下水

Polluted water (general term, often highly contaminated).

Drainage/effluent (any drained water).

Sewer system (the infrastructure).

Both 汚水 and 下水 refer to dirty water. However, 汚水 specifically means 'polluted water' and can refer to any kind of dirty water, not just that from sewers. It often implies a higher level of contamination or pollution.

下水 is specifically water in sewers. 汚水 is generally polluted water.

工場から汚水が流れ出た。 (Polluted water flowed out of the factory.)

排水 means 'drainage' or 'effluent'. While sewage is a type of drainage, 排水 is a broader term that includes any water being drained away, such as rainwater or industrial wastewater, not just domestic waste.

下水 is specifically sewage. 排水 is any drained water.

このパイプは台所の排水に使われる。 (This pipe is used for kitchen drainage.)

廃水 also means 'wastewater', similar to 排水. However, 廃水 often specifically refers to industrial wastewater or water that is no longer usable, rather than general domestic sewage.

下水 is domestic sewage. 廃水 often implies industrial or unusable water.

工場は廃水を浄化して川に流す。 (The factory purifies its wastewater before releasing it into the river.)

下水道 refers to the 'sewer system' or 'drainage system' itself, the infrastructure that carries the sewage. 下水 refers to the actual sewage (the water and waste) within that system.

下水 is the sewage. 下水道 is the sewer system.

下水道工事で道が渋滞している。 (The road is congested due to sewer construction.)

汚物 means 'filth', 'dirt', or 'refuse', and it can include solid waste or excrement. While sewage contains汚物, 汚物 itself is a broader term for anything dirty or unclean, not just the liquid waste.

下水 is liquid sewage. 汚物 is solid filth/excrement.

トイレの汚物を流す。 (Flush the filth in the toilet.)