Biochemical Oxygen Demand ( BOD)
BOD – biochemical oxygen demand – is an expression for the quantity of oxygen required for biological degradation of organic matter in a waste water sample. BOD measurement is therefore used as a basis for the detection of biologically degradable organic matter in water. The difference between BOD and chemical oxygen demand (COD) is that COD additionally registers biologically non-degradable organic matter.
BOD measurement is therefore an important measurement of the effects of domestic and industrial waste water on sewage plants and outflow points.
Respirometric BOD measurement using the AL606
The AQUALYTIC® sensor system AL606 is a 6 sample system that allows precise measurements of BOD based on the manometric principle. Manometric respirometers relate oxygen uptake to the change in pressure caused by oxygen consumption while maintaining a constant volume. Thanks to the modern integral pressure sensors, it is no longer necessary to use mercury for pressure measurements.
Measuring ranges and sample volumes
The BOD level of a sample depends on the quantity of organic matter present, which can vary considerably. The BOD measuring system AL606 is therefore calibrated for the various sample volumes and the corresponding measuring ranges listed in the table below. The overall measuring range of the system is 0 – 4000 mg/l.
For all measuring ranges, BOD is shown directly in mg/l
Respirometric methods provide direct measurements of the oxygen consumed by microorganisms from an air or oxygenenriched environment in a closed vessel under conditions of constant temperature and agitation. Carbon dioxide produced metabolically by the bacteria is chemically bound by the potassium hydroxide solution contained in the seal cup in the bottle.
The result is a pressure drop in the system, which is directly proportional to the BOD value and is measured by the AQUALYTIC® BOD sensor. The BOD level is then displayed directly in mg/l.
The BOD values are stored in the sensor memory and can be called up on the large-format display at any time without the need for time-consuming conversion using factors. This means that test series that end on a Sunday can be evaluated during the following week without any problem.
The measurement period is user-selectable between 1 and 28 days to suit the application. While short measurement periods are useful for scientific applications, standard BOD measurements typically extend over a period of 5 days – and manometric determination of OECD, for example, generally takes place over a period of 28 days.
Evaluation of measurements
If the measuring period is set at 24 hours, the AQUALYTIC® AL606 BOD measuring system records a measurement once every hour. With a measuring period of 48 hours, the unit measures and stores a BOD value once every 2 hours. If the measuring period is between 3 and 28 days, one value is measured and stored each day.
Current values and stored values may be called up at any time. The table/graph below illustrates an example of BOD5 evaluation. The development of BOD over a period of 5 days is easily seen.
Automatic start function
Variations in sample temperature prior to testing result in pressure variations within the measuring system during the temperature equalisation period in the thermostatically controlled cabinet (if BOD measurement is to take place at 20°C, for example). Such variations would normally cause errors during manometric measurement. In order to prevent such errors, the AQUALYTC® AL606 BOD meter is equipped with an automatic start feature:
measurement does not commence until the temperature in the samples is the same as that in the thermostatically
controlled cabinet. This rules out the possibility of temperature (and hence pressure) fluctuations that are not related to the manometric measurement.
The complete AL606 measuring system
In addition to the BOD unit for measurement and storage of BOD levels, the AQUALYTC® AL606 BOD measuring system includes sample bottles, measuring sensors, non-wearing inductive stirring system, overflow measuring flasks for metering of sample volumes, nitrification inhibitor and potassium hydroxide as an absorbent.