Page 34 - Industrial Wastewater Treatment, Recycling and Reuse

Industrial Wastewater Treatment, Recycling, and Reuse: An Overview 17

the standard for comparison, although the test can take longer than 5 days
and is sometime continued for more than 5 days in specific cases. Usually,
for industrial wastewaters, the 5-day BOD is up to 80% of the total BOD
value. The BOD measurement involves oxygen demand by the bacteria
for oxidizing the chemicals. There are certain drawbacks or limitations that
might cause BOD measurement to fail. When industrial wastewaters con-
tain toxic heavy metal ions, cyanides, or other substances that may be
harmful to the living microorganisms oxidizing the chemicals, the BOD
measurement value can be highly misleading and incorrect. Further, if
the nature of a chemical species present in the wastewaters is such that
it is difficult to degrade in the presence of bacteria, the 5-day BOD test
can give a misleading result. This is the case with specialty molecules,
including biodegradable polymers, that require a long period of degrada-
tion. Apart from this, there may be chemicals that can interfere with the
BOD determination. Inconsistent laboratory practices with respect to
the addition of bacteria, the presence of algae, and high ammonia levels
in the sample can result in inconsistent and unreliable BOD values. Also,
while measuring BOD or COD for oxidant-containing wastewaters, espe-
cially with the presence of hydrogen peroxide, it is essential to determine
the residual concentration of hydrogen peroxide prior to measurement of
BOD and COD because H 2 O 2 will interfere in the analysis of both BOD
and COD. The presence of hydrogen peroxide in water will liberate resid-
ual oxygen during the analysis of BOD, consequently creating a lower
value of BOD than actual. Similarly, in the measurement of COD by
the standard potassium dichromate method, it will react with it, thereby
creating a higher value of COD than actual.
The inorganic pollutants in general include heavy metals such as iron,
copper, manganese, chromium, zinc, and lead. Some of the metal pollutants,
such as arsenic, chromium, and mercury, can be potentially toxic. The inor-
ganic pollutants can also include the presence of ammonia. Total dissolved
solids (TDS) and salts such as chloride also need to be included in inorganic
pollutant characterization because some of these can act as potential inhib-
itors. A very important problem with respect to many chemical industries is
reducing ammoniacal nitrogen from wastewaters. Ammoniacal nitrogen
(NH 3 -N) measures the amount of ammonia, a toxic pollutant. Ammonia
can directly poison humans and upset the equilibrium of water systems.
The nitrogen in sewage is assessed as ammoniacal nitrogen. This indicates
the amount of nitrogenous organic matter that has been converted to
ammonia. The average strength of crude domestic sewage will have a