Part 3: Adsorption and chemical alteration techniques
By Ed Butts, PE, CPI
During the past two months of Engineering Your Business we’ve introduced you to various technologies associated with removing unwanted solid material from water and wastewater.
Although the title has been “Solids Separation,” the topic has been expanded to include the removal of undesirable dissolved substances as well as solids from a liquid, usually water. Last month, a more detailed discussion of two of these methods—the straining method using one or more layers of a granular media, and the barrier method using physical impediments such as inline screening devices—to accomplish solids removal were outlined.
This month, we’ll wrap up this topic with the last two methods of dissolved solids removal: adsorption and chemical alteration.
ADSORPTION
This certainly won’t be the first nor probably the last article in an EYB column regarding the science of adsorption. As with other water-related technologies, it is an important aspect of water treatment and one that requires detailed discussion and explanation.
It is interesting to note both adsorption and chemical alterationgenerally use the additional process of granular media, or straining (described in Part 2), to facilitate their solids removal characteristics. Where straining is typically a physical process where the media intercepts and traps particulate matter
within the filter media, adsorption and chemical alteration use subtle changes in chemistry to assist removal of fine and dissolved particulates from a fluid.
As was outlined in Part 1, adsorption is technically defined as the adhesion of a gas, vapor, or dissolved material onto the surface of a solid. Various dissolved organic compounds in water—herbicides, pesticides, industrial waste products—can pose a significant health threat and affect the taste and odor of drinking water.
To remove them, the process of adsorption is often used. Adsorption is a process in which one substance is attached to the surface of another substance. The process of “adsorption” should not be confused with the similar-sounding term of “absorption.”
Using an analogy of a sponge, a sponge will readily soak up or “absorb” water containing offending substances, such as a taste and odor, and when the water is squeezed from the sponge, the taste and odor will generally remain in the water. This process constitutes “absorption.”
In an “adsorption” process using an activated carbon media, water with taste and odor substances is brought into contact with the large pore spaces present on and around carbon particles. The taste and odor constituents “stick to” and remain with the carbon after the water leaves the carbon particles—
due to the adhesion between the offending substances and the surface of the carbon media. This results in water free from bad taste and odor.
More simply stated, the process of absorption involves the volume of the entire “sponge” to soak up material, while adsorption uses the surface or outer area of the “sponge” to soak up material. The ability of a specific material to effect adsorption is known as the affinity of the material.
Adsorption is not only used for taste and odor treatment of water, it is also extensively used to remove various other contaminants, many potentially serious and life threatening, from water as well. This group of contaminants include many of the synthetic and volatile contaminants listed on the “EPA Regulated Water Contaminants” list.
Technically, adsorption refers to the collecting of molecules by the external or internal surface of solids or by the surface of liquids. Occasionally, the word “sorption” is used to indicate the taking up of a gas or liquid by a solid without specifying whether the process is adsorption or absorption.
Adsorption can be either physical or chemical in nature. Physical adsorption resembles the physical condensation of gases to liquids and depends on the physical attraction between the solid adsorbent and the adsorbate molecules. There is no chemical specificity in physical adsorption. Any gas willtend to be adsorbed on any solid if the temperature is sufficiently low enough or the pressure of the gas sufficiently high enough.
In chemical adsorption, gases are held onto a solid surface by the chemical forces that are specific for each surface and each gas. Chemical adsorption occurs usually at higher temperatures than those at which physical adsorption occurs. Furthermore, chemical adsorption is ordinarily a slower process
than physical adsorption and, like most chemical reactions, frequently involves an energy of activation.
Adsorption is present in many natural, physical, biological, and chemical systems. It is widely used in industrial applications such as activated charcoal, capturing and using waste heat to provide cold water for air conditioning and other process requirements. Adsorption, ion exchange, and chromatography
are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column. The two most common methods of adsorption in water treatment use zeolites or activated carbon.
Zeolites
Zeolites are natural or synthetic crystalline aluminosilicates, which have a repeating pore network and release water at high temperature. They are manufactured by a synthesis of sodium aluminosilicate or another silica source in an autoclave followed by an ion exchange process with certain cations (such as sodium or calcium). The ion exchange process is followed by drying of the crystals, which can be
pelletized with a binder to form macroporous pellets.
Zeolites are used in the drying of process air, carbon dioxide removal from natural gas, air separation, catalytic cracking, and catalytic synthesis and reforming.
Non-polar (siliceous) zeolites are synthesized from aluminum-free silica sources or by dealumination of aluminumcontaining zeolites. The dealumination process is performed by treating the zeolite with high temperature steam, at a temperature typically higher than 930°F (500°C). This high temperature
heat treatment breaks the aluminum-oxygen bonds and the aluminum atom is expelled from the zeolite framework.
In addition to adsorption, zeolites were at one time widely used as ion exchange beds in domestic and commercial water purification, softening, and other applications. They are also still widely used as catalysts and sorbents. Their well-defined pore structure and adjustable acidity make them highly active
in a large variety of reactions. These naturally-occurring and synthetic inorganic minerals are anions (ions with a negative electrical charge); therefore, zeolites can exchange only cations (ions with a positive electrical charge) such as sodium (Na) or calcium (Ca2+).
Since the ion exchange capacity of zeolites is not very high, a group of synthetic organic (carbon-based) ion exchange resins were artificially developed during the mid-20th century in an effort to make water softeners more efficient and marketable.
Activated Carbon
Activated carbon is a highly porous, amorphous solid consisting of microcrystallites with a graphite lattice (structure), usually prepared in small pellets or as a powder. Although quite inexpensive to produce, one of its main drawbacks in use is that it reacts with oxygen at moderate temperatures.
Activated carbon can be manufactured from carbonaceous material, including coal, peat, wood, or nutshells (coconut). The manufacturing process consists of two phases: carbonization and activation. The carbonization process includes drying and then heating to separate the by-products, including tars
and other hydrocarbons, from the raw material as well as to drive off any generated gases. The process is completed by heating the material to over 750°F (400°C) in an oxygen-free atmosphere that cannot support combustion.
The carbonized particles are then “activated” by exposing them to an oxidizing agent, usually steam or carbon dioxide at high temperature. This agent burns off the pore-blocking structures created during the carbonization phase and they then develop a porous, three-dimensional graphite lattice structure. The size of the pores developed during activation is a function of the time they spend in this stage. Longer exposure times result in larger pore sizes. The most popular aqueous phase carbons are bituminous because of their hardness, abrasion resistance, pore size distribution, and low cost. All the same, their effectiveness needs to be tested in each application to determine the optimal product.
Besides water treatment, activated carbon is also used for the adsorption of organic substances and non-polar adsorbates and is also used for wastewater treatment. It is the most widely used adsorbent since most of its chemical and physical properties (pore size distribution and surface area) can be fine-tuned according to what is needed. Its usefulness also derives from its large micropore volume and the resulting high surface area of contact.
Activated carbon is commonly used in water treatment for the removal of taste and odor offending compounds or organic materials. It can be found in two forms: granular activated carbon (GAC) or powdered activated carbon (PAC). GAC (Figure 1) is generally used as a granular media bed in a filter vessel where water comes into contact with the material as it flows through the bed. PAC is typically injected into a flow stream where the material attaches to the offending substance, causing the taste and/or odor substance to be removed.
One particle of activated carbon has an extremely large surface area owing to its structure and diversity of pores, similar to those found in a sponge. PAC, a finely ground charcoal, is used for this process. When PAC is added to the water, the organic compounds attach to the surface of the powder granules.The granules of PAC have irregularly-shaped surfaces, which gives PAC a very large surface area to attract organic compounds. It is estimated 1 pound of PAC has a surface area of up to 100 acres. The carbon can then be removed by filtration, taking the unwanted organic compounds with it.
CHEMICAL ALTERATION
The process involving chemical alteration is the last process of solids separation we will discuss in this series. Although this process is not technically the same solids removal process as the barrier or strainer methods, it is often used to alter the characteristics of water to make the removal of solids or substances in the liquid much easier to remove—thus the connection to our theme. Chemical alteration typically involves one or more of the following processes: (1) precipitation, (2) oxidation, (3) ion exchange.
Precipitation
Chemical precipitation in water and wastewater treatment is the change in form of materials dissolved in water into solid particles. Chemical precipitation is used to remove ionic constituents from water by the addition of counter-ions to reduce the solubility. It is used primarily for the removal of metallic
cations, but is also used for the removal of anions such as fluoride, arsenic, cyanide, and phosphate as well as organic molecules such as the precipitation of phenols.
The precipitation process starts with the addition of chemical precipitants, including coagulants and polymers, followed by a second process, flocculation, that is used to increase the particle size through a third process called aggregation. The specific precipitation process can change very fine particles
that are normally held in suspension by electrostatic surface charges into larger and heavier particles that readily settle or are removed by filtration due to their increased mass. These initial electrostatic forces cause clouds of counter-ions to form around the particles, giving rise to repulsive forces that prevent aggregation and reduce the effectiveness of subsequent solid-liquid separation processes (filtration). Therefore, chemical coagulants are often added to overcome the inherent repulsive
forces of the particles.
The three main types of coagulants are inorganic electrolytes (common water treatment agents such as alum, lime, ferric chloride, and ferrous sulfate); organic polymers; and synthetic polyelectrolytes with anionic or cationic functional groups. Typically, the addition of coagulants is followed by a low-shear and gentle mixing device called a flocculator, used to promote and enhance contact between the particles, allowing larger particle growth through the sedimentation phenomenon referred to as flocculant settling.
Flocculant settling refers to a dilute suspension of particles that coalesce, or flocculate, during this operation. As flocculation occurs, the particles continue to combine and increase in mass and settle at an increasing rate. Following flocculation, water is directed to another downstream basin, referred to as
a sedimentation basin. Here, the water is allowed additional time needed to complete the process of gravity settling of the larger particles before the water is directed to the final process—filtration, usually a form of granular media or straining filtration where the remaining finer and aggregated particles
are removed through this final solids separation process. The entire process is shown in graphical form in Figure 2.
The amount of particle removal that occurs depends on the opportunity for coagulant and flocculator contact, sedimentation, and filtration, which varies with factors and specific design criteria—such as the coagulant selection and injection rate, overflow rate, the depth and cross sectional area of the basins, the velocity gradients in the system, the raw and finished water concentrations and range of particle sizes, the time provided for sedimentation, and the filtration rate. The effects of these variables can only be accomplished by theoretical or practical determinations in jar, sedimentation, and pilot tests.
Precipitation or a variable of the process is the most common form of water treatment for surface water supplies and is rapidly becoming one of the most widely selected methods for the removal of heavy metals from groundwater in pump-and-treat operations. It is also used as a pre-treatment process with
other chemical alteration technologies (such as chemical oxidation), where the presence of metals could possibly interfere with treatment. After water is pumped to the surface, precipitation converts the soluble heavy metals to insoluble metals that readily settle or are filtered out of the water.
Oxidation
The majority of oxidation systems involve the removal of iron and manganese and employ the dual treatment processes of oxidation and filtration (Figure 3). The oxidant used, such as chlorine or ozone, chemically oxidizes and converts the state of the offending material (typically iron or manganese),
forming a filterable particle, while at the same time it also kills iron bacteria and any other disease-causing bacteria that may happen to be present.
The filter then removes the particles (precipitates) present in the bed through granular media or other types of filtration. Backflushing or backwashing of the filter is occasionally conducted to remove the filtered material from the filter. Generally, the backflushing procedure is initiated by a pressure
or head differential between the inlet and outlet of the filter. Oxidation followed by filtration is a relatively simple process; however, the levels of contaminants in the source water must be monitored to determine and inject the proper oxidant dosage. The treated water must also be monitored to determine
if the oxidation process was thorough and complete enough and therefore successful.
Oxidant Selection
Before iron or manganese can be removed through filtration processes, they must be oxidized to the electrochemical state needed to form an insoluble (and filterable) compound. Oxidation involves the electrochemical transfer of electrons from the iron or manganese molecule to the oxidizing agent.
For iron, ferrous iron (Fe2+) is oxidized (or chemically stated, is reduced, shown as →) to ferric iron (Fe3+), which then readily forms the insolublecomplex of iron hydroxide: Fe(OH)3. For removal of
manganese, reduced manganese (Mn2+) is oxidized to manganic oxide (Mn4+), which then forms the insoluble manganese dioxide (MnO2) compound.
The most common chemical oxidants used in water treatment are air, chlorine, chlorine dioxide, potassium permanganate, and ozone, along with the use of special oxidizing coatings on media.
Oxidation using chlorine or potassium permanganate is frequently applied for small to mid-size groundwater systems, as the dosing is relatively easy, requires simple and accurate dosing equipment,
and is fairly inexpensive to use.
Chlorination is widely used for the oxidation of iron and manganese, but the possible formation of trihalomethanes in waters with high levels of organics may cause a potential health problem. Chlorine feed rates and contact time requirements can be determined by the use of theoretical calculations
and confirmed by using either jar testing or trial-and-error methods for small systems.
As an oxidant, potassium permanganate (KMnO4) is normally more expensive than chlorine and ozone but, for iron and manganese removal, it has been reported to be as efficient (often more efficient for manganese) and requires considerably less equipment and capital investment than ozone. However,
the required dose of potassium permanganate must be carefully controlled as too little permanganate will not fully oxidize all of the iron and manganese. Too much of an overdose can allow the excess permanganate to enter the distribution system, which can result in a slight pink color or tinge to
the water.
Excessive levels of permanganate can also form group precipitates that can cause mudball formations to form in filters, which are heavier and therefore more difficult to remove. This can compromise filter performance by creating large voids or openings within the filter bed, resulting in possible “shortcircuiting” of the water through the bed, which can often lead to incomplete or inadequate treatment of the raw water.
Ozone may also be used in place of chlorine for iron or manganese oxidation, but it may not be as effective for oxidation in the presence of organic acids in the raw water, such as humic, tannic, or fulvic materials. In addition, if not dosed carefully, ozone can oxidize any reduced manganese that may
already be present and revert it to permanganate which can result in the same pink water condition previously cited.
Manganese dioxide particles, also formed by the oxidation of reduced manganese, must be carefully coagulated to ensure their removal by the filter media. One low-cost method of providing oxidation is through use of oxygen, already present in atmospheric air, as the primary oxidizing agent (aeration).
This can be performed through various methods, such as using an inline venturi or air eductor incorporated into the flowstream or through the injection of air between the well and filter with an oil-less air compressor. As the well pump operates, air is injected and mixed with the water, which
oxidizes the iron and manganese to form a large enough and filterable floc.
The principal advantage to this method is no chemical dosing is required, which allows for unattended and safe operation. But this method is not as effective for water in which the iron is combined with any large organic molecules, since oxygen is not a strong enough oxidizing agent to break the strong
attractions formed between iron and manganese and these large organic molecules.
Furthermore, the amount of oxygen in dry air is generally around 21% by volume (23% by weight), so added air (four to five times the volume of required oxygen) must be introduced to inject the amount of oxygen needed for effective oxidation. The rate of reaction between atmospheric oxygen and iron
and manganese is very slow below pH values of 6.5 and 9.5, respectively, so the application is also limited to those with long contact times and/or higher pH values of >8.2.
Finally, the presence of any additional contaminants, such as hydrogen sulfide, that may increase the oxidant demand must also be accounted for when applying and dosing the oxidant. In general, manganese oxidation is more difficult than iron oxidation because the reaction rate is much slower and
the process is much more pH-dependent. A longer detention time (up to 30-45 minutes) following the addition of the oxidant may be needed prior to filtration to fully allow the reaction to take place.
Ion Exchange (Water Softening)
The final process on the topic of chemical alteration and solids separation involves the well-established method of ion exchange, often referred to as water softening (Figure 4). Although various other methods exist for the so-called softening (sometimes referred to as conditioning) of water, the most common methods used for water well applications are known as ion or anion exchange.
Ion or anion exchange depends on the existence and use of a special type of electrochemical filtration media (usually synthetic), called a resin, which is manufactured with inherent and permanent positive (ion) or negative (anion) electrical charges that are capable of attracting and holding onto ions with an opposite charge—positive to negative and vice versa.
Organic ion exchange resins can be formulated with either anionic or cationic materials of many types, yielding resins with nearly every conceivable functionality and exchange strength and capacity. This means there are now resins formulated specifically to exchange nitrate or fluoride ions, preferentially, as well as for the more common calcium or magnesium ions, or even for the exchange of uranium and
plutonium ions from nuclear wastes—and the potential list goes on.
Ion exchange is most efficient when the media is contained and used inside of cylindrical beds and the incoming water is directed to flow downward (aka: downflow) through the resin. The exchange of ions that occurs between the beads of the resin bed to the contaminant ions in the raw water occurs at
a very rapid pace in the upper reaches of the bed, but there is a transport zone of activity that slowly moves down the bed during operation. Above this zone, all of the previously active sites within or above the resin zone are now said to be exhausted, but below the zone, the resin bed remains in a completely regenerated and usable form to enable exchange.
The thickness or height of the exchange zone is determined by the flow rate—the slower the downflow rate, the narrower the zone. Just as the “per unit (area) flow rate” (rated in gallons per minute per square foot, GPM/ft2) is a critical factor for the hydraulic design of mechanical or granular media filtration, the “per unit (volumetric) flow rate” (rated in gallons per minute per cubic foot, GPM/ft3) is the relevant parameter for the electrochemical or “ion exchange” performance of a media used for adsorption or ion exchange. The usual unit flow rate for ion exchange systems is about 2 GPM/ft3 per bed volume or around 16 bed volumes per hour.
Sodium cycle cation exchange is the most commonly used and “standard” method of residential ion exchange, in which the “hardness” (comprised mostly from ions of calcium and magnesium) is exchanged for sodium ions to produce a “softened” or “conditioned” water result. It is made possible by the availability of an ion exchange resin comprised of individual beads with millions of “active sites” made from a chemical group called sulfonate. Since the sulfonate is electrochemically an anion [made with a negative (–) charge of attraction], it can only exchange cations [ions with a positive (+) charge
or attraction]. As such, this particular chemical arrangement produces a strong affinity for calcium and magnesium (both strong cations) and a weaker affinity for sodium.
The resin is initially treated with around a 5% or 50,000 mg/L strength of a sodium chloride (Na+ + Cl–) salt brine solution. This concentration of sodium (Na) completely overwhelms the effect of any other cations that might be in the water, so the resin loads up with the sodium ion. During the “in service” operation of the softener, this now fully regenerated resin is exposed to the incoming hard water containing the calcium and magnesium ions. The resin employs its electrochemical preference and exchanges two of the sodium ions for each of the individual calcium or magnesium ions that it
encounters—at least so long as there are any sodium ions remaining attached to the resin itself.
When all of the sodium ions are removed from the bed, the resin becomes exhausted and is ready for a bed regeneration (not to be confused with a backwash), which is performed by applying a new concentration (50,000 mg/L) of the salt brine solution to the resin bed. In order to ensure the bed is fully regenerated for the next cycle, the strong brine solution needs to flow through the resin bed for a continuous period of up to 15-30 minutes. This ensures all of the calcium and magnesium ions attached to the resin have the opportunity to be displaced by the overwhelming presence of sodium ions from the resin bed and removed from the softener unit.
This “cycle timing”—which includes the time required for the necessary initial period of backwashing, followed by the regeneration process, and finally the final rinse and resettling of the bed before returning the unit to service—is initiated and performed automatically by the control head and valve and is
started based on either the total water volume passed through the unit (demand-based regeneration) or from a preset automatic timer (automatic regeneration).
Another important design factor that must always be considered is the possible presence of other potential contaminants in the raw water supply. For example, water-softening applications that include iron or manganese in the incoming water supply can result in “fouling” or an eventual plugging of the resin bed over an extended period of operational time.
It is critical the designer fully evaluate any applications that may include iron, iron bacteria, manganese, or hydrogen sulfide in the raw water supply. It is also crucial to use a resin designed and approved for the removal or tolerance of these contaminants, and only allow the use of what is called “ironresistant”
or “iron-out” water softener salt in the unit—salt especially formulated for this type of application.
In many cases, pre-treatment of the raw water to remove iron, manganese, and hydrogen sulfide is indicated to lower the demand on the water softener, improve service, and extend the operational life. Although salt (sodium chloride) has historically been used for creating brine solutions, increasing
concerns due to the potential health impacts caused by higher sodium ingestion, as well as increased salt levels in sewage treatment and wastewater collection systems, have seen a recent increase in the use of alternatives such as potassium chloride (KCl) for creating brine solutions. Although more expensive to use, potassium chloride negates the health concerns related to the ingestion of sodium and potential problems to the wastewater system and replaces it with the far more beneficial health impacts gained from the use of potassium.
As a reminder—depending on the size, application, and treatment intent—water softeners (either ion or anion exchange units) are capable of discharging large volumes of backwash and “rejected ion” water with high levels of removed, sometimes hazardous, contaminants that must be accepted by a local sewer system, septic tank/drain field, or other type of wastewater receiving facility. To guard against any long-term problems, the designer must remember to fully evaluate and confirm the discharged wastewater will not adversely impact the septage receiving facility nor interfere with the ongoing wastewater treatment process.
Ion exchange is most efficient when the media is contained and used inside of cylindrical beds and the incoming water is directed to flow downward.
In addition to ion exchange, another process is also used for the softening of water—lime softening or more commonly called lime-soda softening. As lime in the form of limewater is added to raw water, the pH of the water is raised and the equilibrium of the carbonate species in the water is shifted. Dissolved carbon dioxide (CO2) is changed into a bicarbonate (HCO–3) species and then into carbonate (CO2-3). This action causes calcium carbonate to precipitate due to exceeding the associated solubility product. Additionally, magnesium can be precipitated as magnesium hydroxide in a double displacement
reaction. In this process both the calcium and, to an extent, magnesium in the raw water as well as the calcium added with the lime are precipitated.
This is in contrast to ion exchange softening where sodium is exchanged for calcium and magnesium ions. In lime softening, there is also a substantial reduction in the level of total dissolved solids (TDS) whereas in ion exchange softening, there is no significant change in the level of TDS. Lime softening
can also be used to remove other harmful materials from water—including iron, manganese, radium, and arsenic—although the process requires an intimate knowledge of water chemistry and the proper use and balance of chemicals. Lime-soda softening is not used as commonly as ion exchange softening in today’s water treatment environment due to the ability to now synthesize various types of resin and the
complexity with the process of lime-soda.
This concludes part three of our series on solids separation methods. I hope it has been informative for you so far. We will wrap up the series next month with a fourth installment providing more detail on microfiltration and ultrafiltration membrane methods.
Until then, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 35 years experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.