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How to Build a Small Lagoon for Effluent

Janis Fedorowick

A backyard lagoon is a viable way of replacing or enhancing a septic system. Effluent from the house can be piped directly to the lagoon, or it can be sent to a holding tank as an intermediate step. A general guideline is that the size of the lagoon should be 500 square feet per person. For a four-bedroom home, the surface area of the lagoon should be approximately 2,000 square feet. Typically, the minimum lot size for a lagoon is 1/3 acre so that there is enough space for the lagoon to be surrounded by berms to contain any overflow in the event of a severe storm.

The most significant factors affecting the construction are the soil characteristics and the hydrology. The bottom of the lagoon must be an adequate distance above the water table, and an EPDM pond liner (or equivalent) must be used if the soil is anything other than heavy clay.

Install a grassy berm around the lagoon to contain overflow.

Step 1

Stake out the location of the lagoon based on approved construction drawings.

Step 2

Excavate the lagoon to a minimum depth of 5 feet, and stack the fill to create berms. Ensure that the bottom of the lagoon is flat and the sides have a 3:1 slope.

Step 3

Excavate a 2-foot-wide area for the inlet pipe (and overflow pipe if required), to a depth that will accomodate the pipe plus two layers of aggregate and a layer of fill. Lay down 4 inches of clear aggregate in the trench.

Step 4

Install the inlet pipe from the house to the pond edge, ensuring that there is a positive slope -- the ideal slope is 1.4 inch per foot. Install cleanout valves every 75 feet. Install the overflow pipe with a positive slope to discharge the area. Cover the pipe with clear aggregate and fill the excavated area to the surface.

Step 5

Compact 6 inches of soil in the bottom and on the sides of the lagoon, with a roller. Add two more layers of soil for an additional depth of 12 inches.

Step 6

Extend the inlet pipe to the center of the lagoon and secure it to a poured-in-place concrete slab (2-by-2 feet minimum). Install the EPDM liner (or equivalent), if required.

Step 7

Hydroseed the lagoon sides and berm with the grass mixture. Allow effluent to fill the lagoon to an ideal depth of 2 to 5 feet. Install a safety fence at the perimeter.