Something as common as automotive coolant/"antifreeze" could efficiently travel via thin lines alongside regular plumbing. I think a big part of the challenge

Something as common as automotive coolant/"antifreeze" could efficiently travel via thin lines alongside regular plumbing. I think a big part of the challenge would be in having multiple heat exchangers. Perhaps (if plumbing codes allowed) a plain p-trap could be swapped out for an exchanger unit that also serves as a sewer gas stop. I don't know anything about thermodynamics; how long would it take to transfer a reasonable amount of energy from waste water into the coolant? Another issue again, running all these coolant lines back to a central water heater w/another exchanger. It does all sound pretty complicated and expensive.