Chemical Dosing System for Water & Wastewater

PROGRAM PLC_CONTROL_LOGIC_EXAMPLE
VAR
    // Inputs
    start_button : BOOL;
    stop_button : BOOL;
    system_ready : BOOL;
    error_detected : BOOL;

    // Outputs
    motor_run : BOOL;
    alarm_signal : BOOL;

    // Internal State
    system_state : INT := 0; // 0=Idle, 1=Running, 2=Error
    runtime_counter : INT := 0;


    // Flow Measurement & Totalization
    Instantaneous_Flow : REAL;  // m³/h or GPM
    Flow_Total_Daily : REAL;    // Accumulated flow
    Flow_Total_Monthly : REAL;
    Flow_Totalizer : CTU;

    // EPA Compliance Monitoring
    Effluent_pH : REAL;
    Effluent_BOD : REAL;  // Biological Oxygen Demand (mg/L)
    Effluent_TSS : REAL;  // Total Suspended Solids (mg/L)
    Effluent_Chlorine : REAL;  // Residual chlorine (mg/L)
    EPA_Limit_Violation : BOOL;

    // Chemical Dosing Control
    Chlorine_Dosing_Rate : REAL;  // mg/L or ppm
    Polymer_Dosing_Rate : REAL;
    Chemical_Tank_Level : REAL;
    Low_Chemical_Alarm : BOOL;

    // Level & Pressure Control
    Tank_Level_Percent : REAL;
    System_Pressure_PSI : REAL;
    High_Level_Alarm : BOOL;
    Low_Level_Alarm : BOOL;

    // SCADA Integration
    Remote_Start_Command : BOOL;
    Remote_Stop_Command : BOOL;
    SCADA_Communication_OK : BOOL;
    Telemetry_Update_Timer : TON;

    // Regulatory Reporting
    Sample_Collection_Due : BOOL;
    DMR_Report_Due : BOOL;  // Discharge Monitoring Report
    Last_Sample_Time : DATE_AND_TIME;
    Compliance_Status : STRING[20];
END_VAR

// ==========================================
// BASE APPLICATION LOGIC
// ==========================================

CASE system_state OF
    0: // Idle state
        motor_run := FALSE;
        alarm_signal := FALSE;

        IF start_button AND system_ready AND NOT error_detected THEN
            system_state := 1;
        END_IF;

    1: // Running state
        motor_run := TRUE;
        alarm_signal := FALSE;
        runtime_counter := runtime_counter + 1;

        IF stop_button OR error_detected THEN
            system_state := 2;
        END_IF;

    2: // Error state
        motor_run := FALSE;
        alarm_signal := TRUE;

        IF stop_button AND NOT error_detected THEN
            system_state := 0;
            runtime_counter := 0;
        END_IF;
END_CASE;

// ==========================================
// WATER & WASTEWATER SPECIFIC LOGIC
// ==========================================

    // Flow Totalization for Billing & Reporting
    // Convert instantaneous flow to volumetric pulses
    IF Instantaneous_Flow > Min_Flow_Threshold THEN
        Flow_Pulse := TRUE;  // Generate pulse per unit volume
        Flow_Totalizer(CU := Flow_Pulse);
        Flow_Total_Daily := DINT_TO_REAL(Flow_Totalizer.CV) * Flow_K_Factor;
    END_IF;

    // Reset daily totalizer at midnight
    IF Current_Time = TIME#00:00:00 THEN
        Flow_Totalizer(R := TRUE);
        Flow_Total_Monthly := Flow_Total_Monthly + Flow_Total_Daily;
        Flow_Total_Daily := 0.0;
    END_IF;

    // EPA Discharge Limits Monitoring
    // Typical secondary treatment limits
    EPA_Limit_Violation := (Effluent_BOD > 30.0)      // > 30 mg/L
                          OR (Effluent_TSS > 30.0)     // > 30 mg/L
                          OR (Effluent_pH < 6.0)       // pH below range
                          OR (Effluent_pH > 9.0);      // pH above range

    IF EPA_Limit_Violation THEN
        Compliance_Status := 'VIOLATION';
        // Trigger alarm and regulatory notification
        // Log excursion for DMR reporting
    ELSE
        Compliance_Status := 'COMPLIANT';
    END_IF;

    // Proportional Chemical Dosing
    // Dose chlorine based on flow rate for consistent residual
    Chlorine_Dosing_Rate := Instantaneous_Flow * Target_Chlorine_Residual / 1000.0;

    // Polymer dosing for flocculation based on turbidity
    IF Influent_Turbidity > 100.0 THEN
        Polymer_Dosing_Rate := High_Dose_Rate;
    ELSIF Influent_Turbidity > 50.0 THEN
        Polymer_Dosing_Rate := Medium_Dose_Rate;
    ELSE
        Polymer_Dosing_Rate := Low_Dose_Rate;
    END_IF;

    // Chemical Tank Monitoring
    IF Chemical_Tank_Level < 20.0 THEN  // Below 20%
        Low_Chemical_Alarm := TRUE;
        // Alert operator to refill
    END_IF;

    // SCADA Telemetry Updates
    Telemetry_Update_Timer(IN := TRUE, PT := T#10s);

    IF Telemetry_Update_Timer.Q THEN
        // Send data to SCADA master station
        Send_To_SCADA(Flow_Total_Daily,
                      System_Pressure_PSI,
                      Tank_Level_Percent,
                      EPA_Limit_Violation);
        Telemetry_Update_Timer(IN := FALSE);
    END_IF;

    // Remote SCADA Control Integration
    IF SCADA_Communication_OK THEN
        IF Remote_Start_Command THEN
            System_Enable := TRUE;
        END_IF;

        IF Remote_Stop_Command THEN
            System_Enable := FALSE;
        END_IF;
    ELSE
        // Revert to local control if SCADA comm lost
        System_Enable := Local_Start_Button AND NOT Local_Stop_Button;
    END_IF;

    // Regulatory Sampling Schedule
    IF (CURRENT_DATETIME() - Last_Sample_Time) > Sample_Interval THEN
        Sample_Collection_Due := TRUE;
        // Alert lab technician
    END_IF;

// ==========================================
// WATER & WASTEWATER SAFETY INTERLOCKS
// ==========================================

    // Operational Interlocks
    Production_Allowed := NOT High_Level_Alarm
                          AND NOT Low_Level_Alarm
                          AND NOT Low_Chemical_Alarm
                          AND (System_Pressure_PSI > Min_Pressure)
                          AND (System_Pressure_PSI < Max_Pressure);

    // EPA Compliance Interlock
    IF EPA_Limit_Violation THEN
        // Continue operation but alert operator
        // Log violation for regulatory reporting
        Alarm_Active := TRUE;
    END_IF;

    // Emergency Shutdown on Critical Parameters
    IF (Effluent_Chlorine > Max_Safe_Chlorine) OR
       (System_Pressure_PSI > Burst_Pressure) THEN
        Emergency_Shutdown := TRUE;
        All_Pumps_Stop := TRUE;
    END_IF;