Page 193 - Industrial Ventilation Design Guidebook

4,4 WATER PROPERTIES AND TREATMENT I 5 5

5, Fouling and scale problems
6. Microbiological problems

4.4.4 Water Treatment

In most engineering applications the supply water is not suitable for im-
mediate use without treatment. It is essential that the method of water
treatment selected be the one most suited to the application. If steam is
used as the working medium for a process, it is essential that water treat-
ment be used to prevent the precipitation of substances in the water from
fouling pipe work and heat exchangers; otherwise costly plant damage
will result.
The method of treatment selected depends on many factors, such as the
nature of the salts and the pH of the water, and the assistance of a reputable
specialist company is necessary to carry out regular testing and an analysis
report,
4.4.4.1 Methods of Feedwater Treatment
The treatment may be internal, which involves the addition of chemicals
to the water to make the salts causing scale and sludge less likely to bond to
pipe work and heat exchanger surfaces. In the case of boilers and cooling tow-
ers, blowdown of the water on a continuous or regular basis is required. Care
must be taken in the case of blowdown to ensure that the exiting water tem-
perature does not damage the drains or that the chemicals pollute waterways.
The consent.of the water authorities is required in order to determine what
levels can be discharged into waterways. In certain cases further treatment
may be necessary before discharge.
In the case of boilers operating at low pressure, organic materials such
as natural and modified tannins, starches, or alginates are added to aid
blowdown. For boilers operating at high pressure, synthetic materials such
as polyacrylates and polymethacrylates have been developed. The most
commonly used chemicals for boiler feedwater treatment are phosphates
and hydrazine.
External treatment involves the removal of impurities from the water by
various methods before it enters the plant; this is the most effective method of
water treatment. This category of treatment involves one or more of the fol-
lowing processes.
Sedimentation
In sedimentation the water to be treated flows slowly through a tank,
allowing the suspended material in the water to fall to the base of the tank.
The use of coagulating compounds, such as aluminum and ferric sulfate, in-
creases the efficiency.
Oxidation
It is during oxidation that iron and manganese in suspension are re-
moved from the water. Oxidizing agents (chlorine, ozone, hydrogen perox-
ide, potassium permanganate, etc.) or direct aeration is used; the metals in