All About the Penn State Digester

The Penn State Digester is an on-farm anaerobic digester that was constructed as part of the Department of Animal Science's renovation program. This digester converts animal waste into biogas, which is a renewable energy source, while also supporting Penn State's nutrient management plan and the university's larger sustainability goals. This sheet provides technical details about the Penn State Digester.

What Is an Anaerobic Digester?

An anaerobic digester is a system where microorganisms biologically digest and break down organic material anaerobically, or in the absence of oxygen, to create biogas. This biogas can then either be combusted to generate electricity and heat or can be further processed and refined into renewable natural gas or other fuels. Farm digesters rely on animal manure as the primary feedstock used to create biogas. In addition to creating renewable energy that can be used on-farm or used as an additional revenue source, biogas digesters treat animal waste, reduce odors, and produce helpful co-products that can benefit the farming operation.

What Motivated Penn State to Build a Digester?

There was not a single motivating factor in installing the anaerobic digester as part of the Penn State Dairy Complex. There were three key considerations the University considered prior to initiating this project.

Improve the Waste Handling Process

Penn State was interested in improving the manure waste handling process from the dairy facilities. Prior to the digester, dairy manure was mixed with dry material to make it more easily stored, and then it was trucked to nearby fields for spreading as a fertilizer. This process proved to be inefficient. In the planning for the digester, the University hoped to reduce waste and streamline the manure management process.

Control Odors

Odor control of both the manure storage and the manure field application. Penn State's dairy complex is directly across the road from Beaver Stadium, a popular location during football season (if you look closely at the picture above, you'll see the stadium in the background). Anaerobic digestion is touted for its ability to minimize and contain odors. Furthermore, following digestion, the remaining liquid portion, the digestate, reduces the odor-producing compounds in the manure. Much of the manure from the dairy barns was spread on fields near the State College Airport, which tended to draw birds that were attracted to the insects drawn to the manure. This ultimately could cause a hazard to aircraft. Adding a digester could solve both of these odor-related issues.

Improve Sustainability

In addition to these practical reasons, Penn State is committed to increasing the sustainability of its operations. The digester provides a waste-to-energy system that provides renewable energy in the form of biogas that can be used to create electricity and heat to sustain the anaerobic digestion process. It also creates a more circular system, where bedding is cleaned and recirculated for reuse, a streamlined manure management process, and energy as a key byproduct.

Penn State Digester

Process

At present, the Penn State digester collects manure from 350+ dairy cows and heifers in the freestall housing bedded on sand. While there are many benefits to using sand bedding in dairy operations, it is not compatible with digester pumps and piping. Therefore, the dairy manure and sand bedding are scraped twice daily into a McLanahan Sand Separator system, where sand and manure are sent through a set of steps, including an auger, multiple forms of separation, and screening, so that as much sand as possible is separated from the manure. After this, the sand is then recycled and reused for bedding, while the liquid manure stream is sent to the digester’s mixing tank.

The Penn State Dairy Farm is a bit on the small side to justify the cost of a digester. The EPA recommends a minimum of 500 head of cattle before investing in a digester. Therefore, in the future, the digester will also accept swine manure. This is added to the separated dairy manure in the mixing tank before entering the digester. Here, the manures are mixed and agitated to ensure that the incoming waste is well-dispersed. The digester is designed to take liquid swine manure from the Penn State swine farm, which is a 70-sow farrow-to-finish facility with an average animal inventory of 580 animals of various ages, in addition to the dairy manure.

Once thoroughly mixed, the manure will enter the digester. This is a complete mix digester, which is designed for waste with a low solid content, on average 3–10%. The contents in the digester are mixed by a motor to prevent settling in the tank. This tank is heated to foster a productive environment for the methane-producing bacteria. If you'd like to learn more about how an anaerobic digester works, check out Basics of Anaerobic Digestion as a starting point.

There are two primary products of the digestion process: biogas and digestate. The biogas from the digester will be used to power a combined heat and power plant (CHP), where the heat will be used to keep the digester warm, and biogas will be ignited to turn a generator and create power that can be added to the electrical grid. The nutrient-rich digestate will be used for nutrient application on the surrounding fields.

Farm Facts

• Location: Dairy Complex, Penn State, University Park, PA
• Capacity: Digester volume is 400,000 gallons with an operating volume of 360,000 gallons.
• Digester Type and Manufacturer: Complete mix digester, engineered by Tetra Tech with the 120 kW combined heat and power (CHP) plant from Martin Energy
• Retention Time: 23 days when fully operational, taking 104,000 gallons of liquid manure per week.
• Digesting Temperature: 95 degrees Fahrenheit

Digester Inputs

• Digester Feedstock: Mix of dairy manure from 500+ dairy cows and heifers and swine manure from a 550+ swine finish facility.
• Daily Input: Inflow will receive an average of 11,500 gallons of manure/washwater per day.

Digester Projected Outputs

• Biogas Energy (kWh/m³): Calculated to produce 110 KW of electric power and 130 kW of thermal energy recovery per cubic meter.
• Biogas Output (volume/day): Calculated to be 52,000 SCF/day, anticipated to be 55% methane.
• Annual Energy Output: Calculated to generate approximately 830,000 kW/year.
• Digestate: Liquid digestate will be applied to nearby fields, particularly near the State College airport, to alleviate issues with birds congregating near the airport.

Construction

• Project Cost: Digester: Approximately $735,000 for the digester itself; this does not include upgrades made to the dairy barn and the sand separation facility.
• Financing: Combination of a low-interest loan from the Penn State Energy Savings Program, grant funding from West Penn Power Sustainable Energy Fund (WPPSEF) Sustainable Energy Financing Program, College of Agriculture.

Benefits

• Energy: Renewable source of energy
• Digestate: Liquid digestate will be used as part of Penn State’s nutrient management plan and applied to surrounding fields.
• Sand Separation: Sand separated from McLanahan sand separator system reclaims sand for bedding.
• Odor: Odor reduction both on-site and for liquid digestate fertilization.
• Greenhouse Gas: Provides a greenhouse gas reduction of over 500 metric tons of carbon.

Other Information

Prepared by Siobhan L. Fathel, Assistant Teaching Professor, The Pennsylvania State University, with assistance from Randall Swope, Coordinator of Support Units, and Jeff Spackman, Facilities Project Manager from The Pennsylvania State University.

This information is provided for general informational purposes only, and The Pennsylvania State University shall have no liability whatsoever.