Introduction

Strongly loaded organic industrial waste water, i.e. from sugar- or paper-factories, need to be pre-treated before determining the BOD value. Because of the very high content of organic material in the sample, there is a disparity between the nutrients ratio (C:N:P). The lack of nitrogen (N) and phosphorus (P) causes a strong decrease in microbial capability and abundance, what lead to a subestimate of the BOD as a consequence.
In the following the pretreatment of a sample with so called dilutionwater is described, which contains the missing nutrients and microorganisms and with which an adequate BOD measurement will be obtained.

Operation mode:

The test specimen set-up is mounted on a plate. The plate projects out of the test room and for this reason the door of the test room is cambered. The door of the test room is opened and closed pneumatically. Approx. 10 seconds is needed for opening and closing the door. A special mat is
provided in front of the test chamber. As soon as the mat is stepped on to the door of the testchamber cannot be opened or closed.

Preparation1

A beaker2 for storage and ventilation with air is needed.
Composition:
• Drinking water with 1 % settled wastewater, taken from the effluent of a well working domestic treatment plant
• 4 mg/l urea
• 1,6 mg/l penta-sodium-tri-phosphate.
This composition should be aerated for 3 to 10 days at 20 °C before use.

Sample preparation

To determine the full BOD content, a sample with unresolved ingredients or particles should be homogenised. Dilute the sample with the prepared dilutionwater to an expected BOD value between 100-200 mg/l. In case of unknown BOD calculate with a maximum of 80 % of the COD-value.

Measurement

• Proceed as usual with the pre-treated samples and consider the adequate measuring range (see Instruction Manual)
• A blank value, the BOD of the dilution-water itself, should be determined, because this raises the BOD value and need to be subtracted to obtain the true samplevalue
• The use of nitrification-inhibitor is recommended.
• We strongly recommend to carry out at minimum a parallel determination of each sample for a procedure-check and to calculate the mean value (preparation inaccuracies would be multiplied with the dilution factor later).

ex1

ex2

Tip

We recommend to use preadapted microorganisms, taken from an effluent of a domestic treatment plant, because an artificial cocktail of microbes doesn’t contain all the biochemical capability and complexity of an original, autochtonous microbial population.

1. See German Standard DIN 38 409 – H 51 or international Standard ISO 5815
2. Individual size
3. See table of measurement ranges in the Instruction Manual