CCRO, FRRO, PFRO – How Long will the Membranes Last?

I've previously shared my view that the recovery gains offered by proprietary RO systems - CCRO, FRRO, and PFRO - are, in most cases, only marginal when compared to conventional multistage systems. However, today I want to focus on a different but equally critical concern: membrane longevity. Specifically, how long will the membranes last in these proprietary systems?

Conventional multistage RO processes operate at steady state with a constant flow in one direction and it is well documents that membranes can last for over 10 years on well-run brackish systems and maybe as low as 5 years for highly fouling systems that need frequent cleaning. Proprietary RO processes such as Closed Circuit RO (CCRO), Flow Reversal RO (FRRO) and Pulse Flow RO (PFRO) do not operate in steady state and have either widely fluctuating pressures and/or flow directions, which RO and NF elements were never designed for. This raises the question: what impact will the fluctuating pressures and changing flow directions have on membrane life? I have had a few membrane element manufacturers tell me, off the record, that they are concerned about the mechanical damage these processes could cause (even though it is good for membrane sales).

These proprietary processes have a lot less operating data, having only been installed for several years, at least for municipal systems, so we don’t have any published information yet on how long the membranes last. Industrial processes adopted some of these processes earlier, particularly CCRO, but information on these installations is harder to obtain. I was on a tour of the West Morgan-East Lawrence Water & Sewer Authority CCRO facility during an AMTA Workshop in Decatur AL last year and it was mentioned that they were starting to replace membranes in some of the trains only 3 years after startup. That set off some alarm bells for me since this is a relatively low scaling feed water source from the Tennessee River with pretreatment using a Pall MF system. I believe they said CIPs are conducted on the CCRO system around every 3 months, which is fairly frequent but not unlike a reuse system where membranes should last at least 5 years. The recovery was 85% which is not pushing the CCRO too hard. It raises the question, has mechanical damage shortened the life of these membranes? The application here is PFAS removal, so perhaps removal of this contaminant is more sensitive to mechanical damage than other targeted ions where the goal is to achieve non-detect levels on PFAS in the permeate.

I recently heard of a PFRO system treating MBR effluent at a municipal wastewater plant that is running at 92% recovery and doing daily acid and alkali cleans. The plant manager told me they were told the membrane would last 10 years, but he said they now think they will need to replace these after 2 years! Is it due to the frequent cleaning or mechanical damage? Probably both.

I also know of a CCRO system treating cooling tower blowdown and running at 94% recovery and they are doing CIPs once a week and replacing membranes yearly. There are several issues here, where the pretreatment is only multi-media filters which is inadequate for RO pretreatment on this type of feed water and the recovery rate is probably too high. However, the customer faces significant costs associated with offsite concentrate disposal, creating a tradeoff between minimizing hauling expenses and the increased cost of CCRO membrane replacement and chemical usage. It is worth noting though that the customer was told, when selecting CCRO, that the recovery would be even higher…

What about the valve and pump life?

Another difference between proprietary and multi-stage RO processes is that all the proprietary processes have frequently actuating valves, at least every 30 minutes for many of these compared to a few times per day for multi-stage. The FRRO process also needs many more actuated valves to be able to change flow directions in the housings and alternate stages. The more times a valve actuates, the lower the life of the valve and actuator and higher the maintenance cost. Alternatively, you could use a more expensive valve/actuator, similar to what is used on a MF/UF system to extend the life. It's also important not to overlook the toll that fluctuating pressures and frequent start-stop cycles can take on pump longevity.

Therefore, when considering the proprietary ‘high recovery’ RO processes, you should consider more than the benefits of getting a few more percent recovery. Will O&M costs for these processes be significantly higher than multi-stage processes due to more frequent membrane replacement, higher cleaning costs and added maintenance for the valves and pumps? As these systems accumulate more full-scale operational experience, I’m eager to see published data that sheds light on their actual long-term O&M costs.

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

North Dakota – The Porcelain State!

 

Two More Ceramic Membrane Projects Awarded in ND

With ceramic membrane systems recently selected for two more drinking water projects in North Dakota, we may soon be calling it the Porcelain State! By my count, that’s now at least four ceramic systems in ND, three are using Nanostone and one is with Metawater membranes. I think that currently has North Dakota with more ceramic membrane installations for drinking water treatment than any other US state. There are also a few drinking water systems retrofitted with ceramic membranes nearby in South Dakota.

What is driving the love of ceramic membranes in the Dakotas? Cold water is the reason for a few of the retrofits where existing polymeric systems had capacity challenges in winter. Cold water reduces the flux of polymeric membranes a lot more than ceramic membranes, so if a ceramic module can produce more water than a polymeric module then the capacity is increased – note, ceramic modules have a lot less surface area than polymeric modules, so just because you can get a higher flux with ceramic does not mean you will get a higher output per module – the math needs to be done to confirm this (see my previous post What the Flux!).

Most of the projects that bid in North Dakota allowed either polymeric or ceramic modules and ceramic membrane systems were selected based on an evaluation matrix, including a lifecycle cost comparison and other qualitative evaluation factors.

For two of the projects, despite the selection criteria, ceramic membranes were quite competitive based on capital cost alone. The major factor that allowed competitive capital costs for these bids was the allowed design flux for each type of membrane. On a surface water with coagulation and plate settler pre-treatment, the maximum flux allowed at 2 deg C for polymeric systems was 15 gfd and for ceramic systems was 90 gfd. Ceramic membranes were piloted to determine the design flux. As far as I am aware, there was no parallel pilot of polymeric membranes conducted and the design flux was based on a conservative design for an existing plant installed over 12 years ago. Notably, there are other polymeric membrane systems on the same water source (Missouri River) that have design fluxes over 40 gfd. 

With this spec, not surprisingly, some of the polymeric system OEMs decided not to bid because they had to bid with six times the membrane surface area. With the capital costs very close between polymeric and ceramic systems, based on the evaluation matrix, ceramic won by virtue of having a longer warranty period in both cases, which raises another concern about how these bids are evaluated – if you have a lifecycle cost comparison with the membrane replacement frequency based on the warranty period, why is there a separate evaluation scoring criteria for the warranty length? In that case you are penalizing polymeric membranes twice for a shorter warranty duration – there should not be a separate score for warranty period if there is a lifecycle cost comparison based on warranty length! Unless of course you want to make sure ceramic membranes are selected…

The most recent ceramic membrane project awarded in ND also had an evaluation matrix and while downstream reverse osmosis was also part of the evaluation, most of the evaluation criteria weighting was based on the MF/UF system. In this case, ceramic capital costs were significantly higher than polymeric, but lifecycle costs and non-cost evaluation factors resulted in the selection of ceramic membranes versus polymeric.

A water system owner is free to select a ceramic system if they want it and are happy to pay a premium for capital cost (and also operating cost if you do 21-year lifecycle – see post), just like someone can buy a Mercedes car ahead of a Lexus if that is their preference. If that is the case, flat spec ceramic membranes rather that have a flawed evaluation criteria against polymeric membranes just to get extra bids.

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