Biological
Oxygen Demand
What Is Biological Oxygen
Demand And How Does It Affect Water Quality?
Biochemical oxygen demand is a measure of the quantity of
oxygen used by microorganisms (e.g., aerobic bacteria) in the oxidation of
organic matter.
Natural sources of organic matter include plant decay and
leaf fall.
However, plant growth and decay may be unnaturally
accelerated when nutrients and sunlight are overly abundant due to
human influence.
Urban runoff carries pet wastes from streets and
sidewalks; nutrients from lawn fertilizers; leaves, grass clippings, and paper
from residential areas, which increase oxygen demand.
Oxygen consumed in the decomposition process robs other
aquatic organisms of the oxygen they need to live.
Organisms that are more tolerant of lower dissolved
oxygen levels may replace a diversity of natural water systems contain bacteria, which need oxygen (aerobic) to survive.
Most of them feed on dead algae and other dead organisms
and are part of the decomposition cycle.
Algae and other producers in the water take up inorganic
nutrients and use them in the process of building up their organic tissues.
Consumers like fish and other aquatic animals eat some of
the producers, and the nutrients move up the food chain.
When these organisms die, bacteria decompose the organic
compounds and release into the water inorganic nutrients such as nitrate, phosphate, calcium, and others.
Some of these nutrients end up downstream or in
sediments, but most of them recycle again and again. Most of the bacteria in
the aquatic water column are aerobic. That means that they use oxygen to
perform their metabolic activities of decomposition.
Under normal conditions, dissolved oxygen exists in very
low concentrations. Natural levels of oxygen in aquatic systems are always
somewhat depleted by normal levels of aerobic bacterial activity.
In most cases, if dissolved oxygen concentrations drop
below 5 parts per million (ppm), fish will be unable to live for very long.
 |
| Healthy Water |
All clean water species such as trout or salmon will die
well above this level and even low oxygen fish such as catfish and carp will be
at risk below 5 ppm.
When abnormally high levels of aerobic bacterial activity
takes place the level of dissolved oxygen can drop dramatically. Under what
circumstances do this happen? Generally, this occurs when there is some sort of
abnormal "pollution" introduced into the system.
This can occur in the form of organic pollution for
sources such as domestic sewage, septic tank leakage, and fertilizer runoff, or could be
in the form of inorganics from domestic or industrial sources.
Natural sources of organic compounds can also come into
aquatic systems by means of floods, landslides, and erosion.
One of the most important nutrients, which affected BOD in aquatic systems in the recent past
is phosphate pollution from American households.
 |
| Unhealthy Water |
It was discovered decades ago that the addition of
phosphorous to soaps and detergents made them clean better.
By the 1960's, millions of households and businesses were
dumping tons and tons of phosphate down the drain. Eventually, much of this
important nutrient made its way to the watercourses of America.
Because phosphorous is one of the most important limiting
factors (necessary nutrients) in aquatic systems, there began numerous and
widespread algal blooms.
Algal blooms are dramatic population outbursts of growth
in which often one or two species of algae suddenly find the conditions right
for rapid growth.
Because most unicellular algae reproduce asexually by
rapid cell division, it doesn't take long for a species of algae to suddenly
and literally turn the water green with billions and billions of new cells.
Because the conditions necessary to these algal blooms
are sometimes temporary or because the algae exceed the threshold level of some
other limiting factor, the blooms are only temporary.
They often last only a few days. What happens when the
bloom is over? The algal cells don't have enough nutrients and most of them
die.
At this point, the aerobic bacteria become important and
start to decompose the algae.
Because there is so much food for them, they also
experience a sort of bloom, and they literally suck the oxygen out of the
water.
When the oxygen is gone, the bacteria and most other
aerobic creatures in the aquatic system start to die.
During
the 1960's and the 1970's, this phenomenon was widespread with dramatic fish
kills and large segments of slow-moving rivers and lakes becoming almost
abiotic (lifeless) because of high BOD caused by pollution.
The
procedures followed in this exercise involve the collection of water and the
measurement of dissolved oxygen and pH at the time of the collection.
The
samples are placed in bottles full to the brim and sealed off by a lid.
The
sample bottles are covered completely with aluminum foil and placed in a dark
place. This limits the photosynthesis, which could happen with captured algae.
After
five days, the bottles are uncorked and the dissolved oxygen is probed.
The
difference between the first and the last of the samples is called the BOD.
A
low number generally means little pollution and/or little aerobic activity. A
high BOD means the opposite.
RELATED POSTS:
Gases in freshwater
CLICK HERE . . .
.
How Exactly Does Dissolved
Oxygen
Affect Water Quality?
CLICK HERE . . .
.
AERATION
.
CLICK HERE . . .
.
BACTERIA FOR WASTE WATER
CLICK HERE . . .
.
 |
Multi-Media Filter, Highly-Activated Carbon Filter,
Zeolite-Process Water Softener With Brine Tank,
Fiberglass Ballast-Type Pressure Tank
(fully automatic backwash & regeneration)
|
.
PURICARE
Water
Treatment
Systems
.
.
...
Aganan, Pavia, Iloilo, Philippines
...
CLICK HERE . . . to view company profile . . .
 |
Tachmina Laminated
Sand Filter  |
 |
Reverse Osmosis with Cartridge Pre-Filters
|
 |
Ultraviolet Bactericidal System
with Cartridge Pre-Filters |
No comments:
Post a Comment