I may have misunderstood you though, if you were talking about storm or sanitary sewer rather than fresh water pipes. There is some potential energy that could be recovered from dirty water, but turbines need fairly clean water. People flush things they shouldn't and the city has a crew of people who's job it is to clear condoms and tampons out of lift station pumps. Storm sewers have the same garbage, but also small trees and 2x4's. 

I think this kind of system is only feasible if a city's fresh water source is at an elevation above most of the population.

I posted that before I actually read the article. It does say that this would only work for cities that get fresh water from a higher elevation.

There are devices that filter large debris in stormwater, but they need regular maintenance and it would require an engineering study to determine whether the electricity produced would cover those costs. It would need to be done on one of the underground rivers. A power source that only supplied power when it rains would certainly not be cost effective.

The underground rivers mostly run through brick pipe arches, so it wouldn't be as simple as replacing a section of pipe with a turbine. You would basically need to construct an underground hydroelectric plant. My gut feeling as an engineer is that the idea is possible, but not feasible.