OSU Dairy Farm Biological Digester - Operating Conditions

The OSU Dairy farm has a herd of cattle and they generate a lot of biowaste (cow poop). The

dairy runs a biological digester to turn the biowaste, grass clippings, straw, etc. into manure,

which is then sprayed out on the fields. If you ever bike out by the covered bridge in the Spring

you can both see and smell the result of their efforts!

The digester is a HDPE tank with D=3m and L=3m (L/D=1.0). It is completely insulated on

the outside (q(r)= 0), except for the top, where a 2.5cm thick Styrofoam lid has been added.

The digester biowaste generates heat (10 W/m³).

The optimum temperature for digestion is T(average) > 80°C (353 K).

Some additional important information is given below.

a) Sketch and clearly label the problem. Clearly state all given information and assumptions.

b) What is the average biowaste temperature at steady state when the digester is filled with biowaste?

Does this meet the optimum requirements for digestion, which is T(average) > 80°C (353 K).

Hint: The average temperature in the biowaste can be taken as the average of the temperature

at the top and bottom of the biowaste (Trop + Toot)/2.

c) If the digester was FULL (instead of % full), calculate the average temperature in the biowaste.

Does this meet the optimum requirements for digestion, which is T(average) > 80°C (353 K).