Copper-silver ionization

By Timothy Allinson, P.E.,
Murray Co., Long Beach, Calif.

One of the hospital projects I am working on has a requirement for copper-silver ionization treatment of the domestic water supply. When I first encountered this system some years back on another project, copper-silver ionization was new to me, and I had little understanding of how it worked. What little I knew of it fell into the “smoke and mirrors” category of Legionella control, in my subjective opinion.

I had seen the system used only once, on the return line of a domestic hot water circulation system. The equipment seemed redundant to me, since the water heaters raised the hot water temperature to140 F before sending it through a tempering valve and back to the system, killing the Legionella in the process. What was the point of the copper-silver ionization unit in that application? Belts and suspenders, I supposed, if it did anything at all. Since then, the Cu-Ag system has received recognition in the engineering community, due in part to new guidelines such as ASHRAE 188.

This second, more recent, exposure to the Cu-Ag system has been more in depth and illuminating, and I have learned a lot about it. In this design case, we are weighing the options of treating the domestic water in a storage tank sized for a three-day supply versus treating the water as it is delivered on-demand in the pump discharge piping. The idea of treating cold domestic water for Legionella initially made no sense to me, since the bacteria doesn’t begin to grow until water temperature reaches about 85 degrees, but there are reasons to treat the cold water, which I will get to shortly.

The first thing I wanted to understand about this system was how it worked. I understood that it added copper and silver ions to the water, but how did that translate to Legionella control? Here’s what I was told.

The unit, or flow cell, contains copper/silver electrodes that produce copper and silver ions by alternating current from cathode to anode. The amount of copper is measured by a flow controller to keep it between 0.3 and 0.8 ppm, which is well below the EPA maximum for safe drinking water of 1.3 ppm. Silver is not regulated by the EPA but is generally considered acceptable below 100 ppb. The silver associated with the Cu-Ag system is kept between 30 and 80 ppb.

The ions introduced to the water are positively charged. As they flow through the piping system they are naturally attracted to the negatively charge microorganisms that dwell in the biofilm on the pipe wall. Here’s the cool part. The copper ions adhere to the microbes, which weakens their cell wall. Once the cell walls are weakened, the silver ions impregnate the microbes and kill them by deconstructing their DNA. It’s sort of a one-two punch on an ionic microbial level. This process is not only effective on Legionella, but also on Pseudomonas (a particularly troublesome waterborne bacteria in healthcare facilities), M. Avium, E. coli, Salmonella and a host of other microbes as well.

The key to the system operation is to have enough ions in solution to allow them to be absorbed by the biofilm that coats the pipe wall, creating a lasting residual effect. The ions are not affected by water heating, so as hot water is made from the treated cold water, the hot water system absorbs the ions as well. This is an important distinction from the function of chemicals such as chlorine that dissipate rapidly in warmer water temperatures where bacterial growth is the fastest.

Since hot water systems are known to be more subject to Legionella than cold water, because of the temperature-induced breeding ground, one could consider treating only the supply to the water heaters. However, this would not carry the benefit of killing other pathogens on the cold side, such as the aforementioned E. coli, Salmonella and their friends.

The use of copper and silver to control bacteria is not a new concept. Ancient Greeks used to kill bacteria in wine by lining their vessels with silver, while controlling algae and fungi with copper. This system can also be used as an alternative to chlorine in swimming pools.

Of course, Cu-Ag ionization comes at a price, but surely it is much less expensive than managing a Legionella outbreak. Flow cells are usually rated for about 25 gpm each, and systems cost in the neighborhood of $50,000 per flow cell, including all the necessary controls. Keep in mind that systems need not be designed for peak flow rates. A project with an anticipated peak flow of 400 gpm would be sufficiently served by a 100 gpm Cu-Ag set up (i.e. four flow cells) with a PRV bypass. The idea is that the flow cells would treat all the water during low to average flow periods, allowing peak flows to bypass the system. The key is to impregnate enough ions into the water to have them absorbed by the biofilm over a period of days, weeks and months, not to treat every drop of water that travels through the piping system.

The electrodes within the flow cells need to be changed every few years, as their material is absorbed into the water system. This replacement is also not cheap, costing $3,000 to $4,000 per cell, but still nothing compared with the cost of a Legionella outbreak or other microbial infestations. The material of the electrodes is usually a 70/30 mix of copper/silver; however, the ratio is adjusted depending on water quality. If the utility water already has a high concentration of copper, the ratio might have as little as 5% copper and as much as 95% silver.

One potential residual effect of the Cu-Ag system that was raised on the aforementioned hospital project is its potential impact on sludge disposal. Wastewater sludge is tested for the presence of heavy metals before disposal. Initially, this made no sense to me, since the heavy metals in the sludge came out of the ground to begin with. The reason for regulating heavy metals, however, is that the sludge is frequently used in the agricultural process and there is concern about the heavy metals affecting crops.

Here in California the sludge restrictions are more rigid than the national standard, and sometimes sludge that doesn’t pass the California litmus test has to be carted off to Arizona. In my research, I was unable to come up with the California standards, but I did learn that the federal requirements here in the States are far more lax than in all other developed nations. For example, maximum levels for copper are 750 mg/kg in the U.S., whereas the EU ranges from 50 – 140 mg/kg. Keep in mind that the data I found on the Internet was a bit old and may have changed in recent years, but you get the idea.

So the concern is that, if the utility water already has high copper levels, as it does on the subject project, and if the Cu-Ag system adds more copper, which it may or may not, that it might push the sludge over the maximum level. The theory about the impact of the Cu-Ag system is that much of the Cu added to the water is absorbed by the biofilm, so it should not be additive to the sludge. The counter argument is that the biofilm can sluff off as the microbes die, which might then add Cu to the sludge in concentrated doses. There is no data available as to the reality of this possibility. My thoughts are that as long as the Cu is kept below the EPA drinking water guidelines, there should not be a problem. But the proof would be in the pudding, I mean the sludge.

 

Timothy Allinson is a senior professional engineer with Murray Co., Mechanical Contractors, in Long Beach, Calif. He holds a bsme from Tufts University and an mba from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed accredited professional. Allinson is a past-president of aspe, both the New York and Orange County Chapters. He can be reached at laguna_tim@yahoo.com.