About environmental engineering

1. Water Softening (Lime)
Pb: A dosage of alum in the alum lime coagulation of water is 50 mg/l, it
is designed to react with only in 10 mg/l as CaCO3, of the natural
alkalinity with the alum. Based on the theoretical equations , what will be
the dosage of lime required in addition to 10 mg/l of natural alkalinity to
react with alum dosage?
Sol:
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2. • Unsolved Pb: Alum is applied at a dosage of
20 mg/L, in coagulating a surface water. How
much natural alkalinity is consumed? What is
the change in pH.
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3. Chlorination and water distribution
system
• Water chlorination is the process of adding chlorine or chlorine
compounds such as sodium hypochlorite to water. This method is used to
kill bacteria, viruses and other microbes in water. In particular,
chlorination is used to prevent the spread of waterborne diseases such as
cholera, dysentery, and typhoid.
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4. • The addition of chlorine or chlorine compounds to drinking water
is called chlorination. Chlorine compounds may be applied in liquid
and solid forms—for instance, liquid sodium hypochlorite or
calcium hypochlorite in tablet or granular form. However, the direct
application of gaseous chlorine from pressurized steel containers is
usually the most economical method for disinfecting large volumes
of water.
• Taste or odour problems are minimized with proper dosages of
chlorine at the treatment plant, and a residual concentration can be
maintained throughout the distribution system to ensure a safe
level at the points of use. Chlorine can combine with certain
naturally occurring organic compounds in water to
produce potentially harmful by-products (trihalomethanes). The
risk of this is small, however, when chlorine is applied after
coagulation, sedimentation, and filtration.
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Chlorination and water distribution
system

5. Types of Chlorination
Plain chlorination:
• When the raw water contains turbidity less than 10 NTU, obtained
from unpolluted lakes or reservoirs, the water could be supplied to
the public without any treatment except chlorination.
• Such chlorination is called plain chlorination. The dosage of chlorine
for plain chlorination is about 0.5 mg/L.
Pre-chlorination
• Pre-chlorination is the addition of chlorine to the raw water prior to
treatment to produce residual chlorine after meeting chlorine
demand. The residual chlorine is useful in several stages of the
treatment process – aiding in coagulation, controlling algae
problems in sedimentation basins, reducing odour problems, and
controlling mud-ball formation in filters. In addition, the chlorine
has a much longer contact time when added at the beginning of the
treatment process, so pre-chlorination increases safety in
disinfecting heavily contaminated water.
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6. • Pre-chlorination is generally applied to the water before
coagulation. It improves the coagulation and reduces load on
filters. It also reduces taste, colour, odour, algae and other
organisms.
• The chlorine dose for pre-chlorination should be 0.1 to 0.5
mg/L. The pre-chlorination is always followed by post
chlorination, so as to ensure final safety of water. Until the
middle of the 1970s, water treatment plants typically used
both pre-chlorination and post-chlorination.
• However, the longer contact time provided by pre-
chlorination allows the chlorine to react with the organics in
the water and produce carcinogenic substances known as
trihalomethanes (THM). As a result of concerns over THM,
pre-chlorination has become much less common in the
western countries. Currently, pre-chlorination is only used in
plants where THM formation is not a problem.
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Types of Chlorination

7. Post chlorination
• Post chlorination is the normal process of applying chlorine in the end,
when all other treatments are completed but before the water reaches
the distribution system. At this stage, chlorination is meant to kill
pathogens and to provide a chlorine residual in the distribution system.
Post-chlorination is nearly always part of the treatment process, either
used in combination with pre-chlorination or used as the sole disinfection
process.
• The chlorine dose at post-chlorination stage should be such as to leave a
residual-chlorine of about 0.1 to 0.2 mg/L after a contact period of 20 to
30 minutes. This residual chlorine will ensure the disinfection of water if at
all any recontamination occurs in the transmission and distribution
system. Chlorine dose should not be generally greater than 2.0 mg/L as
the excess residual concentration of chlorine may damage the pipelining
and pump impellers.
• The WHO recommends “a residual concentration of free chlorine of
greater than or equal to 0.5 mg/litre after at least 30 minutes contact time
at pH less than 8.0.” This definition is only appropriate for users who
obtain water directly from a flowing tap.
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Types of Chlorination

8. Break point chlorination
• When chlorine is added to water, number of
reactions taking place in water and the residual
chlorine in water is also changing (increasingly as
well as decreasingly). A typical breakpoint
chlorination curve, showing the chemical reactions
and the residual chlorine levels at various stages is
illustrated in below figure.
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Types of Chlorination

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Types of Chlorination

10. • Chlorine added to water first reacts with any iron, manganese or hydrogen sulphide that may
be present in water. The entire chlorine added will be utilized in reacting with organic
substances (including bacteria). Hence there will not be any residual chlorine (line AB) as
initial chlorine demand.
• When chlorine is further added to water, it reacts with the ammonia present in the water, so
as to produce combined chlorine residual (chloramines). The combined chlorine residual,
increases with addition of dosage (curve BC) until a maximum combined residual is reached
(point C).
• If the addition of the chlorine is continued beyond the point C, the chlorine reacts with
organics and ammonia naturally, found in the water and therefore, the residual chlorine
content suddenly falls down, as shown in the curve CD.
• The point D at which the total chlorine demand is satisfied, as any chlorine added to water
beyond this point, breaks through the water and appears as residual chlorine. This point D is
called break point.
• The addition of chlorine beyond break point is called break point chlorination. The residual
of free chlorine, appearing after break point, is not usually removed except by sun light and
therefore, it takes care of the future recontamination of water. The breakpoint chlorination is
the most common form of chlorination, in which enough chlorine is added to the water to
bring it past the breakpoint and to create some free chlorine residual.
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Types of Chlorination

11. Problems
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13. Do it yourself
Pb 1: How many grams of dry hypochlorite
powder with 70% available chlorine must be
added to 400 L of water to make a 1 % solution.
Pb2: How many pounds of available chlorine are
contained in 1 gallon of sodium hypochlorite
with a strength of 15% ? How many gallons
would be required for a dosage of 0.6 mg/L to 6
million gallons of water?
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