เครื่องวัดค่าบีโอดี [BOD Measurement System]

• APHA, AWWA, WEF Standard Methods 5210 D
• H55 as a supplement to EN 1899-2

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 measure of the effects of domestic and industrial waste water on sewage plants and outflow points.

The Lovibond^® 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.

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.

The result is a pressure drop in the system, which is directly proportional to the BOD value and is measured by the Lovibond^® 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.

The BOD level of a sample depends on the quantity of organic matter present, which can vary considerably. The Lovibond^® 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. For all measuring ranges, BOD is shown directly in mg/l.

If the measuring period is set at 24 hours, the Lovibond^® 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 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.

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 Lovibond^® 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 Lovibond^® 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 Lovibond^® OxiDirect^® BOD meter. The set contains 8 BOD CM11 test tablets that cause a defined oxygen consumption.

The tablets are easy to use. Simply place a tablet in the BOD bottle, start the measurement process, read off the BOD value after 5 days, and then compare with the defined value. If this value is within the quoted tolerance, this means that the BOD measuring system is functioning correctly.

The microprocessor-controlled Lovibond^® inductive stirring system is non-wearing and maintenance-free. In other words, there are no moving parts in the system.

At regular intervals, the magnetic stirring rods are accelerated and slowed down again, taking them up to maximum speed and back down again. This ensures the centralization of the stirring rods. Stirring rods that move away from the centre of the bottle are re-centered quickly and reliably.
The inductive actuation system guarantees maintenance-free operation (no need to replace drive belts or burnt-out drive motors) for many years.

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 a temperature of 20°C.
For temperature equalisation, we recommend Lovibond^® thermostatically controlled cabinets with a user-selectable temperature from 2°C to 40°C.

TECHNICAL DATA

*No standard is available to check the accuracy of respirometric oxygen uptake measurement. Tests with a glucose-glutamic acid solution having a known theoretical BOD have shown that the variation is approximately 5% in the range of 50…100 mg/l BOD, and 3% for higher range. Minimum Response or sensitivity of respirometric systems is about 0.05 … 1 mg/l.

OxiDirect^®, complete unit with 6 sensor heads and control unit with batteries, Inductive stirring unit with power supply, 6 sample bottles, 6 rubber gaskets, 6 magnetic stirring rods, 1 overflow flask, 157 ml, 1 overflow flask, 428 ml, 1 bottle, 50 ml potassium hydroxide solution, 1 bottle, 50 ml nitrification inhibitor solution, 1 instruction