Design and Implementation of PLC Control System for Food Wastewater Treatment

The automation control of food wastewater treatment is directly related to environmental compliance and production efficiency. A control system based on Siemens PLC can achieve stable operation and precise management.

1. Hardware Configuration

Selection of PLC and Expansion Modules

Considering the characteristics of food wastewater treatment, it is recommended to use the Siemens S7-1200 series PLC as the core controller. This series has strong processing capabilities and good anti-interference, making it particularly suitable for industrial environments.

Main Controller: S7-1214C DC/DC/DC (14 digital inputs, 10 digital outputs, 2 analog inputs)

Expansion Modules:

SM1231 Analog Input Module (4AI) × 1: Used for collecting analog values such as pH, dissolved oxygen, turbidity, etc.
SM1232 Analog Output Module (2AO) × 1: Used for control of valves and frequency converters
SM1222 Digital Output Module (8DO) × 1: Used for control of pumps, fans, and other equipment

I/O Point Allocation Table

**Digital Inputs (DI)**:

I0.0: Inlet pump running feedback
I0.1: Aeration fan running feedback
I0.2: Mixer running feedback
I0.3: Dosing pump running feedback
I0.4: Sludge pump running feedback
I0.5: High liquid level alarm
I0.6: Low liquid level alarm
I0.7: Emergency stop button

**Digital Outputs (DO)**:

Q0.0: Inlet pump start/stop
Q0.1: Aeration fan start/stop
Q0.2: Mixer start/stop
Q0.3: Dosing pump 1 start/stop
Q0.4: Dosing pump 2 start/stop
Q0.5: Sludge pump start/stop
Q0.6: Solenoid valve 1 control
Q0.7: Solenoid valve 2 control

**Analog Inputs (AI)**:

IW64: pH sensor (4-20mA)
IW66: Dissolved oxygen sensor (4-20mA)
IW68: Turbidity sensor (4-20mA)
IW70: Level sensor (4-20mA)

**Analog Outputs (AO)**:

QW80: Aeration fan frequency control (4-20mA)
QW82: Dosing pump frequency control (4-20mA)

2. Control Program Design

Variable Definition Specification

To ensure program readability and maintainability, a structured variable naming convention is adopted:

[Device Type]_[Function Description]_[I/O Type]

For example:

Pump_Inlet_Run (Inlet pump running status)
Valve_Chemical_Ctrl (Dosing valve control)
AI_pH_Value (pH analog input)
AO_Blower_Speed (Blower speed control output)

Program Architecture Design

The program adopts a modular structure design, clearly dividing functional areas:

OB1: Main cycle block, responsible for calling various function blocks

FB10: Data collection and processing (analog conversion and filtering)

FB20: Automatic control logic (control of various process segments)

FB30: Manual control logic

FB40: Alarm handling

FB50: Data recording

FB60: Communication processing

Corresponding data blocks:

DB10: Parameter configuration

DB20: Operating data

DB30: Alarm information

DB40: Historical records

Function Block Design Example

Taking the pH control function block as an example, automatic dosing control is implemented:

FUNCTION_BLOCK "FB_pH_Control"

VAR_INPUT

    pH_Value : REAL;            // Actual pH value

    pH_SetPoint : REAL;         // pH set point

    pH_Deviation : REAL;        // Allowable deviation

    Auto_Mode : BOOL;           // Automatic mode

END_VAR

VAR_OUTPUT

    Pump_Acid_Start : BOOL;     // Acid dosing pump start

    Pump_Alkali_Start : BOOL;   // Alkali dosing pump start

    Pump_Speed : REAL;          // Dosing pump speed (0-100%)

END_VAR

VAR

    pH_Error : REAL;            // Error value

    PID_Controller : FB41;      // PID controller instance

END_VAR


BEGIN

    // Calculate pH error

    pH_Error := pH_SetPoint - pH_Value;
    
    // Determine dosing direction

    IF Auto_Mode THEN

        // Determine if dosing is needed

        IF ABS(pH_Error) > pH_Deviation THEN

            // Decide whether to add acid or alkali based on error direction

            IF pH_Error < 0 THEN

                Pump_Acid_Start := TRUE;

                Pump_Alkali_Start := FALSE;

            ELSE

                Pump_Acid_Start := FALSE;

                Pump_Alkali_Start := TRUE;

            END_IF;
            
            // Call PID to calculate dosing speed

            "PID_Controller"(

                Setpoint := pH_SetPoint,

                Input := pH_Value,

                Output => Pump_Speed

            );
        ELSE

            // Stop dosing within allowable deviation range

            Pump_Acid_Start := FALSE;

            Pump_Alkali_Start := FALSE;

            Pump_Speed := 0.0;

        END_IF;
    END_IF;
END_FUNCTION_BLOCK

3. Data Management and Storage

Parameter Configuration Table

Use global data blocks to store system parameters for centralized management and modification:

DATA_BLOCK "DB_Parameters"

{S7_Optimized_Access := 'TRUE'}

VERSION : 0.1

NON_RETAIN

    STRUCT

        // pH control parameters

        pH_Control : STRUCT

            SetPoint : REAL := 7.0;         // pH set point

            Deviation : REAL := 0.2;        // Allowable deviation

            P_Factor : REAL := 1.5;         // Proportional factor

            I_Factor : REAL := 0.8;         // Integral factor

            D_Factor : REAL := 0.1;         // Derivative factor

        END_STRUCT;
        
        // Dissolved oxygen control parameters

        DO_Control : STRUCT

            SetPoint : REAL := 2.5;         // DO set point (mg/L)

            Deviation : REAL := 0.3;        // Allowable deviation

            P_Factor : REAL := 2.0;         // Proportional factor

            I_Factor : REAL := 0.5;         // Integral factor

            D_Factor : REAL := 0.2;         // Derivative factor

        END_STRUCT;
        
        // Sludge return control parameters

        Sludge_Control : STRUCT

            Ratio : REAL := 0.4;            // Return ratio

            Timer_On : TIME := T#15M;       // Running time

            Timer_Off : TIME := T#5M;       // Stopping time

        END_STRUCT;
        
        // System operating parameters

        System : STRUCT

            Auto_Start_Time : TIME_OF_DAY := TOD#6:00:00;  // Automatic start time

            Auto_Stop_Time : TIME_OF_DAY := TOD#22:00:00;  // Automatic stop time

            Data_Save_Interval : TIME := T#30M;            // Data save interval

        END_STRUCT;

    END_STRUCT;

BEGIN

END_DATA_BLOCK

Runtime Data Recording

The system runtime data is recorded in a circular buffer data block, periodically saving key parameters:

DATA_BLOCK "DB_Runtime_Data"

{S7_Optimized_Access := 'TRUE'}

VERSION : 0.1

NON_RETAIN

    STRUCT

        // Current runtime data

        Current : STRUCT

            pH_Value : REAL;          // Current pH value

            DO_Value : REAL;          // Current dissolved oxygen value

            Turbidity : REAL;         // Current turbidity

            Inflow_Rate : REAL;       // Inflow rate

            Blower_Speed : REAL;      // Blower speed

            Chemical_Pump_Speed : REAL; // Dosing pump speed

        END_STRUCT;
        
        // Cumulative runtime data

        Statistics : STRUCT

            Total_Inflow : DINT;      // Cumulative treated water volume (m³)

            Energy_Consumption : REAL; // Cumulative energy consumption (kWh)

            Chemical_Usage : REAL;    // Chemical usage (L)

            Runtime_Hours : DINT;     // System runtime (h)

        END_STRUCT;
        
        // Historical data (circular buffer, 48 points, one every 30 minutes)

        History : ARRAY[0..47] OF STRUCT

            Timestamp : DATE_AND_TIME; // Timestamp

            pH_Value : REAL;           // pH value

            DO_Value : REAL;           // Dissolved oxygen value

            Turbidity : REAL;          // Turbidity

            Inflow_Rate : REAL;        // Inflow rate

        END_STRUCT;
        
        // Historical data pointer

        History_Index : INT := 0;      // Current record pointer

    END_STRUCT;

BEGIN

END_DATA_BLOCK

4. Fault Diagnosis and Troubleshooting

Common Fault Analysis

Common faults and solutions for food wastewater treatment systems:

Abnormal pH Fluctuations

Cause: Sensor calibration errors, dosing system failures, sudden changes in influent water quality
Solution: Verify sensor calibration, check dosing system, set buffer to adjust influent pH

Low Dissolved Oxygen

Cause: Blower failure, aeration head blockage, excessive organic load
Solution: Check blower operation status, clean aeration system, adjust inflow rate

Abnormal Sludge Return

Cause: Pump failure, pipeline blockage, abnormal level sensor
Solution: Check pump motor, clear pipeline, calibrate level sensor

Diagnosis Program Design

Below is an example of a device status monitoring and diagnosis function block:

FUNCTION_BLOCK "FB_Equipment_Diagnosis"

VAR_INPUT

    Enable : BOOL;                 // Enable diagnosis

    Equipment_Running : BOOL;      // Equipment running status

    Current_Feedback : REAL;       // Current feedback

    Current_Normal_Min : REAL;     // Normal current lower limit

    Current_Normal_Max : REAL;     // Normal current upper limit

    Pressure_Feedback : REAL;      // Pressure feedback

    Pressure_Normal_Min : REAL;    // Normal pressure lower limit

    Pressure_Normal_Max : REAL;    // Normal pressure upper limit

END_VAR

VAR_OUTPUT

    Normal_Operation : BOOL;       // Normal operation

    Overload_Warning : BOOL;       // Overload warning

    Underload_Warning : BOOL;      // Underload warning

    Pressure_High_Warning : BOOL;  // High pressure warning

    Pressure_Low_Warning : BOOL;   // Low pressure warning

    Fault_Code : INT;              // Fault code

END_VAR

VAR

    Timer_Delay : TON;             // Delay timer

END_VAR


BEGIN

    // Reset outputs

    Normal_Operation := FALSE;

    Overload_Warning := FALSE;

    Underload_Warning := FALSE;

    Pressure_High_Warning := FALSE;

    Pressure_Low_Warning := FALSE;
    
    // Perform diagnosis only when equipment is running

    IF Enable AND Equipment_Running THEN

        // Current diagnosis

        IF Current_Feedback > Current_Normal_Max THEN

            Overload_Warning := TRUE;

            Fault_Code := 1;  // Overload fault

        ELSIF Current_Feedback < Current_Normal_Min THEN

            Underload_Warning := TRUE;

            Fault_Code := 2;  // Underload fault

        END_IF;
        
        // Pressure diagnosis

        IF Pressure_Feedback > Pressure_Normal_Max THEN

            Pressure_High_Warning := TRUE;

            Fault_Code := 3;  // Pressure too high

        ELSIF Pressure_Feedback < Pressure_Normal_Min THEN

            Pressure_Low_Warning := TRUE;

            Fault_Code := 4;  // Pressure too low

        END_IF;
        
        // Determine if normal operation

        IF NOT (Overload_Warning OR Underload_Warning OR 

                Pressure_High_Warning OR Pressure_Low_Warning) THEN

            Normal_Operation := TRUE;

            Fault_Code := 0;  // No fault

        END_IF;
    ELSE

        Fault_Code := 0;      // Not running state

    END_IF;
END_FUNCTION_BLOCK

5. User Interface Design

Interface Layout Planning

The HMI interface for food wastewater treatment should include the following main pages:

Overview Page: Displays the overall system operating status, including the status of each treatment unit, main equipment, and key parameters
Process Flow Page: Shows detailed treatment processes, equipment status, and real-time data
Parameter Settings Page: Interface for viewing and modifying control parameters, requiring password protection
Alarm Page: Current active alarms and historical alarm records
Trend Curve Page: Historical trends of key parameters
Data Record Page: Function for recording and exporting operating data

Parameter Settings Interface

The parameter settings page should have tiered authorization to avoid misoperation:

Operator Level: Can only view parameters, cannot modify
Engineer Level: Can modify operating parameters, such as pH set point, DO set point, etc.
Administrator Level: Can modify core system parameters, such as PID parameters, system configuration, etc.

After parameter modification, a confirmation mechanism should be in place, and important parameter changes should record the operator and time.

Alarm Handling Design

The alarm system should be designed in tiers:

Prompt Information: Non-critical parameters deviating from normal range, displayed in yellow
Warning Alarm: Important parameters deviating but not reaching dangerous values, displayed in orange with sound prompts
Danger Alarm: Critical parameters reaching dangerous values, flashing red display with sound prompts, may trigger interlock protection

Alarm information should include: alarm time, alarm content, alarm level, confirmation status, and release time.

6. System Debugging Methods

Step-by-Step Debugging Program

System debugging should follow the principle of “single point – loop – system”:

Single Point Testing: Test each I/O point one by one to confirm correct hardware connections and address mapping

Digital Inputs: Simulate field signals to verify PLC reading status
Digital Outputs: Manually set positions to verify actuator actions
Analog Inputs: Simulate 4-20mA signals to verify PLC conversion values
Analog Outputs: Manually set output values to verify actuator response

Loop Testing: Test each control loop to verify control logic

pH Control Loop: Simulate pH changes to verify dosing logic
DO Control Loop: Simulate DO changes to verify blower control
Level Control Loop: Simulate level changes to verify inlet pump and drainage valve control

System Coordination Testing: Overall system operation testing

Automatic/Manual mode switching test
Process parameter setting and response test
Abnormal situation and alarm handling test
Long-term stability test

Parameter Tuning Steps

Taking pH control as an example, the PID parameter tuning steps are as follows:

Initialize parameters: Set I and D to 0, P to a small value
Gradually increase P value until the system shows slight oscillation
Introduce I value to reduce overshoot and eliminate steady-state error
Introduce D value as needed to improve system response speed
Fine-tune the three parameters until satisfactory control effect is achieved
Verify parameter adaptability under different working conditions

The PLC control scheme for food wastewater treatment can effectively improve wastewater treatment efficiency and stability. The technical solutions presented in this article have been validated in multiple projects, and discussions on further optimization solutions are welcome.