Grab a cup of Joe, and chew on this with some thought as to you're tank being a living, breathing
Biochemical Oxygen Demand, or BOD, is a measure of the quantity of oxygen consumed by
micro-organisms during the decomposition of organic matter. BOD is the most commonly used
parameter for determining the oxygen demand on the receiving water of a municipal or industrial
discharge. BOD can also be used to evaluate the efficiency of treatment processes, and is an
indirect measure of biodegradable organic compounds in water.
Imagine a leaf falling into a stream. The leaf, which is composed of organic matter, is readily
degraded by a variety of micro-organisms inhabiting the stream. Aerobic (oxygen requiring)
bacteria and fungi use oxygen as they break down the components of the leaf into simpler, more
stable end products such as carbon dioxide, water, phosphate and nitrate. As oxygen is
consumed by the organisms, the level of dissolved oxygen in the stream begins to decrease
water can hold only a limited supply of dissolved oxygen and it comes from only two sourcesdiffusion
from the atmosphere at the air/water interface, and as a byproduct of photosynthesis.
Photosynthetic organisms, such as plants and algae, produce oxygen when there is a sufficient
light source. During times of insufficient light, these same organisms consume oxygen. These
organisms are responsible for the diurnal (daily) cycle of dissolved oxygen levels in lakes and
If elevated levels of BOD lower the concentration of dissolved oxygen in a water body, there is a
potential for profound effects on the water body itself, and the resident aquatic life. When the
dissolved oxygen concentration falls below 5 milligrams per liter (mg/l), species intolerant of low
oxygen levels become stressed. The lower the oxygen concentration, the greater the stress.
Eventually, species sensitive to low dissolved oxygen levels are replaced by species that are
more tolerant of adverse conditions, significantly reducing the diversity of aquatic life in a given
body of water. If dissolved oxygen levels fall below 2 mg/l for more than even a few hours, fish
kills can result. At levels below 1 mg/l, anaerobic bacteria (which live in habitats devoid of
oxygen) replace the aerobic bacteria. As the anaerobic bacteria break down organic matter,
foul smelling hydrogen sulfide can be produced.
BOD is typically divided into two parts- carbonaceous oxygen demand and nitrogenous oxygen
demand. Carbonaceous biochemical oxygen demand (CBOD) is the result of the breakdown of
organic molecules such a cellulose and sugars into carbon dioxide and water. Nitrogenous
oxygen demand is the result of the breakdown of proteins. Proteins contain sugars linked to
nitrogen. After the nitrogen is "broken off" a sugar molecule, it is usually in the form of ammonia,
which is readily converted to nitrate in the environment. The conversion of ammonia to nitrate
requires more than four times the amount of oxygen as the conversion of an equal amount of
sugar to carbon dioxide and water.
When nutrients such as nitrate and phosphate are released into the water, growth of aquatic
plants is stimulated. Eventually, the increase in plant growth leads to an increase in plant decay
and a greater "swing" in the diurnal dissolved oxygen level. The result is an increase in microbial
populations, higher levels of BOD, and increased oxygen demand from the photosynthetic
organisms during the dark hours. This results in a reduction in dissolved oxygen concentrations,
especially during the early morning hours just before dawn.
In addition to natural sources of BOD, such as leaf fall from vegetation near the water's edge,
aquatic plants, and drainage from organically rich areas like swamps and bogs, there are also
anthropogenic (human) sources of organic matter. If these sources have identifiable points of
discharge, they are called point sources. The major point sources, which may contribute high
levels of BOD, include wastewater treatment facilities, pulp and paper mills, and meat and food
Organic matter also comes from sources that are not easily identifiable, known as nonpoint
sources. Typical nonpoint sources include agricultural runoff, urban runoff, and livestock
operations. Both point and nonpoint sources can contribute significantly to the oxygen demand in
a lake or stream if not properly regulated and controlled.
Performing the test for BOD requires significant time and commitment for preparation and
analysis. The entire process requires five days, with data collection and evaluation occurring on
the last day. Samples are initially seeded with microorganisms and saturated with oxygen (Some
samples, such as those from sanitary wastewater treatment plants, contain natural populations of
microorganisms and do not need to be seeded.). The sample is placed in an environment suitable
for bacterial growth (an incubator at 20o Celsius with no light source to eliminate the possibility of
photosynthesis). Conditions are designed so that oxygen will be consumed by the
microorganisms. Quality controls, standards and dilutions are also run to test for accuracy and
precision. The difference in initial DO readings (prior to incubation) and final DO readings (after 5
days of incubation) is used to determine the initial BOD concentration of the sample. This is
referred to as a BOD5 measurement. Similarly, carbonaceous biochemical oxygen test
performed using a 5-day incubation is referred to as a CBOD5 test.
Fish waste, excess food and even filter media play a vital role in the success or failure of this
environment. But ultimately it is the lack of oxygen that kills fauna, very high levels of C02 can do
this also obviously, but with high levels of C02 you also get lower levels of oxygen, so the proper
amount of surface agitation is vital.
High quality filter and media plays a vital role in this environment also because it helps control
nitrate, nitrite and ammonia..nitrate is also a form of acid, nitric acid.
It is no exaggeration to say that the condition of an aquarium depends very much on the performance
of its biological filter. When the filter's micro-organisms are thriving, the water will be crystal
clear and there is no algae growth.
The chemical reaction that expresses the oxidation process carried out by the nitrobacteria which
converts harmful ammonia into harmless nitrate is NH3; NO2; NO3. The bacteria that converts ammonia
(NH3) into nitrite (NO2) is called Nitrosomonas, and the bacteria that converts that into nitrate
(NO3) is called Nitrobactor. Research shown that the remaining nitrate is about 70 times less toxic
than nitrite, but if enough accumulates in the water it canstill be harmful. Therefore, it is always
necessary to frequently change the aquarium water even when using a top-of-the-line filter.
So we need good filters and media, lots of oxygen or surface agitation, but not to the degree that we
degrade our level of C02 content....which will be evident by algae growth, and or poor plant health based
on the amount of light over the tank, and keeping the tank clean.
Injecting to much C02 into the water to the point of choking fish you are in fact damaging the micro-
organisms at the same time, don't do that.
It is an environmental thing.
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