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
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:

1. Sugars
2. Starches
3. Cellulose & Hemi-cellulose
4. Ligno-cellulose
   
   

Proteins
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.
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 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:

1. Digester design & operating specifications
2. Know current influent makeup – it is more than just a COD number
   1. What are the organic components & their convertibility to methane
   2. Micronutrients
   3. Sulfate/Sulfides
   4. Inhibitory metals
   5. Inhibitory organics
3. How to increase monitoring when adding new solids to the digester – pay close attention to digester ammonia, alkalinity, and VFA. In the off-gas measure CO2 and methane.
4. Gradually increase the FOG addition over time to allow for the microbial population to develop to the new influent makeup. There will be an increase in anaerobic LCFA oxidizers and H2 utilizing methanogens – but they take time to develop into needed populations.

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:

• ​Media provides growth surfaces for the microbes. The media needs to be near neutral buoyancy and composed of stable material (usually polyethylene or polypropylene).
• Loading rates are based on surface area per cubic meter volume. So you want media with higher surface areas per cubic meter. You should also be aware that clogging can be a problem if the media has "too much" surface area. Ask the manufacturer their experiences with various options.
• Normally the media has 85% open space for water and microbial growth. Less plastic means more volume for treatment. But the plastic must be robust enough to handle biofilm weight, friction, and degradation from UV radiation/chemicals.
• For true MBBR systems the media is added to approximately 66% volume. Again, system specific issues may require changes in media volumes added.
• Media lifespan - while long lived, media will eventually need to be replaced. Ten years is an often quoted number.
• Media size - can it washout if screen failure occurs. (Larger media would be more likely to be contained in a clarifier).

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.

• Aerobic
• Anoxic
• Anaerobic

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:

1. Increased treatment capacity with a given foot-print
2. Ability to meet permits by retro-fitting an existing plant
3. Less susceptiblity to shock loadings
4. Can allow for better nutrient removal with respect to both nitrogen & phosphorus

What are the downsides?

1. Initial expense for installing the system
2. Maintenance of aeration system more difficult with media
3. In mobilized media systems the screens must be maintained to prevent media loss
4. All are susceptible to fouling – especially from oil & grease that encapsulates the biofilm

Selecting the proper wastewater treatment system can be challenging, to say the least. The process involves:

1. Understanding influent makeup, concentration, and volume (flow)
2. Current and anticipated future effluent discharge requirements
3. Potential changes in influent
4. Budgetary constrains for capital expenditure and ongoing operations/maintenance
5. Life expectancy of the treatment system and ability to modify if needed

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. 
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Working depths are

• Anaerobic  Depth 2.5 - 4.5 meters
• Facultative Depth 0.9 - 2.4 meters
• Aerobic Depth 30 - 45 cm (note unit change)

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.