By Greg Reyneke, MWS
Granular activated carbon (GAC) has a long-proven track record for use in residential, commercial and industrial water quality improvement applications. Whether dechlorinating, addressing taste and odor, perfluorinated compounds (PFCs), pharmaceutical compounds or other contaminants, GAC is a go-to water quality improvement technology for many water treatment professionals around the world.
Why do we chlorinate?
Waterborne microorganisms vary in size from extremely small viruses in the sub-micron range to relatively large cysts that can approach 50-micron in diameter. Pathogenic microorganisms can occur naturally in and around most surface water sources. Even groundwater supplies are not immune, since the existence of subterranean bacteria has been definitively proven. Enteric viruses and other organisms have the ability to leach into groundwater from the land application or burial of sewage sludge and other residential and industrial wastes.
Municipal chlorination is probably one of the single largest contributors to an increase in human longevity. Once cities started to physically filter and chlorinate their water, they were able to deliver a downstream product to their users that was clean, clear and devoid of most harmful organisms.
As a strong oxidizer, it is believed that chlorination can deactivate microorganisms through a variety of mechanisms, such as damage to cellular membranes, inhibition of enzymes, destruction of nucleic acids and various other currently unknown/undocumented mechanisms. The effectiveness of the chlorination process depends upon a variety of factors, including chlorine concentration, contact time, water temperature, pH value and level of turbidity.
- Monochloramine (Chloramine, NH2Cl), which is the most effective biocide
- Dichloramine (NHCl2)
- Nitrogen trichloride (NCl3)
Chloramines can form spontaneously in water, or be deliberately formed at the municipal level, since a growing minority of central providers actively choose chloramine as their disinfectant technology. The various species of chloramine can rapidly shift from one form to another throughout the distribution system. The predominant species depends on pH, temperature, dissolved oxygen, carbon dioxide, organics in the water and the instantaneous chlorine-to-ammonia ratio. Monochloramine is less reactive with other organics in water than free chloramine, so it will stay active for longer in the water and form significantly fewer trihalomethanes and other undesirable chlorine-related byproducts, which makes it an enticing choice for many providers.
The limited adoption of chloramination at a municipal level in the US speaks to the complexities associated with it and the concerns that many have about byproducts, increased corrosivity toward metals and nitrification. Chloramines are challenging to remove with regular activated carbon, since it takes more than just the simple reducing action of activated carbon. (I’ve had the very best results with various catalytic carbon blends, depending on what other contaminants are in the water.) Bear in mind that nitrification can occur after the dechloramination system; this can be problematic, especially if infants or those with compromised immune systems will be using the water. It can also provide food for certain microorganisms.
Ease of use, low cost and widespread availability make chlorine the most widely adopted disinfection chemical globally; it is truly a cheap and efficient killer. Residual levels of downstream chlorine aid in reducing the proliferation of heterotrophic bacteria (HPC) in plumbing and appliances, while also reducing the chance of pathogenic organisms (like Legionella) finding a safe place to live and grow. Municipal chlorination is beneficial and should never be disparaged by water quality improvement professionals. The downside to municipal chlorination is that many people object to the taste and odor of the water. Some are also concerned about the carcinogenic risk from DBPs, while others are concerned about the potential damage to skin, hair, clothing, piping and seals from chlorine and chloramine.
How does activated carbon work?
GAC can be manufactured from various feedstocks such as coconut shell, wood, bituminous coal and a host of others. The feedstock is harvested, cleaned and inspected. After inspection it is activated using one of many processes that are intended to expose more of the carbon’s micropores, which enable greater reactivity with the world around it. The type of feedstock and method of activation will impact a carbon’s absorptive and adsorptive characteristics. In addition to activation, the carbon might also be surface-treated with certain compounds that enable additional surface reactions between the carbon and certain waterborne contaminants. We intuitively call this type of carbon catalytic and it is used to address a variety of contaminants, but most commonly chloramine and hydrogen sulfide.
When addressing chlorine, GAC doesn’t capture chlorine. The action is a combination of hydrolysis of free chlorine to the hypochlorite ion and catalytic decomposition of the hypochlorite ion on the carbon surface with the result being small amounts of hydrochloric acid. Concentration of chlorine in the water, temperature of the water and rate of flow all influence the amount of bed depth, volume and contact time required to properly dechlorinate.
Chloramine reactions are painfully slow and inefficient when using standard GAC, so the best practice is to utilize surface-treated catalytic carbon, which results in chloride ions and free ammonia. After dechlorination and especially in the lower depths of a media column, bacteria can survive and thrive, with the carbon filter becoming a growth medium for bacteria. It is not uncommon to install UV or ultrafiltration (UF) equipment after dechlorination tanks to protect from downstream bacterial contamination.
The slimy polysaccharide matrix that comprises biofilm is nature’s way of feeding and protecting life. Biofilm protects the stomach from autophagy, helps sustain life in oceans, exists in many inhospitable environments (extremophiles that live in salty, hot and acidic water) and it most certainly exists in safe chlorinated waters. Biofilm will grow wherever it finds slow movement and depending on the specific strain, a comfortable temperature, oxygen and a source of nutrition.
Chlorine has difficulty in penetrating this slime, so anything protected by biofilm is not likely to be killed by chlorinated water. Certain other organisms in water have specially evolved shells that protect from oxidative damage caused by chlorine. While concentrations of bacteria and living organisms in water are significantly reduced through chlorination/chloramination, it is foolish to presume that the water is now 100-percent safe.
On well-water applications, many practitioners will choose chlorination followed by carbon filtration when addressing iron, manganese and hydrogen sulfide. Interestingly, these troublesome compounds are almost inevitably accompanied by iron and sulfur-related bacteria, along with a host of other organisms. The oxidative action of the chlorine is highly effective against the oxidizable metallic species, but not entirely effective against all living organisms, especially those protected by biofilm. It is not unusual for my team and I to consult on complex downstream bacterial contamination in cases like this, especially in warmer climates, where the carbon tank(s) and other downstream equipment is fouled with slime and/or stringy bacterial growth.
Evidence of contamination
Whether on well or city water, bacterial contamination is typically evidenced by slime growth (green, white, clear, gray, brown, black, pink), musty, fishy or swampy odors and possibly even a degradation in the perceived flavor of the water. In extreme cases, this biofilm can form large masses that reduce flow through a media tank (raising the DeltaP), clog distributor assemblies and disrupt the operation of downstream equipment like water softeners and other filters. Low-level biofilm growth can even occlude the surface of ion exchange resins, reducing their effectiveness.
Prevention is better than cure
First, practice sanitary installation techniques:
- Segregate sanitary from unsanitary tools and safety equipment
- Wear disposable nitrile gloves when working with susceptible components
- Physically clean all components before installation
- Thoroughly disinfect the installation site and downstream piping/fixtures after installing any equipment
- Disinfect all treatment equipment at least annually with a manufacturer-approved disinfection protocol.
- Replace or augment the carbon and other media on a regular schedule as recommended by the equipment manufacturer and according to industry best practices. Disinfect after any service/maintenance work is performed.
Coping with disaster
Once you notice downstream contamination, your priority is to protect your client. Ensure that they stop drinking the water and utilize a known safe source of water. The next step is to remove the source(s) of contamination and physically clean bacterial adhesions from as many contaminated surfaces as possible. In my experience, this typically means discarding the GAC tank completely to ensure efficacy. Biofilm adhesions in plumbing can be difficult to remove and the first step is to apply a recirculating chemical cleaner. Sometimes. the only option is removal and replacement of the affected section.
Granular activated carbon is a highly effective tool for improving aesthetic and health-related water quality issues. While it is easy to use, it should be respected by deploying with a full understanding of the downstream consequences, then installing and maintaining it in a sanitary fashion.
About the author
Greg Reyneke, Managing Director at Red Fox Advisors, has two decades of experience in the management and growth of water treatment dealerships. His expertise spans the full gamut of residential, commercial and industrial applications, including wastewater treatment. In addition, Reyneke also consults on water conservation and reuse methods, including rainwater harvesting, aquatic ecosystems, greywater reuse and water-efficient design. He is a member of the WC&P Technical Review Committee and currently serves on the PWQA Board of Directors, chairing the Technical and Education Committee. You can follow him on his blog at www.gregknowswater.com