Water quality and treatment Knowledge-Based Study

Study notes

1. Overview of Water Quality

   Natural water sources contain dissolved gases, mineral ions, and organic matter. Evaluating whether water is safe for municipal, agricultural, or industrial use requires assessing distinct physical, chemical, and biological parameters.

   Key Physical and Chemical Indicators

   • Turbidity: A measure of water clarity caused by suspended solids such as silt, clay, and microscopic organisms. Measured in Nephelometric Turbidity Units (NTU).
   • Dissolved Oxygen (DO): The concentration of free, non-compound oxygen molecules dissolved in water. Cold, fast-flowing clean waters typically contain 8 to 12 mg/L of DO, which is critical for aerobic aquatic life.
   • Biochemical Oxygen Demand (BOD): The quantity of dissolved oxygen required by aerobic microorganisms to biologically degrade organic contaminants over a specific incubation period (commonly 5 days at 20 °C, termed $BOD_5$). High BOD indicates high organic pollution and depletes available DO.
   • Chemical Oxygen Demand (COD): An aggregate measure of all chemically oxidizable substances (both biodegradable and non-biodegradable) determined by strong chemical oxidation using acidified potassium dichromate ($K_2Cr_2O_7$).
   • pH and Electrical Conductivity: Safe drinking water typically falls within a pH range of 6.5 to 8.5. Electrical conductivity reflects total dissolved solids (TDS) and ionic strength.

   Stages of Surface Water Treatment

   Municipal water treatment facilities employ sequential mechanical, physical, and chemical unit operations to render raw surface water potable.

   1. Screening and Pre-sedimentation: Coarse bar screens remove large debris such as leaves, branches, and plastic waste.
   2. Coagulation: Addition of positively charged chemical coagulants, such as aluminum sulfate ($Al_2(SO_4)_3 \cdot 14H_2O$) or ferric chloride ($FeCl_3$). Coagulants neutralize the negative surface charges on colloidal particles, destabilizing the suspension:$$Al^{3+} + 3H_2O ightarrow Al(OH)_3(s) + 3H^+$$
   3. Flocculation: Gentle, slow mechanical mixing encourages destabilized colloids to collide and aggregate into larger, visible masses called flocs. Polymeric flocculant aids are often added to enhance floc bridge formation.
   4. Sedimentation (Clarification): Water flows into quiescent clarifier basins where the heavy floc particles settle to the bottom by gravity to form sludge, which is periodically removed.
   5. Filtration: Clarified water passes through granular media filters (typically layers of anthracite coal, fine silica sand, and supporting gravel) to remove residual suspended solids, parasites, and remaining micro-flocs.

   Disinfection Chemistry

   Disinfection destroys pathogenic microorganisms, including enteric bacteria, viruses, and protozoan cysts.

   • Chlorination: Chlorine gas ($Cl_2$) or sodium hypochlorite ($NaOCl$) hydrolyzes in water to yield hypochlorous acid ($HOCl$), a potent oxidizing germicide:$$Cl_2 + H_2O ightleftharpoons HOCl + H^+ + Cl^-$$Hypochlorous acid partially dissociates into hypochlorite ion ($OCl^-$), which is less effective as a disinfectant. Lower pH favors the active $HOCl$ species. A major benefit of chlorination is providing a persistent disinfectant residual throughout municipal distribution piping, though it can react with natural organic matter to form unwanted disinfection by-products (DBPs), such as trihalomethanes ($THMs$).
   • Ozonation: Ozone gas ($O_3$) is a powerful oxidant that inactivates resistant pathogens (such as Cryptosporidium cysts) rapidly without generating chlorinated by-products, though it leaves no residual protection in the distribution system.
   • Ultraviolet (UV) Irradiation: Germicidal UV light at wavelengths around 254 nm penetrates microbial cell walls and alters pyrimidine bases in DNA/RNA, halting cellular replication without introducing chemicals into the water.

   Causes and Mitigation of Hardness

   Water hardness is caused primarily by dissolved polyvalent metal cations, overwhelmingly calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$). Hardness prevents soap from lathering efficiently by forming insoluble curds (precipitates) and leads to scale accumulation in industrial boilers and domestic heat exchangers.

   Types of Hardness

   • Temporary Hardness (Carbonate Hardness): Associated with bicarbonate salts, $Ca(HCO_3)_2$ and $Mg(HCO_3)_2$. It can be precipitated by boiling:$$Ca(HCO_3)_2(aq) \xrightarrow{\Delta} CaCO_3(s) + CO_2(g) + H_2O(l)$$
   • Permanent Hardness (Non-carbonate Hardness): Associated with sulfate, chloride, and nitrate salts of calcium and magnesium (e.g., $CaSO_4$, $MgCl_2$), which do not precipitate upon boiling.

   Softening Methods

   • Chemical Precipitation (Lime-Soda Process): Addition of slaked lime ($Ca(OH)_2$) neutralizes free carbonate species and precipitates magnesium as $Mg(OH)_2$ and calcium as $CaCO_3$. Soda ash ($Na_2CO_3$) precipitates non-carbonate calcium hardness.
   • Ion Exchange Resins: Synthetic polymeric resins loaded with sodium ($Na^+$) or hydrogen ($H^+$) ions selectively exchange their monovalent counter-ions for dissolved multivalent $Ca^{2+}$ and $Mg^{2+}$ ions, removing hardness without forming sludges:$$2R-Na + Ca^{2+}(aq) ightleftharpoons R_2Ca + 2Na^+(aq)$$
   • Membrane Processes: Nanofiltration and Reverse Osmosis (RO) physically exclude dissolved multivalent salts under high hydraulic pressure across a semipermeable membrane.