Lagoons are one of the oldest and most cost-effective ways to treat municipal wastewater. Lagoon treatment systems are often found in rural and small municipalities where enough land is readily available. Even with the latest generation of wastewater permits with BOD/COD, TSS, ammonia and phosphate removal required for needed effluent treatment; a lagoon system offers the easiest to operate at the lowest cost for many facilities and towns.
There are four types of lagoons seen in field practice:
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.
Commonly encountered microbes are roughly classified into the three temperature groups (1) Psychrotroph (low temperatures), (2) Mesophile (medium temperatures usually 15 - 40 deg C), and (3) Thermophile (Temperatures from 45 - 80 deg C). Most of our wastewater treatment plants operate in the mesophile temperature range for much of the year.
The image above gives the general ranges of the organisms where you should also note the overlap in each group. I have come to see the transition zone from one group to the other as a "Problem Zone" where none of the individual groups is completely in its optimum growth range. Frequently we see this problem as increase in effluent suspended solids (TSS) for both the low and high temperatures. Also with low temperatures slowing metabolic activity in general, the low temperature problem zone also can cause problem with BOD/COD removal.
In my field of bioaugmentation or adding in select cultures to wastewater systems to enhance treatment, we add organisms that are very successful in the problem zone ranges to quickly establish the new microbial population that nature is taking its time to develop. By adding cultures, the system can be rapidly brought back under control for the entire period of high or low temperatures.
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.
Almost all organic compounds can be transformed by microbes into carbon dioxide, water, and new cellular material. Problems arise when the degradation rate occurs only under specific conditions not present at the site or the decay rate is so slow that changes are required for bioremediation to be a viable cleanup option.
A site's bioremediation potential is first evaluated by determining the types and extent of pollutants present. Following site characterization, a remediation plan is developed based on the pollutants present, site characteristics, environmental variables, and cleanup time frame.
The compounds present may include some organics that are slow to degrade or have a degree of toxicity/quasi-toxicity to many microbes. In these cases, the time for cleanup can be reduced by first adding the mixing/aeration and fertilizers as used in biostimulation; then on-site time can be further reduced by avoiding the adaptation and lag phase growth of indigenous microbes via the use of bioaugmentation.
Using data from the site characterization, microbiologists can develop a microbial blend containing organisms having specific metabolic pathways for degrading target organics. Another consideration is the ability of organisms to produce biosurfactants, that improve activity by enhancing hydrophobic compound solubility. The extent of the benefit depends upon the "fit" of the microbial inoculum used and the general state of indigenous microbes already at the site. Overall, usually the decrease in on-site remediation activities including labor, lab testing, and risks associated with an active remediation project can pay for the use bioaugmentation.
Recently, I was doing a microscopic examination of an industrial activated sludge plant's mixed liquor. Microscopic exams are a very good way to monitor a biological system's overall health. With a simple microscope having 10x and 40x objectives, an operator within 5 minutes can obtain very important information that can reveal: (1) warnings of potential toxic upset, (2) estimate changes in soluble BOD & COD, (3) predict issues with effluent suspended solids (TSS) and turbidity, and (4) determine impact of new influent streams on biomass. Knowledge of any one of these variables is important, but the ability to cover all variables with an easy 5 minute test with no extensive equipment or reagents makes it a double benefit to doing frequent microscopic exams.
When doing a microscopic exam at 100x or 400x magnification, the bacteria do not appear particularly distinctive. In fact, free bacteria are the small round, oval, or rods that appear to be bouncing around in the water. Bacteria in biofilm or floc appear as larger aggregates that should be evaluated for size and density. However, what we usually note in microscopic exams are the protozoa that are associated with the bacteria. The observed protozoa are highly dependent upon water quality which is why frequent use of microscopic exam can establish a baseline and any changes is readily apparent.
The organisms pictured above is a stalk ciliate. This protozoa is found in systems with low soluble BOD, good floc formation that allows for stalk attachment, and sufficient dissolved oxygen. Any toxic shocks will cause stalk ciliates to detach from the floc or disappear completely.
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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