Lagoons drainage: which pipe to choose, how to install and connect it so it never clogs

In a treatment bed, we look at the plants and the gravel. But what makes the filter work is underneath: the drain network that distributes or collects water throughout the entire filter bed. If water passes through only part of the gravel, the rest becomes a dead zone, without oxygen, that no longer filters anything and eventually starts to smell bad. Choosing and installing the drains properly is the guarantee of an effective treatment bed for years.

Upflow or downflow: the drain's role changes

Before choosing a drain, you need to know which direction the water flows through your treatment bed.

• In upflow (ascending flow), the pump injects water into the drains at the bottom of the filter bed. The water rises through the gravel and overflows at the surface into the basin. The drains are distributors.
• In downflow (descending flow), water reaches the surface of the treatment bed, passes through the gravel from top to bottom, and is collected by the drains at the bottom, which carry it to a pumping well.

In both cases, the goal is the same: for water to pass through the entire volume of gravel evenly, without a preferential flow path.

Rigid PVC or agricultural drainage pipe?

An agricultural drain is designed to drain water from a field, not to continuously carry water laden with organic matter. For a constructed wetland intended to operate for years, rigid PVC is an investment that quickly pays for itself.

The layout: a regular comb

The most reliable configuration is a comb: several parallel drain lines distributed across the entire constructed wetland area, connected at one end by a perpendicular manifold. Spacing is chosen based on the grain size and depth of the bed, with one simple rule: no area of gravel should be far from a drain. A single drain in the middle of a large constructed wetland guarantees dead zones along the sides.

The perforations

• Size: smaller than the gravel surrounding the drain, so the gravel cannot enter the pipe.
• Total area: the combined area of the holes must significantly exceed the pipe's internal cross-sectional area; otherwise, the perforations limit the flow rate.
• Position: on the sides and underside of the pipe, distributed along its entire length. Downward-facing perforations clog less quickly in downflow systems.
• No geotextile sock around the drains: in a constructed wetland, it becomes covered with biofilm and clogs. The gravel acts as the filter.

Gravel around the drains

Surround the drains with a coarser grade than the rest of the bed, so it remains open and protects the perforations. Lava rock is highly porous, providing considerable surface area for bacteria while remaining lightweight. Beneath the pipes, a protective felt prevents the fittings from damaging the liner.

Connect the network

1. Glue all the fittings with suitable PVC adhesive after cleaning: a fitting that is merely pushed in will eventually come apart under the weight of the gravel.
2. Provide a vertical riser at the opposite end of the manifold, closed with a removable cap above the gravel level. It allows you to flush the network with a water jet or clear out sludge.
3. Pass the liner to the pumping well through a watertight wall penetration, never through a simple hole sealed with silicone.
4. Install a valve on each major branch if the constructed wetland is large: you will be able to balance the flow rates and flush one area at a time.

Maintaining the drains

Even when well designed, a drain network accumulates fine sludge. Once or twice a year, flush it through the vertical risers, ideally in spring and autumn. If the water coming out of an area is black and foul-smelling, this indicates insufficient circulation: increase the flow rate or flush more often.

Also read

To choose the flow direction: Downflow or upflow constructed wetland for my natural swimming pool.

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Frequently asked questions

— What drain diameter is needed for a constructed wetland?

It depends on the pump flow rate and the size of the constructed wetland. The main collector must be able to handle the entire flow without excessive pressure loss; the branches can be smaller. Ask us to size the system if you have any doubts.

— Should the drain be surrounded with geotextile?

No. In a constructed wetland, the sock clogs quickly. Instead, surround the drain with coarser gravel.

— Can agricultural drainage pipe be used?

Technically yes, but it is difficult to clean and may collapse. For a durable installation, perforated rigid PVC is preferable.

— My constructed wetland smells like rotten eggs. Why?

Part of the bed no longer receives enough water and becomes anaerobic. Flush the drains, and check the flow rate and distribution.

In summary

• Perforated rigid PVC rather than agricultural drainage pipe.
• Comb-like layout across the entire surface, with no area far from a drain.
• Holes smaller than the gravel, with a total surface area greater than the pipe cross-section.
• Glued fittings, cleaning riser, watertight wall penetration.

Last updated: September 28, 2026.