Three Oaks Engineering

Subsurface Septic System Components

Septic Tank

The septic tank is designed to provide primary treatment of wastewater by capturing and restricting soil clogging constituents (such as solids, fats, oils, grease, etc.) from reaching the drainfield.  The septic tank provides an environment whereby some of these constituents are further reduced by microbial decomposition.  It is recommended the septic tank be pump out at least every 2 – 5 years.


The drainfield is comprised of a distribution device (distribution box or D-Box, pressure manifold) and one or more drainlines.  Gravel and gravel-less trenches are available. Some gravel-less trenches allow for a 25% reduction in the overall drainline length when compared to a gravel drainline and, therefore, can reduce the size of the septic system drainfield. 

Subsurface Septic System Types

Conventional Gravity Septic Systems 

(Typical cost for a four-bedroom system-$4,000-$5,000)

 Low-Pressure Pipe (LPP) Systems

(Typical cost for a four-bedroom system-$12, 000 to $15,000)

These drip disposal systems are similar to the anaerobic systems except that they have an aerobic pre-treatment unit that further treats the wastewater.  These systems are utilized on the most marginal soils with usable soil depths of 13 to 18 inches.  The system consists of a septic tanks, pump tank /aerobic pre-treatment unit, hydraulic unit/filter module, and drip tubing drainfield.

Types of Septic Systems

Drip Disposal Systems - Anaerobic Treatment 

(Typical cost for a four-bedroom system-$17,000 to $25,000)

Drip disposal systems are generally utilized for sites with usable soil depths of 18 to 24 inches.  The system consists of a septic tank, pump tank, hydraulic unit, and drip tubing drainfield.

Drip Disposal Systems - Aerobic Treatment 

(Typical cost for a four-bedroom system->$40,000)

These systems utilize a separate pump tank and pump to properly distribute wastewater to the drainlines and are most often used when the drainfield is located uphill of the home.  The drainfield can utilize almost any type of gravel or gravel-less drainline.

Pressure Distribution Systems

Pressure distribution systems is designed to create better distribution of wastewater over the extent of the drainfield.  These systems are typically used when soil conditions are less than optimal coupled with steep slopes or limited available space.  Types of pressure distribution systems include:

Pressure Manifold System 

(Typical cost for a four-bedroom system-$6,000 to $8,000)

A conventional gravity system is the simplest form of an onsite wastewater system and requires deep usable soil depths (>30 inches).  A conventional gravel system is comprised of a septic tank and drainfield.  The system is sited to allow gravity to advance wastewater throughout the septic system.  Conventional gravity systems are always the first choice due to their comparatively lower cost and minimum maintenance requirements.  The drainfield can utilize almost any type of gravel or gravel-less drainline. 

Conventional-Pump Septic Systems

(Typical cost for a four-bedroom system-$5,000 to $6,000)