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I know that in places where there are convenient rivers and streams the tendency is to dump the communities sewage into those waterways so that it is carried away.
But what about places where there is no river nearby? So, for example in Phoenix, or other midwestern cities where rivers are either very small or non-existent, what do they do?
Phoenix has two monster "wastewater" treatment plants that produce "highly treated effluent" as output. This water is used to water golf courses, etc. before it soaks into the ground water.
Some of the treated water is piped out to the Palo Verde Nuclear Generating Station which uses it in the cooling towers. When I worked there, if the wind was in the right direction, the mist from the cooling towers would land on your car and you had trouble seeing through the windshield on the way home. PVNGS is a zero (liquid) effluent site. Anything they take from Phoenix that they don't use and all onsite sewage goes to the water reclamation plant where they extract any water they can and the rest goes to the evaporation pond where desert the sun and air do their thing, leaving nothing behind but minerals.
As all the other answers have said, nobody in the developed world dumps sewage in nearby rivers. What is discharged into the river is the treated water that comes out of the Sewage Plant.
If your city is in a very arid area like Phoenix without a river your choices are:
Most of the solids will be digested by the treatment process. The rest can be incinerated, used as fertilizer, or trucked to landfill.
Arizona is rather proud of this, it seems. ☺
After filtration and treatment, the waste water is supplied for various re-use purposes. These can range from various categories of irrigation through snowmaking (sic!) to industrial coolants, and there is a system of categories of waste water and what each category may be used for.
It can also be run off into dry streams and rivers, or dumped into infiltration ponds from which it passes into the groundwater table.
The treatment plant at 91st Avenue in Phoenix supplies water to the Palo Verde nuclear plant as coolant, for example. It is claimed that 100% of the water treated at that and at the 23rd Avenue plant in Phoenix is re-used. According to a 2013 report, as of 2010 over the Phoenix Active Management Area, 82% of all waste water from treatment plants is reused.
There were plans back in 2010 for the 91st Avenue plant to do more recharging into the Agua Fria River. I do not know whether they ever came to fruition.
Three options come to mind : septic tanks and sewage systems and reed bed treatment.
Some properties far away from others ie in rural areas have septic tanks, which get emptied every so often by "sludge gulpers" - tankers designed to suck up and remove the contents.
Being on mains drainage is much easier and the sewage ends up at Water Treatment Plants or Sewage Farms (either name is used). The sewage is treated and the residue can be spread on fields as fertilizer.
Reed beds can treat sewage but needs the ground area and climate for it to work well. One book that explains them is "Lifting the Lid" By Halestrap & Harper.
Dumping your untreated poop directly into the river is actually a bad practice.
A lot of municipalities (at least in my neck of the EU) have a main line running through it which collects the sewage from the buildings and brings it to a water treatment plant.
If that is not an option then a septic system can be installed:
A typical septic system works with a large holding tank which overflows out into the drainage field. Which is a large area where the sewage gets slowly percolated into the groundwater where natural processes take care of it.
Building a septic system required a permit in most areas. This permit is to ensure the field and tank are sized properly, its location is recorded and that the ground can percolate the waste well enough.
