How a deep biological treatment plant works: zones inside the tank
From the outside, a biological treatment plant is a tank buried in the ground with a lid and a compressor. Inside it is a sequence of zones, each of which transforms the wastewater in its own way, and their order is not arbitrary. Understanding that order explains almost every operating rule: why the compressor must not be switched off, why bleach is harmful, and where excess sludge comes from.
Who does the work here
The work is done by microorganisms, not by materials. The community of bacteria and protozoa living in the body of the wastewater is called activated sludge: to the eye it is a suspension of brown flocs, each floc a colony.
For that community the organic matter in wastewater is food. The bacteria oxidise it for energy and grow their own biomass. Two consequences follow, and they shape the whole design. First, the sludge needs oxygen, in an amount proportional to the incoming organic load. Second, the biomass grows, so it has to be taken out from time to time or it will fill the tank.
Mechanical treatment works the other way round: the process is physical and the result depends on particle size. Dissolved organic matter will not come out on a screen, and that is exactly why settling alone never brings domestic wastewater to a state fit for discharge onto the ground.
The zone without oxygen: denitrification
First in the sequence is the lower zone, where the oxygen content is low. There are two reasons to put it first.
One is primary biological reduction: large, complex organic molecules break down here into simpler compounds that the next stage can oxidise. Wastewater arriving from the house is too "heavy" for aerobic bacteria, and this preparation speeds up everything that follows.
The other reason is denitrification. Nitrogen in domestic wastewater arrives mainly as ammonium, and bacteria in the aeration zone turn it into nitrates. Nitrates are no longer ammonium, but the nitrogen is still in them, and left to itself it would leave with the treated water. In a zone low in oxygen, bacteria breathe the oxygen bound up in the nitrates — and the nitrogen is released to the atmosphere as a gas. Hence the practical arrangement: part of the contents is returned from the aeration zone to the oxygen-free one, so the nitrates end up where they can be broken down.
This is why simply blowing air through wastewater is not enough for deep treatment. Aeration turns ammonium into nitrates; what takes nitrogen out of the water is the zone without oxygen.
The aeration zone: where the main load is removed
This is the part of the process the compressor exists for. Three elements work together:
- ✓fine-bubble aeration — the air is delivered so that the bubbles come out small. The finer the bubble, the more air-to-water contact surface you get from the same volume of air, and the more of the oxygen dissolves. A coarse bubble rises fast and carries most of its oxygen straight back into the atmosphere;
- ✓bio-media — a large surface on which microorganisms settle as a film. Attached biomass is hardier than the free-floating kind: it is not washed out with the water at peak flow, and it survives breaks in the inflow better;
- ✓active microflora in suspension — the sludge itself, which oxidises the dissolved organic matter.
Aeration does a second, less obvious job as well: mixing. Without it the sludge would settle to the bottom and the wastewater would slide past above the layer, barely touching it.
Ammonium is oxidised in this same zone, by a separate, slow-growing group of bacteria. They are more sensitive to a shortage of oxygen and to harmful substances than the ones that break down organic matter. A practical sign of this: when the sludge is overloaded or poisoned, ammonium shows up in the treated water before any of the other readings go off.
The clarifier and what becomes of the sludge
What leaves the aeration zone is a mixture of treated water and sludge, and the two have to be parted: the sludge stays in the plant, the water goes. In the settling zone the flow slows, the sludge flocs sink under their own weight, and the clarified water is drawn off from above and sent to discharge or to further polishing.
The settled sludge goes back to work, but not all of it: biomass grows on the organic matter it has eaten, and the surplus is taken out. That is a routine operation, not a sign of a fault, and it is nothing like emptying a sealed holding tank — what leaves is only the extra biomass, not the whole contents.
How well the clarifier separates is what you see at the outlet. Healthy sludge settles as dense flocs and leaves clear water behind. Sludge damaged by chemicals or by starvation settles badly and goes out with the water — brown turbidity appears at the outlet. Sludge carry-over is therefore a diagnostic sign: it tells you about the state of the biology, not about the clarifier.
Why the operating rules follow from the design
The sequence described above explains restrictions that would otherwise look like nitpicking.
| What happens | What suffers | How it shows |
|---|---|---|
| Power cut of a day or more | aeration stops, aerobic sludge suffocates | odour, turbidity at the outlet, ammonium |
| Chlorine-based cleaners, solvents, medicines in noticeable amounts | sludge inhibited or poisoned | sludge carry-over, treatment lost for weeks |
| Standing idle with no inflow | no food for the sludge, biomass dies off | once the load returns, treatment takes time to come back up |
| Peak discharge above the design figure | the mixture passes the zones faster than the process runs | turbidity, sludge carry-over |
| Filter backwash water with a high salt content | microflora inhibited | treatment degrades gradually |
Every row has the same thing in common: what gets damaged is neither the tank nor the compressor but the living part of the plant. Mechanical trouble can be seen and heard; biological trouble cannot. That is why the problem is usually spotted in the water at the outlet, long after the moment that caused it.
What next
Understanding the process before you buy pays off, because it puts the questions you will have to answer anyway: how reliable the power supply is, how the house is occupied, what goes into the wastewater and where the treated water will end up.
Worth collecting:
- ✓the number of permanent residents and the calculated daily wastewater volume;
- ✓the occupancy pattern and the expected interruptions in inflow;
- ✓reliability of the power supply and whether there is a backup;
- ✓what besides domestic wastewater may enter the drain — a pool, backwash from water treatment filters, a workshop;
- ✓the receiving point for treated water and its height relative to the outlet.
The model range, with capacities and user numbers, is in septic tanks for the home. If half of that list is still missing, start with a site survey rather than a model: a biological plant places more demands on the plot than a sealed holding tank does, and those are settled before the tank is chosen.
