RO Membrane Process for Salt Content Equalization

Reverse Osmosis (RO)

• RO is used to equalize salt content by blocking salt ions and allowing water passage.
• Osmotic pressure of salt solution (Pos) is the pressure applied to the side of higher salt content to prevent water flow.
• RO uses high pressure (ΔP) to increase water permeability (Q α (Papplied – Pos)).
• Papplied is 80-120 psi for demostic use and 350-150 psi for industrial use.
• Water product is 70-90 % of feed (brackish) and 30% of feed (sea).
• Membrane materials are:
  • Cellulose acetate (high Q/A in spiral-wound flat sheets)
  • Aromatic polyamide (lower Q/A in hollow fibers to increase A ~ 15 spiral wound membrane)
• Water product (permeate) and brine (reject) are the outputs of the RO process.

Scaling Potential

• Scaling potential is determined by the concentration of ions in the brine (calcium sulfate, calcium carbonate, and silicon dioxide).
• Example: Ca2+ 200 mg/l in feed, 25% recovery water product with 7 mg/l and reject 779 mg/l (4 times).

Chemical Treatment Processes

Definitions:

• Atomic weight: weight of an element relative to C-12
• Valence: combining power of an element relative to that of hydrogen atoms of 1 (H+)
• Flocculation: coagulation reduces the net electrical repulsive forces at particle surfaces using coagulant chemicals
• Coagulants:
  • Aluminum sulfate for removal of organic matter
  • Iron salt for removal of color, taste, and odor
  • Polymer coagulant used as aids

Aluminum Sulfate (Filter Alum)

• Formula: Al2(SO4)3 · 14.3 H2O
• Available as a powder or liquid alum syrup
• Every 1 mg/l of alum decreases water alkalinity by 0.5 mg/l (as CaCO3) and increases CO2 by 0.44 mg/l (corrosive and undesired)

Ferrous Sulfate (Copperas)

• Formula: FeSO4
• Green crystal for dry feeding
• Lime is usually added to raise pH to precipitate ferrous ions as ferric hydroxide by caustic alkalinity

Ion Exchange Softening

• Removes Ca2+ and Mg2+ (hardness) by replacing them with sodium using a cation resin (R)

Cation Exchange Softening Process

• Ca2+ and Mg2+ are removed from the resin using a backwash solution of sodium chloride
• The resin is restored to its original condition

Chlorination

• HOCl + NH3 → H2O + NH2Cl
• Free available residual chlorine (FRC) = residual chlorine in water as HOCl or OCl
• Combined available residual chlorine (CRC) = residual chlorine in water as chlorine or with organic N compounds
• Chlorine demand (CD) = Chlorine added - (FRC + CRC)

Chlorine Residual Curve

• Chlorine reacts with reducing agents, ammonia, and other organic compounds
• Breakpoint chlorination is used to determine the minimum amount of chlorine required to achieve a residual

Ozone

• A strong oxidizing gas that reacts with most organic and inorganic molecules
• Ozone decomposes in water: O3 + H2O → O2 + 2OH
• Ozone is generated on-site since its half-life is ~10-30 minutes
• Ozonation system components:
  • Air preparation or O2 feed
  • Electrical power supply
  • Ozone generation
  • Ozone contactor
  • Ozone destructor

Reduction of Dissolved Salts

• Flash Distillation
• Reverse Osmosis
• Electro dialysis
• Process selection depends on size, source of energy, capital, and cost.