The waste water treatment industry has 3 levels of treatment here on Earth. Primary was what was done in the 60's and before (if any treatment). Solids were ground and held to allow bacteria to digest it (the septic tank method) and it was dumped in the river to dilute it for downstream, with a shot of Chlorine. Then secondary treatment came online in the 70's and later, which is what most municipalities do today, where the solids are filtered out by vacuum or pressure filters and burned or buried, but you'd still be able to tell that the chlorine treated effluent was far from potable.

Finally there is tertiary treatment, which yields water so pure you could drink it (disgusting as it might seem), and this is what is implemented at locations such as Lake Tahoe CA. The water flowing out of the waste water treatment is cleaner than that in the lake itself, after the calcium filtration, etc. There are also de-nitrogenation and de-phosphoration processes to "scrub" the effluent of excess Nitrogen and Phosphorus.

How did you think the Mission to Mars was going to supply water to the crew? Certainly could not tanker enough fresh water to make the multi-year trip to Mars AND BACK.