Choosing between a sewage lagoon or septic field (also called a lateral field or drain field) on-site sewage treatment system is based more on necessity than on preference. Government authorities that regulate on site sewage disposal systems decide which system is appropriate for your location.
Both lagoon systems and drain field systems include septic tanks. The tanks allow the heavy solids to settle out of the effluent and the lighter solids to float to the surface. The liquid between the two flows out of the tank for the next treatment stage. Anaerobic bacteria (the kind that can work without air) break down the solids in the tank over time. You need to have the tank pumped out periodically to remove the sludge that accumulates on the bottom.
Both types of systems also have pipe that gets the effluent from the building to the tank and then from the tank to the septic field or lagoon.
Septic fields require soil that is porous and that drains well. You do a percolation test to determine if the soil at your site is suitable for a septic field. Sand to loam types of soil work best for septic fields. Clay, stone and soils with a high water table are limiting soil types for septic fields.
Lagoon systems are not as dependent on soil type to do their work since they are an alternative for sites where septic fields will not work. However, they are limited by the local water table. If there is water close to the surface of the soil, then lagoon systems may not work. Soil morphology tests are the best way to determine if a lagoon system is appropriate for your site.
Septic fields use a system of pipes laid in gravel, or plastic domes set below the soil surface to do the final treatment of the effluent. Aerobic bacteria followed by soil filtration clean the effluent so it is no longer a polluting substance.
Lagoon systems use a pit of a calculated size to handle the final stages of effluent treatment. The waste water flows into the center of the pit from the bottom. Sun, wind, and aerobic bacteria process the effluent. In some cases overflow from the lagoon is sent to an adjacent septic field for final treatment.
Septic fields cost less to construct than lagoons. A septic field relies more on the surrounding soil to do its job, so its size and amount of components needed are less. Lagoons have to be leak proof and may require pumps, manholes for clean outs and heavy liners. These items drive up the costs quickly.
Septic fields require less space because they are installed in locations where soil handles the job. Lagoons depend more on the environment and aerobic activity to clean the effluent so their space needs are greater. Depending upon the soil conditions a lagoon for a two bedroom home could require more than 5,000 square feet. Both systems are limited by the size of the lot, but lagoons are more limited.