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Water Purification Technology: What is "Green" & What is Not

by Stephen Wiman

What constitutes sustainable or "green" technology in water purification systems? For a water treatment solution to be considered green, it must meet the criteria of not adding anything to the water and not using any additional water in the process. This article covers green technologies currently available and the contaminants for which green purification technologies are, and are not, an option. In the marketplace, there is an abundance of "greenwashing" of conventional technology and often a failure to disclose all the parameters of a purification system touted as being environmentally responsible. But just as conventional water purification is (or certainly should be) based on water chemistry, so is green technology in water purification. If you don't know what is in the water, you cannot possibly be successful in treatment.

Why should you be interested in green water purification? Many municipalities and individuals are focused on reducing water consumption. But why not do your part to make it even lower by using green water purification techniques if applicable? In thinking about options, remember water quality varies both locally and seasonally. For example, the seasonal mix of Santa Fe municipal system water at any connection point is a combination of physical location (proximity to reservoirs and well fields) and seasonal levels of reservoir water.

The growing presence of naturally occurring contaminants makes water purification of interest to people with health-related water contaminant concerns and to those who experience infrastructure damage from contaminants (primarily hardness "scale," iron and manganese staining) in both municipal and well (aquifer) water supplies (see Table 1). There are some contaminants for which there is simply not a sustainable solution. Several technologies merit additional explanation.

Table 1 - Common contapossible health riskts and their suitability for sustainable treatment solutions.
Contaminant Concern Non-Sustainable Solution Sustainable Solutions
Chlorine
taste and color

X

if the carbon system is discharging

X

Carbon block filter (requires annual, or more frequent change of filter

Hardness
pipe and appliance damage

X

only ion exchange softens water

X

Anti-scalants and other salt-free systems

Fluoride
possible health risk

X

RO most effective in household applications

 

Iron & Manganese

 

Hydrogen Sulfide

staining; odor if bacteria present

"rotten egg" smell

X

conventional oxidation systems

X

Dissolved Oxygen generator

High TDS (Total Dissolved Solids)

pipe and appliance damage

X

conventional softener; or RO if TDS > 1,000 mg/L

 

Nitrate
possible health risk; septic tank discharge

X

anion or negative ion exchange; RO

X

Disposable cartridge

Arsenic
possible health risk

X

oxidation before anion or negative ion exchange; oxidation before reverse osmosis if Arsenic III present

X

some disposable cartridges remove both varieties of Arsenic

Coliform Bacteria
possible health risk

X

shock-chlorination

X

UV treatment

Radium (naturally occurring)
possible health risk

X

some softeners NSF-certified for radium reduction

 

Uranium (naturally occurring)
possible health risk

X

uranium reduction is not certified by NSF; post system testing establishes that RO is highly effective

X

Turnkey Solutions (testing and certified safe media disposal)

GAC and Carbon Block Filters The most effective method of removing chlorine is by using carbon filters (often improperly identified as “charcoal"). Some carbon filters are predominantly made from coal (look for "iodine content" as an indicator of the presence of coal), but the very best carbon filtration media is GAC (or granulated activated carbon) made from coconut shells, a renewable resource. Depending on the processing and particle size, the surface area of activated carbon can range from 500 to 1,400 square meters per gram. Several companies are now manufacturing GAC with sustainable technology by capturing the greenhouse gases from the ovens used to toast the coconut shells (to create the high volume of surface area) and by using the byproduct gases for other industrial applications.

Granulated activated carbon (GAC) is effective but it can provide a base for the growth of bacteria, has to be replaced (or wastefully backwashed) on a regular basis, and it becomes less effective through time because of channeling (erosion). An increasingly popular method of removing chlorine is by whole-house filtration using canister filters and tanks. Both methods are limited by the type of carbon used. One method uses in-line canisters (typically 4"x20") containing carbon block filters that are changed annually. These filters are created by compressing very finely pulverized activated carbon in a binding medium and fusing it into a solid block. Carbon block filters can remove particles down to 1 micron and also remove Giardia and Cryptosoridium if present and eliminate the problems of channeling of the carbon. These filters are so dense that they minimize the potential for bacterial growth.

Other whole house, chlorine-removal devices are standalone carbon tanks used in complex treatment sequences, or in tanks that may be combined with embedded electromagnetic devices. These latter systems are frequently backwashed and may waste as much water as a conventional softener. Carbon block filters are effective in removing chlorine and VOCs (Volatile Organic Compounds), but they do not protect you from other harmful contaminants that might be present in water. GAC, the media found in most household (faucet and pitcher) filters, are also very limited in their contaminant removal capabilities. Because of the potential for bacterial growth and its inability to remove metals and radionuclides in water, carbon filtration is a poor choice for most well water except where chlorine is used for specific oxidation applications (such as for iron, manganese or arsenic III) and then removed by dedicated carbon block filters that are changed on a scheduled basis.

Anti-scalant systems are designed to reduce or eliminate lime scale accumulation, but water softening, or ion exchange, is the only proven method for softening water. Salt-free systems are not a substitute for softeners and are poor choices for consumers accustomed to softened water; but they are ideal for customers who want to conserve water, avoid the hassles of salt use and the discharge of chlorides into the environment. Some systems that are touted as salt-free are actually wasteful backwashing systems. The key is the presence of an electronic control head, which is used to time and control backwashing. Magnets and other catalytic systems are not generally accepted methods for mitigating hardness, but they do work in some cases and may offer the consumer a more cost-effective solution to lime scaling (but not contaminant removal).

Dissolved Oxygen Generators By creating high levels of dissolved oxygen in the water, these oxygen-generating devices are effective in oxidizing iron and manganese minerals, thus enabling easy removal by filtration, while eliminating unpleasant iron, manganese and hydrogen sulfide odors. An electrical current is passed between an anode and cathode positioned some distance apart in the water solution. The electrical current pulls the positive and negative charges of the water molecule apart, causing it to split into hydrogen and oxygen, efficiently producing extremely small oxygen microbubbles, which are unable to break the surface tension of the water. As a result, virtually all the released oxygen remains dissolved in solution and available for oxidizing contaminants. Test case experiments are in progress to confirm that this process will also oxidize Arsenic III, which will allow it to be removed, along with Arsenic V, by reverse osmosis.

Ozonation Ozone (O3) is a relatively unstable molecular "free radical" of oxygen, which readily gives up one atom of oxygen and becomes a powerful oxidizing agent that is toxic to most waterborne organisms. Ozone oxidizes by attaching the extra oxygen atom to anything that can be oxidized. The only by-product of ozone is pure oxygen. The high oxygen content of ozonated water provides numerous benefits including disinfecting viruses, algae spores, fungus, mold, and yeast spores on contact. Ozone also oxidizes and precipitates iron, sulfur, and manganese so they can be filtered out of solution. Ozonation is commonly applied by installing an ozone generator in an atmospheric surface tank or cistern.

Ultraviolet (UV) Treatment UV light works by attacking the genetic core (DNA) of bacteria and viruses, destroying their ability to function and reproduce. The process is simple but effective and destroys 99.99 percent of harmful microorganisms within microseconds without adding chemicals or changing the taste or color of the water. UV is more effective than chlorine in disinfection against organisms such as Cryptosporidium, which is resistant to chlorine.

The main disadvantage of the use of UV radiation is that it leaves no residual disinfectant in the water and hence is not widely used in municipal water purification. It is an ideal disinfection process, however, for residential and small commercial applications. The effectiveness of a UV system in eliminating microbiological contamination is directly dependent on the physical qualities and/or clarity of the water supply. Suspended solids or particulate matter can cause a shielding problem in which a microbe may pass through the UV filter without actually having any direct UV penetration. Untreated iron and manganese may cause staining on the quartz sleeve that causes the UV bulb and may impair its disinfection capabilities. UV filters are best used after adequate sediment, turbidity, iron, and manganese filtration.

Turnkey Solutions For both arsenic and uranium removal, turnkey solutions are available with dual tanks in a lead-lag configuration. When simple testing reveals that the removal media in the active tank is being depleted, the homeowner switches to the 2nd tank. The spent first tank is then harmlessly handled and shipped to the manufacturer, who dispenses of the waste in a certified hazardous waste site and then regenerates the media for future use. Turnkey solutions are a promising trend in dealing with these contaminants that constitute both a risk to the consumer and a disposal risk to the environment. It is likely that awareness of both water supply shortages and environmental concerns will accelerate the development of new and sustainable technologies in water purification.

Stephen K. Wiman, Ph.D., is a geologist and owner of Good Water Company in Santa Fe. He was recently appointed to the City of Santa Fe's Water Conservation Committee. His primary interest is in using water chemistry to determine the most sustainable methods for treating specific water purification issues. Stephen may be reached at 505-471-9036 or skwiman AT goodwatercompany.com.

 

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