Septic tank installation and start-up: pit, anchoring, first winter
A correctly sized plant still works badly if it goes into the ground wrong. Installation mistakes differ from sizing mistakes in that they do not show up straight away, and the only way to put them right is to dig the tank out again. Here are the stages in order — from the pit to the first winter.
The pit, the base and backfilling
The location is chosen before any earthworks, and four constraints define it at once: the distance to where the drain leaves the house, the distance to the water source, the fall on the inlet pipe, and whether a vacuum truck can reach the access hatch. The last of these is what people think of last, and it deserves to come first: if the hose cannot reach the hatch, pumping out excess sludge becomes a problem at every service visit.
The pit is dug with an allowance beyond the tank dimensions so that there is room for backfilling around the perimeter. The base is levelled and compacted — the tank must bear on its whole bottom area, not on three points. Uneven support produces a tilt, and a tilt in a vertical tank disturbs the zones inside it: their levels are designed for a vertical position.
Backfilling is done in layers, with the tank being filled with water as the work goes on. The point is to balance the pressure: soil backfilled before the tank is filled presses on the walls from one side and can deform them. Water inside balances that pressure from within.
The backfill material deserves separate attention. Frost-susceptible soil expands when it freezes and presses on the tank; sand, which does not behave that way, is preferable for backfilling around the perimeter. Heavy machinery is kept off the ground above the tank, both during the works and afterwards.
Anchoring: why an empty tank is pushed out
Here is a phenomenon that seems impossible until you meet it: a buried tank rises out of the ground. The cause is buoyancy. A tank filled with air and submerged in water-saturated soil behaves like a float: the water tends to push out a body whose volume it would otherwise occupy.
Two situations are dangerous:
- ✓the spring groundwater maximum — the level rises to its seasonal peak and the soil around the tank becomes saturated;
- ✓a completely emptied tank — during servicing, winterisation or media replacement there is air inside instead of water.
It is the coincidence of those two conditions that pushes the plant up. Two practices follow. The first: where the groundwater level is high, the tank is anchored — strapped to a concrete slab or to anchors rated for the buoyant force. The second is operational: the tank must not be fully drained for a long period, and if that is unavoidable for works, the backfill and the water level outside are taken into account in advance.
The groundwater level is taken at its seasonal maximum, not from what is visible in a dry summer month. The difference between the summer and the spring level on the same plot can exceed a metre, and the decision on anchoring is made on the spring figure.
Connections: inlet pipe, ventilation, electrics
The inlet pipe. Laid with a constant fall towards the plant, with no counter-slopes and no sagging between supports. A local reverse slope is a place where water collects and a blockage forms; in winter it freezes first. The pipe must lie below the frost line or be insulated, and on a long run an inspection point is put in so that it can be cleared.
Ventilation. Biological treatment gives off gases and they need a way out. This works in combination with the internal plumbing of the house: a vent stack taken up to the roof keeps air moving through the system. Non-functioning ventilation is the most common cause of an odour that gets blamed on the plant. The second most common has nothing to do with the plant at all: a dried-out trap in a rarely used drain inside the house lets air from the drain into the room.
Electrics. The compressor needs a permanent supply, and the cable is laid in a protective conduit with an allowance for ground settlement. A dedicated circuit with a protective device is preferable to plugging into an extension lead outdoors: aeration stopping because of a tripped socket costs more than the circuit itself.
Discharge of treated water. The gravity option requires the receiving point to be below the outlet along the whole route. If that condition is not met, the water is pumped out instead — and the system gains one more component that depends on electricity.
Commissioning and the first weeks
After installation the plant does not start treating wastewater instantly: the activated sludge has to grow. A full biological community forms over weeks rather than days, and special rules apply during that time.
- ✓The tank is filled with water and aeration is switched on before wastewater is admitted, not at the same time.
- ✓The load is increased gradually; whether the plant has reached its design treatment quality is judged by the water at the outlet, not by the calendar.
- ✓In the first weeks no chemicals go down the drain at all: young sludge is more sensitive than mature sludge.
- ✓Clarity of the water at the outlet is the main available indicator in the first month. Turbidity and a brown tint mean either that the process has not settled yet or that the load has grown faster than the biomass.
- ✓Seeding the plant with sludge from another working plant speeds up the start-up, but requires the source to be healthy.
A restart after a long idle period follows the same rules. A plant that has stood without load for months is effectively a new start-up: the sludge there has died off and has to grow again.
The first winter and winterisation
How the plant winters depends on whether the house is lived in.
| House occupied year-round | House unoccupied in winter | |
|---|---|---|
| Aeration | runs continuously | stopped, compressor taken indoors |
| Water level in the tank | working level | left in place, never fully drained |
| Inlet pipe | heat of the wastewater prevents freezing | emptied, fall checked |
| Main risk | a long power cut | the empty tank floating up in spring |
| In spring | nothing | a new start-up: the sludge has to grow |
The warmth of domestic wastewater is a factor that gets underrated: in a house occupied year-round it protects both the pipe and the contents of the tank from freezing. As soon as wastewater stops arriving, that protection disappears and insulation becomes mandatory.
What next
Check the following before installation:
- ✓the groundwater level at its seasonal maximum and the decision on anchoring;
- ✓the soil type and the backfill material;
- ✓the fall on the inlet pipe and its depth relative to the frost line;
- ✓working ventilation of the drain stacks in the house;
- ✓a dedicated power circuit with protection;
- ✓the height of the receiving point for treated water relative to the outlet — this determines whether a pump is needed;
- ✓access for a vacuum truck and the hose length to the hatch.
Installation, commissioning and servicing of the home septic tank range are all done in Georgia. If the plant has not been chosen yet, the order is the reverse of the intuitive one: site data and occupancy pattern first, model second — half the options are ruled out by the groundwater level and the receiving point rather than by capacity.
