I recently read a great article by Dr. D.G. Cirne et al, titled "Anaerobic Digestion of Lipid-Rich Wastes - Effects of Lipid Concentration." This article answers questions on the impact of increased fat, oil & grease concentrations have on anaerobic digester performance with the goal being methane generation.
I was curious about this article since many digesters receiving high FOG loadings experience problems with fatty acid buildup and lower than expected methane production.
In the past we have tried to remedy this problem by adding enzymes (lipase), micronutrients, and acid forming bacteria in an effort to break down fatty acids into short-chain forms readily used by methanogens.
In this study, Cirne evalued the impact of various lipid loadings (5 - 47% of total COD). Also evaluated was the impact of adding free lipase the digester. The authors found that lipid loadings from 5 - 18% had no impact on methane production. However higher grease loadings from 31- 47% exhibited inhibition that increased with higher lipid loadings. In all cases, the microbes eventually converted all the fatty acids to methane, but the lag associated with the process is what causes problems with digesters run in continuous flow operation.
Adding free lipase to the digester increased the amount of inhibition. This counter-intuitive finding reveals that the methanogen inhibition is caused by a long chain fatty acid buildup from the initial grease splitting. The addition of lipase immediately splits the fats into glycerol and 3 long chain fatty acids which exacerbates the already increased VFA production from the indigenous break down of grease.
For operations, the study shows that a digester can handle high grease loadings if the residence time is long enough. In practice, most digesters do not have this level of retention time so grease loadings should be limited to below 18%.
Non-filamentous bulking is often overlooked as a wastewater treatment problem. Operators facing viscous bulking often complain of "gelatinous" floc, that billows over the clarifier weir and is difficult to dewater on the secondary press.
We see viscous bulking when the bacteria begin to produce excessive quantities of extracelluar polysaccharides (EPS). In normal good floc, it is the EPS that acts as the glue to hold bacteria cells and adsorbed (outside cell wall) particles. When cells begin to overproduce the EPS, the previous glue begins to hold excessive amounts of water and is very vulnerable to sheer. The resulting sludge does not compact well and can easily be carried over secondary weirs. The return sludge (RAS) concentration is lower as we are returning more water relative to biological solids which further compounds the problem.
You can see viscous bulking under normal light microscopy as "fingers" and gelatinous appearance. The appearance can be enhanced for better observation by adding India Ink to the slide. The India Ink will not penetrate the EPS and will appear as clear zones around the floc.
Causes of Viscous Bulking
Rotifers are among the largest indicator organisms commonly found in wastewater treatment plants. I realized the other day after looking at storm water pond samples that I had never talked about rotifers on this blog.
In wastewater treatment, rotifers are associated with "good" treatment where all major treatment parameters have been met. As a large - 0.5 - 2mm organism, the rotifer is interesting in that it is a multicellular creature with visible mouth, digestive system, and discrete organs. They can be attached to floc where they feed on bacteria and other organic particles.
In wastewater, we see rotifers appear under low F/M (Food/Microorganism Mass) or "older" sludge ages. There also has to be sufficient dissolved oxygen and bacteria floc. So, we have an indicator organisms found in "good" system health that is easily observed - what makes them a "less than ideal indicator organism?" I'll list the pros and cons below:
Easily observed Somewhat resistant initial spills
Seen with low F/M Usually indicate an older sludge than optimal
Ponds often develop surface films often develop in warmer months. Environmental conditions promoting film formation include warm weather, low flows, and sunlight promoting algae blooms. While a slight film may cause no problems it can promote conditions for odor complaints (H2S & volatile organic acids) and be a precursor to an algae bloom. In this post I will cover what compromises the film and how it arises in the pond.
While algae is important for normal pond activity. Excessive nutrients and summer temperatures can lead to a cyanobacteria bloom. These "algae" are actually a photosynthetic bacteria. They tend to form a bright green, blue-green surface scum and increase suspended solids in the pond. Besides causing problems with effluent TSS, cyanobacteria produce chemicals that contribute to odors and off-flavors in ponds. If the bloom is substantial, cyanobacteria can also prove deadly to wildlife. In 2013, an elk herd in New Mexico drank from a pond with a cyanobacteria bloom and was found dead in the immediate vicinity.
The second source of film is less obvious from cursory visual clues. In a quiescent lagoon with anaerobic activity often forms a film containing fatty acids. Since the fatty acids are naturally hydrophobic, we get in effect a "soap scum" on the surface. This film layer prevents normal oxygen transfer which further increases the size of the anaerobic zone. As a result, the pond often produces H2S and other malodorous compounds.
Dissolved Oxygen Uptake Rate (DOUR) and Specific Oxygen Uptake Rate (SOUR) are often used to monitor biological system health on a daily basis. When used frequently, oxygen uptake rates show changes in bacteria growth/respiration rates. I have included the protocol in the at the end of this post, but I wanted to discuss what OUR monitors and what various changes mean to daily operations.
What Oxygen Uptake Rate tests monitor
The DO meter reads oxygen changes in a saturated sample over a set period of time. The changes in dissolved oxygen are primarily due to the use of oxygen by microbes in the sample. (Chemicals can exert some demand, but in most biological units the change is due to microbes.)
If the microbes are dividing and growing rapidly (log phase growth), the DOUR will be high. A system with a sudden increase in OUR indicates more soluble organics (BOD) which can impact effluent BOD, nitrification, and TSS.
Following a spill with more toxic influents, the OUR can suddenly drop which indicates the biomass is actually not reproducing and/or in the lag phase where growth is low or stopped. We also see low OUR when the influent loadings are reduced (low influent BOD) - as seen during process unit shutdown.
By doing OUR tests on a daily basis, we are collecting baseline data that ties oxygen uptake to other parameters including effluent BOD/COD, nutrient removal, TSS removal, and effluent toxicity. When operators see OUR numbers outside normal ranges, they can immediately begin to search for causes and begin protocols to protect effluent quality.
Retention ponds are used in a number of situations to remove pollutants prior to discharge into receiving streams. Small ponds are often seen near parking lots, gas stations, or neighborhoods where the ponds are used to treat for oil sheen, fertilizer run-off, and other pollutants. Larger ponds are often found at the end of wastewater treatment systems and are vital for "polishing" the effluent by removing residual TSS while removing small quantities of BOD, ammonia, phosphate, or other criteria pollutants.
Waste treatment involves microbes modifying chemical compounds with the majority end products being new cells, carbon dioxide, and water. The process of modification requires enzymes which function as a catalyst to make the necessary chemical reactions occur in the given environment. The typical enzymes is a large molecular protein structure that functions in both providing building block molecules for the cell or taking cellular building materials and assembling them into vital cellular components.
Enzymes are manufactured by cells to be substrate specific and function under a set range of environmental conditions. If the conditions such as pH or temperature are not in the correct range or an inhibitory compounds bind with the enzyme active site, the enzyme's effectiveness is compromised and it is rendered useless.
In waste treatment we often refer to a general group of enzymes that function on components of domestic waste. These include:
When treating wastes, cell yield is considered because the biosolids produced must be eventually processed. Sludge disposal can be via composting, land application, incinerated, or sent to a landfill. No matter which disposal method is used, the costs of handling solids are of concern.
In selecting between aerobic and anaerobic treatment options, cell yield from influent organics are often considered. For a given substrate (carbon source) the cell yield for anaerobic treatment is between 6 - 10x less than aerobic waste treatment. This reduction in solids production is often used to justify the higher costs of building an anaerobic system.
The yields for an example substrate is given below: Notice that a lightly loaded aerobic system has much lower cell yield than highly loaded systems.
High Loading Conditions - no carbon limitations (high BOD)
1 unit substrate carbon —> 0.5 units CO2 carbon + 0.5 units cell carbon
Low Loading Conditions - liming substrate supply (low BOD)
1 unit substrate carbon —> 0.7 units CO2 carbon + 0.3 units cell carbon
1 unit substrate carbon —> 0.95 units (CO2 + CH4) carbon + 0.05 units cell carbon
When referring to anaerobic treatment, I am talking about the process where archea organisms produce methane as a part of waste stabilization. The chemistry of the anaerobic process on the surface appears much more complex than the aerobic system, but it is very similar except we are not using elemental oxygen as a terminal electron acceptor. (I'll get back to this biochemistry section in a future post). When considering an anaerobic digester unit you should first know the advantages and disadvantages of anaerobic waste treatment:
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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