Iron & Manganese Removal Methods

Next to hardness, the presence of iron is probably the most common water problem faced by consumers and water treatment professionals. The secondary (aesthetic) maximum contaminant levels (MCL) for iron and manganese are 0.3 milligrams per liter (mg/l) and 0.05 mg/l, respectively. Iron and manganese in excess of the suggested maximum contaminant levels (MCL) usually results in discolored water, laundry, and plumbing fixtures.

Small amounts of iron are often found in water because of the large amount of iron present in the soil and because corrosive water will pick up iron from pipes. Clothing washed in water containing excessive iron may become stained a brownish color. The taste of beverages, such as tea and coffee, may also be affected by iron. Manganese produces a brownish color in laundered clothing, leaves black particles on fixtures and as with iron, affects the taste of beverages, including coffee and tea.

Well water from the faucet or tap is usually clear and colorless. However, when water containing colorless, dissolved iron is allowed to stand in a cooking container or comes in contact with a sink or bathtub, the iron combines with oxygen from the air to form reddish-brown particles (commonly called rust). Manganese forms brownish-black particles. These impurities can give a metallic taste to water or to food.

The rusty or brown stains on plumbing fixtures, fabrics, dishes, and utensils cannot be removed by soaps or detergents. Bleaches and alkaline builders (often sodium phosphate) can make the stains worse. Over time, iron deposits can build up in pressure tanks, water heaters, and pipelines, reducing the quantity and pressure of the water supply.

Unluckily, iron and manganese can often be quite difficult to treat. This is due primarily to the fact that iron can be present in several forms, and each form can potentially require a different method of removal..

Types of Iron

There are three main forms of iron and manganese. Other types are much rarer:

1. Ferrous - This type of iron is often called "clear water iron" since it is not visible in poured water. It is found in water which contains no oxygen, such as water from deep wells or groundwater. Carbon dioxide reacts with iron in the ground to form water-soluble ferrous bicarbonate, which, in the water, produces ferrous ions (Fe++).
2. Ferric - Ferric iron is also known as "red water iron". This type of iron is basically ferrous iron which has been exposed to oxygen (oxidized), usually from the air. As carbon dioxide leave the water, oxygen combines with the iron to form ferric ions (Fe+++). These oxidized particles are generally visible in poured water.
3. Bacterial Iron - Slime depositing in toilet tanks or fouling water filters and softeners is a good indication of the presence of bacterial iron. Better described as iron bio-fouling, the iron bacteria problem is both complex and widespread. It attacks wells and water systems around the world in all sorts of aquifer environments, both contaminated and pristine. In some places, it causes great damage; in others, it is considered a minor nuisance

Treatment Methods

Iron Bacteria

Iron bacteria can be controlled by periodic well chlorination or it can be treated in the building. The treatment involves the following: Chlorination, retention, filtration. Activated carbon is usually used as the filter material so the excess chlorine can also be removed.

Ferric Iron

In theory, the elimination of ferric iron is simple - use a properly sized media filter to filter it from the water. In practice, however, there may be other issues:

1. Some iron may be present in colloidal form. Unlike ferric iron, which will generally stick together to form large flakes, the tiny particles of colloidal iron do the opposite. Their large surface area and charge relative to their mass causes the individual particles to repel one another. As a result they will not coagulate. Their small size, then, makes them difficult to filter, and a coagulating agent is often required to obtain adequate filtration.
2. Most water containing ferric iron also contains ferrous iron. This can add complexity to the process, since some of the methods for removing ferrous iron will also remove ferric iron.

Ferrous Iron

There are a variety of ways for removing ferrous iron, each with its own strengths and limitations. These methods fall into two categories: Ion exchange and Oxidation / filtration

Ion Exchange (Water Softener)

Ion exchange should be considered only for the removal of small quantities of iron and manganese. For practical purposes in an everyday working softener, the upper limit is about 5 to 7 parts per million. Ion exchange involves the use of synthetic resins where a presaturant ion on the solid phase (the "adsorbent," usually sodium) is exchanged for the unwanted ions in the water. One of the major difficulties in using this method for controlling iron and manganese is that if any oxidation occurs during the process, the resulting precipitate can coat and foul the media. Cleaning would then be required using acid or sodium bisulfate

Where the concentration of iron is above 5 or 6 parts per million, or when there is both dissolved and precipitated iron or manganese in the water, a different approach is needed.

Oxidation / Filtration

Oxidation followed by filtration is a relatively simple process. The oxidant chemically oxidizes the iron or manganese (forming a particle), and kills iron bacteria and any other disease-causing bacteria that may be present. The filter then removes the iron or manganese particles.

1. Oxidation - Before iron and manganese can be filtered, they need to be oxidized to a state in which they can form insoluble complexes. Oxidation involves the transfer of electrons from the iron, manganese, or other chemicals being treated to the oxidizing agent. Oxidation methods fall into two groups: those using additives like chlorine, ozone or air; or those using an oxidizing filter media.
2. Ozonation - An ozone generator is used to make ozone that is then fed by pump or by an air injector into the water stream to convert ferrous iron into ferric iron. Ozone has the greatest oxidizing potential of the common oxidizers. This is followed by a contact time tank and then by a catalytic medium or an inert multilayered filter for removal of the ferric iron.
3. Chlorination - Chlorine can be introduced into water in one of several forms: a gas; as calcium hypochlorite; or commonly, as sodium hypochlorite. The treated water is then held in a retention tank where the iron precipitates out and is then removed by filtering with manganese greensand, anthracite/greensand or activated carbon. If applied this way, a dosage of one part of chlorine to each part of iron is used and 0.2 parts of potassium permanganate per part of iron is fed into the water downstream of the chlorine. The potassium permanganate and any chlorine residual serve to continuously regenerate the greensand.
4. Aeration - Air is also used to convert dissolved iron into a form that can be filtered. This approach mimics what happens when untreated dissolved iron comes into contact with the air after leaving a faucet. Aeration methods can be of a two-tank or a single-tank variety. In a two-tank system, air is introduced into the first tank using a pump or other injection device. The dissolved iron precipitates in the first tank and is carried into the second tank where it is filtered in a Birm or multi-media filter. One drawback to this system is that water bearing the precipitated iron goes through the head of the first unit and the piping between the units. Particularly at lower flow rates, the sticky ferrous hydroxide tends to foul the valve on the first unit and may require cleaning every 6-24 months. A single-tank system essentially combines the two tanks of a single tank system into one. The iron is oxidized at the top of the tank before falling into the filter medium at the bottom. There is no potential fouling of the head. The iron is filtered before it goes through the outlet port of the valve. For very high levels of iron, chlorination with continuous regeneration is the only practical approach.

Oxidizing Filtration Media

1. Manganese Greensand - the most common chemical oxidant used, it has a relatively high capacity for iron removal and can operate at high flow rates with moderate backwash requirements. Greensand is a processed material consisting of nodular grains of the zeolite mineral glauconite. The material is coated with manganese oxide. The ion exchange properties of the glauconite facilitates the bonding of the coating. This treatment gives the media a catalytic effect in the chemical oxidation reduction reactions necessary for iron and manganese removal. This coating is maintained through regeneration with potassium permanganate – about 1.5 to 2 oz. per cubic foot of greensand.
2. Birm - acts as a catalyst to promote the reaction between the oxygen and iron dissolved in the water. It requires no regeneration but needs a relatively high level of dissolved oxygen and works best at a pH above 6.8.
3. Pyrolox - a natural ore that oxidizes and then filters the resulting insoluble iron. It does not need to regenerate, therefore, it doesn’t need other chemicals. However, it needs, ideally, to backwash at 25 to 30 gallons per sq. ft.

Iron - Removal P&ID

Iron Removal from Water

Iron is one of the most abundant metals of the Earth's crust. It occurs naturally in water in soluble form as the ferrous iron (bivalent iron in dissolved form Fe²⁺ or Fe(OH)⁺) or complexed form like the ferric iron (trivalent iron: Fe³⁺ or precipitated as Fe(OH)₃). The occurrence of iron in water can also have an industrial origin ; mining, iron and steel industry, metals corrosion, etc.

In general, iron does not present a danger to human health or the environment, but it brings unpleasantness of an aesthetic and organoleptic nature. Indeed, iron gives a rust color to the water, which can stain linen, sanitary facilities or even food industry products. Iron also gives a metallic taste to water, making it unpleasant for consumption. It can also be at the origin of corrosion in drains sewers, due to the development of microorganisms, the ferrobacteries.

In aerated water, the redox potential of the water is such as it allows an oxidation of the ferrous iron in ferric iron which precipitates then in iron hydroxide, Fe(OH)₃, thus allowing a natural removal of dissolved iron.

4 Fe2+ 3 O2 --> 2 Fe2O3

Fe2O3 + 3 H2O --> 2Fe(OH)3

The form of iron in water depends on the water pH and redox potential, as shown in the Pourbaix diagram of Iron below. Usually groundwater has a low oxygen content, thus a low redox potential and low pH (5.5- 6.5)

However ground waters are naturally anaerobic: so iron remains in solution and therefore it is important to remove it for a water use.

The elimination of the ferrous iron, by physical-chemical way, is obtained by raising the water redox potential by oxidation thanks to oxygen of the air and this by simple ventilation. In the case of acid water, the treatment could be supplemented by a correction of the pH. Thus, the ferrous iron is oxidized in ferric iron, which precipitates in iron hydroxide, Fe(OH)₃. The precipitate is then separated from water by filtration on sand or decantation. The stage of precipitation by chemical oxidation can also be carried out with the stronger oxidants such as the chlorine dioxide (ClO₂), ozone (O₃) or the potassium permanganate (KMnO₄).
This elimination can be carried out by cascade or spraying open-air systems (for an acceptable maximum content of Fe²⁺ of 7mg.L^-1) known as gravitating systems. Those systems require a significant place on the ground, but, in addition to an easy and a cheap exploitation cost, they also make possible aggressive CO₂ and hydrogen sulfide (H₂S) removal. There are also pressure systems, which in addition to their compactness, make possible to treat water whose Fe²⁺ concentrations between 7 and 10mg.L^-1.

Iron is often found in water in complexed forms. In order to be eliminated, iron complexed requests a coagulation stage, which comes in between oxidation and filtration.

Remark : Thanks to microorganisms, it is possible to remove iron from water by biological way. Indeed, there are many bacteria, whose metabolism and thus their survival, are related to the oxidation of iron. However this biological removal requires conditions specific for the pH, the temperature, the redox potential, etc

Membrane Bio-Reactor - Oman

Oman's Haya Water plans next week to officially open the Al Ansab water treatment plant, which has a large membrane bioreactor (MBR) at its heart and is the main part of the Bausher Water Reuse Project.

Commenting on the forthcoming event on 19 February 2011, Haya's CEO, Omar Al-Wahaibi, said that the current stage was the cornerstone of the water-reuse project, adding that the company was currently working on the completion of the main trunk line to connect areas from Al-Hamria Roundabout to the complete line near Al Ghubra Power & Desalination Plant.

The production capacity of the plant is about 55,000 m³/d of treated water in the first stage, which will be followed by other stages with the production capacity increasing according to requirements. Once commissioned, it will be one of the largest of its kind worldwide using MBR technology.

Solar-Powered Aeration Circulation - Saved $100,000 + $20,000 yearly utility costy

In the Buckeye State, the village of St. Henry, Ohio, is bucking the trend of many rural farming communities. Instead of losing residents and local businesses to bigger cities, this community 40 miles northwest of Dayton is growing. Local industries such as turkey processing are expanding as well. Consequently, the city of 2,700 was outgrowing the capacity of its wastewater treatment plant. But St. Henry is still a small town with a limited tax base, so when city officials learned that new wastewater treatment equipment would cost almost half a million dollars and add $49,000 in annual energy costs, they looked elsewhere for a solution. 

Not only did they find an answer, they also saved on their utility bill. Instead of adding energy-consuming aeration equipment, they saved $100,000 in capital equipment costs by adding solar-powered circulation equipment to the existing plant. SolarBee circulation equipment thoroughly mixes the ponds and significantly reduces energy consumption.

“The problem with the old system was that it was undersized,” said Stan Sutter, public utilities supervisor for St. Henry. “We needed more aeration and we needed a new cell for extra storage capacity. Originally, we considered installing a diffuser blower system that would replace the surface aerators.” Three big blowers would blow air through a common header along the whole outside of the lagoon. “But the blower system would have increased our horsepower by 30 percent and our utility bill as well. We were determined to solve our problems without all that extra expense.”

The treatment plant not only had to meet the needs of the growing population, it also had to smell better. The solution that St. Henry staff and consulting engineers developed was to reconfigure the first pond, reduce mechanical aeration run time and add solar-powered circulation to thoroughly mix the ponds and reduce odors.

Solar-powered circulation technology can displace up to 40 hp of grid-powered mixing energy.The unique technology from SolarBee combines solar power with long-distance, near-laminar-flow circulation to provide radial, horizontal and vertical pond mixing. SolarBee circulators operate day and night to circulate and mix wastewater ponds of all types. They help to conserve dissolved oxygen by mixing and distributing saturated surface water throughout the pond, replacing 20 to 40 horsepower of aeration and mixing run time per unit.

As a result of this mixing efficiency, St. Henry officials can offload a significant portion of their energy-intensive aeration and mixing functions, while significantly reducing biochemical oxygen demand (BOD), total suspended solids (TSS) and ammonia. Near-laminar-flow circulation also solved the odor problem and reduced sludge buildup.

SolarBee circulation equipment is designed around pumps capable of moving up to 10,000 gallons per minute, or 14.4 million gallons per day (mgd). Because of the unique hydraulic design, the system uses only 36 watts to power a one-half horsepower direct-drive motor that is 90 percent or more efficient. Three 80-watt photovoltaic panels charge an onboard battery, thus enabling the units to run day and night on solar power.

Reconfigured plant eliminates odor problems and saves $20,000 annually in utility costs

Reconfiguring St. Henry’s wastewater treatment plant meant dividing the first pond into two sections. The purpose of dividing the eight-acre, rectangular-shaped pond was to concentrate the treatment in a small, total-mix, high-solids lagoon and essentially create an activated-sludge basin without the expense of building one. In section A of the first pond, eight aerators and one SolarBee unit concentrate the aeration and provide deeper mixing. Approximately 800 to 1000 mg/l of carbon biochemical oxygen demand (CBOD) run through the first cell and about 100 mg/l of CBOD leave the pond after a three-day detention time.

After the first pond, the system returns to a facultative system. During the day, the gentle and continuous mixing of the SolarBee units brings nutrients to the surface of the pond to promote an increased and highly beneficial algae growth. The algae produce energy-free pure dissolved oxygen—up to 250 lbs. of dissolved oxygen per acre per day. A high pH also occurs near the surface. The high dissolved oxygen and high-pH water is continuously and thoroughly mixed throughout the pond, instead of being mostly underutilized, as in all-natural ponds. The higher dissolved oxygen throughout the pond helps reduce BOD by 70 to 90 percent. And, most noticeable to St. Henry residents is the fact that the bad odors have been virtually eliminated.

In total, seven SolarBee units and 11 aerators mix and aerate the lagoons to meet permit requirements. A new fourth pond was also installed, which provides an additional 67 million gallons of storage. By the time wastewater reaches the fourth pond, there is not enough carbon, ammonia or phosphorous to cause permit problems, or to support algae growth high enough to result in BOD and TSS problems. Better yet for the St. Henry municipal budget, aeration run-time has been cut by 60 percent and the utility bill by $20,000 per year.

To Stan Sutter, that’s a “green” payback. “Green goes hand in hand with operational cost savings,” he said. “We’re not consuming nearly as much power as we were originally looking at. In fact, we’ve decreased our horsepower rather than increasing it, and any time we can use fewer resources, it benefits the entire community.”

With a renewed wastewater treatment system designed with an eye on saving big bucks, St. Henry is well prepared for future population growth and new industries and to meet stringent regulatory requirements.

Asia's Water Crisis will affect the Economy

If the water crisis in Asia is not addressed it could be a significant setback to the region's economic growth, according to Asian Development Bank (ADB).

ADB says there is a widening gap between demand and supply of water, and that the estimated gap, 40% by 2030, is a reasonable expectation.

In China, India and the Philippines, per capita availability of water per year has fallen below 1,700m³, the global threshold for water stress.

The water shortage could become more serious because 80% of Asia's water is used for irrigation, which could have a serious bearing on food supplies in the region.

According to ADB, the efficiency in water usage in agriculture and industry has improved by only 1% a year since 1990, according to independent.co.uk.

How to Treat oil sands in Water

GE and FilterBoxx have signed an agreement to develop innovative water treatment solutions for the oil sands in Alberta, Canada.

Calgary-based FilterBoxx will work alongside Connecticut-based GE on heavy oil-producing water treatment projects for de-oiling and water treatment options, using in-situ thermal methods such as steam-assisted gravity drainage.

In-situ methods are required because 80% of the oil sands in Alberta cannot be mined through open pit due to the bitumen being too deep under the surface.

The in-situ processes will use thermal energy, steam or solvents to make the bitumen flow so that it can be pumped by a well to the surface.

The new system will give higher recoveries of produced water, requiring 30-50% less water for steam-assisted gravity drainage process compared with 0.3-0.4bbls of water required for each barrel of bitumen produced