Causes

Large concentrations of humans in cities produces large quantities of human waste.

Animal husbandry in large scale and concentrations [feedlots] produces large quantities of animal wastes.

Effects

If improperly treated and discharged, these waste accumulations will pollute water bodies and endanger aquatic ecosystems [flora and fauna].

The untreated sewage will have a biochemical oxygen demand (BOD) [usually 300 ppm] which will tax the dissolved oxygen (DO) [usually 8 ppm] of the body of water.

[Sewage treatment is expensive. Most developing countries usually cannot afford treatment. Example: the New River, IBWC's sewage treatment plant north of the U.S.-Mexico border at Imperial Beach].

Solution

Research and develop suitable alternatives to the flush toilet.

Research and develop more effective sewage treatment processes.

Sanitation alternatives include low-flow toilets, air toilets [if energy source is sustainable], and dry ecological latrines [for rural areas and developing countries].