The biochemical oxygen demand (BODn) is the mass of oxygen which is needed by microorganisms during a test in closed flasks over a period of 5 (BOD5), 10 (BOD10) or 20 days (BOD20) at a temperature of 20 °C, pH 7-8 and after the addition of nutrients. Additional bacteria have to be injected into samples which contain too few bacteria. In these cases, biologically treated wastewater or contaminat-
2.3 Methods for Measuring Dissolved Organic Substances as Total Parameters 35
ed surface water can be added to domestic wastewater after settling. The BOD5 of these supplements must be taken into account (Fresenius et al. 1988).
Depending on the concentration of biodegradable substances, it may be necessary to dilute with water containing various nutrients. Four different solutions with special nutrients should be prepared, which can be used simultaneously as dilution water. This should be saturated with oxygen.
Predominately, two different methods are used to measure the amount of oxygen needed. The quantity of dissolved oxygen consumed is determined by measuring the difference of dissolved oxygen, either electrometrically using an electrode or manometrically (Ramalko 1983; Fresenius et al. 1988).
In the first case, glass flasks of 110-130 ml or 250-300 ml (with stoppers) are prescribed, which must be totally filled after they are diluted accordingly with water.
In the second case, the sample is put into a vessel leaving a space for air, which can be closed off from the atmosphere. The sample must be stirred or shaken during the measurement in order to avoid any influence of mass transfer on the O2 consumption rate. The CO2 produced, which is not dissolved or converted to HCO-, is adsorbed by potassium hydroxide. According to the volume of the air enclosed in the bottle, a certain amount of oxygen is available, so that higher dilutions are not necessary.
Usually, BOD measurements are made after removing the settleable solids from the sample by sedimentation. Therefore, the measurement includes not only the dissolved organics but also the organic colloids and the non-settleable solids, which may partly be hydrolyzed and used by aerobic microorganisms. In addition, bacteria use oxygen by endogenous respiration; and some of them die and thus produce new substrate after cell lysis. The most important disadvantage of the method, however, is its long measuring time. BOD5 is not suitable as a control parameter for treatment plants if rapid measurements are necessary.
Several rapid BOD measuring devices have been developed to improve response time. The ZAW-LAB and ZAW-FIX (GIMAT) are semi-continuous analyzers for the on-line determination of respirometry rates. A sample is saturated with oxygen and mixed with an adapted culture. The oxygen consumption rate is determined from the sinking dissolved oxygen concentration over time and is used for the calculation of the BOD5. The Bio Monitor from LAR (Laser and Analytical Research GmbH) works continuously. Two small, four-step cascades are operated in parallel. One is supplied with air, activated sludge and sample, and another only with air and activated sludge. After a retention time of only 1-4 h, the substrate is nearly completely consumed, resulting in a corresponding oxygen consumption, which is calculated from the difference between the two cascades. The short measuring time results from the elevated temperature of 30-35 °C and the higher bacterial concentration.
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