1/11/2016
Topic V: Chemical Treatment Processes
Equivalent Weight
Learning Objectives:
1. Review chemical principles as related to
water treatment.
2. Describe hardness and alkalinity and
illustrate methods of softening.
3. Identify disinfection byproducts and apply
the concept of C·t product for disinfection
of portable water.
Reading Assignments:
Chapter 11 (pp.373-382, pp.399-412, pp.419452)
Example 1: Moles and Chemical
Reactions
Calculate the amount of chlorine required to
oxidize 2.5 mg of H2S.
1
1/11/2016
Concentration Units
Chemical Equilibrium and Its
Applications
Alkalinity
The carbonate System
2
1/11/2016
Chemical Reactions Involving Carbonate
Species
3
1/11/2016
Example 2: Expression of Alkalinity
Hardness
A water contains 122 mg/L HCO3-, 60 mg/L
CO32-, and 17 mg/L OH-. Calculate its total
alkalinity.
Example 3: Expression of Hardness
Coagulation
A water contains 70 mg/L Ca2+, 9.7 mg/L Mg2+.
Calculate its total hardness.
4
1/11/2016
Attraction
Force
Repulsion
Theory of Coagulation
5
1/11/2016
Coagulants
6
1/11/2016
Coagulation Reactions
Example 4: Alum Coagulation
A surface water is coagulated with a dosage of
30 mg/L of aluminum sulfate and an equivalent
dosage of lime. (a) How many pounds of alum
are needed per mil gal of water treated? (b)
How many pounds of quicklime are needed,
assuming a purity of 70% CaO? (c) How many
pounds of Al(OH)3 sludge are produced per mil
gal of water treated?
Jar Test
7
1/11/2016
Water Softening
Example 5: Excess Lime Treatment
Chemistry of Lime–Soda ash Process
Determine lime and soda ash requirements for the
following water.
mg/L
CO2
8.8
Ca2+
70
Mg2+
9.7
Na+
6.9
SO42-
96
-
meq/L
Lime
Soda Ash
10.6
Cl
-
Alk(HCO3 ) = 115 mg/L as CaCO3 = 2.3 meq/L
8
1/11/2016
Carbonate Hardness Removal
Minimum Practical Limits of Precipitation
Softening
Non–Carbonate Hardness Removal
Ion Exchange Softening
9
1/11/2016
Regeneration
Example 6: Chemical oxidation
Iron and Manganese Removal
Disinfection and By-Product Formation
A well water supply contains 3.2 mg/L of iron
and 0.8 mg/L of manganese at pH 7.8. estimate
the dosage of potassium permanganate
required for iron and manganese oxidation.
10
1/11/2016
HOCl (%)
Break Point Chlorination
Other Disinfectants
11
1/11/2016
Control of DBPs
Concept of the C·t Product
12
1/11/2016
13
1/11/2016
Example 7: Disinfection
A conventional surface water plant with coagulation,
flocculation, sedimentation, and filtration produces a
filtered water with a turbidity less than 0.3 NTU, pH 8,
and temperature 10°C on a day when the peak hourly
flow is 2.0 mgd. After filtration, the water is chlorinated
in a baffled storage tank and then pumped to the first
customer in one mile of 12-inch diameter pipe as
shown in Figure 11.29. The residual chlorine concentration at the end of the storage tank is 0.6 mg/1
and at the end of the pipeline is 0.2 mg/l. The effluent
response to a step input of tracer in a tracer analysis of
the storage tank is given in Table 11.13.
14
1/11/2016
a. Determine whether the required C • t value has been
met.
b. If the water system wanted to use a chloramine
disinfectant in the pipeline to control DBP formation, and
maintained a 0.4 mg/1 concentration at the first customer,
would the C • t requirements be met?
c. An ozone disinfection system is being considered in
place of the chlorination and storage tank. What contact
time would be required in the ozone contact tank if the
ozone residual at the end of the tank was 0.2 mg/1 and the
C • t requirements are to be met before the pipeline?
Example 8: Chlorine Contact Tank
Positive total colifom samples in the
distribution system of a public water supply
from groundwater has caused a utility to
provide a 4-log (99.99%) virus reduction.
Sketch a plan for a continuous-flow chlorine
contact tank housed in a building for a water
temperature of 10°C and a well capacity of 400
gpm.
15
1/11/2016
Taste and Odor Control
Membrane Treatment Processes
16
1/11/2016
17
1/11/2016
Reverse Osmosis
Activated Carbon Adsorption
Example 9: Ultrafiltration Plant Concept Design
An ultrafiltration treatment plant is to be designed to
produce 19,000 m3/d of treated water. Piloting
demonstrates that a UF membrane system can operate
at a flux of 100 L/m2-h at 20°C with a filter run length of
75 min. The selected UF manufacturer offers the
following full-scale system:
18
1/11/2016
Example 10: Adsorption by Activated
carbon
Determine the Freundlich model coefficients
and find the dose of PAC per liter of water to
produce a concentration at equilibrium of 15
mg/L. Volume = 200 mL of water, Co = 200 mg/L
19