I can confirm that small amounts of oil going down the drain will not be a problem. Most hair and shower products, washing powders and soaps contain fat, which wash into the sewers every day. The fat and oil that can causes problems with the sewers are cooking fat and oil; this must not be put down the drain.
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As I expected, they seem to have no problems with the small amounts of mineral oil that would be required.
A cut from a thread in uk.d-i-y from 2006: Payback was 2 years, by a real user. "I cut my DHW heating by somewhere between 1/3 and 1/2 by installing a waste-water heat-exchanger."
Nice one daestrom, you obviously did your homework on payback. This company have been threatening to setup in the UK (or have an agent) for a time. Googling brings up some very encouraging feedback, and seems a better investment than solar. The US government sites speak highly of it.
Unfortunately most homes here don't have basements, as they do in the USA, so difficult to locate in a house. I see they have just brought out a batch processing unit that can be located anywhere, so clawing back bathwater is possible not just shower water. They use a controller and pump, so more complicated. As my 4 year girl likes splashing about in a bath, the payback might be a bit longer for me using the shower version.
But looks like they can be DIYed. One copper inside another, differential controller and a pump. They work by the water spiralling around the pipes and sticking to the sides as gravity pulls the water down. If a pump forced water through too fast maybe it would not do this and heat wasted. Just a thought. The pump's speed would have to be calculated.
Currently most people just would not believe they actually work.
Yes, apparently it softens the scaley coating, which must account for something like less than 0.1% of the sales.
So why would you oil a healthy baby?
It was/is a by-product, an industrial waste until some smarmy sales man found a market. Of course, every good mother knows that a healthy baby must be kept clean and well-oiled, like a guardsman's rifle.
They don't want any amount of oil of mineral origin in their drains and the Environment Agency are even more keep to keep it out of surface water drains. Compressors only loose a 'small amount of oil' to the condense drain. So are all these oil absorbing filters a con? If I fill the sump with mineral oil, will they find that acceptable.
You are evading the main point; putting oil into a bath will leave a coating on the sides. It is a daft idea, dreamt up by the clueless.
I have never suggested that it is a good idea as originally described! Merely pointed out that a small quantity of mineral oil going into the sewer isn't an issue the water companies are bothered about. And it is the person's choice whether or not they care about oil on their bath.
I have no experience of the oil used in compressors, oil absorbing filters, etc. hence no comment on that bit.
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