Setting up a resilient wastewater network for Mackay

Undertaking the largest repair project for the city, Mackay Regional Council navigated heavy rainfall to find a long-term solution for one of its essential sewage rising mains. But finding such a solution required equipment specifically designed to withstand damage and heavy rainfall, including custom-built pipework along with a gate valve and VS4 air valves.

Mackay Regional Council conducted repairs on a section of damaged sewage rising main in June 2022. The southern main is a critical pipeline for the Queensland city, with over 18 million litres of sewage being transported every day through the city to the Mackay South Water Recycling Facility, and servicing 27,000 connected customers.

The 914mm mild steel cement-lined rising main stretched 11km long, with an isolation valve at Sydney Street pump station and another at the Mackay South Water Recycling Facility. The repairs involved replacing 18m of pipe with a custom-made section and installing new isolation valve and air valves.

The southern rising main repairs became some of the largest undertakings by Mackay Regional Council. Despite facing challenges, the work proved to be a learning experience for the council to better understand how to set up a resilient wastewater network.

Finding a long-term repair solution

The repairs had to be undertaken after a community member informed Council about a leak adjacent to Paradise Street. Following investigations, it was determined that the main’s high point was 4m from the air valve offtake point, meaning decade’s worth of corrosive gas cavitation had caused the release of raw sewage and Council took immediate action to stop the leak with a repair clamp.

Mackay Regional Council Water and Waste Services Chief Operating Officer, Stuart Boyd, said the team had to find a long-term solution to avoid more leaks in the future along this essential pipeline, but any work would inevitably require releasing raw sewage.

“A 20m section of the main was to be replaced with custom- built pipework, including new fixtures. But how to safely and responsibly isolate or bypass the main without an uncontrolled release of raw sewage?” Mr Boyd said.

“In under 12 hours, we had to drain and redistribute 995,000L of raw sewage from four scour points to adjacent catchments, and remove and replace it with the new custom- built pipework section and valves.

“While planning to manage this critical risk, Mackay Water and Waste Services (WWS) assessed isolation methodologies to establish an agreed permanent repair method for isolation to allow for repair.”

The draining involved turning off 88 sewerage pump stations in Mackay, which meant an overuse of water in the area could have inundated the pump stations and caused harmful discharges to the local environment.

“Corporate risks were considered and evaluated with a cost- effective way of managing all the design, flow management, asset isolation and construction in-house. Being a regional council, we wanted to provide the best solution to our ratepayers while considering the environmental, safety, and financial risks,” Mr Boyd said.

Procuring the right equipment

Preventing further leaks in the future along the pipeline meant finding the right equipment that could withstand both everyday demands and high wastewater flow during storm events.

Mackay Regional Council decided to use a custom-built pipework that could suit precise dimensions and be coated for extra protection, in addition to installing a 900mm gate valve. While the custom pipework was procured from a third party supplier, the gate valve was sourced using a three- quote method.

“As part of the 20m custom-built pipework section, we took the opportunity to install a 900mm mechanical gate valve. The pipework was epoxy-coated 540° internally and for extra protection, we decided to coat the inside and outside of the 900mm valve,” Mr Boyd said.

On top of the custom pipework and gate valve, Mackay WWS procured two new VS4 air valves which ideally suited for the pipeline.

Following the works, the valves were kept as a council store-stocked item. “The design of these air valves is perfect to use on sewage-rising mains. The valves are accessible and sit within a rectangular concrete pit with a lockable aluminium lid,” Mr Boyd said.

Overcoming challenges

However, before repairs could begin, works were postponed from the original start date in May to June because of unprecedented wet weather, which caused a significant increase in wastewater flowing through the city.

“Sewage infiltration after the rain event saw higher flows that were not manageable. Any inundation could cause harmful discharges to our environment – something we want to avoid at all costs,” Mr Boyd said.

“The rain had soddened the ground at the construction site, which meant heavy vehicles could not access it. We had to redistribute and redo the radio ads, all while maintaining the community’s interest.” These repairs proved to be one of the largest undertakings for Mackay WWS thanks to the challenges posed by heavy rainfall.

“We had 55 crew members working across five locations on the night of the shutdown. Communication was maintained through the use of mobile phones and two-way radios.” With repairs unable to take place, Mackay Regional Council had to work with the community to reassure them works would go ahead and would require residents to cut down on water use to prevent inundation of the pump stations.

“Humour was used for the ‘if it’s yellow, let it mellow’ campaign. The use of memes helped engage our community to get our message across to minimise water usage across the catchment on the night of the shutdown and construction.”

Knowledge for future projects

Despite the delays, responding to heavy rainfall and planning the works around the community helped to gain insights for when Council has to undertake repairs on other mains in the future. Mr Boyd said the shutdown offered a chance for Mackay WWS to better understand the water network and what would be required during further works.

“The capture of newfound data and knowledge was obtained on the night through shutting down and ponding within the sewerage network and draining and pumping down of the main,” Mr Boyd said.

In terms of future works, Mackay Regional Council intends to improve permanent draining infrastructure, install inline 900mm gate valves at strategic locations, and upgrade air valves along the network.

Mr Boyd said the newfound knowledge from this project of works gave WWS an opportunity to learn from challenges that would not normally affect works, which would greatly benefit upcoming works if the city is once again hit with unprecedented rainfall.

“The size and complexity of this project and having an operational and planning team undertake and complete this work while conducting business as usual is a testament to the committed effort of the project team.”