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.
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
pH has a significant impact on the microbial makeup of a wastewater system. Most biological wastewater treatment is accomplished at pH range of 6.5 - 8.5, which is where a majority of our common environmental microbes thrive. Today, I want to detail how (1) long run high or low pH can impact microbial populations and (2) what rapid shifts in pH can do to biomass performance.
Many systems such as pulp mills operate at an inlet pH > 9.5 which is not often mentioned in literature. This constant loading creates an environment where alkaliphile, organisms that thrive at pH from 8.7 - 11.0. Alkaliphiles function like "normal" wastewater organisms except their enzyme systems are optimized for high pH environments. As with temperature, many organisms can be facultative alkaliphiles which thrive from pH 6.5 - 10.5 (common in Bacillus sp).
Systems that operate in lower pH are also seen in in systems with influent from metal finishing, fruit processing, and some chemical operations. If a system is operated below pH of 5.5, we start to see increasing number of fungal cultures. Other organisms that thrive in acid pH with high levels of sulfides include Thiobacillus sp. which create their own acid environment by converting H2S into H2SO4 when oxygen levels are sufficient. All of the acidophile organisms have enzymes and cellular operations that thrive with the low pH.
What we do not see in either low pH or high pH wastewater systems is the growth of autotrophic nitriiers which thrive at pH from 7.2 - 8.0 when you go below 6.5 or above 8.5 the autotrophic nitifiers (Nitrosomonas and Nitrobacter) will "washout" or "die" and ammonia oxidation will become compromised.
The biggest challenges for operators occur when a system undergoes a rapid pH shift. While a biological system can often adapt to long run high or low pH operation, a rapid shift in pH from of can have immediate impact on the biomass. Often the first observed change following a pH shock is increased TSS & turbidity at the effluent. The bacteria in response to unfavorable conditions begin to lose beneficial biopolymer bonds that form floc and biofilms. If rapidly corrected to normal pH levels, the biomass should rapidly come back to proper floc/biofilm formation. In cases where the pH swing also causes cell death and more complete biopolymer failure, operators often find a benefit to adding adapted cultures to avoid the time lag seen to bring the system back into proper biomass conditions (also known as decline phase) where microbes are at maximum density and proper biofilm conditions.
The microbes in the wastewater treatment plant include:
As small prokaryotic (no cell nucleus or organelles), bacteria form the "backbone" of the wastewater treatment plant in that they are the most common organisms and they do most of the work in converting pollutants into non-hazardous forms. The species vary according to system temperature, pH, inlet chemical makeup, dissolved oxygen, and other environmental factors. Usually we classify wastewater bacteria based on their ability to grow under various temperatures (psychrophiles, mesophiles, thermophiles) and the ability to utilize oxygen or other electron acceptor for cellular respiration (aerobic, facultative anaerobic, obligate anaerobe).
A more complex organism than bacteria, Fungi can be unicellular (yeasts) or multicellular with hyphae. In waste treatment we usually have higher concentrations of fungi under low pH conditions (pH <5.0). Other factors that can favor fungi include complex organics (lignin and other complex biopolymers) and low concentration of macronutrients (nitrogen & phosphorus). Usually fungi are found in much lower concentrations than bacteria in wastewater.
Once classified as an unusual bacteria group, over the past 20 years scientists have moved archaea into a separate kingdom. Possessing unique cell membranes and chemistry, archaea microbes are found in such environments as ocean thermal vents, hot springs, anaerobic digesters, ruminant digestive systems, and other diverse environments. In waste treatment we most often see archaea in methane producing microbes in anaerobic digesters. These methanogens, produce methane from short-chain organic acids and H2 which are produced by facultative and obligate anaerobic bacteria. The activity of the methanogens is vital for COD/BOD reduction in anaerobic digesters and for production of methane gas. Most other archaea are found in low concentrations in wastewater treatment plants and are secondary to bacteria in importance.
Anammox is one of the newest technologies for treating both ammonia and nitrite in wastewater. In the past operators running systems with long sludge ages noticed that nitrification did not consume as much oxygen and alkalinity as was calculated by normal Nitrosomonas and Nitrobacter ammonia oxidation. Furthermore, the denitrification process to remove nitrate/nitrite had less of both compounds entering the anoxic/anaerobic zone. What was happening?
Anaerobic Ammonia Oxidation (ANAMMOX) is pictured in the nitrogen cycle graphic at left. The process is as follows (forgive the lack of subscript):
NH4 + NO2 --> N2 + 2H2O
The microbes responsible for ANAMMOX conversion of ammonia and nitrite have been recently isolated and we are still in the process of understanding their microbiology in wastewater. Here is what we know:
Often waste professionals faced with difficult treatment criteria, cost concerns, or a new waste stream find themselves pressured to purchase strange additives. Today I will address one of the most common that I see being marketed to industrial and municipal wastewater professionals.
What I offer is like a "Bug Steroid" - natural extracts and vitamins that make bugs grow faster, degrade more compounds, reduce odors, and produce less sludge. Of course it sounds great, but I am of the curious/scientific type and always ask.... "well how does it actually work?" In this case the answer is almost a uniform... "We don't know the exact mechanisms, but it does what we say it does.... Can I have your purchase order."
I am not suggesting that vitamins and micronutrients cannot increase microbial growth rates and in some cases be necessary for proper function. The most common use of micronutrient additives is in operating anaerobic systems where methane production is often increased by adding rare elements such as Co, Ni, & Mo. Again; before adding micronutrients the system should be examined to see if the additives are needed based on influent characterization.
In addition to anaerobic systems, I have seen adding some micronutrient/vitamin formulations make slight improvements to floc formation and microbial activity in systems with some lacking micronutrients. The most cost effective way to get these micronutrients is not some expensive additive. In my lab tests on pulp mill wastewater, I have found that adding glacial rock powder provides an excellent supply of minerals that can be used by the microbes when needed. With glacial rock powder readily available in 50 lb bags - it is used by organic farmers - you can often solve issues with lacking micronutrients by adding glacial rock at a rate of 5 - 10 pounds per million gallons flow ~ 0.7 - 1.2 parts per million (PPM).
To conclude - There is no such thing as a bug steroid. If you are found lacking micronutrients, add the lowest cost source of these minerals which is often glacial rock or even a metal salt (example magnesium sulfate).
Of course, if someone has a bug steriod and can tell me how it works on a cellular basis, I am willing to evaluate how it can help improve waste treatment.
Biofilms develop as microbial growth slows and the cells begin to excrete polymers that act like a glue between cells. The cells and other solids in the water begin to form a biofilm on surfaces (also called floc in suspended growth systems). Biofilms contain a relatively low percentage of "active" living microbes usually between 5 - 12% in wastewater with the remainder being dead cells, biopolymers, particulates, and other debris found in the water. When exposed to harsh conditions such as pH changes, toxic chemicals, or adverse temperatures; the biofilm acts to protect the microbes from harm. Only the top layers are killed in the by chemicals such as disinfectants. Wastewater treatment requires the effective formation of biofilms as they indicate proper BOD/COD removal and provide a means to remove suspended solids from the water flow. Loss of biofilm consistency and seeing free cells in the water column are the first indications of upset condition.
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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