Chapter 8: Water Treatment
8.1 The Environmental Protection Agency (EPA) provides reports (sometimes referred to as
consumer confidence reports) that explain where your drinking water comes from and whether
any contaminants are in the water. Go to this information at the “Local Drinking Water
Information” page of EPA’s Web site, (http://water.epa.gov/drink/local/index.cfm). Look up the
utility that serves your university and the largest city near your hometown. (a) What is the source
of water? (b) Are there any violations? (c) If so, are they for physical, biological, or chemical
constituents?
Solution:
Students’ responses will vary based on their university and home town location. An example of
8.2 Nitrate concentrations exceeding 10 mg NO3– as N/L are a concern in drinking water due to
the infant disease known as methemoglobinemia. Nitrate concentrations near three rural wells
were reported as 5 mg NO3–/L, 35 mg NO3–/L, and 50 mg NO3–/L Do any of these wells exceed
this 10 ppm regulatory standard?
Solution:
8.3 What are the major differences and similarities between the water quality of a typical surface
water and typical groundwater source?
Solution:
Information from Table 7.7 and sections 8.1-8.3 used to create this table.
Source of Water
Similarities
Surface water
Presence of:
pathogens, particles,
fluoride, sulfur,
natural organic matter
(NOM), and Iron and
Manganese. Chloride
concentrations similar
to ocean from
saltwater intrusion of
trapped brine. Can
contain high
concentrations of
nitrate and other
nitrogen.
• High flows.
• Relatively high suspended solids
(TSS).
• TOC, a measure of NOM, (1-20
mg/L).
• Large seasonal fluctuations in water
quality.
• Magnesium (less than 10, up to 20
mg/L).
• Easy to contaminate.
Groundwater
• Lower flows.
• Natural filtering capacity that removes
suspended solids (TSS).
• TOC, a measure of NOM, (0.1-2.0
mg/L).
• May be high in dissolved solids (TDS),
including Fe, Mn (less than 30, up to
40 mg/L), Ca, and Mg (hardness).
• Difficult to clean up after
contaminated.
8.4 Jar testing was performed using alum on a raw drinking-water source that contained an
initial turbidity of 20 NTU and an alkalinity of 35 mg/L as CaCO3. The optimum coagulant
dosage was determined as 18 mg/L with a final turbidity of 0.25 NTU. Determine the quantity of
alkalinity consumed as CaCO3.
Solution:
8.5 Jar tests were performed on untreated river water. An optimum dose of 12.5 mg/L of alum
was determined. Determine the amount of natural alkalinity (mg/L as CaCO3) consumed. If
6
50 10 gal./ day×
of raw water are to be treated, determine the amount of alum required (kg/yr).
Solution:
8.6 A utility is trying to achieve 25% removal of TOC and is using jar test to determine the
optimal coagulant dose. The following table contains their jar test data (data from EPA 815-R-
99-012, 1999). What is the optimal coagulant dose (mg/L)?
The minimum of the graph is the optimal coagulant dose which occurs at an alum dosage of 100
mg/L.
0
1
2
3
4
5
6
020 40 60 80 100
Settled water TOC (mg/L)
Alum dose (mg/L)
8.7 Ferric sulfate is available as a commercial coagulant and is popular at removing turbidity
and color. The chemical reaction for its addition to water is:
Fe2(SO4)3 + 3Ca(HCO3)2 —–> 2 Fe(OH)3 (s) + 3CaSO4 + 6CO2
Results of a jar test to determine the optimal coagulant dose are provided below. The initial
water sample has a pH = 6.5, turbidity of 30 NTU, and alkalinity of 250 mg/L as CaCO3.
Ferric Sulfate dose,
mg/L
5
10
15
20
25
Turbidity, NTU
15
5
1
0.9
2
(a) What is the optimal mass of ferric sulfate you would need to purchase every day to treat 1 x
106 gallons/day to a turbidity below 1 NTU (assume 100% purity of the coagulant). (b) Do you
have to add alkalinity to the system? If so, how much (in concentration units as mg CaCO3/L)?
Solution:
Use unit conversions to convert 20 mg/L to mass:
20 𝑚𝑔
𝐿×1 𝑔𝑎𝑙𝑙𝑜𝑛
3.785 𝐿× 1 ∗106𝑔𝑎𝑙𝑙𝑜𝑛𝑠
𝑑𝑎𝑦 ×1 𝑔
1000 𝑚𝑔 ×1 𝑘𝑔
1000𝑔=𝟓.𝟑𝒌𝒈
𝒅𝒂𝒚
b. A good assumption is that in many natural waters, the alkalinity can be approximated by being
set equal to the concentration of HCO3–
0
5
10
15
0 5 10 15 20 25
Turbidity (NTU)
Ferric sulfate dose (mg/L)
8.8 A mechanical rapid-mix tank is to be designed to treat 50 m3/day of water at a temperature
of 12°C. Using typical design values in the chapter, determine the (a) tank volume and (b) power
requirement.
Solution:
( )
( )
233
2
N s 1 kN kN m
900 / s 1.2388 10 0.0347 m 0.035 35 W
1,000 N s
m
−
⋅⋅


= ×× × × = =




8.9 An in-line mixer is to be used for rapid mixing. The plant flow is 3,780 m3/day, the water
viscosity is 0.001307 N – sec/m2, and the RMS velocity gradient is 104/s. Estimate the daily
power requirement for the in-line mixer.
Solution:
8.10 The city of Melbourne, Florida has a surface water treatment plant that produces 20 MGD
of potable drinking water. The water source has hardness measured as 94 mg/L as CaCO3 and
after treatment, the hardness is reduced to 85 mg/L as CaCO3. (a) Is the treated water, soft,
moderately hard, or hard? (b) Assuming all the hardness is derived from calcium ion, what
would the concentration of calcium be in the treated water (mg Ca2+/L). (c) Assuming all the
hardness is derived from magnesium ion, what would the concentration of magnesium be in the
treated water (mg Mg2+/L).
Solution:
1 𝑒𝑞𝑣 ) = 𝟐𝟎.𝟒 𝒎𝒈 𝐌𝐠𝟐+
8.11 A laboratory provides the following analysis obtained from a 50-ml sample of raw water.
[Ca2+] = 60 mg/L, [Mg2+] = 10 mg/L, [Fe2+] = 5 mg/L, [Fe3+] = 10 mg/L, Total solids = 200
mg/L, suspended solids = 160 mg/L, fixed suspended solids = 40 mg/L, and volatile suspended
solids = 120 mg/L. (a) What is the hardness of this water sample in units of mg/L as CaCO3?
(b) What is the concentration of total dissolved solids of this sample?
Solution:
8.12 A source water mineral analysis shows the following ion concentrations in the water:
22
33
Ca 70 mg / L, Mg 40 mg / L,and HCO 250 mg / Las CaCO
++ −
= = =
. Determine the water’s
carbonate hardness, noncarbonated hardness, and total hardness.
Solution:
3
100 /
g CaCO mole
mg mg
8.13 (a) Calculate the lime dosage required for softening by selective calcium removal for the
following water analysis. The chemical constituents in the water are
( )
22 2
2 3 34
CO 17.6 mg / L,Ca 63mg / L, Mg 15mg / L, Na 20mg / L,Alk HCO 189 mg / Las CaCO ,SO 80 mg / L,and Cl 10 mg / L
+ ++ − − −
= = = = = = =
. What is
the finished-water hardness?
Solution:
8.14 A municipality treats
6
15 10 gal./ day×
of groundwater containing the following:
( )
22 2
2 3 34
CO 17.6 mg / L, Ca 80mg / L, Mg 48.8mg / L, Na 23mg / L, Alk HCO 270 mg / Las CaCO ,SO 125 mg / L,and Cl 35 mg / L
++ + − −
= = = = = = =
. The water
is to be softened by excess lime treatment. Assume that the soda ash is 90 percent sodium
carbonate, and the lime is 85 percent weight CaO. Determine the lime and soda ash dosages
necessary for precipitation softening (kg/day).
Solution:
Chemical
Conc.mg/L
MWCaCO
3
/MW
Conc. mg/L As
CaCO3
CO2
17.6
100/44
40
Ca+2
80
100/40
200
Mg+2
48.8
100/24.4
200
Na+
23
100/23
100
Cations
500
Alk(HCO3–)
270
SO42-
125
100/96
130
Cl–
35
100/35.5
100
Anions
500
Ca(HCO3)2 = 200 mg/L as CaCO3; Mg(HCO3)2 = 270 – 200 = 70 mg/L as CaCO3; MgSO4 = 200
– 70 = 130 mg/L as CaCO3
Lime required: (30m g/L is added for pH adjustment)
3 3 3 33
6
6
3
40 200 2 70 130 30
56 / 15 10 3.78 lim 20,172
100 / 10 0.85
mg CaCO mg CaCO mg CaCO mg CaCO mg CaCO
L L L LL
g mole CaO Kg gal L kg bulk e kg
g mole CaCO mg day gal kg CaO

+ +× + +


×
× × × ×× =
Soda ash required:
6
3 23
6
3
106 / 15 10
130 100 / 10
3.78 8, 681
0.9
mg CaCO mg Na CO mole kg gal
L mg CaCO mole mg day
L kg bulk soda ash kg
gal kg sodium carbonate



×

× ×× ×




 




×=




8.15 Water contains 7.0 mg/L of soluble ion (Fe2+) that is to be oxidized by aeration to a
concentration of 0.25 mg/L. The pH of the water is 6.0, and the temperature is 12°C. Assume the
dissolved oxygen in the water is in equilibrium with the surrounding atmosphere. Laboratory
results indicate the pseudo first-order rate constant for oxygenation of Fe2+ is 0.175/min.
Assuming steady-state operations and a flow rate of 40,000 m3/day, calculate the minimum
detention time and reactor volume necessary for oxidation of Fe2+ to Fe3+. Perform the
calculations for both a CMFR and a PFR. (You should be able to work this out from information
provided in Chapters 3 and 4.)
Solution:
8.16 Calculate the settling velocity of a particle with 100 µm diameter and a specific gravity of
2.4 in 10°C water.
Solution:
8.17 Calculate the settling velocity of a particle with 10 µm diameter and a specific gravity of
1.05 in 15°C water.
Solution:
8.18 A water treatment plant processes 21,000 m3 of water per day. Assume two types of
flocculated particles enter a rectangular sedimentation basin that has dimensions of: depth = 4 m,
width = 6m, and length = 40 m. The first type of particle has a settling velocity of 0.5 m/hr and
the other type has a settling velocity of 1.8 m/hr. What percent of particles are removed for each
of the two types of particles?
Solution: