What happens to detergent after it goes down the drain?

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Nigel Sheridan

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Nigel Sheridan  asked the Naked Scientists:

Hello Naked Scientists.

I have a question about detergent, i.e. biodegradable washing up liquid, household cleaners etc.

I use biodegradable washing up liquid at home as well as many other eco-friendly products. I know that detergents attach themselves to grease so that the become easier to remove when doing the dishes etc. My question is: when the detergent breaks down does it liberate this grease back into the environment or has it been altered in some way so as to become harmless?

All of my waste water etc. ends up in a septic tank and if the grease is liberated would the microbes in the septic tank be able to break it down or would it end up sticking onto the sides of the tank or congealing into a general mess? I normally wipe as much of the grease out of frying pans etc as I can with kitchen paper before washing them up, I normally throw the  kitchen paper in the general rubbish bin as I am not sure if it could  be composted or disposed of in another more eco friendly way. I must add that if I do add oil to my cooking it is normally only in small amounts.

Look forward to your answers.

Great show by the way, keep up the good work.

Many thanks

Nigel & Rusty

What do you think?
« Last Edit: 22/05/2008 22:29:09 by chris »


Offline chris

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What happens to detergent after it goes down the drain?
« Reply #1 on: 24/05/2008 00:25:04 »
I think some detergents / soaps can be cleaved by water-borne and environmental microbes, which liberates useful trace elements like phosphorus. Consequently, I think detergent pollution of waterways has been linked to eutrophication. I need to check this, but, in the meantime, does anyone else know?

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Offline Bored chemist

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What happens to detergent after it goes down the drain?
« Reply #2 on: 24/05/2008 14:41:02 »
At the risk of being gross, bodies decay and they have quite a lot of fat in them, so fat is biodegradable. On the other hand, life (bacteria or whatever) can only dgrade it if they can get to it. Lumps of fat will decay more slowly than finely dispersed stuff. From this point of view the detergent probably helps and, provided you don't put lots of fat down the drain, a septic tank should work fine.

Practically all detergents these days are biodegradable.

The problem with phosphate in detergents wasn't a problem with the detergents themselves. Phosphates were added to washing powders as water softeners and because they are relatively chearp. To much phosphate in the water can upset the balance of the local ecosystem.
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