WATER RESOURCES
By
Eng.Miral alshraideh
A high quality stream or river is clear and clean; the
water has no odor or color and has pleasant taste.
A “good” river flows fast and freely over a clean
river bed, contains high levels of dissolved oxygen,
and supports abundant fish life. It has a low
bacterial count.
On the other hand, low quality water body is often
characterized by being stagnant, looking murky,
having a foul odor and taste, and containing
numerous chemicals and bacteria.
One of the best measures of the quality of water in
natural environment is its level of dissolved
oxygen. Dissolved oxygen (DO) is impartant to
aquatic life. When water is In contact with air,
oxygen from the air dissolves into water. Water can
only hold a small concentration of DO, but this
small concentration is sufficient to support all fish
and other aquatic life.
The solubility of DO depends mainly on
temperature and salinity as shown in table 4.3
There are four rate processes affect the DO level
in natural water bodies:
1. Aeration (Reaeration).
2. Photosynthesis.
3. Respiration.
4. Biodegradation of wastes.
The two processes that most affect DO levels
are aeration and waste biodegradation.
Water pollutants can be classified into four
general categories:
1. Physical
2. Chemical
3. Biological
4. Radiological
Individual pollutants from each of these four
general categories can come from either point
sources (municipal and industrial wastewater
discharges) or nonpoint sources ( runoff from
urban, residential, and agricultural lands).
In many cases, nonpoint sources of pollution
contribute more than point sources to certain
receiving bodies of water.
The four general categories can be subdivided into
more specific groupings, including the following :
Suspended solid (particles): one of the major
groupings of water pollutants .these may be natural
soil or silt particles or waste products of various kinds.
One measure is the total suspended solids (TSS),
which refer to particles that are suspended in the
water and can removed by filtering the water through
a fine filter.
Total dissolved solids (TDS): this group includes
any solid materials that have dissolved and
cannot be removed by filtering.TDS includes any
thing from table salt (NaCl) to a fertilizer (e.g.,
potassium nitrate- KNO3), to an organic
compound like sugar (C11H22O11) and many
others.TDS are measured by evaporating a sample
of the water and weighting the residue.
Turbidity and Color: Two physical parameters that
affect water quality. Turbidity measures the
water clarity (which is degraded by very small
suspended solids). Color is affected by both
suspended and by dissolved material.
Taste and odor: are two other physical
properties of water, and are typically assessed
for potable water(water used for human
consumption). Even after some degree of
treatment, water may have unpleasant taste or
odor. Obviously these tests are subjective, and
require the use of human panels.
Nutrients: primarily refer to nitrogen and
phosphorus. These are key elements in
fertilizers, and if they wash into lake or river,
they encourage the growth of aquatic plants.
Pesticides: ( insecticides and herbicides) have been
widely used for decades in the united states and other
countries to improve agricultural yields or simply to
make homeowners’ yards more beautiful.
Toxic metals: that are regulated by the U.S. EPA
include arsenic, barium, cadmium, copper, lead,
mercury, nickel, and others. In very small
concentration some of those metals are essential for
life, but in slightly larger concentrations, many of
these metals are poisons to humans and to a number
of other organisms.
Pharmaceuticals: are examples of modern
pollutants. Endocrine disruptors are human
made substances that may mimic or interfere
with the function of hormones in the body.
Excess Heat: can be considered a pollutants in
some situations. As we seen in table 4.3,
warmer water holds less oxygen than cooler
water. Thus excess heat may warm the water so
much that is reduce the oxygen content enough
to disturb the ecology of the waterway. Mostly,
heat is added by industry (especially electricity
generation) when the use local water supplies to
dissipate the heat from their operations.
Natural system:
“Oxygen-demanding wastes” is a catch –all phrase meant
to identify all those organic materials that when the get
into water would be biodegraded by aquatic
microorganisms.
Over millions of years, a very strong ecology has evolved
with thousands of different kinds of bacteria that utilize
complex organic compounds as a food source, and in the
same time biodegrade those compound back to CO2 and H2O
and other simple compounds.
This important role of microorganisms as “decomposers”
prevent massive accumulation of organic wastes in any part of
our ecosystem.
During this biological process, some or all of the dissolved
oxygen (DO) In the water is consumed, and can results in
little or no DO left for fish and other aquatic life.
The term used to describe and quantify such wastes is
biochemical oxygen demand, or BOD.
BOD is direct measure of oxygen usage
BOD is indirect measure of the strength (concentration ) of
organic wastes in water.
The concentrations of organics in water are reported as BOD in
units of mg/L.
Engineered system:
Proper waste disposal is one of the chief tasks caring of
humans basic needs.
For last 150 years or so, newer cities installed sanitary
sewer systems to carry human excreta away from living
areas to areas where it could be treated and disposal.
Engineers must design sanitary landfills to handle the city’s
solid wastes, and are some times involved with bioremediation
projects to clean up contaminated soils or underground water
supply.
Ultimate BOD or (BODu) is the amount of oxygen used to
completely oxidize the compound.
Chemical oxygen demand (COD) is the oxygen need to
chemically oxidize the compound in water.
For many compound , COD is identical to BOD.
One main advantages of COD is that COD tests can be
completed in few hours in the lab, whereas the
determination of BODu can take weeks.
Major disadvantages is that if the wastewater contains
compounds that are toxic to bacteria or not readily
biodegradable the COD test will not show that.
Total organic carbon(TOC) measures the carbon content of the
all organic compound in the sample.
If the compound has nitrogen in it, the nitrogen will also be
oxidized.
The complete oxidation process is occurring in two stages, with
the first stage oxygen usage commonly called the carbonaceous
BOD (CBOD) and the second stage refer to as the nitrogenous
of BOD (NBOD) .together they make total BOD.
During the first stage, the carbon is oxidized to CO2 and water
to H2O and organic nitrogen only gets to ammonia (NH3)
In the second stage, ammonia is oxidized to nitric acid (HNO3)
and water.
Organics (often called substrate) in water are oxidized in a
complex set of microbiological reactions, BOD consumption
does not happen instantaneously .it can take days or weeks to
completely biodegrade some substrates.
BODu is usually thought of as the amount of oxygen that was
consumed or exerted at the end of the biodegradation process
of a particular substrate.
It can also be thought of as the potential amount of oxygen
remaining to be used before the biodegradation of the substrate
even start.
Figure 4.7 displays BOD progression
At time t the symbol yt is given to the BOD that has been used
and the symbol Lt to the BOD remaining .
At any time, the BOD has been used plus the BOD remaining to
be use must sum to BODu as shown in equation(4.8)
yt + Lt = BODu (4.8)
Where:
yt is the BOD that has Been used.
Lt is the BOD remaining.
The oxygen consumed at any time during the BOD process is
equivalent to the amount of substrate (measure as BOD) that has
reacted:
yt = Lo ـLt (4.13)
Where:
yt = BOD exerted at time t (mg/L)
Lo = initial BOD remaining = BODu (mg/L)
The general equation to define the BOD progression as a function
of time:
yt = Lo (1- e –kd t )
Where:
kd is the deoxygenation rate constant (day –1)
The numerical value of kd reflects the case with which the organics
are degraded.
For municipal wastewaters, kd can range from about 0.1 to about
0.23 day –1
It is observed that the BOD exerted at time infinity is equal to
BOD potential at time zero and it is equal to the BODu as
shown in eq.(4.15)
y∞ = Lo = BODu (4.15)
The five day BOD exerted (y5, or more commonly BOD5 ) is
the one most often used in testing , the point is that BOD5 is
widely recognized throughout the world and it is standard
time for doing BOD tests.
The BOD test measure the oxygen content of the water at the
start of the test and again after five days to determine the
amount of oxygen used.
The test is conducted in a closed bottle stored in the darkened
area to prevent transfer from the atmosphere and exposure to
light (to exclude photosynthesis).
The temperature is maintained at 20 °C to ensure
standardization of the results.
The sample of the wastewater to be tested is diluted with
seeded dilution water, which contains a bacterial seed and all
necessary inorganic nutrients to support the desired
biological reaction.
The dilution water does not contain any organic materials, so
that the only source of substrate in the diluted sample is from
the water to be tested.
The dilation water must be aerated prior to use to ensure an
adequate supply of the oxygen to maintain aerobic conditions
throughout the BOD test. That is, for the test to be valid, the
bottle must have at least 1.0 mg/L of dissolved oxygen left in
it after five days.
At the beginning of the batch test, a measured volume of the
sample is placed in a BOD bottle which is then filled with
seeded dilation water and sealed to prevent oxygen transfer
from the atmosphere.
The dissolved oxygen is measure carefully at the beginning
and the end of the test (five days) the difference in the
dissolved oxygen concentrations represents the oxygen
consumption.
The biological reactions may require in excess of 30 days to
reach completion, in which case of the five day oxygen
consumption (BOD5) would represent a fraction of (BODu)
Time for your questions!!
