House wastewater: from site conditions to a drainage scheme

Water supply for a house is usually planned well in advance, while wastewater is left for later and then settled in a hurry, once the house is already standing. Yet the choice has little to do with your budget or the size of the house: it is decided by the plot itself — the soil, the groundwater level and the lie of the land. Here is what you are choosing between, and what you need to know about the site before any particular installation is even mentioned.

Four ways to deal with wastewater

Municipal sewer. If a main runs along the street and a connection is technically possible, the question is closed: what follows is a connection design and a contract with the utility. The rest of this article is about the case where there is no main.

Sealed holding tank (cesspit). This is not a treatment facility but a buffer. No treatment takes place in it at all: the whole flow collects and is carted away by a vacuum truck. The advantage is that it depends on nothing — not the soil, not the power supply, not what you pour into it. The drawback is the running cost, which is directly proportional to how much water you use. In a permanently occupied house the volume of wastewater is close to the volume of water consumed, and the truck becomes a regular line in the budget.

Septic tank with soil treatment. A multi-chamber vessel in which solids settle, fats float and organic matter partly ferments without oxygen. Clarified water leaves the last chamber for a drainage field, a soakaway well or an infiltrator, and is finally treated by the soil itself. This arrangement needs no electricity. But it works only as far as the soil is willing to take the water.

Biological treatment plant. An aerated unit in which the wastewater is worked on by activated sludge — a community of microorganisms that live on the organic matter in the flow. A compressor feeds air continuously, the sludge oxidises the pollutants, and the solids separate in a settling zone. What comes out is water that, if the local regulator permits, goes to a drainage ditch, onto the surface or into a tank for irrigation. It needs an uninterrupted power supply and an uninterrupted flow of wastewater.

What actually decides the choice

The order here is the opposite of the usual one: look at the plot first, and only then at a catalogue of installations.

Soil type and how well it absorbs. Sand and sandy loam take water readily, loam noticeably less so, and dense clay takes practically none. You establish this not by eye but with a percolation test: dig a trial pit to the depth of the future drainage field, pour in a measured volume and time how long it takes to soak away. If the soil will not take water, soil treatment is ruled out entirely — and no amount of extra septic tank volume will change that.

Groundwater level. What matters is not the level you see in summer but the seasonal peak, usually in spring. High water means two things at once. A drainage field stops working, because saturated soil leaves the water nowhere to go. And the vessel itself can be pushed up out of the ground by buoyancy when it is emptied, which is why it is anchored to a concrete slab.

The lie of the land. Wastewater runs by gravity, so if the outlet ends up higher than the house, the scheme gains a pumping station — one more unit that depends on electricity. A hollow on the plot collects surface water and makes conditions for soil treatment worse.

Distances. To the borehole or well, to the house, to the boundary, to a watercourse or a roadside ditch. The actual figures come from the local regulator and differ from site to site — take them from the requirements for your plot, not from an article on the internet. One engineering principle holds in any jurisdiction: put the installation downstream of the water intake in the direction the groundwater flows, never upstream.

Volume and character of the flow. The number of permanent residents sets the daily volume; the appliances set the surge load — a bath emptying, a dishwasher and a washing machine running, showers in two bathrooms at once. The installation is sized against both figures, and it is usually the surge that turns out to be the constraint.

Why permanent occupancy matters so much

Biological treatment is a living process, and that is not a figure of speech. Activated sludge needs two things without a break: organic matter from the wastewater and dissolved oxygen from the compressor. With no flow coming in, the sludge starves and dies off. With no electricity, aeration stops and the sludge suffocates within hours.

This has a direct consequence for a weekend house. A biological treatment plant that works two days a week and sits idle for five will degrade. After a long idle spell it has to be restarted, and it will not reach its design quality straight away — the sludge needs time to build biomass back up. For seasonal living, a septic tank with soil treatment takes it better: the anaerobic process copes with pauses far more easily, and the soil does not lose its properties over the winter.

The same principle explains the limits on what may go down the drain. Chlorine-based cleaning products in quantity, solvents, medicines, filter backwash water with a high salt content — all of it knocks the sludge back. A holding tank barely notices, a septic tank notices a little, a treatment plant notices a great deal.

What happens if untreated wastewater goes into the ground

The commonest and most expensive way to save money is to send the flow "straight into a drain": a ditch, a bottomless pit, an old barrel with holes punched in it. There are two consequences, and both land on the owner rather than the neighbours.

The first is your own water source. Wastewater filters down vertically into the very aquifer that the borehole or well on your plot draws from. Wells and shallow boreholes into perched water are the most vulnerable. It surfaces in the water analysis as rising nitrates and microbiology, and no amount of paperwork will fix it: a contaminated source has to be re-drilled into another aquifer or replaced by delivered water.

The second is silting. Untreated wastewater carries solids and fats that blind the soil — they seal its pores. The absorption capacity of the site drops to zero in a matter of seasons, not years. Flushing will not bring it back: the field has to be rebuilt from scratch, with the soil dug out and replaced. That is precisely why mechanical and anaerobic clarification ahead of soil treatment is not an optional upgrade but the condition for the field lasting longer than two years.

Running it: emptying, winter, smell

None of these schemes works without servicing, and how much servicing they need differs enormously.

Emptying. A holding tank is emptied in full and often. A septic tank has the sludge that builds up in the first chamber taken out from time to time. A biological plant has its surplus sludge drawn off — less often and in smaller amounts than the contents of a holding tank. One question always gets remembered last: can the vacuum truck get in, and where can it stand relative to the access hatch. If the hose will not reach, the whole thing becomes harder to live with, whatever is buried in the ground.

Winter. The vessel goes below the frost line or is insulated, and the inlet pipe is laid at a constant fall, with no counter-slopes where a blockage can form. A plant shut down for the winter is not drained dry: an empty vessel gets pushed out of the ground by groundwater. The compressor comes off for the idle period and is kept somewhere warm.

Smell. A facility that is working properly hardly smells at all. A smell that appears is a diagnosis, not a feature: either the soil stack vent is blocked, or the biology has died, or the tank is full, or a trap under a rarely used drain in the house has dried out. Deal with the cause instead of pouring in air freshener.

What next

Choosing a scheme starts with data about the plot. Collect it before any particular installation comes up in conversation — without it, every answer is guesswork:

✓the number of permanent residents and the pattern of use: year-round, seasonal or weekends;
✓the daily volume of wastewater and the largest surge — bath, washing machine and dishwasher, number of bathrooms;
✓whether a municipal sewer runs along the street and whether a connection is technically possible;
✓the soil type at the depth of the future installation and the result of a percolation test in a trial pit;
✓the groundwater level at its seasonal peak;
✓the lie of the plot and the height difference between the outlet point and the point where the pipe leaves the house;
✓distances to the borehole or well, to the house, to the boundary, to a watercourse or ditch;
✓where the treated water is to go: a ditch, the surface, a tank for irrigation, a drainage field;
✓how reliable the power supply is and whether there is a backup;
✓whether a vacuum truck can get to the tank;
✓what the local regulator requires for a discharge on this particular plot.

Send us these figures and it will be clear which of the four schemes is workable on your site at all, and we will go through the options together. Starting with a model of installation before you know the soil and the groundwater level is pointless: half the options fall away on those two points alone.

If the choice has already settled on a biological treatment plant, the models, their throughput and the number of users they serve are gathered in the septic tanks for the home section.