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.