Chapter 4 - Organic effluent

Abstract

When waste-water including urban runoff is discharged into a watercourse, it exerts a polluting load on that water body. Micro-organisms present in the natural water and the wastewater break down (stabilize) the organic matter. As the early forms of wastewater treatment developed are aerobic, so the simplest way of estimating the biodegradability of a wastewater is to estimate the amount of oxygen required to stabilize the waste. This type of system is used to assess the oxygen demand of wastes including organic matter from urban runoff and the test developed from this system is known as the biochemical oxygen demand (BOD) test. Traditionally, the BOD test is carried out for five days and the resulting oxygen demand is referred to as the BOD5. In practice, the test is often modified slightly in that a quantity of seed micro-organisms are added to the BOD bottle to overcome the initial lag period. The disadvantages of the BOD test have led to the development of a simpler and quicker test that is known as the chemical oxygen demand (COD) methodology. In this test, strong chemical reagents are used to oxidize the waste. The COD test oxidizes material that micro-organisms cannot metabolize in five days or that are toxic. Most wastewater treatment processes operate best in pH ranges between 6.8 and 7.4. The two most important nutrients in wastewater treatment are nitrogen and phosphorus, both are needed for cell growth.