Bromide in Dowflake vs Macroalgae Export

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RF Staff
Dec 15, 2004
Duluth, Minnesota
Randy made some new calcuations today for us on the old Bronmide issue.

The paper below describes bromide in seaweed, and we can use that to compare the addition of bromide via Dowflake to the potential export by macroalgae:

Bromine content in some seaweeds of Goa (Central West Coast of India). Naqvi, S. W. A.; Mittal, P. K.; Kamat, S. Y.; Solimabi; Reddy, C. V. G. Natl. Inst. Oceanogr., Goa, India. Botanica Marina (1979), 22(7), 455-7.


Nineteen species of seaweeds were examd. for their Br content by the spectrophotometric method involving ashing with Na2CO3, oxidn. of bromide to Br withchloramine-T, and measuring the absorbance in CHCl3 at 400 and 650 nm. The Br content, on a dry wt. basis, varied from 0.024 to 0.247% for the green algae, from 0.020 to 0.400% for the red algae, and from 0.015 to 0.54% for the brown algae. Only 2 species, Chondria armata and Codium elongatum, showed relatively high Br contents of 0.400 and 0.247%, resp. A large fraction of the Br in these species appears to be organically bound.

Dowflake is about 7,000 ppm bromide and 280,000 ppm calcium. So bromide added is 280,000 / 7,000 = 0.025 times the calcium added.

I've estimated that a typical low demand tank uses/adds about 4 ppm calcium per day a high demand tank might be 40 ppm per day.

So we can put bounds on the bromide added per day:
0.1 to 1 ppm per day.

So lets calculate for a 100 gallon system.

0.1 ppm bromide/day means about 38 mg of bromide per day.

1 ppm bromide/day means about 380 mg bromide per day.

Take the low end of bromide in green algae, 0.024%. That is 0.24 mg bromide per g of algae.

How much algae would need to be harvested each day to export this amount of added bromide?

That gives 3.8/0.24 = 16 dry grams of macroalgae per day at the low demand tank and 38/0.24 = 158 dry grams per day for the high demand tank.

The highest green algae was just over 10 times as concentrated as the lowest, so assuming the highest content algae gives 1.6 and 16 dry grams per day.

Converted to a per month basis, these are:

low demand tank
50-500 dry grams per month

high demand tank
500-5000 dry grams per month

The range is obviously quite large, but the numbers seem almost unattainable to me for a high demand tank, but might be attainable in a low demand tank.

Also,this calculation assume that there was NO export of bromide aside from he macroalgae, which is unlikely. I also do not know how Caulerpa or Chaeto compare to the various algae species that they tested.

Overall, I'd be reluctant to use the high bromide Dowflake for a high demand system.
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