Respirograms - Applications


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OXYGEN UPTAKE RATES MEASUREMENTS

Oxygen uptake measurements usually are made using respirometers that consist of a reaction vessel to which wastewater and seed organisms are added on a batch or semi-continuous basis. Reaction vessels can be almost any shape and size that will accommodate the oxygen uptake measurement technique. Variations range from a 300-mL BOD bottle, say when using the conventional OUR test, to 10 L as used with sophisticated respirometers systems. Oxygen consumption is measured by a change in dissolved oxygen in the test slurry, by a change in the composition/content of the headspace gas, by a decrease in pressure within the respirometer vessel caused by depletion of oxygen in the headspace gas, or by monitoring the amount of oxygen needed to maintain a constant oxygen environment/level in the reaction vessel.

Batch oxygen uptake data can be easily plotted to produce respirograms. Mixtures of wastewater and mixed liquor solids are placed into a respirometer vessel followed by monitoring the oxygen uptake rates until the biodegradation reaction has been completed. Respirograms frequently show initial high oxygen uptake rates caused by oxidation of readily biodegradable oganic compounds. Second steps or peaks usually correspond or depict oxygen uptake rates associated with compounds having slower rates of biodegradation. After all the soluble substrates have been depleted, oxygen uptake rates typically fall to respiration/endogenous rates only. The end of biodegradation of soluble organics may occur sharply, slowly, or may not be obvious at all depending on the oxidation rates. This mode of operation give valuable information on biodegradability of the constituents in the wastewater being analyzed and the time needed to oxidize the soluble wastewater constitnuents. Respirograms also can be used to detect and/or monitor nitrification taking place if ammonia oxidation occurs simultaneously with oxidation of organic compounds.

STARTER ILLUSTRATION

This is a very simple/crude graph showing dissolved oxygen levels as about 1,400 mgVSS/L of unfed bugs basically do nothing but stay alive so to speak.

The very same numerical data set can be arranged or plotted to basically witness the oxygen consumption pattern, perhaps the simplest respirogram possible, i.e. nothing happens except biomass respiration uptakes, no biodegradable substrate present..