Managing Septic Lagoons During Drought

Jim Crawford, University Extension

With the extended dry conditions, we are experiencing, I am receiving inquires regarding septic lagoons for single family homes going dry.  People are concerned they will not function correctly, do they need to add water, will it hurt the lagoon long term or if they should be concerned at all.  Rainfall for most of the state since last summer is far below normal so these are very valid questions.

Lagoons lose water two ways: evaporation into the air or through leaks in the structure.  Lagoons in Missouri are sized based on the number of bedrooms in the structure it is serving with a three-bedroom home needing 1320 square feet of surface area. Proper operating depth is a minimum of 3 feet deep. 

The average summertime evaporation from the surface of a small pond or sewage lagoon is approximately 1/3 of an inch per day.  It would take an average daily inflow of 275 gallons per day of wastewater and rainfall to maintain the proper operating depth in a lagoon with 1320 square feet of surface area.

Design standards for lagoons in Missouri state that percolation (leaks) cannot exceed 1/8 inch per day.  This equates to approximately 103 gallons of water per day.  Assuming both evaporation and percolation are occurring, a total of 378 gallons of inflow or rainfall are required to maintain proper depth. 

The Missouri Department of Natural Resources estimates that the average person uses 50 gallons of water per day with 80% of that entering the waste stream.  This water usage does not include watering the lawn, washing the car etc. as that water does not enter the lagoon.  For a family four, that amounts to 160 gallons per day entering the septic lagoon.  I believe with the adoption of low flow faucets, showers, and toilets as well as washing machines that use much less water than the older tub style, plus the fact many homes have parents working all day and kids in school, the actual usage at home is much less.  This total is almost 2½ times less that what is needed to keep the lagoon at operating level. 

Average rainfall in Missouri is about 40 inches per year or 0.1 inches per day.  But as we know, rainfall does not come every day or in such small increments, so it is difficult to count on rainfall to help maintain lagoon level especially when we are in a drought.  

Low water levels affect the efficiency of the biological processes occurring and can cause structural issues.  If the level drops to a point where the bottom is exposed for an extended period of time, it can dry and crack, reducing its water holding capabilities in the future.  If the water level drops to an unacceptable level, you can add water to the lagoon from another water source.  Keep in mind that adding water from your well may stress it in during a dry period.  If you are on rural water, this can greatly increase your water bill.  

For more information consult our MU Guide WQ402 Residential Sewage Lagoon Systems: A Homeowner's Guide to Installation and Maintenance.