Domestic sewage: Quantity Estimation and Quality Parameters (BOD, COD and TOC)

DOMESTIC SEWAGE

(Quantity & Quality (BOD, COD, TOC) estimation)

Quantity of domestic sewage

The flow of sanitary sewage alone in the absence of storms in dry season is known as dry weather flow (DWF).

Quantity= Per capita sewage contributed per day x Population
Usually 80% of the water supply may be expected to reach the sewers.

There will be fluctuations in dry weather flow.
It can be assumed that
-Maximum daily flow = 2 x average daily flow and
-Minimum daily flow = 2/3 x (average daily flow)

Quality of sewage
The quality of sewage is generally discussed in terms of BOD, COD and TOC which are discussed below-

BOD

BOD stands for Biological Oxygen Demand.It refers to the amount of Oxygen required oxidation of biologically active organic matter. BOD test directly gives the amount of biologically active organic matter present. BOD is defined as the oxygen requirement of bacteria to decompose organic matter under aerobic conditions. BOD measures the amount of food (organic carbon) that bacteria can oxidize. BOD test requires 5 days. BOD is used to

• measure waste loading of treatment plants and
• evaluate efficiency of treatment processes

COD

COD stands for Chemical Organic Demand. COD gives the total of biologically active as well as biologically inactive organic matter. COD is the total measure of all chemicals that can be oxidized. COD test can be carried out in two hours. Hence COD is preferred to BOD. 

TOC

TOC stands for Total Organic Carbon. It is an important method of expressing organic matter in terms of organic content. Organic Carbon can be both biodegradable and non-biodegradable. TOC is the measure of organic carbons. A new technology used for TOC analysis is use of closed loop photocatalytic oxidation