Woodend WWTP Expansion Project

Client: Waimakariri District Council
Contract value: $1.4million
Contract period: Jan 2017 – July 2017

Scope:
• Construction of aeration Pond 1C (7,600m3) and settling Pond 2B (25,400m3)
• Construction of associated pipework and chambers
• Supply and installation of eight aerators in Pond 1C and 2B
• Supply and installation of associated structures and equipment/instruments
• Modification of the existing inlet structure
– Construction of a new channel at the existing inlet structure
– Installation of a second inlet screen (Screen 2)
– Relocation of the bypass channel to Pond 2B
• Tie-ins and related construction
• Dry and wet commissioning of the entire system

Technical Difficulty:
Waimakariri District Council contracted Grant Hood Contracting to construct an expansion to the Woodend Wastewater Treatment Plant. This is the second in a series of upgrades with the aim of increasing the capacity of the plant to cope with the predicted increased population loading.

The Woodend Wastewater Treatment Plant needed additional ponds to keep up with the demands for treated wastewater in the area. The new ponds needed to be completed with the same surface area and volume as the existing aeration pond and settling pond.

The ponds were constructed from the in situ sand, which was extremely prone to wind erosion and very moisture sensitive. Pond 1C received a HDPE liner while pond 2B was lined using a clay liner. The clay liner was constructed during one of the wettest Autumns of recent times, which provided its own problems. Part of the construction of pond 2B involved constructing a reinforced concrete waveband around the entire 480m perimeter. Constructing this slab on a 3 in 1 grade proved challenging and the construction methodology evolved as the project progressed.

Aeration equipment, additional instrumentation and floating finger piers were procured for the aeration basin and settlement pond to allow for safe servicing of aerators. A new wetland bypass chamber and associated pipework was constructed to connect the new ponds to the existing system. New access roads were constructed and some existing roads realigned to fit the new plant layout.

The project involved dry and wet commissioning of the new system while working alongside a fully operational plant. This involved constant liaison with the engineer and client, as well as stringent management of the flow of wastewater throughout the plant.