By the 16th and 17th centuries, it was determined that potassium nitrate needed for gunpowder could come from human waste. European cities soon used their public latrines as a way to collect materials that would then be used to create potassium nitrate used for gunpowder. This industry helped to create a way to recycle and remove waste from public areas, helping cities to become more health and grow by the late Medieval and early modern periods.
Overall, however, the system that was already in ancient societies in the Near East, Asia, and Rome largely did not develop further until the modern Industrial Age in the late 18th and 19th centuries. The main innovation that developed was the introduction of mechanical pumps. Large sewer systems could not be created with better tunneling technologies and pumping stations could be used to move water irrespective of the terrain slope. Cities during this time became unhealthy as populations expanded and migrated to cities.