Co-digestion of organic material with dairy manure represents an opportunity to provide both a revenue stream to anaerobic digester operations, through the collection of a tipping fee and/or increased biogas/electricity production, as well as a means for waste generators to dispose of their product in a beneficial way.
However, there are many factors for an operator to consider when deciding on whether to accept organic waste. A major consideration is the volume of biogas that the material will generate when co-digested. This can be used both to assign a value to the waste through increased biogas production and/or electricity sales, as well as to size equipment for producing, treating and potentially selling/using the biogas. Estimating the biogas produced is a complicated process, encompassing many different factors of digester design, waste characteristics, and environmental factors.
To assist in this estimation, we have developed software that allows a user to predict the biogas production from mixed wastes and dairy manure based on changing herd sizes, as well as providing the ability to vary the timing and volume of addition of multiple organic wastes, throughout the course of a simulated year. With this user-friendly tool, we hope to enable producers to better explore the opportunities that co-digestion offers.
What Did We Do?
The originally developed Cornell Anaerobic Digester Simulations software allowed the user to input a herd size and to select how much (if any) of seven wastes would be co-digested with the dairy manure. This rudimentary method of simulation assumed that the same volume/mass would be applied to the digester in a steady-state constant fashion for the entire year that the simulations were run for. However, that is unlikely to be the case in a real-world production environment.
In the new version of the software, we have incorporated the characteristics of over 200 wastes into a user selectable interface. Once a waste type is selected, the user has the option to select when the waste is placed into the digester, whether that be on an everyday, weekly, monthly or custom basis with the option to select to which months of the year the additions occur. When selecting a weekly or monthly basis, the user can select which day(s) of the week or month wastes are added, and in the custom basis, the user can select which days of the year additions occur.
Once the timing of addition is completed, the user can select how much of the waste is applied during each addition. Whether that be a constant volume for each addition, or a custom volume for each addition.
The data for the specific wastes includes the dry matter and organic matter content as well as the biogas and methane yields. Based on the type of waste we have also assigned a “digestibility” curve to the particular waste which when assuming a first order kinetic model of gas production, can provide the production of gas a function of time. The production of biogas from all added wastes and the added manure is then summed for each day of the year to provide an estimate of the biogas production, on a daily basis, that can be summarized with a minimum/maximum/average on a monthly and annual basis.
What Have We Learned?
During the process of developing the software, we examined a few different techniques for estimating the timing of biogas production from co-digested wastes. There are more complicated models available such as Anaerobic Digestion Model #1 (ADM1), however many more parameters must be known/estimated for each waste type, (not to mention requiring a much more complicated user interface). We felt that using a simplified first order kinetic model provides a good way to add the necessary complexity to model biogas production over time without overly complicated calculations. The simplification allowed us to include a more complicated and yet more real world means of modeling the addition of wastes to a digester that wouldn’t be possible with more complicated digestion/biogas production models.
Currently, the Cornell Dairy Anaerobic Digestion Simulation Software is capable of predicting the amount of heat necessary to maintain digester temperatures, as well as the parasitic electrical load. Future additions will include modeling the energy usage (and effects on biogas) of treatment processes to produce Renewable Natural Gas (RNG) from biogas.
We would also like to include the ability to track nutrients through the process of digestion. Nutrient additions from the co-digestion of wastes also represent an important consideration for farm as they may or may not have the land base/crop requirements to use all of the imported nutrients. The cost of treatment of the effluent from the digester to remove nutrients, or the shipment of effluent off site may have to be added into the determination of how much of a “tipping fee” a farmer would need to charge for taking an organic waste for co-digestion.
We hope to make the program freely available to the public to use. Currently, the software is written in MATLAB which ordinarily requires a license to operate, however it is possible to create an executable standalone program that can be shared and run without the need to purchase MATLAB.
Timothy Shelford, Extension Associate, School of Integrated Plant Science, Cornell University
Corresponding author email address
Curt Gooch, Senior Extension Associate Emeritus, Department of Biological and Environmental Engineering, Cornell University
Peter Wright, Agricultural Engineer, Department of Animal Science, Cornell University
Lauren Ray, Agricultural Energy Systems Engineer, Cornell University
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