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

• microorganisms
• hardness
• pH
• salinity
• suspended solids
• dissolved metals
• dissolved organic compounds

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

Turbidity

Suspended solids

Dissolved solids

pH

Dissolved oxygen

BOD

• Primary Treatment

Screening

grit removal

sedimentation in primary settling basin

to remove both settleable and floating material

• Secondary Treatment to remove BOD

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)

• Tertiary treatment (advanced waste treatment). Removes:

suspended solids

dissolved organics

dissolved inorganics (esp. nutrients)

infectious agents

• Physical/chemical treatment plants

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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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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

• Large particles

settling

filtration

• Smaller particles

microscreening (to 10 m m)

• Colloidal particles

coagulation with iron and aluminum salts

polyelectrolytes

sodium silicate

sand filtration

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Metals Removal

• Ca and Mg salts (hardness). Water softening

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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Strong cationic exchanger

Strong anionic exchanger

Weak cationic exchanger

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Water softening by chelation

• Removal of Fe and Mn

• Electrodeposition

• Reverse osmosis

• Cementation

• Ion exchange

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Problem:

NTA is used to keep CaCO₃ from precipitating. If Ca²⁺ is present at 0.002M total and CO₃^2- 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

• Oxidation byproducts--trihalomethanes and organo oxygen compounds

• Humic substances

• Removed by activated carbon filtration

• Synthetic polymers

• Chemical or electrochemical oxidation.

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Dissolved Inorganics

• For water recycling salt removal is important

• N and P need to be removed even if recycling is not being done

• Electrodialysis: Must remove large molecules before to prevent fouling

• Ion exchange

• Reverse osmosis

• Phosphorous removal

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• Nitrogen removal

Ammonia air stripped

NH₄⁺ removed by ion exchange

biosynthesis

bacterial nitrification--denitrification

Chlorination

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Sludge Disposal

• Anarobic digestiom

• Thickening

• Dewatering

Use as soil amendment

Problem with:

PCB, pesticides

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

Pathogenic organisms

• Air drying, composting, lime treatment

• Other types of sludge

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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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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• Chlorine dioxide as alternative disinfectant

No trihalomethanes produced, but ammonia also is not removed

• Ozone

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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Planned or unplanned

Direct or indirect

Irrigation water

Cooling water

Groundwater recharge

Considerations for water reuse

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