Like many other cities in New Zealand, Whangarei's aging sewer reticulation network suffers from extensive infiltration during heavy rainfall. This often resulted in spillage of untreated sewage into the upper reaches of the Whangarei Harbour. Growing public pressure saw local authorities adopt a multi-faceted approach to resolve the problem.
As part of the solution, the Whangarei District Council let a contract to United Civil Construction in 2010 for the upgrade of the Okara Park Sewer Pump Station. The pumping capacity of the station, which was originally constructed in 1967, was approximately 60% of the incoming pipelines, leading to substantial and frequent overflows following heavy rain from this facility into the immediately adjacent Hatea River (Whangarei Harbour). This had become a focus of ongoing community concern and media interest.
Given the importance of the pumping station – it is from here that the majority of the city’s wastewater is pumped to the nearby treatment plant – extensive pre-planning, risk analysis and the development of comprehensive contingency measures were an essential prerequisite to the actual construction, so as to ensure the facility remained operational at all times. These pre-planning measures extended to the fabrication of scale models of key pipe components, to confirm the geometry of these were compatible with the existing pipework in what was a very confined space that left no room for error.
- Extensive and detailed pre-planning, risk analysis and contingency planning took place prior to construction, including scale modelling of key pipework.
- Construction operations were typically undertaken during periods of minimum flow, around 2am. Critical tasks had to be completed within a four-hour window, after which time the limited upstream storage capacity of the system would be exceeded.
- A significant change to the design of the pump suction pipework arose as a result of a United Civil initiative, eliminating the need for staff to enter the wet well (i.e. sewage tank). The primary driver for this change was to eliminate the HSE hazards associated with working in a live sewer.
- The project work was completed within tight and inflexible deadlines, on budget and without incident – efforts which were acknowledged with a New Zealand Contractors’ Federation Construction Excellence Award for 2011 (for projects with a value between $1 million and $10 million).
The scope of the upgrade work which all had to be undertaken whilst maintaining the capacity and functionality of the existing facility, included:
- The decommission of existing pumps and the installation of four new pumps
- Retrofitting and upgrading existing mechanical systems
- The fabrication and installation of new pipework valves, flow meters, lifting beams, and external rising main
- The supply and installation of associated power and instrumentation components, including a new motor control centre.
There was an ongoing need for redesign due to numerous challenges that presented themselves during construction, and the specified methodology was subject to frequent modification.