Page 443 - Industrial Power Engineering and Applications Handbook

Switchgear and controlgear assemblies 13/41 7
The sedimentation and secondary settling tanks Nos (Na2C03) or calcium hydroxide (Ca(0H):). i.c. lime
h and 7 are designed to provide sufficient retention time water. The basic (alkaline) effluents can be treated with
for settling of biomass. The settled biomass is recycled any mineral acid, preferably sulphuric acid (H2S04), until
to aeration tank No. 5 to maintain the desired mixed the required pH value is obtained.
liquor suspended solids (MLSS). The sludge settled in
tank No. 6 can be drained to the sludge drying beds No. For phosphating and passivating effluents
8, where it can be dried, scrapped and disposed on land. (phosphate as P)
The filtrate from the sludge drying beds is taken back to
sedimentation tank No. 6. The overflow from the These are also acidic in nature, and are taken care of
secondary sedimentation tank No. 7 is treated water. free automatically when the pH value of the effluents is
from effluents and can be discharged into the drains or maintained at the required level. If the passivation is
farmland or can also be recycled. carried out by using chromic acid, chromium must be
treated separately by using sodium tnetabisulphite.
For level of chemical oxygen demand (COD)
To remove solids
COD provides a meawre of the oxygen equivalent to
that portion of the organic matter in a sample that is By using sedimentation (settling) tank\. the suspended
susceptible to oxidation by a strong chemical oxidant. A solids can be removed as they settle at the bottom. Proper
higher level of COD can be degraded (neutralized) coagulants and llocculants such as lime and alum are
chemically. The level of COD, however, automatically utilized to convert the amount of dissolved matter into
reduces with the level of BOD and no further treatment suspended solids.
i\ normally necessary. The numbers and sizes of tanks will depend upon the
amount of discharge and the type of effluent\. For a
To neutralize the alkali and acid effluents normal paint shop, the size should be such that it is able
to allow enough retention time for the suspended solids
The alkalinity or acidity in wastewater is usually deter- to settle at the bottom. To avoid turbulence in the tanks.
mined bb titration of' the wastewater with a standard the arrangement of inlet and discharge pipes can be as
alkali or acid to the transition point of methyl orange illustrated in Figure 13.12. From the polishing tank No.
(pH range 3.1 to 4.4). Almost every effluent which is 7 clear water, which will meet the desired standards. can
free from caustic alkalinity (pH less than 8.3) requires be discharged into the drains.
addition of acid to bring it to the methyl orange transition The scum of oil and grease, if any. collected at the
point. If the wastewater has a pH greater than 8.3, a surface of the tanks may be scooped out and destroyed
titration with acid using a phenolphthalein indicator should with the waste paints. The sludge (solids) that settle at
be made and the result recorded as alkalinity to the the bottom can be collected from the underflou of the
phenolphthalein. tanks or pumped out.
If the liquid i\ acid to methyl orange, alkalinity is
absent and the acidity must be determined. When methyl For zinc and other heavy metals
orange is used as an indicator for titration, the result
may be recorded as mineral acid acidity and for a few The treatment for zinc can be carried out with normal
organic acids. which give acidic solutions, the result may coagulants until it has formed any complex (chemical
be recorded as acid to methyl orange. In some cases it compound). Other heavy metals and cyanide constituents.
may be desirable to determine total acidity by titration however. will require special attention.
with alkali using phenolphthalein as an indicator, in which
case the result is recorded as total acidity. This would Disposal of waste primer, paint, oil and grease
include acidity due to carbon dioxide. as well as that due scum
to tnineral and organic acids.
The alkalinity and acidity is generally controlled, based These areconsidered to be highly hazardous (\trong) wastes.
on the amounts and also by maintaining the pH value of The oversprayed paint that collects at the bottom of the
the effluent discharge between 5.5. and 9.0, the ideal spray booth, and on its baffles, as well as the scum of oil
being around 7.5. as far as possible. The pH can be and grease may be collected and stored in drums and sealed.
checked with the help of a pH meter. It is then disposed of at a recognized dumping ground for
Acidic effluents can be treated with sodium carbonate industrial wastes or destroyed in an incineration plant.