water treatment math

What is the formula to convert Feet of water to PSI?
I also need to convert cubic feet to gallons?

Starting from metric units:

10 m water = 1 bar = 14.5 psi.

10 m = 32.81 ft.

Therefore 1 ft = 0.442 psi

I have alot of questions but i will start with one) The inflow of raw sewage into the wet well of a lift station is continuous(ie. even during pump-out), and is measured to be 290 L/min. The pumps installed are capable of pumping 780L/min. The wet well is 2m by 2m and the start/stop floats that activate the pumps are 90cm apart, vertically. a) how long do the pumps have to run between the start and stop floats? and b) how long dose it take after the pumps stop befor the start switch is activated again?.

[JohnS]

Originally Posted by TYoneon

I have alot of questions but i will start with one) The inflow of raw sewage into the wet well of a lift station is continuous(ie. even during pump-out), and is measured to be 290 L/min. The pumps installed are capable of pumping 780L/min. The wet well is 2m by 2m and the start/stop floats that activate the pumps are 90cm apart, vertically. a) how long do the pumps have to run between the start and stop floats? and b) how long dose it take after the pumps stop befor the start switch is activated again?.

This looks like homework, so I'm only going to give hints.
1) What is the volume between the limit switches? This is the amount of inflow or net outflow required for a pump cycle. PS: You can't deal with a mix of cubic meters and liters. You need to convert to one or the other.

2) With the pump off, inflow is as stated 290 L/min. How long does it take to fill the volume in (1)

3) With the pump on, NET outlow is 780-290 L/min. How long does it take to empty the volume in (1)

thanks, thats all i need is a path to go on. I have 28 more just like it. its hard cause that don't tell me much in the book. But thank you i need all the help i can get. Happy new year

[scottrussell01]

I am looking for a conversion formula. We went over a question in class that I have an answer for, I am trying to use it to get the answer to another question, but the formula used in class doesn't make enough since to me to help me. Can you explain it to me.

Question: An operator wants to dose a 1,700 GPM raw water flow at 1mg/L. The neat coagulant specific gravity is 1.23. What should the dosing pump rate be set at in mL/min?

Formula used: (QC)1 + (QC)2 = (QC)3
Q3= 1,700 GPM (the raw water flow)
C3= 1 mg/L (the Dose)
C2= 1.23gm/mL (the specific gravity)
Q2= the unknown
C1= 0 (the dosage in the raw water)
Q1= 1,700 GPM (the raw water flow)
Q1 & C1 cancel out cause 1,700 GPM x 0mg/L = 0

which leaves you with (QC)2 = (QC)3

then it was worked out this way.

(1mg/L x 1,700 gal/min x 3785 mL/Gal)/(1.23gm/mL x 1000mg/gm x 1000mL/L)
after cross canceling the answer is Q2 = 5.23mL/min

Can you clarify this please?

[JohnS]

Originally Posted by scottrussell01

I am looking for a conversion formula. We went over a question in class that I have an answer for, I am trying to use it to get the answer to another question, but the formula used in class doesn't make enough since to me to help me. Can you explain it to me.

Question: An operator wants to dose a 1,700 GPM raw water flow at 1mg/L. The neat coagulant specific gravity is 1.23. What should the dosing pump rate be set at in mL/min?

Formula used: (QC)1 + (QC)2 = (QC)3
Q3= 1,700 GPM (the raw water flow)
C3= 1 mg/L (the Dose)
C2= 1.23gm/mL (the specific gravity)
Q2= the unknown
C1= 0 (the dosage in the raw water)
Q1= 1,700 GPM (the raw water flow)
Q1 & C1 cancel out cause 1,700 GPM x 0mg/L = 0

which leaves you with (QC)2 = (QC)3

then it was worked out this way.

(1mg/L x 1,700 gal/min x 3785 mL/Gal)/(1.23gm/mL x 1000mg/gm x 1000mL/L)
after cross canceling the answer is Q2 = 5.23mL/min

Can you clarify this please?

I find that equation very confusing. I think it is better written as
Q1*C1 + Q2*C2 = Q3*C3

It is not true that Q1 = Q3, because something is added. The amount added is small, but not zero. However Q1 and Q3 are close. I will assume Q3 is the exit volume. The inlet volume is slightly less, by about Q2, although volumes are not strictly additive. There is none of the additive in the inlet water, so C1, and hence the term Q1*C1 are zero. Q2*C2 = Q3*C3

But there is a units mess as english and metric units are mixed. 1 gallon = 3.7854 L so the 1700 GPM is 1700 gal/min x 3.7854 L/gal = 6435.2 L/min
Q3*C3 = 6435.2 L/min * 1 mg/L = 6435.2 mg/min = 6.4352 g/min.

Q2 = (Q3*C3) x 1/C2 = 6.4352 g/min x 1 mL/1.23 g = 5.23 mL/min

This is the same as the worked example, but getting rid of the units mess, then plugging in seems clearer (at least to me), than resolving the unit mess inside the formula.

[scottrussell01]

Ok, I will try it that way and see how it works for me.

What kind of a formula would you recommender for this:

An operator uses 2 Gallons of a neat coagulant over 26 hours at a flow rate of 4 MGD. The neat coagulant specific gravity is 1.34. What was the dose rate in mg/L?

[JohnS]

Originally Posted by scottrussell01

Ok, I will try it that way and see how it works for me.

What kind of a formula would you recommender for this:

An operator uses 2 Gallons of a neat coagulant over 26 hours at a flow rate of 4 MGD. The neat coagulant specific gravity is 1.34. What was the dose rate in mg/L?

I'd start by teaching him to use consistent units. However, you are probably supposed to approach it as a nightmare conversion problem.

I would not recommend a formula as it is a conversion problem. Convert whatever crap units you are given to consistent units that relate to the answer you want. You want a dose in mg/L, so in some identical time period, you need to know mass of coagulent used and volume of water treated. Any period would work, I will use 1 hr, as it is common to both 26 h and 1 day.

Is MGD million gallons per day? If so, for water
4 MGD x 3.7854 L/gal x 1 day/24 h = 630 900 L/h

For coagulent,
2gal/26 h x 3.7854 L/gal x 1.34 kg/L = 0.3902 kg/h or 390 200 mg/h

Concentration
390 200 mg/h x 1 h/630 900 L = 0.6185 mg/L

If you try to reduce it to a formula, it will be different when people use different units for the input conditions. Determine what units you need for each input to answer the question, and progressively work towards them with proper conversion factors. In the concentration line, note that I wrote it as multiplying by the reciprocal, but that is the same as dividing. It is a limitation of typing here.

[scottrussell01]

Thank you for your help