramic membrane systems have a lot smaller footprint. I will prove to you that is absolute baloney! Let’s look at the comparative footprints of polymeric and ceramic systems. For polymeric, I’m going to use Toray’s HFUG-2020AN module which has 969 sq.ft. of surface area and is probably the most popular polymeric membrane on the market currently. Compare this with a Nanostone ceramic module at 258 sq.ft. per module. A Nanostone module has around the same diameter as a Toray module and is around 9 inches shorter, so the footprint of a membrane rack is the same for both modules (ie. a rack with 40 Toray modules is the same size as a rack with 40 Nanostone modules). Therefore, a Nanostone module needs to have 3.8 times the flux of a Toray module just to have the same footprint based on surface area per module. So, if the polymeric module is designed for a 50 gfd flux, the flux through the Nanostone module needs to be 190 gfd to match the Toray module footprint. If the design flux for ceramic is 150gfd, the Toray system at 50 gfd will have a smaller footprint.

 Now let’s look at a Cerafiltec flat sheet submerged system. These membranes are supplied as 64.6 sq.ft. modules that have a footprint of 28” x 22.7”. Based on Cerafiltec’s website, these modules can be stacked in towers 16 high, so that would add up to a surface area of 1034 sq.ft. From the photos I have seen on the website, the tallest I saw was 8 high, but I will be conservative and compare the footprint of a 16 high tower versus Toray modules in the same footprint. The Toray modules are 8.5” diameter, so within the footprint of the ceramic flat sheet tower, you could conservatively fit 4.5 Toray modules allowing for spacing between the modules (see Fig 2). Therefore a Cerafiltec tower at maximum height needs to have 4.2 times the flux of a Toray module to have the same footprint, i.e. if the Toray module flux is 50 gfd, the submerged ceramic system needs to flux of 210 gfd to match the footprint.

Figure 2: Polymeric versus Ceramic Footprint Comparison

So, I hope I have made it clear that flux is just a number and a high flux does not mean that a membrane system will have a smaller footprint. You also need to consider the amount of membrane surface area that will fit within a given footprint and that ceramic modules have a lot lower surface area than polymeric modules. The price of a ceramic membranes compared to polymeric modules on a membrane surface area basis is also a lot higher, so you can’t assume a higher flux also means a lower cost.

 Of course, there are other important considerations when comparing polymeric to ceramic membranes such membrane longevity and lifecycle cost. I have mentioned in a previous post that the longevity of the newer polymeric membranes is much improved over earlier polymeric membranes which has narrowed the lifecycle benefits of ceramic over polymeric. I’ve also calculated that when you consider the requirement of a coagulant dose ahead of ceramic membranes, the lifecycle cost of polymeric membranes can be lower than ceramic. I will elaborate on that in a future post.

*Shoutout to Stuart Leak from Avista who first used this acronym in his presentation What the Foulant.

The comments and opinions in this post are my own and not those of my employer.