Wetland Systems

Constructed wetlands or plant-rock filters are categorized as a conventional system in the State of Kentucky. They generally consist of a primary treatment unit, usually a septic tank with a rock bed or cell containing approximately 12 inches of the rock media and a small over flow lateral field, or second wetland cell. With the rock media, aquatic plants are planted which in turn treat the effluent, from the pretreatment unit, to a very high degree. Any excess effluent is then disposed of into the overflow lateral field or second wetland cell.

Wetland cells are typically sized using one cubic foot of gravel area for each one gallon of design wasteflow. A typical size for a three bedroom home would be 360 square feet of interior area. Various length to width ratios are acceptable with generally a relatively narrow width to longer length being preferable. Examples of lengths and widths would be 4x90, 5x76, or 6x60.

The system functions primary by wastewater entering the pretreatment unit, (septic tank), where some treatment occurs. The partially treated wastewater then enters the wetlands or plant rock filter through solid PVC piping. The wastewater is then distributed through an inlet pipe called an "inlet header" where it is distributed equally from side to side in the inlet area. The wastewater then comes into contact with the aquatic plantings, which are growing in the gravel media. The plants within the system act to introduce oxygen into the wastewater through their roots. As the wastewater becomes oxygenated, beneficial micro-organisms and fungi can thrive and reproduce in the system. They attach to the plant roots and the rock media, where they in turn digest organic matter from the wastewater. The microorganisms and plants utilize organic matter and nutrients from the wastewater as food and fertilizer. In addition, fairly large amounts of water may be lost through evaporation.

Currently the State of Kentucky requires that an overflow lateral field or second wetland cell be installed after the wetland system. It is necessary to contain any excess wastewater that the wetland does not remove as well as to possibly treat the wastewater further if it is needed. Should an overflow rock lateral field be utilized, approximately 20 % of normally required footage will be necessary.

Wetland systems have been successful on severe and limiting sites in Kentucky where conventional type septic systems have not been usable. This system also seems to be very space efficient and can be used on small lots was well as irregularly shaped lots. In many instances, the cell or bed portion of the system has been installed in areas with shallow bedrock, shallow restrictive horizons and watertables, compacted graded or filled areas, and irregularly sloped areas. Since the wetland cell is usually lined with a plastic liner, it is possible to locate this portion of the system in undesirable soil conditions. This would leave the best soil condition locations on the property for the installation of the overflow lateral field.

The anticipated end result of the wastewater treatment process from the wetland system is that through filtration, absorption, decomposition, retention, nutrient utilization and evaporation, the effluent is treated to a very high degree.

Intermittent Sand Filters (typically built below ground level)

Sand filter systems have been used for wastewater treatment in the US since the late 1800s. They are a viable addition / alternative to conventional methods when site conditions are nonconductive for proper treatment and disposal of wastewater through percolative beds. Sand filters can be used on sites that have shallow soil cover, inadequate permeability, high groundwater, and limited land area.

Intermittent sand filters ( ISF ) have a 24 inch deep filer beds of carefully graded media, but anthracite, mineral tailings, bottom ash, etc., have also been used. The surface of the bed is intermittently dosed with effluent that percolates in a single pass through the sand to the bottom of the filter. After being collected in the under drain, the treated effluent is transported to a line for further treatment or disposal. The two basis components of an ISF system are a primary treatment unit - a septic tank or other sedimentation system, and a sand filter.

ISF's remove contaminants in wastewater through physical, chemical, and biological treatment processes. Although the physical and chemical processes play and important role in the removal of many particles, the biological processes play the most important roll in sand filters.

ISF's are typically built below grade in excavations 3 to 4 feet deep and lined with an impermeable membrane where required. The under drain is surrounded by a layer of graded gravel and crushed rock with the upstream end brought to the surface and vented. Pea gravel is then placed on top of the graded gravel, and then sand is laid over top of the pea gravel. Another layer of graded gravel is laid down, with the distribution pipes running through it. A flushing valve is located at the end of each distribution lateral. Lightweight filter fabric is placed over the final course of rock to keep silt from moving into the sand while allowing air and water to pass through. The top of the filer is then back-filled with loamy sand that may be planted with grass.

There are three varieties of intermittent sand filters. The first variety of buried ISF's is the gravity discharge. This variety is usually located on a hillside with the long axis perpendicular to the slope to minimize the excavation required. The second variety of buried ISF's are the pump discharge. This variety is usually sited on level ground, but its location in relation to the drain field is not critical since a pump located within the sand filter bed allows effluent to be pumped to a drainfield at any location or elevation. Discharge piping goes over - not through - the sand filter liner, so the integrity of the liner is protected.

Advantages:
1. ISF's produce a high quality effluent that can be used for Drip irrigation or can be surface discharged after disinfection.
2. Drainfields can be small and shallow
3. ISF's have low energy requirements and are easily accessible for monitoring
4. No chemicals are required and ISF can be installed to blend into the surrounding landscape.

Disadvantages:
1. The land required may be a limiting factor.
2. Regular maintenance is required.
3. Odor problems could result from open filer configurations and may require buffer zones from inhabited areas.
4. Clogging of the filter media is possible and the ISF could be sensitive to extremely cold temperatures.
5. ISF's may require a permit with facility number, issued and registered with the state, if the effluent is to be discharged on the surface.

Other Topics:
Alternate Household Cleaning Solutions | Caring for Your Septic System | Septic Tank Additives Danger in Disguise | Engineering & Design for Difficult Sites | Improve Your System Practice Water Conservation | Checklist for Evaluating Your Septic System | Ten Most Asked Questions Septic Systems | Potential Property Owner's Reference | Potential Home Owners Reference | Wetland Plants | Why Systems Fail | Common Onsite Regulations | Corrective Action for a Failing Septic System | Fill & Wait - Why? What? How? | Health Departments - Friends not Foes | Septic Tanks - How do they Work? | Sewage Protection Program

Source of information and picture: National Small Flows Clearinghouse, West Virginia University (800) 624-8301

Barrow Company specializes in the engineering and design of all types of systems. If you would like additional information on these or any other system, please use the Contact Us section on this website, e-mail us at info@barrowcompany.com or contact us by telephone or fax. Office hours are 7:30 a.m. to 4:30 p.m. Monday thru Friday