Common Water Impurities

The following factors indicate the problems that poor-quality water may cause to the engineering plant and to human health:

• Metal corrosion

• Scale formation on the heat transfer surface

• Dezincification

• Plumbosolvency

• Biological health hazards

Water supplies should never be assumed to be chemically pure. Groundwa­ter from wells and springs contains dissolved impurities. Its properties depend

TABLE 4.13 Common Water Impurities

Constituent

Formula

Problems caused

Methods of treatment

Alkalinity

Bicarbonates (HCOj) Carbonates (CO-j) Hydroxyl (OH) as CaCO,,

Steam systems: foaming and solid carry over Steel embrittlement (HC03) and (CO,) Corrosion

Distillation Demmeralization lime and lime soda Dealkalization (ion exchange)

Acid treatment hydrogen zeolite

Ammonia

NH,

Corrosion of copper He zinc alloys

Cation exchanger (hydrogen zeolite) Chlorinating Deaeration

Carbon

Dioxide

CO,

Severe corrosion in condensate lines

Aeration

Deaeration

Alkalies (neutralization! Filming and neutralization amines

Chloride

Cl

Increases solid contents Produces a corrosive solution

Distillation

Demmeralization

Color

Boiler foaming Presents problems with iron removal Discoloration of

Manufactured product

Adsorption

(activated carbon) Coagulation Filtration Chlorination

Conductance

US

Due to ionizing solids in solution; an increase in conductivity occurs resulting in corrosive water

Reduce dissolved solids by lime softening or demineralization

Dissolved solids

Caused by evaporation, in steam generation resulting in blockage and foaming Frequent blow down Loss of treated water L, oss of heat

Lime softening Distillation Cation exchange.

(hydrogen zeolite) Demineralization

Fluoride

F

Few major industrial water problems Reduces dental decay

Alum coagulation Magnesium Hydroxide reaction Anion exchange Membrane separation

Free mineral acids

HC1

H2so4

Corrosion

Any process using alkalines to neutralize

Hydrogen sulphide

H2S

Corrosion (rotten egg smell)

Aeration

Chlorination

Ozone

Iron

Fe(II) (ferrous) Fe(III) (ferric)

Discolored water Deposits formed will foul surfaces

Coagulation and filtration Catalytic filtration Lime softening Aera tion

continues)

Constituent

Formula

Problems caused

Methods of treatment

Manganese

Mn(Il)

See iron

See iron

Nitrate

NO,

Increased solid content assists in the reduction of metal embrittlement Health problems with infants if used in foods

Distillation

Demineralization

Oil

Scale

Sludge

Foaming

Fouling of pipe work and heat exchangers

Baffle separators Strainers Coagulation Diatomaceous earth

Oxygen

0,

Severe metallic corrosion

Addition of corrosion inhibitors, sodium sulfite Automatic air vents, deaeration

PH

PH = log-^ H+

Graded into acidic or alkaline water Scale 0-14:

0 = highly acidic

7 = neutral

14 = highly alkaline

Increased by alkaline addition Decreased by

The addition of acid

Silica

Si 0,

Scale buildup on

Surrounding surfaces, reducing flow and heat transfer

Removed by applying hot magnesium salts Demmeralization

Processes

Sulphate

So4

Increased solid content Combines with Ca to form calcium sulfate salt

Distillation

Demineralization

Suspended

Solids

Clogs pipelines Fouls heat exchanger Surfaces

Settling

Filtration

Total solids

Sum of dissolved and suspended solids

See dissolved and suspended solids

On the nature of the ground over which it flows or passes through. Surface wa­ter from lakes and rivers contains silt, dissolved impurities, and organic matter; its quality varies widely depending on flow rate.

Table 4.13 lists and briefly describes some of the impurities found in typical water supplies. As can be seen, a wide range of problems have been considered, and no one method of treatment is suitable for all cases. The

Pharmaceutical industry and silicon chip manufacturers require the greatest purity in the water used.

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