Page 32 - Industrial Wastewater Treatment, Recycling and Reuse

Industrial Wastewater Treatment, Recycling, and Reuse: An Overview 15

manufacturers and industries/people using these chemicals. These toxic pol-
lutants persist over a longer period of time in a natural environment and have
to be destroyed/eliminated from the streams preferably by incineration.
Odors associated with wastewater are difficult to quantify. The odor
can be qualitatively differentiated on the basis of its origin: A wide variety
of compounds give off foul odors apart from their possible harmful nature.
It is often easy to detect very low concentrations of odorous substances in the
air (sulfides/other sulfur compounds, ammonia, amines, etc.). Hydrogen
sulfide toxicity is comparable to that of hydrogen cyanide; even a low level
of exposure to the gas induces headaches and nausea, as well as possible eye
damage. At higher levels, life-threatening conditions set in, and a number of
fatal accidents, attributable to the buildup of sulfide in sewage systems, have
been reported. Hydrogen sulfide gas is also soluble, and weak acids that form
after it is absorbed can cause corrosion.
The discussion below applies to nontoxic wastewaters, in general.
There are separate and independent parameters for the characterization
of wastewaters. These mainly include organic components, inorganics,
and total dissolved and suspended solids. At the outset, one needs to resolve
issues pertaining to contaminants requiring specific pretreatment such as pH,
alkalinity, acidity, and suspended solids. The presence of organics in the
wastewaters is conventionally indicated in terms of BOD or COD/TOD
(total oxygen demand). In recent years, the total organic carbon (TOC)
measurement, which represents the total organic fraction in terms of carbon,
is also emphasized for better accuracy and evaluation of wastewater quality.
The BOD is representative of the total oxygen requirement for oxidizing
those chemicals that can be oxidized by bacterial/biological means. These
are typically biodegradable substances such as food organic matter. Since
not all chemicals are biodegradable, it is required that we have another com-
prehensive parameter that can represent the amount of pollution due to
organics. This parameter is COD, which represents the oxygen requirement
for oxidizing all the chemicals in the wastewaters. Obviously, COD is always
a higher value than BOD. However, there are certain chemicals, such as
benzene and its derivatives, which are difficult to oxidize completely.
Therefore, even COD measurement is incomplete and inaccurate at times.
Also, it should be noted that inorganics such as sulfides and ferrous iron also
get chemically oxidized, and thus their oxygen demand also gets incorpo-
rated in the COD value. So, the COD value in such cases is not truly rep-
resentative of the presence of organics in wastewaters. In this respect
measurement of the TOC is important and is truly representative of organics
in the wastewaters. Also, it should be noted that since COD values many