I. Case Background

A mineral water production enterprise adopts an “Ultrafiltration + Reverse Osmosis” combined water treatment process. Groundwater is used as the raw water source (directly filtered by ultrafiltration), and ozone disinfection is applied to ensure water quality safety.

The core monitoring device selected is the German-made DOZ1.2MA2 ozone sensor from DABECO, integrated with a foreign-brand online transmitter for real-time ozone concentration monitoring and process control.

1. Basis for Equipment Selection

Based on the ozone concentration requirement in mineral water production (typically stable at 0.1–0.5 mg/L, i.e., 0.1–0.5 ppm), the key specifications of DOZ1.2MA2 are highly compatible with the application scenario:

2. Initial Process Settings

• Ultrafiltration to Reverse Osmosis ratio: 8:2
  (80% ultrafiltration water directly filtered from groundwater; 20% reverse osmosis water for deep desalination)

• Ozone control target: 0.3–0.5 ppm
  Ensuring effective sterilization while avoiding excessive residual ozone.

II. Fault Phenomenon

During system commissioning, abnormal issues occurred:

1. Concentration Instability
   After ozone generator startup, concentration rose to 0.5 ppm, then continuously declined to below 0.1 ppm within 10 minutes (current < 4.8 mA), failing to maintain the target range.

2. Instrument Consistency Verification
   Offline iodometric titration analysis showed the same concentration trend as the DOZ1.2MA2 current output, excluding sensor measurement error or transmitter malfunction.

3. Calibration Obstruction
   Since calibration requires stable concentration conditions, the sensor could not complete the standard analytical calibration procedure, raising concerns about measurement reliability.

III. Troubleshooting Process

Step 1: Excluding Sensor Failure

Comprehensive inspection confirmed:

• Installed in dedicated flow cell

• Flow rate: 22 L/h (within required 15–30 L/h)

• Working pressure: 0.6 bar (<1.0 bar limit)

• Ambient temperature: 25°C (within 0–55°C range)

• No vibration or pressure pulses

• Membrane cap (M20.2, Art.no.11011.1) newly replaced (2 months in service)

• Electrolyte (EOZ1/W, Art.no.11101) normal, no leakage or contamination

• Stable 24 VDC supply, correct M12 wiring, no signal interference

Conclusion: Sensor installation, status, and signal were normal. The root cause was not equipment-related.

Step 2: Identifying Process & Water Quality Correlation

Considering ozone’s chemical reactivity with alkaline substances:

• Ultrafiltration water alkalinity (as CaCO₃): 180 mg/L

• Reverse osmosis water alkalinity: 25 mg/L

With the initial 8:2 ratio, high-alkalinity ultrafiltration water dominated the system.

Ozone reacts rapidly with bicarbonate and carbonate ions:

O₃ + HCO₃⁻ → O₂ + CO₃²⁻ + H₂O

This accelerated ozone consumption.

Verification Test

Process ratio adjusted to 2:8 (20% UF, 80% RO):

• Ozone stabilized at 0.5 ppm (8 mA) for over 30 minutes

• Total alkalinity reduced to 52 mg/L

• Ozone consumption rate significantly decreased

Root cause confirmed: excessive alkalinity due to process ratio.

IV. Solution and Optimization

1. Process Optimization

• Fixed UF:RO ratio at 2:8
  Maintaining total alkalinity at 50–60 mg/L.

• Flow rate adjusted to 20–25 L/h
  Ensuring stable sensor response (T90 ≈ 15 seconds).

2. Closed-Loop Control via 4–20 mA Output

3. Calibration & Maintenance

• After 2 hours of stable concentration, calibration performed using 0.25 ppm iodometric standard solution.

• Transmitter adjusted so output current equals 6.0 mA at 0.25 ppm.

• Measurement error controlled within ±2%.

Maintenance plan:

• Replace membrane cap every 6 months

• Refill electrolyte every 3 months

• Monthly inspection of cable and flow cell cleanliness

V. Case Summary

1. Performance Validation
   DOZ1.2MA2’s low measuring range (0.005–2.000 ppm), high resolution (0.001 ppm), stable 4–20 mA output, wide pH compatibility (2–11), and automatic temperature compensation fully meet low-concentration ozone monitoring requirements in mineral water production.

2. Key Troubleshooting Strategy
   When abnormal data occurs, first eliminate sensor-related causes (installation, consumables, signal), then analyze process and water chemistry factors to avoid unnecessary equipment replacement.

3. Replicable Experience
   For groundwater-based mineral water production, balancing UF and RO ratio is essential to control alkalinity when using ozone disinfection. Combined with DOZ1.2MA2 sensor, closed-loop control, and regular calibration, long-term stable ozone concentration control can be achieved.