Porosity Loss in Subsurface -Flow Treatment Wetlands

On-site wastewater systems remain the only feasible solution for wastewater treatment in rural settings even though many of the settings are not appropriate for traditional septic tank and leachfield applications. One alternative design is the addition of a subsurface-flow wetland (SSFW) between the septic tank and the leachfield to improve the treatment and then dispersing the wastewater with low-pressure dosing or drip irrigation. One drawback to the application of a SSFW is the finite lifespan due to solids build up in the media and loss of porosity. This study examines the loss of porosity in several different SSFW systems and the relationship between treatment efficiency and porosity loss.

A large (10 feet by 50 feet by .75 feet) SSFW was dosed with wastewater from a 1500 gallon 2-chambered septic tank at 500 gallons/day for 3 years and periodically drained and filled to assess porosity loss. The treatment efficiency was evaluated by comparing percent reduction of CBOD, TSS, N and P with porosity loss.

Five (5) smaller (2 feet by 10 feet by 1.25 feet) SSFWs were dosed proportionally and porosity was measured in a similar fashion to the large wetland. The SSFW system variables in the smaller wetlands included media, plants, dosing mechanisms and aeration. The systems were sampled weekly over 9 months for CBOD, TSS, N and P.

Although the large wetland experienced a loss in porosity the treatment efficiency did not appear to decrease significantly. Some of the smaller wetlands showed significant differences in treatment efficiencies but differences in porosity losses were not readily distinguished. These results indicate concerns over the finite lifespan of subsurface-flow treatment wetlands may not be as serious as previously thought.