Sounds like they recover the energy quite far down the line when they talk about sewage being 70F degrees, not much left to recover by that time. It's already mixed with solid waste and cold sewage, energy is lost to surrounding soil etc. But if at apartment building level you had a system that separated out hot water from sinks, baths etc and let it heat the building before going down the drain, that could be quite significant. It's a lot more complicated pipework of course, but if you plan it from the get-go, it would probably pay itself off easily enough.