Rakaia Waste Water Treatment Plant - Sludge Beds

Overview

Grant Hood Contracting was proud to partner with the local Council on the delivery of a key infrastructure upgrade at the Rakaia Wastewater Treatment Plant (WWTP). As part of the Council’s WATE0209 project, we were awarded the contract to construct and commission a new sludge drying bed facility, designed to improve the treatment plant’s capacity for processing biosolids and managing waste sustainably.

This major infrastructure project involves the complete construction of 12 concrete sludge drying beds, along with mechanical systems, electrical and instrumentation works, drainage networks, and supporting access infrastructure. The works are being delivered under a Measure and Value contract in accordance with NZS 3910:2013, requiring precision in both construction quality and cost control.

Project Objectives

The primary objective of this project is to enhance biosolids management at the Rakaia WWTP through the construction of a dedicated drying bed facility. Sludge drying beds provide a cost-effective, low-energy solution for dewatering sludge, reducing both volume and handling costs prior to final disposal or reuse. This upgrade supports the Council’s broader goals for sustainability, resilience, and compliance with environmental regulations.

Scope of Works

The Rakaia Sludge Drying Beds project is a multi-disciplinary undertaking, requiring close coordination between civil, mechanical, electrical, and commissioning teams. Grant Hood Contracting’s scope includes:

Siteworks and Civil Construction

• Site preparation, set-out, and earthworks, including:
• 2,700 m³ of bulk cut
• 1,500 m³ of imported fill supply and placement
• Construction of 12 reinforced concrete sludge drying bed structures, including:
• 400 m³ of in-situ concrete
• 1,526 m² of formed floor and wall surface area
• Supply and placement of 1,200 m² of site concrete beneath the new drying beds
• Construction of gravel roads and a new internal accessway connecting the beds to the existing site road
• Installation of permanent site fencing and access gates for security and operational safety
• Construction of associated stormwater drainage infrastructure

Mechanical and Process Installations

• Supply and installation of filter media within each drying bed
• Installation of PVC gravity pipework from each of the 12 bunkers, feeding into a custom-built pump station, which then pumps filtrate to the plant’s clarifier
• Installation of 63mm PE pressure lines from the existing sludge feed pump, distributing sludge to each drying bed
• Distribution within the bunkers via 50mm stainless steel pipework
• Supply and installation of all associated valves, supports, and fittings

Electrical, Instrumentation & Controls

• Connection to the existing WWTP power supply
• Installation of a new electrical junction box and integration of controls for the pump station
• Supply and installation of instrumentation and control systems for the pump station and monitoring of filtrate flow

Pump Station Installation

• Supply and installation of a packaged pump station, complete with a submersible pump and level monitoring
• Electrical connection, testing, and integration into the existing site systems
• Connection of pump station discharge lines to existing infrastructure

Water Supply and Maintenance Access

• Connection to existing site water infrastructure
• Installation of new water supply pipework and two hose reels for cleaning and maintenance access

Commissioning and Handover

• Comprehensive testing and commissioning of all new systems to ensure functionality and compliance
• Surveying of key levels to inform final as-built documentation
• Provision of a complete Operating & Maintenance Manual
• Full system handover to Council operations

Project Management and Coordination

In addition to self-performed works, Grant Hood Contracting is responsible for the procurement, management, and coordination of nominated subcontractors, ensuring that all trades and disciplines work cohesively to meet project specifications and delivery timelines.

Challenges and Considerations

This project required careful planning and sequencing due to the integration with the existing WWTP infrastructure. Maintaining operational continuity of the plant during construction was critical, and our team worked closely with the Council’s project managers and plant operators to minimise disruptions.

The site’s environmental sensitivity and the need for robust stormwater and leachate management added further technical complexity. Our in-house experience with similar wastewater and civil works allowed us to proactively address these challenges and deliver solutions tailored to the site’s unique conditions.

Delivering Community Value

At Grant Hood Contracting, we recognise the importance of investing in essential public infrastructure that supports healthy communities and sustainable environments. The Rakaia WWTP Sludge Drying Beds project is a practical and long-term solution that will help the Council reduce sludge volumes, lower operating costs, and enhance overall plant efficiency.

By applying our expertise across civil, mechanical, and electrical disciplines, we’re proud to help deliver a project that aligns with both best practice engineering and community expectations.

Why Grant Hood Contracting

With over 20 years of experience in delivering complex infrastructure across Canterbury and beyond, Grant Hood Contracting is trusted by councils, utilities, and private clients to deliver high-quality construction services with a focus on:

• Technical capability across multiple disciplines
• Collaborative approach to project delivery
• Commitment to health, safety, and environmental best practices
• Proven track record in wastewater and civil infrastructure