Wastewater is composed primarily of natural organic substance, which are by-products of human, animal and plant processes. The primary elements in domestic wastewater are nitrogen, phosphorous, ammonia and carbon, these elements are released with the growth of bacteria in standing water. The bacteria consume the oxygen present in water and as a result, living organisms in the water start to die. We know that sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease-causing bacteria and other microbes.
| Organic impurities | Human feces, animal waste, oil, urea (urine). |
| Inorganic impurities | Nitrates, phosphates, metals. |
| Nutrients | Phosphorus and nitrogen. |
| Bacteria | Such as which cause cholera and typhoid. |
| Other microbes | Such as which cause dysentery. |
