Homework help please!!!!!

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OK, I wouldn’t do this if I wasn’t at my wit’s end, and I can already hear you all screaming about not doing other peoples homework but I really really really need a hand here so,

PLEASE help me with my homework!!

QUESTION
A sewage treatment plant has a concrete holding tank of one million litres capacity. Initially the tank contains 750 000 litres of sewage which consists of 60 000kg of organic material suspended in 690 000kg water.
Fresh sewage, with a concentration of suspended organic material of 2 wt%, runs into the tank at 30 000 litres per hour, and sewage leaves the tank at 20 000 litres per hour. The bottom of the tank is covered with a sludge of 40 000kg of precipitated organic material. The stirring caused by the liquid flow causes this precipitate to go into suspension at a rate proportional to the difference between the saturation concentration of 0.10 kg of suspended organic material per kg sewage and the actual
concentration of the suspended organic material in the tank. When there is no suspended organic material the precipitated sludge goes into suspension at a rate of 0.0005kg per minute per litre of solution.

(Hint: Use the second sentence in the previous paragraph to write an algebraic relationship between the rate of re-suspension and the concentration of suspended material. You will see that there is a missing parameter, but the last sentence in the paragraph enables you to evaluate this unknown parameter.)

Provide a material balance solution to this problem.

here’s that in pictures:
http://img.photobucket.com/albums/v66/thecat/flowchart.jpg

OK, so you can see there’s 6 unknowns: the masses of sludge, dissolved solids, and water in the tanks after t=0, and the flowrates out of the tank and (here’s the key) from the solid sludge to the dissolved sludge. I’ve solved everything except for that flowrate:

dMwt/dt = 29400 - (Mwt * 20000/(Mwt+Mot))

dMot/dt = (600 + Fst) - (Mot * 20000/(Mwt+Mot))

dMst/dt = 0 - Fst

Fst is proportional to 0.1 of the total mass of water and dissolved sludge in the tank, minus the mass fraction of dissolved sludge in the tank at any given time. Expanding out,

Fst = k (0.1*(Mot+Mwt) - Mot/(Mot+Mwt))

I need to solve for k in order to express Fst in a relationship involving mass rather than flows, so that I can solve all the equations simultaneously in MATLAB. I just can’t figure out how :?. I know it’s got something to do with Fst being 0.03 kg/hr (0.0005kg/min) when no sludge is dissolved in the tank, but the system is already past this point. So, anyone?

-timmo

Oh God help me!!!

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Homework…that should be a forbidden word/topic in this forum (along with school, college, math, etc.).

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Brain… .popping… coma indused… heels over into a coma

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Blender wouldn’t exist without math and people who understand it well. If you think Plane trigonometry was bad, try that in 3d.

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I know that, but math is not really a topic I like (and I have to do anything I can to prevent it from becoming a popular one ,or we’ll be doomed :stuck_out_tongue: )

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I know that, but math is not really a topic I like (and I have to do anything I can to prevent it from becoming a popular one ,or we’ll be doomed :stuck_out_tongue: )[/quote]
Anything that’s 3D basically is a product of math, it takes a lot of complex formulas and other equations to draw the effect of 3D in a correct fashion.

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No need to repeat what shbaz meant to say. I KNOW THAT ALREADY.

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Hmmm, I was a little put off by the fact you denoted the rate of suspension of organic material as Fst instead of Fos as it is in the image and I might have picked it up wrong but it looks simpler than you make it out. I’m not entirely sure why you need the first 3 equations - perhaps they’re for another problem.

Anyway, you are correct with the equation that comes from the paragraph that defines the rate of suspension:

Fst (or Fos) = k (0.1*(Mot+Mwt) - Mot/(Mot+Mwt))

and it says that when there’s no suspended organic material (Mot=0), the rate of suspension=0.0005kg/min/L, which you correctly point out should be changed to hours -> 0.03kg/hr/L.

So why not plug it into the equation for Fst or Fos?

0.03=k(0.1*(Mwt)-0) -> k=0.3/Mwt

-> Fst = (0.03/Mwt)*((Mot+Mwt)-Mot/(Mot+Mwt))

BTW, are you in high school or university?

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No need to repeat what shbaz meant to say. I KNOW THAT ALREADY.[/quote]
Yes, please SHUT UP ALREADY. I’m tired of you constantly posting crap, JUST TO BOOST YOUR POST COUNT!!! :<

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ROTFL and ROTF again…

%<

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No need to repeat what shbaz meant to say. I KNOW THAT ALREADY.[/quote]
Yes, please SHUT UP ALREADY. I’m tired of you constantly posting crap, JUST TO BOOST YOUR POST COUNT!!! :<[/quote]

People rarely post to boost their post count.
It’s a very popular accusation by people who lack the thinking skills and creativity to analyze the person and state the real reason. His posts don’t strike me as spam at, but as someone just posting their reply.

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Valarking you should look at kansas’s post per day count.
I think it was 50. To me he posts to boost his post count.

May i ask what math that is?
I would like to know so when i take it Ill have a lil bit of an idea how hard it will be.

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You can make that many useful posts a day.
It’s summer and I guess he’s bored.

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Actually summer’s over for me now, now only around 180 days till it starts again. And my PPD is more like 30 last time it was exposed.

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Actually summer’s over for me now, now only around 180 days till it starts again. And my PPD is more like 30 last time it was exposed.[/quote]

Last time it was exposed? Just click the profile button, there’s no need for it to be “exposed.”

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Actually summer’s over for me now, now only around 180 days till it starts again. And my PPD is more like 30 last time it was exposed.[/quote]

Last time it was exposed? Just click the profile button, there’s no need for it to be “exposed.”[/quote]

SHH, ITS A SECRET.

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thanks for the only useful reply i got in among ppl going on about hating maths…

anyways i knew there was something simple i was missing but i just couldnt link it…anyways thanks heaps now hopefully i can get it into matlab and make it work.

-timmo

oh btw its uni maths…dynamic systems modelling

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What kind of engineering degree are you going for?

Electrical here…

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So im guessing I wont take that math in high school lol.

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Lol, try to learn python and let it caculate it for you. :stuck_out_tongue: