From Volume 21, Issue 5 – May 1998
Filters, chlorine and ozone have their advantages.
by: Howard Conner
Few contaminants are more objectionable to the nose than hydrogen sulfide.
Hydrogen sulfide is one of those troublesome constituents that’s almost universally unacceptable at any detectable level. It is not only objectionable to the senses, but is chemically very active. Although it is detectable in concentrations as low as 0.5 milligrams per liter (Mg/L), certain applications might require removal to even lower levels.
Before selecting a treatment strategy it is helpful to review some basic facts. Hydrogen sulfide is a gas that is present in two forms, depending on the pH of the water. At higher pH levels H2S is present in the alkaline sulfide form and at lower pH levels it is present in the gaseous form.
At pH 5.0 about 90 percent is present as gaseous H2S and 10 percent as alkaline sulfide. At pH 8 only 6 percent is present as a gas. You can begin to see the effects that pH would have on different treatment strategies.
For instance, you would not attempt to use aeration for off-gassing of high levels of H2S if the pH is 8. At any time you would have access to only 6 percent of the contaminant as a gas. Although long detention time and continuous aeration might significantly reduce contaminant levels, aeration would not be cost-effective.
Never attempt to treat hydrogen sulfide without first testing for sulfur with a high-quality hydrogen sulfide field test kit. It is not possible to treat sulfur simply by trying to gauge the strength of the odor. At high pH levels you may not detect much of an odor. Although the impact on your nose at a pH of 8 might not be as significant as it would be at a pH of 7, your system has to treat the full level of sulfur. Often the result is failure.
Another common mistake is incorrectly determining the source of the sulfur. Hydrogen sulfide in most areas is usually present in the source water itself. However, in some instances it can actually be produced in the hot water system or in other parts of the distribution system when sulfates are converted to sulfide by bacterial reduction.
These bacteria are part of a group of microbes known as chemoautotrophs. The most notable of this group are a species of bacteria known as Desulfovibrio desulfuricans, which add an additional wrinkle to the problem. It is possible to completely remove the H2S at the point-of-entry (POE) only to have it re-formed in the water heater.
To determine if the water heater is involved, run the cold water only inside the house – preferably in a shower stall. Make sure that if you are using a mixing valve only cold water is being run. If no odor is detected, turn off the cold water and run the hot water only. The presence of sulfur in the hot water but not the cold indicates that bacterial activity is the cause of the sulfur.
It is also possible to have a combination of source water sulfur and water heater sulfur. In such cases, proceed with source water treatment and solve the hot water problem later.
Several treatment strategies for source water sulfur have been successful:
– Oxidizing filters
An oxidizing filter is one of the traditional treatment strategies used. However, these filters are severely limited as stand-alone systems for maximum contaminant levels. Use them for special cases, such as when low levels of 1 mg/L or less are detected, or in portable exchange tanks at low levels when the customer needs a rental system.
For oxidizing filter calculations, consider that sulfur will require three times the amount of oxidizing capacity as iron. Compounding this problem is the fact that a number of manufacturers have designed their oxidizing filters to regenerate with less potassium permanganate than the media manufacturer states as the optimal dose per regeneration.
This is an attempt to lessen the chance of an incomplete final rinse that could leave a potassium permanganate residual in the product water after regeneration. Media manufacturers give a rated capacity per cubic foot per regeneration as 8,000 to 10,000 parts per million (ppm) iron or 2,700 to 3,333 ppm hydrogen sulfide.
This is true if regenerated according to media manufacturers recommendations. The actual capacity when regenerated at the lower potassium permanganate dose equipment manufacturers recommend often is only 2,000 ppm for iron or 667 ppm for sulfur.
Consider the following example using an oxidizing filter with 2,000 ppm capacity for iron per regeneration per cubic foot:
Example: 240 gallons per day use.
4 ppm iron
1 ppm sulfur x 3 = 3 ppm compensated iron equivalent demand
4 ppm iron + 3 ppm iron equivalent = 7 ppm compensated total
7 ppm compensated total demand x 240 gallons per day = 1,680 ppm demand
Since the total compensated demand is 1680 ppm and the total capacity is 2,000 ppm, the unit will have to regenerate every day. The next factor to consider about hydrogen sulfide is its unfortunate tendency to fluctuate. In my experience, it is all too common for hydrogen sulfide to increase or decrease during the course of a season.
If in the above example the sulfur content increased to 1.5 ppm, the total compensated demand would be 2,040 ppm. Since the total maximum capacity is 2,000 ppm, the system would fail and the failure will compound.
The first day of service the filter would have a deficit of 40 ppm. The next day the deficit would be 80 ppm and then 120 ppm the third. As time goes on, the unit would be ruined due to insufficient regeneration.
Eventually the manganese dioxide coating would be stripped from the media and regeneration would become impossible even if the hydrogen sulfide subsides to manageable levels.
For this reason, oxidizing filters should be used on low levels of hydrogen sulfide. With sulfur, you must allow yourself a margin of error to accommodate for possible increases.
With moderate to high levels of hydrogen sulfide, use oxidizing filters as part of a treatment system employing an external oxidizer such as chlorine or ozone. The oxidizing filter is then free to do what it does best – filter.
Chlorine is a powerful oxidizer that can be administered in stoichiometrically equivalent amounts. Extremely high amounts of H2S can be treated with enough detention time and proper dosing of chlorine.
The primary problem with this type of system is maintenance. Some homeowners are not able to maintain the system over time. Adding to the maintenance problem are variable amounts of sulfur in the raw water. Sulfur levels tend to fluctuate, and each fluctuation requires adjustment to chlorine feed.
Chlorine can be fed at higher levels than the sulfur equivalent and removed with a carbon filter, but this can cause other problems. Have a chlorine residual in the distribution lines to reduce the potential for sulfur reducing bacteria causing mischief.
For successful application, make sure you follow these three steps:
1. Test for oxidizable materials. Identify and quantify total levels of iron, manganese, hydrogen sulfide and any other material oxidized by chlorine.
2. Gauge the maximum flow rate of the system. This is used to determine the size of the detention tank. Insufficient detention time can lead to incomplete oxidation.
Low detention times are not recommended on most water, although good results have been achieved with detention time as short as five minutes on water with relatively simple water chemistry and H2S levels of no more than 5 to 6 mg /L. Waters with combined iron, manganese, hydrogen sulfide and other oxidizable materials would require more conventional detention times such as 20 minutes or more and sometimes substantially more.
Because many variables might be encountered in the field, there is no set rule for the proper detention time. If you are unsure of proper detention times in a given area, there is no substitute for a small pilot system or a simple jar test. The more unsure you are about the variables of the problem, the more detention time should be built into the system.
3. Determine where sulfur is being produced. It is important to note if the sulfur is present only in the source water or is being produced in whole or in part in the hot water heater.
To achieve the reaction H2S + Cl2 = 2(HCl) + S, 2.1 mg/L of chlorine is required to oxidize 1 mg of the sulfur in H2S.
For initial calculations, use 3 mg/L of chlorine for each mg/L of H2S. Since chemical feed pumps are typically sized by the number of gallons per day of output when run continuously for 24 hours, it is advisable to convert your chlorine requirement to 24 hours as well.
8 gpm x 60 min per hour x 24 hours per day will produce a total of 11,520 gallons per day (gpd) Assuming an H2S level of 3 ppm, the daily continuous rate use would be 3 mg/L of H2S x 3 milligrams of chlorine per liter of H2S = 9 mg/L chlorine demand. Assuming a solution strength of 2 percent sodium hypochlorite (20,00 mg/L) in the feed pump tank, the daily feed requirement of the feed pump would be as follows:
(103,680 mg chlorine demand)/(20,000 mg chlorine solution strength) = 5.18 gpd of chlorine feed required.
If the customer can operate the system, chlorine has the major advantages of relatively low cost, high-efficiency adjustable dosing and residual oxidation in the distribution lines.
Determining when to use chlorination depends on how capably the customer can operate a chlorinator. Remember that even after you oxidize the hydrogen sulfide with chlorine, you must still filter the precipitate.
Ozone is an even more powerful oxidizer than chlorine. It is extremely effective in oxidizing H2S. However, it often requires a more costly investment in equipment. The systems can be automated to a level that is almost unachievable with a chlorination system. The key here is to calculate the exact oxidant load required by the water analysis.
In ozone applications, two different strategies are used. The first and most simple type of ozone system is inserted into large atmospheric storage tanks (2,000 to 10,000 gallons). These tanks are common in warm weather states such as California where freezing is not a factor.
If this type of tank is part of a pre-existing water system, it is extremely inexpensive to install a small, continuous-production ozone system. These small ozone systems typically cost around $1,500 installed. Although they produce relatively small amounts of ozone, they are extremely effective for treating sulfur.
Most of these systems produce from 3 to 6 grams of ozone per day. Ozone demand calculations for these “aeration type” ozone systems do not seem to make sense at first glance. It is only when the aeration factor is considered that the apparent inconsistency is solved.
Remember that aeration can account for considerable sulfur off-gassing at moderate to low pH levels. On a continuous 24-hour basis, aeration in a 2,000 gallon or larger tank can be very effective. When the capacity of aeration is coupled with low levels of ozone production, the outcome can be very effective. When the simplicity of use, ease of maintenance and efficiency of design are considered, few treatment strategies can compare.
The one major drawback is the need for a large atmospheric storage tank. If the storage tank is not already in place, the system is not as cost effective.
The other type of ozone system is the immediate production system. These systems make economic sense when large atmospheric tanks are not available or not applicable due to local factors, such as cold weather or limitation in available space.
In these systems the ozone generator must be sized considerably larger, the systems typically are much more expensive than aeration-type systems, using large pre-existing atmospheric storage tanks.
These systems must be capable of producing ozone in capacities as large as 20 grams per day or more, depending on flow rate and oxidant demand. Such systems typically are more complex and expensive and require that ozone be produced with corona discharge. Virtually little or no aeration is used. These systems must bulldoze their way through the oxidation requirements of sulfur removal.
Because of the complexities of ozone design, a competent ozone supplier should be consulted for help in applying ozone systems. The manufacturer should have experience in solving rural water problems and should guarantee effective sulfur removal.
Howard Conner is president of Rayne of Santa Cruz, Inc., Santa Cruz, CA.