What is the cost-benefit relationship regarding sewage treatment as a method to fight water pollution?
To treat sewage is much cheaper for society. The non treated sewage pollutes rivers, lakes and the sea, being a cause of diseases transmitted by water. For the society the costs of these diseases are much higher than the cost of the sewage treatment.
One of the most economical systems to treat sewage is the aerobic treatment system, reservoirs kept much oxygenated for aerobic bacteria to decompose organic material.