There’s not really a more efficient system than gravity. In this case, that ‘efficiency’ is in costs and materials. Most systems of moving water require pumps, and pumping water can be very costly. However with gravity, the water will carry itself, so the only maintaining you need to do is of the pipes themselves. This is an instance where the resource wasteful system is the only one that makes sense for city governments to fund, and if we want to change that, we’ll have to make an argument for cities to completely gut their existing systems to fully replace them with something else. Or, or simplify that, we have to convince people who’re bound to their budgets to spend money on something that either they or their voters don’t see as necessary. The waste management system my town uses was installed in the early 1900s, and hasn’t been changed much since. A few years ago the city renovated nearly all of the pipes, but it was only a modernization of the existing system, and it would be nearly impossible to convince the city that a different system would be more efficient for them.

As well systems that use drinkable water is only ‘wasteful’ in places with a lack of drinkable water, so solutions to this issue will have to be case-by-case. The only method of dealing with human waste that I’m aware of, other than conventional sewers, is ‘night soil’ - or the storing of human waste in the ground, where it can be converted into dirt. But this isn’t a solution in cities, and cities are the bigger concern. Perhaps there’s something I’m not aware of, I’m only a mechanical engineer (or, more accurately a firearms engineer, but the two are very similar) and am only into civil engineering because it’s interesting. A civil engineer would be far far more knowledgeable on this than I.