Refuse is household waste. Sewage is mainly Hunan excreta. These wastes are mainly organic. People living near rivers tend to dump these wastes into the river to get rid of them. Often, the sewage people living inland is also dumped into rivers and seas by sewers large underground pipes that carry sewage emptied into it by water closet (WC) lavatories.
Normally, sewage from water closets should be treated in sewage works, where the solid organic matter is removed, leaving only the liquid. The latter is then emptied into rivers. However, there are insufficient sewage works to handle all sewage. Even the liquid from treated sewage contains phosphates which cause pollution problems.
As sewage and most refuse are biodegradable, they are broken down by decomposers, mainly bacteria. During this process, they use up oxygen and produce products like carbon dioxide and water. As long as the amount of this organic waste is small compared to the volume of water, the decomposers can eventually remove the waste from the aquatic habitat. A large amount of organic waste in the water body, however causes an increase in the bacteria population. This reduces the level of oxygen in the water , causing aquatic organisms with a narrow tolerance range for oxygen (mainly fish) to die. As these organisms die the amount of waste increases, causing the level of oxygen to drop further. Eventually most of the aquatic organisms die. The bacteria can still break down the organic waste in the absence of oxygen, ie. Under anaerobic conditions. However, the end products are gasses like hydrogen sulphide, ammonia and methane that have a foul smell and give water an unpleasant taste. The water body thus ends up as a lifeless , foul smelling ugly scene. Its water cannot be used for any purpose.
Water polluted with sewage is also a health risk if it is drunk without proper treatment. This is because sewage teams with harmful and infectious bacteria like those that cause typhoid, dysentery and cholera. In fact, polluted water is one of the Important ways of spreading certain infectious diseases.
