The study published Monday in the journal Earth System Science Data, estimated 359 billion cubic meters of wastewater is produced each year -- which is "equivalent to 144 million Olympic-sized swimming pools -- Edward Jones, a doctoral researcher at Utrecht University and the study's lead author, said in a statement.
Researchers found that 52% of that wastewater is treated for reuse, leaving 48% untreated, which is substantially lower than previous estimates of 80%.
The authors warned that rates of treated water in developing countries are still very low due to lack of financial resources to build infrastructure and pay for expensive advanced treatment technologies.
"We see that particularly in the developing world, where most of the future population growth will likely occur, treatment rates are lagging behind," Jones said.
"In these countries in particular, wastewater production is likely to rise at a faster pace than the current development of collection infrastructure and treatment facilities. This poses serious threats to both human health and the environment," Jones said.