Oxygen continued consumption

Another technique for organics measurement that overcomes the long period required for the BOD test is the use of continuous respirometry. Here the waste (full-strength rather than diluted as in the standard BOD test) is contacted with biomass in an apparatus that continuously measures the dissolved oxygen consumption. This test determines the ultimate BOD in a few hours if a high level of biomass is used. The test can also yield information on toxicity, the need to... 

Another possibility is to immobilise enzymes either on the sensor element itself or in the vicinity of the sensing element. The operation principle is in most cases a semi-continuous spectral difference measurement in combination with a kinetic data evaluation. A sample containing the analyte of interest is recorded by the sensor immediately after contact with the sample and again after a certain time. Provided that no other changes in the composition of the sample occur over time, the spectral differences between the two measurements are characteristic for the analyte (and the metabolic products of the enzymatic reaction) and can quantitatively evaluated. Provided that suitable enzymes are available that can be immobilised, this may be a viable option to build a sensor, in particular when the enzymatic reaction can not (easily) be monitored otherwise, e.g. by production or consumption of oxygen or a change of pH. In any case, the specific properties and stumbling blocks related to enzymatic systems must be observed (see chapter 16). 

Steady-State Mass Balance Method In theory, the Ki a in an apparatus that is operating continuously under steady-state conditions could be evaluated from the flow rates and the concentrations of the gas and liquid streams entering and leaving, and the known rate of mass transfer (e.g., the oxygen consumption rate of microbes in the case of a fermentor). However, such a method is not practical, except when the apparatus is fairly large and highly accurate instruments such as flow meters and oxygen sensors (or gas analyzers) are available. 

In the fed-batch operation of fermentors (which is also commonly practiced), the feed is added either continuously or intermittently to the fermentor, without any product withdrawal, the aim being to avoid any excessive fluctuations of oxygen demand, substrate consumption, and other variable operating conditions. 

Oxygen Consumption during Exercise A sedentary adult consumes about 0.05 L of 02 in 10 seconds. A sprinter, running a 100 m race, consumes about 1 L of 02 in 10 seconds. After finishing the race, the sprinter continues to breathe at an elevated (but declining) rate for some minutes, consuming an extra 4 L of 02 above the amount consumed by the sedentary individual. 

---