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
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 The feed stream is introduced under pressure at the inlet end face and is withdrawn as retentate at the downstream end face.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