Septic tank retrofit: turning an existing tank into biological treatment
On many plots the on-site sewage system is already built: chambers in the ground, an inlet pipe, an outlet. Odour, cloudy water at the outlet and frequent emptying usually point not to a bad tank but to a tank that works only as a settler. If the shell is intact and the volume matches the load, the tank can often stay in place while the process inside it changes.
Why a septic tank settles but does not treat
A conventional septic tank works by settling. Heavy particles sink to the bottom, fats and light matter float, and the water passes to the next chamber or goes to soil polishing. Dissolved organic matter mostly stays in the water.
There is almost no oxygen in a closed tank, so conditions inside are anaerobic, meaning without dissolved oxygen. Organic matter breaks down into hydrogen sulphide and other odorous compounds. Emptying removes the accumulated sludge but does not change the process: a few months later the picture repeats itself.
To change treatment quality, part of the volume has to become an aerobic bioreactor — a zone where activated sludge (a community of microorganisms that oxidise pollutants) receives oxygen and stays in constant contact with the wastewater. Converting an existing structure this way without replacing it is called a retrofit.
How an airlift module works
The core of a retrofit kit is a vertical guide pipe inside the aerated chamber. The pipe has inlet openings at the bottom, a disc aerator slightly above them and outlet openings at the top. Air is supplied to the aerator by a compressor placed outside the septic tank.
The bubbles rise up the pipe and drag water with them. The water-air mixture inside the pipe is lighter than the surrounding liquid, so a steady upward flow forms in the pipe — this is the airlift. Water and sludge are drawn in near the bottom, pass the aerator zone, leave at the top and sink back through the main volume of the chamber to the inlet openings. The air both saturates the water with oxygen and mixes it without a mechanical stirrer.
Circulation is what distinguishes such a module from a diffuser simply lowered to the bottom. A single diffuser creates a strongly aerated patch around itself, while in the far corners of the chamber the water barely moves: sludge settles there and turns anaerobic again. That is why the design depends not only on airflow and aerator size but also on the pipe diameter, its immersion depth, the area of the inlet and outlet openings and its position in the chamber.
Drawing water from the bottom brings into circulation the zone most prone to silting. But if a thick layer of old mineral sediment lies on the bottom, the module will start lifting it. So an old septic tank is inspected before a retrofit and cleaned if necessary.
The compressor sits in a utility room or an outdoor cabinet, and only an air line runs into the tank. There are no electric motors or rotating parts inside. A long line increases pressure losses, so the compressor is selected by airflow at the actual operating pressure rather than by the rated figure in l/min.
Aeration cycle, nitrogen and bio-media
The compressor does not have to run around the clock. For many sites the starting setting is 45 minutes of aeration and 15 minutes of pause: it runs 18 hours a day instead of 24. During aeration the water circulates and pollutants are oxidised; during the pause the microorganisms keep consuming oxygen and its concentration falls. The 45/15 cycle is a starting point, not a universal setting: cycle length depends on load, temperature, chamber volume and sludge concentration. In an extended system it is adjusted by a dissolved oxygen (DO) sensor.
In the aerobic phase nitrifying bacteria develop. They convert ammonium nitrogen to nitrites and then to nitrates: NH₄⁺ → NO₂⁻ → NO₃⁻. When the oxygen runs out, denitrification becomes possible — the reduction of nitrates to nitrogen gas. There is a limitation here: when the air is switched off the airlift stops too, so how complete the oxygen-free stage is depends on the design. If deep removal of total nitrogen is required, the scheme is calculated separately, sometimes with additional zones or mixing.
The chamber can be supplemented with fixed or moving bio-media — a carrier with a large surface area on which a biofilm grows. It keeps more biomass in the reactor and makes the process more resistant to load swings. This helps where wastewater arrives unevenly: in a house that is not occupied all the time, a guest house, a seasonal hotel, and also when the chamber volume is small. Whether media is needed, and how much, is decided by calculation.
No treatment without settling
Aeration alone does not produce clean water at the outlet. After biological treatment the sludge has to be separated from the water; otherwise it leaves with the effluent and raises suspended solids and COD (chemical oxygen demand, a measure of organic matter) at the outlet. That is why the secondary settling zone is identified during the survey: the next chamber, a separate compartment, part of the existing tank or an additional vessel. A complete retrofit means aeration, circulation, biological treatment and sludge separation together.
A retrofit does not make up for too small a volume. If the chamber is small for the daily flow, a more powerful compressor will not help: the biology will not have enough time to do its work. Nor does it fix constant hydraulic overload, a damaged or leaking shell, a badly arranged outlet or substances toxic to bacteria entering the system.
After biological treatment the water is still wastewater. If the site needs disinfection, phosphorus removal, deep denitrification or water for reuse, polishing is provided after the main stage. Where to send the water afterwards is covered in the article on discharging treated water.
Which septic tanks can be retrofitted
An existing tank sets constraints that do not exist when a new plant is designed: the shape of the tank is already given, and the equipment is selected to fit it. A compact round or square chamber usually needs one central module; a long one may need two or more. Before the calculation, the following are checked:
- ✓condition and water-tightness of the shell
- ✓working volume and water depth relative to the daily flow
- ✓position of partitions and overflows
- ✓layer of accumulated sediment
- ✓whether aerobic and settling zones can be separated
- ✓where the treated water will be discharged
- ✓space for the compressor and length of the air line
- ✓access to the manholes for maintenance
If the tank passes these checks, the most expensive part of the system is kept — the tank itself, and often the inlet sewer, overflows, outlet and drainage as well. Earthworks do not have to be repeated. A retrofit suits private houses and guest houses, small hotels, cafés, cottage settlements and old on-site treatment plants — wherever the shell is in good condition but treatment quality is not satisfactory.
What next
For a preliminary assessment, prepare photos of the septic tank, the number of chambers, the dimensions and water depth of each chamber, and the position of the inlet and outlet pipes. Add the number of permanent residents, the approximate water consumption and a description of the problem: odour, cloudy water, overflowing, sediment. Give the distance to the place where the compressor could be installed. From this data it is clear whether a retrofit makes technical sense and which scheme to consider.
If the survey shows that the shell is damaged or the volume is too small for your load, replacing the tank is the more sensible option. How a new plant is built is described in the article on the zones of a deep biological treatment plant, and ready-made models are collected in septic tanks for the home.