Sewage Treatment

Sewerage and sewage must be defined at the outset because they are often used incorrectly. Sewerage is a system of pipes used to collect and carry sewage, which is the wastewater discharged from domestic premises. Domestic sewage consists of human wastes, paper, and vegetable matter. This type of waste is organic because it consists of compounds containing carbon and can be broken down by microorganisms into simpler compounds, which are stable and not liable to cause a nuisance. Sewage can consist of 99.9% water and 0.1% solids.

Raw sewage is a health and environmental concern. It carries a host of bacteria and viruses, causing diseases such as typhoid, cholera, and dysentery. Decaying organic waste is broken down by microorganisms that require substantial amounts of oxygen. If raw sewage is released directly into rivers, lakes, and oceans, it will significantly, and often catastrophically, reduce the oxygen levels in the water, killing fish, native microorganisms, and plant life.

In natural sewage decay, organic waste is consumed by microorganisms such as bacteria and fungi. Initially, this decay is aerobic (requiring oxygen). If the quantity of material is too large, however, the oxygen is depleted and the decay mechanism becomes anaerobic (carried out in the absence of oxygen). Anaerobic decay is slower than aerobic decay, and produces toxic reduction compounds like methane and hydrogen sulfide. The natural process is acceptable for very limited amounts of sewage but impractical for the quantities produced by municipalities. As a result, bulk treatment methods have been developed.

In general, municipal sewage treatment is an iterative process. The process begins by screening out large solids, such as trash, with bars or large mesh screens. Next, grit is settled out in preliminary settling tanks. The sewage then proceeds to further separation by moving through a series of holding tanks where the heavy matter (sludge) falls to the bottom, where it is later removed, and the floating matter rises to the top where it can be skimmed off. Once filtered, sewage is sent to tanks where it is processed biologically, using aerobic organisms. In addition, sewage can be chemically treated to bring pH to an acceptable region, and to remove hazardous wastes.

These are methods of sewage treatment, but not all of them are employed at every sewage treatment plant. The specific methods of treatment is dependant upon both the location of final release and the nature of the sewage being treated.