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:
I have found MBBR technology is a great way to improve existing treatment plant efficiency or add biological pre-treatment to a facility discharging to a POTW. In both cases, the media helps maintain a stable, concentrated biomass with fewer operational issues found in most suspended growth systems.
This brings us to how much media is required, what media materials are suitable, and where should it be purchased.
Here are my quick bullet points on MBBR media:
One of the oldest wastewater treatment methods, trickling filters, works on the same biological principals as one of the latest treatment technologies – the MBBR. All systems relying of attached biofilms can be termed fixed-film systems. At the core, you have microbial colonies attached to a support structure. The film tends to have three distinct ecosystems inside the biofilm depending upon the location inside the biopolymer matrix supporting the colony.
The biofilm acts to protect the microbes from toxic influent compounds, pH swings, hydraulic washout, and other factors that would kill individual microbial cells. Much like floc in suspending growth systems, the fixed-film biomass consists of living bacteria, inorganic materials, extracellular polymers (the glue), and adsorbed organics. The percent of the mass that is truly living microbes depends upon influent makeup, organic concentration, and biofilm sloughing rates.
The fixed film system overcomes solids seperation problems often seen in suspended growth system’s secondary clarifiers. In effect, the fixed film allows for a higher MLSS (up to 3,000 mg/L higher) than a comparable suspended growth system. This directly translates to:
Selecting the proper wastewater treatment system can be challenging, to say the least. The process involves:
Recently, I came across an interesting table from Mara & Pearson (1998) comparing capital expense, O&M, and land requirements for various treatment systems. While the figures are in DM (1996), the relative costs can be extrapolated to current figures.
An older but interesting reference on sizing and loading rates for wastewater ponds without mechanical aeration.
Working depths are
Over the past decade, we have seen cases where petrochemical whole effluent tests are compromised by higher than expected effluent nitrite. In most wastewater treatment systems, ammonia oxidation to nitrite is the rate limiting step and nitrite is quickly converted into less toxic nitrate. The solution has been to add nitrite oxidizing cultures (NOB) when effluent nitrite levels start to increase. Until now, there has been no answers on trigger events that create the NOB activity problem. This past month, I noticed an interesting article in Environmental Science & Technology Journal. The paper by Dr. Sylvia Schaefer and Dr. James Hollibaugh covers their research into temperature decouples ammonium and nitrite oxidation in coastal waters. While coastal waters are not wastewater treatment plants, they have some interesting findings.
In coastal waters, with sufficient oxygen and mixing, when temperatures increase to between 20 - 30 Deg C, NOB oxidizing bacteria start to lag behind the AOB - resulting in nitrite buildup. The full abstract is available here: http://pubs.acs.org/doi/abs/10.1021/acs.est.6b03483
I intend on reviewing data from industrial and municipal wastewater treatment facilities to see if we have a similar phenomenon occurring during periods with high water temperatures. If we could identify trigger conditions, actions can be taken to prevent the nitrite increase before effluent quality is impacted.
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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