Adding grease trap waste to an anaerobic digester to increase methane production - how much can I add?
Anaerobic digesters have very complex, interdependent microbial communities including organisms in four distinct ecological niches with the major work being done by both bacteria (hydrolysis, acid formation) and archaea (methane production). Assuming we have sufficient micronutrients, ideal temperatures, and pH/alkalinity. I wanted to look at how the influent COD composition can impact methane production.
Carbohydrates such as sugars, starches, and cellulose are among the ideal feeds for anaerobic digesters and often improve methane yields. Operators should keep in mind the ease at which carbohydrates are converted simple acids in the digester. In order of ease:
Proteins readily degrade to amino acids that are further converted by bacteria into acetate. The amine groups do lead to the generation of ammonia. Ammonia in un-ionized form (NH3-N) can rapidly become toxic to methanogens at 100 mg/L. Substantial amounts of un-ionized ammonia form only when pH increase well above the target 6.7 – 7.4 range. Ammonium (NH4+-N) is much less toxic to methanogens and can be tolerated above 1,500 mg/L. System treating high protein wastes, should monitor ammonia and pH closely.
Fat & Long Chain Fatty Acids (LCFA)
Digesters receiving mostly wastewater treatment plant biological solids tend to have lower than needed methane production. With high energy potentials, grease trap waste offers a way to increase methane production. Remember FOG used to characterize grease trap waste contains mainly long chain fatty acids rather than pure fats. Most grease traps have microbes producing lipase which breaks down the grease molecule into constituent glycerol (which is used quickly) and long chain, insoluble fatty acids forming the grease cap. In the anaerobic digester, the long chain fatty acids are converted into both acetate and hydrogen via a multiple step anaerobic oxidation process. The LCFA pathway is slower than that of carbohydrates and proteins, and the produced H2 must be removed by H2 utilizing methanogens as increasing levels of H2 in the digester make further anaerobic oxidation thermodynamically impossible. On a good note, the methanogens using H2 grow more rapidly than the acetate using methanogens. Another challenge with adding concentrated LCFA wastes is the potential buildup of volatile fatty acids and associated low pH. The acid formation can continue well below a pH of 6.5 where methane production becomes inhibited.
Back to how much FOG (LCFA) can I add to an anaerobic digester
It should be clear that the amount of FOG that can be added to an individual digester depends on site-specific factors. Operators need to consider the following:
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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