By Michael Havelka
Summary: Controlling the pH rise in water run through a fresh carbon bed can be frustrating for water treatment dealers. The author offers a theory on the cause and how to avoid the problem. In the next issue, PACS Inc., of Pittsburgh, takes a look at causes of pH rise and carbon.
The pH spike associated with the startup of a fresh bed of virgin carbon is a common problem with activated carbon purification used in water treatment, particularly ultra-pure applications. The spike can range from 1-3 pH points and can remain for several thousand bed volumes of water.
According to one theory, the adsorptive forces in the activated carbon are the most probable cause of the pH spike. Leading scientists in the carbon industry have also theorized that the pH rise effect was caused by the ash in the carbon washing off and re-combining to create basic compounds that increase the pH of the effluent stream. This was consistent with the progression of the pH rise phenomenon since activated carbon has a limited amount of ash material present. Testing of the carbons after the pH rise, however, didn’t verify this fact. Instead of less ash, most of these carbons contained the same or slightly more ash constituents than virgin carbon upon startup. The ash concentration should have been reduced significantly to account for this pH spike.
Another theory by leading scientists is that the pH rise was due to an ion exchange effect in the carbon. While the theory has much merit, the fact is that every activated carbon product elicits this pH rise effect although activated carbons are manufactured in dramatically different ways from a variety of original materials. Due to these variations, it’s not possible that each of these carbons has nearly identical surface properties. While the theory may hold for all bituminous coal-based carbons, it doesn’t explain the phenomenon for coconut shell and lignite-based carbons.
The first evidence the author experienced that the adsorption forces in the activated carbon are the most probable cause of the pH spike, emerged during a startup at a bottling facility in 1999. During that startup, a sustained pH rise of 2-3 points lasted for over 40,000 bed volumes. If the pH rise was due to high ash or the type of ash products, the ash concentration of that carbon should have reduced substantially after the incident. On the contrary, the ash levels were identical for the carbon both prior to startup and after the pH spike episode. As a result, some carbon users switched to acid-washed carbon to mitigate the problem. In some cases, this shortened the time for the pH rise but, in many cases, only exacerbated the problem. The next change-out at this facility involved an acid-washed carbon with half the ash content of the prior material. The duration and magnitude of the pH spike was significantly worse than would have been expected if the ash constituent explanation were correct.
Activated carbon is a widely used media for both organic chemical and chlorine removal. One of the forces by which activated carbon removes organic chemicals from air and water is known as adsorption. This isn’t a chemical reaction between the media and the compounds; rather, it’s a physical reaction, like gravity. The forces that bind organic molecules to activated carbon are a weak form of gravity, known as Van Der Waals Forces. While there are many interpretations as to why some compounds adsorb to carbon and others don’t, the fact is that activated carbon has a limited capacity for all organic compounds. Some compounds are so poorly adsorbed it appears that carbon has no capacity. A good example of this phenomenon would be isopropyl alcohol (IPA). Although IPA is infinitely soluble and a small, simple molecule, carbon has very little capacity for it. On the other hand, carbon has tremendous capacity for a compound such as naphthalene—which is poorly soluble in water and, compared to IPA, a very large molecule with many carbon-carbon bonds. For some users of activated carbon, a good method of determining a compound’s adsorptive capacity is to look at a combination of water solubility combined with molecular size.
With this as a basis of understanding adsorptive forces, it’s now important to note that even if carbon has poor capacity for a compound, such as IPA, it still has some limited capacity. For an organic removal application, this generally isn’t enough to justify the cost of utilizing activated carbon as the capacity is orders-of-magnitude lower than traditional organics that are considered generally adsorbable. The range of organics in water is wide and diverse. Although many carbon users worry about regulated compounds like benzene and methyl tertiary butyl ether (MTBE), a wealth of other organics exists in all water. These compounds operate just beneath the radar of traditional carbon users’ interest.
A buffering effect
Many of these compounds are the small organic acids in water that are instrumental in keeping water buffered at its nominal pH range. Water chemistry is very complex and the entire spectrum of compounds present creates the balance that we refer to as pH. Wide ranges of inorganic and organic compounds increase and decrease the ionic strength of the water and create the pH balance. There are various pH ranges in surface and groundwater throughout the United States. This is due to the different chemistry situations that create water. In the northeast for example, iron-rich ground tends to dissolve high concentrations of iron in the water, thus somewhat lowering the pH.
Carboxylic acids, products of decomposition of plant and animal matter in water, are examples of small organic acids. During startup of a fresh bed of carbon, these carboxylic acids are removed and the water loses its buffer. This is a transient effect as carbon has a very limited capacity for carboxylic acids. Figure 1 details the pH spike for a test performed on groundwater at a municipal drinking water facility in West Virginia using standard virgin coal-based granular activated carbon.
Confronting pH spikes
Most activated carbon users either backwash for extended periods or put the system on-line to waste for several thousand bed volumes, until the pH dips back to its influent level. This technique, while generally effective, is costly and often impractical.
Pure water type systems are especially vulnerable to pH rise because the influent water is either municipal or very clean groundwater. In both cases, removal of the small amount of these buffering compounds by the activated carbon tends to exacerbate the pH rise phenomenon. It isn’t uncommon for a pH rise to last 5-10 thousand-bed volumes in these applications. Based on research data derived for testing of both surface water and groundwater sources, Figure 2 demonstrates the pH spike associated with the various types of carbon, including acid-washed carbon.
One company discovered that many of its customers that utilized custom reactivated products weren’t experiencing this same pH spike. These were traditionally groundwater and surface water treatment systems. After studying a broad range of activated carbons, it was discovered that the lower the adsorption of the activated carbon, the lower the pH spike (and often the absence of a pH spike). From the data, the company did follow-up research on the applications where the pH rise was more prevalent. When combining all data, it determined this adsorptive process was the predominant cause of the pH spike effect and any remedy had to block adsorption of these buffering compounds without interfering with removal of desired organics or chlorine.
A stabilizing effect
A process of pre-treating the virgin or custom reactivated carbons was developed to create a condition whereby the carbon no longer has an affinity for the weakly adsorbed organic acids, yet still has its same adsorptive properties for chlorine and the organics that are to be removed. This process entails treating virgin or reactivated carbon with a dilute carboxylic acid to pre-fill these high-energy adsorption sites. This pre-filling technology has been rigorously tested on a myriad of influent water types including groundwater, surface water and process water with similar results. In Figure 2, the pH rise of the typical activated carbon for the above test was nearly two full pH points, from 7 to almost 9. The pH-stabilized carbons didn’t exhibit any discernable pH rise during the same period. They’ve also been ANSI/NSF Standard 61 approved for use in drinking water and food products processing.
Eliminating concerns of pH spikes in activated carbon water treatment process through use of a pH-stabilized activated carbon can be a boon to water treatment professionals who can demonstrate better control and water savings to their customers, particularly on startup operations.
About the author
J. Michael Havelka heads up marketing at Envirotrol Inc., of Sewickley, Pa., which has developed the NoRise™ activated carbon product to eliminate pH spikes. It has already been awarded contracts for supply of this material for both groundwater and surface water treatment facilities. Havelka has a bachelor’s degree in chemistry from West Liberty State College and a master’s degree in business administration from Franciscan University. He also has three patents pending associated with activated carbon technology. He can be reached at (412) 741-2030, (412) 741-2670 (fax), email: email@example.com or website: www.envirotrol.com