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Natural organic detritus and organic waste from waste water treatment plants, failing septic

systems, and agricultural and urban runoff, acts as a food source for water-borne bacteria.

Bacteria decompose these organic materials using dissolved oxygen (DO, thus reducing the

available DO for fish and other a!uatic organisms.

Biochemical Oxygen Demand (BOD is a measure of the oxygen used by microorganisms to

decompose waste. "he more organic waste present, the more bacteria there are decomposing

this waste and using oxygen, so the BOD level will be high. "he oxygen may diminish to levels

that are lethal for fish and a!uatic insects. #s the river re-aerates due to atmospheric mixing and

as algal photosynthesis adds oxygen to the water, the oxygen levels will slowly increase

downstream. "he drop and rise in DO levels downstream from a source of BOD is called the DO

sag curve.

Nitrates and phosphates in a body of water can contribute to high BOD levels, by providing the

nutrients for plants and algae to grow !uic$ly. "his contributes to organic waste in the waterwhen the plants die, which are then decomposed by bacteria.

BOD is determined by measuring the loss of oxygen from the beginning to end of a % day test.

"he amount of oxygen consumed by these organisms in brea$ing down the waste is $nown as

the biochemical oxygen demand or BOD.

&hemical Oxygen Demand (&OD measurements can be made in 'ust a few hours instead of

the % day BOD test to estimate BOD levels. &OD does not differentiate between biologically

available and inert organic matter, and it is a measure of the total !uantity of oxygen re!uired tooxidie all organic material into carbon dioxide and water. &OD values are always higher than

BOD values. )any wastewater treatment facilities use the faster &OD test to estimate BOD

levels. "he *+# re!uires wastewater treatment plants to bring BOD within limits before

discharging treated wastewater, thus measuring BOD or &OD in treated water is an important

part of the monitoring process.

Biochemical Oxygen Demand (BOD)/Chemical Oxygen Demand (COD)

Natural organic detritus and organic waste from waste water treatment

plants, failing septic systems, and agricultural and urban runoff, acts as a food

source for waterborne bacteria! Bacteria decompose these organic materials

using dissol"ed oxygen, thus reducing the DO present for fish! Biochemical

oxygen demand (BOD) is a measure of the amount of oxygen that bacteria will

consume while decomposing organic matter under aerobic conditions!

Biochemical oxygen demand is determined by incubating a sealed sample of

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water for fi"e days and measuring the loss of oxygen from the beginning to the

end of the test! #amples often must be diluted prior to incubation or the

bacteria will deplete all of the oxygen in the bottle before the test is complete!

$he main focus of wastewater treatment plants is to reduce the BOD in the

effluent discharged to natural waters! %astewater treatment plants aredesigned to function as bacteria farms, where bacteria are fed oxygen and

organic waste! $he excess bacteria grown in the system are remo"ed as sludge,

and this &solid' waste is then disposed of on land! Chemical oxygen demand

(COD) does not differentiate between biologically a"ailable and inert organic

matter, and it is a measure of the total uantity of oxygen reuired to oxidie

all organic material into carbon dioxide and water! COD "alues are always

greater than BOD "alues, but COD measurements can be made in a few hours

while BOD measurements ta*e fi"e days!

+f effluent with high BOD le"els is discharged into a stream or ri"er, it will

accelerate bacterial growth in the ri"er and consume the oxygen le"els in the

ri"er! $he oxygen may diminish to le"els that are lethal for most fish and

many auatic insects! s the ri"er reaerates due to atmospheric mixing and

as algal photosynthesis adds oxygen to the water, the oxygen le"els will slowly

increase downstream! $he drop and rise in DO le"els downstream from a

source of BOD is called the DO sag cur"e!