Every biological treatment process — activated sludge, trickling filters, ponds, digesters — runs on the same thing: a living community of microorganisms doing the actual work. Here's who's in that community, how they grow, and what watching them tells an operator about process health.
Wastewater treatment is, at its core, a managed ecosystem. Operators don't remove BOD and ammonia directly — they create the conditions for a community of microorganisms to do it for them, then harvest and remove the resulting biomass. Understanding who's in that community, how they behave, and what their presence or absence indicates is one of the more conceptually heavy parts of the operator exam, but it's also some of the most practically useful knowledge on the job.
Wastewater treatment relies on five broad groups of organisms, each playing a different role in the process.
| Group | Cell Type | Role in Treatment |
|---|---|---|
| Bacteria | Prokaryotic | The dominant workforce — break down organic matter, drive nitrification/denitrification |
| Archaea | Prokaryotic | Methanogens drive anaerobic digestion; structurally distinct from bacteria |
| Protozoa | Eukaryotic | Polish effluent by feeding on free bacteria; key indicator organisms |
| Fungi | Eukaryotic | Tolerate low pH/low nitrogen; can cause filamentous bulking if they dominate |
| Algae | Eukaryotic | Produce oxygen via photosynthesis; dominant in pond systems |
Bacteria are the most abundant and important microorganisms in any biological treatment process. They're prokaryotic, meaning they lack a true nucleus, and they reproduce by binary fission — one cell splitting into two.
All cells need a carbon source and an energy source. Bacteria split into two broad groups based on where they get each:
Nitrosomonas oxidizes ammonia (NH₃) to nitrite (NO₂⁻). Nitrobacter oxidizes nitrite to nitrate (NO₃⁻). Both are chemoautotrophs — they get their energy from oxidizing inorganic nitrogen compounds, not from organic carbon. That's why nitrifiers grow far more slowly than heterotrophic bacteria and need much longer solids retention times to establish a stable population.
| Group | Growth Range | Optimum |
|---|---|---|
| Cryophilic (Psychrophilic) | Below 20°C, usually 12–18°C | — |
| Mesophilic | 25–40°C | 35°C |
| Thermophilic | 50–75°C | 55°C |
Most municipal activated sludge and digestion processes run in the mesophilic range. Bacteria will still grow — more slowly — outside these ranges, and can survive temperatures as low as 0°C if cooled gradually.
| Bacteria Type | Genus | Importance |
|---|---|---|
| Nitrifying | Nitrosomonas, Nitrobacter | Oxidizes ammonia to nitrite, then nitrite to nitrate |
| Denitrifying | Pseudomonas, Bacillus | Reduces nitrite and nitrate to nitrogen gas |
| Iron bacteria | Leptothrix, Crenothrix | Oxidizes ferrous iron to ferric iron |
| Sulfur bacteria | Thiobacillus | Oxidizes sulfur and iron — causes corrosion of iron sewer pipes |
| Indicator bacteria | Escherichia, Enterobacter | Indicates fecal pollution (e.g., E. coli) |
| Pathogenic | Salmonella, Vibrio cholera, Legionella | Cause salmonellosis, cholera, Legionnaires' disease |
Run a batch experiment with a fixed amount of food and a population of bacteria, and you get a predictable growth pattern — the same curve that underlies sludge age, F/M ratio, and process control logic across the entire field.
Bacteria adjust to the new environment and produce the enzymes needed to break down the substrate. Readily biodegradable substrate = short lag phase. Hard-to-degrade substrate can mean a long lag phase, or — if the bacteria can't synthesize the right enzymes — the substrate may be toxic and the cells die instead.
Population doubles at regular intervals due to abundant substrate and optimal conditions. This is rapid, unconstrained growth.
Substrate concentration drops, growth rate decreases, and growth rate equals death rate — a dynamic equilibrium with no net population increase. Corresponds to low substrate concentration.
Substrate or nutrients are exhausted. Cells die and lyse, releasing soluble organics that surviving bacteria use briefly. Death rate keeps increasing until the population collapses.
Most biological wastewater treatment processes — including activated sludge run as a continuous-flow stirred tank reactor (CSTR) — operate somewhere between the stationary phase and the death phase. That's intentional: it corresponds to low residual BOD in the effluent, which is exactly what a permit requires. Running too far into the growth phase means high effluent BOD; running too deep into the death phase produces a weak, dispersed floc that won't settle.
Archaea are structurally prokaryotic, like bacteria, but evolutionarily distinct — different cell wall, cell material, and RNA composition. In wastewater treatment, the group that matters most is the methanogens: Methanobacterium, Methanosarcina, and Methanothrix, which produce methane gas from the breakdown of organic matter under anaerobic conditions. These are the organisms that make anaerobic digestion work and that you monitor through pH, alkalinity, and VFA levels.
Protozoa are mostly unicellular eukaryotes that lack cell walls. Most are aerobic heterotrophs; some tolerate low oxygen; a few are obligate anaerobes. They're roughly an order of magnitude larger than bacteria, ranging from a few to several hundred microns.
In wastewater treatment, protozoa act as polishers — they feed on free-swimming (dispersed) bacteria, algae, and particulate organic matter, clarifying the effluent and improving treatment quality. This is exactly why microscopic examination of activated sludge is a standard process control tool: the protozoa population tells you a lot about how the system is performing.
In a mixed biological culture, larger organisms feed on smaller ones, forming a simple food chain that shifts as sludge age increases:
As a treatment system ages — meaning sludge age (SRT) increases — the dominant predator type shifts up this chain. A young, low-SRT system is dominated by free-swimming and flagellated organisms. A mature, well-stabilized system develops stalked ciliates and eventually rotifers. This shift is why microscopic exams are used as a real-time indicator of sludge age and process stability, often faster than waiting on lab results.
| Organism | Size | Disease / Concern |
|---|---|---|
| Amoeba | Variable | Moves via pseudopods ("false feet"); causes amoebic dysentery |
| Giardia lamblia | 8–18 µm long, 5–15 µm wide | Causes giardiasis — cramps, diarrhea, fatigue. Removed via coagulation-flocculation, filtration, disinfection |
| Cryptosporidium | 4–6 µm oocyst | Causes cryptosporidiosis. Chlorination does NOT kill the oocysts — requires enhanced coagulation or ozone |
This is a commonly tested fact: standard chlorination is not effective against Cryptosporidium oocysts because of their thick protective wall. Removal relies on physical processes — enhanced coagulation/filtration — or alternative disinfection like ozone or UV. The 1993 Milwaukee outbreak, which sickened roughly 400,000 people, is the case study most training programs reference for this exact reason.
Fungi are multicellular, nonphotosynthetic, heterotrophic eukaryotes — most are obligate or facultative aerobes. They reproduce sexually or asexually by fission, budding, or spore formation. The three main groups are molds, yeasts, and mushrooms.
Fungi tolerate conditions bacteria don't handle well — low nitrogen, low moisture, and low pH (optimum around 5.6, tolerant range 2 to 9). They can degrade cellulose, which makes them useful in composting. In activated sludge, however, fungal overgrowth is a process problem: when pH drops or nutrients become limiting, filamentous fungi can outcompete floc-forming bacteria and cause bulking sludge that won't settle properly in the clarifier.
Algae are autotrophic, photosynthetic eukaryotes (with one notable prokaryotic exception — blue-green algae/cyanobacteria, which can produce toxins harmful to fish and birds). They have no roots, stems, or leaves, and range from single-celled to multicellular filaments and colonies.
Algae matter most in pond and lagoon systems, where they supply the dissolved oxygen aerobic bacteria need through photosynthesis during daylight — and consume oxygen overnight through respiration, which is exactly why pond DO swings between day and night.
Excess nutrients (nitrogen and phosphorus) cause algae blooms, leading to eutrophication — a green mat forms on the water surface, blocks sunlight, and after the algae die and decompose, dissolved oxygen crashes. The Chesapeake Bay is the textbook example of large-scale eutrophication driven largely by agricultural runoff.
A virus is a noncellular genetic element — metabolically inert outside a host cell, and an obligate intracellular parasite. It consists of a nucleic acid core (DNA or RNA) surrounded by a protein shell called a capsid. Viruses of concern in wastewater are those excreted in large numbers in human feces, including polio virus, hepatitis A, and various enteroviruses.
Drinking water treatment is held to a 99.99% virus removal standard — far stricter than typical wastewater discharge requirements, but it underscores why disinfection at the wastewater plant matters for downstream water reuse and public health protection.
| Topic | Value |
|---|---|
| Bacteria reproduce by | Binary fission |
| Heterotrophic bacteria use as carbon/energy source | Organic compounds (both) |
| Autotrophic bacteria use as carbon source | Inorganic compounds (CO₂, HCO₃⁻) |
| Nitrosomonas converts | Ammonia (NH₃) → Nitrite (NO₂⁻) |
| Nitrobacter converts | Nitrite (NO₂⁻) → Nitrate (NO₃⁻) |
| Mesophilic bacteria range | 25–40°C, optimum 35°C |
| Thermophilic bacteria range | 50–75°C, optimum 55°C |
| Four phases of bacterial growth curve | Lag → Log (exponential) → Stationary → Death |
| Most treatment processes operate at | Between stationary and death phase |
| Protozoa size relative to bacteria | ~10x larger |
| Predator hierarchy | Bacteria → Protozoa → Rotifers/Crustaceans |
| Chlorine effective against Cryptosporidium? | No — requires enhanced coagulation/filtration or ozone |
| Minimum DO for game fish | 4 mg/L |
| Drinking water virus removal standard | 99.99% |
200 multiple-choice questions covering every major wastewater treatment topic — no math required. Detailed explanations with every answer.
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