Water Chemistry Introduction

8/13/2019 Water Chemistry Introduction

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Alkalinity

Alkalinity is comprised primarily of carbon dioxide, bicarbonate, carbonate and

hydroxides. Naturally occurring alkalinity is the earths natural buffering system inthat small doses of strong acids (e.g. acid rain) react with alkalinity and result in

relatively small changes in p. !arbon dioxide and bicarbonate are in a balance

between the p range of "." and #.$. At a p of "." or lower, all alkalinity is in theform of carbon dioxide. At a p of #.$, there is no carbon dioxide and all alkalinity

is bicarbonate. %icarbonate and carbonate are in a balance between the p range of

#.$ and &.'. At a p of &.', there is no carbon dioxide or bicarbonate and all

alkalinity is carbonate. As the p increases above &.', hydroxyl alkalinity due to thepresence of the hydroxide ion starts to occur. ost naturally occurring water

sources have a p between ' and #.", so the presence of hydroxides is the result of

manmade activity. Alkalinity, especially by boiler water chemists, can be reportedas Alkalinity and *Alkalinity. Alkalinity measures the +otal Alkalinity in a

water in terms of ppm as calcium carbonate- based on an acid titration to a p of

".$ using a ethyl orange indicator endpoint. *Alkalinity measures the amount ofbicarbonate, carbonate and hydroxyl alkalinity based on an acid titration to a p of

#.$ using a *henolphthalein pink indicator endpoint.

Aluminum (Al):

Aluminum, based on its low solubility, is typically not found in any significant

concentrations in well or surface waters. Aluminum, when present in an / feed

water, is typically colloidal in nature (not ionic) and is the result of alum carryoverby an onsite or municipal clarifier or limesoftener. Alum (aluminum sulfate) is a

popular coagulant that is effective in the absorption and precipitation of naturallyoccurring, negatively charged colloidal material (e.g. clay and silt) from surface

waters. Alum, when introduced into water, disassociates into trivalent aluminum

and sulfate. +he hydrated aluminum ion reacts with the water to form a number ofcomplex hydrated aluminum hydroxides, which then polymeri0e and starts absorbing

the negatively charged colloids in water. 1ouling by aluminumbased colloid

carryover can occur, with alert levels for the / designer ranging from 2.3 to 3.2

ppm aluminum in the feed water. Aluminum chemistry is complicated by the factthat it is amphoteric. Aluminum at low ps can exist as a positively charged

trivalent cation or as an aluminum hydroxide compound. Aluminum at high pscan exist as a negatively charged anionic compound. +ypically, the range of leastsolubility for aluminum compounds is in the p range of 4.4 to 5.4.

Ammonium (NH4):

A monovalent cation. Ammonium salts are very soluble and do not cause a /

scaling problem. +he ammonium ion is the result of very soluble gaseous ammonia

(N6) being dissolved in water of higher p. Ammonia ioni0es in water at high pto form the ammonium ion and hydroxide ion. At lower p the ammonia gas is


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prevalent and being a gas will not be re7ected by a / (similar to carbon dioxidegas). Ammonium is typically not found in well water sources, having been

converted by bacterial action in soils to the transitory nitrite (N/$) ion and then

oxidi0ed into the more prevalent nitrate ion. Ammonium is found in surface watersources at low levels (up to 3 ppm as the ion), the result of biological activity and the

breakdown of organic nitrogen compounds. 8urface sources can be contaminated

with ammonium from septic systems, animal feed lot runoff, or agricultural fieldrunoff from fields fertili0ed with ammonia. Ammonium is prevalent in municipal

waste facilities with levels up to $2 ppm as the ion in the effluent, the result of high

levels of organic nitrogen compound compounds and biological activity. Another

source of ammonium is the result of adding ammonia to chlorine to form biocidalchloramines.

Barium(%a)9

A divalent cation. +he solubility of barium sulfate (%a8/") is low and can cause a

/ scaling problem in the backend of a /. %arium sulfate solubility is lower withincreasing sulfate levels and decreasing temperatures. +ypically, barium can be

found in some well waters, with typical concentrations less than 2.24 ppm to 2.$

ppm. :t is important that barium be measured with instruments capable of 2.23 ppm

(32 ppb) minimum detection levels. ;ith saturation at 322


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%rackish water, in one sense, is defined as a fresh low +>8 water source thatexperiences a large increase in normal +>8 due to seawater intrusion. :n the /

field, brackish water can be defined as feed water with low to medium +>8 levels

(up to 32,222 to 34,222 ppm) that can be treated with a brackish / element-designed for '22 psi maximum applied feed pressure.

Calcium (Ca):

A divalent cation. !alcium, along with magnesium, is a ma7or component of

hardness in brackish water. +he solubility of calcium sulfate (!a8/")(gypsum) is

typically limited to $62< with the use of an antiscalant. +he solubility of calciumcarbonate is typically limited to a =8: (=anglier 8aturation :ndex) value of positive

3.# to $.4.

Carbon Dioxide (CO):

!arbon dioxide is a gas that when dissolved in water reacts with the water to formweak carbonic acid ($!/6). :f a pure water was completely saturated with carbon

dioxide, its concentration would be about 3'22 ppm and the p would be about ".2.

A typical source for carbon dioxide in natural waters is the result of a balance with

bicarbonate alkalinity based on the p of the water. +he concentration of carbondioxide in water is typically indirectly determined by graphical comparison to the

bicarbonate concentration and p. !arbon dioxide and the bicarbonate ion are in a

balance between the p range of "." and #.$. +he alkalinity is all carbon dioxide at

p "." and is all bicarbonate at p #.". +he / design program calculates thecarbon dioxide level based on the bicarbonate level and p of the water. !arbon

dioxide, being a gas, is not re7ected or concentrated by a / membrane, therefore itsconcentration will the same in the feed, permeate and concentrate. Acidifying the

/ feed water will lower p by converting bicarbonate to carbon dioxide.

Carbonate (CO3):

A divalent anion. +he solubility of calcium carbonate is low and can cause a /

scaling problem in the backend of a /. !alcium carbonate solubility is measuredusing =8: (=anglier 8aturation :ndex) for brackish waters or 8>8: (8tiff>avis

:ndex) for seawaters and is lower with increasing temperature and increasing p.!arbonate is one component of alkalinity and its concentration is in a balance withbicarbonate between the p range of #.$ and &.'. At a p of &.' and higher, there is

no carbon dioxide or bicarbonate, with all alkalinity being in the carbonate form.

!ations and Anions9

!ations are ions with a positive valence state (they are willing to accept electrons)

and have the ability to react with anions which are ions with a negative valence state

(they have extra electrons to share). +he sharing of electrons createselectroneutrality. 1or example, the calcium ion is a divalent cation and will combine


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with two monovalent chloride ions to form the electrically neutral salt known ascalcium chloride. A balanced water analysis will have the same concentration of

cations as anions when reported as

ppm as calcium carbonate- or as me?@l. 8ilica, a very weak anion, is not used tocalculate the ionic balance of cations and anions (though it is used in the calculation

of +>8). !hloride (!l)9

A monovalent anion. +he solubility of chloride salts is high and does not create a

/ scaling problem. !hloride, in seawater, is the prevalent anion. !hloride is the

anion used to automatically balance a / feed water analysis. +he recommended

upper limit for chloride in potable water by the 8 B*A and ;/ is $42 ppm basedon taste issues.

COD (Chemical Ox!"en Demand):

!/> is a nonspecific test that measures the ?uantity of both biodegradable andnonbiodegradable organic matter and is reported as ppm as oxygen-. +he test

measures the ability of a hot chromic acid solution to oxidi0e organic matter.

!olor9

!olor is a nonspecific test that measures the relative level of organic compounds in

water based on their contribution to adding color and is reported in A*A unitsrelative to the platinum standard.

Conducti#it!:

!onductivity is a measurement of the ability of water to transmit electricity due tothe presence of dissolved ions. Absolute pure water with no ions will not conduct an

electrical current. !onductivity is measured by a conductivity meter and is reported

as micromhos@cm or micro8iemens@cm. !onductivity is a convenient method of

determining the level of ions in a water but is nonspecific in what the ions are. +heelectrical conductance of ions will vary by ion and will decrease as the concentration

of ions increase. +>8 (+otal >issolved 8alts) meters utili0e conductivitymeasurements with a conversion factor applied. !onductivity can also be estimatedusing individual conversion factors from the reported ion concentrations of a water

analysis or by using a single conversion factor based on the sum of the ions (+>8).

!arbon dioxide conductivity can be estimated by taking the s?uare root of the ppmconcentration and then multiplying by 2.'. +he silica ion does not contribute to

conductivity. +he most accurate conductivity readings for high ?uality / permeate

are obtained onsite since carbon dioxide levels, being a gas, can vary when exposed

to the atmosphere.


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1luoride (1)9

A monovalent anion. 1luoride is found naturally at low levels in some well waters,

but normally its presence is due to in7ection into municipal water to provide aresidual up to $.4 ppm for the control of dental caries. 1luoride levels in potable

waters above 4 ppm can cause mottled and brittle teeth. +he re7ection of fluoride by

a / membrane is p dependent. e7ections with polyamide membranes in thebasic p range can be greater than &&< due to fluoride being in the salt form.

e7ections in the acidic p range can drop below 42< due to fluoride being in the

acid form.

Crains (per gallon)9

:on exchange and boiler water chemists fre?uently report the concentration ofhardness as Crains per Callon (as calcium carbonate e?uivalents)-. /ne Crain per

.8. Callon (as calcium carbonate) is e?ual to 35.3 ppm (as calcium carbonate).

ydrogen 8ulfide ($8)9

ydrogen sulfide is a gas that causes the noticeable rotten egg- smell in feed

waters, with a threshold odor level of 2.3 ppm and a noticeable offensive odor at 64ppm. ydrogen sulfide is readily oxidi0ed to elemental sulfur by oxidants (e.g. air,

chlorine or potassium permanganate). 8ulfur acts as a colloidal foulant and has a

history of not being removed well by conventional multimedia filtration. +he

preferred / system design suggests leaving the hydrogen sulfide in its gaseousform, let it pass through the / into the permeate, and then treat the permeate for its

removal.

:onic 8trength9

+he solubility of sparingly soluble salts increases with increasing feed +>8. +o

account for this effect in calculating the solubility of a salt (e.g. calcium sulfate,

barium sulfate, strontium sulfate or 8>8:), the :onic 8trength of a water is

calculated. +he :onic 8trength of each ion is derived by taking the ppmconcentration of each ion (as calcium carbonate) and multiplying each monovalent

ion by 3 x 324 and each divalent ion by $ x 324. 8umming the :onic 8trength ofeach ion then derives the total :onic 8trength of the water.

:ron (1e)9

:ron is a water contaminant that takes two ma7or forms. +he watersoluble form is

known as the ferrous state and has a D $ valence state. :n nonaerated well waters

ferrous iron behaves much like calcium or magnesium hardness in that it can be

removed by softeners or its precipitation in the back end of the / system can becontrolled by the use of a dispersant chemical in an / feed water. +he water


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insoluble form is known as the ferric state and has a D 6 valence state. +ypically,/ manufacturers will recommend that combined iron levels be less than 2.24 ppm

in the / feed. :f all iron is in the soluble ferrous form, iron levels up to 2.4 ppm in

the feed can be tolerated if the p is less than 5.2 (though an iron dispersant isrecommended). +he introduction of air into water with soluble ferrous iron will

result in the oxidation to insoluble ferric iron. 8oluble iron can be found in deep

wells, but can be converted into the more troublesome insoluble iron by theintroduction of air by being placed in tanks or by leaky pump seals. 8oluble iron can

be treated with dispersants or can be removed by iron filters, softeners or lime

softening. :nsoluble ferric iron oxides or ferric hydroxides, being colloidal in nature,

will foul the front end of the / system. 8ources of insoluble iron are aerated wellwaters, surface sources, and iron scale from unlined pipe and tanks. :nsoluble iron

can be removed by iron filters, lime softening, softeners (with limits), ultrafiltration

(with limits) and multimedia filtration with polyelectrolyte feed (with limits).*recautions are re?uired with the use of potassium permanganate in manganese

greensand iron filters in that potassium permanganate is an oxidant that could

damage any polyamide membrane. *recautions are also re?uired with a cationicpolyelectrolyte in that they can irreversibly foul a negatively charged polyamide

membrane. !orrosion proof vessels and piping (e.g. 1*, *E! or stainless steels)

are recommended for all / systems, / pretreatment, and distribution piping

coming to the / system. :ron as foulant will ?uickly increase / feed pressurere?uirements and increase permeate +>8. :n some cases, the presence of iron can

create a biofouling problem by being the energy source for ironreducing bacteria.

:ronreducing bacteria can cause the formation of a slimy biofilm that can plug the

/ feed path.

agnesium (g)9

A divalent cation. agnesium can account for about a third of the hardness in a

brackish water, but can have a concentration five times higher than calcium in seawater. +he solubility of magnesium salts is high and typically does not cause a

scaling problem in / systems.

anganese (n)9

anganese is a water contaminant present in both well and surface waters, withlevels up to 6 ppm. anganese, like iron, can be found in organic complexes in

surface waters. :n oxygenfree water, it is soluble. :n the oxidi0ed state, it is

insoluble and usually in the form of black manganese dioxide (n/$) precipitate.An alert level for potential manganese fouling in a / aerated / feed waters is

2.24 ppm. >rinking water regulations limit manganese to 2.24 ppm due to its ability

to cause black stains. >ispersants used to control iron fouling can be used to help

control manganese fouling.


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e?@l9

A method of reporting the concentration or e?uivalent- weight of an ion or

substance in a given volume of water as millie?uivalents per liter. e?@l iscalculated by dividing the mg@l by the e?uivalent weight of the ion or substance.

eporting the concentration of ions as me?@l is popular by / chemists for

determining whether a water analysis is balanced- where the sum of the cations e?uals the sum of the anions.

Nitrate (N/6)9

A monovalent anion. Nitrate salts are highly soluble and do not cause a / scaling

problem. Nitrate, along with ammonia gas and ammonium, is a nitrogenbased ionwhose presence is tied with natures nitrogen cycle. +he primary sources of nitrogen

introduction in a feed water come from decomposing animal and plant waste, septic

systems, animal feed lot runoff, or agricultural field runoff from fields fertili0ed withammonia. :n well water sources, ammonia and ammonium are not found, having

been converted to the transitory nitrite ion by certain types of bacteria in soils and

then oxidi0ed into the more prevalent nitrate ion. 1re?uently, nitrate concentrations

are reported as ppm as nitrogen- in water analysis and not as ppm as nitrate- asre?uired for / pro7ections. +o convert ppm as nitrogen- to ppm as nitrate-,

multiply ppm as nitrogen- by "."6. +he 8 B*A has set a maximum recommended

limit of nitrate at 32 ppm as nitrogen ("".6 ppm as nitrate) for potable drinking

water. Nitrates are harmful in that they compete with oxygen for carrying sites inblood hemoglobin. +he reduced oxygen content can result in the bluebaby

syndrome- which is why babies and pregnant women are at higher risk to the effects of

nitrates.

p9

+he p of the feed water measures the acidity or basicity. A p of 5.2 isconsidered neutral. A p between 2.2 and 5.2 is acidic. A p between 5.2 and 3".2

is basic. +o the analytical chemist, p is a method of expressing hydrogen ionconcentration in terms of the power of 32 with the p value being the negativelogarithm of the hydrogen ion concentration. +o the water chemist, p is important

in defining the alkalinity e?uilibrium levels of carbon dioxide, bicarbonate,

carbonate and hydroxide ions. +he concentrate p is typically higher than the feeddue to the higher concentration of bicarbonate@carbonate ions relative to the

concentration of carbon dioxide. +he />B8:CN program allows the user to ad7ust

the p of the feed water using hydrochloric and sulfuric acid. =owering the feed p

with acid results in a lower =8: (=anglier 8aturation :ndex) value, which reduces thescaling potential for calcium carbonate. 1eed and concentrate (re7ect) p can also


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effect the solubility and fouling potential of silica, aluminum, organics and oil.Eariations in feed p can also affect the re7ection of ions. 1or example, fluoride,

boron and silica re7ection are lower when the p becomes more acidic.

*otassium (F)9

A monovalent cation. :t is typically found at much lower concentrations thansodium. +he salts of potassium are highly soluble and do not cause a / scaling

problem.

ppb (parts per billion)9

A method to report the concentration of an ion or substance in a water. +he

following conversions apply for dilute waters with a specific gravity of 3.29 /neppb is e?ual to one microgram per liter (ug@=). /ne ppm is e?ual to 3,222 ppb.

ppm as !a!/69

A method of reporting the concentration or e?uivalent- weight of an ion or

substance in a given volume of water as ppm as calcium carbonate-. eporting the

concentration of ions as ppm as calcium carbonate- is popular by ion exchangechemists for the calculation of ionic loading of cation or anion resins. :t is also

popular in determining whether a water analysis is balanced- where the sum of the

cations e?uals the sum of the anions when the concentration of the ions are reported

as calcium carbonate e?uivalents. ;ater chemists use the concept of e?uivalency-when balancing cation and anion electroneutrality levels since ions combine in

nature based on their valence state and available electrons, not on their actual-weight. !alcium carbonate was arbitrarily picked because its molecular weight is

322 and its e?uivalent weight is 42 since its divalent. +he formula to convert an ion

reported as mg@l as the ion- to ppm as calcium carbonate- is to multiply mg@l asthe ion- times the ratio of the e?uivalent weight of the ion- by the

e?uivalent weight of calcium carbonate-.

As an example, a water with sodium at 322 ppm as calcium carbonate and chlorideat 322 ppm as calcium carbonate are in ionic balance since every sodium ion has a

corresponding chloride ion. owever, sodium concentration at 322 ppm as calciumcarbonate is only "5 mg@l of actual substance (since its e?uivalent weight is $6.2)and 322 ppm of chloride as calcium carbonate is only 53 mg@l of actual substance

(since its e?uivalent weight is 64.4). +he calculated +>8 of this solution is 33#

mg@l.

8>: (8ilt >ensity :ndex)9

An empirical test developed for membrane systems to measure the rate offouling of a 2."4 micron filter pad by the suspended and colloidal particles in a


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feed water. +his test involves the time re?uired to filter a specified volume offeed at a constant 62 psi at time 0ero and then after 4 minutes, 32 minutes and 34

minutes of continuous filtration. +ypical / element warranties list a maximum

8>: of ".2 at 34 minutes for the feed water. :f the 8>: test is limited to only 4 or32 minute readings due to plugging of the filter pad, the user can expect a high

level of fouling for the /. >eep wells typically have 8>:s of 6 or less and

turbidities less than one with little or no pretreatment. 8urface sources typicallyre?uire pretreatment for removal of colloidal and suspended solids to achieve

acceptable 8>: and turbidity values.

8ilica (8i/$)9

8ilica (silicon dioxide), in some cases, is an anion. +he chemistry of silica is acomplex and somewhat unpredictable sub7ect. :n similar fashion as +/! reports the

total concentration of organics (as carbon) without detailing what the organic

compounds are, silica reports the total concentration of silicon (as silica) withoutdetailing what the silicon compounds are. +he +otal 8ilica- content of a water is

composed of eactive 8ilica- and nreactive 8ilica-. eactive silica (e.g.

silicates 8i/" ) is dissolved silica that is slightly ioni0ed and has not been

polymeri0ed into a long chain. eactive silica is the form that / and ion exchangechemists hope for. eactive silica is the form of silica to be used in / pro7ection

programs. eactive silica, though it has anionic characteristics, is not counted as an

anion in terms of balancing a water analysis but it is counted as a part of total +>8.

nreactive silica is polymeri0ed or colloidal silica, acting more like a solid than adissolved ion. 8ilica, in the colloidal form, can be removed by a / but it can cause

colloidal fouling of the frontend of a /. !olloidal silica, with si0es as small as2.22# micron can be measured empirically by the 8>: (8ilt >ensity :ndex) test, but

only that portion that is larger than 2."4 micron or larger. *articulate silica

compounds (e.g. clays, silts and sand) are usually 3 micron or larger and can bemeasured using the 8>: test. *olymeri0ed silica, which uses silicon dioxide as the

building block, exists in nature (e.g. ?uart0es and agates). 8ilica, in the polymeri0ed

form, also results from exceeding the reactive silica saturation level. +he solubility

of reactive silica is typically limited to $22622< with the use of a silica dispersant.eactive silica solubility increases with increasing temperature, increases at a p

less than 5.2 or more than 5.#, and decreases in the presence of iron which acts as acatalyst in the polymeri0ation of silica. 8ilica re7ection is p sensitive, withincreasing re7ection at a more basic p as the reactive silica exists more in the salt

form than in the acidic form.

8odium (Na)9

A monovalent cation. +he solubility of sodium salts is high and does not cause a

/ scaling problem. 8odium, in seawater, is the prevalent cation. 8odium is thecation used to automatically balance a / feed water analysis. >ietary sodium


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levels can range from $222 mg@l for lowsodium diets to 6422 mg@l for averageconsumption levels. +he 8 B*A has set a >;B= (>rinking ;ater B?uivalent

=imit) of $2 mg@l for potable water but is reevaluating the limit as to low. >aily

consumption of $ liters (2.46 gallons) of water with 322 mg@l of sodium would beonly $22 mg. A relatively hard water- with 32 grains per gallon (353.$ mg@=) of

hardness (as calcium carbonate) results in only an additional 5& mg@= of sodium

when softened.

8trontium (8r)9

A divalent cation. +he solubility of strontium sulfate is low and can cause a /scaling problem in the backend of a /. 8trontium sulfate solubility is lower with

increasing sulfate levels and decreasing temperatures. +ypically, strontium can be

found in some well waters where lead ores are also present, with typicalconcentrations less than 34 ppm. ;ith saturation at 322


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solubility of sparingly soluble salts. As a rough rule of thumb, every 32degree1ahrenheit decrease in feed temperature increases the feed pump pressure

re?uirement 34 at # ppm.

+urbidity9

+urbidity is a suspension of fine colloidal particles that do not readily settle out ofsolution and can result in a cloudiness-. +urbidity is determined by a

Nepholometer that measures the relative amount of light able to pass through a

solution. +urbidity is reported as N+ (Nepholometric +urbidity nits). +ypical/ element warranties list a maximum of 3.2 N+ for the feed water.


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*arametersGofGtheG/G*rocess

+he performance of membrane elements operating in a reverse osmosis system isaffected by the feed water composition, feed temperature, feed pressure, and

permeate recovery ratio. *erformance at a given set of system operating parameterscan be calculated from nominal membrane performance at reference test conditions.

1BB> ;A+B 8A=:N:+H

/smotic pressure of the feed water is directly proportional to feed water salinity.*ermeate flux is proportional to the available net driving pressure (N>*). N>* is

the difference between the applied feed pressure and the average osmotic pressure at

the membrane surface. +herefore, higher feed salinity will re?uire higher feedpressure to produce a given permeate flow. *ermeate salinity is proportional to the

average feed salinity at the membrane surface. +herefore, an increase in feed salinity

will result in a correspondingly higher permeate salinity.

1BB> ;A+B +B*BA+B

+emperature affects the diffusion rate of water and dissolved ions across asemipermeable membrane. +he change in water flux with temperature is about 6 1BB> ;A+B *B88B

Applied feed pressure affects N>*, which in turn directly affects permeate flux.

igher feed pressure will result in higher permeate flux. owever, feed pressuredoes not affect salt transport across the membrane. +herefore, an increase in feed

pressure will result in higher permeate flux with a lower concentration of salt in the

permeate stream (higher dilution of a given amount of salt passing through the

membrane).

*BBA+B B!/EBH A+:/

+he maximum recovery rate at which an brackish / system can operate is defined

mainly by water chemistry9 the ion composition of the feed water. An excessive

recovery ratio may result in formation of mineral scale on membrane surface. :nseawater / systems, the recovery ratio is limited by the re?uired permeate salinity

and feed pressure limitations. :ncreasing system recovery ratio results in higher

average salinity at the membrane surface with a proportional increase in the average

feed osmotic pressure. +his higher osmotic pressure re?uires a higher feed pressureto maintain a design permeate flow. ;ith this higher average feed salinity, passage

of dissolved ions across the membrane also increases.

Documents

Water Chemistry Introduction