Building a house in the countryside or owning a rural plot without a sewer connection raises the same question sooner or later: what do we do with the wastewater? For decades the almost automatic answer was the septic tank, because it was cheap and it was what everyone knew. Today there is a certified alternative that really treats the water, works without chemicals and costs less in the long run: the biological treatment plant.
The decision matters more than it seems. It is not just about completing a formality, but about protecting the soil where your children play, the groundwater that many neighbours draw their water from and the value of your property. It is worth taking the time to understand what each solution does and why a septic tank that seems good enough today can become a problem within a few years.
What a septic tank does and what it does not
A septic tank is essentially a holding tank. Wastewater flows in, the solids sink to the bottom and form sludge, and the water on top overflows into a soakaway or a drain, from where it seeps into the ground. That is all it does: separate and let through.
What it does not do is treat the water. Inside the tank, without oxygen, the organic matter breaks down anaerobically, the same process that produces a rotten smell and gases. The water that leaves the tank is still heavily polluted and reaches the soil practically untreated. At best, a septic tank is a preliminary step, not a treatment.
Why a septic tank fails over time
The underlying problem is that a septic tank relies on the ground to get rid of the water. As long as the soil around the drain absorbs well, everything seems to work. But the fine solids and fats that the tank does not hold back gradually clog the pores of the soil and the infiltration lines. Little by little the soil stops absorbing.
When that happens, the water no longer goes down: it comes up. It starts to overflow, surfaces on the ground and floods the garden, right where children play and animals roam. The bad smell appears and, worst of all, there is hardly any fix: saturated soil cannot be washed clean. On ground with poor absorption this can happen within one to three years. The septic tank did not break down, it simply did the only thing it knows how to do until the ground gave up.
The biological alternative
A biological treatment plant works the other way round: instead of hiding the water underground, it treats it. A compressor blows in air and keeps millions of aerobic bacteria alive, which use that oxygen to consume the organic matter in the wastewater. It is pure biology, without chemicals and without additives.
The result is clear, odourless water, with a treatment efficiency of around 95%. This effluent leaves the plant with safe values and no longer clogs the ground the way untreated water from a septic tank does. Through infiltration into the ground it reaches the roots of plants and trees and returns to the natural water cycle. Irrigation is only permitted in this way, from below through the soil, not from above onto plants, trees or crops.
That is why the biological plant solves exactly the cases where the septic tank fails: clay soils that absorb poorly, a high groundwater table that leaves no room for infiltration, and sites with little available land. Where a septic tank clogs up, a biological plant keeps delivering clean water.
Septic tank versus biological plant.
| Criterion | Biological plant | Septic tank |
|---|---|---|
| Treatment efficiency | Around 95% | Low, only settles solids |
| Load on the groundwater | Low, clarified water leaves the plant | High, seeps in untreated |
| Odour | None, aerobic process | Frequent, anaerobic process |
| Maintenance | Predictable, one visit a year | Uncertain, until it clogs up |
| Tank warranty | Up to 25 years from the manufacturer | Often only 3 to 5 years |
| Lifetime costs | Predictable, no risk of collapse | Rising, with a risk of remediation |
What the regulations require: the example of Chile
Every country regulates this in its own way; you will find the rules for yours on its country page. Chile is a good example. There, wastewater from a home without a sewer connection is governed by the Código Sanitario (Sanitary Code) and the Reglamento General de Alcantarillados Particulares (General Regulation on Private Sewer Systems, Decreto 236 of 1926, still in force and updated). Its article 5 is clear: before it is infiltrated or discharged, the water must be treated until it is free of putrescible organic matter and contains fewer than 1,000 faecal coliforms per 100 millilitres. The regulation does not require a particular technology, it requires a result.
That difference changes everything. A simple settling tank rarely achieves these values reliably, while a biological plant meets them comfortably. In addition, the health authority may inspect the installation every year. Seen this way, the regulation is not a threat: it is the best description of what you need to protect your soil, your family and the value of your plot. Complying with it is, above all, looking after what is yours.
What the cheap option really costs
There is an irony that repeats itself on many building sites: a lot of money goes into a beautiful, solid house, and when it comes to wastewater treatment people look for the cheapest option. The septic tank wins that comparison on day one, because it costs little to install. The problem comes later.
When the ground becomes saturated, costs go up: more frequent emptying, drains that have to be rebuilt and, in many cases, a complete job to replace the system, with the garden dug up. In the end you pay twice. The biological plant requires a higher initial investment, but its lifetime costs are lower and predictable, and it maintains, and even increases, the value of the property. Saving on the wastewater solution is often the most expensive saving of the whole build.
Septic tank or biological plant, answered.
A septic tank only separates: it holds back the solids and lets the water seep into the ground almost untreated. A biological plant treats: with oxygen and aerobic bacteria it consumes the pollution and delivers clear, odourless and safe water. One lets solids settle, the other really treats the water.
Over the years, fine solids and fats clog the pores of the soil and the ground stops absorbing. Then the water does not go down, it comes up: it overflows, surfaces in the garden and the bad smell appears. Saturated soil can hardly be fixed. On ground with poor absorption this can happen within one to three years.
Yes, these are exactly the cases where it works best. Because it delivers water that has already been treated, it does not depend on the soil absorbing large volumes of untreated water. It works on clay soils, with a high groundwater table or with little space, where a septic tank clogs up quickly.
Yes. Water treated with AQUATO can be used through infiltration into the ground, directing it towards the roots of plants and trees, always in line with local conditions and the regulations in force. But we must make this absolutely clear: treated water must not be spread from above onto plants, trees or crops. Only irrigation from below is permitted.
A biological plant needs a periodic check, usually one visit a year, plus sludge removal depending on use. It is predictable maintenance that can be covered by a service contract. A septic tank, on the other hand, usually runs without any checks until it fails.
The septic tank costs less on day one. But when the ground becomes saturated, frequent emptying, rebuilt drains and often a complete replacement job follow. The biological plant requires a higher initial investment, followed by lower and predictable costs, and it preserves the value of the property.
That depends on the country. In Chile, for example, the Reglamento de Alcantarillados Particulares (Decreto 236) requires the water to leave free of putrescible organic matter and with fewer than 1,000 faecal coliforms per 100 ml, without prescribing a specific technology. The permit for a domestic plant is granted by the competent authority. Ektennsa prepares the technical documentation together with the professional in charge of your project.
Usually not. The old septic tank is taken out of service and removed, not refurbished, because old tanks are almost always heavily contaminated. The new plant arrives pre-assembled and can often be installed using the existing connection, without digging up the whole plot. Depending on the size, installation takes one to three days.
