More on High Solids Anaerobic Digestion


Alpha-Gamma is the international licensee for an exciting new recycling technology developed at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) called High-Solids Anaerobic Digestion (HSAD).


The continuous HSAD Process is a new unit operation that can be readily integrated into existing industrial plants and municipal solid waste sorting facilities. An HSAD system is almost completely automated requiring little attention each day. Integrated into an existing operation, the anaerobic composting process will soon return money and energy for its owners.

HSAD is a new application of a proven, conventional technology: low-solids anaerobic digestion systems are used throughout the world as the preferred technology in wastewater and municipal sewage treatment plants.

The HSAD system applies this same technology to create a new high-solids process that can successfully utilize solid organic waste feedstocks. The solid phase fermentation reduces the required equipment volume and associated capital and operating costs, while the bioreactor's volumetric productivity is factors higher. Conventional anaerobic digestion feedstocks typically contain 1-2 percent solids. In contrast, HSAD can handle feedstocks with solids content up to 45 percent.

The centerpiece of the HSAD process is a proprietary closed system design developed after 10 years of sponsorship at NREL. The system utilizes equipment modified from the chemical processing industry to prepare organic solid wastes and load the HSAD bioreactor. Inside the bioreactor, a unique blend of thermophilic microorganisms converts the organic carbon into cell mass and biogas, hence the descriptive term "Anaerobic Composting."

The biogas is captured and converted to electricity or steam and heat by standard industrial equipment. The effluent from the bioreactor produces two products: a moist, compost material rich with valuable elements like nitrogen and phosphorus, and a liquid fertilizer. Both by-products further enhance the economic viability of the technology. A proprietary supervisory control system completes the integrated system and provides near total automation.

While the HSAD process is specifically designed to recycle solid organic wastes, it easily accommodates combinations of solid and liquid, industrial or municipal wastes. Blends of rapidly degrading feedstocks, like fat, oil and grease, and slower-degrading materials, like paper and yard waste, make superior feedstocks. Blended feedstocks provide consistency of composition with improved process control and higher conversion rates.

The enclosed, odorless system converts organic carbon in solid organic waste material into biogas, which can be used to run plant systems or generate electricity. The process reduces the volume of solid feedstock by as much as 65 percent depending on feedstock, and the resultant compost is a salable product rather than a disposal liability.