Determination of BOD of water

Determination of BOD of water

Aim

To determine the dissolved oxygen concentration and biological oxygen demand of pond water

Principle

Dissolved oxygen of water is of paramount importance to all living organisms and is considered to be a factor which to a greater extent can reveal the nature of the whole aquatic system at a glance.  Dissolved oxygen is used as an indicator of the health of a water body.  Higher dissolved oxygen concentrations are correlated with high productivity and little pollution.  The presence of DO in water is mainly due to Direct diffusion from air and by Photosynthetic evolution by aquatic autotrophs.

The first one is purely a physical process and depends on the solubility of oxygen under the influence of temperature, salinity, water movement, whereas the later is a biological process and depends on the availability of light and rate of metabolic processes resulting in diurnal fluctuations.

Two methods are commonly used to determine DO concentration: The iodometric method which is a titration-based method and The membrane electrode procedure.  In the Iodometric method, divalent manganese solution is added to water, followed by addition of strong alkali in a glass-stopper bottle. DO rapidly oxidize an equivalent amount of the dispersed divalent manganese hydroxide precipitates to hydroxides of higher valence states. In acidic conditions, in the presence of iodide ions, the oxidized manganese reverts to the divalent state and iodine equivalent of the original DO content get liberated. The iodine is then titrated with a stranded solution of thiosulfate and the titration end point can be detected using a starch indicator.  Winkler Method uses titration to determine dissolved oxygen in the water sample. 

Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed or demanded by aerobic organisms to break down organic material present in a given sample at certain temperature over a specific time period.  It is a good index of organic pollution.  If the amount of organic matter in a sewage is more, then more oxygen will be utilised by bacteria to degrade it.  

BOD is calculated by keeping a sample of water containing a known amount of dissolved oxygen for five days in dark under aerobic conditions at 20^OC.   The oxygen content is measured again and BOD calculated.  Since nitrification consumes oxygen significantly, it will result in over estimation of BOD and must be checked by adding 1 ml of 0.5% solution of Allyl thourea.  In the water Samples where more than 70% of initial oxygen is consumed, it is necessary to aerate/oxygenate/dilute the sample with BOD free water to avoid oxygen stress.

BOD level in mg/litre                        Quality of water

            1 - 2                                         - Very Good

            3 - 5                                         - Fair

            6-9                                           - Poor

            100 or more                             - Very Poor

Reagents required

·         Manganese Sulphate solution - 48 g MnSO4.4H2O or 40 g MnSO4.2H2O or 36,4 g MnSO4.H2O in 100 ml distilled water

·         Alkali - iodide – azide reagent

o   Solution A - 500 mg NaOH and 135 g Sosium Iodide in 1 litre distilled water

o   Solution B - 10 g Sodium Azide in 40 ml distilled water

o   Solution A and B are mixed to get alkali -iodide - azide solution

·         Concentrated sulphuric acid

·         0.025 N Sodium Thiosulphate solution

·         Starch Solution - 2 g soluble starch in 100 ml distilled water.

·         Allyl Thio Urea Solution (0.05%)

·         Sulphuric acid (1N) - 2.8 ml of concentrated H2SO4 added to 100 ml distilled water

·         Sodium hydroxide (1 N) - 4 g of NaOH in 100 ml distilled water

Materials required

Water Sample, Pipette, Titration assembly, BOD bottle, BOD Incubator, Flasks, etc

Procedure

• pH of water sample was adjusted to neutral using 1N acid or 1 N alkali Solution.
• The pond water sample was filled in 2 BOD bottles without any bubble formation.
• 1 ml alkyl thiourea was added to 1 bottle.
• 2 ml of manganese sulphate was added slowly to the bottle by inserting pipette below the surface of water so that no air bubbles are introduced via the pipette.
• 2 ml of alkali - iodide - azide reagent was added in the same manner.  The bottle was stoppered with care so as not to allow air entry in to the bottle and mixed the sample by inverting several times.  If oxygen is present, a brownish - orange colour of precipitate or floc will appear.  
• The precipitates formed are dissolved by adding 2 ml of concentrated sulphuric acid by keeping the pipette very near to sample surface.  Again after carefully stoppering, the bottle was inverted several times to dissolve the floc.  
• 50 ml of the sample from the bottle was titrated with 0.025 N sodium thiosulphate, till a pale straw colour is formed.
• 2 ml of starch solution was added, and a blue colour formed
• Titration is continued until the sample colour turns clear.
• The burette reading was noted.

·         Calculation of D.O. in mg/Liter = 8 X 100 X N/V X v

§  V = Volume of sample taken 

§  v = Volume of 0.025N Sodium thiosulphate solution used 

§  N = Normality of titrant = 0.025

§  8 is constant (1ml of 0.025N Sodium thiosulphate solution is equivalent to 0.2mg oxygen).

• The concentration of dissolved oxygen in the sample is equivalnet to the volume of 0.025 N sodium thiosulphate used.   Each ml of 0.025 N sodium thiosulphate added is equal to 1 mg/L Dissolved oxygen. This is recorded as Dissolved Oxygen (D1).  
• The other 2 bottles are kept in BOD incubator at 20^OC and after 5 days, the dissolved oxygen was determined by the above method and the value was recorded as Dissolved Oxygen (D2).
• The BOD of water in mg/Liter was calculated as BOD = D1-D2
• D1 is the initial dissolved oxygen concentration of the sample before 5 days incubation and D2 is the concentration after 5 days of incubation.