I am planning to install a high power dish washer. This dishwasher runs hot and the water is heated to 180 degrees during the wash cycle. One potential problem with this is that I have a septic system, so if I send such hot water into the septic tank, it will kill the helpful bacteria there.

How can I cool down the water before it reaches the septic tank?

  • How many gallons a day will it use?? any heat recovery systems you can tie it into? what about a holding tank?
    – JACK
    Oct 11, 2019 at 1:01
  • 3
    Even if the water is at 180 degrees when it leaves the dish washer, it will be well below that after traveling through the drain pipes on its way to the septic tank. Once it gets there it will be mixed with 500-1000 gallons of much lower temperature water and your bacteria will be safe. The few gallons of hot water will do next to nothing to the overall temperature of the septic.
    – jwh20
    Oct 11, 2019 at 1:19
  • consider 80°C hot water isn't that high also a dishwasher water usage, compared to a septic tank volume, is minimal: a tank usually holds at least 1m^3 of water while the washing machine uses few litres, so there'll be a 1 thousand to 1 diluition ratio
    – DDS
    Oct 11, 2019 at 16:04
  • No septic exp, but intuition says it'll be fine. What would be bad is if you cooled it down: grease plugs.
    – Mazura
    Oct 12, 2019 at 4:38

1 Answer 1


You could enlarge your wdv piping. If you put a 4" abs trap under your sink (it would look crazy but you could do it), the 4" trap would hold (pie*4*4)* length of pipe = ~ 608 cubic inches * 0.0043 gallons per cubic inch = 2.59 gallons. Dishwasher uses 4-6 gallons. It would take quite a bit of work to upsize your venting and stack if not already at 4" at the tie in.

Make sure you don't have PVC pipe in your wdv as the max temperature it handles is 140. 180 is the limit for abs.

The other thing that might help is to put a watercycles heat recovery stack on the vertical portion of your stack that the dishwasher is eventually draining into. These units pull cold water from your supply through a copper pipe around another copper pipe that is absorbing heat from your drain.


Converting some of your wdv to metal would allow a large portion of the heat to dissipate, choose the portions of pipe where the water moves slowest.

I'd do some real world measurements of the temperature at the junction to the septic to see how hot the water is once it reaches that point. 82C water that is traveling through wdv at ambient say 20C pipe depending on the length of the pipe is going to loose a fair amount of heat to the trap contents and the surrounding materials.

There is of course manual intervention of running some cold water at the same time your dishwasher drains. Super manual would be to have it drain into a 6 gallon bucket and manually dump the bucket later once the water has cooled.

A grease trap would probably work, they are large boxes that would have the effect of tempering the water coming out of the dishwasher with existing water and they have a fairly large capacity.

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