Water Iron Manganes And Sulphur

Sometimes referred to as the "fly in the ointment," iron can be the most bothersome domestic
water treatment problem to deal with. Perhaps that is why so many different labels have been given to the various categories of iron in water chemistry. 1 Table 8-1 lists four basic forms of iron in water along with other alternate names in each

Table 8-1

Forms of Iron (Fe) in Water

1. Ferrous, Fe 2+
Clear-water Iron
Dissolved Iron
Soluble Iron

2. Ferric, Fe 3+
Oxidized Iron - Iron Oxide
Insoluble Iron
Precipitated Iron
Red-water Iron - Rust

3. Organic Iron
Bacterial Iron
Chleated Iron
Heme Iron

4. Colloidal Iron
Dispersed Iron

Geological and geographical circumstances have a distinct bearing on the nature of iron found in water supplies. By and large, excess iron in water will not be found in public municipal water supplies. The most common occurrence of iron in domestic water is in private wells.

Iron is one of the most common elements found in nature, accounting for at least five percent of the earth's crust. It is understandable, therefore, that just about all water supplies, surface or ground, contain some measurable amount of iron. In nature, iron usually occurs as an insoluble oxide, ferric oxide (Fe 2 O 3 ). Under favorable conditions on the earth's surface, the iron is converted
to a soluble form and dissolves in water will which it comes in contact. For this reason, iron can be found in almost every natural
water source, but particularly in well waters. Well waters are usually high in carbon dioxide (CO 2 ) and low in dissolved oxygen (0 2 ), which contributes to the conversion of insoluble iron oxide to the soluble form of ferrous bicarbonate [Fe(HCO 3 ) 2 ]. Ferrous iron is colorless in solution, and the sample is clear when drawn.

When clear water containing ferrous bicarbonate is exposed to the atmosphere for a period of time, it will adsorb oxygen from the air and react to form insoluble iron, most often as ferric oxide (commonly referred to as rust). Because iron oxidizes readily and precipitates as an insoluble substance, it will cause red-brown staining of laundry and porcelain fixtures. In addition, iron will impart a metallic taste to drinking water and beverages.

Water that contains iron at 0.1 ppm-mg/L or lower may be considered acceptable for domestic uses. However, if the total Fe level is 0.3 ppm-mg/L or higher, staining can result on kitchen and bathroom fixtures, dishes, cookware, laundry, and masonry surfaces. Several industrial applications call for an almost total absence of iron content in water used for process work.

In addition to natural sources of iron in water, this metallic substance can result from corrosion of exposed steel or iron. Corrosion can also dissolve other heavy metals. The more corrosive the water, the more iron and other heavy metals will dissolve from metal surfaces with which the water supply comes in contact. Where oxygen is present in a low pH value water, corrosion of iron, steel, cadmium, copper, lead, and zinc will be accelerated. Also, higher water temperatures, such as in hot water heaters and boiler heating water, add another dimension for corrosion. As a result of corrosive conditions, iron can be present most often as rust particles, and to some extent as dissolved iron. The iron content of a water sample should always be analyzed and reported as the total iron ____ the combination of all forms present. All forms of iron create an unacceptable characteristic in water for drinking, dishwashing, bathing, and laundering- Another serious problem of water corrosion occurs where lead-soldered piping and brass faucets are installed. Evidence of lead in drinking water has been reported at kitchen faucets. 2 Where corrosive conditions of this nature do occur, a safe practice is to let the cold water run for a few minutes each morning before drawing water for consumption/ cooking. This will reduce the contact time during which lead may dissolve.

Another step, of course, is to treat the water at the kitchen tap for removal of heavy metals with a cartridge-type filter or suitable whole-house treatment.

As mentioned in Chapter Five, a cation exchange water softener regenerated at 10 pounds (4.5 kg) or more of NaCI will reduce and remove dissolved iron, plus soften the water. Likewise, other dissolved cations, including cadmium, copper, lead, manganese, and zinc, can be reduced to acceptable drinking water standards by cation water softening.

Even at very low levels, iron can produce a favorable climate for the growth of what is called iron bacteria, These microorganisms, such as Crenothrix, Leptothrix, and Gallionella utilize energy obtained from the oxidation of ferrous to ferric iron to "fix" dissolved carbon dioxide into organic
molecules necessary for their existence. These organisms need only a continuous supply of

ferrous iron and air or oxygen to metabolize ferric iron into their cell structures, and to deposit gelatinous ferric hydroxide iron compounds. The growth of these organisms will result in the formation of a jelly-like mass, cause pipe encrustation, and can produce foul-tasting drinking water. If the interior of a water closet has a gelatinous sludge and the surface reflects an iridescent (rainbow) slick, it is usually a telltale sign of the presence of iron bacteria. Because of its organic nature, iron bacteria by whatever name, is one of the most difficult forms of iron to remove and control.

While colloidal iron can be observed visually in a water sample, as can ferric iron and, to some degree, organic iron, it does differ from the other two. Colloidal iron stays in suspension, giving a red-pink, turbid cast to the water sample. It is very highly dispersed and has a very low specific gravity almost equal to that of water. The specks of iron appear to be floating, and sometimes are attached to silica. The colloidal particles can have a slight negative charge. It may take a water sample containing colloidal iron 48 hours for the iron to drop out or begin to settle at the bottom of the container. In municipal/industrial water treatment plants, colloidal iron is removed by adding a coagulant such as ----- allowing it to coagulate, form a floc with the colloids, and partially settle out; then passing the water through a granular medium filter system.

The presence of manganese in groundwaters, like iron, is generally attributed to the solution of rocks and minerals, chiefly oxides, sulfates, carbonates, and silicates, that contain some degree of manganese. Manganese-bearing minerals are less abundant than iron-bearing minerals; consequently, manganese is found less frequently than iron in water sources. The hydroxides and carbonates of manganese are, on the other hand move soluble than the corresponding iron complexes. Despite these differences, concentrations of manganese seldom run higher than 2.0 ppm-mg/L, and those of iron are seldom above 15.0 ppm-mg/L. Comparative test data of many public water supplies in Illinois show that the concentration of iron encountered is roughly 10 times the manganese concentration in the same source water. 3 The solution of manganese-bearing minerals into a water source is often attributed to the action of carbon dioxide in groundwater ____ the
same general theory as iron. Often groundwater carbon dioxide is presumably generated by the bacterial decomposition of organic matter leached from soils. The solution of manganese and iron may take place under anaerobic conditions and in the presence of reducing agents (organic substances, hydrogen sulfide) that are capable of reducing the higher oxides of manganese and iron into the manganous manganese (Mn 2+ ) and ferrous iron Fe 2+ states. 4 Manganese is a vital micronutrient for both plants and animals. When manganese is not present in sufficient quantities, plants exhibit a yellowing of leaves (chlorosis). While the average daily intake of manganese for humans is 10mg, large ingested amounts are reported to have caused some liver damage. 5 Manganese will be found in water in the same forms as iron: dissolved (clear water) chiefly as manganese bicarbonate, precipitated (oxidized) manganese, and organic manganese.

Manganese is rarely found alone in a water source; it is generally found in conjunction with iron. Concentrations of 0.1 ppm-mg/L are considered troublesome in both homes and businesses. The EPA has listed the maximum level of manganese at 0.05 ppm-mg/L in the Secondary Drinking Water Regulations. Both iron and manganese are stain-causing substances. While oxidized manganese is a dark brown, accumulations on a surface will also appear as black. This is why manganese-bearing water may be referred to as "black water." A quick clue to the presence of manganese in a water supply is the occurrence of a black stain on the inside of the toilet.

Laundries, textile mills, and paper mills are among the industries that are most critically affected by manganese in the process water supply. Food process water must also be extremely low in manganese concentration.

Organic (bacterial) manganese behavior is similar to that of iron bacteria. There is an old saying in the water treatment industry that "manganese holds up iron." It doesn't, but organic matter can sometimes hold both metals in a water solution. 6

The third link of the trio is the offensive odor ___ producing substance of hydrogen sulfide ___ often called sulfur water in the water treatment trade. Unlike manganese and iron, which are inorganic dissolved metals, hydrogen sulfide is a gas that dissolves readily in water. When released, such as at a water faucet, it's disagreeable "rotten egg" odor is very noticeable, even at low concentrations.

This gas is also flammable and, in higher atmospheric concentrations, poisonous to humans. In addition, H 2 S is corrosive to most metals and can tarnish silverware (notice the blackening of forks and knives after eating poached eggs). At the level of only 0.25 ppm-mg/L, hydrogen sulfide is detectable by most persons.

Despite its unpleasant and corrosive characteristics, hydrogen sulfide has played a role in spas and hot sulfur springs over the years. Hot Springs National Park in Arkansas, Warm Springs in Georgia, and Hot Sulfur Springs in Colorado were known for their role potential therapeutic cures.

Occasionally a hydrogen sulfide odor may be detected in a softened or filtered water supply that showed no H 2 S in the raw water sample analysis. This condition generally occurs in the morning when the first water is drawn at a hot water faucet. The cause is the presence of a harmless sulfur bacteria, usually in the hot water heater tank, which is discussed more fully later in this chapter.

Because of the changing nature of gases dissolved in water, hydrogen sulfide is sensitive to water temperature and pH value. Raising the water temperature from, say, 50°F (10°C) to 70°F (21°C) will result in 25 percent less H 2 S in the water (see Table 2-1). Hydrogen sulfide is most readily removed at a pH value of 5.5. Ninety-eight percent of the sulfide in water at pH 5.5 value exists as H 2 S, and only two percent exists as the bisulfite ion. On the other hand, at 9.0 pH value, just a little over 0.05 percent would be present as hydrogen sulfide. 7

Quite a few options are useful either as single-step treatment or in combinations of two or more techniques to reduce and remove iron, hydrogen sulfide, and manganese to acceptable levels. Depending upon conditions, the overall treatment methods can be grouped as follows:

• Ion exchange
• Aeration plus filtration
• Chemical oxidation plus filtration
• Catalytic oxidation filtration

Ion Exchange for Iron Removal ____ See Chapter Five

Cation exchange softening used for removal of dissolved (clear water) manganese and hardness follows the same principles as described in Chapter Five for removal of dissolved iron in conjunction with softening. When dissolved manganese and iron (as well as hardness) are removed during the softening run, their combined cation exchange removal can be illustrated by the following reaction:

4R-Na + Mn(HCO 3 ) 2

Ca(HCO 3 ) 2

2R-Mn

2R-Ca + 4NaHCO 3 + H 2 O

Sodium

Cation

Exchanger

Manganese

and Calcium

Bicarbonate

Manganese and

Calcium Cation

Exchanger

Sodium

Bicarbonate

Water

R=Cation Exchanger

Regeneration of the manganese-calcium cation exchanger with sodium chloride brine would follow this reaction:

R 2 -Mn

R 2 -Ca + 4R-Na

4R-Na + MnCl 2

CaCl 2 + H 2 O

Manganese

Cation and Calcium

Cation Exchanger

Sodium

Chloride

Salt

Sodium

Cation

Exchanger

Manganese and

Calcium to Waste

Water

R= Cation Exchanger

While limited data is available, an ion exchange process does exist for hydrogen sulfide reduction and removal with the use of a strong base anion exchange resin regenerated with NaCl salt. Where total solids (TDS) in the source water are very low and the H,S present is less than 1.0 ppm-mg/L, this scheme has demonstrated ability to reduce the objectionable "rotten egg" constituent to negligible levels. Published data shows a sulfide capacity of 3.7 kilograins (kgr) per cubic (cu) foot (ft) for a stream of 200 ppm-mg/LTDS water at pH of 8.0, with the sulfide being 80 percent of the total anions. At higher pH values, the effectiveness of this system increases, because H 2 S more completely ionizes to a bisulfite ion (HS - ) or sulfide ion (S -2 ). In field applications, however, both iron and hydrogen sulfide are often present in the same source water. Some H 2 S reduction can be achieved in the dual-resin bed of cation exchanger and a top layer of anion resin.

Another method of ion exchange for hydrogen sulfide reduction is with a special cation exchanger in the manganese form. While information on this process is limited and field data scarce, a strong acid cation exchange resin converted to the manganese form in the same manner as manganese synthetic zeolite (or a dual-function single-media bed, as discussed in Chapter Nine) has the potential for modest hydrogen sulfide reduction while also softening water. With this special Mn form cation resin, part of the softening capacity is available for softening in addition to the manganese catalytic phase. Regeneration for this cation media calls for two-step methods. First, the bed is regenerated and rinsed down with potassium permanganate (KMNO 4 ), followed by the sodium chloride salt stage. The softening capacity of this special resin would be in the 20 kgr/cu ft range at the 10 lbs/cu ft salting level. 8

Aeration Removal of Hydrogen Sulfide, Iron, and Manganese

The aeration process as a physical water treatment method has been discussed in Chapter Two. Both the open-gravity and closed-pressure aeration systems are covered in
regard to how they function and the results that can be expected. This portion will cover the operating conditions and limitations of aerators for removal of the troublesome trio.

In the aeration process, either water is brought into intimate contact with air or is brought into intimate contact with water. Aeration can either strip (release) troublesome substances (gasses) from water or add air constituents, such as oxygen, to a water source. There are two basic aeration processes: (1) the open-gravity system, and (2) the closed pressure scheme. In municipal and large industrial water treatment plants, the open gravity aerator has long been used to reduce unwanted gasses, such as H 2 S, and some volatile organic chemicals from a water source. Any aeration process also adds oxygen to water, which will oxidize clear-water iron and dissolved manganese and cause them cancel to precipitate out. This is the most economical process for large volumes of water to be treated. However, open-gravity aerators call for repumping and repressurizing the water circuit.

Even though open-gravity aerators require repumping, several residential applications of this technique are now utilized in the United States, especially in the more temperate southeastern and southwestern regions. These systems represent scaled-down versions of the municipal-type plants. A typical design (Figure 8-1) for home or small business application includes (in additions to the basic well pump setup) an aerator-reservoir tank, a second pump to repressurize the system, and a second pressure tank. To polish the water for home use, an oxidizing filter (plus perhaps a softener) is usually installed in a typical application.

Open aerators such as these are capable of processing water to remove such gases as hydrogen sulfide, methane, and radon, as well as some volatile organic chemicals (VOCs). Because this type of system is not closed and is often installed outdoors in warm climates, there is a tendency for airborne organic species and other air pollutants to nest and harbor in the stored aerated water vessel. For this reason, it is fairly common practice to chlorinate the water after the storage tank to control organic compounds and microorganisms.

A major municipal water treatment plant in Florida, many years ago, had to abandon its open aerator when it was determined that "chironomidae eggs were on the surface of water in the basins and the larvae (or 'blood worms'), after hatching, appeared at water consumers' faucets." 9

In open aeration, the substance being stripped or vaporized from the water must be volatile. Oxygen, carbon dioxide, and hydrogen sulfide are volatile, and their concentrations in water are readily affected by the open aeration process, which is basically a "degasifying" technique. Since

higher water temperatures increase the volatility of dissolved compounds and decrease their saturation values, it follows that removal of volatile substances is more effective in warmer than in cold waters. Similarly,
the removal of such volatile gases as CO 2, H 2 S, and NH 3 is strongly dependent upon the pH value of the water. By and large, low pH values favor removal of these compounds by open aeration. 10 The actual interchange of the substances from water to air, or from air to water, takes place at the exposed air-water interface surface. the rate at which the interchange occurs depends on concentrations of the substance at that surface, its volume, and the speed which new surfaces develop and are exposed. 11

The reaction that takes place first is the oxidation of the hydrogen sulfide as follow

H 2 O + ½ O 2 H 2 0 + S

Hydrogen Oxygen Water Sulfur

Sulfide

In practice, the sulfide is converted to sulfur and precipitates as elemental sulfur. 13 Open aeration with the smaller, domestic-size aeration system is generally not too successful at removing dissolved iron and manganese. Closed-pressure aeration is usually the choice for these residential applications.

One critical requirement for successful operation of the open-gravity aeration method (as well as for closed-pressure systems) is a sufficient pump capacity (gpm-L/min.) and water flow. In residential open-gravity aerators, a good back pressure at the spray nozzle or nozzle grid is the key factor in creating a very fine mist, atomizing the water. Before sizing an aerator, the well pump capacity must be determined. As a rule of thumb the minimum sustained pump capacity needed is six to eight U.S. gpm (22.7-30.3 L/min.) Systems operation must be at better than 25 pounds psi. (See suggested method on page 6 of Chapter One.)
More details and operating conditions, the various styles of open-gravity aerators and their limitations can be found in Chapter Two.

In commercial water treatment, the closed-pressure aerator has been used for man years (see Figure 2-3). In this case, the air is brought into intimate contact with the water must by the use of compressed air in a fully pressurized system. This type of aerator is very suitable for oxidizing dissolved inorganic substances, such as iron and manganese.

In recent years, scaled-down domestic-size closed-pressure aeration systems have been designed much like the system shown in Figure 8-2.

When large amounts of iron are found in water sources, it is usually in the form of ferrous bicarbonate Fe(HCO₃ )

2 . This substance is a soluble, colorless salt that exists only a solution. As the CO₂ content of water increases, so does the solubility of ferrous bicarbonate. Ferrous bicarbonate expressed as iron has been reported as high as 50 ppm-mg/L with a 5.0 pH value. 14 An unaerated glass of water containing ferrous bicarbonate will appear clear when first drawn from a faucet ____ thus the term "clear water iron." After exposure to air, it will turn yellowish,then reddish. It is on the principle of air exposure that iron and manganese can be reduced in a water source. As the elements precipitate, they drop out or are filtered out.

The amount of manganese (Mn 2+ ) found in water sources is generally less than three ppm- mg/L and seldom over five ppm-mg/L. 15 In theory, one ppm-mg/L of oxygen will oxidize seven ppm- mg/L of dissolved iron (as Fe). Comparatively, it takes 1.5 ppm-mg/ L of oxygen to oxidize one ppm-mg/L of manganese (as Mn). The reaction time for aeration of manganese is quite slow compared to that of dissolved iron, but this can be improved greatly when the water's pH value is around 9.5 or higher. 16 The oxidation and precipitation of iron is similarly enhanced when the water's pH is 7.2 or higher.

The amount of dissolved iron and manganese that good closed-pressure aerators can handle is usually much greater than the amount that can be treated with the catalyst procedure. Well-engineered aerator/precipitator/filter systems can handle as much as 25 ppm-mg/L combined dissolved Fe and Mn. The filter medium mix in these systems often includes a calcite-type product that will help increase the pH value.

The aeration process will not be effective for organic-bacterial iron and/or manganese. In fact, these organic species will tend to plug the venturi air eductor and also clog the filter media bed. Chemical oxidation is generally the more effective treatment of organic, iron and manganese, as the encapsulating sac around the Fe and Mn molecules must be destroyed in order for oxidation to occur.

Operationally, the advantage of the closed-pressure acrator is that no repumping of the water is necessary. Thus, for domestic water processing, the closed-pressure system is often employed, especially in the less temperate regions of the northern United States and Canada. A typical example of one pressure aerator system is shown in Figure 8-2. In this system the water goes directly from the well pump through the aspirator/eductor fitting, with a venturi tube creating a slight pressure differential that causes a needle valve to open (like an inner-tube valve). This allows air to be educted (drawn) in and mixed with the ------ing water. This air-saturated water, after pneumatic storage, then enters the precipitator/aerator vessel, where the excess air separates in the top of the unit.

When a high enough back pressure builds up, it automatically vents this excess air to the exhaust. During this air-enriched stage, the aerator will act as a degasifier. On acid water high in carbon dioxide, some free CO 2 will be released, slightly raising the pH value and establishing a more favorable environment for oxidation. Likewise, during this air ------- stage, some dissolved hydrogen sulfide in water may be set free as a gas and ------ vented to exhaust.

Typical Residential Closed-Pressure Aeration/Filter System

From the aerator/precipitator, the water flows through a filter that uses various media to entrap and screen out the oxidized particulates of iron, manganese, and/or some elemental sulfur. The enhanced water can then be used for service or additionally treated.

Another advantage of the closed-pressure aeration scheme is that no chemical solutions are used and no new chemical by-products are created, as occurs with chlorination. Additionally, no consumable chemical cost is involved, as air is free. The atmosphere contains about 21 percent oxygen. The most important maintenance step involved in operation is periodic backwashing of the filter.

The most critical factor for successful operation of this style of pressure aerator is pumping capacity. Most of these systems require a constant minimum of six to eight U.S. gpm to generate enough air intake through the venturi eductor. For sizing an aerator, will pump capacity must be determined. (See suggested method on page 6 of Chapter One)

Chemical Oxidation: Iron and Manganese Reduction

The oxidation of dissolved iron or manganese in a water source can also be accomplished by feeding solutions of chemical oxidizing agents, such as chlorine bleach, potassium permanganate, or hydrogen peroxide, into the water. Table 8-2 lists the commom chemical solutions used for oxidation and suggested dosage levels.

Small chemical (proportional) feed pumps that are electrically wired to start up when the well pump comes on are used for this method of treatment (see Figures 8-3 and 8-4). As illustrated here, two points of injection of chemical oxidant can be used. The choice of above-ground injection (Figure 8-3) or direct-into-the-well scheme (figure8-4) depends on circumstances. One other direct-into-the-well technique involves the use of pellet oxidant feed, as shown in Figure 8-5.

When liquid chlorine bleach (5.25 percent sodium hypochlorite) is used as an oxidant, enough is fed to produce a chlorine residual. for iron, the usual dosage is 0.6 to 1.0 ppm-mg/L of Cl 2 , per ppm-mg/L of Fe at a pH range of 6.5 to 75 or higher. For manganese, the feed is 1.7 to 2.0 ppm-mg/L per ppm-mg/L of Mn, but a pH above 8.0 is often needed. Each of these chemical solutions is fed ahead of the pressure storage tank and retention tank, which allows time for the chemical oxidation to take place. A minimum of 20 minutes of contact is required for the iron and/or manganese to come out of solution and become oxidized. After the retention tank, the water is passed through filter to remove the ferric iron and oxidized manganese particles. The filter media used can be calcite, sand, anthracite, aluminum silicate, or granular activated carbon. Where any of the first four of these media are used, it is best to keep the residual Cl 2 to a minimum because of the unpleasant taste. When activated carbon is used this excess chlorine residual will be adsorbed. All of these filter types

Typical Chemical Feed Pump Arrangements

Direct to Well

must be backwashed at regular intervals to rid the media bed of the accumulated iron floc. Here again, it is best to install automatically operated filters to be sure the task is done on a regular schedule.

A potassium permanganate solution is another good oxidizing agent and is used frequently in iron removal. This chemical is fed with a small feed pump the same as described for chlorine feed. The filter media used for removing the precipitated iron is often a special manganese-coated aluminum silicate product or an eight-inch layer of anthracite on top of a bed of manganese-treated greensand. A word of caution on KMNO 4: this compound is temperature-sensitive while going into solution or upon standing. The best range is between 50°F and 72°F (or 10°C to 23°C) for effective and stable water solution of potassium permanganate.

Besides oxidizing troublesome metal constituents, the foregoing chemical treatments also will disinfect the water, as both Cl 2 and KMnO 4 deactivate bacteria. Good performance of these methods calls, of course, for keeping the chemical solution tanks for the feed properly filled. In situations where some organic iron is present, longer retention times may be necessary as well as higher ppm feed levels in order to fully penetrate the protective organic sac that encapsulates the iron. In some stubborn organic/colloidal iron- and manganese-bearing waters, feeding low levels of aluminum sulfate, (alum) to coagulate and filter these species has been very effective. Alum will tend lower the pH, so it is best used in water above pH 7.5 value.

The chlorine chemical feed treatment lends itself well to multiple water problems that need treatment. For instance, soda ash and liquid bleach may be injected in sequence to neutralize an acid water and, at the same time, oxidize the iron and manganese. These chemical feed systems are both useful and more economical in treating higher concentrations of iron and manganese than ion exchange or catalyst filters. Where very large volumes of water are to be treated, however, such as in food processing plants and commercial laundry applications, aeration followed by chemical oxidation feed is the preferred choice of water treatment.

Hydrogen Sulfide Reduction by Chemical Feed

As little as 0.1 ppm of hydrogen sulfide (H 2 S) in cold water is noticeable and offensive. Less odor is detectable from a sulfide-bearing water when the pH value is 8.0 or higher. This generally indicates the presence of ionized sulfide, which does not have the usual rotten-egg odor of H 2 S. The minimal detectable taste of H 2 S in a water sample is considered to be 0.05 ppm. 17 In some regions in the United States, the appearance of H 2 S in water sources can be seasonal. For example, in the New England area, H 2 S shows up in the spring and fall, when water tables tend to be at their highest levels. Hydrogen sulfide frequently occurs in well water sources that also contain appreciable levels of iron, hand especially along the eastern U.S. coastal plains.

The common practice in home and business applications is to feed 2.0 to 3.0 ppm- mg/L of Cl 2 for each ppm of H 2 S to be removed. Where potassium permanganate is used, the usual dosage is 4.0 to 6.0 ppm-mg/L of KMNO 4 per ppm of H 2 S to be removed. Best results for either oxidant treatment can be attained where the alkalinity of the water has been adjusted to a pH value of 8.0 or higher. Again, if pH adjustment is required in addition to oxidation of H 2 S, a combination of soda ash and chlorine bleach can accomplish both. Table 8-3 shows the stoichiometric quantities of various oxidants needed to precipitate iron and manganese.

As mentioned in the discussion of oxidation by aeration, hydrogen sulfide cannot usually be completely removed by aeration. That is why many municipal water supplies

Table 8-3

Stoichiometric Amounts of Various Oxidizing Agents Required for the Oxidation of Iron and Manganese 18

Amount to Oxidize 1 mg/L

Oxidant Iron Manganese
Oxygen (O₃ )
Chlorine (Cl₂ )
Potassium Permanganate (KMNO₄ )
Chlorine Dioxide (ClO₂ )
Ozone (O₃ )

0.14
0.62
0.91
1.21
0.86
0.29
1.30
1.92
2.45
0.87

in the Southeast have a lingering trace of rotten-egg odor. Removal of trace quantities of H 2 S can often be accomplished with either the catalyst process or chemical solution oxidation.

Sulfur Bacteria and Hydrogen Sulfide

When a hydrogen sulfide odor occurs in a treated water (softened or filtered) and no H 2 S is detected-in the raw water, it usually indicates the presence of some form of sulfate-reducing bacteria in the system. These anaerobic, single-cell sulfate-reducing bacteria (such as Desulfovibrio desulfuricans) can exist in home piping systems, especially on the hot water side. It is most noticeable on the first hot water drawn in the morning. Water softeners provide a convenient harbor and environment for anaerobic (oxygen-depleted) bacterial growth. A strong rotten-egg odor in waters drawn from an anaerobic zone usually indicates the presence of hydrogen sulfide (H 2 S), which results from a chemical process that involves three primary components: sulfur, electrons, and bacteria. Sulfur often exists in water as sulfate ions (SO 4 2- ). However, sulfate can convert to sulfide (S 2- ) and hydrogen sulfide (H 2 S) gas in the presence of excess electrons and sulfate-reducing bacteria according to the following formulas.

SO 4 2- + 8 electrons S 2- + H 2 O + CO 2

(from decay of anaerobic organic matter sulfate-reducing or corrosion of metals) bacteria
S 2- + 2H + H 2 S

Note that the necessary excess electrons may occur in water as the result of the decay of organic matter or the corrosion of metals. However, the concentration of organics in the raw water can be below detectable levels. 19 Sacrificial anodes in hot-water heaters, being designed to concentrate corrosion, can often provide a ready abundance of available electrons.

When H 2 S is present in hot water, the initial task is to heavily chlorinate the entire piping system, including storage and hot water tanks. Usually, a dose of household bleach left standing in the piping system (hot and cold) overnight will destroy the sulfur bacteria. Should the situation persist after more than two chlorination treatments, then a constant chlorination feed will be necessary to keep the condition under control.

It also has been reported that hot water can develop traces of H 2 S odor after water softeners are installed. Where softened water is the feed to hot water heaters, this condition has been overcome by removing the magnesium anode element from the water heater and replacing it, where necessary, to protect against water heater corrosion, with an aluminum or zinc sacrificial anode rod. Aluminum and zinc have a slightly lower electrode potential than magnesium and, therefore, will sacrifice fewer electrons as the energy source to reduce the sulfates.

Dry Pellet Chemical Feed

As an a alternate to liquid chlorine feed (for iron, manganese, and hydrogen sulfide reduction), a dry pellet-type of feed is often used in domestic, farm, and business applications for oxidation (as well as disinfection). This feeder is installed right at the well head and dispenses pellets of calcium hypochlorite (70 percent Cl 2 and 30 percent ------) directly down the inner well casing to the water level in the well (refer to Figure 8-5).

The dry pellet feeder is electrically wired to operate simultaneously when the main pump operates. Oxidizing the Fe 2+ , Mn 2+ , and H 2 S at the well source allows more contact time, as the water standing in the well zone in this technique is pretreated before even being pumped. These pellet feeders are installed outside at the well head. The dry chemical material is handled more easily and conveniently than liquid bleach solutions. Theoretically, under some conditions, early oxidation of the iron and manganese in the well zone can cause precipitate deposits in the piping and pump, creating possible clogging of well screens.

Ozone Feed Treatment

The ozone (O 3 ) oxidation process, over 100 years old and widely used among European countries, has been used mostly in municipal water and wastewater treatment in the United States. However, there appears to be a place for this unique process in residential or business market. Scaled- down, safe, and dependable versions of this process are becoming available for small (5-15 U.S. gallons per minute) flow rate ranges. Ozone, as an oxidating step, will also provide extremely powerful disinfection (see Table 8-3). The ozone process leaves no residual taste/odor, such as occurs with chlorine and

Typical Dry Chemical Chlorinator Installation at Wall Head

chloramine feeds. Where the atmosphere can be used, a very inexpensive source of oxygen can be utilized to generate ozone on site. 20 Some trace by-products (e.g., formaldehyde and bromoform) can be generated with the ozone process, however.

Ozone must be generated on site, at point of injection into the water or waste stream. It cannot be packaged, stored, and shipped as ozone. Commercial and industrial ozonation systems generally use liquid oxygen as the feed to generate the O 3 gas, usually passing the oxygen through a dryer unit first. Domestic-size ozonation units use air with a desiccant-type dryer and depend upon the available free oxygen in the atmosphere, which on average runs around 20-21 percent. Corona discharge ozone generators produce O 3 - air concentrations greater than one percent, whereas ultraviolet light ozone generators yield less than one-tenth of one percent ozone in the airstream. The higher the concentration of O 3 in the airflow bubbled into the water, the more ozone will transfer into the water. Bubble (O 3 ) contact time and total bubble surface area are also critical to the resultant ozone transfer into water. 21 The most vital step in ozonation is the proper and full distribution of the generated O 3 into the water stream.

Ozone (0 3 ) is not very stable and, accordingly, an O 3 residual in the treated water cannot be maintained, as can be done with chlorine, iodine, and bromine. For this reason to establish full disinfection on a continuing basis, municipal water supplies must have downstream supplemental chlorine feed in conjunction with ozone. The ppm-mg/L level of chlorine feed is very low, of course, because the primary oxidation step is accomplished by the ozone treatment. The modern, 600 million gallons per day municipal water treatment plant for the city of Los Angeles, which began operation in 1986, employee two-step disinfection process. Ozone provides the primary disinfection step, with limited downstream chlorine feeds to establish a low Cl 2 residual. This facility is one of the largest ozone systems in the world, and this two-step process is capable of meeting product water quality below EPA limits for trihalomethanes.

Oxidizing Catalytic Media

Because chemical and atmospheric conditions vary greatly from region to region in North America, many products and techniques have been tested and used to try to reduce the "troublesome trio" over the last three quarters of the 20th century. Among filtering materials used are those called chemically reactive media (see Table 1-4, Chapter One These catalytic media speed up chemical reactions by serving as a catalyst.²²

Among the earliest materials used in industrial process water for reduction of iron, manganese, and/or hydrogen sulfide was manganite ore. The higher oxidation states of manganese and the available oxygen from the manganite (MnO[OH]) in this material causes oxidation. The oxidation capacity of the ore was periodically replenished by either the backwash water, which contained a good concentration of O 2 or by passing a mild solution of chlorine bleach through the ore-filtering bed. Also adding to the replacement of available oxidant was the physical rubbing and scrubbing of the manganite particles against one another during the backwashing of the bed. This scraping of the particles exposed fresh surfaces to release new oxidation materials. 23 The very high density of manganite and the inability to control the size of the particles left a lot to be desired.

One of the principal objections to manganite for steam production was its potential to impart hydrogen sulfide to the treated makeup water due to the breakdown of iron sulfide contained in the manganite ore itself. This was recorded years ago when hydrogen sulfide production in steam led to heavy losses by a manufacturer, who used the steam for food processing.²⁴

Thus, the next step was to synthesize the product by chemically bonding a manganese coating on a stable medium. Currently, a manganese dioxide-coated pumicite medium, as well as several cation exchangers coated with manganese oxide, are in common usage. Pyrolusite (MnO 2 ) ores are considered useful oxidizers for iron, manganese, and hydrogen sulfide, as explained under the heading "manganese dioxide media."

Ordinary greensand zeolite, some forms of silica gel zeolite, and cation polystyrene resin may all be alternately treated with solutions of manganese sulfate and potassium permanganate to produce a coating on the base material, which consists mostly of manganese oxide. In a similar manner, pumicite medium is coated with higher oxides of manganese dioxide. Filter systems that use any of these media are commonly called "oxidizing catalyst filters."

All oxidizing catalyst filter media function on the same principal: the shiny black manganese coating on the media particles or beds provides a catalytic adsorptive surface on which one or two chemical reactions may occur. First, the manganese oxide surface has the ability to attract to it, or adsorb, ferrous (dissolved) iron and/or manganese. The higher oxidation states of manganese can serve to oxidize and, thereby, precipitate the lower oxidation states of dissolved iron, manganese, and sulfide. This reaction takes place when there is little or no dissolved oxygen in the water being treated. Regeneration of these manganese oxide-coated media with potassium permanganate will also oxidize the adsorbed iron or manganese and allow these metal ions to be removed via subsequent backwash and rinse. The media in this group are the cation exchangers coated with oxides of manganese.

Another reaction that may take place is one in which dissolved oxygen in the water maintains higher oxidation states in the oxides of manganese on the surface of the medium. The oxidants in their high oxidation states are then available to oxidize water- soluble iron and manganese. The precipitated forms of manganese and iron will be filtered out by the media bed and subsequently removed by backwashing. Most water will contain some dissolved oxygen by the time it passes through the pump and pressure tank system. Thus, some oxidizing catalyst filters do not require a regeneration step with potassium permanganate. 25 These nonregenerable catalyst systems find widest application in the Central Plains States region in the United States, and also in Europe and some South American countries. A minimum dissolved oxygen content of 15 percent

* Sold under trade name BIRM.

of the dissolved iron is recommended for effective application. (Example: Where iron is 10ppm,dissolved oxygen must be 1.5 ppm-mg/L or more.) This product is not recom-mended for hydrogen sulfide removal.

Another manganese dioxide mineral ore called pyrolusite (sold under various trade names) is also being used for nonregenerable iron and manganese removal systems. Pyrolusite has powerful and effective capacity as a sacrificial mineral to reduce the "troublesome trio." It is an
extremely dense material, requiring very high backwash rates of 20 gallons (75 L/min.) or more per square foot of filter bed surface area. In domestic water applications, it is used in multiple small- diameter tanks operating in parallel. In this manner, each tank can be backwashed independently. An eight-inch diameter filter tank for example, would need 7 U.S. gpm (26 L/min.) for backwashing.

The regenerable cation base catalyst media, in addition to reducing dissolved iron and manganese, can also be effective for hydrogen sulfide reduction and removal. Table 8-4 illustrates the capacity of one of the most widely used catalyst media-manganese

Table 8-4

Typical Capacity of Manganese Greensand Zeolite

Average Gallons of Treated Water (Thru-Put)
Per Cubic Foot of Manganese Greensand Zeolite
(based on 4 oz. KMnO 4 per cu. ft. regeneration)

Influent
Content

For Total
Iron Only (Fe 2+)

For Total of Iron
Plus Manganese
(Fe 2+ and Mn 2+)

For Hydrogen
Sulfide Only (H 2 S)

0.5 ppm
1.0 ppm
1.5 ppm
2.0 ppm
3.0 ppm
4.0 ppm
5.0 ppm

18,000 gal.
12,000 gal
9,000 gal
6,000 gal
4,000 gal
3,000 gal
1,800 gal

10,200 gal.
6,800 gal
5,200 gal
3,400 gal
2,260 gal
1,700 gal
1,020 gal

5,800 gal
3,900 gal
2,900 gal
1,950 gal
1,300 gal
975 gal
580 gal

Note: Water should contain minimum of 4.0 ppm alkalinity for every ppm of iron or manganese.

greensand zeolite (sold under trade name BIRM.)

The manganese form of synthetic gel zeolite has a capacity equal to the natural manganese greensand product. However, the synthetic gel product requires less backwash water and also can simultaneously soften water while removing low levels of iron and manganese (Patent 4,260,487, see Chapter Ten). Water to be treated by manganese synthetic zeolite should contain at least six ppm silica, otherwise the raw water will tend to leach out the silica content in the zeolite, making the gel decompose.

When the synthetic gel and the cation resin products are converted to the manganese form, not all the ion exchange sites become Mn sites. A good portion are still sodium exchange sites available for softening. For this dual function, a two-stage regeneration process is used. First, the system goes through the KMnO̟ 4 regeneration and rinse down. Next, in a separate brine tank, the usual salt regeneration step takes place with rinse down and return to service. All of the manganese-coated media for catalyst/filtration products are very pH-sensitive, and each calls for a pH of 7.0 or higher in the influent water.

Catalyst/filter media installed on water supplies with low pH due to the presence of mineral acids or excess carbon dioxide will not function properly. First, the acid will prevent precipitation of the iron or manganese oxides, allowing a portion of these ionic species to pass through the filter system unchanged. Second, the low-pH acid condition will strip away and destroy the manganese coating on the filter particles on any of these media, necessitating catalyst/filter media replacement. Catalyst/filter media systems should not be placed in service where the influent pH is below 7.0. The most effective iron removal results with catalyst/filter media will be obtained where the inlet water has been adjusted to 7.5 pH or above. For the most efficient manganese reduction, a pH of 8.5 is recommended. 26

Calcite also can be used as a catalyst-sacrificial iron removal medium. This product, when used for pH modification on an iron-bearing water, will, as a rule, remove one-third of the total ferric and ferrous iron present, as well as increase the pH value of an acid water. The calcite mineral precipitates dissolved ferrous iron in the upper portion of the bed. Acting as a filter, the calcite bed will then screen out ferric iron particulate material.

In this combined function, a higher backwash rate of the calcite medium is needed to fully flush out the bed periodically at the rate of 10 to 12 U.S. gpm (38-45 L/min.). As soon as there is a 10 percent decrease in bed depth (due to calcite dissolving), replacement calcite should be added.