Septic systems with drainage field lines are commonly utilized to handle sewage discharge from households in South Carolina. These septic systems may fail when water infiltration rates into the soil are inadequate for drainage filter field lines to work properly. In extreme cases, the sewage water will actually surface, possibly causing a public health risk. One method to reduce this problem is to treat the sewage water prior to in-ground disposal. This can be done with a subsurface flow constructed wetlands system.
Bennettsville Demonstration Site
Pre-installation Site Conditions
Untreated sewage water surfaced in the backyard from the septic system drainage field lines during extended rainy periods. The lot is nearly level and has a soil with a high clay content. The soil is unsatisfactory for conventional septic systems because of slow water infiltration rates and a high water table.
This design is based on a 3-bedroom house allowing for 360 gallons of water use per day. A 1,000 gallon baffled septic tank with a sewage filter was installed to minimize solids and organic loading to the constructed wetlands system. Because of site conditions, a submersible pump was used to carry sewage water from the septic tank up to the treatment cell.
Labor & Equipment $1,600
The treatment cell is lined with 45 mil synthetic rubber which prevents seepage into ground water. In the lined treatment cell, the sewage water flows through gravel and the roots of aquatic plants. The water is maintained at a depth of 12-inches. The top 3-inches of the gravel surface remains dry. Sewage water is pumped into the treatment cell. Micro-organisms that grow in the gravel bed digest the organic material and the aquatic plants absorb nutrients and assist with water disposal through transpiration into the atmosphere. Some of the water will pass into the atmosphere through evaporation. Any remaining treated water is released into the underground disposal cell, which consist of 18inches of sand and gravel.
Aquatic vegetation consists of canna lily, blue and yellow flag iris, and giant bulrush.
Property owners agree to--
-maintain designated water level in treatment cell
-care for aquatic vegetation in treatment cell
-perform periodic pump-out of septic tank and cleaning of sewage filter.