A dedicated pasteurisation system supplied by HRS Heat Exchangers is playing a key role in operations at New Zealand’s first and only commercial-scale anaerobic digestion (AD) facility, operated by Ecogas.
The Reporoa-based plant processes around 75,000 tonnes of organic waste each year from council kerbside collections, as well as commercial and industrial food producers. The facility produces biogas and digestate biofertiliser; the biogas is upgraded into biomethane for injection into New Zealand’s gas grid, while captured carbon dioxide is supplied to a nearby tomato greenhouse to aid crop growth.
Digestate produced at the site is pasteurised using the HRS Digestate Pasteurisation System (DPS) before it is used as biofertiliser. This step neutralises pathogens and weed seeds, ensuring the fertiliser is safe for agricultural use. The system uses ‘waste heat’ from the site’s combined heat and power (CHP) plants to heat digestate to more than 70°C for one hour before cooling.
Unlike traditional tank-based systems that discard heat after use, the HRS system recovers energy by transferring heat from pasteurised to unpasteurised sludge, reducing energy consumption and removing the need for additional heat sources. Its corrugated-tube heat exchanger design improves heat transfer and reduces fouling, cutting maintenance and downtime.
The fully automated DPS provides an auditable ‘kill-step’ for pathogens such as Salmonella, Staphylococcus, and E. coli, while also helping to reduce odours and maintain fertiliser quality. The system was delivered and installed in February 2022, with the heat exchanger section split into two stacked modules to minimise the plant’s footprint.
Alzbeta Bouskova, general manager at Ecogas, said pasteurisation was a key step in achieving quality standards for the company’s biofertiliser. “Ecogas adopted UK-based PAS110 digestate standards as our primary design and operation principle to ensure that the biofertiliser produced from the Reporoa facility is of the highest quality, safety and value for the potential users,” she said.
“The Reporoa plant is the first of its kind in New Zealand, and with that comes the process of introducing our new biofertiliser product, Fertify, to the farming and horticulture sector,” she added. “We chose HRS based on their reputation and strong track record in digestate pasteurisation, supported by a highly engaged local team.”
Ella Taghavi, sales engineering manager at HRS Heat Exchangers, said the Reporoa facility represented a significant development for New Zealand’s bioenergy sector. “We believe that if renewable energy from biogas and anaerobic digestion is to reach its full potential globally, it needs to be as efficient as possible, including extracting value from digestate as a renewable organic fertiliser,” she said.
Ecogas’ Reporoa facility was officially opened in October 2022. In July, it won the AD Hero of the Year award at the 2025 AD and Biogas Industry Awards in Birmingham, UK.
