Explain dilution to my boss
November 20, 2014 1:17 PM Subscribe
I'm preparing a report on a proposed open ocean fish farm, and am having trouble explaining particulate dilution rates to my boss.
Here's the offending paragraph: (the applicant) has included calculations of the dilution factor for estimated daily particulate waste products (feces and uneaten food) from an individual cage at maximum fish capacity. Given a typical fish feed assimilation efficiency of 87% and a maximum standing stock single cage biomass of 154,000 lbs fed at 3 % per day, approximately 600 lbs per day of uneaten feed and feces would be released to the environment. At the observed current speed of 1 cm/sec, the flow through the cage would be 168,000 m3 of seawater per day and the particulate dilution would be approximately one part in 600,000.
To me, this makes sense. Yet no matter how many times I rewrite this, my boss can't grasp it. I get questions along the lines of one part of what? 600,000 molecules of water? and the more basic I don't get it and explain this better.
One of us is being obtuse, though I don't know if it's him or me. Regardless, my challenge is: how do I make this clear in layperson terms?
Here's the offending paragraph: (the applicant) has included calculations of the dilution factor for estimated daily particulate waste products (feces and uneaten food) from an individual cage at maximum fish capacity. Given a typical fish feed assimilation efficiency of 87% and a maximum standing stock single cage biomass of 154,000 lbs fed at 3 % per day, approximately 600 lbs per day of uneaten feed and feces would be released to the environment. At the observed current speed of 1 cm/sec, the flow through the cage would be 168,000 m3 of seawater per day and the particulate dilution would be approximately one part in 600,000.
To me, this makes sense. Yet no matter how many times I rewrite this, my boss can't grasp it. I get questions along the lines of one part of what? 600,000 molecules of water? and the more basic I don't get it and explain this better.
One of us is being obtuse, though I don't know if it's him or me. Regardless, my challenge is: how do I make this clear in layperson terms?
Yeah, that would be my sticking point, that you switch from mass to volume without specifying which you use in the final comparison.
posted by RobotHero at 1:27 PM on November 20, 2014 [1 favorite]
posted by RobotHero at 1:27 PM on November 20, 2014 [1 favorite]
Just a question of units, really. You're talking about a ratio between two quantities. What are the two quantities?
posted by empath at 1:29 PM on November 20, 2014 [1 favorite]
posted by empath at 1:29 PM on November 20, 2014 [1 favorite]
"A 600 gallon tank is a cylinder roughly 64" across and 50" high. A one-gallon container is a cylinder roughly 6.5" across and 6.5" high," (this is a straightforward comparison you could probably draw for him). Now take 1000 big tanks and put an equal amount of the substance in the 1-gallon container in each. That's the dilution we're talking about here - I'm just putting it in language that fisheries management people & hydraulics geeks are used to seeing."
Maybe it's better to try to get him to see plainly what you're actually saying and he'll stop trying to wordsmith his way out of his own lack of understanding.
posted by Emperor SnooKloze at 1:55 PM on November 20, 2014
Maybe it's better to try to get him to see plainly what you're actually saying and he'll stop trying to wordsmith his way out of his own lack of understanding.
posted by Emperor SnooKloze at 1:55 PM on November 20, 2014
The folks above have a good point. You're thinking the waste is becoming 600,000 tines more dilute, but you don't have an initial concentration. "1 part per 600,000" doesn't mean much without units. What is your unit? You want something like grams of waste per cubic meter of water, probably.
In some contexts you can get away with just saying "parts per million" or whatever, but those are situations where there is an industry convention that allows people to agree on a unit so that they don't have to keep saying it all the time, or where you have the same unit in the numerator and the denominator so they cancel out and leave a unitless measure.
Usually these are situations where they are literally talking about molecules, like when you want to know the concentration of carbon dioxide in the air, e.g. if I sample a million molecules of air, how many molecules of carbon dioxide will I find?
Your situation doesn't really work like that since presumably you aren't counting individual molecules of fish waste so you can't compare them to molecules of water. You need to start with a concentration of fish waste, e.g. 1kg/m^3, and then you can say it becomes "600,000 times more dilute" over the course of the day or else that after one day the concentration drops to 1.8e-5kg/m^3.
Your boss has the right of this one.
posted by Anticipation Of A New Lover's Arrival, The at 2:00 PM on November 20, 2014 [1 favorite]
In some contexts you can get away with just saying "parts per million" or whatever, but those are situations where there is an industry convention that allows people to agree on a unit so that they don't have to keep saying it all the time, or where you have the same unit in the numerator and the denominator so they cancel out and leave a unitless measure.
Usually these are situations where they are literally talking about molecules, like when you want to know the concentration of carbon dioxide in the air, e.g. if I sample a million molecules of air, how many molecules of carbon dioxide will I find?
Your situation doesn't really work like that since presumably you aren't counting individual molecules of fish waste so you can't compare them to molecules of water. You need to start with a concentration of fish waste, e.g. 1kg/m^3, and then you can say it becomes "600,000 times more dilute" over the course of the day or else that after one day the concentration drops to 1.8e-5kg/m^3.
Your boss has the right of this one.
posted by Anticipation Of A New Lover's Arrival, The at 2:00 PM on November 20, 2014 [1 favorite]
It sounds like the units are bothering him, so just state all the units. "One gram of waste per 600kg of clean seawater." Or express it in grams per liter or gallon, since that's a unit of volume he can probably get his head around.
posted by tchemgrrl at 2:00 PM on November 20, 2014
posted by tchemgrrl at 2:00 PM on November 20, 2014
It's not clear to me how all the numbers in that paragraph are intended to relate to each other.
Is there an equation that takes 600lbs/day and 163,000m3 per day and spits out the 1 in 600,000 number?
posted by sparklemotion at 2:04 PM on November 20, 2014
Is there an equation that takes 600lbs/day and 163,000m3 per day and spits out the 1 in 600,000 number?
posted by sparklemotion at 2:04 PM on November 20, 2014
Best answer: Also, I'm not sure that you're calculating the right thing. Do you really want to know the dilution rate of the waste as a function of time? If so, it's not really right to say that all the waste is 600,000 times more dilute based on the flow rate through the cage. Presumably the waste is being generated more or less continuously, so some of it is going to have drifted away, perhaps into faster currents, and become much more dilute, while some of it will still be actually in the cage at its initial concentration.
To me it would be more sensible to calculate the dilution as a function of distance. How far away from the farm do you have to go before the waste concentration drops below some critical level? How does your waste plume disperse as it moves out into the water? This is a more complicated question involving fluid dynamics, but it would model reality much more closely. Does it really matter how much water is moving through the cage, or does it more matter what actually happens to the waste once it enters the surrounding environment?
posted by Anticipation Of A New Lover's Arrival, The at 2:10 PM on November 20, 2014 [1 favorite]
To me it would be more sensible to calculate the dilution as a function of distance. How far away from the farm do you have to go before the waste concentration drops below some critical level? How does your waste plume disperse as it moves out into the water? This is a more complicated question involving fluid dynamics, but it would model reality much more closely. Does it really matter how much water is moving through the cage, or does it more matter what actually happens to the waste once it enters the surrounding environment?
posted by Anticipation Of A New Lover's Arrival, The at 2:10 PM on November 20, 2014 [1 favorite]
Response by poster: I think mbrubeck hit on the simple explanation that was escaping me. Though ... I'll see if the boss signs the report before I mark this as "answered!"
re: Anticipation. These are good points, and I should probably add this into the discussion. I'm not the one doing the actual studies; I'm the one who has to summarize the applicant's environmental studies. The important question, as you noted, is how much waste will be present in nearby waters.
posted by kanewai at 2:36 PM on November 20, 2014
re: Anticipation. These are good points, and I should probably add this into the discussion. I'm not the one doing the actual studies; I'm the one who has to summarize the applicant's environmental studies. The important question, as you noted, is how much waste will be present in nearby waters.
posted by kanewai at 2:36 PM on November 20, 2014
Major sticking point: fish waste sinks, it is not soluble. Unless you are talking strictly ammonia there is no reason to assume your waste will act as a water soluble liquid. Because it won't.
posted by fshgrl at 6:05 PM on November 20, 2014
posted by fshgrl at 6:05 PM on November 20, 2014
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posted by mbrubeck at 1:24 PM on November 20, 2014 [2 favorites]