Heat Exchanger using Laundry drain water

[cityclean]

I am trying to pre-heat my incoming water by circulating it through a tank that holds my drain water. I have not purchased anything yet so design size is up in the air. I am looking for a 400 - 1000 gal tank to store the waste water (110-120 deg F). I have an Incoming 1-1/2" line (40 psi & 40-50 deg F). The more pre-heat I can put into this water the less my boiler has to work to give me 160 deg F wash water. What I am thinking is running the incoming line throught the waste water tank but I do not know how long the line has to be to give me the proper surface area to get the most heat exchange I can get. I have looked into buying some 3/4" tube and making my own header. Or buying some pre -finned tubes. Please let me know if you have any ideas

[Joe Steam]

These waste heat coils are used a lot where I am from, NYC, to grab the heat from hot condensate where it is not returned (district heating use). Basically you have a simple coil or tubes in the collection tank. Many companies make tank coils pre-made (all of the major heat exchanger manufacturer's do).
However, you may have a problem with the 'stuff' in your drain water. It does not sound clean to me, and if you hold it in a tank, where does all this stuff go? To the bottom of the tank? How will you get it out? Also, if you can figure out the above problem, you probably don't want to use finned tubes because they will get fouled.
I think it would be a nice test to use some 3/4" tube inside the tank and see what happens. For how much, you will need your GPM of the flow and how much Delta T you want....