Waste Water Treatment

WHAT IS WASTE WATER/SEWAGE?

Waste water/sewage is used water of a community. Since water is used for washing it contains dirt, sweat, food particles, fats, carbohydrates and proteins and soap and detergents. Water is also used for flushing toilets and therefore contains faeces, urine and paper. Sewage contains large numbers of bacteria, some of which originate from the human digestive tract and contain pathogenic bacteria. Always wash your hands after handling sewage or sewage treatment equipment.

WHY TREAT SEWAGE?

Sewage treatment was first started to prevent water-borne diseases e.g. cholera. If drinking water is contaminated with sewage it will have a high bacterial content. Some bacteria make humans very sick and may eventually kill them. Since rivers are used as a drinking water supply and for recreational purposes it is best to treat sewage to reduce its bacterial content before discharge.

More recently with rising populations it was found that sewage discharged into rivers also damaged the rivers appearance and aquatic life. Sewage is contaminated primarily with organic material. This organic material is food for bacteria which are universally present in the environment. In consuming food the bacteria use oxygen and multiply their numbers until the food supply is exhausted. Sewage has enough organic matter in it to consume 300mg/l of oxygen in the presence of bacteria. Oxygen also dissolves from the air in clean water to about 9mg/l. Hence if sewage is discharged untreated to rivers the bacteria can quickly reduce the oxygen content of the river to zero.

Fish require dissolved oxygen to live. They absorb the dissolved oxygen through their gills. In a similar way we absorb oxygen from the air through our lungs. Whereas fish die without dissolved oxygen, many bacteria do not. They have an ability to change their metabolism to live without free oxygen. In doing so they generate unpleasant odorous compounds such as hydrogen sulphide (also known as rotten egg gas). Hydrogen sulphide reacts with iron to form black iron sulphide. Hence, when sewage is discharged untreated it can kill fish and make the river odorous. Black odorous muds can also result.

An important part of sewage treatment is removing sewage’s ability to consume oxygen. This oxygen demand is a measure of the sewage strength and is called the biochemical oxygen demand (BOD) and is normally measure over five days. Sewage treatment effectiveness is measured in terms of five day BOD (written BODs).

HOW IS SEWAGE TREATED?

Bacteria will proliferate in sewage, particularly if there is an ample supply of dissolved oxygen. Furthermore, bacteria are slightly denser than water and many bacteria have the ability to clump together when concentrated into relatively fast settling particles. Many sewage treatment processes use this property of bacteria cultures to purify the water. The sewage is mixed with a culture (called “activated sludge” or the “bugs”) of not only bacteria cells but a large number of larger naturally occurring organisms and this “mixed liquor” is vigorously agitated in the presence of air to dissolve oxygen (this is called “aeration”)

The culture grows on the food or organic matter in the sewage until all the organic matter has been removed from the water. This is called “cell synthesis” and during this process about one half of the organic matter is converted to carbon dioxide and energy for cell life and the other half is converted to more cells. The mixed liquor is then allowed to settle. The activated sludge settles to the bottom leaving clear clean water on top. This treated water can then be removed from the settling tank.

Activated sludge has another interesting property. If aeration is continued after the food supply has run out, the activated sludge eats itself away. The bacteria cease multiplying and after a while they die. The larger organisms eat the dead bacteria and eventually die themselves. This process is called “endogenous decay”.

The extended aeration process uses both cell synthesis and endogenous decay in the same tank to remove dissolved and suspended organics from the water and also the greatly reduce the volume of activated sludge.