class 4

Tank Sizing and Configuration

  • Importance of Adequate Retention Time

    • Retention time is crucial for gauge measurement and reagent effectiveness.
    • Proper retention ensures water has enough time in the tank for treatment, especially during pH adjustment.

  • Typical Tank Configuration

    • In an ideal tank, the treatment tank's inlet and outlet should be positioned at opposite sides.
    • One inlet should be located low in the tank while the outlet remains high to promote effective treatment.
    • Purpose of Opposite End Location:
    • Prevents short-circuiting.
    • Short-circuiting occurs when untreated wastewater flows rapidly across and out of the outlet, leading to ineffective treatment.
    • A bottom inlet (as seen in the preferred configuration) maximizes flow path and ensures thorough treatment.
    • Alternative configuration options exist (e.g., top in, bottom out) but have shorter flow paths.

pH Probes Placement

  • Placement of pH Probes in Tank Systems

    • Typically involves two tanks:
    1. Mixed Tank: Roughly adjusts pH to neutral range (approximately 6-9).
    2. Trim Tank: Fine-tunes the pH closer to neutral (around 7).
    • Effluent pH Probe: Monitors the pH of discharged water, crucial for regulatory compliance.

  • Regulatory Concerns

    • Regulators, such as the MWA or GLSD, prioritize the pH of water discharged to ensure compliance with standards.

Process Control Methods

  • Types of Process Control
    • Feed Forward Control:
    • Measures pH at the inlet point.
    • Predicts changes in the process before they occur.
    • More complex since proper probe location analysis is needed.
    • Feedback Control:
    • Measures pH at the discharge point.
    • Reacts after disturbances occur, leading to potentially less control.
    • More straightforward and commonly used in many applications.

pH Sensor Structure and Function

  • Components of a pH Sensor

    • Measurement Electrode: Acts as the main sensor for voltage generation.
    • Reference Electrode: Provides a stable reference potential for comparison.
    • Reference Liquid Junction (Salt Bridge): Facilitates ion flow between the electrodes.
    • Gel Coated Glass Membrane: Protects the electrode.

  • Measurement Mechanism:

    • Compares electrical potentials between electrodes to determine hydrogen ion concentration.
    • Voltage generation occurs due to ion movement between the electrodes through the salt bridge.
    • Greater electrical potential differences yield higher voltage readings, which translate into pH readings.

  • Calibration and Temperature Effects

    • Probes may display a millivolt reading instead of direct pH units, requiring further conversion.
    • Temperature changes affect pH readings:
    • Higher temperatures decrease pH readings, and vice versa.
    • Most pH sensors include temperature compensators but can be slow to adjust.
    • Calibration best practices: Ensure buffer solutions are at similar temperatures to the wastewater being measured.

Factors Affecting pH Sensor Accuracy

  • Carbon Dioxide Influence

    • Alkaline solutions absorb CO2, potentially lowering pH by up to 0.1 units over time.
    • Importance of using fresh buffer solutions during calibration to prevent errors.

  • Response Time and Maintenance

    • Response time assessments can indicate probe functionality and cleanliness.
    • Fouling occurs when contaminants build up on electrode surfaces, degrading performance.
    • A clean pH probe is essential for accurate and timely measurements.
    • Lifespan of a pH probe typically ranges from 9 months to 2 years depending on maintenance and wastewater characteristics.
    • Response times are non-linear and depend on buffering capacity, pH change size, and direction of the change.
    • Excessive buffering can slow response time, especially during significant pH shifts.

Instrument Control Types

  • Four Types of Instrument Control
    • On/Off Control: Simple activation based on set points.
    • Proportional Control: Regulates speed of chemical feed based on pH levels.
    • Proportional Integral and Proportional Integral Derivative Controls: More precise and adaptive control systems, if required.

Maintenance of pH Systems

  • Importance of Preventive Maintenance

    • Regular checks and maintenance are crucial to ensure sensor accuracy and longevity.
    • Primary factors to maintain include:
    • pH sensors and level sensors.
    • Chemical feed pump components.
    • Automatic valves and their functionality.
    • System mixing equipment.

  • Monthly Maintenance Checklist:

    • Calibrate pH sensors and record effluent gauge readings.
    • Check probe response times and fouling.
    • Perform alarm tests and ensure that all functions, including if chemical drums are changing as needed.

Troubleshooting pH Adjustment Issues

  • Steps for Troubleshooting

    1. Check mechanical operation of equipment and pumps to ensure they are running correctly.
    2. Evaluate pH electrode performance, including response times and cleanliness.
    3. Determine if any changes occurred in wastewater characteristics that might require adjustment in system settings.

  • Importance of Checking Chemical Supply

    • Ensure adequate chemical supply to prevent improper treatment and monitor drum levels.

  • Equipment and System Behavior

    • Make sure mixers and pumps are functioning properly to ensure accurate chemical mixing and pH control.