I think doing adequate filtration of the full waste water flow without losing most of the heat would be a challenge. And remember you're also sending soap suds

I think doing adequate filtration of the full waste water flow without losing most of the heat would be a challenge. And remember you're also sending soap suds down the pipe, which will pass the filter and leave deposits inside the exchanger. Also, if you're just using gravity to push the waste water through, what flow rate would you get on one of those units, and how many would you need to handle the full flow? The most persuasive waste water heat exchangers i've seen are based around the a large-diameter pipe, with conical sections connecting it to the input and output, where the waste water flows down the inner wall of the pipe in a thin layer, and the cold water flows up the outside. It's not as space-efficient as a conventional heat exchanger, but it avoids the problem of blockage. That would also be fairly easy to improvise, i think, given that you can buy copper pipe, or even buy copper foil and wrap it into a large, thin-walled cylinder.