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re: venturi(how to make??)
28 jul 1998
jeff boyd  posted some useful information:

> - store site

>reprinted from 1996 akca seminar binder

>most fish keepers know they need some type of aeration.
>but, unless you've studied this science (and who has?)

good question. somebody must have. limnologists?
commercial trout or catfish or tilapia farmers?

>you are probably assuming that your stream, venturi, waterfall, or
>fountain is taking care of your aeration needs. they may be, but
>are your sure? almost anything that assists the transfer of oxygen
>into water could be called an aerator. but is the aeration effect
>significant and is the energy expended cost effective?

i wonder (1) how many grams per hour of oxygen naturally diffuses into
a square meter of 25 f pond via the surface, if the actual average oxygen
concentration in the pond is 6 ppm, vs the 8.38 ppm saturation level at
25 c, and (2) how that changes if the pond is gently stirred with no
aeration to keep the oxygen concentration uniform with depth, vs having
a saturated surface layer and decreasing concentration with depth. 

it seems to me that the first situation requires no electrical energy,
and the second may require less than frothy surface aeration or bubbles
or fountains, for the same oxygen uptake rate.

>...theoretical oxygen budget for a 1000 gallon (3800 liter) pond.
>1000 gallons of water at 6 ppm contains about 24 grams of oxygen.

saying how much it contains is different from saying what the rate
of oxygen uptake is, in say, grams per hour...

>10 lbs. of fish need about 18 grams of oxygen per hour.

you mentioned this depends on temperature... would this be at 20 c?

>1000 gallons of water with a chlorophyll a of 20 mg/l (light green water),
>a b.o.d. of 10 mg/l (uneaten food, bacteria, etc.) will consume about
>2 grams per hour.

that's 20 grams per hour so far.

>feeding 3% of the fish's body weight per day will add 60 grams of
>food which contains 24 grams of protein, which converts to about 2 grams
>of ammonia, which will consume about 14 grams of oxygen in the biofilter.

10 pounds is 4,536 grams, and 3% of that is 136 vs 60 grams...

if we feed the fish 60 grams per day, is their ammonia output 2 grams
per day or 2 grams per hour? does the biofilter require another 14 grams
per hour, or 14 grams per day, ie another 0.6 grams per hour?

>pollen, bird droppings, leaves, etc. may add 2 more grams.

per day or per hour? i guess these all increase with the pond surface.
a 5' cube would collect less than an 8x8x2' deep pond...

>36 divided by 24 equals 1.5 grams per hour.

i'm confused... 

>this example shows the fish consuming the most oxygen and the biofilter
>the 2nd largest amount, but be aware that very green water can cause
>a fish kill in a stagnant, non aerated pond even if it contains only
>very few fish!

it's my impression that algae add oxygen on the whole, but they can also
severely deplete it at night. how about maintaining a separate algae pond
or bog with no fish for daytime aeration, and circulating water between
fish and aeration ponds during the day, and turning the pump off at night?
how would we keep the algae out of the fish pond? we might control the
pump with an oxygen sensor. is that very expensive?

maybe biofilters add oxygen to ponds, but it seems to me they would need
a lot more horizontal surface to make lots of oxygen by photosynthesis.
does water circulate through them at night?

>if you had an aerator maintaining the oxygen level at 6 ppm and you
>turned it off at 8:00 pm, you would lose about 6% (1.5 grams) of the
>pond's oxygen per hour. by 8:00 am, the next morning, the oxygen level
>could be as low as 1 ppm... 

but the pond will still absorb oxygen through the surface at night, no?
and the oxygen consumption rate will decrease and the absorption rate at
the surface will increase as the oxygen concentration drops, in proportion
to the difference between actual and saturation concentrations.

table 10-20 on page 645 of metcalf and eddy's 1991 wastewater engineering
book shows a 1 acre 20 c pond with 100-260 mg/l of algae treating about
100 pounds of bod5 per day. i think that means it is taking up oxygen at
about twice that rate, ie at about 200x454/43560 = 2 grams per square foot
per day, with a much lower oxygen concentration, perhaps 1 ppm, and a 20 c
saturation level of 9.17 ppm. so that kind of pond has an o2 surface
diffusion constant kl = 2/(9.17-1) = 0.25 g/ft^2-day.

so we might expect a pond with 6 ppm and that kind of algal concentration
to collect about kl(8.5-6) = 0.625 g/ft^2-day at 25 c, ie 10 pounds of
fish requiring 36 grams of oxygen per day would require 58 ft^2 of pond
surface, eg an 8'x8'x2' pond with no aeration. not much surface. but how
much does the daily oxygen uptake rate decrease with no algae, eg in a
shaded pond?  


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