Raw Water Treatment III, Sodium Zeolite Softening
Alex McDonald
Steam Generation Systems, Inc.

Raw Water Treatment III PDF

Introduction

Ion exchange is the process in which materials exchange one ion for another, hold it temporarily, and release it to a regenerating solution. These materials are widely used to treat raw water supplies that contain dissolved salts. Today, the most commonly used material is an ion exchange resin. Resins are plastic beads to which a favorable ion has been chemically attached which can be exchanged for unwanted ions dissolved in the raw in the water supply. Once the resin has given up or exchanged most of its favorable ions, it is said to be exhausted and needs to be regenerated by coming in contact with a strong solution of ions called the regenerant.

Sodium zeolite softeners historical used natural zeolite materials but have now been replaced with exchange resins made of polystyrene. These resins have sodium ions loosely attached and will readily give up the sodium for a more desirable ion such as calcium and magnesium. This exchange is only for cations of positively charged ions. This is why sodium zeolite resin is referred to as a cation exchange resin. .

The primary exchange reactions occurring at the surface of a sodium cation exchange resin are

...where "R" represents the solid resin.

The water to be softened is passed through the vessel containing resin. Calcium and magnesium ions are exchanged for the sodium ions in and on the resin beads. The sodium then takes the place of the calcium and magnesium in associating with the anionic species in solution to provide charge balance.

Although sodium zeolite treated water is nearly free of all hardness, some hardness leakage normally does occur. The amount of leakage depends primarily on the raw water hardness, sodium concentration and softener regeneration level.

A plot of the softener effluent profile shows a low, nearly constant effluent hardness level until the ion exchange resin nears exhaustion. At this point, the hardness level usually increases quite rapidly and resin regeneration is required.

The resin bed is regenerated by reversing the softening reactions. Exhausted sodium zeolite resin is regenerated by exposure to a concentrated brine solution - sodium chloride. Analogously, strong acid cation resin is regenerated by exposure to a strong acid (H2SO4).

Concentrated brine used for sodium zeolite regeneration is 26% by weight sodium chloride. However, because of the dilution of the brine as it is forced through the softener vessel, the actual application strength of the brine is closer to 10% when it reaches the resin bed.

Various cations are exchanged at resins to different degrees. The order of preference or selectivity for a specific cation to be exchanged is a complex function of charge and strength of solvation. In dilute solutions, the general rule is that ions with the highest charge and the lowest strength of solvation are preferentially exchanged. A partial list of relative affinities for ion exchanged at cation exchange resins are contained in the adjacent table.

Equipment

Softener systems consist of a tank, valves and a means of transporting the brine to the softener tank. Usually the softener tank is a steel, vertical shell, pressure vessel with dished heads. The interior of the tanks are sometimes lined with corrosion resistant coatings to prevent attack by the
brine regenerating solutions. The tank includes a service and rinse water inlet distributor, freeboard or the headspace from the top of the resin tank, a regenerant distribute, a bed of ion exchange resin, and sometimes a supporting medium or outlet distribution system.

The inlet distributor located in the top portion of the tank, consists of a baffle plate or hub and lateral system similar to the spokes on a wagon wheel. Strainers are frequently incorporated as parts of the lateral design. Special care is taken to avoid directing the water against the tank wall that could cause channeling of the flow around the tank perimeter.

The inlet assembly is designed to distribute the incoming water, preventing it from impinging in the resin bed and hollowing out cavities that would cause the flow to channel, reducing capacity.
and effluent quality. The distributor also acts as a collector for the backwash water that goes into the sewer.

The freeboard space allows the resin to expand without loss to drain during backwashing and should be at least 100% of the resin bed volume.

The regenerant distributor, which is normally located about 4 to 6 inches above the resin bed, usually consists of a header lateral system that spreads the brine uniformly over the resin.

The bed of resin, operating in the sodium cycle, softens the water. The quantity of water to be treated per regeneration, flow rate, and the regenerant level employed. A minimum bed depth of 24 inches is recommended for all systems.

The underdrain system, located in the bottom of the softener, includes the media, which support the resin bed, and/or underdrawn strainers. It collects the treated water, waste brine and rinse water, and distributes the backwash water. The underdrawn system must collect water evenly from all portions of the bed during the service, brine and rinse operations. If the collection is not uniform, channeling may occur which will lower capacity and increase hardness leakage. Resin waste may occur either through carryover with the backwash water or as the result of an upset in the support material.

Softener Operation

A sodium zeolite softener operates through two basic cycles: the service cycle, which produces soft water, and the regeneration cycle, which restores the exhausted resin to capacity:

During the service cycle, raw water enters the softener through the inlet distributor, flows through the resin bed, is collected by the underdrawn system and is transferred to the point of use. The flow to the softener should be as constant as possible and sudden large surges or frequent on-off operations should be avoided.

The softening cycle is conducted most efficiently at a flow rate of approximately 6 to 8 gallons per minute per square foot of resin surface area, with all parallel units on line. Equipment manufacturers generally design zeolite softeners for normal operation within this range, but provide for periodic flow rates as high as 15 gallons per minute per square foot. This allows total softened water requirements to be met while parallel units are regenerated.

Continuous operational or above 15 gallons per minute per square foot may result in channeling bed compaction, leakage, or premature hardness breakthrough. Channeling is a very involved subject; it can generally result in poor softener operation and loss in total throughput .

Conditions which can contribute to channeling include poor backwash flow rates, which do not adequately fluff up the resin bed to prepare for regeneration, as well as broken or partially obstructed inlet headers which "channel" the water flow through one particular area of the softener. Operation significantly below normal service flow rates (3 gallons per minute per square foot or less) can also produce difficulties such as decreased capacity and leakage. This potential exists because at low flow rates, water is not forced through the inlet distributor at a velocity high enough for uniform distribution across the whole bed.

Softener Regeneration

Softener regeneration consists of four steps: backwash, brining, slow rinse and fast rinse.

Backwash. During the exhaustion or service cycles, the downward flow of raw water caused suspended matter to accumulate on the resin bed. The resin is an excellent filter medium. Backwashing is an upward flow of water which passes through the underdrain system, up through the resin bed, and out the service water distributor to waste. This reverse flow lifts and expands the resin bed by placing each bead in motion. In this manner, the bed is regraded while particulates and resin fines are removed.

Regrading or classification of the zeolite resin brings smaller beads to the top of the unit; larger beads go to the bottom. This enhances proper brine distribution. Expansion releases material accumulated within the resin bed and fluffs the bed to allow for efficient brine-resin contact. Particulate matter and resin fines must be removed to prevent channeling, high pressure drop and poor kinetics.

Backwashing should be carried out for a minimum of 10 minutes or until the backwash water effluent is clear. The backwash water flow rate should be sufficient to produce a minimum of 50% bed expansion, yet not excessive enough to cause loss of resin.

The percent bed expansion resulting from a set flow rate is a function of the backwash water temperature. At a given flow rate, the lower the temperature, the more the bed is expanded. Due to increased viscosity of the water, adjustments in backwash water flow rates should be made as water temperatures vary seasonably. Backwash rates usually vary from 4 to 8 (ambient temperature) and 12 to 15 (hot service) gpm per square foot of tank area, but each manufacturer's recommendations should be carefully followed.

Brining The brine regenerant stream enters the softener through the regenerant distributor, flows downward through the resin bed, is collected by the underdrain system and then discharged to waste.

Applications and Limitations

The potential for scale and deposit build in boilers has created a large demand for softened water. The ability of the sodium zeolite softener to satisfy this demand economically makes its use appealing for preparing boiler feedwater, and many types of chemical process waters. Compared to other softened methods, sodium zeolite units offer many advantages:

• The treated water has a very low scaling tendency because this method reduces the hardness level of most water supplies to less than 2 ppm.
• Operation is simple and reliable; automatic regeneration controls are available at reasonable cost.
• Regeneration is accomplished with inexpensive, easy-to-handle salt.
• Waste disposal usually presents not problem.
• Within limits, variations in the water flow rate have little effect on treated water quality.
• Efficient operation can be obtained in almost any size unit, making sodium zeolite softeners suitable for both large and small installations.

While sodium zeolite softeners efficiently reduce the calcium and magnesium content of a raw water supply, they have little effect on silica or dissolved solids content and no effect on alkalinity Sodium zeolite softeners will not function efficiently on turbid waters. City and well water supplies are usually quite suitable for use but surface water often must be clarified and filtered.

Particulate contaminants such as iron or aluminum in raw water supply can foul the resin surfaces and be detrimental to the exchange process. Aluminum may be present in the raw water supply, but problems associated with it generally arise from the use of aluminum compounds for flocculation. Whenever iron or other metallic contaminants are present, the softener must be backwashed thoroughly and a suitable resin cleaner applied during regeneration.

Also strong oxidizing agents in the raw water will degrade ion exchange resin and shortens its total service life. Chlorine is a powerful oxidant and if present, should be eliminated (de-chlorination) with a reducing agent such as sodium sulfite.