Hi, it depends on several factors:

The first to consider is if the residential area has growth expectations, the distance that may currently be acceptable is unacceptable in a short time.

Second depends on the type of treatment.

Third factor is the direction of the prevailing winds.

Also influences the quality of the operation of the plant.

With all this it is deficil to give a number but at least several hundred meters.

 Right dear Ricardo Isla, but when referring to EPA or WHO there must be a number.Here in Iraq it is 3 km (minimum) South East of the town as the dominant wind direction is North West. Also the effluent towards a river must not precede an influent to a water treatment plant. Best regards.

not less than 3 km, but could be less if use new and advance technology like odour removal and so on

Yes dear Ali hadi Ghawi. Especially,that odor removal technology is common now,or by using the MBBR technology which is with no dour. This will lead  us to the subject of Decentralized WWTP which is now dominating as it is more economical than the centralized one reducing the cost of trunk sewers and maintenance costs of sewerage systems due to the long distance to the centralized WWTP. Best regards. 

800 metres - provided action is taken to reduce sources of odours (eg inlet area, primary settling tanks, sludge processing) to < 5 OU (one hour 99.5th percentile) at the plant perimeter. 

Dear Dr. Nabeel Al-Saati; it depends up on treatment type (technology), treatment capacity, dominant climate conditions and topography. The Iraqi legislation (No.3 at 2011) is now been changed. I am working on finalizing the Iraqi Code of wastewater collection and treatment. There is a chapter deals with the safe environmental distances from WWTP and pump stations in details. It take in to consideration various treatment technologies and capacities. I 'll provide you with a copy of as it is permitted.

By the way, we should adopt the decentralized treatment system to overcome costs of trunk lines construction and operation (including related pump stations), and to permit effective reuse. In Najaf, we succeeded (after fighting the routine) to construct a WWTP adopting IFAS technology with a capacity of 50,000 cu m per day. It has no (never) odor sensible even be standing on the basins of the plant. It was constructed within one calendar year and started in operation since 2015. No considerable quantity of sludge is produced. Effluent BOD is within only 3 - 8 ppm, and sometimes not detectable.

Have you found any answer? I'm interested in this too, I have found many answers but different to each other.