Installation details for a septic system such as size, location, depth and configuration are determined by soil testing. Percolation tests and deep observation pits are required by the Health Code for both new and repaired septic installations. Site testing for existing homes is generally conducted by a TAHD sanitarian and by a Professional Engineer for new home construction. A brief explanation of the percolation test and observation pit is noted below:
The percolation test consists of a hole 6-12 inches in diameter dug in the area of the proposed septic system. The depth of this hole varies depending on the soils encountered but it is generally not greater than 24 inches. The hole is initially filled with water (presoak) in an attempt to saturate the soil, allowed to drain away and then refilled with approximately 12 inches of water. The rate at which the water drops in the hole is measured at intervals over a period of time ranging from 30-60 minutes. The uniform slowest rate of drop of the water level over a measured time interval is converted to minutes per inch and used as a basis of design in determining the septic system size. For example, if the water dropped uniformly 1\4 inch every five minutes the rate would be 20 minutes per inch. The Health Code provides a simple table that determines the size of the system based on the measured perk rate and the number of bedrooms in the home. The greater the number of bedrooms and the slower the percolation rate, the larger the system required. Commercial systems are sized using the perk rate and projected estimates of water usage in gallons per day.
Observation pits are dug in the proposed septic area in order to be able to see the various soil layers. These holes are dug with a backhoe machine generally to a depth of 7ft and observations are made of the soil layers, color and texture and recorded by the sanitarian or engineer. Particular attention is given to the depth to ledge rock or ground water since both are specifically addressed in the Health Code. The bottom of the leaching system must be maintained a minimum of 18 inches above ground water or impervious soil and 4 feet above ledge rock. The deep test pit information is used to determine the depth of the system in the original ground. In this part of Connecticut the soils encountered are such that a majority of septic systems will require fill to comply with the separating distances to ground water or ledge. Fill used in the installation of a septic system is sandy in consistency and must meet minimum specifications set forth in the Health Code. This type of material is in limited supply and as a result the cost of a septic system is influenced greatly by the amount of fill required.
The percolation test results, observation pit data and the slope of the property in the septic system area are used to determine the Minimum Lateral System Spread (MLSS). In simple terms, MLSS is the length of leaching system spread along the contours of the land that is necessary to allow the liquid waste to seep naturally into the ground without surfacing. For example, a single 100ft long leaching trench is more efficient than two 50 foot long trenches set one below the other. MLSS was incorporated into the Health Code in 1995, and all new septic systems must comply with this requirement.