Date :
Title: BOD/COD ratio of both natural and synthetic organic chemicals.
Aims:
· To show the necessesity of making dilutions for samples of unknown BOD /COD.
· To show that organic chemicals can exert a BOD and also have bactericidal and toxic effects.
·To calculate the BOD:COD ratio of various samples.
Introduction
In this experiment both a BOD and a COD test will be carried out on synthetic organic chemicals , in this case a coke sample.
The BOD test was originally developed to stimulate conditions which occured following the discharge of an organic effluent into a river. The test now has a wider application particullarly in the evaluation of waste treatment. It is used for measuring waste loadings to waste treatment plants and for determining the efficiency of the waste treatment plants by measuring the BOD removal during the process.
BOD is defined as the amount of oxygen required by bacteria to stabilise the decomposable organic matter in the sample under aerobic conditions. The test has been standardised internationally. It involves measuring the dissolved oxygen of the sample initially (by the Winkler or electrode method) and then following incubation for five days +/-1 hour at 200c +/-10c.
BOD (mg/l O2) = DOinitial - Dofinal or (DO0 - DO5)
BOD is thus a measure of the amount of biodegradable organic matter in the sample. Some common organic materials have very high BOD values as the results of this experiment should demonstrate.
As both the coke sample to be tested and the other samples( ) from the class will have an expected BOD of greater than 7mg/l dilution is necessary to ensure that all the dissolved oxygen is not used up during incubation. For this dilution a standard dilution water will be used containing nutrients necessary for bacterial growth (Na,K,Mg,Ca,Fe,S,N,P in ionic form) and buffers which control the pH and provide some of the nutrients. This is to ensure that growth of the bacteria is not limited by a lack of nutrients or by a change of pH.
Usually if a sample of unknown BOD is being determined, a series of dilutions should be carried out covering a wide range. The number of dilutions will be reduced if a known sample is being used , as is the case here with the expected BOD for coke being between 60,000 and 90,000 mg/l O2. Generally three dilutionsare carried out which overlap one another. Results for diluted samples are calculated on those samples with a DO uptake of at least 2 mg/l , leaving a residual of at least 1 m/l.
As the synthetic organic chemicals to be tested do not contain natural populations of microorganisims, to break them down it is necessary to provide these in the dilution water in the form of seed. The most commonly used seed is sewage effluent, but in the case of this experiment an activated sludge based seed will be used and this will be added to the dilution water before use. The dilution water blank , which will be made at the same time that the samples are analysed, provides quality control on dilution water at the time of analysis as well as on the cleanliness of apparatus such as BOD bottles. The O2 uptake of the blank should not exceed 1mg/l following incubation.
Also to be carried out in this experiment is a COD test on the coke sample. The COD test is used for measuring the organic strength of sewage and industrial wastes also. It is used as an alternative test to the BOD test. The main advantage is that it is a rapid determination compared to the 5 day BOD test. Also it can be used for toxic wastes where the BOD may not be suitable. If the sample contains readily oxidisable organic matter and no toxic matter , then the COD result can be used to appreoximate the carbonaceous BOD value. Once the BOD:COD ratio has been established for a particular effluent of uniform quality the COD can replace the BOD in routine monitoring of the effluent. The main dissavantage of COD is that the test cannot differentiate between biologically oxidisable and biologically inert organic matter. Also it does not give any indication as to the rate at which the biologically active material would be stabilised under natural conditions. In this experiment the hack method will be used for the COD determination as opposed to the sometimes dangerous refluk method
Materials
BOD bottles Dissolved Oxygen meter
Dilution water Various pipetts
Seed material ( Activated sludge) Volumetric flasks
Methods
As per manual except for,
Sample dilution and preparation
As the estimated BOD of the coke sample is 60,000 - 90,000 mg/l O2 The sample must be diluted to give a BOD of betwen 2 and 8 mg/l O2 therefore three dilutions of the original sample were made namely
1/100 ,made by bringing 10 mls of coke to 1l with dilution water in a 1l volumetric flask
1/10,000 made by bringing 1ml of 1/100 to 1l with dilution water in a 1l volumetric flask
1/20,000 made by bringing 0.5ml of 1/100 to 1l with dilution water in a 1l volumetric flask
Results
BOD RESULTS:
Sample used = coke
Table1
Dilution / DO 0 / DO 5 / Difference / BOD 5, mg/l O2Blank / 8.7 / 7.9 / 0.8 / 0.8
1/100 / 8.9 / 0.0 / 8.9 / 0.0
1/10.000 / 8.9 / 2.4 / 6.5 / 57,000
1/20.000 / 8.8 / 4.3 / 4.5 / 74,000
Calculation:
BOD = (DO0 - DO. 5 sample) - (DO.o - DO. 5 blank) * Dilution factor
BOD = (8.8 - 4.5) - (8.7 - 7.9) * 20,000 = 74,000
Average BOD = 65,000 mg/l O2
COD RESULTS:
Table 2
Dilutions / COD mg/l1/250 / 118,000
1/500 / 124,000
1/1000 / 117,000
Average COD = 119,667 mg/l
From a combination of the results the BOD:COD Ratio = 1:1.82
CLASS RESULTS:
Table3
SAMPLE / BOD mg/l D.O / BLANK BOD mg/l / RANGESMILK / 116,000 / 0.8 / 40,000 - 120,000
PLAX / 897,000 / --- / >600,000
COKE / ------/ 5.3 / 60,000 - 90,000
PLAX / 170,000 / 2.7 / >600,000
ANTIFREEZE / 220,000 / 0.0 / 0.5 - 2,000,000
COKE / ------/ 1.8 / 60,000 - 90,000
ETHANOL / ------/ 1.2 / 0.6 - 1,500,000
ETHANOL / 1,600,000 / 1.8 / 0.6 - 1,500,000
TOOTHPASTE / 51,000 / 1.2 / 10,000 - 100,000
ANTIFREEZE / ------/ 2.0 / 0.5 - 2,000,000
FORMALDEHYDE / 255,000 / 3.3 / 1.5 - 2,000,000
COKE / 38,000 / 2.7 / 60,000 - 90,000
CLASS RESULTS FOR BOD:COD RATIO
Table 4
SAMPLE / BOD / COD / BOD:COD RATIOCoke / 65,000 / 119,667 / 1:1.82
Milk / 116,000 / 212,250 / 1:1.83
Plax / 1,500,000 / 295,000 / ------
Coke / ------/ 973,333 / ------
Plax / 170,000 / 340,000 / 1:2
Antifreeze / 1,340,000 / 1,132,500 / 1:0.84
Coke / ------/ 101,250 / ------
Ethanol / ------/ ------/ ------
Milk / 71,500 / 151,000 / 1:2.11
Coke / 54,000 / 112,000 / 1:2.07
Ethanol / 1,600,000 / 2,240,000 / 1:1.4
Toothpaste / 51,000 / 18,750 / 1:.3.6
Antifreeze / ------/ 216,000 / ------
Formaldehyde / 255,000 / 579,200 / 1:2.27
Discussion