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~cal oxidant 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 oxldis- ing 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 % avall- able chlorine. Bleaching powder is readily soluble in water and lose 11s availaMe strength upon storage. It is necessary to deter- mine 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 or- ganic matter or reduced inorganic substances such as am- monia, 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~c contact 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 can- not 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~dual toxicity that remains after chlorination with bleaching powder.

Excess chiorine may be immed~ately neutralised by the add~ton of reducing agents such as sod~um thiosulphate. 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~ded by sunl~ght and aeration. Physical aeration of water is the cheapest and the best method of dechlorination available to shrimp farmers

SUGGESTED PROCEDURE FOR THE USE OF BLEACHING POWDER IN SHRIMP FARMING SYSTEMS

a) Treatment during pond preparation

The use of chlor~ne as dis~nfectant is be~ng practised 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 en- sured. However, this may not be cost-effective for low produc- tion 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 com- bination 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 ( 1 ha w.s a) Atieast 6-8 numbers 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 indi- cated earlier must be ensured to eliminate detrimental effects of chlorine on shrimp in grow-out ponds.

Chlorination of reservoir water must be done dur~ng the 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.. ~lng powder commercially available hyphochlorite or stabi- lized chlorine dioxide. Treat the water with chlorine concentra- tion 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 14% Brand E 24% Brand B 26% Brand F 25% Brand C 30% Brand G 30% Brand D 24% Brand H 24%

Brand l 27% Average 26%

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) Chlorine (%)

30 25.5

Tabie 1 The quantity ( in kg) of bleaching powder ( availabie chlorine 26% ) required to be used in reservoir ponds 7

10,000(1 ha 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~sease causing 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 Institute of Brackishwater Aquaculture, No 141, Marshalls Road, Egmore, MadrasGOO 008

Telephones, Director (Personal) 855 4851 Off ice 855 4891

855 4866 Computer Room 855 3002

Telex : 041-6054 CiBA IN Telegram: MONODON Fax: 91 -441855 4851 Email : ciba@x4OO,nicgw.nic.in

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-752 002 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.