by Gino Iori, Derek Cunningham, Catherine Rout and Elline Camilet, Sydney Water
Hydrogen sulphide (H2S) gas in sewer systems can cause significant corrosion and odour issues, but dosing chemicals into sewer networks is an effective means of controlling gas levels. Sydney Water Corporation recently undertook a project to optimise the chemical dosing in the Malabar Wastewater Network through the development of online monitoring equipment.
To reduce concrete corrosion rates to less than 1mm per year, Sydney Water’s Corrosion and Odour Strategy specifies an average gas-phase H2S level of less than 5ppm, and average dissolved sulphide levels less than 0.5mg/L.
Sydney Water operates 60 chemical dosing units (CDUs) across the wastewater networks as a means of minimising H2S gas production to manageable levels.
Proper and efficient management of H2S gas in wastewater networks requires reliable data capture and modelling of key parameters at strategic locations. A project was undertaken to optimise chemical dosing in the Malabar Wastewater Network, Sydney Water’s largest wastewater system, which covers an area of 50,000 hectares and services 1.6 million customers in Southern and South Western Sydney.
The Malabar Wastewater Network was chosen as the focus of this optimisation project due to the large number of ferrous chloride-dosing CDUs within the network (eight), and therefore, the high potential for improvement and cost savings.
A review of the Malabar Wastewater Network was completed in two parts: analysing long-term H2S gas levels at various monitoring sites in the network, and modelling a number of scenarios using the Integrated Sewer Network (ISN) model, to determine the optimal chemical dose rate at each CDU site.
Historical H2S data from monitoring sites at strategic locations within the network were inputted into the ISN tool to validate that it was properly calibrated prior to performing a desktop study of the optimisation.
During the monitoring process, the Thermo Fisher Odalog TM H2S logger provided continuous monitoring of gas phase H2S levels. This data was sent via fibre optic cable to the Odatrack TM adaptor, which then outputs the corresponding signal on a 4-20mA loop.
This equipment was selected due to proven reliability and availability within the Sydney Water business.
Optimisation of chemical dosing
Following ISN modelling and the development and installation of reliable online monitoring equipment at strategic locations, dosing at selected CDUs was progressively optimised, with modelled dosing flow rates used as an initial starting point.
In order to avoid a system-wide, uncontrolled change in dosing, which could have resulted in enormously negative impacts on H2S gas levels, the optimisation followed the same division of sub-systems as was used during modelling.
Beginning with the Upper Northern Georges River Submain (the sub-system furthest from the Malabar Wastewater Treatment Plant) chemical dosing was reduced incrementally, with changes in H2S gas levels monitored continuously via the portable online monitoring equipment and on-
site liquid sampling at selected sites.
This process was repeated until the frequency and magnitude of H2S gas spikes downstream of that point increased, or, the average daily H2S gas concentration exceeded 5ppm. At this point, dosing at the CDU would be returned to the previous increment.
Modelling suggested that average H2S gas levels in the system could be maintained below 5ppm if total dosing was reduced by approximately 33 per cent.
Incremental dosing reductions resulted in the dosing at two CDUs within the Upper Northern Georges River Submain being reduced by up to 50 per cent, and one CDU decommissioned.
Mascot, the largest CDU in the Malabar Wastewater Network, was the final CDU to be optimised.
Review of the H2S gas levels revealed consistency in diurnal pattern of average dry weather flows and H2S gas levels. This indicated that implementation of profiled dosing at CDUs in the network would be beneficial in reducing variability and peaks in downstream H2S gas levels.
Diurnal chemical dosing profiles were developed and implemented at CDUs.
Sydney Water has successfully optimised chemical dosing within the Malabar Wastewater Network through:
- The review of historical H2S gas levels at various monitoring sites in the network
- Use of the ISN model to simulate a number of scenarios to determine optimal chemical dose rates at each CDUs site
- The development and installation of an innovative system of online liquid-phase and gas-phase monitoring equipment
The ISN model enabled prediction of H2S gas levels in the Malabar Wastewater Network and the development of chemical dosing scenarios which formed the starting point for the optimisation.
Sydney Water has now installed an additional 19 permanent online H2S gas monitoring units at strategic locations in two of its largest wastewater systems.
In addition, eight relocatable online H2S gas monitoring units have been deployed downstream of high-risk trade waste customers.
Installation in, and optimisation of, chemical dosing in the remaining wastewater systems will follow in the future.
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