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What is a mound system?
 
A septic tank mound system is a technology used for treating and disposing of domestic wastewater in areas unsuitable for conventional septic tank soil absorption systems. Mounds are pressure-dosed sand filters placed above, and discharging directly to, the natural soil. Their main purpose is to provide additional treatment to the wastewater before it enters the natural environment. Mound systems are designed to overcome site restrictions such as:

 Slow or fast permeability soils

 Shallow soil cover over creviced or porous bedrock

 A high water table

The three components of a mound system are a pretreatment unit(s), dosing chamber, and the elevated mound. The pretreatment unit is usually a septic tank, which removes solids from the wastewater. The dosing chamber (pump chamber) follows the septic tank and contains a pump, which uses pressure to evenly distribute the wastewater over the infiltration surface of the mound. The mound is made up of a soil cover that can support vegetation and a fabric-covered coarse gravel aggregate in which a network of small diameter perforated pipe is placed. The network of perforated pipe is designed to distribute the effluent evenly through the gravel from where it trickles down to the sand media and hence, into the plowed basal area (natural soil). Treatment occurs through physical, biological, and chemical means as the wastewater filters down through the sand and the natural soil.

What are the advantages and disadvantages of using mound systems? 
 
Advantages:

 The mound system enables use of land that would otherwise be unsuitable for in-ground or at-grade onsite systems.

 The natural soil utilized in a mound system is usually the top layer, which is typically the most permeable.

 A mound system does not have a direct discharge to a ditch, stream, or other body of water.

 If care is taken, construction damage can be minimized since there is little excavation required in the mound area.

 Mounds can be utilized in most climates.

Disadvantages:

 Construction costs are typically much higher than those of conventional systems.

 Since there is usually limited permeable topsoil available at mound system sites, extreme care must be taken not to damage this layer with construction equipment.

 The location of the mound may affect drainage patterns and limit land use options.

 The mound may have to be partially rebuilt if seepage or leakage occurs.

 All systems require pumps or siphons.

 Mounds may not be aesthetically pleasing in some cases.
 

What determines the performance of a mound system?

Years of monitoring the performance of mound systems have shown that mounds can consistently and effectively treat and dispose of wastewater. One factor that determines good performance is the selection of sand media that can be dosed at a reasonable rate and can adequately treat the wastewater. Usually coarse sand with the right characteristics provides the best treatment because the wastewater will percolate through the sand, allowing time for adequate treatment.
Successful performance also depends on proper design, installation, and maintenance. For design of residential mounds, the daily wastewater volume is determined by the number of bedrooms in a house. Typical design flow requirements for individual homes are up to 150 gallons per day (gpd) per bedroom. 
Are mound systems easy to operate and maintain?
When a mound system is properly installed and maintained, it should last for a long period of time. In general, the maintenance required for mounds is minimal. However, as with any system, poor maintenance could lead to system failure. 

Possible problems that can occur in a mound system include: 

 Ponding in the absorption area of the mound;

 Seepage out of the side or toe of the mound;

 Spongy area developing on the side, top, or toe of the mound;

 Clogging of the distribution system.

  
The septic tank and dosing chamber should be checked for sludge and scum buildup and pumped as needed to avoid carryover of solids into the mound. The dosing chamber, pump, and floats should be checked annually and replaced or repaired as necessary. It is critical that the septic tank and dosing chamber be watertight. In addition, electrical parts and conduits must be checked for corrosion. 
Follow all of the manufacturer's operation and maintenance (O&M) instructions. All equipment must be tested and calibrated as recommended by the equipment manufacturer. A routine O&M schedule should be developed and followed for any mound system.

 

What is an intermittent sand filter?
 
Sand filter systems have been used for wastewater treatment in the U.S. since the late 1800s. They are a viable alternative to conventional methods when soil conditions are not conducive for proper treatment and disposal of wastewater through percolative beds/trenches. Sand filters can be used in sites that have shallow soil cover, inadequate permeability, high groundwater, and limited land area. 
An assessment conducted by the U.S. Environmental Protection Agency of intermittent sand filter (ISF) systems in 1985 revealed that sand filters are a low-cost, mechanically simple alternative. More recently, sand filter systems have been serving subdivisions, mobile home parks, rural schools, small communities, and other generators of small wastewater flows. In ISFs, wastewater is applied in intermittent doses to a bed of sand or other suitable media. The wastewater first receives primary treatment in a septic tank or an aerobic treatment unit, and then is pumped from a screened vault in the septic tank or separate dosing tank to the sand bed where it is evenly distributed over the top of the sand filter bed. As the wastewater passes through the sand filter, treatment is accomplished through physical and chemical means, but mainly by microorganisms attached to the filter media. The treated wastewater is collected in underdrains at the bottom of the sand filter and is then transported to a line for further treatment or disposal. 
ISFs are designed such that the pre-treated wastewater passes through the sand filter bed once.

Common Types of ISFs 
 
Gravity Discharge ISFs

One variety of buried ISFs, the gravity discharge ISF, is usually located on a hillside with the long axis perpendicular to the slope to minimize the excavation required. Because the effluent leaving the sand filter flows out by gravity, the bottom of the sand filter must be several feet higher than the drainfield area. To achieve that difference in elevations, a sand filter may be constructed partially above ground.

Pumped Discharge ISFs

Another type of buried sand filter, the pumped discharge sand filter, is usually sited on level ground, but its location in relation to the drainfield 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.

Bottomless ISFs

A third type of buried sand filter has no impermeable liner and does not discharge to a drainfield, but rather directly to the soil below the sand.

What are the advantages and disadvantages of using ISFs?

Advantages:

 ISFs produce a high quality effluent that can be used for drip irrigation or can be surface discharged after disinfection.

 Drainfields can be small and shallow.

 ISFs have low energy requirements.

 ISFs are easily accessible for monitoring and do not require skilled personnel to operate.

 No chemicals are required.

 If sand is not feasible, other suitable media could be substituted that may be found locally.

 Construction costs for ISFs are moderately low, and the labor is mostly manual.

 The treatment capacity can be expanded through modular design.

 ISFs can be installed to blend into the surrounding landscape.

 The soil cover prevents odors.

Disadvantages:

 The land area required may be a limiting factor.

 Regular (but minimal) maintenance is required.

 Odor problems could result from open filter configurations and may require buffer zones from inhabited areas.

 If appropriate filter media are not available locally, costs could be higher.

 Clogging of the filter media is possible. ISFs could be sensitive to extremely cold temperatures.

 ISFs may require an NPDES Permit when the effluent is surface discharged.

What determines the performance of an ISF?

ISFs produce a high quality effluent by removing a very high percentage of the contaminants. The performance of an ISF depends on how biodegradable the wastewater is, the environmental factors within the filter, and the design of the filter. The most important environmental factors that determine the effectiveness of treatment are media reaeration and temperature. Reaeration makes oxygen available for the aerobic decomposition of the wastewater. Temperature directly affects the rate of microbial growth, chemical reactions, and other factors that contribute to the stabilization of wastewater within the ISF.

Several process design parameters that affect the performance of ISFs are the degree the wastewater was pretreated prior to going through the sand filter, media size and depth, the hydraulic loading rate, and dosing techniques and frequency. Although physical, chemical, and biological processes are all at work to some degree in an ISF, the biological processes play the most important role since bacteria are the primary workers in sand filters.

Are ISFs easy to operate and maintain?

ISFs require annual maintenance, although the complexity of maintenance is generally minimal. Buried sand filters used for residential application can perform for an extended period of time. The majority of operation and maintenance involves monitoring the influent and effluent and checking the dosing equipment periodically. Pumps and controls should be checked every 3 months, and the septic tank or aerobic unit should be checked for sludge and scum buildup and pumped as needed. Control panels with programmable timers and hour and event meters simplify troubleshooting and diagnosis.

To learn more about the cost of an ISF please click here.

What is a low-pressure pipe system?

A low-pressure distribution (LPD) system is a shallow, pressure-dosed soil absorption system with a network of small diameter perforated pipes placed in narrow trenches. Originating in North Carolina and Wisconsin, LPD systems were developed as an alternative to conventional soil absorption systems to eliminate problems such as: clogging of the soil from localized overloading, mechanical sealing of the soil trench during construction, anaerobic conditions due to continuous saturation, and a high water table.

The LPD system has the following design features that overcome these problems:

1) shallow placement, 
2) narrow trenches, 
3) continuous trenching, 
4) pressure-dosed with uniform distribution of the effluent, 
5) design based on area loading, and 
6) resting and reaeration between doses.

The main components of an LPD system are:

 A septic tank or an aerobic unit;

 Pumping (dosing) chamber (submersible effluent pump, level controls, high water alarm, and supply manifold);

 Small diameter distribution laterals with small perforations (holes).

The septic tank is where settleable and floatable solids are removed and primary treatment occurs. Partially clarified effluent then flows by gravity to a pumping chamber where it is stored until it reaches the level of the upper float control, which turns the pump on. The level controls are set for a specific pumping sequence of one to two times daily, allowing breaks in between doses for the soil to absorb the wastewater. However, the dosing mechanisms and frequency may vary for different systems. The pump moves the effluent through the supply line and manifold to the distribution laterals under low pressure. The laterals are a network of PVC pipes that have small, drilled holes through which the wastewater is distributed evenly. The laterals are placed in narrow trenches that allow enough storage volume so that the depth of the wastewater does not exceed 2 or 3 inches of the total trench depth during each dosing cycle.

What are the advantages and disadvantages of using LPDs?

Advantages:

 Shallow placement of trenches in LPD installations promotes evapotranspiration and enhances growth of aerobic bacteria.

 Absorption fields can be located on sloping ground or on uneven terrain that would otherwise be unsuitable for gravity flow systems.

 Improved distribution through pressurized laterals disperses the effluent uniformly throughout the entire drainfield area.

 Periodic dosing and resting cycles enhance and encourage aerobic conditions in the soil.

 Shallow, narrow trenches reduce site disturbances and thereby minimize soil compaction and loss of permeability.

 LPDs allow placement of the drainfield area upslope of the home site.

 LPDs have reduced gravel requirements.

 There is a significant reduction in land area required for the absorption system.

 Costs are comparable to other alternative distribution systems.

 LPDs overcome the problem of peak flows associated with gravity-fed conventional septic systems.

Disadvantages:

 In some cases, the suitability could be limited by soil, slope, and space characteristics of the location.

 There is a potential for clogging of holes or laterals by solids or roots.

 LPDs have limited storage capacity around their laterals.

 There is the possibility of wastewater accumulation in the trenches or prolonged saturation of soil around orifices.

 LPDs could experience moderate to severe infiltration problems.

 Regular monitoring and maintenance of the system is required; lack of maintenance is a sure precursor to failure.

What determines the performance of an LPD system?

Two critical factors that affect the performance of an LPD system are dosing and distribution of the effluent. The dosing and resting periods help maintain aerobic conditions in the soil and around the distribution trench. An LPD system cycles back and forth between aerobic and anaerobic conditions. The distribution of wastewater must be uniform (spread evenly over the soil absorption field) without hydraulically overloading it.

The four factors that affect the suitability of an LPD system for a given site are soil, slope, available space, and anticipated waste flow. An LPD system should be located in soils that have suitable texture, depth, consistence, structure, and permeability, according to state/local regulations. Although the size requirements for an LPD system will vary depending on the site, in general an undeveloped lot smaller than 1 acre may not be suitable for an LPD system.

Are LPD systems easy to operate and maintain?

A properly designed and installed LPD system requires very little ongoing maintenance. However, regular monitoring is necessary to ensure proper performance. Some states, such as North Carolina, require professionally trained operators to inspect and maintain LPD systems on a minimum of a 6-month frequency. The septic tank and pumping chamber should be checked for sludge and scum buildup and pumped as needed. Screens or filters can be used to prevent solids from escaping the septic tank. An important operation and maintenance consideration is that LPD systems are very susceptible to hydraulic overloading due to excess infiltration of rainwater, shallow perched soil waters, and/or groundwater into the tanks. In areas with improper drainage, leaky pump tanks can become sinks for nearby groundwater and can overload the drainfield with more water than the soil can absorb. Therefore, it is important that tanks be watertight to overcome this potential problem.

LPD piping network must be cleaned periodically to prevent clogging of the orifices. This is typically done by attaching a vacuum hose from a pumper truck to the ends of each lateral and applying vacuum to the laterals. This ensures that solids accumulation and plaque build-up on the walls of the pipe is thoroughly cleaned out.