What the Flux! – Part 2

Polymeric vs Ceramic Membranes – Lifecycle Cost Comparison Myths

 

In my last post I discussed that just because ceramic membranes can operate at much higher fluxes than polymeric membranes, a ceramic system will not necessarily have a smaller footprint since ceramic modules or stacks have a lot less surface area than polymeric modules – often less than a quarter the surface area in the same footprint. Therefore, higher flux does not necessarily translate into a lower capital cost.

In this post I will focus on the operational cost comparisons and dispel some myths that ceramic membrane systems have lower lifecycle costs due to longer membrane longevity. Let’s start with the longevity. In polymeric versus ceramic membrane lifecycle cost comparisons, membrane replacement frequencies are typically based on warranty lengths which are often 10 years for polymeric and 20 years for ceramic membranes. A 10-year life for polymeric membranes is OK, but there are plenty of examples of Pall (Aria Filtra) and Toray installations still operating with the original modules after 13-15 years. To my knowledge, only Metawater has ceramic membrane installations over 20 years’ old and therefore can guarantee this lifespan. They say the strength of a chain is based on the weakest link and while I don’t doubt the ceramic membranes used in Nanostone, Ovivo and Cerafiltec systems are very durable and would probably last 20 years, what about the plastic components, polymers and glues used to construct and house these membranes? Will these parts last 20 years? I don’t know if any of these systems have been operating more than 3-5 years so far, so it is taking a great leap of faith in believing these systems will last 20 or more years. The housings on the Metawater modules on the other hand are made of stainless steel, so as long as you don’t drop a module (where it would be like dropping a ceramic pot) there isn’t a weak link to fail before the membranes fail.

 The lifecycle calculations I have seen in bid documents are typically over 20 years, and assume a polymeric replacement after 10 years, and no ceramic replacement over that period, so that is where ceramic systems have an advantage. What if we run the lifecycle cost over 20 years and one day, so we include 2 polymeric replacements and 1 ceramic replacement in the lifecycle cost? That’s fair isn’t it?

 From what I have seen, the cost of a ceramic module is at least twice that of a polymeric module while having around a fourth of the surface area. For a replacement cost comparison, let’s assume the following:

• A ceramic module has 1/4 the surface area of a Toray or Dupont polymeric module (see previous post).
• A ceramic module can get 3 times the flux of a polymeric module.
• A ceramic module costs twice as much as a Toray or Dupont module. Say the polymeric module replacement cost is $2600, Ceramic is $5200.
• Let’s say the base case is a 2 x 2 MGD train polymeric system, design flux of 50 gfd, 45 modules per train. Equivalent capacity ceramic skids will have 60 modules (3 x flux but 1/4 surface area per module).
• Cost to replace all polymeric modules is $234,000.
• Cost to replace all ceramic modules is $624,000.

 Based on the above assumptions, for a 10-year polymeric replacement period and 20-year ceramic replacement period, over 20 years and one day, the replacement cost for polymeric membranes is $156,000 less (two replacements of polymeric versus one ceramic replacement). Maybe ceramics can get a little higher flux and maybe the cost assumptions are a little off, but there is no way you are saving much if anything on membrane replacement costs with ceramics. If ceramic membranes can get four times the flux, the replacement cost is just breakeven using my pricing assumptions.

Known Fact: we know there are Pall and Toray polymeric membrane systems that have had their membranes last 13-15 years. Can we be certain the new ceramic membranes on the block will last 26-30 years to break even with the polymeric membrane replacement cost?

Every water is different, and the flux difference may vary one way or the other to shift the economics, but you can’t say there will be a significant savings in membrane replacement costs using ceramics unless you are looking at the polymeric membranes of yesteryear. Kind of like comparing the fuel efficiency of a car from the 70s with today’s modern cars.

I was going to include a comparison of lifecycle costs with coagulant included, where ceramic membranes often need a coagulant dose to achieve high fluxes, while polymeric membranes do not. This can make the lifecycle cost of a polymeric system significantly lower than a ceramic, but I have rambled on for too long already so will leave that to Part 3.

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