It’s not just Hobbits who are flocking to the Auckland, New Zealand, area—the city’s population is estimated to increase by at least 500,000 to 2 million inhabitants in the next two decades. The population boom presents challenges for treating wastewater in the area, but the Mangere Wastewater Treatment Plant (WWTP) has the situation well in hand.
Approximately 75% of Auckland’s wastewater flows to the Mangere WWTP, the largest WWTP in the country. Construction of the original plant—then known as the Manukau Sewage Purification Works—started in 1956, with the plant opening in 1960. Since then, it has undergone substantial upgrades to modernize and increase capacity.
The plant is run by Watercare Services Ltd., a water and wastewater services provider owned by the Auckland Council, and is funded through charges to Watercare’s customers. It treats 320 million liters of wastewater per day from 1.1 million Auckland residents, discharging it into Manukau Harbour on an outgoing tide.
An Ambitious Project
The plant operated using one of the world’s largest oxidation pond systems until the completion of “Project Manukau,” a $450 million endeavor that took place between 1998 and 2005. The project removed 500 hectares of oxidation ponds, returning them to Manukau Harbour, and restored 13 kilometers of shoreline. Other Project Manukau upgrades included the addition of new, 3-mm subscreens for the plant’s headworks; eight new activated sludge reactors/clarifiers; new biofilters; UV disinfection and filtration; a sludge thickening and dewatering facility; and new pumping stations.
The improvements reduced the wastewater treatment cycle from 21 days to 12 hours. The introduction of the UV disinfection plant—at the time, the largest such plant in the world—reduced the number of pathogens in the treated water 10,000-fold.
The Mangere WWTP’s treatment process now includes primary, secondary and tertiary treatment. Primary treatment includes fine screens, pre-aeration tanks, 12 primary sedimentation tanks and gravity thickening. Secondary treatment includes biological nutrient removal (BNR), nine reactor/clarifiers, dissolved air flotation (DAF) thickeners, anaerobic sludge digesters and a dewatering plant. Tertiary treatment entails the use of sand (anthracite) filters and UV disinfection via 7,776 UV lamps.
An interesting feature of the WWTP is its ability to generate electricity from waste. Anaerobic treatment of solids removed in the treatment process produces biogas, which supplies up to 55% of the plant’s energy requirements. Post-digestion, dewatered biosolids are used to fill a former quarry on nearby Puketutu Island, with the eventual goal of transforming it into a regional park.
“By 2025, Watercare plans to run the Mangere plant and its second-largest wastewater treatment plant in Rosedale entirely on self-generated electricity,” said Watercare Wastewater Manager Shane Morgan. This “will see Watercare reduce its electricity demand on the grid by about 37 GWh every year,” he said.
Eye on Growth
“With Auckland’s rapidly growing population, the main challenge is ensuring the wastewater network stays ahead of growth, while keeping costs to customers at a minimum,” Morgan said.
Current upgrades will enable the plant to treat wastewater from an additional 230,000 people. A $136 million project also is underway to add an additional BNR facility, which is scheduled to be operational in December 2017.
Additionally, Watercare is embarking on a $538 million wastewater pipeline project known as the “Northern Interceptor.” When complete, it will divert some of the flow to the Mangere WWTP to the Rosedale WWTP, located 15 miles away.
“This has two major benefits,” Morgan said. “It utilizes spare capacity at the Rosedale plant and it gives the Mangere plant greater capacity to support growth in the central and southern areas. It also provides additional resilience across the wastewater network.”