Melbourne Water finds an energy-saving solution

Sewage and wastewater treatment is a highly energy-intensive process, presenting a challenge for water utilities seeking to meet net zero goals.

Melbourne Water’s Eastern Treatment Plant (ETP) treats almost half of all Melbourne’s sewage, an average of 330 million litres a day. Located in the Melbourne suburb of Bangholme, approximately 19 miles south-east of the city centre, the 1000 ha site was upgraded in 2012 to treat sewage to an advanced tertiary standard, producing Class A recycled water.

Shooting for net zero

Since opening in 1975, the ETP has always generated energy from sewage gas, and the site houses seven power generators that are capable of running solely on biogas — produced through the anaerobic digestion of sludge during the first and second phases of water treatment.

In 2020, Melbourne Water began a project with John Holland-KBR Joint Venture to upgrade the biogas handling system at the Eastern Treatment Plant to make it more efficient, resilient and futureproof. The project aimed to expand capacity, prevent corrosion and sedimentation, and extend the working life of equipment — with specific requirements to remove liquid droplets and moisture and to minimise the size and cost of any necessary cooling system.

Sewage transfer and treatment are responsible for about 85% of Melbourne Water’s total greenhouse gas (GHG) emissions, so contributing to the company’s net zero commitment was a main target for the project. “However, with existing assets nearing the end of their service life, ensuring that the new asset can service future production growth and plant upgrades was also essential,” said Nick Fung, Senior Project Manager at Melbourne Water.

Increasing reliability for the operations team onsite and improving safety across both operation and maintenance were further aims.

Selecting an energy-saving solution

The HRS BDS formed part of a wider upgrade to the biogas handling equipment at Melbourne Water’s Eastern Treatment Plant.

Biogas contains hydrogen sulfide (H₂S) gas, which condensates out to form a highly corrosive liquid. Including a system to remove most of the moisture from the gas was therefore essential to minimise the amount of corrosion and degradation of the power station generators, while also limiting operational downtime and reducing the need to import electricity from the grid.

HRS Heat Exchangers was one of five companies invited to tender for the moisture removal equipment in the project. The HRS Biogas Dehumidification System (BDS) removes water from biogas, protecting combined heat and power (CHP) engines and generator sets from corrosion and cavitation. According to HRS, it condenses more than 90% of the water present in biogas by reducing the temperature to leave a clean gas. The addition of heat regeneration technology means the cold biogas produced can be used to pre-cool the incoming warmer biogas. This reduces the load on the final cooling heat exchanger and saves valuable energy.

The BDS supplied for the project has a maximum capacity of 4161 m³/h (4710 kg/h), while the inclusion of an energy recovery section subsequently reduces the eventual chiller load by 30%. This BDS consists of two heat exchangers, a regeneration unit and a final cooler, supplied on two skids. One skid — located in a hazardous area zone — contains the heat exchangers, a condensate knockout pot, a condensate drain line and all the relevant IECEx-rated instrumentation. The second skid contains a buffer tank, standby and duty glycol pumps, and all the relevant glycol line process control valves and instruments.

HRS also supplied an additional chiller and detailed process control descriptions, enabling the joint venture engineers to provide overall control and automation systems for the client.

Overcoming complex design parameters

The HRS BDS at Melbourne Water’s Eastern Treatment Plant incorporated some complex design elements.

The BDS needed to meet the requirements of two different operational stages, each of which contains four duty requirements. This meant a complex thermal design process was necessary to make sure that the supplied unit could meet all eight of the potential design scenarios.

Ella Taghavi, Project Manager and Technical Lead at HRS Heat Exchangers, explained: “Designing the BDS for the Melbourne Water project posed a significant challenge as it needed to cater to both current and future demands, with two very different conditions in each phase.

“The current first phase uses biogas supplied by the compressors and aftercoolers at an average pressure of 66 kPa. In the second (future) scenario, the gas is supplied following a number of additional treatments at an average pressure of 5 kPa and then supplied to the compressors. We therefore had to deal with two very different inlet conditions.

“The addition of heat regeneration added further complexity in terms of design. However, thanks to our cutting-edge technology, not only were capital costs reduced as a smaller chiller could be specified, but regeneration is also helping to lower ongoing operational costs.”

An efficient result

From being awarded the contract in July 2021, it took less than 12 months for HRS to install the bespoke BDS onsite, with commissioning completed in June 2022.

The John Holland-KBR team noted that of the five tendered solutions, the technology of the HRS BDS stood out, including the excellent technical support it provided to help develop the project further with Melbourne Water.

“Our experience in project management, construction and logistics enabled us to overcome early design challenges and respond to necessary changes during the design and construction process. As a result, we were able to deliver a high-quality product that has performed to specification since its installation,” Taghavi said.

The upgrade project has improved the quality of the biogas supply to the power station at ETP, providing more confidence in its ability to deliver more than 36,000 MWh (approximately 130 TJ) of thermal energy for process heating. This means a reduced reliance on natural gas, cutting down on emissions and making the operation more self-sufficient.

Top image caption: The HRS Biogas Dehumidification System (BDS) is specifically designed to improve the operating life and energy efficiency of biogas engines. Images courtesy of HRS Heat Exchangers.