North Canberra Effluent Reuse Treatment Scheme
The North Canberra Water Re-use Facility (NCWRF) treatment process consists of the following steps:
- Raw effluent transfer
- Pre screening
- Chemical pre-dosing
|2. Membrane filtration
- Membrane filtration
- Filter back pulse system
- Clean-in-place system
- Chemical post dosing
- Chlorine disinfection
- Treated effluent transfer
- Sewage disposal
Raw effluent is pumped through a pre-filter screening system to the immersed membrane process tank. Chemical dosing upstream of the membrane process tank includes sulphuric acid to control the pH of the effluent and sodium hypochlorite solution to reduce the rate of biofouling and increases the operating time between membrane cleans.
The membrane filtration technology in use is a proprietary Zenon process that produces high quality treated water by drawing raw water through immersed Zeeweed® 500 Series ultrafiltration modules. The ZeeWeed® 500 series membranes are reinforced membranes that are extremely robust. Within the immersed membrane process tank, individual membrane modules are combined to form cassettes. This design is flexible and can be easily upgraded to meet increased demand by simply adding additional membranes to the process tank.
The existing NCWRF process tank has a capacity to house a total of ten immersed hollow fibre membrane cassettes. Currently only five cassettes of membranes are used, with the initial throughput of the plant of 20 L/s (approximately 1.6 ML/day). Future expansion by installation of additional cassettes could increase plant capacity to 40 L/s (approximately 3.2 ML/day).
Filtration is achieved by drawing water to the inside of the membrane fibre under low-pressure suction. The treated water (permeate) is conveyed to the main permeate collection pipes. ZeeWeed® 500 membrane utilise "Outside-In" flow, through a reinforced, hollow-fibre membrane that has nominal and absolute pore sizes of 0.04 and 0.1 microns respectively. The small pore size excludes particulate matter including solids, bacteria, pathogens and certain viruses.
The membrane surface is kept clean though aeration scouring and membrane back-pulsing. Diffused air is introduced from the bottom of the membrane module and travels along the surface of the membrane, scouring solids away from the surface of the membrane. At pre-set time intervals (10 to 15 minutes), the membranes are back-pulsed. This is accomplished by briefly reversing the flow of permeate through the membrane to remove any particles. About 3 times per year the membranes are soak cleaned to remove the build up of solids on the membranes.
The filtered water (permeate) is disinfected to eliminate any remaining bacteria, with chlorine gas dosing to ensure that licence limits for chlorine residual is achieved.
The treated effluent from the NCWRF is pumped into the Lower Russell Reservoir, which supplies the irrigation water for the North Canberra Water Re-use Scheme.
During the membrane filtration process, as water is extracted from the membrane filter tank, the concentration of solids and waste material increases. Therefore a percentage (generally 10%) of the inlet water is required to be continuously blown down to the waste line by the filter reject pump to prevent solids build-up. This reject stream is discharged to the main sewer for treatment at the Lower Molonglo Water Quality Control Centre (LMWQCC).