CHLORINATION IN BRACKISHWATER
AQUACULTURE
Chlorine is commonly practised In brackishwater
aquaculture systems lor disinfection and oxidation. But It is
doubtful whether the use of chlorine is used on any scientific
basis. indiscriminate use leads to increase in cost, inhibit
phytoplankton production and cause mortality of the prawn
post-larvae.The negative environmental Impact of the residual
chemicals as strong chem~caloxidant also can not be ruled out.
Effect of Chlorine:
Chlorlne is used in aquaculture medium for disinfection
and oxidation of organic matter of source water Disinfection
refers to the selective destruction of disease causing organisms
Chlorination will aslo successfully eliminate toxic plankton
(dinoflagellates) and aquatic animal pests and competitors that
come in with the water suppy Since chlorine is a strong oxldising agent, fi will react with iron (Fe),manganese (Mn), hydrogen
sulphide (H2S), free amonia (NH3) and organic matter in the
water
Bleaching powder:
The chemical expression for the bleaching powder or
calcium hypochlorite is Ca(OC1)z It is a white powder High
grade calcium hypochlorite contains at least 70% available
chlorine But commonly available types have only 24-30 % avallable chlorine. Bleaching powder is readily soluble in water and
lose 11savailaMe strength upon storage. It is necessary to determine the strength of bleaching powder before it is used for disin
fection ol water.
When chlorine compound is added to water,
hypochlorous acid (HOCI) and hypochlorte ions (OCf) are
formed. These 'free available chlorine' forms are responsible for
the disinfection Hypochlorous acid is 100 times more toxic than
hypochlorite ion. Since hypochlorous acid is more potent a1
lower pH, t is better to have the pH range of water between 6
and 8 and addition of lime should be limited during chlorination
Other compounds that result from chlorination are the
chloramines This happens when chlorine reacts with ammonia
These compounds are called 'combined available chlorine'
These are also disinfectants, but not as strong as free available
chlorine
Since chiorine residuals are oxidizing agents, they react
with most reduced substances in water Therefore, the con.
centration of bleaching powder which must be added lo water
for disinfection increases with increasing concentrations of organic matter or reduced inorganic substances such as ammonia, The chlorine demand of water is the difference between
the amount of chiorine applied and the quantity of free available
chlorine remaining after a specif~ccontact period.
Optimal conditions for application of Bleaching Powder:
a) pH: The action of chlorine is directly proportionate to the pH
value of water. If pH values are high say above 8.5, chlorine cannot be relied upon to act for bacterial removal On the other
hand, disinfection by chiorine is rapid when pH value of water IS
below 7.
b) Temperature: Temperature has a noticeable effect. Delay in
disinfection occurs in cold climate
c) Suspended solids : The water to be treated lor disinfection
should be clear as far as possible The presence of suspended
matter retards the action of bleaching powder by giving protec.
tlon to bacteria
d) Organic maner and impurities: Chlorine isabsorbed or
destroyed by certain impurities present in water such as organic
matter, suiph~des,nitrites, ammonia. iron, etc Therefore,
enough bleaching powder should be added to obtain free avail
able chlorine residual for effective dtsinfection
Dechlorination:
It IS necessary to dechiorirlate the treated water before
use to reduce the res~dualtoxicity that remains after chlorination
with bleaching powder.
Excess chiorine may be immed~atelyneutralised by the
add~tonof reducing agents such as sod~umthiosulphate. For
every 1 ppm chlorine detected. 7 ppm of sodium thiosulphate
will suffice for neuteraiizatlon
However, without any treatement, chiorine will dissipate
gradually. This can be a~dedby sunl~ghtand aeration. Physical
aeration of water is the cheapest and the best method of
dechlorinationavailable to shrimp farmers
SUGGESTED PROCEDURE FOR THE USE OF BLEACHING
POWDER IN SHRIMP FARMING SYSTEMS
a) Treatment during pond preparation
The use of chlor~neas dis~nfectantis be~ngpractised
during pond preparation, between crops. Chlorination should be
done before applying the lertiiizers After routine pond drying,
water is introduced into the pond and retained for 3 days. This
will allow all water-borne organisms to develop and thus be
more susceptible to chlorination. The water should be treated
with 10-15 ppm of chlorine (Table-1). Three days following
chlorination, the water shouid be clear and the pond bonom
visibie. After manuring and fertilization, stocking can be done.
b) Treetement in reservoir ponds:
It is safer to treat water in reservoirs and before taking it
into grow-out ponds, complete dechiorination has to be ensured. However, this may not be cost-effective for low production targets This practice is generally recommended for the
semi- intensive and intensive systems only
Reservoir shouid consist of two separate ponds. In the
first pond the source water is to be allowed to settle the
suspended solids and then treated with 5-20 ppm of
chlorine(Tabie-I) The chlorination must be carried out In combination with aeration
This reservoir pond(t ha water spread area) shouid have
atieast 4-5 paddle wheei aerators to facilitate proper mixing ot
chlorine Allow chlorination of water for at least 12 hours and
upto 24 hours. The supernatant water from this pond is applied
to the second reservoir pond ( 1ha w.s a) Atieast 6-8numbers of
paddle wheei aerators must be placed here to ensure sufficient
aeration to remove the residual chlorine from water Proper
chemicai tests to estimate chiorine in the treated water as indicated earlier must be ensured to eliminate detrimental effects of
chlorine on shrimp in grow-out ponds.
Chlorination of reservoir water must be done dur~ngthe
monsoon months and during other critical periods. It is however,
advisable to chlorinate the required water throughout the year
rather than in times of crisis.
C) Treatemenl in hatcheries:
In hatcheries, disinfection of water can be done using
Mei.. ~lngpowder commercially available hyphochloriteor stabilized chlorine dioxide. Treat the water with chlorine concentration between 5 pprn to 20 ppm ( table.?) depending on
suspended solids and organic load of water. Allow chlorination
of water for at least 12 hrs and upto 24 hours and then check the
residual chlorine levels. Neutralise remaining chlorine with
sodium thiosulphate (Na2Sz03) before using the water.
Permissible level of chlorine residuals In treated water
for use in shrimp grow-out ponds is less than 0 001 ppm and tor
hatcheries is nil or traces.
Testing of chlorine residuals
Before allowing the treated water to enter the grow.out
ponds or hatcheries, the residual chlorine should be tested with
chlorine test kits (exclusively meant lor sea water) available In
the market.
Testing of available chlorine In bleaching powder
The amount of available chlorine In bleaching powder
can be tested as follows.
1. Prepare a 100 ppm solution of bleaching powder by
dissdving 100 mg of bleaching powder in 1 It of distilled water.
Dilute 5.0 ml of this solution to 50 0 ml to get 10 ppm solution
2. Test this solution for availaMe chlorine by test kits and
estimate the %age of chlorine.
The percentages of chiorine estimated in some corn.
rnercial brands of Meaching powder amlable in the market are
given below :
Brand A
Brand B
Brand C
Brand D
14%
26%
30%
24%
Average
Brand E
Brand F
Brand G
Brand H
Brand l
26%
24%
25%
30%
24%
27%
Availability of chiorine in waters ol difierent salinities
The availability ol chlorine in waters of different saiinilies by
dissolving a particular brand of bleaching powder is as follows.
Water Salinity (ppt)
30
Chlorine (%)
25.5
Tabie 1 The quantity ( in kg) of bleaching powder ( availabie
chlorine 26% ) required to be used in reservoir ponds
7
10,000(1ha pond
water column)
Table 2 : The quantity ( in grams ) of Meaching pawder to be
used for disinfection of water for use in hatcheries.
Vdume of water
in tons
(L'tres)
5 ppm
/ 10 ppm 1 15 ppm / 20 ppm 1
I
I
1
Laboratory trials showed that applications of commer.
cially available bleaching powder at a concentration of 5ppm
and 20 ppm of chlorine kills d~seasecausing Vibrio sp, in filtered
and unfiltered seawater respectively.
For further details write to the addresses given overleaf
9
CENTRAL INSTITUTE OF BRACKISHWATER AQUACULTURE
(Indian Council of Agricultural Research )
Headquarters
Director
Central Instituteof Brackishwater Aquaculture,
No 141, Marshalls Road, Egmore,
MadrasGOO 008
Telephones,
Telex :
Telegram:
Fax:
Email :
Director (Personal)
Off ice
Computer Room
041-6054 CiBA IN
MONODON
91-441855 4851
ciba@x4OO,nicgw.nic.in
855 4851
855 4891
855 4866
855 3002
Muttukadu Experimental Station of CIBA, Madras
Off Icer-~n-charge
Muttukadu Experimental Station of CIBA.
Kovaiam post.603112
Chengai MGR Dist
Tamil Nadu
Telephone:
Kakdwip Research Centre
Officer-in-charge
Kakdwip Research Centre of CIBA
KAKDWIP-743 347
West Bengal
Telephone:
Puri Research Cemre
Officer-In-charge
Puri Research Centre of ClBA
15,B,S,Nagar,Talbania,
PURI-752002
Orissa.
Telephone :
Narakbl Research Centre
Officer-in-charge
Narakkal Research Centre of ClBA
NARAKKAL-682505
Kerala.
Telephone:
Published by :
Director, CIBA, Madras
Ten by:
Dr. K. 0 . Joseph, Dr. B. ?I Gupta,
Dr. S. S. Misra and Dr. M. Muralidhar
Editorial Cornminee: Dr. R. D. Prasadam, Dr. K. V. Ramakrishna,
Dr. L. Hanurnantha Rao, Dr. K. Goplnathan.