August 2025 What's on Tap Employee Newsletter

What to Do With Biogas? Why Not Fuel the Buses Raleigh Water upgrades solids handling with thermal hydrolysis and anaerobic digestion, reducing biosolids volume and producing RNG for city buses

By Steve Lund , Treatment Plant Operator Magazine

When Raleigh Water decided to change how it handled biosolids at its Neuse River Resource Recovery Facility, it did so in a big way. “ This is a comprehensive biosolids upgrade project, ” says Erika Bailey, P.E., assistant director. “ We ’ re using an advanced process — thermal hydrolysis pretreatment — that pre conditions the solids upstream of anaerobic digestion. It makes digestion more efficient. It does a better job of destructing the organic solids, and it produces more biogas. ” Previously, the plant in North Carolina ’ s capital used a mix of aerobic

Raleigh Water installed a membrane gas holder (WesTech Engineering) at its Neuse River Resource Recovery Facility as part of its bioenergy recovery project.

digestion, lime stabilization and composting. That required significant energy for aeration and costly trucking of the final biosolids. Now anaerobic digestion with preconditioning produces energy instead of using it, and it reduces the volume of solids. And the utility will convert the biogas into renewable natural gas to fuel the city ’ s fleet of buses. A better way The Neuse River facility (75 mgd design, 50 mgd average) is the largest of Raleigh Water ’ s three wastewater treatment plants. The Smith Creek (3 mgd) and Little Creek (2.2 mgd) plants both send their waste activated solids to Neuse River. There, the biosolids are dewatered in centrifuges (GEA) before thermal hydrolysis (Cambi) which uses pressure and heat to condition the material. The pretreatment pasteurizes the solids, breaks down the organic compounds and hydrolyzes the solids into a pourable liquid. The solids are pumped into two new 2.2 million - gallon digesters and mixed by chopper pumps (Vaughan). After 15 days, the solids are dewatered again with centrifuges and belt presses (Alfa Laval). The Class A final product is spread on city - owned land. The ability to produce Class A biosolids in just 15 days was one attraction of the process. “ This is considered an intensification technology, ” says Bailey. “ It lets us fit more capacity in the same size digester. ” The process also reduces biosolids volume, and the material dewaters better. The larger volume of biogas is stored in a membrane gas holder (WesTech Engineering) and runs through a gas treatment process (DMT International) to meet renewable natural gas standards. Developing the entire process took many years and significant research by Raleigh Water staff and consultants. Preliminary workshops and design began in 2014, and ground was broken in 2019. The thermal hydrolysis technology came online in August 2024, and the RNG connection to the gas grid is expected in spring 2025. Raleigh Water doesn ’ t sell the RNG to the utility; it pays a transportation fee to have the gas piped to the bus fueling station. Nathan Howell, operations superintendent, says the COVID pandemic slowed the project down, but the utility also invested significant time visiting other utilities that had installed processes Raleigh was considering.

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