d. water biological properties

8/7/2019 d. Water Biological Properties

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ENVIRONMENTAL

TECHNOLOGY (CE20110)WATER

Biology


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LESSON OBJECTIVES

Biological Oxygen Demand (BOD).

Chemical Oxygen Demand (COD).

Water Hardness.


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1a. Biological Oxygen Demand (BOD)

Oxygen (O2) both in the dissolved andgaseous forms are necessary on Earth.

It is a crucialelement for life in all livingorganisms, e.g. humans, plants, animals,bacteria etc.

Dissolved Oxygen is necessary for aquaticlife. So enough dissolved oxygen must bepresent in natural water


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1b. Biological Oxygen Demand (BOD)

BOD is the amount ofoxygen required by

bacteria.

Bacteria uses oxygen to:

Decompose (break-down)organic matter.

Make food to sustain

their lives.


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1c. Biological Oxygen Demand (BOD)

Example of a chemical equation forbacterial breakdown of an organicwaste:

C50H100O40N (s) + 54.25 O2(g)

50 CO2(g) + 48.5 H2O (l) + NH3(g)


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1e. Biological Oxygen Demand (BOD)

Testing method of BOD (contd):

Bottle B is kept for 5 days after which theamount of O2 is measured.

The difference in O2 readings in bothbottles is the BOD of the sample.

Measurement of O2 is done with a DissolvedOxygen (DO) probe.


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1f. Biological Oxygen Demand (BOD)

Testing method of BOD (contd):

Results shown will be represented asBOD5.

Refers to the amount of oxygen used bybacteria in the sample in the 5 days

period of the experiment.


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1f. Biological Oxygen Demand (BOD)

So BOD5 = BOD results after 5 days

BOD3 = BOD results after 3 days

BOD20 = BOD results after 20 days


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1g. Biological Oxygen Demand (BOD)

Samples frequently used for BODmeasurement includes:

Domestic wastewater. Industrial wastewater.

Sewage water.

Tap water.Soil.


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1h. Biological Oxygen Demand (BOD)

Measurement units of BOD is in terms ofmg/L or ppm.

Typical range of BOD5 readings:

Clear lake = < 2mg/L.

Tap water = 5 to20 mg/L.

Sewage = 100 to 800 mg/L.

Industrial wastewater = 1000 to 9000 mg/L.


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2a. Chemical Oxygen Demand (COD)

COD is the total amount of O2 needed tooxidize all organic matter into CO2 and H2O.

Compared to BOD values, COD values arealways higher.

Organic matter are those that contain oxygenin their molecular structure.


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2b. Chemical Oxygen Demand (COD)

Organic matter can be natural, e.g. humans,animals etc.

OR

can also be man-made (artificial), e.g.plastics etc.


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2c. Chemical Oxygen Demand (COD)

Testing method of COD:Put sample of known weight in a bottle.

Pour in a measured amount of oxidizingchemical (excess amount) onto the sample.

Leave the sample for 3 hrs for completereaction.


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2d. Chemical Oxygen Demand (COD)

Testing method of COD (contd):

Mathematical calculation of amount of

oxygen present.

Measurement of COD is written as mg/L

COD.


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3a. Water hardness

Hardness:

Measurement of the amount of ions in

water, e.g. magnesium (Mg2+), calcium(Ca2+).

Higher the amount of these ions, theharder is the water.


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3b. Water hardness

Levels of water hardness:

Hardnessclassification

Amount of calciumcarbonate (mg/l)

Soft 0 -75

Moderately hard 75 -150

Hard 150 -300

Very hard > 300


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3c. Water hardness

Effects of water hardness in industry:

Scalingin industrial water pipes. Deposits of magnesium hydroxide

[Mg(OH)2] and calcium carbonate(CaCO3).

Causes pipes diameter to decrease andpressure to build up.


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3e. Water hardness

Prevention of water hardness:

Addition of softening chemicals. Exchange of insoluble Ca2+ and Mg2+ ions

with soluble sodium (Na+) ions.

Using an ion-exchange column to removesalts.


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4a. Other unwanted chemicals

Chemical Source Problem

Arsenic (As) Pesticides Toxic

Cadmium (Cd) Industrial waste Toxic

Copper (Cu) Pipes/herbicide Poor taste

Iron (Fe) Pipes Clogging

Lead (Pb) Industrial waste ToxicMercury (Hg) Industrial waste Toxic

Chloride (Cl) Wastes Corrosion


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4a. Water testing

When testing water samples, samplesshould be stored at 4 0C

d. water biological properties

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