Total oxygen demand

In the instrumental total oxygen demand (TOD) test, organic and some inorganic compounds are converted to stable end products, such as CO2 and H2O, in a platinum-catalyzed combustion chamber. The TOD is determined by the loss of oxygen in the nitrogen-carrier gas [11]. 

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Another test is the total oxygen demand (TOD) test, which oxidizes the waste in the presence of a catalyst at 900°C in a stream of air. Under these harsh conditions, all the carbon is oxidized to CO2. The oxygen demand is calculated from the difference in oxygen content of the air before and after oxidation. The resulting value of TOD... 

Several methods have been developed to estimate the oxygen demand in waste water treatment systems. Commonly used laboratory methods are biochemical oxygen demand (BOD), chemical oxygen demand (COD), total oxygen demand (TOD), total organic carbon (TOC), and theoretical oxygen demand (ThOD). 

Total oxygen demand (TOD) The total amount of molecular oxygen consumed in the combustion of oxygen-demanding substances at about 900°C. 

The chemical compositions of materials are usually expressed in terms of simple oxides calculated from elemental analysis determined by x-ray fluorescence. For spent foundry sand, the chemical parameters include bulk oxides mass composition, loss on ignition, and total oxygen demand. Table 4.6 lists the general chemical properties of spend foundry sand. It is shown that spent foundry sand consists primarily of silica dioxide. 

Total oxidation reactions, in the presence of water, 19 87 Total oxygen demand (TOD) in wastewater treatment, 25 885 Total phosphorus content, determining,... 

In the Philips total oxygen demand meter the zirconium oxide is specially prepared and is in the form of two series connected tubes or cells through which a nitrogen carrier gas is allowed to flow. Each cell is provided with two pairs of annular (internal and external) electrodes and the whole assembly is maintained at about 600°C. 

Voorn and Marlow [176] applied this technique to industrial and municipal waste waters. Fig. 12.16 shows TOD/COD ratios obtained from some waste water samples. Where the total oxygen demand value is... 

Marty and Aim [177] described an automatic determination of the total oxygen demand of waste water. The method gives rapid results and correlates well with chemical oxygen demand and biochemical oxygen demand. Measurements of corrections are made for nitrogen compounds which are the principal interferents. 

Other workers who have discussed the determination of total oxygen demand include Wells [178] and Ravenscroft [179]. 

When nitrification occurs 4.57 gram oxygen are used per gram of ammonia nitrogen oxidised. Thus the total oxygen demand is then the sum of the autotrophic and heterotrophic respiration rates. 

The oxygen demand by the nitrifying bacteria is therefore a measure of the ammonia concentration in the sample. For this rapid determination method a detection limit for ammonium in the ppb-range was observed. Because this biosensor reacts also to nitrite and urea, but shows only little reaction to other C-compounds (see Table 7), it is further suitable for the summary quantitation of nitrifiable N-compounds, the so-called N-BOD [85,861. This parameter can be useful for the control of aerobic wastewater treatment processes, due to the fact that the oxygen demand for nitrifiable N-compounds can make up to 20-40% of the total oxygen demand. 

Total oxygen demand (TOD) tells us how much 02 is required for complete combustion of pollutants in a waste stream. A volume of N2 containing a known quantity of 02 is mixed with the sample and complete combustion is carried out. The remaining 02 is measured by a potentiometric sensor (Box 17-1). Different species in the waste stream consume different amounts of O,. For example, urea consumes five times as much 02 as formic acid does. Species such as NH3 and H2S also contribute to TOD. 

In contrast to the BOD test, which is designed to measure the oxygen demand of only the biochemically oxidizable carbon compounds present in the sample, the COD test gives a close measure of the total oxygen demand of the sample. It is also a practical method to obtain a valid oxygen demand result when the sample contains toxic substances, which cannot be easily neutralized. A standard BOD test conducted under these conditions would give a low or zero result from the toxicant action on the microorganisms, even with biochemically oxidizable material present. 

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