Biological Oxygen Demand BOD Teating and Measurement in water and waste water

 PURPOSE:

To describe the Laboratory standard operating procedure for measurement of Biological Oxygen Demand of Water and Waste Water samples.

SCOPE:

Applicable to the Laboratory for analysis of Water and Waste Water samples, where measurement of Biological oxygen demand is required.

 

RESPONSIBILITY: 

Lab chemist: Analysis of sample for measurement of Biological Oxygen demand.

Technical Manager:   Review of activity 

Quality Manager: Implementation and compliance of SOP.

PROCEDURE: 

Principle:

Biochemical Oxygen Demand (BOD) is defined as the amount of O2 required by microorganisms while stabilizing biologically decomposable organic matter in a waste under aerobic conditions.

Instrument and Equipment:

BOD bottles 300ml capacity, BOD Incubator to be controlled at 27oC ± 1oC.

Reagents: 

Phosphate buffer: Dissolve 8.5 g KH2PO4, 21.75 g K2HPO4, 33.4 g Na2HPO4.7H2O and 1.7 g NH4Cl in distilled water and dilute to 1000ml. Adjust its pH to 7.2.

Magnesium sulfate: Dissolve 82.5 g MgSO4.7H2O and dilute it to 1000ml.

Calcium chloride: Dissolve 27.5 g CaCl2 and dilute it to 1000ml.

Ferric chloride solution: Dissolve 0.25 g FeCl3.6H2O and dilute it to 1000ml.

Sodium thiosulfate solution 0.025 N: Dissolve 6.205 g Na2S2O3 and dilute to 1000ml.

Test Method:

Preparation of dilution water:

Aerate the required volume of distilled water in a container by bubbling compressed air for 1-2 days to attain DO saturation. 

Add 1ml each of phosphate buffer, magnesium sulfate, calcium chloride and ferric chloride solution for each liter of dilution water. Mix well.

In the case of the wastes which are not expected to have sufficient bacterial population add seed to the dilution water. Generally, 2ml settled sewage is considered sufficient for 1000ml of dilution water

Dilution of sample:

Neutralize the sample to pH amount 7.0 if it is highly alkaline or acidic.

The sample should free from residual chlorine. If it contains residual chlorine   remove it by using Na2S2O3 solution as follows.

Removal of residual chlorine:

Take 50ml sample and acidify with 10ml of 1 + 1 acetic acid.

Add approximately 1g of KI

Titrate with Na2S2O3 using starch as indicator.

Calculate the amount of Na2S2O3 required per ml of the sample and add accordingly to the sample tested for BOD.

Samples having high DO content, i.e., DO 9 mg/L due to either algal growth or some other reason, reduce the DO content by aerating the agitating the samples.

Make several dilutions of the prepared samples so as to obtain about 50% depletion of DO in the dilution water but no less than 2 mg and the residual oxygen after 3 days of incubation should not be less than 1mg/L.Prepare dilution as follows:

Siphon out seeded dilution water in a measuring cylinder or volumetric flak half the required volume. Add the required quantity of carefully mixed sample. Dilute to the desired volume by siphoning dilution water and mix well.

The following dilutions are suggested:

  %   dilution                 BOD Range

      0.01                         20,000 -70,000

      0.02                          10,000 - 35,000

      0.05                          4000 -  14,000

       0.1                           2000 - 7000

       0.2                           1000 - 3500

       0.5                            400 - 1900

         1                              200 - 700

         2                              100 - 350

        5                               40 - 140

       10                               20 - 70

        20                              10 - 35

        50                               Up to 14

        100                             1 - 7

Siphon the dilution prepared as above labeled BOD bottles as demonstrated and stopper immediately.

Keep 1 bottle for determination of the initial DO and incubate 3 bottles at 27oC for 3 days. See that the bottle has a water seal.

Prepare a blank in duplicate by siphoning plain dilution water (without seed) to measure the O2 consumption in dilution water.

Fix the bottles kept for immediate DO determination and blank by adding 2ml MnSO4 followed by 2ml NaOH + KI + NaN3 as described in the measurement of DO

Determine DO in the sample and in the blank on initial day and after 3 days

Calculation:

Calculate BOD of the ample as follows

Let D0    =    DO in the sample bottle on 0th day.

               D1    =    DO in the sample bottle on 3rd day.

                   C0    =    DO in the blank bottle on 0th day.

                   C1    =    DO in the blank bottle on 3rd day   

 C0 - C1 =     DO depletion in the dilution water alone

 D0 - D1   =    DO depletion in the sample + dilution water

 (D0 -   D1) -  (C0 - C1) = DO depletion due to microbes

 BOD mg/L = (D0 - D1) – (C0 – C1) mg X decimal fraction of sample used.

If the sample is seeded find out BOD of seed in the above manner and apply corrections, as per demonstrated.