Household Septic System Designs, Applications, Costs, Pros, Cons, and Comparisons

 

     The suitability of septic system design type is dependent on several factors including soil quality for leaching, seasonal water table depth, space available for system size including replacement area, and use parameters. Cost, including ongoing O&M costs, is always a consideration. Reliability is a concern as well. A low-cost, low-maintenance, high-reliability septic system is obviously the most desirable choice. However, the abovementioned factors (soil quality, water table depth, and available space) will determine whether the preferred option can be installed.  

     Septic system sizing includes recommended tank sizes and length of leaching trenches in soil absorption systems (traditional leach fields, usually with plastic open chambers). Trench length and number of trenches is based on soil quality and system size.

 

 

System Costs

 

     Septic system costs vary considerably based on system size, cost of additional components, and type of system. I recently saw a spray irrigation system for a large 6-bedroom cabin that cost about $35,000. Average system costs for say a 2 bedroom are much less than that and the lowest cost, lowest maintenance system is a soil absorption system that uses leach trenches with chambers. The cost estimates in my table below are probably low to current prices.

 

 

 

The Regulatory Process of Permitting, Siting, Sizing, Designing, and Approving Household Sewage Treatment Systems

 

     Here in Southeastern Ohio the most desirable household sewage treatment system (HSTS) is a gravity fed system with one septic tank of adequate size for expected use and appropriately sized leach trenches. These are usually filled with open chambers consisting of plastic half corrugated half tubes 8-12 inches high and 12 to 24 inches wide. 8” high X 24” wide is a common size. At my house, the trenches are 34 inches wide. These are open to allow oxygen to get into the system and assist the filtered effluent to seep into the soil along the trench. Instead of chambers, trenches filled with perforated pipe and gravel may be used. These used to be common but open chambers are now the norm where applicable.

     Permits are given after site visits and soil evaluations to determine site suitability and soil suitability for different types of systems. The most desirable systems are those that do not have electrical or other components such as pumps, aerators, chlorinators, UV lights, and alarms. They also use power and can short out. Operation and maintenance costs for those systems are much higher. Often, the owner and installer may be given a menu of design options based on site and soil. After the system type is approved the system is installed, inspected, and approved.

     Systems with components, especially pumps and aerators, that are often in need of repair or replacement, may be required to carry extended service contracts with service providers. One-year inspections are common with systems still under bond or warranty by the installer to fix any issues. I think 18 months liability is common.

     Septic systems are designed based on available on-contour land with soil of sufficient permeability and free of restricting layers often indicative of seasonal water tables. If these conditions are not ideal as is common, then mounded systems, drip systems, spray systems, and pre-treatment systems are considered.

 

 

Gravity-Fed Chamber Leach Line Systems

 

     These very common systems are most often open-bottom plastic chambers but may also be fabric-wrapped pipe (such as the EZ Flow design), and synthetic materials such as expanded polystyrene media. These systems used to be trenches filled with gravel, but most are now gravelless systems. Microbes on or near the soil treat the sewage effluent as it flows along and into the soil. Some of these systems may require a lift station if the available topography does not support gravity-feeding. That adds the expense of pump maintenance and electricity.

 

Drip Distribution Systems

 

     These are low-pressure systems with small piping that drips the effluent through the soil. The advantages are that they can be built on shallow soils or soils with a high water table without mounding. Disadvantages are expense, high O&M costs, and the large dosing tank that is required in addition to the septic tank.

 

 

Aerobic Pretreatment Units

 

     This type of system mimics a municipal sewage treatment system which utilizes aerobic pretreatment. Aerobic pumps called aerators inject oxygen into the effluent water to increase the rate of aerobic decomposition. Disinfection with chlorinators (often passive flow through a chamber with solid chlorinated disks) and UV lights are also used. There are several design types of tanks, aerators, and filters. 

 

 

Mound Systems

 

     These systems are used where there is shallow soil depth, high groundwater, or shallow bedrock. A sand and/or gravel mound is constructed with leaching trenches installed within. The effluent percolates through the highly permeable sand for a time before entering the soil below. Common types of mound systems include AES’s Presby (sand-lined) and ATL Systems. These types often do not require a dosing tank or a pump tank and so are considered to be low maintenance since passive slow percolation through the sand and soil absorption are the mechanisms of treatment. They do require venting, both a low vent from the mound and a high vent, usually the house vent. Presby systems (but not ATL systems) are eligible for a 1-foot soil depth credit, which means they can be built closer to the surface in areas where the water table is high and/or the soil quality is poor. They are also mounded up, with the sand mound being lightly covered with topsoil. They have a low failure rate. They can be replaced onsite by removing and disposing of the sand and adding clean sand. This is expensive but does not require more space.

 

 

Recirculating Sand Filter System

 

     This is similar to the sand-lined systems but also utilizes low-pressure pumping. Effluent is recirculated back into a pump tank ahead of the sand filter.  After the sand goes through the sand-lined chamber (the sand filter) it goes on to a soil absorption leach field. Presby systems, on the other hand, utilize special-made chambers with geotextile fabric and other design features to enhance treatment within the sand body. I don’t think I have ever seen one of this type.

 

 

Constructed Wetland Systems

 

     According to EPA:

 

“The wetland cell typically consists of an impermeable liner, and gravel and sand fill, along with the appropriate wetland plants, which must be able to survive in a perpetually saturated environment.”

 

“A wetland system can work via either gravity flow or pressure distribution. As wastewater flows through the wetland, it may exit the wetland and flow into a drainfield for further wastewater treatment into the soil.”

 

 

Wastewater Treatment Lagoons

 

     I have seen sewage treatment lagoons. They are not too uncommon in rural areas and communities with shared treatment. EPA has specific requirements for them. Below is a 'sewage pond' in West Virginia. 

 

 

 

 

Spray Systems

 

     Spray systems are a newer design that pretreats the effluent with aeration, chlorination, and UV lights then runs the effluent to sprayers that spray the treated effluent onto the open ground for final treatment on a timing schedule, The sprayers are usually programmed to spray at night when no humans are around. These systems are considered a “green” alternative, but they can be very expensive.

 

 

References:

Ohio Sewage Program Updates. Victoria Miller. Ohio Dept. of Health. Presented at 2024 Contractors Workshop, Logan Ohio. Feb. 15, 2024.

Anua Systems Operation & Maintenance. Sean McGuigan. Anua. Presented at 2024 Contractors Workshop, Logan Ohio. Feb. 15, 2024.

Alternative System Applications: Spray, Drip, Mounds, Anua, and Sand-Lined Systems. Katie Wasky & Jennifer Valentine. Fairfield County Health Dept. Ohio. Presented at 2024 Contractors Workshop, Logan Ohio. Feb. 15, 2024.

Presby vs. ATL: An Installers Perspective. Tyler Duncan. Duncan & Daniels Enterprises. Presented at 2024 Contractors Workshop, Logan Ohio. Feb. 15, 2024.

Pumping and Maintenance of Transient Rental Systems. Keary Helber. K.G. Helber Construction. Presented at 2024 Contractors Workshop, Logan Ohio. Feb. 15, 2024.

How Much Does It Cost to Replace a Septic Tank and Leach Field?: Common Range: $6,790 – $17,100. National Average: $12,840. Updated: November 17, 2023. Written by: Steve Hansen. Costimates. November 17, 2023. Septic Tank and Leach Field Installation Cost | Costimates 

Types of Septic Systems. U.S. EPA. Types of Septic Systems | US EPA

How Septic Systems Work. U.S. EPA. How Septic Systems Work | US EPA

Small and Rural Wastewater Systems. Lagoon Wastewater Treatment Systems. U.S. EPA. Lagoon Wastewater Treatment Systems | US EPA