Week 4

Water

Water is the main condition for the existence of life on Earth

Properties

Freezing and boiling temperatures which are far apart,
Maximum density at 4 deg C. Therefore, ice floats .
Permits layering in ponds
High specific heat, higher than any liquid except ammonia. 5 times of solids
Serves as a ballast to prevent fast temperature changes
High heat of vaporization, among the highest.
Serves as an excellent heat sink
The best solvent

Hydraulic cycle

97.2% of the Earth water is in the oceans
70% of Earth surface is covered by water
About 50% of the entering solar energy serves to evaporate water
It is about 4000 of the energy use of people
88% percent of the evaporating water is from oceans
The rest is from upper water and leaves
Most of the evaporating water returns to Earth as rain

General uses

Irrigation
Urban use
Power stations
Industry

Water for industry

Industries:
Steel
Petroleum
Paper
Power stations
Chemical

Uses:
Cooling
Steam
Solvent
Raw material
Transport of solids
Dilution

Sources:
Ground water
Rivers and lakes
Sea water
Saline water
Recovered waste water

Water treatment

Filtration
Chemicals for preventing of corrosion and growth of plants.
Chemicals for settling of dispersed solid particles.
Softening
High purification

Waste water treatment

Dilution
Filtration and settling
Chemical oxidation.
Biological aerobic and unaerobic treatment.
Specific chemical treatment.

Treatment of cooling water

Why needed?
Prevention of salts from crystallizing in piping.
Prevention of corrosion.
Prevention of plant growth in cooling towers and pipes.

A. Initial settling of salts

Treatment with lime:
Ca(HCO3)2 + Ca(OH)2 = 2 CaCO3 + 2H2O
Treatment with lime and soda:
CaCl2 + Na2CO3 = CaCO3 + 2 NaCl
MgCl2 + Na2CO3 = MgCO3 + 2 NaCl
MgCO3 + Ca(OH)2 = Mg(OH)2 + CaCO3
Ion exchange.

B. Prevention of settling

  1. Dispersion agents prevent agglomeration (1-5 ppm, detergents)
  2. Stabilization agents prevent settling and enable operation above saturation (1-5 ppm, polyphosphates, tanins, lignin, starch)
  3. Complexing agents cage insoluble ions (2-10 ppm, polyphosphates, fluorides, versene)
  4. Blowdown: Periodic withdrawal of part of the water in order to prevent increased concentration of salts.

C. Corrosion prevention

Corrosion is caused mostly by contact between two different metals or sediments, or two areas kept at different temperatures, or low pH.

  1. Prevent contact of different metals.
  2. Inhibitors (For low pH cases)
  3. Cathodic protection, by sacrificial metal or low DC current.
  4. Prevention of settling.

The anodic reaction is: Fe0 = Fe+2 +2 e-

The function of the inhibitor is to form a protective film. Unfortunately, oxides of Fe++ are not dense enough to protect the underlying metal. Chromates can form a protective layer. The blackening process of guns forms a protective later.

The cathodic reaction is: O2 + 2 H2O + 4e- = 4 OH-
Here again, the function of the inhibitor is to prevent oxygen from contact with the iron. This is done by painting or plating. Thin layers of CaSO4 or CaCO3 may be used.

D. Flora inhibition

Algae use CO2 and water and sunlight
Micro organisms, including microbes, and iron consuming microbes, fungi, yeasts, and slimes. need low levels of light and consume waste and other organics.

Flora inhibition by poisoning

  1. Chlorine – cheap for low levels of organics. An excess of 0.5-1 ppm above stoichiometry is required. Expensive for high levels of organics. Some organics may cause the formation of cancer promoting chloro amines. An initiating period of over 15 minutes is required. Can be used as a one time shot, or in a continuous or periodic manner. Can be in the form of chlorine gas or liquid or hypochloride.
  2. Bromine – similar to chlorine, but stronger, more poisonous and more expensive. More useful for one shot cases.
  3. Modern compounds that are in equilibrium with their solution and maintain a constant low concentration of the active ingrediate, while it is consumed. Very expensive and used mainly in small swimming pools.

Algae poisons:
Copper salts (Disadvantage: they settle at high pH and cause corrosion).
Permanganates (Disadvantage: MnO2 settles)

Wooden structures may be protected against fungi by reactions of solutions absorbed by the wood to form fungi poisons entrapped within the wood. For instance: reacting CuSO4 and Na2CrO4 to form CuCrO4.

Water for steam

The most stringent standards are for steam used in atomic power stations – less than 1 ppm of total salts.

Problems:

  1. Settling of salts in the steam boiler and pipes.
  2. Corrosion by CO2 and oxygen.
  3. Erosion of turbine blades by small particles, mainly of silica
    Requirements:
    <25 ppm of silica for 600-800 PSIA
    <3 ppm of silica for over 1500 PSIA
  4. Intrusion of oils and dirt into the condensate.

Treatment:

  1. Removal of ions, mainly by a combination of cationic and anionic ion exchange.
    Often cheaper lime and soda treatment is used before the ion exchange.
    The use of polyphosphates or versene to prevent settling.
  2. Silica settling by cold settling, adsorption on zeolites, use of special ion exchangers.
    The concentration of silica determines the frequency of the blow down.

Waste water

Problems:

1. Organic waste.

The saturation concentration of oxygen in the waste water is 8-15 mg/liter, depending on salts concentration and temperature.
The concentration required to maintain live fish is 5-8 mg/liter for very active fish like trout, down to 3 mg/liter for less active fish like carp.

The level of organic waste is measured by B.O.D. (BIOCHEMICAL OXYGEN DEMAND) in a varity of units:
lb oxygen per cu m or cu ft
lb oxygen per 100 lb water at 20C for 5 days
lb BOD per population units

The specification of BOD depends on its use:
Drinking water, irrigation, swimming or fish farming.

Another standard is COD (Chemical oxygen demand) : The amount of oxygen required for chemical oxidation of the waste.

Treatment
Dilution
Filtration of organics
Aerobic ponds with large residence times
Aerobic ponds with oxygen addition
Trickle filters
Wet oxidation in reactors
Active sludge ponds (aerobic or anaerobic)
Incineration

2. Accumulation of salts and other chemicals.

Treatment
Dilution
Pumping into old inactive wells far from underground water
Selective settling
Ion exchange
Electrochemical reactions for silver, copper, zinc and chrome
Concentration by evaporation, extraction or crystallization for the
recovery of some components.

3. Floating solids

Treatment
Cheap sand filters
Coagulation, using fresh aluminum hydroxide
Settling ponds
Burial or incineration of the concentrated solids.

4. Petroleum

Treatment
For large quantities: Settling , Phase separation, Electrostatic breaking of emulsions.
For medium quantities: Adsorption on fresh aluminum hydroxides, Adsorption on active carbon.
For small quantities: Digestion by special bacteria.

5. pH

Ideally: react acids with bases
Practically CaCO3 for acids and H2SO4 for bases.

6. Radioactive materials

Better not have any.
High dilution practiced once is not permitted today.
Concentration and burial in long lived containers.

7. Changing compositions.

Intermediate storage in order to balance concentrations.

8. Pathogens

City water may have microbes, viruses, protoza and parasites.
Chlorine treatment is usually adequate.

Water desalination

General

Close to 99% of the water on Earth is in the sea or in glaciers.
The salt concentration in the oceans is 3.2-4%
The yearly rain on Earth would have been more than sufficient for the population had it been distributed evenly and captured. However, about 75% is re-evaporated and there exist relatively dry regions on Earth.

The minimum amount of water required to produce 2.5 lb of food a day in the form of bread and vegetables is 1.2 cu m (300 gallons), but 2500 gallons are required to produce 1 lb of meat.
The minimal (thermodynamic) amount of energy required to separate 1000 gallons
of pure water from sea water is 2.8KWH. In practice it is 20 times as much.

Any driving force may be used and has been tried for desalination: Temperature, pressure, electrical or magnetic fields, gravitation.

Desalination processes

1. Removal of water

a. Multi stage evaporation:
Evaporation with vapor compression
Multi stage flash
Solar evaporation
b. Freezing:
Direct freezing (crystallization)
Indirect freezing (using intermediate fluids)
Using hydrates
c. Reverse osmosis (Microfiltration under pressure)
d. Extraction

2. Removal of salt
a. Electrodialysis
b. Osmionic separation.
c. Ion exchange.

The processes used in recent years were mainly reverse osmosis and evaporation with compression.
Currently, due to advances in technology, the most used process is reverse osmosis

The desalinated water costs more than water from natural sources