Methods for Analyzing Organic Matter in Water Bodies - Total Oxygen Demand (TOD)

The total oxygen demand refers to the substances that can be oxidized in the water, mainly the amount of oxygen required when organic substances turn into stable oxides during combustion. The result is expressed in m8 of 02.

The principle of measuring TOD with a TOD meter is to inject a certain amount of water sample into a quartz combustion tube equipped with a platinum catalyst, and pass in a carrier gas (nitrogen) containing a known oxygen concentration as the raw material gas. Then the reducing substances in the water sample are instantly burned and oxidized at 900°C. By measuring the decrease in oxygen concentration in the raw gas before and after combustion, the total oxygen demand value of the water sample can be obtained.

The TOD value can reflect the amount of oxygen required for almost all organic substances to become C02, H20, N0, S02... after combustion. It is closer to the theoretical oxygen demand value than BoD, CoD and permanganate index. But there is no fixed correlation between them. Some researchers pointed out that BODs/TOD=0.1-0,6; CoD/TOD=0.5-0.9. The specific ratio depends on the properties of wastewater.

The proportional relationship between TOD and TOC can roughly determine the type of organic matter. For carbon-containing compounds, because one carbon atom consumes two oxygen atoms, that is, Oz/C = 2.67, so theoretically speaking, TOD = 2.67TOC. If the TOD/TOC of a certain water sample is about 2.67, it can be considered to be mainly carbon-containing organic matter. If TOD/TOC>4. o, it should be considered that there is a large amount of organic matter containing S and P in the water; if TOD/TOC <2.6, it should be considered that the content of nitrate and nitrite in the water sample may be large. They decompose and release oxygen under high temperature and catalytic conditions, causing a negative error in TOD measurement.