Wastewater with no sewer: what the regulation actually demands

Working out where the water comes from is half the problem. The other half is where it goes, and it tends to be left until the end of the budget as a single line reading «septic tank» with a round number beside it.

That line is wrong on two counts. The first is technical: what the code requires is not a tank but four units in a row, and the last one needs land and a soil that will absorb. The second is procedural: there is a permit behind it, it is granted by the environmental authority and not by the town hall, and it comes first.

The starting rule has no threshold

A common belief holds that a single house with few occupants falls below any requirement. The articles say the opposite, and say it without qualification:

The general rule on the permit is just as broad: it reaches anyone whose activity discharges into surface water, marine water or into the ground. There is no minimum flow below which the article stops applying.

Notice what the exception really does: it does not lift the technical obligation, it moves it. You stop needing a permit and start needing a design that complies with the technical code. So in both scenarios the question ends up being the same one: what does that code say.

The code nobody could read for months

That code is Resolución 0330 de 2017, issued by the housing ministry and known in the trade as the RAS. And here this blog settles a debt: the article on water stated, inside its own text, that it had not been able to read it. The PDF published by the ministry is a set of scanned pages with no text layer.

The full text is published, with a text layer and with currency notes article by article, in the legal database of the Comisión de Regulación de Agua Potable y Saneamiento Básico. Every figure in this article comes from there. So does the trail of amendments, which is the first thing to look at: 0330 was amended by Resolución 799 de 2021 and corrected by Resolución 908 de 2021, and several of the articles quoted here carry that newer wording, not the original one.

What people call a «septic tank» is four units

This is the part that changes a budget. The septic tank on its own does not comply, and this is nobody’s opinion: it is two articles working together. Paragraph 1 of article 173 requires, in individual systems, a grease trap at the head and an anaerobic filter after the tank. Article 176 then requires a complementary treatment after that.

1 · The grease trap

It goes as close to the kitchen as possible and upstream of the tank. Its job is to keep grease from blocking everything downstream, which is precisely how most of these systems fail.

• —Retention time of 2.5 minutes minimum.
• —Length-to-width ratio between 1:1 and 3:1, depending on its geometry.
• —Working depth to suit the volume, with 0.35 m as a minimum.
• —Cleaning every time it reaches 75% of its grease holding capacity. That is an operating duty, not a suggestion.

2 · The septic tank

It settles and digests. The code allows it expressly for areas with no public sewer network and for scattered rural housing with enough surrounding land to fit the tank together with its post-treatment. That phrase is what turns available land into a requirement rather than a convenience.

• —Hydraulic retention time between 12 and 24 hours.
• —If rectangular, a length-to-width ratio between 2:1 and 5:1. Any other geometry calls for a justified hydraulic design.
• —Two chambers minimum, and the first must hold two thirds of the total volume.
• —A device for venting gases, and a design allowing inspection and maintenance.

What the code does say about the water well is item 7 of article 173, and it is a rule of position rather than of metres: the tank «must be sited downhill of any well or spring used for the supply of water for human consumption». On a sloping plot that decides where the house sits as much as the view does.

3 · The anaerobic filter

The upflow anaerobic filter — FAFA on the drawings — is built either as a chamber attached to the end of the tank or as a separate one. Its filter bed is sized between 0.02 and 0.04 m³ for every 0.1 m³ of daily wastewater, and with plastic filter media half that volume may be used.

4 · The post-treatment, which decides whether it fits

This is where the project is won or lost. The code offers four options and leaves the choice to the designer, «according to the permeability of the soil, the land available and the effluent quality required»:

How much land it takes, using the code alone

It can be bounded without inventing anything, by chaining three articles. The first sets how much drinking water is allotted to each occupant and — this surprises people — it depends on height above sea level, not on income bracket or house size:

The second is the return coefficient: what fraction of the water coming in leaves as wastewater. The code orders it to be estimated from local information and, absent field data, taken as 0.85. The third is the loading rate of the infiltration field: no more than 100 litres per day per square metre.

With that, a house of five people in a warm climate: 5 × 140 = 700 litres of drinking water a day; × 0.85 = 595 litres of wastewater a day; ÷ 100 = 6 m² of absorption area. And the tank, at 24 hours of retention, starts at 0.6 m³ of working volume, which under Table 25 is resolved with a working depth between 1.2 and 2.2 m.

The studies that come before drawing the solution

Article 171 lists five of them. They are worth reading slowly, because three happen on the plot itself and none can be replaced by a supplier’s quotation:

1. 1A site visit to identify nearby surface water, buildings and property boundaries.
2. 2Survey work locating the system in plan and in height.
3. 3Locating the post-treatment, not only the tank.
4. 4Ground characteristics: peak water table levels and, where needed, the infiltration capacity of the subsoil.
5. 5Settling the environmental procedures and obtaining whatever authorisations are required.

The permit: what it asks, how long it lasts, how long it takes

The discharge permit application asks for twenty-two items. These are the ones that surprise anyone expecting a form:

• —An up-to-date land registry certificate on ownership of the property, or sound proof of possession.
• —A land-use statement issued by the municipal authority. So the town hall does appear — as the body that certifies, not the one that authorises.
• —Technical reports, engineering design and detail drawings of the treatment system, with its efficiency conditions.
• —Discharge flow in litres per second, frequency in days per month and duration in hours per day.
• —A risk management plan for the discharge.
• —And for discharge into the ground: field results and data from the infiltration tests, the operating and maintenance manual, the disposal area on a georeferenced drawing, and a closure and abandonment plan for that area.

The procedural deadlines are in the code and they add up. They are not how long it will take — that depends on the authority — but they are the legal floor, and they tell you when a delay has stopped being normal:

What this article will not give you

A price. And it is not squeamishness: there is no published Colombian source with a methodology for the cost of an individual treatment system, and the line item depends on the one thing nobody knows before measuring it, namely how that soil absorbs. An infiltration field in sand and one in clay, for the same house, match neither in area nor in excavation nor in invoice.

Nor do I publish the assessment fee the regional authority charges to process the permit: each one works it out under the system and method of article 96 of Ley 633 de 2000, and there is no national figure. It is one phone call, and it is free. Why this blog gives no construction figures without a methodology is set out in what a square metre costs.

What can be said about the shape of the cost: the precast tank is the cheap, visible part; excavation, post-treatment and the soil test are the expensive part, and they are exactly the three absent from a tank supplier’s quotation. It is the same logic that governs the rest of a rural budget, set out in what a country house costs.

The order worth solving it in

1. 1Ask the regional authority, in writing and naming the plot, whether your case falls under the scattered rural housing exception or needs a permit. The answer changes the whole calendar.
2. 2Run the infiltration test before the concept design, not after. It fixes the area, and the area fixes where the house can stand.
3. 3Measure the water table in the wet season, not in the dry one. It is the figure deciding whether the tank floats and whether a soakaway is viable.
4. 4Place the tank downhill of the water well from the very first sketch. Fixing it later means moving one of the two.
5. 5Budget the four units, not the tank. Trap, tank, filter and post-treatment, each with its excavation.
6. 6Treat the permit as a parallel route that comes first, with its floor of sixty-eight working days.

Whoever settles this at concept stage is spared the awkward conversation. Whoever settles it with signed drawings sometimes finds that the infiltration field will not fit where the house is — and then it is the house that moves.