Water Treatment and Use

The processes involved are similar--need to deal with such characteristics as:

The desirable level of each of these depends on the use planned

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Industrial Water

 

May need reduction in dissolved oxygen by addition of hydrazine or sulfite

May need chelating agent to prevent Ca2+ precipitation

May add corrosion inhibitors

pH may need adjustment

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Sewage Treatment

Turbidity

Suspended solids

Dissolved solids

pH

Dissolved oxygen

BOD

Screening

grit removal

sedimentation in primary settling basin

to remove both settleable and floating material

Trickling filter or rotating bioreactor

Activated sludge tank

some BOD is metabolized by bacteria for energy,

some is converted to biomass

(Sludge disposal is a problem)

suspended solids

dissolved organics

dissolved inorganics (esp. nutrients)

infectious agents

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Problem

A sewer plants treats a flow of 8000 l/hr, containing 250 mg/l of biodegradable {CH2O} It oxidizes 50% of this waste to CO2 and H2O. How many liters of CO2 at 20 oC and 1 atm are produced?

 

 

 

 

 

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Industrial Wastewater

different problems from domestic sewage

refractory synthetic organics

heavy metals

Inorganics such as cyanide

May require neutralization, carbon filtration, solvent extraction, air stripping, steam stripping, peroxide or ozone treatment, resin filtration. Microorganisms in bioreactors may need acclimatization.

 

Methods used in various types of treatment plants

Solids Removal

settling

filtration

microscreening (to 10 m m)

coagulation with iron and aluminum salts

polyelectrolytes

sodium silicate

sand filtration

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Metals Removal

To prevent scale formation

The lime soda process

 

 

 

 

 

 

 

Recycling the calcium carbonate

 

 

 

Recarbonating to lower pH

 

 

 

Problem: If calcium carbonate or magnesium carbonate are still present, they will form bicarbonate as pH is lowered.

 

 

 

 

Chemically stabilized water vs aggressive water

 

Ca efficiently removed by addition of phosphate

 

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Water softening by Ion Exchange

Strong cationic exchanger

 

Strong anionic exchanger

 

Weak cationic exchanger

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Water Softening by Ion Exchange

 

 

 

 

 

 

 

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Water softening by chelation

 

 

 

 

 

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Problem:

NTA is used to keep CaCO3 from precipitating. If Ca2+ is present at 0.002M total and CO32- is present in solution at 8.0 x 10-5 M will CaCO3 precipitate? If the solution pH is 8, how much NTA excess over the stoichiometric amount is needed to keep CaCO3 solid from forming?

 

 

 

 

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Dissolved Organics

 

 

 

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Dissolved Inorganics

 

 

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Ammonia air stripped

NH4+ removed by ion exchange

biosynthesis

bacterial nitrification--denitrification

Chlorination

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Sludge Disposal

Use as soil amendment

Problem with:

PCB, pesticides

Heavy metals (Pb, Cd, Hg, Cr, Ni, Zn etc)

Pathogenic organisms

Treatment of water produces sludges of iron and aluminum hydroxides, also other metal hydroxides--hard to dewater, may be possible to reclaim the metals.

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Disinfection of water

Chlorine

Chloramine formation

 

 

 

 

enough excess of Cl over NH3 is required to reach the breakpoint at which enough excess Cl is still present to disinfect.

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No trihalomethanes produced, but ammonia also is not removed

 

 

 

 

 

 

 

produced on site electrically

Decomposes too rapidly to provide residual disinfection power.

Some Cl is still needed

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Natural Water Purification Process

soil as a natural water purifier

Land disposal of agricultural waste, manure

soil organisms degrade many organics--phenols, organophosphates

Not useful for strong acids, bases, salts, heavy metals, volatile organics

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Reuse and Recycle

Planned or unplanned

Direct or indirect

Irrigation water

Cooling water

Groundwater recharge

Considerations for water reuse

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