Dear Aquarists

It would appear that I was wrong. There is enough interest in my question
concerning the required concentration of chlorine to sterilise seawater and
the required concentration of thiosulphate to neutralise the chlorine for
the replies to have gone to the list. Here then are copies of the replies
that I have received. My apologies to those who have no interest in this
topic.
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Dear David,

My experience growing algae is mostly in shrimp hatcheries and we used
between 2 and 10 ppm active calcium hypochlorite. It depends on the amount
of organic matter in the water but you would like an active reading of 2 or
above for 4-6 hours. Thiosulfate needed to neutralize chlorine is a dosage
of 0.5 thio : 1.0 cl. A lot of people may give you a different dose base on
atomic weights etc, but if you do the titration you will find that .5 is all
you need. I have don't the titration in seawater in 10 different sites in 5
different countries and it always works. It is important with penaeid
shrimps because thiosulfate is toxic at 2:1 thio to cl levels and will
weaken or kill nauplii. hope this helps. Hank Bauman, farm mgr, Belize
Aquaculture Ltd.
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Hi David,

To sterilise seawater we use bleach (liquid) and Sodium thiosulphate. The
amount of each is dependent on the amount of water you need to sterilise.

Bleach: 0.5 mls of bleach for every liter of seawater, this has to bubble
for at least 1/2 an hour.

Sodium Thiosulfate: 0.175 grams of Sodium thiosulfate for every liter of
seawater, this has to bubble for at minimum of 20 minutes before the water
is used.

Hope this is of some assistance.

Cathy Williams
Research Technician
Aquaculture Research and Development Facility
Ocean Sciences Centre
Memorial University fo Newfoundland
St. John's, NF
Canada, A1C 5S7
Phone: (709)737-2678
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Hi David,
please take a look at O.I.E. http://www.oie.int/fr/normes/fcode/F_00066.htm
(unfortunatly it is written in french).

They suggest using clorine at 30ppm for 3 hr, and neutralizing it using an
ammount of Sodium
thiosulphate 2.85 time greater the weight of clorine.
I guess they mean pure clorine, so you have to make correct calclulcation.
Also clorine in water is a gas that naturally escape the water after some
time.

Please send me any other information you would get on this subject.

Kind Regards
Lucio Fariano
Italy
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David,

We are a shellfish hatchery in NY, USA growing oysters, clams and scallops.
We chlorinate our seawater for algae culture and it works very well. We use
commercially available liquid bleach (5.25% sodium hypochlorite) at a rate
of 5ml for 20 liters, 100ml for 200 liters and 1000 ml for about 3200
liters. All are bleached during the day and left till the following morning.
Dechlorination is with sodium thiosulphate stock solution (30g per liter of
distilled water) at a rate 1/4 of the chlorine dose, or 1.5ml, 25ml and
250ml. We use more than is necessary, based on the formulas we originally
received, but this has worked well fro us for over 10 years. Good luck.

John Aldred
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Dear David,
some points need to be cleared:
I mentioned using 30ppm of chlorine.
This means 30gr/cubic meter of pure chlorine.
If you use bleach, you have to know the titration of active chlorine (in
weight). If you just know the % of active chlorine in volume, you must
consider that the weight of 1 liter of Cl2 (gas) is 3.17g.
But normally they give you the % of active chlorine in terms of weight of
clhorine/ volume of solution.
To know if you have well neutralized chlorine with thiosulphate, you ought
to use potassium iodure. You dissolve a small ammount of it in a small
ammount of water, then you drop a small quantity of this solution in the
wter you want to analyse. If it turns yellow, it means that chlorine is
still active and you should add more thiosulphate.
Consider that organic matter and time will neutralize chlorine, so Hank
Bauman may be right that 0.5 thiosulphate to 1 Cl is enough (instead of what
I told you: 2.85 to 1).
I suggest in any case to make a small trial appart from the main system.

An important issue could be sea water. My experience is just in fresh water.

Kind Regards
Lucio Fariano
Italy
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Once again, my thanks to all those that replied

David
____________________________________
David Barnes
Port Nolloth Sea Farms
PO Box 112, Port Nolloth, 8280, South Africa
Phone: +27 27 851 7870
Fax: +27 27 851 8797
Cell: +27 72 325 2551
E-mail: pnsf@telkomsa.net