Heat transfer due to aeration in waste water basin

[rubsie]

I'd like to calculate the amount of heat transferred from air when it is pumped into a (biological) waste water basin (8m depth).

I know the temperature, pressure and volumetric flow of the air. It is coarse bubble aeration, not very fine bubbles.

How do I start with this calculation?

 

[racookpe1978]

You have not told us anything useful (Yet.) So we cannot help you. (Yet)

What are your two starting temperatures (of the air, and of the sewage water?
What is the volume of the sewage tank?
Can you ignore the thermal mass of the concrete walls and ground around the tank? (What latitude and what are the regional temperature changes at that location?)
What is the assumed (starting point for iterations) flow rate for the air?

No information = no help. Sorry, we can't help if we don't know your conditions.

Start by making assumptions, and by comparing thermal "mass":
How much water at what temperature needs to be changed to what temperature and what is the coef. of the suspended sewage + water? (Or can you use water thermal properties?)
What is the thermal mass of air needed? (this tells what flow rate of air, IF (big if there!) all of the air is 100% efficient at heating/cooling the water.

Then calc flow rate of air. Iterate from that point using assumed efficiencies for heat transfer.

 

[rubsie]

The air at the inlet of the surpressor ('roots blower') is 20 °C at ambient pressure and compressed to 800 mbar and about 85 °C, we pump 6000 Nm³/h of air in the waste water basin which has a volume of 6800 m³. The starting temperature of the sewage water is 30 °C, it has about 0,5 to 1% of organic material in it so I guess it is safe to assume the same thermal properties as water.

You can ignore the thermal mass of the concrete walls and the ground around the tank.

The purpose of the air is not to heat the water but to dissolve oxygen so it can be taken up by bacteria. What I would like to estimate is how much of the heat in the compressed air is transferred to the water, at the moment I have an average temperature increase between inlet and outlet of the basin of about 4,5 °C. I would like to change the roots blowers to screw compressors because they have an outlet temperature (same 800 mbar) of 10 °C less (for same airflow) compared to roots blowers. Does this mean a significant decrease in temperature for the waste water? That's the question I'd have to answer in my investment proposal.

Is this information helpful? How do I calculate the thermal mass of air and water?