As an engineer you have a responsibility to supply your clients with the best possible choice for their wastewater system. Due to failing infrastructures, big pipe is not always the best one to choose.
With the EPA tightening the requirements on municipal (central) wastewater plants, community or decentralized systems are making more sense.
By way of explanation; a mobile home park has 75 units with small lots and no room for any kind of septic/treatment tank. All the sewage flows to a final collection point and into a treatment system. The treatment is sized for 15,000 gallons per day.
Now suppose you need to design for 100 homes and a small strip mall which uses 2,500 gpd.
You could build a community plant to treat 47,500 gpd (100X450gpd + 2,500 gpd). This would be a fairly large footprint.
However you could put in a small package plant for the mall and septic tanks, with individual treatment units for each house (or connect 2-4 houses to a slightly larger tank) and run the treated effluent to a discharge tank(s) for final disposal. This is decentralization. It becomes much simpler and more cost effective. There is no upfront cost for the big system and the smaller systems (as homes are built) just need to be connected to the final dispersal collection line.
The discharge from the disposal tank could be used to help irrigate green belt areas (Check your local codes for requirements).