EXPERIMENT ON BIOCHEMICAL OXYGEN DEMAND (B.O.D) TEST

Experiment on determination of  Biochemical Oxygen Demand (BOD) in a given water sample.

AIMS AND OBJECTIVE

The aims and objectives of the experiment includes:

·        To determine the Biochemical Oxygen Demand of a given sample of Wastewater.

·        To establish the concentration of organic matter in waste water samples.

·        To assess the quality of surface waters.

THEORY

BIOCHEMICAL OXYGEN DEMAND (BOD)

Biochemical oxygen demand or BOD is a chemical procedure for determining the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period.

 It is not a precise quantitative test, although it is widely used as an indication of the organic quality of water. It is most commonly expressed in milligrams of oxygen consumed per liter of sample during 5 days (BOD) of incubation at 20°C and is often used as a robust surrogate of the degree of organic pollution of water. BOD directly affects the amount of dissolved oxygen in rivers and streams. The rate of oxygen consumption is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water. The greater the BOD, the more rapidly oxygen is depleted in the stream. This means less oxygen is available to higher forms of aquatic life. The consequences of high BOD are the same as those for low dissolved oxygen: aquatic organisms become stressed, suffocate, and die.

Sources of BOD include topsoil, leaves and woody debris; animal manure; effluents from pulp and paper mills, wastewater treatment plants, feedlots, and food-processing plants; failing septic systems; and urban stormwater runoff.

BOD is affected by the same factors that affect dissolved oxygen. BOD measurement requires taking two measurements. One is measured immediately for dissolved oxygen (initial), and the second is incubated in the lab for 5 days and then tested for the amount of dissolved oxygen remaining (final). This represents the amount of oxygen consumed by microorganisms to break down the organic matter present in the sample during the incubation period.

METHODOLOGY

APPARATUS

Apparatus used in the experiment include:

·        BOD bottle

·        Water bottle

·        Pipette

·        Conical flask

·        Burette and burette stand

·        Measuring Cylinders

·        Incubator.

MATERIALS USED

·        10% Manganese chloride

·        Alkaline Iodide solution ( 20% NaOH, 15% KI)

·        25% Hydrochloric Acid

·        0.05M Sodium thiosulphate

·        Starch Solution.

  

PROCEDURE

DISSOLVED OXYGEN CONTENT      

·       About 20cm3 of the water sample was pipette from the bottle.

·        10% v/v of MnCl solution was added to the already pipette water sample.

·        5.0cm3 of alkaline iodide solution (20% NaOH, 15% w/w KI) was then added to the solution.

·        The bottle was stoppered to avoid trapping air bubbles and mix several times by inverting.

·        10cm3 of 25% HCl solution was pipetted into the bottle and again stoppered and mixed.

·        The content was then poured into a 500cm3 conical flask and the liberated iodine was titrated with 0.05M Sodium thiosulphate using starch as end-point indicator.

·        Using appropriate relation, the Dissolved Oxygen content can thus be calculated in mg/litre.

o   DO = (titre × molarity × 8000)/ V(cm3) of sample taken

·        The same procedure highlighted was used to determine the dissolved oxygen contents of the stored samples after 5days.

·        The BOD is the difference in the two dissolved oxygen contents.

OBSERVATION

The following were observed during the course of the experiement:

·        A dirty brown colour was noticed during stopper process

·        Upon addition of HCl, a wine colour was observed.

  

RESULTS

DATA:

Volume of water sample used = 20 cm³

Titre value obtained for day 1 = 25 ml (Given)

Titre value for day 5 = 18 ml (assumed)

CALCULATIONS:

DO = (titre × molarity × 8000)/ V(cm3) of sample taken

For Day 1:

DO₁ = (25ml × 0.05M × 8000) / 20cm³

       = 500 ml/gm

For Day 5:

          DO₅ = (18ml × 0.05M × 8000) / 20cm³

                       = 360 ml/gm

Thus,

          BOD = DO₁ - DO₅

                              = 500 – 360

                   = 140 ml/gm

CONCLUSION AND RECOMMENDATION

According to Standard A and Standard B by Environmental Quality (Sewage and Industrial Effluent) Regulations 1978, the BOD of the sample (140 mg/ml) falls out of the acceptable range according to these standards. Hence, it can be safely concluded the water sample has been polluted.

With this, it is recommended that the water sample should be adequately treated before use.

Note: Standard A (20mg/l) is the indicator for drinking water.

Standard B (50mg/l) is the indication for inland water quality.

REFERENCES:

• ·       https://en.m.wikipedia.org/wiki/Biochemical_oxygen_demand
• ·        www.freedrinkingwater.com/water_quality/quality1/1-bod-effects-on-water-quality.htm
• ·        www.gaepd.org/Files_PDF/techguide/wpb/devwtrplan_b.pdf
• ·        serc.carleton.edu/microbelife/research_methods/environ_sampling/oxygen.html
• ·       www.ut.ee/ams/wpcontent/uploads/2012/09/Irja_Helm_PhD_Dissolved_Oxygen_Gravimetric_Winkler.pdf