Codigestion is a term heard often these days, and a concept that will likely increase in popularity as we move through the current tense times towards a friendlier, more renewable economy.
In general, codigestion refers to the anaerobic digestion (AD) of multiple biodegradable substrates (feedstocks) in an AD system. The more contemporary definition refers to the digestion of a combination of select biodegradable feedstocks with a base substrate that an AD system was designed to handle. The general idea is to maximize the production of biogas in an AD plant by adding substrates that produce much more biogas per unit mass than the base substrate. Two readily available substrates – municipal biosolids and agricultural manure – are the base substrates most often utilized and are located near the bottom of the “biogas per unit mass” scale. However, the benefits that can be realized from codigestion, as well as the potential pitfalls that can be encountered, need to be carefully evaluated. In the case of low-cost or free high-energy potential substrates, it pays to look the gift horse in the mouth.
One reason for the increased interest in codigestion is the creation of opportunities for the use of biodegradable wastes due to the tremendous number of AD plants online and currently being constructed in the United States.
Designers and operators of AD systems have a wide variety of potential substrates from which to choose when considering how to boost biogas production. In addition to the base substrate used, numerous wastes are available, many of which have been tested to benchmark fundamental characteristics.
The information is provided in units of methane produced per unit dry mass (volatile solids, or VS) applied. Note that data provided in the literature and from various system suppliers will be presented in many forms, requiring a great deal of evaluation before use in any comparative assessment. Also, all substrates, such as food wastes, are not created equal; they possess different levels of protein, fat/oil/grease (FOG), and carbohydrates. Testing of your specific waste stream is required to obtain a realistic idea of its biogas potential.
Unfortunately, when examining potential substrates for codigestion, most attention is paid to such characteristics as biodegradability and biogas production. Other characteristics of critical importance are organic nitrogen, presence of chemicals, sulfur, levels of K, N, and other cations, pH and alkalinity, phosphorus, FOG, and gross solids.
Once the base substrate and supplemental codigestion substrates have been identified and are ready to be fed to the AD system, care must be exercised in blending and feeding them to minimize process upsets. For optimum AD process performance, it is advised to follow these guidelines:
- Test individual codigestate loads for characteristics of concern. If any load quality is suspect, segregate it and run a BMP test.
- Store various codigestates in separate mixed and heated tanks or areas. This minimizes downstream preparation time and allows more precise blending and careful control of organic loading.
- Develop a preferred feed ratio for the various substrates, making small changes to it as the availability of the various base substrates and/or codigestates changes. This reduces the chance of AD process upsets.
- Collect full-scale system data and use it to adjust feed ratios and digester operating parameters.
Numerous full-scale systems have been operating successfully around the United States, codigesting a variety of substrates. The East Bay Municipal Utility District in California operates a profitable acceptance program for industrial and commercial organic wastes, preprocessing them, and then feeding them to its existing biosolids anaerobic digesters to boost biogas production. A number of POTWs in California manage programs that accept FOG-type wastes and feed them to their existing biosolids digesters.
In Wisconsin, the availability of dairy production wastes from a number of small dairy operations has helped develop codigestion. For years, the Madison MSD accepted cheese whey from a local dairy and fed it directly to existing biosolids digesters. Similarly, POTWs in Beaver Dam, Sheboygan, South Milwaukee, and Waupun have accepted dairy and/or other wastes and used them in existing biosolids digesters to boost biogas production. In a unique codigestion application, the Milwaukee MSD has been handling spent deicing fluid from Mitchell International Airport at its South Shore treatment plant since 2000, codigesting it with biosolids.
Companies have been active in employing codigestion in the private sector as well. Microgy has three thermophilic digesters in Wisconsin handling dairy manure along with alcohol production wastes, glycerin, waste FOG, and other wastes. More recently, a farm in Wisconsin doubled the size of its AD system to handle additional dairy manure, milking parlor wastewater, cheese production wastewater, and cheese whey.
The benefits of codigestion are numerous and the current availability and variety of possible substrates will generally improve the economic factors for an AD plant. Competition for the more common codigestates will increase, driving up prices and forcing us to look at non-standard substrates. The search for and use of more unique substrates should be based on a careful assessment (testing) protocol to define biodegradability. Permitting and design issues are evolving as more codigestion systems are proposed and become operable, so spend time looking into these.
Employ an AD technology that is flexible in its ability to handle high TS/FOG substrates. If the amount of FOG codigested in an AD system exceeds 10-20% of the overall feedstock, plan on increased monitoring of system performance and higher maintenance costs. Once your AD plant is operating, collect more data than you think you will need, as it will be useful in making day-to-day adjustments, fine-tuning the system to achieve maximum efficiency, and troubleshooting problems.
This is an excerpt taken from Dennis Totzke’s article published in the May 2009 issue of Biocycle magazine. Visit the Biocycle website at www.jgpress.com to order a copy of the magazine.