What is the best decentralized treatment solution for a city which is already built without a centralized sewage system?

Most of the cities in Afghanistan are built without a proper sewage system, for example Kabul is a metropolitan city with only 3% of the houses connected to sewage and treatment plant. 97% of the city does not have sewage systems and treatment plant for wastewater and they only collect the toilet wastewater: in septic tanks (18%), pit latrine/cesspit (65%), and dry toilets (14%). All other domestic wastewater flows to the open channels which are connected to the river, therefore in dry season the river becomes a sewage channel. Most of the time people illegally drop the sludge from their cesspits, septic tanks and dry toilets to the river and landfill.  What is the best decentralized household wastewater treatment solution? Are there any case studies of cities which have successfully implemented such a solution? All experiences are much appreciated.