According to how the other half lives why did sinks stink in tenements

Jacob Riis did not give away the explicit reason for the stench of the sink in the tenements. His only clue was the line "that all the tenants may have access", the reader would have to understand the living condition of the tenements for them to realize that the stench is the result of the over-populated tenement which lacked new plumbing.

They were filled with waste water ~Apex