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BIOCHEMICAL OXYGEN DEMAND (BOD)INTRODUCTIONThe level of pollution (industrial or domestic wastewater) is measured with a BOD (Biochemical Oxygen Demand) test. BOD is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. It is usually performed over a 5-...

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BIOCHEMICAL OXYGEN DEMAND (BOD)

INTRODUCTION The level of pollution (industrial or domestic wastewater) is measured with a BOD (Biochemical Oxygen Demand) test. BOD is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. It is usually performed over a 5-day period. It is used in water quality management and assessment, ecology and environmental science. A BOD of 10 mgL-1 represents a litre of a sample requires 10 mg of oxygen for bacteria to successfully undergo oxidization. Most pristine rivers will have a 5 days BOD below 1 mgL-1. Municipal sewage that is efficiently treated would have a value of about 20 mgL-1. Untreated sewage varies, but averages around 600 mgL-1. The BOD test is carried out by diluting the sample with de-ionized water saturated with oxygen, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen and sealing the sample (to prevent further oxygen dissolving in). The sample is kept at 20 ° C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, and the dissolved oxygen is measured again. The difference between the final dissolved oxygen (DO) and the initial DO is the BOD. In Malaysia, the BOD test is typically prepared at 30 ° C. Example A BOD test was conducted for three (3) samples collected from a river. The samples have an initial dissolved oxygen value of 7 mgL -1. At the end of incubation period (5 days), the dissolved oxygen values were 0, 3, and 7 mgL-1. Determine the BOD values for each sample. Case 1 : No result, unless BOD is assumes above 7 mgL-1. Case 2 : BOD5 = 7 – 3 = 4 mgL-1. Case 3 : BOD5 = 7 – 7 = 0 mgL-1. BOD5 is defined as BOD of a sample incubated for 5 days. If BOD value is above 7 mgL -1, then the sample should be diluted. For example (in second case), of 1 mL is diluted to become 100 mL with dilution water (dilution is 100 times original) the dilution factor, P = 1/100 = 0.01, therefore the BOD is 1

(7 – 3)

× 100 = (7 – 3) / 0.01 = 400 mg/L

First order reaction is used to explain the BOD kinetics; dLt / dt = -k Lt Where Lt is oxygen equivalent for organic matter at time t, k is a constant with unit t -1 (per day). Integration of the equation results; Lt / Lo = exp (-k t) Or, log Lt – log Lo = -k t Lt / Lo = 10-kt Lt = Lo (1 - 10-kt) Where Lt is oxygen equivalent for organic matter in the sample at time = 0 and this value is also known as ultimate BOD. As a result, BOD transferred is; BODt = Lt - Lo The constant, k for a wastewater can be estimated by plotting (time / BOD t)1/3 versus time. From the graph, k = 2.61 B/A where B is the slope of the linear graph and A is intersection at (time / BODt)1/3 axis.

OBJECTIVE To estimate the constant, k for a wastewater sample (1 litre).

EQUIPMENT 1) Volumetric flasks 2) Beaker 3) Three (3) 300 ml BOD bottles 2

4) DO meter 5) Incubator

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PROCEDURE 1) Seeded dilution water is prepared by adding 1 ml of the following prepared solutions per litre: phosphate buffer, MgSO4, CaCl2, FeCl and settled sewage (the seed) to distilled water that has been saturated with oxygen by aerating at room temperature. 2) The sample (wastewater) pH is determined using pH meter. The sample is neutralized by adding HCL and the volume added is recorded. 3) The sample is diluted with seeded dilution water (prepared in step 1 above) in 1 litre volumetric flasks. (Example : 1/50 the dilution is equivalent to 20ml of sample in 1 litre volumetric flasks). (Note* It is always good practice to use volumetric flasks for accurate reading; however 1.5 litre cylinder also can use for this experiment). 4) The solution is transferred to BOD bottles (3 bottles). Initial day (zero day) of dissolved oxygen on all three (3) bottles is determined using calibrated dissolved oxygen meter. 5) The bottles are incubated for 3 days at 30 ° C. 6) The residual dissolved oxygen is determined after incubation period of one day, two days and three days (each day only one (1) bottle is determined for its dissolved oxygen). 7) Initial day (zero day) and after 3 days of incubation period of dissolved oxygen is determined for prepared seeded dilution water as well.

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RESULTS 1) Sample pH reading : 7.24 Volume of HCL added : 2.0 ml (t / BODt)1/3

N

DOo

Time, t

DOt

BODt = (DOo – DOt) / P

o

(mgL-1)

(day)

(mgL-1)

(mgL-1)

1

7.8

1

6.6

60

0.255

2

7.9

2

5.0

145

0.240

3

8.0

3

4.8

160

0.266

Calculation BODt = (DOo – DOt) / P = ( 7.8 – 6.6 ) / ( 1/50 ) = 60 mg/L

(t / BODt)1/3 = ( 1 / 60 ) 1/3 = 0.255

2) Plot the graph of (t / BODt)1/3 versus t.

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3) Determine value k by using the graph in (2) above. Given k = 2.61 (B/A), where; A : intersection at the axis B : slope of the linear graph

A = 0.234 B=

=

y2 − y1 x2 − x1

0.266−0.247 3−1.5

= 0.013

k = 2.61 (B/A) = 2.61 (0.013 / 0.234) = 0.145

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DISCUSSION 1)

List factors which may affect the experiment. There are many factors that effect the experiment such as human error, apparatus and environmental error. Firstly, is human factor that is parallax error. The eye level not perpendicular to the scale of burette reading while adding the HCL and would make this parallax error. The sample that we get is in condition of alkaline. So we add the hydrochloric acid to the sample by using the burette. Besides, students maybe not stir the sample properly. This can affect the result. In the order way, students maybe not draw or calculate the result carefully. Next, the apparatus factor. The apparatus might not be cleaned thoroughly with distilled water before used and the lit of BOD bottles are removed to early before the reading is taken. Besides, the apparatus could make calibration before use. This can affect to the result of experiment. We also need to wait for a while until the meter gave a static reading to get an accurate reading. The last one is the environment factor. This happened when we take the sample that is not exposed to the sun light. The temperature could be low and this can make the microorganism in the sample die and the BOD level could less accurate. We should fine sample that we can see there are microorganisms in there. Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose this waste. If there is a large quantity of organic waste in the water supply, there will also be a lot of bacteria present working to decompose this waste. In this case, the demand for oxygen will be high (due to all the bacteria) so the BOD level will be high. As the waste is consumed or dispersed through the water, BOD levels will begin to decline. The factor affecting BOD level in the sample is :8

1. The main factor that contributes to a high BOD in a food processing plant waste water is the presence of high level of organic matter or food in the waste water. Other organic matters that contribute to high BOD in a body of water are dead plants, leaves, grass clippings, manure and sewage. As mentioned above, when organic matter level in the water supply is high, the bacteria will begin the process of breaking down this waste. When this happens, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live. 2. Nitrates and phosphates - in a body of water can contribute to high BOD levels. Nitrates and phosphates are plant nutrients and can cause plant life and algae to grow quickly. When plants grow quickly, they also die quickly. This contributes to the organic waste in the water, which is then decomposed by bacteria. This results in a high BOD level. 3. The temperature of the water - can also contribute to high BOD levels. For example, warmer water usually will have a higher BOD level than colder water. As water temperature increases, the rate of photosynthesis by algae and other plant life in the water also increases. When this happens, plants grow faster and also die faster. When the plants die, they fall to the bottom where they are decomposed by bacteria. The bacteria require oxygen for this process so the BOD is high at this location. Therefore, increased water temperatures will speed up bacterial decomposition and result in higher BOD levels. When BOD levels are high, dissolved oxygen (DO) levels decrease because the oxygen that is available in the water is being consumed by the bacteria. Since less dissolved oxygen is available in the water, fish and other aquatic organisms may not survive. Therefore, if a food processing plant waste water has a higher BOD level when it is 9

discharged to the city waste water system, it will cost the company a lot of money that the state government charges them to treat this water and decrease its level BOD to a safe level before discharging it into the natural water reservoirs, so it will not negatively affect the life of the fish and other aquatic organisms.

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SUGGESTIONS/ COMMENTS 1) Take the readings when the meter gives a static reading. 2) Clean up the beakers before we use by using distilled water. 3) DO meter should be clean up before we use by distilled water. 4) Cover all abrasions and wear good quality latex gloves when in direct contact with raw wastewater. 5) Read all labels carefully and know what to do in case of a spill. 6) Always use a pipette bulb. NEVER pipette anything by mouth. 7) Always pour acids or bases into water, never add water to the acid or base. Mixing concentrated acids or bases with water can create a significant amount of heat.

CONCLUSION Based on the experiment conducted, the water is moderately polluted since the DO value is in the range of 2 to 8 mgL -1 but the initial reading is 7.8, 7.9, and 8.0 for the samples of three bottles. Besides that, to get more accurate value of BOD level we could analyse the factor that can affect the experiment. As example, we should the clean all the apparatus using distilled water before start the experiment. Besides, we could make a calibration to the apparatus before us, the calibration process should do twice per month or before use it. Next, we should truth each other in the group and do the experiment together. The value of BOD is in range 0.2 to 0.25 mg/L. The value of constant, k for a wastewater sample is 0.145. BOD Level (in ppm)

Water Quality

1-2

Very Good There will not be much organic waste present in the water supply.

3-5

Fair: Moderately Clean

6-9

Poor: Somewhat Polluted Usually indicates organic matter is present and bacteria are decomposing this waste.

100 or greater

Very Poor: Very Polluted Contains organic waste. 11

REFERENCES 1) Labsheet Environmental Laboratory, 2015/2016/1 UTM Kuala Lumpur 2) Dr Shreeshivadasan a/l Chelliapan and Mrs Nor Syaziana binti Mohd Radzi; Notes and Lecture Module DDPA 2522 Environmental Engineering 2014/2015/2 UTM Kuala Lumpur. 3)

https://water.usgs.gov/owq/FieldManual/Chapter7/NFMChap7_2_BOD.pdf

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APPENDIX

Take the sample of water

Take the pH value of water sample

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Take Do (initial) reading for all three bottles

BOD bottles

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DO meter

15...