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.

PFAS Discharges into Sand Creek from Suncor’s Denver Refinery Must Stop

Suncor Refinery alongside Sand Creek - Photo by Hyoung Chang, Denver Post

I have been stewing over whether to do a post on an article I saw in the Denver Post on July 27, 2023, regarding discharges of PFAS from Suncor’s refinery in the Denver area into Sand Creek which eventually makes its way to the South Platte.  For years I have been hearing local news stories of concerns from nearby communities about air emissions from the refinery and allegations of exceeding EPA and Colorado Department of Health and Environment (CDPHE) permit levels. I have wondered if Suncor has been given some slack due to its position as a major fuel supplier in the region. Whenever the refinery is offline due to maintenance, etc., fuel prices spike which impacts the wider community’s hip pocket (does anyone keep their wallet in their hip pocket anymore?). Is that allowing Suncor some leverage over CDPHE’s permitting process? According to the Denver Post article, Suncor had been operating on an air quality permit from 2006 that is supposed to be updated every 5 years. The permits for water and stormwater discharges were last updated in 2012.  Of course, having a permit does not mean Suncor adheres to it and there have been reported incidences of benzene spills into Sand Creek over the years as well as air permit exceedances.

South Platte’s PFAS Problem

Being in the water industry and seeing the great expense many water utilities and communities are facing to meet upcoming PFAS regulations, when I read about how such high levels are being discharged from Suncor into a drinking water source used by so many Coloradans it really hit a raw nerve for me. Pretty much any water treatment system taking water from the South Platte in the Denver Metro Area and East, including nearby wells, will have to implement some sort of treatment for PFAS removal. The cost for treatment is in the millions to tens of millions of dollars each, depending on system size. While the drinking water PFAS regulations are still being finalized, many water systems are already making plans to install treatment, since it can take years to get the funds and construct the required treatment equipment (typical solutions are GAC, Ion Exchange or Reverse Osmosis).

Suncor’s source of PFAS is likely firefighting foams used onsite, although I’m not familiar with refining to know if any raw materials contain PFAS also. This contaminates groundwater under the refinery and according to the Denver Post, Suncor treats this groundwater before releasing to Sand Creek, although obviously it not treated for PFAS removal yet. Admittedly, PFAS has only been identified as a concern in drinking water relatively recently (since 2016) and drinking water regulations are still being finalized. But Suncor was issued a draft permit by CDPHE in 2020 to release no more than 70 parts per trillion into Sand Creek. Note in June 2023 Suncor reported to the CDPHE a discharge level of 2,675 ppt…this is after Suncor apparently installed in interim treatment system to reduce PFAS to 70 ppt in early 2022.

The Denver Post article reported that Suncor estimated it would take 3 years and millions of dollars to build a permanent system to remove PFAS from wastewater before discharge into Sand Creek. Suncor also said PFAS removal is extremely difficult and treatment technologies are still in development. These statements really raise my hackles. First of all, established treatment technologies for PFAS are available now – Reverse Osmosis, Granular Activated Carbon (GAC) and Ion Exchange are well proven and already in use for PFAS removal by water utilities. Secondly, if the stormwater is already being collected for treatment, a lot of the hard work is already done and it would not be difficult to add GAC or ion exchange to the treatment train. Much larger treatment systems have already been installed on contaminated Californian ground water supplies in a quick response to the detection of PFAS and the interim regulations. So don’t try to say the technologies are not yet developed! Locally, there are also treatment systems installed at water utilities south of Colorado Springs where they detected PFAS in the ground water supply originating from a local military base.

While Suncor continues to discharge PFAS into Sand Creek, communities downstream are paying the price with their health and their money where the local water treatment plants must pay for treatment.

The article in the Post was written in late July, so Suncor may very well have accelerated installing treatment for PFAS removal since then, since the media is quick to report a violation but often slow to report on a resolution. If so, then I retract some of my vitriol for Suncor not taking action.

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

Gasson Spices up Membrane Technology Conference Opening Session

(Not one of the keynote speakers)

This year’s Membrane Technology Conference (MTC) in Knoxville TN, February 20-23, saw a spike in attendance, almost back to pre-covid levels, with a definite buzz around the presentations and exhibit hall where attendees were excited to be back networking with colleagues and technology suppliers.

Christopher Gasson, Publisher of Global Water Intelligence, was a keynote speaker for the Opening General Session, along with Harry Seah, CTO of PUB. Christopher’s ‘State of the Global Membrane Industry’ presentation certainly provided a spicey opening to the conference with his description of ‘What’s Hot and What’s Not’. Some exhibitors in the audience that were on Christopher’s ‘Not Hot’ list may have begged to differ. These included manufacturers and developers of ‘Fancy Membranes’ which I assume referred to new chlorine resistant membranes and fouling resistant membranes among others. From a global market share perspective, he is probably correct, but companies such as ZwitterCo are likely not trying to take the place of traditional RO membranes and are content targeting niche markets.

Other technologies or technological trends on the ‘Not Hot’ list included higher recovery for seawater, higher flux RO membranes and lower pressure desal membranes.

On the ‘Question Mark’ list included ceramic membranes and Universal/Open Platform low pressure systems.

During questions, Hary Seah agreed to disagree on the potential for ceramic membranes where PUB is a big advocate of ceramic membranes at its plants in Singapore. I’ve given my thoughts on the ceramic market previously and copped some flak for saying it is a niche technology, but I would agree with Christopher on his position.

I also agree that the Universal/Open Platform low pressure market may have cooled a little now that there are many direct replacement modules available for Asahi (Pall), Memcor, Toray and Dupont modules, which gives some flexibility for future membrane replacements without needing a membrane rack to accommodate modules of different configurations. Also, the proliferation of non-proprietary MF/UF systems using modules from Toray, Dupont and others has taken some steam out of the need for Universal racks. I will flesh this out further in a separate post.

 On the ‘Hot’ list were higher recovery RO in industrial applications (not seawater) which I assume is technologies such as CCRO, Pulse RO and FRRO, polymeric NF (NX Filtration), RO/NF membrane spacers, brine mining and digital monitoring (AI). Christopher pointed out that NX Filtration is capitalized at over €500M with revenues of €8M last year, having investment characteristics of a start-up tech company and a lot of pressure to perform.

I must admit I haven’t been to many opening sessions at MTC, but this one was very well attended, possibly in anticipation of the speakers’ topics. The audience was not disappointed, and Christopher’s thought-provoking statements provided a great catalyst for discussion afterwards and set the stage for a very lively conference.

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

Filling the Conference Gap

In past years at this time I would be writing about technology developments and industry news I had picked up at the AMTA/AWWA Membrane Technology Conference (MTC) and the California WateReuse Conference in March but as we all know, this year these conferences and many more have been cancelled due to the Coronavirus outbreak. These two shows were one week away when most of the country started to go into lockdown, which was a shame for the many who had put so much hard work into planning these events, but obviously the right decisions were made to not cram so many people together in meeting rooms, exhibit halls and social events during these times. Also, as there had been so many attendee and exhibitor withdrawals, it was probably not feasible for the shows to proceed anyway.

Now with AWWA’s ACE20 and many other regional shows being cancelled through July so far, how will this impact the water industry in the short and long term?

In the short term, personally, I am missing out on the networking at these conferences where stronger relationships are developed from direct interactions with engineers, water utilities and manufacturers/service providers. It is through these interactions that you can find out about upcoming projects and industry developments well in advance of public announcements. I know Dupont was really looking forward to being the major sponsor at MTC following the acquisition of several membrane businesses in late 2019 and as I noted in a previous post, I was hoping to spend some time with Dupont at the show to see how these acquisitions would impact the relationship with its existing OEM customers. I am sure many similar meetings would have been planned at MTC. In the everyday hustle and bustle of working on projects and chasing new opportunities when you feel like you are always in firefighting mode, these conferences are important opportunities to break away and spend some quality time to look at new technologies, develop new relationships and strategize with current and future business partners.

The Coronavirus cancellations hit just when the busy spring conference season was starting and now that season is wiped out, we will have a void of at least five months with no shows. For manufacturers and consultants with well-established networks I don’t think there will be long term damage to industry relationships and finding new projects. This pause in shows is actually providing an opportunity to explore and utilize video conferencing and ancient means of communication like talking on the phone. Companies looking to introduce new technologies or enter new markets will be more impacted where the shows are an important avenue to launch these companies or technologies.

So far, I am not seeing a pause in projects bidding, although a few bid dates have been extended, but nothing cancelled yet. If anything, the bidding seems a little busier, maybe because engineers and owners have more time to get these bids on the street and are hoping by the time contractors are awarded the shutdowns will be relaxed and the projects can break ground in late summer. I don’t know if that means some projects are being brought forward so that here will be a lull in bids over summer/fall. I will update on that in a few months.

Long term there could be an impact on the attendance at conferences. Personally, with less travel I have been able to work on a few papers, write a few project case-studies, sort out some projection software issues, and other items I have had on the backburner for years. It has me wondering if it would be more productive for me to possibly cut back on some of the shows. I’m sure others will be thinking the same. There will also be reluctance to travel as much due to the risk of catching a virus, at least for the next year or two if not longer. For exhibitors, this pause in shows is probably also providing time for some reflection on the return they are getting from committing expenditure and resources to these shows and could result in some prioritization on what shows they exhibit at and redirecting these resources into webinars, etc – I am seeing a lot more webinars on new technologies and product information lately, obviously in response to less conferences and sales travel. Normally I wouldn’t have time to watch these, but with no travel I have been happy to sign in.

More Virtual Conferences?

We are seeing some of the cancelled shows now going to be presented virtually and depending on the success of the online versions, there could be more demand for this format. If so, the conference organizers will have to work out how to monetize the virtual offerings to make up for much-needed revenue from the conferences to keep these organizations going. As an industry we need organizations like AWWA and AMTA, plus local AWWA and WEA Sections, to be healthy and active so we have forums for technology exchanges, networking, training, government lobbying, regulation setting and many other services these groups provide. It is in our best interests to keep these organizations strong. I also do work on the industrial water treatment market and you only have to look at how fragmented that market is in terms of conferences and networking opportunities to really appreciate what we have in the municipal market with organizations such at AWWA, WEA, AMTA and WateReuse.

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

2019 AMTA/AWWA Membrane Technology Conference Wrap

The AWWA/AMTA Membrane Technology Conference was held in New Orleans, February 25-28 and attendance was a little down this year. I put the lower attendance down to two main factors - scheduling the week before Mardi-Gras made accommodation and travel expensive (although it was fun seeing some of the festivities) and; because the region isn’t a hub for membrane installations, at least municipally which is where most attendees come from, there were very few local attendees. Further to the last point, there were no technical tours this year because suitable nearby membrane installations could not be found and that may have made the conference less attractive to some potential attendees.

Despite a few hundred less attendees, the technical program was still strong and most of the key engineers working in the municipal membrane field were in attendance. In terms of new developments, there were two ceramic membrane sessions, one of which I attended, but I didn’t see a lot of new information presented as I had already seen versions of these projects presented in the past year. Nanostone has completed a few more retrofits of their membrane at polymeric installations but overall, I don’t see large scale adoption of ceramic membranes at new facilities in the near future where costs are still prohibitive – there is a lot of interest in ceramic membranes though. In the exhibit hall there were a few more companies promoting ceramic membranes and I would like to see some case study posters or presentations from these firms in the future.

Niche Technologies Poorly Represented

Most of the sessions were more around interesting applications of existing membrane technology or optimization of performance and waste minimization, which is all very important, but I didn't see much on the full-scale application of new technologies (although there were some good research presentations on new membrane developments). Walking around the exhibition, there are some interesting technologies available such as Berghof’s tubular UF membranes which are used in challenging filtration applications and it would be nice to have a session to showcase these niche technologies. Perhaps these companies don’t submit abstracts because they are intimidated by the selection process, so maybe there should be a dedicated session for the more niche membrane products.

One beef I had about the exhibition was how far it was located from the presentations which was a disincentive for those attending the presentations to make it back down to the exhibit hall during breaks or for those in the hall to go to the presentations. For this reason, I didn’t get to see as many presentations as I normally do as I had to be at a booth. The hall was slower than I have seen it in the past 8 years and many exhibitors were not happy. The New Orleans Convention Center is fine for a huge event like WEFTEC but was too large and spread out for a smaller specialty conference like MTC. I think the logistical issues were realized by the organizers and will be taken into account at future shows.

So, it was still a worthwhile conference to attend and it is good to try new locations, and some will be better than others. With MTC in Phoenix next year in a strong membrane using region, I am sure attendance numbers will be back up.

Why do so many new water technology companies struggle?

I was at an American Membrane Technology Association (AMTA) workshop last December focused on the use of membranes in the oil & gas and mining industries and in a discussion with a professor who had done a lot of research on forward osmosis (FO) applications I learned of the demise of Oasys Water in the past 12 months. Oasys was one of the water tech shooting stars a few years ago, promising great new applications for forward osmosis for seawater desalination and oil and gas produced water treatment. The professor I was speaking to was a big proponent of forward osmosis ten or so years ago and his group at the Colorado School of Mines had done a lot of work looking at the use of FO for produced water treatment applications.

Over that time he said the producers of the FO membranes have failed to deliver a robust product with issues such as contaminant leakage, fouling and membrane integrity not being solved. I believe another FO company called Hydration Technologies (HTI) has been restructured and refocussed. I don’t want to get into the technical issues of FO but rather why did these start-ups with a lot of investor backing and seemingly great technology fail?

Here are a few of my thoughts as to why I think many water tech start-ups struggle:

Technology is brought to market too soon

In the excitement over the great potential for a technology, it is brought to market too soon without enough testing in real world applications to iron out technical flaws and while product development is still underway. I suspect executives are too impatient to sort out these details and want to get the product to market ASAP and assume the technical flaws can be worked out on field trials for commercial opportunities. I agree that you don’t want to delay launching a product until every minor technical detail is finalized but word travels fast in the water industry and a failed pilot study can be a critical set back to a new technology. There is a delicate balance between getting a product to market ASAP to capture an opportunity and start bringing in some revenue and continuing costly product development until the technology is perfect. I assume that in some cases start-ups are pressured by investors to commercialize a product too soon, so they can start getting a return on their investment.

Once brought to market, all product development stops

I have experience working for a company where once the new technology was commercialized, all efforts were put into field demonstration testing to get some sales while research and product development efforts were put on hold. Even when there was feedback from the field testing that some product improvements were needed to eliminate some issues, there were no resources available to work on these issues. I can see how in the urgency to demonstrate market acceptance and make investors happy after years of sinking money into product development there would be pressure to focus solely on making sales, but inevitably the field testing will find some product flaws and there must be resources allocated to further product development to keep the product competitive. Also, customer needs change and companies must adapt to keep meeting these needs. At the AMTA workshop one presenter explained how fracking chemistry has been improved to allow high salinity produced water to be reused in fracking whereas several years ago it was necessary to desalinate this water for reuse. That pretty much killed a key market Oasys Water, HTI and other forward osmosis companies were targeting!

Poor understanding of market

The municipal water treatment market is a conservative industry. If that is the target for a new technology the startup must be very patient and prepared for a very long sales cycle starting with small installations and very slowly building up to larger systems. Even if the technology is amazing and appears to have little competition, end users will be wary of buying an important piece of process equipment from a company that is new to the market without a track record and evidence that the company will still be around in several years to provide the customer with the technical support, parts/consumables necessary to keep the equipment operating in the future. Process equipment is typically a large capital investment and very few if any water utilities want to be the first to adopt it, even if it is widely used overseas. Industrial customers can be a bit more open to new technology and are often a good proving ground before entering the municipal market. But I still have seen companies with technologies proven overseas enter the US market expecting to hit the ground running and start selling to larger systems and end up very disappointed by the lukewarm reception from engineers and end users.

Other Start-ups to Watch

There are a few water tech startups I am watching with interest to see how they hold up in the next few years.

Nanostone Water, a ceramic membrane manufacturer, has been around for over 5 years now and in the last 1-2 years has been more aggressively pursuing commercial installations. In the early days its activities focused on pilot studies to prove the technology and also finish product development without racing to get sales, which I think was very prudent and showed great patience by the company’s investors. In recent years it seems like the development phase is mostly finished and the expectations are to start getting some sales, primarily in the drinking water market. I am not sure why the conservative drinking market has been initially targeted unless it is considered that the technology is ready for that market so that immediate sales can be pursued while development continues on more challenging applications. I would think for relatively non-challenging drinking water treatment in general it will be very difficult for a ceramic membrane to economically compete with a polymeric membrane. Especially considering that there are some very good polymeric membranes on the market that have minimal integrity issues on drinking water, where superior integrity is the main benefit offered by ceramic membranes. There could still be some legacy Memcor and Zenon systems out there that have to pin a lot of membranes, but polymeric membranes have come a long way since the late 90s/early 2000s and these systems should not be used as the standard for polymeric membranes.

There are some niche challenging potable applications such as backwash water recovery that Nanostone has done a good job in getting some early retrofits as mentioned in my previous post, but I don’t think they are ready for competing against polymeric membranes for mainstream filtration applications. Another big advantage of ceramic membranes is the significantly higher flux possible compared to polymeric membranes. I would therefore look at the wastewater reuse market where organics result in reduced fluxes for polymeric systems, more cleaning and therefore more chance of irreversible fouling and a shorter membrane life – all issues that ceramic membranes claim not to have. It seems too obvious be me, so there must be some issue with wastewater filtration using ceramic membranes that I am not aware of. I do know the Parker Water & Sanitation District's (Colorado) Metawater ceramic membrane system has had some fouling issues and the feedwater has a wastewater input so perhaps the characteristics of these organics are not favorable.

PolyCera has been a more recent entrant to the market after being spun off by Water Planet in 2017 to sell its membranes developed from a unique material. The main product is an organic polymeric UF membrane with similar properties to inorganic ceramic membranes which allow the membrane to have similar fouling resistance to ceramic membranes as well as being able to tolerate more aggressive cleaning regimes than conventional polymeric membranes. I saw a presentation from PolyCera a few years ago on a produced water treatment application in California where a full-scale system has been installed and this seemed like a very good market for PolyCera’s membrane. The thing is, I haven’t seen anything new presented since then other than a few pilot studies. It may be that PolyCera's membranes have applications in industries that I don't work in.

I do not know the strategies of any of the companies mentioned in this post and have just thrown a few theories out there based on my own experience on why some of these companies may be struggling. I am excited by the innovations out there in the water treatment market and really hope that they can be successfully brought to market when they can offer real benefits to end users.

The opinions expressed in my posts are mine and not those of my employer.

Has Adoption of Forward Osmosis Stalled?

Lack of Significant Progress the Past 6 Years



After seeing several papers presented at the WateReuse Symposium in Dallas in 2008, I was intrigued with the potential for forward osmosis in water and wastewater treatment with a lot of research completed on these applications and a lot of promising research underway (see 2010 post). I did wonder though if FO would be more of a niche technology due to process limitations such as the complexities of recovering water from saline and organic draw solutions. My prediction in 2010 that FO would be a niche technology appears to be spot on so far. While Oasys Water, one of the FO pioneers, has found some applications for produced water treatment in the O&G market, I am not aware of any adoption of FO in the municipal market.

 
I scanned the proceedings from the AWWA/AMTA Membrane Technology Conference the past several years to see if there were any trends in papers presented on FO – this conference has a good mix of research and full-scale case studies and I think is a good barometer of what is upcoming in the membrane industry and what is being commercialized. The reducing and flattening number of FO presentations suggests interest in this technology for mainstream water treatment is waning, perhaps due to some of the technological barriers preventing commercialization. Also, in contrast to the several presentations I saw at the 2008 WaterReuse Symposium, at the 2016 Symposium I did not see any FO papers on the proceedings.

I did a quick scan of the program for the upcoming 2017 AWWA/AMTA Membrane Technology Conference and found three platform papers and one poster on FO but for very niche applications and all presented by universities – so nothing to demonstrate that the technology is moving towards mainstream commercial implementation.


Forward Osmosis Papers & Posters at AWWA/AMTA Membrane Technology Conferences

 This trend in papers presented is in contrast to an article published in ‘World Water: Water Reuse & Desalination’s Autumn 2015 issue titled “Will forward osmosis always be a niche market” (O’Callaghan and Pearce). This article, based on a BlueTech Research Insight report, showed how there had been an uptick in the number of research papers published and patents registered from 2011 to 2013, suggesting increasing interest in building IP positions in the technology. A graph showing the number of published research papers from 2011 to 2014 did show a steady increase to 2013 but there was a slight drop to 2014 and I would not be surprised that this drop has continued to 2016 based on the US conferences I have looked at. The article does say that despite the research interest there are technical hurdles such as membrane fouling that have prevented the technology making any mainstream commercial impact and limited FO to some niche applications. It also makes a good point that the players in the FO market have been venture backed smaller companies, while the larger establish players in the membrane market have been watching from the sidelines.

The lower energy costs of FO compared to RO are certainly enticing although since 2008, significantly lower energy and oil costs may have reduced the attractiveness of this benefit, keeping the big players out of the FO market. One big change since 2008 is that the public is a lot more accepting of potable reuse following severe droughts in Texas and California. That could make the application of FO at combined seawater desalination/wastewater outfalls to reduce the energy costs of desalination, as described in my 2010 post, a lot more feasible. We will probably still need to see energy costs escalate again to get some serious interest from the larger players to invest in FO, sort out the technical issues and finally bring it to large scale commercialization. It will be interesting to see where FO is at in another six years!

Biofiltration Dominates Discussion at 2016 WateReuse Research Conference

At the recent WateReuse Research Conference in Denver, I had expected much of the discussion to be around the use of membranes (low and high pressure) for wastewater reuse applications, but was surprised that much of the research focus was on biological filtration. Perhaps since there are a lot of full scale membrane systems already installed and in the pipeline for water reuse, this is starting to be considered as a more mature technology and not needing as much research. If that is the reason I think it is mistaken, but I will come back to that later in this post.

I haven’t been keeping up with recent developments in biofiltration, so I was very interested in what was presented at this conference. Around 10-15 years ago when I was working in ion exchange I recall there was interest in looking at biofiltration for nitrate and perchlorate removal from drinking water as an alternative to IX but this treatment generally wasn’t seriously considered for full-scale systems, due to the perhaps misguided concern about using bugs to treat the water when you were also trying to pull them out to protect public health. This did not take into account that many existing filters were probably operating in biological mode to some degree anyway.

Wastewater Reuse Helps Acceptance of Biofiltration

Today the landscape has changed significantly with the current drought on the west coast and the recent drought in Texas helping the public to be more accepting of direct and indirect potable reuse of wastewater and with reuse becoming an essential component of the water supply. If the public will accept potable reuse of biologically treated wastewater, acceptance of biological filtration should be a walk in the park!

Wastewater reuse has also opened up new opportunities for biological filtration, where micropollutants (MPs) not removed in wastewater plants need to be removed if the treated water is to be used for drinking directly or indirectly. In combination with pre-oxidation to break down the MPs to biologically assimable components, biological filtration can be an attractive alternative to high pressure membranes where it has a significantly lower waste volume and does not produce a saline waste, which is a considerable benefit in non-coastal areas where NF/RO waste disposal options are limited and/or expensive.

Wastewaters Ain’t Wastewaters!

There are plenty of technical articles today about potable reuse, so you don’t need to read another one from me. I do think it is important to mention that I believe the use of membranes for wastewater reuse is not yet a mature technology and is worthy of further research. We have all read a lot about Orange County Water District’s Groundwater Replenishment System that has been using membranes for decades for treating wastewater to drinking water standards and maybe some think you can just throw membranes at the end of any secondary wastewater treatment plant and start making high quality recycled water….but wastewaters ain’t wastewaters and membrane treatment is not that simple…. The quality of secondary effluent can vary significantly, from inland WWTPs with stringent nutrient removal requirements to ocean discharge WWTPs with less stringent requirements. This can result in widely varying cleaning requirements which also varies by membrane type. While there have been a lot of pilot studies on different secondary effluent sources, I haven’t yet seen a dedicated study evaluating the impact of specific secondary effluent constituents and the impact on low pressure membrane permeability and cleaning effectiveness.

Perhaps with all of the recent reuse pilot studies, this information will start to come together. Or maybe the key results will be kept from the public as a competitive advantage for those with the pilot experience, which is understandable when OEMs and/or engineering firms have generated this knowledge. This is where no-strings-attached research can help the reuse industry. After being involved in several recent pilot studies my take-away is that wastewaters ain’t wastewaters and don’t assume you can simply take the performance of a membrane at one WWTP and apply it to another.

2016 AMTA/AWWA Membrane Technology Conference Wrap

Ceramic Membranes the Hot Topic

Ceramic membranes seemed to be the hot topic of this year’s AMTA/AWWA Membrane Technology Conference, with a dedicated session of presentations and several other presentations spread throughout the program. There were also at least four companies in the exhibition offering ceramic membranes or systems using ceramic membranes. The companies I saw were Metawater, Nanostone, PWN and Meiden. There may have been more, but I didn’t have a lot of time to thoroughly walk the show floor this year.

After a lot of fascination with ceramic membranes in the U.S. for at least the past 10 years the technology is finally gaining some traction with the first large-scale system (10 MGD) starting up at Parker, CO in 2015 and another large scale installation under construction at Butte, MT. Both of these systems will be using Metawater’s ceramic membrane.

Reuter-Hess Water Purification Facility, Parker, CO

Everyone loves the performance of ceramic membranes in terms of strength and cleaning tolerance – it is much more forgiving than polymeric hollow fiber membranes. But the economics have been the obstacle with capital costs very high, even taking into account the projected long life of the membranes. Perhaps with the introduction of some competition, some new lower cost manufacturing methods and the ability for OEMs to buy ceramic membrane modules to build their own systems, much like the direction the polymeric hollow fiber MF/UF market is going, ceramic membranes will become a lot more cost competitive with polymeric membranes.

Not all Ceramic Membranes are the Same!

I also learned at the show that not all ceramic membranes are the same and therefore the benefits provided vary.

Metawater:

The ceramic membrane I think most of us are familiar with is Metawater’s, who probably has the most and longest operating municipal installations (initially as NGK), mostly in Japan, and now will have their membranes in the first two large US systems. I covered some of the history of NGK/Metawater in a previous post several years ago. Since I wrote that post, Metawater has split its ties with Kruger and is going direct to market selling complete engineered ceramic membrane systems. These membranes are manufactured from aluminum oxide as a complete monolithic ceramic element complete with feed tubes and permeate conduits (see previous post for a more detailed description). Each of these ceramic elements are encased in a stainless steel housing, allowing these membranes to withstand extreme temperatures and chemical exposures and therefore the ability to be used in some pretty dirty applications because they can be cleaned with almost anything. The membranes also supposedly will last forever – at least 20 years.

Metawater Ceramic Membrane Element

PWN Technologies:

Based in the Netherlands, PWN sources its ceramic membranes from Metawater but rather than putting these in individual housings, PWN combines the elements in a CeraMac® block of up to 192 elements in a single stainless steel vessel. The vessels look like large circular cartridge filter housings (or pressure cookers). The membrane system has all the benefits of the Metawater ceramic element and PWN says it has a lot smaller footprint and is more economical with the elements all bundled on a single housing. Like Metawater, PWN is also selling complete membrane systems. The CeraMac system certainly looks a lot different to your typical membrane plant and I am not sure how U.S. regulatory authorities would feel about having so many elements in a single vessel for integrity testing for drinking water applications. I know Metawater/PWN will say that integrity testing is not an issue because there will never be a break, but that will be a hard one to get past the regulators.

CeraMac Vessels with Multiple Ceramic Membrane Elements

Nanostone Water:

A relatively new player in the ceramic membrane market, Nanostone has taken a different approach in developing a lower cost ceramic membrane, with a goal of being closer in capital cost to polymeric membrane systems and not relying as much on the long membrane life as Metawater and PWN for competitive 20-year or longer lifecycle cost comparisons against polymeric membranes. Nanostone’s business model is also to just sell their ceramic membrane modules to OEMs who build membrane systems.

Segment of a Nanostone Ceramic Membrane

Nanostone’s ceramic membranes are manufactured as flat sheets with multiple rows of feed tubes. These flat sheet segments are incorporated into a PVC pressure housing, similar to that used for polymeric hollow fiber membranes. The segments are ‘glued’ together at each end using a potting material similar to polymeric hollow fiber membranes. The ceramic membrane module looks very similar to a polymeric hollow fiber membrane module and Nanostone is looking at the potential to use these modules in Universal MF/UF racks designed for polymeric hollow fiber membrane modules.

Ceramic Segments Potted Together

Nanostone Ceramic Membrane Module

I can see how this ceramic membrane will be lower cost with the manufacture of flat sheets rather than more complex circular ceramic elements and with the use of PVC housings rather than stainless steel. Header piping for the modules can also be PVC or HDPE rather than stainless steel. However this lower cost comes with some strings attached. Because of the potting of the ceramic segments and the PVC housings, these modules cannot withstand the extreme temperatures and chemical exposure of the Metawater/PWN elements, so probably can’t be used for the same ‘dirty’ applications that require extreme cleanings. While the membrane itself is ceramic and I assume will last as long at Metawater’s, will the potting last as long as the membranes? In my initial assessment, for a robustness ranking, I would place the Nanostone ceramic membrane above the polymeric hollow fiber membranes but below Metawater’s. So you get what you pay for. Nanostone is still optimizing aspects of the module design, including the potting, so in the future they may rise further up the robustness scale. I do like the business model of selling the modules to OEMs who will be able to build the systems more economically than I think Metawater or PWN can.

 

Meiden, who has a flat sheet ceramic membrane, was also was exhibiting at the show. These immersed flat sheet ceramic membranes operate as outside-in membranes.  The main application for these membranes is as MBRs and they would have all the robustness advantages over flat sheet and hollow fiber polymeric membranes used in MBR applications.

 

Other 2016 MTC news:

Probably next hottest topic at MTC was Universal/Open Platform MF/UF systems where there was also a dedicated session plus a few other presentations in the program. Since the 2015 MTC, there have been two large scale Universal UF systems start up at Clifton, CO (H2O Innovation) and Santa Barbara, CA (Wigen Water Technologies) and presentations on these systems were presented by Carollo and CDM Smith the respective design engineers.

 

The other interesting development was seeing Metawater and Aqua Aerobics with a combined booth. I wasn’t aware at the time that Metawater had acquired Aqua Aerobics with the announcement only a few weeks before the show. Aqua Aerobics had been promoting UF systems using BASF/Inge’s Dizzer multi-bore PESM membranes which are likely the most robust of the polymeric membranes on the market. I don’t know if this acquisition was for the U.S. engineering and manufacturing capabilities and sales network of Aqua Aerobics or if Metawater is looking to be positioned as providing the most robust of membrane systems, both ceramic and polymeric, or both. I will be interested to see if Metawater now starts promoting membrane systems using the Dizzer module.

As usual, there is never a dull moment at MTC and 2016 was no exception. It will be interesting to see what happens in the next 12 months and what the big news will be at Long Beach, CA in 2017. Here are my predictions:

• Ceramic membranes continue to gain steam
• Reuse, Reuse and more Reuse in California as El Nino does not deliver the needed moisture (First DPR system in CA announced?)
• Some consolidation amongst membrane OEMs as larger companies try to get in on the fast growing membrane market

 

 

 

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.

Ceramic Membranes: Mainstream or niche low-pressure membrane player?

Ceramic membranes have been a hot topic at recent American Water Works Association (AWWA) and International Water Association (IWA) conferences. In more and more cases, ceramic membranes are being piloted alongside polymeric membranes in technology evaluations for new water treatment plants. Claims from manufacturers such as NGK–MetaWater of higher fluxes, a greater tolerance to foulants and particulates and no membrane replacement for the life if the facility, has drawn much interest from the most conservative of consulting firms.

I have been involved in a few pilot studies that have included ceramic membranes and there is no doubt that the technology is effective. The claimed advantages also seem to be valid. But are ceramic membranes an economically viable alternative to low pressure polymeric membranes? Why are there still not any installations in the United States?

History of Ceramic Membrane use for Drinking Water
NGK Insulator, Ltd., traditionally a manufacturer of ceramics for the automotive, power and electronics industries, began research into the development of ceramic membranes for drinking water treatment in the early 1990s. In 1996, NGK began production in Japan of the first commercial ceramic membrane water purification systems and installed the first small-scale system. In 2008, NGK and Fuji Electric merged their water businesses to form Metawater Co., Ltd. whose product range focused on NGK’s ceramic membranes and Fuji’s ozone generation systems. As of December 2009, NGK-Metawater had installed 76 ceramic membrane systems in Japan. Most of these are very small, with only eight systems over 1-MGD capacity and the combined capacity of these eight systems around 28-MGD (Freeman, et al), although the MetaWater website indicates that the combined capacity of systems installed and under construction is 112 MGD. Outside of Japan, there has been virtually no adoption of ceramic membranes in drinking water treatment, although according to Kruger Inc. who has the rights to NGK-Metawater’s ceramic membrane technology in the United States, there are two systems currently in design in the U.S.

How Ceramic Membranes Work

The ceramic membranes used for water treatment are made from aluminum oxide and are tubular, similar to hollow fiber polymeric membranes, but with a much larger diameter (Figure 1).


Water passes down the parallel tubes from the feed inlet to the outlet end face. The surfaces of the tubes are coated with a ceramic membrane material that has a uniform pore size to provide microfiltration or ultrafiltration. The feed stream is introduced under pressure at the inlet end face and is withdrawn as retentate at the downstream end face. Permeate passes through the membrane into the porous monolith ceramic structure. The combined permeate from all of the tubular passageways flows through the monolith support to permeate conduits within the monolith that transport the permeate through slots to an external collection zone (Figure 2).

Figure 2: Schematic of Ceramic Membrane Operation (Source: Kruger Inc.)

Pros and Cons of Ceramic Membranes
Two presentations by Freeman, et al and Kommineni, et al at the 2010 AWWA Annual Conference in Chicago provided information on side by side pilot study comparisons of ceramic and polymeric membranes and some independent insight into the advantages and disadvantages of ceramic membranes.

The major advantages of ceramic membranes are as follows:

• Longer Membrane Life: There are no membrane fibers to be broken so the membrane life should be significantly longer than polymeric membranes and maintenance requirements significantly less. MetaWater claims that no membrane elements have needed replacement since the first system was installed in 1996.
• Easier to Clean: The high mechanical strength of the membranes allows aggressive cleaning regimes with acids, alkalis, oxidants, high temperatures and high backwash pressures to recover membrane performance. The membranes can therefore tolerate high foulant and particulate loadings. Chemical cleaning frequency is only 2 to 6 times per year.
• Higher Flux: Ceramic membranes can be operated at flux rates over 100 gfd and reportedly as high as 175 gfd compared to polymeric membranes which are typically operated in the range of 40 to 60 gfd (Freeman, et al). Less membrane surface area is therefore required to provide a given throughput.
• Higher Recoveries: Less frequent backwash cycles and shorter cycles result in recovery rates of around 98% for ceramic membranes compared to 90-92% for polymeric membranes.

Looking at these advantages you would wonder why there isn’t wide adoption of ceramic membranes for drinking water treatment. Well there is one important factor that needs to be considered – the cost! In the pilot study reported by Freeman, et al, ceramic membrane capital costs were of the order of 2 to 2.5 those of polymeric membranes. Taking into consideration the additional membrane replacement cost for polymeric membranes, the present worth (20 years @ 6%) for ceramic membranes was still twice that of polymeric membranes. If the feedwater had a high particulate loading or required activated carbon dosing, the added costs from additional pretreatment required for polymeric membranes did narrow the present worth difference somewhat.

As the water industry is also conservative, and understandably so where ratepayers’ money and health are at stake, there is also some reticence to recommend this relatively new technology without a better understanding of long term membrane life and long term fouling characteristics. Both presentations also indicate that limited supplier options in North America are a concern.

So a mainstream or niche technology?
With Kruger’s sales and marketing reach in North America, I am sure there will be ceramic membrane installations outside of Japan in the not too distant future, but… while ceramic membranes are very effective at providing drinking water filtration, unless initial installation costs can be significantly reduced, it will remain a niche technology. The niche will be limited to drinking water sources that are difficult to filter with polymeric membranes (high particulate and organic levels) and would otherwise require significant pretreatment. Remote systems may also benefit from the significantly longer membrane life and lower maintenance requirements. Outside drinking water applications, I see much more potential in applications such as wastewater recycling and recovery of oil and gas produced water.

Freeman, S; Henderson, R; Delphos, P; Clement, J; “When are Ceramic MF/UF Membranes Cost-Competitive with Polymeric MF/UF”, Proceedings of AWWA Annual Conference and Exposition, June 20-24, 2010, Chicago IL

Kommineni, S; Hoffman, R; Karnik, B; Stringer, C; DelRegno, K; Myers, N; “A Collaborative Evaluation of Ceramic Membranes – An Emerging Water Treatment Technology”, Proceedings of AWWA Annual Conference and Exposition, June 20-24, 2010, Chicago IL



ACTIFLO® CARB: Advanced Organic Carbon Removal or Smoke & Mirrors?

I have been intrigued with Kruger Inc.’s (part of Veolia Water) ACTIFLO® CARB process ever since it was selected as pretreatment to ceramic membranes for a new water treatment plant at Parker Water & Sanitation District (PWSD) in Colorado. ACTIFLO® CARB was selected based on what I thought was very shaky science – a ‘trial’ on a high total organic carbon (TOC) water source that consisted of dosing 4000 mg/L of virgin powdered activated carbon in front of the membranes operated in crossflow mode and recirculated for eighty hours. The results showed TOC removal starting at 90% and finishing at about 50% after the eighty hours. Somehow Kruger was able to convince the consulting engineer and PWSD that these results could be extrapolated to predict the performance of a continuously operating full-scale plant with a 25 mg/L fresh PAC make-up dose.

The ACTIFLO® CARB process is an extension of Kruger’s ACTIFLO® process where powdered activated carbon (PAC) is added in a contact stage at the front of the process followed by coagulant addition and then microsand and polymer to provide ballasted flocculation/clarifiation (see Figure 1). Most of the PAC is recirculated while a portion is wasted, which allows more of the carbon adsorption sites to be utilized. Make-up PAC doses range from 15 to 40 mg/L. The addition of the “CARB” step to ACTIFLO® provides greater TOC removal when the coagulants used in the ACTIFLO® process cannot achieve treated water quality goals.

Figure 1: Schematic of the ACTIFLO® CARB Process

What was astounding to me was that the engineer and District accepted the manufacturer’s recommendation after such a short trial of a process that has no operating installations in the US, where the pilot plant did not even simulate how the full scale system would operate and where more proven treatment alternatives, such as ion exchange, tested over longer periods reliably demonstrated equivalent levels of TOC removal…. Such is the influencing power of a large established technology provider!

Finally a Fair Evaluation

In the past few months, more than two years after the PWSD trial, I finally had the opportunity to see the performance of ACTIFLO® CARB in a trial in Georgia operated for at least several weeks on a pilot plant representative of the full-scale process. The objective of the project was to improve TOC removal of the existing water treatment plant to meet tightening EPA standards for disinfection byproducts. As with the PWSD trial, ion exchange pre-treatment was evaluated in parallel but this time it was a fair side-by-side comparison over similar operating periods.

Lo and behold….under steady-state operating conditions the ACTIFLO® CARB process only achieved 52% TOC removal at very high PAC make-up doses and about 40% removal at economically realistic make-up doses. This compared to 66% TOC removal for the ion exchange pretreatment process. As the trial has only recently been completed, specific results will likely be publicly available in early 2011.

In this Georgia project the power and influence of the manufacturer could not overshadow the results of a well thought out and executed trial. While ACTIFLO® CARB has been shown to remove more TOC than coagulation alone, I can't see how the marginal improvement in removal justifies the considerable extra expense.