How a septic system is sized in Costa Rica

Ask three builders in a Costa Rican beach town how big your septic tank should be and you may get three different answers. The law, however, only accepts one method. Decree 42075-S-MINAE, the national regulation on disposing of treated wastewater into the ground, sets out how the daily flow is estimated, how the tank is dimensioned and how long the drain field must be. The engineer signs a calculation report, called the memoria de cálculo, that follows those steps and goes in with the construction plans.

This guide explains that method in plain English. It does not repeat the basics of how a septic system works or the general rules; those are in our guide to septic systems in Costa Rica. Here we focus on the numbers, so you can read a design, ask the right questions and spot one that was sized "by eye". The figures below come from the decree as published in 2020; it was later amended, so your engineer should always work from the current consolidated text.

Step 1: estimate the daily flow

Everything starts with how much wastewater the house will produce. For a new home, Decree 42075 accepts estimates based on the water supply standards of AyA, Costa Rica's national water and sewer institute. For an existing property, it accepts the average of the last six months of water bills from the water operator.

Table 3 of the decree's Annex 3 gives the figures per person per day:

Two details matter for foreign owners. First, beach properties use the coastal figure, which is as high as the capital's; you cannot use the lower rural number just because the lot is far from town. Second, the wastewater flow assumes that 80 % of the water you use comes back as wastewater. The decree sets that return factor at no less than 0.8.

The design population

The table gives litres per person, not per bedroom. Choosing the number of people is part of the engineer's job, and it is where many rental villas go wrong. A house used by a couple most of the year can sleep ten guests at New Year. The system has to work on that week, so the design population should reflect the maximum realistic occupancy, not the average.

Step 2: check the 190 m³ line

Decree 42075 defines a septic tank as a solution for single-family homes and other activities with flows up to 5 cubic meters per day, which it equates to 190 cubic meters a month of drinking-water use with the 0.8 return factor.

• Up to 190 m³ a month: a septic tank (or another system approved by the Ministry of Health) with a drain field is acceptable. The tank, the drain field calculation and the percolation test report go in with the construction plans (Article 7).
• Over 190 m³ a month: the wastewater must go through a treatment system approved under Decree 39887-S-MINAE, the drain field needs a sampling and flow-measurement box at its start, and the treated water must meet quality limits with periodic operational reports (Article 8).

A single home is normally far below the line. A small hotel, a group of villas on one system or a restaurant can cross it. If that is your case, read our guide on boutique hotel wastewater treatment.

Step 3: size the tank

Annex 1 of the decree sets minimum requirements for the tank itself:

• Minimum useful volume: 1.2 m³.
• Hydraulic retention time: at least one day. Retention time is the useful volume divided by the average daily design flow, so the tank must hold at least one full day of wastewater.
• Liquid depth: between 1.00 and 2.80 m.
• Length-to-width ratio: between 3:1 and 4:1, with a minimum width of 70 cm.
• Free board: at least 30 cm between the liquid surface and the top of the structure.
• Two-chamber tanks are allowed if the first chamber meets the table; it should be at least half the length, and 60 to 75 % of the design volume is recommended.

An improved septic tank adds an upflow anaerobic filter after the tank to polish the effluent before it reaches the ground. It does not change the flow calculation, but it gives a rental property more margin during busy weeks. See our improved septic tank service.

Step 4: read the percolation test

The drain field is sized from the percolation test, called prueba de infiltración in Spanish. Annex 3 describes it: at least two tests where the field will go, with the hole kept full of clean water for at least 24 hours before measuring, so the result reflects wet-season soil. The rate is reported in minutes per centimeter.

Only rates between 2 and 24 min/cm allow infiltration trenches (Article 14). Each rate has a maximum application rate in Table 2 of Annex 3:

More on how the test is done and what we check on site is on our percolation test page.

Step 5: size the drain field

With the flow and the application rate, Annex 3 sets the calculation:

1. Infiltration area = daily wastewater flow ÷ application rate.
2. Required surface = infiltration area × a precipitation factor of at least 2.5, so the field still works in the rainy season.
3. Total trench length = infiltration area ÷ the trench's effective perimeter. For a standard trench 0.50 m wide with 0.30 m of gravel under the pipe, the effective perimeter is 0.77 m. Other widths and depths use the formula in the decree.
4. Spacing between trenches comes from the required surface divided by the total length.

Annex 2 adds layout rules: trench bottoms and pipes must be level, the longest trench cannot exceed 60 m, there must be an inspection box every 20 m, centerlines must be at least the trench width plus 1.5 m apart (never less than 1.8 m), and on slopes between 1 % and 30 % the trenches follow the contour lines at least 5 m apart. Above 30 % slope, trenches are not allowed at all (Article 16).

A worked example

The numbers below are illustrative only, to show how the method behaves. They are not a design for any real property.

Imagine a three-bedroom rental villa on the coast, designed for a peak of 8 guests:

• Daily wastewater flow: 8 × 300 L = 2,400 L/day (2.4 m³/day).
• Monthly drinking water: 2.4 ÷ 0.8 × 30 = 90 m³, well below 190 m³, so a septic system with a drain field can apply.
• Tank: one day of retention means at least 2.4 m³ of useful volume. With 1.20 m of liquid depth, that is 2.0 m² of floor, for example about 0.82 m wide by 2.45 m long (a 3:1 ratio).

Now the drain field, assuming the test gave 10 min/cm (40.40 L/m²/day):

• Infiltration area: 2,400 ÷ 40.40 = 59.4 m².
• Total trench length: 59.4 ÷ 0.77 = about 77 m of standard trench, for example four lines of roughly 19.3 m each.
• Required surface with the 2.5 factor: 148.5 m², which, together with the minimum spacing and the setbacks, tells you how much garden the field will occupy.

The same villa on different soil:

The lesson is simple: on slow clay the same house needs twice the trench length it would need on fast soil. That is why a percolation test belongs in the decision to buy and design, not at the end of construction.

Where the field can go

The length is only half the problem; you also need a place for it. The field must sit in private green areas, never under driveways, parking, decks or pavers. The water table must be at least 1.5 m below the trench bottom. And the decree's setbacks apply: 1 m from property lines and the nearest building, 1.5 m from drinking-water pipes, 2 m from slopes steeper than 25 %, 3 m from water storage tanks, and the protection areas the Water Law and Forestry Law set around springs, rivers and streams.

On a narrow hillside lot with an ocean view, those rules can decide where the house goes. It is cheaper to place the drain field on the site plan first than to move it after the foundations are poured.

What to ask your designer

If someone hands you a septic design, ask for:

1. The design population and the per-person flow used, with the reason.
2. The percolation test report: dates, locations, depth, water table and the professional's signature.
3. The calculation report showing area, length, spacing and the precipitation factor.
4. A site plan with the field, setbacks, slope and the inspection boxes.

If any of those is missing, the system was probably sized by habit. Problems usually show up as smells or overflows in the first heavy rains; our guide on septic smells and overflows explains what to do. For budget questions, see what drives septic tank cost.

Ingeniería de las Aguas, based in Palmares, Alajuela, designs and builds septic systems, drain fields and treatment plants for homes and rental properties across Guanacaste, the Pacific coast, the Central Valley and Limón. If you would like us to review your numbers or size a new system, send us the location, the number of bedrooms and the expected occupancy on WhatsApp, or use our contact page.