Handbook for the Operation of Water Treatment Works Notes

Purpose and Target Audience of the Handbook

• Primary Objective: To provide encyclopedic information on the operational aspects of water treatment for domestic use, focusing on management, performance assessment, and optimization rather than process design.

• Core Goals for Operators:

  • Assess raw and process water quality.

  • Understand consumer health implications of sub-standard water.

  • Calculate dosages and parameters for varying raw water conditions.

  • Identify and correct causes of poor unit performance.

  • Manage water loss and safety programs.

• Target Audience: Specifically designed for personnel at the BSc/BTech level, including plant supervisors, process controllers, engineers, and scientists.

Introduction to Water Quality and Treatment

• Definition: Physical, chemical, and microbiological characteristics determine water quality and its fitness for specific uses (e.g., human consumption vs. industrial boiler feed).

• Requirements for Domestic Use:

  • Safety: Free from harmful micro-organisms and toxic substances.

  • Aesthetics: Must be clear, tasteless, and odorless.

  • Chemical Stability: Must not corrode pipes or form excessive scale.

• Human Right: Provision of clean water is a basic human right under the South African Constitution.

• Historical Milestones:

  • 1832: First municipal filtration plant (Scotland).

  • 1855: Dr. John Snow identifies water-borne cholera transmission.

  • 1905-1908: Commercial development and adoption of chlorination.

  • 1960s: Development of Reverse Osmosis (RO) membranes for desalination.

Physical Water Quality Parameters

• Turbidity: Measured in Nephelometric Turbidity Units ($NTU$). Indicates colloidal particle concentration. Goal for drinking water is < 1\,NTU, ideally < 0.1\,NTU.

• pH: Logarithmic measure of hydrogen ion concentration $[H^+]$. Formula: $pH = -\log[H^+]$. A change of 1 unit equals a 10-fold change in concentration.

• Electrical Conductivity (EC): Ability to conduct current. Measured in $mS/m$. Related to Total Dissolved Solids ($TDS$).

  • Conversion: $mS/m = \mu S/cm \times 0.1$.

  • Estimate: $TDS\,(\text{mg/l}) \approx EC\,(\text{mS/m}) \times 6.5$.

• Colour: Classified as "True" (dissolved) or "Apparent" (colloidal). Measured in $mg/l\,Pt$ or Hazen units.

• Taste and Odour: Measured by Threshold Odour Number ($TON$).

  • Formula: $TON = \frac{\text{ml sample} + \text{ml odour-free water}}{\text{ml sample}}$.

Chemical Water Quality and the Carbonate System

• Alkalinity: Measure of acid-neutralizing capacity. Predominantly determined by bicarbonate ($HCO_3^-$), carbonate ($CO_3^{2-}$), and hydroxide ($OH^-$).

  • Total Alkalinity: Titrated to $pH\,4.5$.

  • Phenolphthalein Alkalinity: Titrated to $pH\,8.4$.

• Hardness: Caused by divalent cations, mainly $Ca^{2+}$ and $Mg^{2+}$. Expressed as $mg/l\,CaCO_3$.

  • Classification:

    • Soft: < 50\,mg/l.

    • Hard: $250 - 350\,mg/l$.

    • Very Hard: > 350\,mg/l.

• Chemical Stability: Assessed to prevent corrosion or scale.

  • Indices: LSI (Langelier) and RI (Ryznar).

  • CCPP: Calcium Carbonate Precipitation Potential. Target is $+4\,mg/l\,CaCO_3$ for a thin protective layer.

• Water Equilibrium Formulas:

  • $K_w = [H^+][OH^-] = 10^{-14}$ at $25^\circ\text{C}$.

  • Solubility Product ($K_{sp}$): For $CaCO_3$, $K_s = [Ca^{2+}][CO_3^{2-}]$. If the ion product exceeds $K_s$, precipitation occurs.

Microbiological Quality and Pathogens

• Indicator Organisms: Used because detecting every pathogen is impractical.

  • Total Coliforms: General indicator of treatment efficacy and distribution integrity.

  • Faecal Coliforms/E. coli: Specific indicators of recent human or animal waste pollution.

  • Heterotrophic Bacteria: Indicates general microbial growth and process performance.

• Major Pathogens:

  • Bacteria: Vibrio cholera (Cholera), Salmonella typhi (Typhoid).

  • Viruses: Hepatitis A, Rotavirus (Gastroenteritis).

  • Protozoa: Giardia lamblia, Cryptosporidium (extremely resistant to chlorine).

Conventional Treatment Processes

• Coagulation: Addition of chemicals to destabilize colloids. Common coagulants:

  • Aluminium Sulphate (Alum): $Al_2(SO_4)_3 \cdot 16H_2O$. Requires $pH\,6.0 - 7.4$.

  • Ferric Chloride: $FeCl_3$. Operates across $pH\,5 - 8$.

• Flocculation: Aggregation of particles via slow stirring. Efficiency relies on the velocity gradient ($G-value$).

• Sedimentation: Gravity separation in basins. Important parameter: Surface Overflow Rate ($V_s = Q/A$).

• Filtration: Final clarification using sand.

  • Rapid Gravity Sand Filtration: Standard for conventional plants.

  • Slow Sand Filtration: Biological/mechanical removal without prior coagulation.

• Disinfection: Destruction of pathogens.

  • Chlorine: Formulated as gas ($Cl_2$), sodium hypochlorite ($NaOCl$), or HTH ($Ca(OCl)_2$).

  • $C \times t$ Concept: Residual concentration multiplied by contact time. Required for 99% inactivation.

Advanced Treatment Technologies

• Membrane Filtration:

  • Reverse Osmosis (RO): Non-porous; removes dissolved salts ($Pore\,size\,0.1\,nm$).

  • Nanofiltration (NF): Softening/ion removal ($Pore\,size\,1 - 10\,nm$).

  • Ultrafiltration (UF): Removes viruses/colloids ($10 - 50\,nm$).

  • Microfiltration (MF): Removes bacteria/particles (> 50\,nm).

• Activated Carbon: Adsorption for removal of dissolved organics, tastes, and odours.

• Ion Exchange: Resins used for softening (SAC/WAC) or nitrate removal (SBA).

• Ozonation: Powerful oxidant for organics/disinfection, though leaves no residual.

Post-Treatment and Stabilisation

• Recarbonation: Adding $CO_2$ to lower pH after lime softening.

• Fluoridation: Adding compounds like Sodium Fluorosilicate ($Na_2SiF_6$) to reach approximately $0.7\,mg/l\,F$ for dental health.

Maintenance and Safety

• Safety Legislation: Compliance with the Occupational Health and Safety Act (Act 85 of 1993).

• Hazardous Chemicals: Strict protocols for chlorine handling to prevent gas leaks.

• Preventative Maintenance: Scheduled lubrication, cleaning of screens/weirs, and calibration of flow meters to ensure plant integrity.