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Are swales effective for cooling hot water enough for plants?

 
dan long
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Context: I will be in Souther California. It is very hot. Water will cool relatively slowly. The hottest water i will likely use is dishwasher water.

If the dishwasher empties directly into mulched swales, is it likely to burn plant roots?
 
Jennifer Wadsworth
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Hi Dan:

There are two problems with recycling water from a dishwasher - heat AND dishwasher detergent.

Heat can be dissipated - I'd probably run the water through something before dumping it into a swale to cool it off - like a long pipe, or a woodchip filter.

Detergent is the bigger issue. Here's what Greywater Action says about dishwashers:

Can I use the water from a dishwasher?

Dishwashers are not a good sources of greywater because the detergents for dishwashers are high in salt. Salt will destroy the structure of your soil over time. If you are able to find a dishwashing detergent that does not have salt or boron in it, then the greywater can be reused. If you have to use the dishwater greywater, send it to a dedicated portion of the yard and plant with salt tolerant plants. Dishwashers are not included in most greywater codes (Montana is an exception), since the kitchen sink is excluded entirely.

 
dan long
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I have no intention of using detergent. The dishwasher is just for sanitizing.

Woodchip filters probably wouldn't be terribly effective because the hot water would kill off any microorganisms that would otherwise make it effective. A long pipe is more infrastructure than the water is worth (the swale goes in wether or not i use greywater, btw). When you think about it, water cools so slowly that a "long" pipe would have to be a REALLY long pipe! Again, if that were the case, the water isn't worth enough to justify the infrastructure. You would be better off just growing fewer water loving plants.

Are there any other methods to cool water before it burns the s*** out of my berm plantings?
 
Jennifer Wadsworth
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I have an outdoor shower that I use exclusively. It has hot and cold running water. The water flows down three separate drains in the shower floor to mulched basins that grow trees and vines. I've been using this shower since 2008. Only one vine has shown any sensitivity to the hot water going down the drain (or maybe it's hyper sensitive to soap - I try to use "bio-compatible" Oasis soap).

Water cools pretty quickly. The black, ABS pipes my water goes through to get to the mulched basins ranch from 5-15 ft. That seems to be enough time/movement to take the brunt of the "heat" out of the shower water.

I haven't had a dishwasher in a couple of decades but I believe you can turn down the heat on some of the newer models.

 
Peter Ellis
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Water coming out of a dishwasher may be as hot as 180F. That is pretty hot to put directly on any living tissue.
But - the volume isn't very large, so the amount of heat to be dissipated is not all that large.

I know we have people here who can do the math on how much 5gallons of180 F water would raise the temperature of 5 cubic feet of wood mulch, but I am not one of them.

I would suggest that a two foot sprayer bar to distribute the water into a cubic yard mulch pit, and from there into a swale with growing plants, would assure safe temperatures by the time the water reached plants. With the bar below the mulch surface, I would expect marsh plants to be unharmed by the hot water, even growing there in the mulch basin.
 
Rebecca Norman
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I thought that many greywater systems have a "surge tank" to hold several gallons of liquid, to mix and dilute discharges. If your dishwasher discharges say, 5 gallons of water, would a surge tank of 10 or 15 gallons let the hot water mix with other discharges and give it time time to cool?

We don't use a surge tank, and our kitchen sink water goes straight through a pipe underground to an outlet next to two trees. One is a willow and one is an apricot and they don't seem to mind a bit. We sometimes drain large pots of boiling hot water (though if there is salt or food value we try to remember to put it in the slops bin instead) Large pots means probably 5 gallons. The pipe discharges into an irrigation canal that has a nice flow of clean water once or twice a week to share the wealth with other trees.
 
Jennifer Wadsworth
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Rebecca Norman wrote:I thought that many greywater systems have a "surge tank" to hold several gallons of liquid, to mix and dilute discharges. If your dishwasher discharges say, 5 gallons of water, would a surge tank of 10 or 15 gallons let the hot water mix with other discharges and give it time time to cool?.


A surge tank is a good idea - I've seen them mostly used on "laundry to landscape" projects to capture the pressurized water before it goes into the landscape. This helps prevent scouring because laundry water is pressurized. There is no storage in the surge tank because greywater can get stinky FAST.

That's why my first suggestion was to flush this water through a barrel filled with woodchips. I think that a lot of microbes would still survive. When I've measured my compost pile, the core temp is usually around 145-160 degrees. Water percolating through the woodchips would cool it down pretty fast, I would think, because instead of being a solid body of water, the water would be dissipated amongst the chips and thus more surface area exposed to cooling.

What do others think?
 
dan long
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Jennifer Wadsworth wrote:
That's why my first suggestion was to flush this water through a barrel filled with woodchips. I think that a lot of microbes would still survive. When I've measured my compost pile, the core temp is usually around 145-160 degrees. Water percolating through the woodchips would cool it down pretty fast, I would think, because instead of being a solid body of water, the water would be dissipated amongst the chips and thus more surface area exposed to cooling.

What do others think?


I think that is an amazing idea. I hadn't taken into consideration that themophilics prefer that tempature anyways. Hot water really wouldn't be much of a problem for them. I love this idea so much, i just might steal it from you!
 
Jennifer Wadsworth
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Steal away and let us know how it goes! Take some pics too. I'm sure there will be more than one permie who wants to know if this works out.
 
Dave Turpin
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Dan,

If your dishwasher is to be used for sanitizing, why not use a dedicated sterilizer instead? They use steam instead of hot water. Much less water usage, much less water that you need to cool before it hits the plants.

They are difficult to find in a store in America but they are commonly used in Asia.
 
dan long
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My thoughts exactly, Dave. I see those in almost every house here in Taiwan, but i had never seen them before i came here. My farm will be in the States though, so a sterilizer is just wishful thinking.
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