How Wastewater is Treated

The upgraded treatment plant no longer uses oxidation ponds to treat waste water. The old secondary and tertiary treatment processes at the plant have been replaced with new technology (reactor clarifiers and UV filtration) while other areas of the plant have either been replaced or comprehensively upgraded.

Much of the operation at Mangere has been computer automated. The Wastewater Business Unit has also completed a 3-year programme of sophisticated computer modelling of the entire network performance.

Screens building. Six 3mm revolving drum screens intercept raw wastewater flowing into the plant.

Primary sedimentation tanks. Twelve of these units and associated pre-aeration or grit tanks at the beginning of the treatment process separate the bulk of the solids from the liquid waste stream.

Gravity thickeners. Two units thicken sludge from the primary tanks for pumping to the anaerobic digester.

Interstage pump station. Conveys effluent from the primary sedimentation tanks at a rate of nine cubic metres per second, to the reactor/clarifiers for secondary treatment.

Reactor/clarifiers. Nine doughnut-shaped units use biological nutrient removal (BNR) technology to remove harmful nutrients such as nitrogen and to extract solids from the waste stream.

DAF (dissolved air flotation ) tanks. Three units thicken waste sludge from the reactor/clarifiers before it is passed to the anaerobic digesters.

Anaerobic digesters. Seven of these units use biological processes to treat primary sludge received from the gravity thickeners and secondary sludge from the DAF system before passing it on to the dewatering plant. All the units have new mixing equipment installed.

Blower building. Five electrically driven blowers provide the air to aerate the nine reactor/clarifiers.

Co-generation plant. Four new Jenbacher engine/generators provide 7 megawatts of electricity from digester biogas to help offset plant power requirements. The plant is also designed to run on natural gas.

Filtration facility. Filters the clarified wastewater from the reactor/clarifiers before it proceeds to the UV plant.

UV disinfection plant. The UV plant is believed to be the largest of its type in the world and is designed to kill pathogens in the wastewater. A concrete chamber about three times the size of a tennis court, it is filled with 7776 fluorescent lamps that deliver a computer-controlled dose of UV radiation to the treated effluent before it is discharged.

Biosolids dewatering building. A complex of equipment supporting six centrifuges which spin-dry sludge from the digesters. The product is lime treated and becomes "process biosolids".

Biofilter beds. Foul air from different areas of the treatment plant is passed through the biofilter beds to extract odours.

Biosolids storage building. This is the final stage of the solid waste treatment. The complex has the latest odour extraction technology and storage capacity for 96 hours of biosolids production before they go to landfill or to other beneficial uses that are being developed.