A Wisconsin University Explores the Frontiers of Efficient Anaerobic Digestion

A research center at the University of Wisconsin Oshkosh focuses on ways to maximize digestion efficiency and biogas production.

A Wisconsin University Explores the Frontiers of Efficient Anaerobic Digestion

Gregory Kleinheinz

Aaerobic digestion is a high-potential growth area in water resource recovery facilities. It’s a source of renewable energy, a promising new stream of revenue and often the key to energy self-sufficiency.

But how can anaerobic digestion and biogas-to-energy be made as cost-effective as possible? How much can gas production be increased through co-digestion of biosolids with food waste and other organic substrates? How can digester retention times be shortened? Volatile solids reduction optimized? Operating costs kept to a minimum?

Such questions are the concern of the Environmental Research and Innovation Center, or ERIC, at the University of Wisconsin Oshkosh. Led by Gregory Kleinheinz, R.S., Ph.D., chair of the university’s Department of Engineering Technology, ERIC is a research and testing center for the campus, the public, and industry partners. Its staff works to serve clients while training students in ecology, environmental and public health, biology, limnology and microbiology.

Among its functions, ERIC operates three anaerobic digesters as a centerpiece for research on topics like optimizing solids management and evaluating materials for biogas production potential. The center provides expertise across multiple disciplines and offers access to state-of-the-art laboratories. Kleinheinz and Brian Langolf, biogas director, talked about the center and its biosolids and biogas research in an interview with Treatment Plant Operator.

  Urban Dry Digester Small Farm Digester Large Farm Digester
System type Dry, batch Mixed plug flow Complete mix
Temperature Mesophilic Mesophilic Mesophilic
Target retention time 28 days 18 days 18-20 days
Volume treated 10,000 tons 6,600 tons/year 146,000 tons
Feedstock 53 percent food waste
23 percent yard waste
24 percent farm bedding
82 percent manure scrape
12 percent bedpack/manure
6 percent industrial food waste
100 percent manure scrape
CHP size 370 kW 64 kW 1,426 kW
Electrical production 178 MWh/month N/A N/A
Biogas methane content 58 percent 58-60 percent 53 percent
Digestate volume Liquid: 300,000 gallon/year
Solid: 7,000 tons/year
1.4 million gallon/year 18 million gallon/year
Digestate use Liquid: Land-applied
Solids: Composted/sold
Land-applied Land-applied

TPO: What was the rationale for creating the ERIC?

Langolf: Several years ago, the university was doing sustainability projects on campus in water conservation, lighting retrofits, energy savings and building design modifications. Part of that was an evaluation of how to use the campus food waste. That led to the installation of an anaerobic digester, built much bigger than just for campus needs to enable us to work with the community and develop programs around biogas and organic diversion. We now operate three digesters and have the ability to provide research and testing services to industry.

TPO: What does the center offer in addition to the digesters and related services?

Kleinheinz: We operate as a contract research and development laboratory, basically as a place to transfer technology development. We also offer all the laboratory tests that wastewater treatment plants require and support parties who have an interest in managing organic materials.

TPO: On the anaerobic digestion side, what are some of the services offered?

Kleinheinz: We can do pilot projects in the lab involving multiple technologies, including dry and continually stirred systems. We can evaluate materials, taking into account feedstock variation and physical, chemical, and biological considerations, and then develop recipes and test them at full scale. Having control of the digester systems, we can take ideas developed and tested in the lab and do proofs of concept and demonstrations in the real world.

TPO: Can you give a specific example of the kinds of projects you undertake?

Kleinheinz: We’ve worked with wastewater treatment plants that want upgrade their biosolids from Class B to Class A. Or they’re looking at mixing local organic resources with biosolids and need to know how to make such a mixture work. What are the chemical and biological issues related to that? What are the economic metrics? We’ve also done projects for wastewater treatment plants that aren’t related to anaerobic digestion, like helping them put in biofiltration systems for odor control. It’s like a comprehensive consulting relationship.

TPO: What anaerobic digestion technologies does the center operate?

Langolf: We have three basic technologies. Initially we built a dry fermentation system that is the first of its kind in the U.S. Its primary goal is diversion of food and yard waste from landfills. It takes feedstock from the campus and from more than 100 institutions and facilities — grocery stores, schools, restaurants, hospitals, corporations, and other University of Wisconsin System campuses. It treats 10,000 tons of yard waste, food waste and farm bedding per year to produce 3,300 MWh of electricity, about 8 percent of the university’s needs.

TPO: What are the other two digester technologies?

Langolf: We also operate a digester on a dairy farm with 130 cows. It’s designed to demonstrate a technology for providing energy for small farms and helping with nutrient management plans. It digests mainly cattle manure but is co-fed with a small amount of industrial food processing waste. Finally, we have a large manure digester at a farm with about 8,500 dairy cows. It’s a traditional complete-mix digester that treats 100 percent manure scrape.

TPO: In the big picture, what are you learning from your anaerobic digestion research?

Kleinheinz: First and foremost, there is no one-size-fits-all solution. Everybody needs to understand enough about their system and their operations and how it might be different. Sometimes seemingly small differences can have rather large impacts. People need to evaluate their issues and technologies on their own merits in their own systems.

TPO: What basic procedure do you follow when performing testing for a client?

Kleinheinz: The first step is to ask: “What’s the problem?” For example, a treatment plant manager might say, “A cheese manufacturer wants me to take their byproduct into my digester, and here’s what they’re looking to pay me. Do you think that makes sense?” Those are the kinds of things we can help them evaluate.

TPO: Once you define the problem, how do you proceed from there?

Kleinheinz: In almost all cases, we advocate a stepwise approach. The steps are bench scale test, pilot scale and then full-scale. These are all stop-go points. When bench scale or pilot scale results show that what’s being proposed may not be feasible, we have a jumping-off point to reevaluate, without diving in and wasting time and effort funding a full-scale project that has a low probability of success. We function as an impartial third party. We’re not selling anything. We’re here to evaluate and offer advice to clients to maximize their chance of success without spending a lot of money to get the answers they need.

TPO: How and where are the smaller-scale tests performed?

Kleinheinz: Most of the pilot and bench scale tests are done in the lab. We have a continuously stirred digester pilot, a dry digestion pilot, and drying capabilities for biosolids and compost materials. We have a composting pilot dryer system located at one of the digesters. A lot of these systems are adaptable. So for example, if a client wanted to test an aerated static pile composting system, that would be an easy adaptation.

TPO: Have you made any major discoveries related to anaerobic digestion?

Kleinheinz: Anaerobic digestion has been around since the first microbes crawled out of the primordial ooze. We look at it as exactly like the wastewater treatment process. Wastewater treatment is basically controlling microbial metabolism to achieve BOD reduction, pathogen kill, solids reduction or denitrification. In a similar way, anaerobic digestion is straightforward. But how can we manipulate the biology and use chemical treatment to make the process more efficient — such as by increasing methane production or decreasing retention time? That’s where research on anaerobic digestion is going.

TPO: Where do you see growth trends in anaerobic digestion in wastewater treatment?

Langolf: We’re seeing more waste reduction projects where utilities are looking to save cost on waste disposal or produce energy they can use on site. There’s substantial development in converting biogas into natural gas for vehicle fuel or injection into pipelines. Many sites are looking to develop value-added end products like bagged soil amendments they can sell. We’ve seen people explore using digestate as a substrate for products like fiberboard and oriented strand board. One other area of exploration is using biogas to create bioplastics — the digester as a chemical factory of the future.


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