Wanganui Wastewater Treatment Plant consists of two circular Clarifier tanks and one rectangular Contact Stabilisation tank divided into four sectors. The tanks are designed to be water tight with specific emphasis on crack control to protect the structural elements in an aggressive acidic environment.
The Clarifier tanks are 33m in diameter with vertically pre-stressed precast panels that are stitched together to form the open-air tanks. These are then post-tensioned horizontally to maximise the strength of the concrete elements while maintaining a water tight solution. The tank sits on conventional shallow foundations with a conventionally reinforced concrete slab.
The Contact Stabilisation tank is 78m by 26m with partially two-way spanning precast panel walls. These walls are also pre-stressed and post-tensioned to form an efficient water tight solution. The tanks have a number of distribution channels at the top of the walls that have been designed as capping beams to reduce the loads within the walls, these are tied across the structure with a number of post-tensioned tie beams. Due to the large depth of water in this tank, the floor has been designed as a hybrid post-tensioned and conventionally reinforced slab to limit crack width while maintaining the required strength to resist the differential settlements expected in the soil below.