Drainage Services
Department
Harbour Area Treatment
Scheme Stage 2A –Advance Disinfection Facilities (ADF) at Stonecutters Island
Sewage Treatment Works
Mechanism for
controlling dosage of sodium hypochlorite and sodium bisulphite to avoid
over dosage and adverse environmental impacts on water quality and marine
ecology
1.
Introduction
1.1 Background
In accordance with Condition 3.1 of the Environmental Permit no. EP-295/2007/A, the proposed mechanism for controlling dosage of sodium hypochlorite and sodium bisulphite in the disinfection process to avoid over dosage and adverse environmental impacts on water quality and marine ecology is required to be presented to the Director of Environment Protection.
Chlorination followed by dechlorination is adopted for the disinfection at Stonecutters Island sewage treatment works (SCISTW). For the advance disinfection facilities, the existing effluent culvert system will be used as the chlorine contact basin. Sodium hypochlorite solution as the disinfection agent will be dosed at Flow Distribution Chamber while sodium bisulphite as the dechlorination agent will be dosed at Chamber 15 prior to the outfall discharge. Based on literature review and test results gathered during the ADF Environmental Impact Assessment Study, dechlorination generally takes place almost instantaneously. Dosing of sodium bisulphite at Chamber 15 before discharge to the outfall should provide sufficient contact time for the dechlorination agent to remove the residual chlorine in the effluent. A general layout plan showing the dosing locations is at Annex A.
1.2 Objective
and General Description
This paper will outline the mechanisms in controlling the dosages of sodium hypochlorite and sodium bisulphite to avoid over dosage and adverse environmental impacts on water quality and marine ecology.
2 Disinfection
Chemicals Dosage Control Philosophy
A computerized control system will be set up to monitor and control the dosing of sodium hypochlorite and sodium bisulphite solutions. The operation will normally be operated in automatic mode where dosage will be controlled based on the total residual chlorine (TRC) level of the chlorinated effluent measured by the on-line TRC analyzers installed at Chamber 15 before dechlorination. In case of failure in the on-line TRC analyzers, the dosage will be controlled based on the TRC level measured manually.
2.1 Chlorination
Dosage Control
Sodium hypochlorite dosage is affected by the chlorine demand[1] of the effluent which varies from time to time depending on the effluent characteristics and other factors. Sodium hypochlorite dosage will be set with an aim to maintain the TRC level at Chamber 15 (before dechlorination) within the target design range (i.e. 2mg/l to 4mg/l) for sufficient bacterial kills. In case the TRC level at Chamber 15 falls outside the target design range, the dosage of sodium hypochlorite will be adjusted automatically to restore the TRC level back to the target design range. The target dosage range will be reviewed during the commissioning stage to allow optimization of the process.
2.2 Dechlorination
Dosage Control
Sodium bisulphite dosage will be set with an aim to maintain zero TRC level at discharge. The dosage of sodium bisulphite will be adjusted according to the measured TRC level at Chamber 15 (before dechlorination) to ensure that sufficient sodium bisulphite is provided to remove the TRC. The sodium bisulphite dosage is determined by multiplying the measured TRC with the adjusted stoichiometric weight ratio of sodium bisulphite (i.e. a predefined constant of 1.61, which will be subject to fine-tuning during the commissioning period). The dosing pumps will control the sodium bisulphite dosing rate based on the sewage flowrate and the sodium bisulphite dosage determined. The designed weight ratio applied will be verified and reviewed during the commissioning stage to allow optimization of the dosage.
Figure 1 shows a schematic plan of the chemical dosing control and monitoring system and Figure 2 shows a schematic plan of the monitoring locations at the Dechlorination Plant for the advance disinfection facilities.
3 Design
Features to Enhance System Reliability to Avoid Overdosing
3.1 Dual TRC Analyzers
As TRC
measurement plays an important role in the dosage control, dual TRC analyzers
will be provided for on-line TRC monitoring to enhance measurement accuracy and
system reliability. The TRC analyzers will be calibrated daily initially to
ensure reliability and consistency of the measurements. The calibration
frequency shall be reviewed after operation of the ADF for a period of six
months. Such measures could
minimize the possibility of over dosing due to measurement inaccuracy. In case both on-line TRC analyzers
failed, alarm will be activated; the plant operator
will adjust the sodium hypochlorite
dosage used in the control system based on the TRC level measured manually by
them and a slightly higher sodium bisulphite stoichiometric weight ratio will be adopted to safeguard zero TRC
discharge so as to avoid causing any adverse
environmental impacts on water quality and marine ecology.
3.2 Dechlorination
Simulation System
A simulation system, in the form of a pipeline which collects dechlorinated effluent from the drop shaft entry culvert before discharging to the outfall, will be provided to simulate the dechlorination process in the outfall. Manual measurement of TRC level at the end of the simulation system will be conducted by the plant operator for compliance with the TRC standards in the discharge licence. Bisulphite level at the end of the simulation system will also be measured by on-line analyzer to facilitate optimization of the sodium bisulphite dosage.
3.3 Standby
Units and Uninterruptible Power Supply
Standby units and dual power supply are provided for the sodium hypochlorite dosing and control system to enhance system reliability. Similarly, standby units are provided for the sodium bisulphite dosing and control system. An uninterruptible power supply is provided for the sodium bisulphite dosing system which will keep the bisulphite dosing system operate for a period of 30 minutes after a power failure. Dosing of sodium hypochlorite will be terminated immediately in case there is power failure at the sodium bisulphite dosing system. Given that the traveling time of the effluent between the Flow Distribution Chamber and Chamber 15 would be less than 15 minutes, the continuous dosing of sodium bisulphite for 30 minutes after the termination of sodium hypochlorite dosing will enable all the chlorinated effluent within the box culvert to react with bisulphite before discharge to avoid any adverse environmental impacts on water quality and marine ecology.
4 Failsafe
Measures to Avoid Chemical Overdose
4.1 Failsafe
Measures to Avoid Overdosing of Sodium Hypochlorite Solution
To avoid overdose of sodium hypochlorite, the control system will automatically terminate the dosing of sodium hypochlorite when abnormal conditions, such as activation of “no flow” alarm, power failure and/or system failure at the sodium bisulphite dosing system, and losing of communication link between the two dosing and control systems, are detected. Warning signals will also be activated by the control system to notify the plant operator at the control room located within SCISTW in case abnormal conditions detected. The plant operator could then terminate the dosing through controls in the control room if the automatic termination system fails.
4.2 Failsafe
Measures to Avoid Overdosing of Sodium Bisulphite Solution
To avoid overdose of sodium bisulphite, the control system will automatically terminate the dosing of sodium bisulphite when abnormal conditions, such as activation of “no flow” alarm, failure of the chlorination system, and losing of communication link between the two dosing and control systems, are detected. Warning signals will also be activated by the control system to notify the plant operator at the control room in case abnormal conditions detected. The plant operator could then terminate the dosing through the control system in the control room if the automatic termination system fails.
Nevertheless, if the dosing system for sodium
hypochlorite is running or has just been stopped for not more than 15 minutes,
the dosing system for sodium bisulphite cannot be stopped manually except when
“Emergency Stop” button[2]
is pressed. The chemical dosing pump for sodium bisulphite will continue to run
for at least 15 minutes until the TRC reading of the TRC analyzer at Chamber 15
(before Dechlorination) has dropped to an acceptable level. This arrangement will ensure that all
the chlorinated effluent will react with bisulphite before discharge to avoid
any adverse environmental impacts on
water quality and marine ecology.
4.3 Warning
and Alarm System
In addition to the automatic termination mechanism discussed in paragraphs 4.1 and 4.2 above, in case any system failures or system abnormalities identified, the computerized control system will activate alarms and warning signals and display them on the screen of the System Control Station to notify the plant operator for further actions. Alarm or warning signals would also be activated at local panels for any local failures or abnormalities detected.
System alarms will be activated in case there are any system failures such as pump fault, TRC analyzer fault, activation of the “Emergency Stop”, etc. The plant operator will carry out necessary inspection and remedial actions according to the Operation and Maintenance (O&M) Manual and the emergency response plans as stipulated in the Environmental Monitoring and Audit (EM&A) Manual.
Other than the system failure alarms, alarm or warning signals will also be activated within minutes for situations, such as very high or low[3] TRC level (before dechlorination), calibration drift of TRC analyzer, very high or low sodium hypochlorite dosage, very high or low sodium bisulphite dosage, etc. Those warning signals will enable the plant operator to have early arrest of any potential problems that might cause overdosing or under-performance of the disinfection system and to make necessary adjustments or rectifications. In case of such abnormalities are detected, the plant operator would immediately switch the operation to manual operation mode to maintain the disinfection operation, carry out investigations to find out the cause(s) of the abnormalities and rectification works before returning the system to the automatic operation mode.
5.
Conclusions
The dosage of sodium hypochlorite and sodium bisulphite will normally be controlled automatically by a computerized control system based on the on-line TRC reading at Chamber 15 (before Dechlorination). Various features were included in the disinfection facilities design to enhance system reliability and to allow dosage optimization. Besides, different failsafe measures are provided for the sodium hypochlorite and sodium bisulphite dosing systems to avoid over dosage and adverse environmental impacts on water quality and marine ecology due to power or system failure.
Drainage Services Department
September 2009
[1] Chlorine demand is the difference between the amount of chlorine applied and the amount of free, combined, or total chlorine residual remaining at the end of the contact period. The chlorine demand is caused by the organic material and some of the inorganic reductants in the effluent.
[2] The “Emergency Stop” button would normally only be pressed in case of emergency where there are safety and hazard concerns.
[3] “very high or low” TRC level or chemical dosage shall mean the level or dosage beyond the pre-defined range.