Where treated water goes: gravity, pumped discharge, soil polishing
Treating wastewater is half the job. The other half is what to do with the water once it has left the plant, and that is settled by the geometry of the plot, not by the specification of the tank. The awkward part is that the question tends to surface only after installation, by which time the options have narrowed to one expensive answer.
Four receiving points and what each requires
A drainage ditch or roadside channel. The simplest option where a ditch exists and lies below the outlet. It requires permission from the local regulator: it is the regulator who grants the right to take water beyond the plot boundary, and the conditions in the permit differ from site to site. The ditch has to work — one overgrown and silted up will not accept water, and in winter a channel that freezes stops being a receiving point at the least convenient moment.
An infiltration field, filter well or infiltrator. The water goes into the soil on the plot and is polished within it. This depends entirely on the infiltration capacity of the soil: sand and sandy loam accept water readily, clay loam less so, dense clay practically not at all. This is checked by a percolation test in a trial pit at the depth of the future plant, not by the look of the surface layer.
A storage tank for irrigation. Treated water is collected in a tank and used on the plot during the season. An attractive solution, but an incomplete one: irrigation is needed for a few months a year while wastewater arrives all year round. That means a second receiving point for the rest of the time, and the storage tank becomes an addition rather than a replacement.
The ground surface. Discharge onto the surface of your own plot. What matters most here is where the water goes next: a slope towards the neighbours or towards the water intake turns the solution into a problem that will have to be redone. Like a ditch, it requires permission.
All four have one thing in common: the receiving point must take the water in the worst season of the year, not on average across it. In spring, when the soil is saturated and the ditch is full, the receiving capacity is at its lowest — and that is the state to design for.
Gravity or a pump
The range offers both ways of discharging: by gravity and pumped.
Gravity works where the receiving point is below the outlet along the whole route and the fall is maintained without local rises. It needs no electricity, adds no components and has nothing in it to fail.
Pumped discharge is needed where the receiving point is above the outlet or the route cannot hold a fall. A pump solves the problem but adds a dependence on electricity and one more item to service.
| Gravity | Pumped | |
|---|---|---|
| Applicable when | receiving point below the outlet, fall along the whole route | receiving point above the outlet, or fall unattainable |
| Dependence on electricity | none | yes, in addition to aeration |
| Items to service | none | pump, level control |
| Behaviour during a power cut | keeps working | discharge stops until power returns |
| What is checked at installation | fall, absence of counter-slopes | supply, level control, non-return valve |
One practical consequence of power cuts. When the supply fails, aeration stops and so does pumped discharge, which means the plant stops treating and stops discharging at the same time. If cuts on the site are a regular event, a gravity scheme is the better bet — and that is an argument for hunting down a receiving point below the outlet even at the cost of a longer route.
What levels decide, and capacity does not
The difference between the two schemes is set by geometry, and it is established by levelling rather than by eye. Four heights have to be measured:
- ✓where the drain leaves the house;
- ✓the inlet into the plant;
- ✓the treated water outlet from the plant;
- ✓the receiving point: the bottom of the ditch, the top of the infiltration field, the neck of the storage tank.
Every step must go downwards. One section where the receiving point turns out to be higher is enough to make the whole scheme a pumped one. Hence a common survey result: the plant has been chosen correctly, but the route requires a pump because the plot is flat or the house sits in a hollow.
The second thing levels decide is the depth of the inlet. The plant is positioned so that the inlet pipe lies below the frost line with a constant fall; over a long distance from the house that takes the inlet to a considerable depth, and the outlet follows it down. The deeper the outlet, the smaller the chance that the receiving point will be below it.
What is not done with treated water
Three decisions look like a saving and go on to create problems that cost more than the saving was worth.
Discharge into a bottomless pit, or into the soil without a proper receiving point. What separates this from an infiltration field is not the shape but the calculation: a field is designed around the infiltration capacity of the soil and spreads the water over an area, whereas a pit takes everything at a single spot. The soil there clogs up — the pores seal with suspended solids — and its capacity drops to zero within a few seasons.
Discharge towards your own borehole or well. Even treated water must not find its way into the aquifer you draw water from. The engineering principle holds whatever the jurisdiction: the receiving point goes downgradient of the water intake with respect to groundwater flow, never upgradient. Check this on your own plot and with the neighbouring ones in mind.
Connecting into a stormwater system without permission. Storm drainage and treated domestic wastewater are different flows with different rules. Routing the second into the first is possible only where the local regulator explicitly permits it for that particular site.
What next
The receiving point is decided together with the model, not after installation. Collect:
- ✓levelling data: the outlet from the house, the inlet and outlet of the plant, the receiving point;
- ✓the soil type at the depth of the plant and the result of a percolation test in a trial pit;
- ✓the groundwater level at its seasonal maximum;
- ✓whether there is a ditch or channel, and what state it is in during winter;
- ✓the area available for an infiltration field, if the soil is to be the receiving point;
- ✓reliability of the power supply — it determines whether a pump is acceptable in the scheme;
- ✓the local regulator's conditions for discharge beyond the plot boundary.
Models with gravity and pumped discharge are collected in the household wastewater treatment section. If the levels are what is missing, start with them: levelling a plot takes an hour and answers a question that otherwise comes up after the tank has been buried.
