Hypochlorite Disinfection Monitoring and Adjustment Procedure

Purpose. This procedure tells plant operators how to monitor, verify and adjust sodium hypochlorite disinfection at the Kialla plant and the bore field so that the disinfection critical control point stays inside its limits.

1.Disinfection targets

The disinfection critical control point is measured as free chlorine residual at the outlet of the contact tank with the contact time calculated from the flow. The target, alert level and critical limit are recorded in the Critical Control Points Register. The operator must know the current values before starting a shift.

2.Shift checks

At the start of every shift the operator must compare the online chlorine analyser reading with a bench test using the colorimetric method, record both in the shift log and in the Water Quality Sampling Register, and recalibrate the analyser if the difference exceeds 0.1 milligrams per litre. Check the day tank level, dosing pump stroke and speed, and the injection point for crystallisation or blockage.

• Online analyser versus bench test difference within 0.1 mg/L
• Day tank level and pump running
• pH at contact tank outlet within 6.8 to 7.8
• Flow and contact time calculation reviewed

3.Adjusting the dose

Adjust the dosing pump in small steps and wait for the contact time plus analyser lag before adjusting again. Record every adjustment with the time, the reason and the resulting residual. A dose change of more than 20 percent must be reported to the Water Treatment Manager. Hypochlorite strength decays over time and in heat, so recalculate the dose after each delivery and record the delivered strength.

4.Alert level response

If the residual reaches the alert level, the operator must check the analyser, the pump and the day tank, adjust the dose and confirm the residual is returning to target within 30 minutes. Notify the control centre and log the event.

5.Critical limit breach

If the residual falls below the critical limit or the dosing pump stops, the operator must switch to the duty standby pump and, if the residual is not restored within the time set in the register, stop the plant outlet valve to the clear water storage and notify the control centre and the Water Treatment Manager immediately. Water that has left the contact tank below the critical limit must be traced and the Water Quality Incident Response Procedure applied.

6.Chemical safety

Sodium hypochlorite is corrosive and releases chlorine gas if it contacts acid. Wear a face shield, chemical gloves and apron when working on the dosing system, keep the acid used for cleaning away from the hypochlorite area, and use the safety shower and eyewash for any contact. Deliveries must be received by an operator who checks the delivery docket against the bulk tank and connects the correct fill point.

7.Records

Shift log entries, bench tests, adjustments and any excursion must be recorded and reviewed by the Water Treatment Manager each week. Excursions are verified in the Critical Control Points Register review each month.