Water Treatment Automation: pH and Chlorine Control

Why Automate Water Treatment in India?

India faces enormous water quality challenges. With over 1.4 billion people and rapid industrialisation, automated water treatment is not a luxury — it is a necessity. From municipal water supply in Bengaluru to industrial effluent treatment in Vapi, automation ensures consistent water quality, regulatory compliance, and reduced chemical wastage.

Key parameters that require automated control:

• pH: Must be maintained between 6.5-8.5 for drinking water (BIS IS 10500:2012)
• Chlorine residual: 0.2-1.0 mg/L at the consumer end as per CPHEEO guidelines
• TDS (Total Dissolved Solids): Maximum 500 mg/L desirable, 2000 mg/L permissible
• Turbidity: Less than 1 NTU for treated drinking water
• Dissolved Oxygen: Critical for biological treatment processes

Manual chemical dosing, still common in many Indian towns, leads to under-dosing (health risk) or over-dosing (chemical waste and taste issues). Automated systems maintain precise control 24/7.

pH Control System Architecture

An automated pH control system consists of:

1. pH sensor/probe: Measures the current pH of water. Industrial pH probes for Indian water treatment cost ₹5,000-25,000 and require replacement every 12-18 months.
2. pH transmitter/controller: Converts the probe signal to a standard output (4-20mA or Modbus). Brands popular in India: Hanna, Eutech, Yokogawa, and Indian-made Apsara.
3. Chemical dosing pump: Peristaltic or diaphragm pump that adds acid (HCl) or alkali (NaOH/lime) to adjust pH. Prominent, Grundfos, and Indian-made Unique Dosing Systems are popular in India. Cost: ₹5,000-50,000.
4. PLC/Controller: Runs PID control loop, reading pH and adjusting pump speed.
5. Mixing tank: Where chemicals are dosed and mixed before reaching the next treatment stage.

The PID controller modulates the dosing pump output to maintain pH at the setpoint. Due to the logarithmic nature of pH (each unit represents a 10x change in H+ concentration), precise control near neutral pH (6.5-7.5) requires careful tuning and often a split-range control strategy — acid on one side, alkali on the other.

Chlorine Dosing and Residual Control

Chlorination is the most common disinfection method in Indian water treatment. The Central Public Health and Environmental Engineering Organisation (CPHEEO) mandates chlorine residual at the consumer end.

Automated chlorination system components:

• Chlorine analyser: Online residual chlorine measurement. Amperometric or colorimetric sensors. Cost: ₹50,000-2,00,000.
• Chlorine gas or sodium hypochlorite system: Gas systems are cheaper per kg of chlorine but require safety infrastructure. Sodium hypochlorite (liquid) is safer and more common in Indian municipal plants.
• Dosing pump: Controlled by the PLC based on residual chlorine reading and flow rate.

The control strategy: Chlorine dose = (Demand + Desired residual) × Flow rate

Where demand varies with water quality (higher turbidity and organic content = higher demand). In Indian conditions, monsoon season dramatically increases chlorine demand due to higher turbidity and organic loading in surface water sources.

Arduino-Based Water Quality Monitor

// Basic Water Quality Monitor
// Reads pH, temperature, and TDS
// Displays on LCD and logs to SD card

#include <LiquidCrystal_I2C.h>
#include <OneWire.h>
#include <DallasTemperature.h>
#include <SD.h>
#include <SPI.h>

#define PH_PIN A0
#define TDS_PIN A1
#define TEMP_PIN 2
#define SD_CS 10

OneWire oneWire(TEMP_PIN);
DallasTemperature tempSensor(&oneWire);
LiquidCrystal_I2C lcd(0x27, 20, 4);

float phValue, tdsValue, temperature;

void setup() {
  Serial.begin(9600);
  lcd.init();
  lcd.backlight();
  tempSensor.begin();
  SD.begin(SD_CS);
  lcd.print("Water Quality Mon.");
  delay(1000);
}

float readPH() {
  int raw = 0;
  for(int i = 0; i < 20; i++) {
    raw += analogRead(PH_PIN);
    delay(10);
  }
  float voltage = (raw / 20.0) * 5.0 / 1024.0;
  // Calibration: adjust offset and slope
  return 3.5 * voltage + 0.28;
}

float readTDS() {
  int raw = 0;
  for(int i = 0; i < 20; i++) {
    raw += analogRead(TDS_PIN);
    delay(10);
  }
  float voltage = (raw / 20.0) * 5.0 / 1024.0;
  // Temperature compensation
  float compensationCoeff = 1.0 + 0.02 * (temperature - 25.0);
  float compVoltage = voltage / compensationCoeff;
  return (133.42 * pow(compVoltage, 3) - 255.86 * pow(compVoltage, 2) + 857.39 * compVoltage) * 0.5;
}

void loop() {
  tempSensor.requestTemperatures();
  temperature = tempSensor.getTempCByIndex(0);
  phValue = readPH();
  tdsValue = readTDS();
  
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("pH: ");
  lcd.print(phValue, 1);
  lcd.setCursor(0, 1);
  lcd.print("TDS: ");
  lcd.print(tdsValue, 0);
  lcd.print(" ppm");
  lcd.setCursor(0, 2);
  lcd.print("Temp: ");
  lcd.print(temperature, 1);
  lcd.print(" C");
  
  // Log to SD
  File logFile = SD.open("waterlog.csv", FILE_WRITE);
  if (logFile) {
    logFile.print(millis()/1000);
    logFile.print(",");
    logFile.print(phValue);
    logFile.print(",");
    logFile.print(tdsValue);
    logFile.print(",");
    logFile.println(temperature);
    logFile.close();
  }
  
  delay(5000);
}

Sensors for Water Quality Monitoring

Monitoring water treatment processes requires reliable environmental sensing. These sensors help track temperature — a critical parameter that affects pH readings, chemical reaction rates, and chlorine decay:

Temperature compensation is essential for accurate pH and TDS measurements. A 1 degC change in water temperature causes a 0.01 pH unit shift. For Indian water treatment where inlet water temperature varies from 10 degC (winter in North India) to 35 degC (summer), this compensation is critical.

BIS Standards and Compliance for Indian Water Treatment

Indian water treatment plants must comply with:

• IS 10500:2012: Drinking water specification — covers 28 physical, chemical, and biological parameters
• CPCB effluent standards: Industry-specific discharge limits for pH, BOD, COD, TSS
• National Green Tribunal orders: Real-time monitoring and data upload for certain industries
• CPHEEO Manual: Design and operation guidelines for municipal water supply

Automated monitoring helps with compliance documentation. Many Indian regulatory bodies now require continuous online monitoring with real-time data upload to central servers. The Central Pollution Control Board’s (CPCB) OCEMS (Online Continuous Emission Monitoring System) mandate covers both air and water effluent monitoring for 17 categories of industries.

Scaling Up: Industrial Water Treatment Automation

For larger Indian installations, the automation architecture scales as follows:

[Field Instruments]
  pH sensor → 4-20mA → PLC AI
  Chlorine analyser → Modbus → PLC COM
  Flow meter → Pulse → PLC DI
  Level sensors → 4-20mA → PLC AI
  Turbidity meter → Modbus → PLC COM

[PLC (Delta/Siemens)]
  PID loops: pH control, chlorine control
  Sequence: backwash, CIP, startup
  Alarms: parameter exceedance, equipment fault

[SCADA Server]
  Data logging (1-second resolution)
  Trend displays
  Regulatory reports (monthly/quarterly)
  SMS/Email alerts

[Remote Access]
  VPN → SCADA for off-site monitoring
  Mobile app for operator notifications

Total cost for a 10 MLD (million litres per day) automated water treatment plant in India: ₹15-50 lakhs for the automation system (instruments, PLC, SCADA). This is typically 5-10% of the total plant civil and mechanical cost.

Frequently Asked Questions