BOD-OxiDirect BOD Measurement System
Accurate, Direct, Easy to use Flexible, (m=1-28 days) Environmentally-friendly.
Applications: Waste Water, Determination of Biological Activity, Waste Water Treatment plants, Analytical Laboratories, Research Centers, Universities
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 the COD additionally registers biologically non-degradableorganic matter.
BOD measurement is therefore an important measure of the effects of domestic and industrial waste water on sewage plants and outflow points.
Manometric, respirometric BOD measurement using the AQUALYTIC® OxiDirect® BOD meter
The AQUALYTIC® sensor system OxiDirect® is a 6 sample system that allows precise measurement 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 measurement.
The principle of respirometers by AQUALYTIC® OxiDirect®
Respirometric methods provide direct measurement of the oxygen consumed by microorganisms from an air or oxygen-enriched 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.
Measuring ranges and sample volumes
The BOD level of a sample depends on the quantity of organic matter present which can vary considerably. The AQUALYTIC® BOD measuring system OxiDirect® 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.
Evaluation of measurements
If the measuring period is set at 24 hours, theAQUALYTIC® OxiDirect® 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 2 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 AQUALYTIC® OxiDirect® 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.
In addition to the BOD unit for measurement and storage of BOD levels, the AQUALYTIC® OxiDirect® 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.
We also supply a test set to check for correct operation of the AQUALYTIC® OxiDirect® BOD meter. The set contains 8 BOD CM1 test tablets that cause a defined oxygen consumption.
The microprocessor-controlled AQUALYTIC® inductive stirring system is non-wearing and maintenance-free. In other words, there are no moving parts in the system.
Temperature equalisation during BOD measurement
Temperature equalisation is essential prior to biological testing, as temperature has a major effect on biological activity. BOD measurements, for example, are always performed in a thermostatically controlled cabinet at atemperature of 20°C.