By Richard Fox

Summary: Existing technologies for hydrogen sulfide (H2S) removal have significant limitations, and their improper application and operation have long been a source of frustration to dealers and customers alike. A new technology, presented here, eliminates the problems that have long plagued these removal processes. This article will describe how it’s applied, how it works and how it compares to existing technologies.


In the potable water industry, there has been little, if any, improvement in hydrogen sulfide (H2S) removal technologies for quite a while. Yet according to an industry survey, hydrogen sulfide—characterized by its rotten egg odor—is the second most complained about contaminant by homeowners.1

A new media employing a “reaction on demand” (ROD) process is poised to take over the elusive job of H2S removal from residential water. It’s referred to as ROD because of its ability to respond automatically to changes in levels of H2S. As levels of H2S rise, as they often do, only the exact amount of media is consumed to completely eliminate the odor; thus, if H2S levels decrease, the media is indefinitely preserved until needed.

Distinctive characteristics
The media is defined by three distinctive characteristics. First, it’s a completely passive system with no chemical additives, no injection pumps (chemical or air) and no moving devices of any kind whatsoever. Second, the media is completely selective to H2S and will not react with—nor be affected by—any other constituent that may be present in the water. Third, the media uses a “chemical reaction” mechanism to indefinitely retard the H2S molecule.

This unique combination of characteristics—passive, selective, and permanent reaction with H2S—is what makes the media invaluable to the potable water treatment industry.

Properties, use and reaction
This reaction media is black, substantially dry and granular. From a distance it looks similar to carbon, although there’s no carbon in the product. The basic ingredient is an insoluble synthetic iron compound. It weighs 70 pounds per cubic foot and the bulk is sized at 4-to-40 mesh. The media, although not “dusty” in appearance, contains a substantial amount of fine particles, smaller than 1 micron.
The chemical reaction between the media and H2S is unlike any other technology in that it doesn’t form elemental sulfur (see Figure 1). Rather, the reaction creates an insoluble iron disulfide, i.e., ferrous sulfide. The disulfide is not a new particle, but instead is simply the original particle with a new molecular form. Therefore, since no solids particles are created, the media bed remains porous. This means no filtration, no backwashing, and virtually nothing for the customer to operate or maintain.

It is used in an up-flow, vertical tank system (see Figure 2). Water enters the bottom and flows up through the media, exiting the top of the tank free of any trace of H2S. The tank is divided into a turbulence eliminating section, a reaction section and product water storage section.

The turbulence eliminating section, which contains a gravel under-bedding, allows the water to spread uniformly over an entire cross section of the tank. As the water rises through the reaction section, the H2S is converted by the media into an insoluble iron disulfide. The treated water then enters the purified water storage section of the tank, where it’s reintroduced to the household system. This up-flow, three-section system is unique to ROD media, as most other solid bed processes use a down-flow system.

The tank is sized according to H2S concentration, i.e., the higher the concentration, the longer the contact time and thus the larger the tank (see Figure 3). However, reaction time decreases logarithmically with higher concentrations (i.e., doubling bed height squares removal capacity, tripling it cubes capacity, etc.), thus extremely high concentrations of H2S can be removed with relatively short bed depths. The media is exhausted when the customer smells a slight odor. At that time, it’s extracted by vacuum and replaced.

Other contaminants, such as iron bacteria and organics, won’t affect this media, and no pretreatment of any kind is necessary. The system isn’t affected by pH unless the level of hydrogen sulfide exceeds 100 parts per million (ppm), in which case pH should be below 8.0. At that or above, the media removes forms of the sulfide ion but with less efficiency. Another important feature of this media is that it’s “bacteriostatic,”2 which means it won’t allow bacteria to grow. This is important because many H2S removal processes will breed bacteria; thus, while they may solve the sulfur problem, they can create a potentially worse bacteria problem.

History of ROD media
Although this media is new to the potable water industry, it has a long and proven track record in other industries. It has been used successfully for nearly 30 years in industries such as oil and gas production, as well as the wastewater, landfill and environmental industry in general. It removes H2S from drilling fluids, natural gas, air, liquid hydrocarbons (i.e., propane, butane, diesel, etc.) and non-potable discharge water.

Reasons for removing H2S are many and include safety (see Table 1), equipment and pipeline preservation (H2S) in water can be extremely corrosive), and odor control (H2S) can be detected by the human nose as low as 20 parts per billion, or ppb). The same reasons for removing H2S in other industries are also present in the potable water industry. It not only corrodes metal pipes (at varying levels depending on material and water chemistry), but its fumes can corrode electric wiring in homes, tarnish jewelry and have been known to make people extremely ill.3 For these reasons, homeowners and dealers need an acceptable, long-term solution for H2S removal.

Current methods
There are basically three methods to remove hydrogen sulfide from potable water in use today: chemical injection, water filtration and aeration.

Chemical injection methods use an oxidizing agent to react with H2S. Chemicals, such as chlorine or hydrogen peroxide, are added by injection directly into the water. The reaction product of this method is usually elemental sulfur. The solidified sulfur is filtered out with a carbon filter that’s periodically backwashed. Sometimes a retention tank is required to give the chemical sufficient time to react with the H2S.

Water filtration methods use absorption or adsorption technology to remove H2S rather than a chemical reaction. These usually involve fixed bed filters such as carbon and manganese greensand. Once the sulfur is adsorbed or absorbed onto or into the surface of the filter media, the systems are backwashed and the process starts over. Some types of filter media are regenerated with chemicals such as potassium permanganate.

Lastly, aeration uses air to strip the H2S from the water. This process can be performed at either atmospheric conditions (called an “open” system) or under pressure (called a “closed” system). The water can be either cascaded like a waterfall, atomized through spray nozzles or purged with air bubbles. Closed systems are less effective at stripping H2S because its transferability from the liquid phase to the vapor phase is more effective at lower pressures.

Prior method drawbacks
There are so many problems with H2S removal technologies, selecting a process often has been based more on one’s ability to control and manage those of a particular system than selecting the best one for the application. Common problems with chemical injection methods include insufficient chemical injection rates, short residency times, chemical depletion and injection pump failures. Both chemical injection and filtration processes have serious problems with the backwash function. Most of these problems involve the backwash valve becoming clogged with debris including precipitated sulfur, bacteria growth or other solid contaminants. A bad backwash valve will virtually shut down a typical H2S removal system. Another problem with existing methods is the need for pretreatment or preconditioning of the water. Many systems won’t work with contaminants such as iron bacteria or organic matter in the water. Others require certain constituents such as iron or dissolved oxygen to work properly. Aeration systems don’t eliminate the odor; they simply transfer it to another location causing nuisance odors elsewhere.

Unfortunately, however, dealers are unfairly blamed for customers’  failure to properly maintain the systems. With prior methods, customers must actively maintain systems by replenishing oxidizing chemicals, maintaining backwashing equipment and sanitizing aeration tanks. When they fail to do so, H2S bleed-through or bacteria regrowth will occur. Then, the dealer is dependent on the customer to diagnose the situation. If a system fails even briefly, bleed-through will contaminate downstream pipes, water softening tanks and hot water heaters, resulting in additional problems and non-productive work for the dealer. Bottom line, customer call backs for improperly working systems can be an enormous cost to dealers both in lost time and referrals.

ROD media benefits
The greatest benefit of this  media is it requires virtually no customer or dealer maintenance or operation. It’s completely passive and maintenance-free except for infrequent media replacement. There are no pretreatment or preconditioning requirements so it has wide applicability. (If there is sand or mud in the water, a sediment filter is recommended.) And since there are no precipitated solids, there is no filtration and backwashing necessary. This means no backwash valves to clog or break down. There are no moving parts, no spray nozzles, no pumps (liquid or air) and no electrical requirements.

The media’s selectivity to H2S makes it the perfect pretreatment for other types of processes like iron or manganese removal. Those processes can become prematurely exhausted by the H2S to the detriment of removing other contaminants effectively. By using this media as an H2S pretreatment to those processes, those contaminants (i.e. iron, manganese, etc.) can be removed much more effectively.

This reaction media will remove any level of HH2S so long as the equipment is designed with sufficient contact time. In current operation, it’s removing up to 350 ppm in potable water and in excess of 1,000 ppm in non-potable water applications.

Conclusion
“Reaction on demand” media offers solutions to many problems that have long plagued existing hydrogen sulfide removal technologies. The lack of chemical additives and regenerating agents mean a more desirable water supply for the customer. And the media is bacteriostatic. With no H2S limit, no backwashing and no maintenance, it’s the first real development in H2S removal technologies in  a long time.

References

  1. “1998-99 POU/POE Industry Survey Results: Statistics Reflect Consolidation,” WC&P, November 1998, p. 41.
  2. “Final Report—Bacteriostatic Challenge Study Project, ASI No. 99194-031,” Analytical Services Inc., Wiliston, Vt., November 2, 1999.
  3. Smith, E.B., “Stench Chokes Nebraska Meatpacking Towns,” USA Today, February 14, 2000.

About the author
Richard Fox is president and CEO of The SulfaTreat Company in St. Louis, Mo., an international manufacturer and owner of proprietary technologies for the removal of hydrogen sulfide and other sulfur compounds from gases and liquids. The company manufactures WaterSweet, a reaction-on-demand media for hydrogen sulfide removal from potable water. Inquiries can be directed to (636) 532-2189, (633) 532-2764 (fax) or email: info@watersweet.com

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