Every mechanical treatment plant is a container for organisms doing the actual work. Knowing which ones are present, and what each one prefers, turns a microscope from a curiosity into the earliest warning system you have.
Operators don't treat wastewater. Organisms treat wastewater — operators manage the conditions those organisms live in. Every process control decision you make, from the DO setpoint to the wasting rate, is really a decision about which community you're selecting for. That reframing is what makes the microbiology worth learning rather than memorizing.
Bacteria accomplish the overwhelming majority of treatment. They fall into two groups that behave very differently, and the difference drives most of your process decisions.
| Heterotrophs | Autotrophs (nitrifiers) | |
|---|---|---|
| Carbon source | Organic carbon — the BOD in your influent | Inorganic carbon — CO₂ and alkalinity |
| Energy source | Oxidizing organic material | Oxidizing ammonia (AOB) or nitrite (NOB) |
| Growth rate | Fast | 10–20× slower |
| Sets | How much BOD you remove | The minimum SRT your plant can run |
| Sensitive to | Toxicity, extreme pH | Temperature, DO, pH, alkalinity — nearly everything |
Nitrifiers grow so much slower than heterotrophs that they are always the constraint. If your sludge age drops below what nitrifiers need at the current water temperature, they wash out faster than they can reproduce and nitrification fails — while BOD removal carries on looking fine. This is why winter is when ammonia limits get missed, and why the answer is almost always a longer SRT rather than more air.
The heterotrophs that matter most are the ones that secrete exopolymer — a sticky polysaccharide that glues cells together into floc. Floc is what makes activated sludge work: it lets you separate the biomass from the treated water by gravity in a clarifier and return it to the front of the process.
Without good floc formation you have dispersed growth — organisms treating the water perfectly well and then leaving with it, producing a turbid effluent and a TSS violation. Exopolymer production is affected by sludge age at both extremes: too young and the community hasn't developed it, too old and endogenous decay has broken it down into pin floc.
Protozoa don't do much of the treatment. What they do is tell you where the sludge is in its life cycle — because different groups dominate at different sludge ages, in a reliable sequence.
| Organism | Appears when | What it tells you |
|---|---|---|
| Amoebae | Very young sludge, startup, after an upset | The community is rebuilding from near zero |
| Flagellates | Young sludge, high F/M | Short SRT, recent seeding, or a toxic event that reset the process |
| Free-swimming ciliates | Sludge is maturing; soluble food declining | Transition — the process is heading in the right direction |
| Crawling ciliates | Established floc present to crawl on | Good sign; floc structure has developed |
| Stalked ciliates | Mature, stable sludge | Healthy, well-operated process. They attach to floc and polish the effluent by eating free bacteria |
| Rotifers | Older, well-stabilized sludge | Stable process, adequate DO, low toxicity |
| Nematodes | Very old sludge, long SRT | Sludge age may be running long — check against pin floc |
The progression amoebae → flagellates → free-swimming ciliates → crawling ciliates → stalked ciliates → rotifers → nematodes tracks sludge age from youngest to oldest. You don't need to identify species. Knowing roughly where your dominant population sits on that line tells you whether your sludge age is where you think it is — and it will tell you a day or two before the settleometer does.
Filaments get treated as villains, which is only half right. A modest number of filaments is beneficial — they act as a backbone that larger floc particles form around. The problem is excess: filaments extending beyond the floc hold particles apart, reduce density, and slow settling. SVI climbs, the blanket rises, and solids risk going over the weirs.
The diagnostic value is that different filaments favor different conditions. Identify the organism and you've identified the cause.
| Filament | Condition it favors | What to check |
|---|---|---|
| S. natans, H. hydrossis | Chronically low DO | DO profile through the basin — not just at the probe |
| M. parvicella, Nocardioforms | Long sludge age, high FOG | SRT and grease loading. Often paired with thick brown foam |
| Thiothrix, Beggiatoa, Type 021N | Septicity — H₂S in the influent | Collection system detention; look upstream |
| Type 021N, Type 0041 | Nutrient deficiency, low F/M | N and P against BOD load; sludge age |
Chlorinating the RAS line knocks filaments back, but if the underlying cause is a fouled diffuser starving the basin of oxygen, they return as soon as you stop — and you've damaged your floc formers in the meantime. Chlorination is a bridge while you fix the actual problem, not the fix itself.
Weekly microscopic examination is one of the highest-value habits available to an operator, and one of the least practiced. What it gives you that lab data doesn't is lead time — biological changes show up under the scope one to two weeks before they appear in effluent quality.
What to record each time:
Pair it with the settleometer and SVI. The two together are far more informative than either alone: SVI tells you settling is degrading, microscopy tells you why.
| Condition | Effect on the community |
|---|---|
| Dissolved oxygen | Below ~1.0 mg/L, low-DO filaments gain the advantage. Nitrifiers slow markedly below 2 mg/L |
| pH | Most activity between 6.5 and 8.5. Nitrifiers are the first to suffer outside 6.8–8.0 |
| Temperature | Rates roughly double per 10 °C rise within the normal range. Cold weather hits nitrifiers hardest |
| Sludge age | Selects the whole community. Too short washes out nitrifiers; too long produces pin floc and old-sludge filaments |
| Nutrients | Roughly 100:5:1 BOD:N:P as a working target. Deficiency favors filaments |
| Toxicity | Industrial slugs, extreme pH, and some metals can crash the community in hours. Recovery takes days to weeks |
In Wisconsin this material sits inside subclass A1, Suspended Growth Processes, under the theory and principles chapter.
Activated Sludge: Bugs, Basins & Beyond includes a full microbiology subtopic — indicator organisms, filament identification, and what each condition selects for — inside 150 questions covering the whole process, each with a detailed explanation.
Get the Activated Sludge Guide — $17Instant PDF download · 150 questions · Class I & II tagged