Sustainability success for Dairygold: mixers deliver biogas system results that matter

Flygt mixers are a critical component at the heart of this world`s largest ADI-BVF® digester

Dairygold is Ireland’s leading farmer-owned co-operative, processing around 1.43 billion litres of grass fed milk each year. Based in Mitchelstown, Cork in Ireland, sustainability is at the heart of everything at Dairygold. From its farming practices, through to production and processing of milk, butter, cheese, whey products and more at its Castlefarm complex.

Challenge

As one of Ireland’s largest dairy producers, to consider the installation of an anaerobic digestion wastewater treatment system to be a success, it was critical to get the right mixing in the process. Dairygold turned to Xylem for solutions. With ambtitious energy consumption reduction targets to meet its environmental goals and mitigate the impact of increasing energy costs, the operating team turned its focus to the site’s wastewater treatment plant. Dairygold needed to find an alternative that met its operational demands. With several treatment streams producing a total flow of 9,800 m3/d, the biological nutrient removal (BNR) system was periodically overloaded. Foul odour was also a growing concern. They wanted to address these and follow their push to reduce energy consumption and achieve an operating model for circular economy.

Solution

In partnership with Xylem, Dairygold installed the world’s largest above ground Evoqua ADI-BVF^® digester. This anaerobic digestion (AD) wastewater treatment system is a highly effective way to treat the industrial wastewater, reduce sludge production and produce biogas that can be used for energy. For Dairygold, the pre-treatment digester serves to remove the high COD loading from the wastewater. This reaches up to 20,000 kg of COD a day - a 90% removal rate. This process produces up to 14,000 m3/d of biogas. The effluent from the digester then goes on to the site’s BNR treatment plant, where nutrients such as phosphorus are polished and removed before discharge.

Central to the performance of the system is the digester’s mixers. Two Flygt 4680 mixers were selected, for high efficiency mixing. One was installed at a low and one at a medium level of the AD tank to ensure the sludge blanket on the bottom of the 10m deep, 80m diameter digester tank always remains uniform. It is critical that the sludge, which can reach up to 4% solid content, is always uniform, so there is good contact with the incoming feed.

Result

Since 2012, the Xylem Flygt 4680 mixers have been delivering exceptional results, year after year. Their reliable operation has consistently ensured peak wastewater treatment performance, with COD levels averaging below 200 mg/L and BOD levels under 50 mg/L. Remarkably, this performance is achieved with just a few minutes of intermittent mixing per day. The results speak volumes:

• 50% reduction in sludge volume
• Odour eliminated
• Zero digester tank cleaning required
• Smart, low-maintenance operation

The mixers have stabilized and optimized anaerobic digestion, simplifying plant operations and reducing energy use by about 7% for Dairygold.