Municipalities use chlorine/chloramine to control biological activity in their water systems. They often inject chlorine gas (Cl 2 ) which reacts with water to form hypochlorous acid (HClO) which is a strong biocide.
Hypochlorous acid is also created by injecting a sodium hypochlorite (NaOCl) solution, like bleach (5.25 %) or pool chlorine (14 – 16 %). NaOCl dissolves to release sodium ions (Na + ) and hypochlorite ions (OCl - ). The hypochlorite ion combines with the hydrogen ion (H + ) from water to form hypochlorous acid. At higher pH, some of the hypochlorite ion stays as hypochlorite, which is not as effective a biocide.
Hypochlorous acid will tend to oxidize (i.e., take electrons from) any organic contaminants in the water. This can create trihalomethanes, THMs, which are carcinogenic. To prevent this, many municipalities now inject ammonia (NH 3 ) with the chlorine gas. Their reaction forms chloramine (NH 2 Cl), which is not as strong an oxidizing agent. It will not create THMs, but is also not as effective as a biocide.
A free chlorine test will measure hypochlorous acid and the hypchlorite ion. A total chlorine test measures both free chlorine and chloramines. [Ch. 3 THM Removal by Act. Carbon]
The speed of settling of suspended particles can be increased by adding a coagulation agent.
Metal ions with high charge characteristics, such as iron (Fe+2 which becomes oxidized to Fe+3) or aluminum (Al+3), are attracted to the negative charge that is characteristic of most suspended particles.
When they group, the charge on the particles is neutralized. This allows these larger particles to group even more. The larger the resulting particle, the faster it will settle.
It is common for natural waters to contain some concentration of charged metal ions. Some coagulation will occur as the metals group with suspended particles.
If these materials are able to get through filters in the RO pretreatment system, they will likely fall out of suspension on the RO membrane as they become more concentrated (due to the removal of permeate water).
A concentration can be given in many ways. Some give the weight of the contaminant per weight/volume of water/solution, such as milligrams of contaminant per liter of solution (mg/L).
A similar unit is parts per million (ppm), meaning parts contaminant by weight per million parts of water by weight.
In dilute solutions, a liter of solution will have a weight of 1000 grams, so that a concentration expressed as mg/L is almost exactly the same as one expressed as ppm.
Concentrations may be based on the number of molecules or ions, as in molar concentrations, also known as molarity.
Similarly, some concentrations are based on the number of charges, either positive or negative. This is the case with equivalent concentrations and with concentrations expressed as calcium carbonate (CaCO 3 ) equivalent.
An ion with 2 charges, such as sulfate (SO 4 -2 ), will have twice the equivalent concentration as its molarity.
Concentrations expressed as CaCO 3 are based on the ion's molecular weight and charge as compared to CaCO 3 (100 g/mole with a charge of 2).
To convert from molarity to a CaCO 3 equivalent concentration, multiply by 50,000 and by the ion's charge.
In a water analysis, the concentration of positive ions, called cations, should approximately equal the concentration of negative ions, called anions, when expressed as CaCO 3 .
Suspended Solids / Particles
Particles that are suspended in water behave differently from dissolved ions or dissolved organic molecules. For example, they may add noticeable cloudiness to the water, depending on their concentration.
They are larger than dissolved materials, and can be filtered from the water, with the right filter. They may be either organic or inorganic in origin.
The particles are suspended in the water by collisions with water molecules combined with the repulsive force of other like-charged particles.
Typically, the suspended particles in natural waters have a slightly negative charge (not as much as the charge of a dissolved ion).
The repulsion between the particles prevents them from grouping into larger particles that might have sufficient weight to overcome the bombardment by the water molecules.
However with enough time, even the smaller particles might drift down to the container bottom.
As water containing suspended solids passes through the membrane elements of an RO system, the particles become more concentrated. This forces them closer together.
If close enough, they will overcome their repulsive charges and group into a larger particle. They are then more likely to fall out of suspension on the membrane surface.
Thus, it is important to reduce their concentration upstream of an RO system to control the RO fouling rate.
Ph and Water Dissociation
The pH of water is a measurement of the hydrogen ion concentration that is present in water.
This is also considered the acidity of the water.
The p in pH stands for -logarithm and the H stands for hydrogen, so that pH means the negative log of the hydrogen ion molar concentration ( pH = - log [H+] ).
This can also be expressed as [H+] = 10-pH.
Logs are used because the concentration can vary all the way from 0.000000000000001 (1 x 10 to the minus 14th) moles per liter at a pH of 14 to about 1 (1 x 10o) mole per liter at a pH of 0.
Notice how the pH is equal to the number of decimal places in the value for concentration, and to the exponent when expressed in exponential notation.
We have previously discussed how the polarity of water molecules can break apart salts into their ions.
The forces are strong enough to even break apart some of the water molecules into their components, the hydrogen ion (H+), and the hydroxide ion (OH-).
This breaking apart is called dissociation.
Molecules will dissociate until the hydrogen ion times the hydroxide ion equals 1 x 10 to the minus14th.
In neutral water, the hydrogen and hydroxide ion concentrations are equal so that their concentrations would each be 1 x 10 to the minus 7th moles/liter.
The pH would be 7. Dropping this to a pH of 5 would require 100 times the hydrogen ion concentration.
Filter Fouling and Pressure Drop
Pressure is the driving force for filtration. The filter media is the resistance to this force. Some amount of the filter inlet pressure energy will be lost as water is forced through the filter pores.
As foulants are removed by the filter, they also will provide resistance to the flow of water through the filter. This will result in a greater loss of pressure across the filter media and its foulant layer, if the same flow rate is maintained.
The loss in pressure from filter inlet to outlet can be used to monitor the extent of filter fouling. When using cartridge filters or media filters, pressure drops larger than 15 pounds per square inch differential (psid) should be avoided. Collapse of the foulant matrix is likely at these pressure drops, resulting in shedding of some of the previously filtered contaminants.
Permeate flux rate is simply the permeate flow rate per square foot of membrane area.
It is usually expressed as gfd, gallons per square foot per day.
It is a measure of how much water is being pushed through the membrane.
The permeate flux has a large impact on fouling rate.
With more water going through the membrane, the ability of cross flow to minimize the membrane surface boundary layer is limited.
Suspended solids will build up on the membrane surface, possibly resulting in membrane fouling.
The permeate flux is proportional to the net driving pressure, and is a function of temperature. In warm waters, it is often necessary to reduce the driving pressure so as to reduce the permeate flux in order to control fouling.
This is particularly a concern with water sources that contain a high concentration of suspended solids, such as waste waters or surface waters.
Although the design specifications from the membrane manufacturer may give permeate flow rates based on flux rates as high as 25 gfd, it is rare that membrane elements are pushed this hard.
This would only occur with extremely pure feed water .
RO systems with poor quality feed water will operate with lower permeate flux rates, possible as low as 10 gfd.
The term, fouling, in the membrane industry is generally used to describe the loss of membrane performance caused by water contaminants, that does not depend on their concentration.
This is to distinguish fouling from scale formation, which occurs when salts precipitate (i.e., fall out of solution) on the membrane surface because their concentration has become excessive.
Suspended solids tend to collect in the boundary layer next to the membrane surface. They may be brought closely enough to overcome their repulsive charge that normally keeps them apart.
They may then group into a larger particle that is more likely to stick to the membrane surface.
Bacteria growth within the membrane elements may cause fouling.
This may occur in the boundary layer or in the spacing material between the membrane sheets.
The bacteria survive by living off organic materials in the water.
Fouling may cause the permeate flow rate and/or the concentrate pressure to decrease.
Fouling will typically not affect the salt rejection (i.e., ability to reject salts) unless it affects the ability of the cross flow to assist the movement of ions from the boundary layer back into the bulk stream.
Performance losses by fouling can usually be reversed by cleaning. One exception is when it has become too extreme .
Scaling of the membrane can occur when the concentrations of the oppositely-charged ionic components of a dissolved salt exceed their solubility.
At this point, the high ion concentrations result in the individual ions becoming close enough, so that the attraction between them is greater than the attraction they have for the water molecules. The oppositely-charged ions are drawn together to form a salt crystal.
This crystal then attracts more ions to its growing surface. When it becomes sufficiently large, it will tend to stick to the membrane surface, or to the spacing material between the membrane envelopes.
Scale formation may cause the permeate flow rate to decrease, and it may cause the concentrate pressure to decrease.
Calcium carbonate scale seems to have a greater impact on the permeate flow rate, whereas sulfate scales seem to affect the concentrate pressure more significantly.
Fouling or scale formation may, or may not, affect the membrane rejection of dissolved ions.
Scale formation can usually be prevented with the proper RO pretreatment.
When it does occur, the ability to clean it from the membrane system, will depend on its severity and on the type of scale.
Carbonate scales are relatively easy to clean, whereas sulfate and silica scales can be very difficult to effectively clean.