The drain and sewage systems are separated. Drain water flows directly into the city aboveground rivers, while shit flows to filtering stations.
The first sewer tunnels were opened only in 1898, with aeration fields in the unlucky Lublino district of Moscow. Brick sewage tunnels were constructed nearly until WWII, because the concrete at that time wasn’t strong enough.
Now the sewage system includes several pumping stations and has two main filtering stations and two small ones.
This map depicts the city’s utility networks:
The sewer system is nearly unexplorable due to flow rate and gas (if we exclude all Darvin prize nominees). The photos below are from two of the three channels from Moscow to the Lyubertsy filtering station while they were closed for reconstruction.