Alum Reaction

Intoduction to Environmental EngineeringDavis and Cornwell

Shown below are the results of water quality analyses of the Thames River in London. If the water is treated with 60.00 mg/L of alum to remove turbidity, how much alkalinity will remain? Ignore side reactions with phosphorus

and assume all the alkalinity is HCO-3 ion. Does the final pH of the water rise or fall ? Why ?

ConstituentTotal hardnessCalcium hardnessMagnesium hardness

Total ironCopperChromiumTotal alkalinityChloridePhosphate (total)SilicaSuspended SolidsTotal SolidspHa

Expressed asCaCO3CaCO3CaCO3FeCuCrCaCO3ClPO4SiO2---

mg / l260.0235.025.0`

1.80.050.01130.052.01.0

14.043.0495.07.4

a Not in mg/L

Solution: When alum is added to water containing alkalinity the following reaction occurs

carbon dioxide

Al2(SO4)3.14H2O + 6HCO3

- <====> 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO4-2

alkalinity precipitatealuminum hydroxide

wateralum sulfate

3-37 alkalinity reactions.mcdlast save 10/4/99 / 7:59 AM

D:\WINMCAD8\Mathcad applicationareas\CE314\D&CNELL\CHAP3\3-37

alkalinity reactions.mcd

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flash mix flocculation

settlingbasin

treated waterto

filtration

flash mix basin - purpose is to achievecomplete mixing of coagulant with raw

water. Short detention time, highmixing intensity

flocculation basin - purposeis to promote collisionsbetween floc particles -medium to low mixing

intensity. Detention timeapproximately 20 minutes

settling basin - purpose is to allow floc andcolloids to settle out and be removed from

system

CLASSIC COAGULANT ADDITION SEQUENCE OF OPERATIONS *

* Many existing/older plants may lack one or more of these processesor have flocculation and settling combined into a single unit

sludge - to disposal

We see from the reaction that the following occurs:

a. 1 mole alum uses up 6 moles bicarbonate alkalinity

b. carbon dioxide is released which will require additional alkalinity to prevent a drop in the pH, why?

c. Aluminum hydroxide, a precipitate, is created, which settles, hopefully taking turbibity causing suspended solids with it.

d. 1 mole alum produces 2 moles aluminum hydroxide

MWalum 2 26.98⋅ 32 4 16⋅+( ) 3⋅+ 14 18( )⋅+[ ]gm

mole⋅:=

MWHCO3 1 12+ 3 16⋅+( )gm

mole⋅:=

MWCaCO3 100gm

mole⋅:= EWCaCO3

MWCaCO3

2:=




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Aluminum hydroxide is the precipitate (floc) MWfloc 26.98 17 3⋅( )+[ ]gm

mole⋅:=

note: Alkalinity given as: mg/liter as CaCO3.

bicarb

130mg

liter⋅

EWCaCO3MWHCO3⋅:= bicarb 158.6

mg

liter= mg/liter as the HCO-3 ion:

alum

60mg

liter⋅

MWalum

:= alum 1.01 10 4−×

mole

liter=

The alkalinity used up by the alum and that remaining is:

Alkalinity - mg/liter as the ion HCO3- used up in the reaction

alkHCO3 6 alum⋅ MWHCO3⋅:=

alkHCO3 36.972mg

liter=

Alkalinity used up in the reaction expressed as calcium carbonate alkCaCO3

alkHCO3

MWHCO3EWCaCO3⋅:=

alkCaCO3 30.305mg

liter=

alkremaining 130mg

liter⋅ alkCaCO3−:=

alkremaining 99.695mg

liter= NOTE : The creation of carbon dioxide will use up additional alkalinity




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How much alum sludge will have to be disposed of if 12 Mgd of water are treated and 100% of the total suspended solids in the raw water are removed.

Q 12 106⋅

gal

day⋅:= TSS 43

mg

liter⋅:=

dry_sludge Q alum 2⋅ MWfloc⋅( ) TSS+ ⋅:=

dry_sludge 5.884 103×

lb

day= dry solids

If the alum sludge coming from the clarifier is, in fact 98% water, by weight, what is the volume of sludge to be disposed of ?

solids_content .02:=

dry_sludge solids_content total_weight⋅=

total_weightdry_sludge

solids_content:=

total_weight 294198.522lb

day= weight of solids and water

Convert lbs of wet sludge to gallons of wet sludge by dividing by the unit weight of the wet sludge. This is obtained as the product of the unit weight of water * specific gravity of the wet sludge




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Assume the specific gravity of the wet mixture is 1.05. This will depend on the degree to which the sludge is thickened, i.e. the solids content.

sg 1.05:=

vol_sludgetotal_weight

sg 7.48⋅lb

gal⋅

:=

vol_sludge 3.746 104×

gal

day=

solids_content .005 .01, .09..:=

vol_sludge solids_content( )dry_sludge

solids_content sg lb⋅( )⋅ 7.48⋅gal⋅:=

0.5 2.2 3.9 5.6 7.3 91 .10

3

1 .104

1 .105

1 .106

solids content of sludge

slu

dg

e vo

lum

e, g

al./d

ay 79018

38986

1 2

Notice that a seemingly small increase in solids content of the sludge initially results in a major decrease in sludge volume. However, as the sludge solids content increases further thickening becomes a process of diminishing returns.




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Alum Reaction