Hihi Wastewater Treatment Plant Interim Upgrade

Hihi is a small coastal settlement in Te Tai Tokerau Northland, where ageing infrastructure was presenting a serious risk to environmental and public health. The local wastewater treatment plant (WWTP) was facing structural failure of its main reactor, and with a permanent upgrade in early planning the Far North District Council wanted to give more time for community engagement ion the full plant upgrade.  They faced a pressing challenge: how to protect the environment and maintain treatment, while giving time more time for community engagement and planning for a full plant replacement.

WSP was engaged to help identify and deliver an affordable, temporary solution that could be implemented quickly, meet environmental standards, and improve operator safety—all within a tight budget.

Following a collaborative options assessment involving FNDC, contractors, WSP engineers, and community representatives, the team selected a modular above-ground tank system as the preferred solution. The tank—constructed from interlocking HDPE panels with a flexible liner—offered a 100m³ capacity and required only a compacted gravel base, avoiding the need for extensive civil works or a concrete foundation.

Assembly was completed in just three days by a small crew, demonstrating the rapid deployment advantage of modular systems. Despite its temporary designation, the tank’s structural design meets a 25-year service life, providing a generous safety margin over the intended five-year use. Importantly, the tank can be dismantled and reused elsewhere, reducing the risk of stranded investment.

Since commissioning, the interim WWTP has performed reliably, preventing untreated discharges and reducing the risk of environmental non-compliance. The new storm tanks provide increased holding capacity during rain events, further supporting consent compliance.

Community and mana whenua stakeholders were engaged throughout the project, helping to build trust and support for the solution.

The Hihi WWTP interim upgrade demonstrates how small communities can address urgent infrastructure needs through innovative design, collaborative planning, and sustainability thinking. By installing a modular tank system with supporting improvements, the project averted a potential environmental crisis and created a bridge to a future permanent upgrade.

The solution has met its objectives—maintaining compliance, improving safety, and doing so within budget—while ensuring that many components can be reused or integrated into the final plant design.