North Carolina Septic Tank Installation Regulations

A worker installs a sewer manhole on a septic tank made of concrete rings. Construction of sewerage networks for country houses
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When installing a septic tank in the state of North Carolina, property owners must be aware of the Tar Heel State’s specific regulations. A septic system must work with a location’s soil conditions and topography. Septic tanks are used in areas across the state where municipal sewage disposal systems are unavailable.

Before installing a septic system, property owners will need more than one type of permit, and if the system meets certain factors, they'll need to hire an engineer to design it. A septic system needs to be several feet from a home's foundation, basement, swimming pool and private or public water supplies, such as streams and reservoirs.

What a Septic Tank Does

Sewage is liquid and solid waste from water-using fixtures and appliances. A septic tank is a receptacle that is covered, water tight and designed to collect sewage from a building. Septic tanks and systems are mainly used in rural locations that don’t have centralized sewer and wastewater treatment systems.

A septic tank separates settled and floating solids from liquid, digests organic matter via anaerobic bacteria, and stores solids that have been digested through a detention period, allowing clarified liquids to pass for further treatment and disposal. A septic tank system is a sanitary sewage system that is below ground and has a septic tank and disposal field.

Septic System Types Used in North Carolina

There are several different types of septic systems used in North Carolina. What kind of system a property owner installs depends on the soil and site conditions and whether the local health department can issue permits for the system.

A conventional septic system, with a septic tank and several trenches buried up to 3 feet deep is used by half of the homesites with septic systems in the state and has an average installation cost of $650 to $2,000 depending upon where they are located.

Alternative septic systems include:

  • Low-pressure pipe (LPP) systems: This pressure-dosed, shallow system has a network of perforated pipes in narrow trenches and was developed to eliminate issues with soil clogging, mechanical sealing, anaerobic conditions as a result of continued saturation, and a high water table. This system costs double what a conventional system costs.
  • Fill systems: Sand-filter spray irrigation systems that require five to 10 acres or more. Fill systems cost between $7,000 to $10,000.

Getting a Septic System Permit

Before installing a septic system, a property owner must get an improvement permit (IP). An environmental health specialist will conduct a comprehensive site and soil evaluation, and either issues or denies the permit depending on what they find.

After issuance of an IP, the property owner needs to get a construction authorization (CA) permit before issuance of a building permit can occur. However, an IP and a CA may be issued at the same time.

Issuance of an Operating Permit

An improvement permit is valid for 10 years before it needs to be renewed, unless it was issued under special conditions for a professionally surveyed lot. A permanent IP is issued to property on which the conditions or intended use will not change.

After the septic system’s installation is approved by the health department, it must issue an operation permit (OP) before electricity can be connected to it.

Once the property owner begins using the system, they will need to contact their local health department if they plan to add a home, install a swimming pool or outbuilding, or engage in activity requiring a building permit.

Hiring an Engineer for a Septic System Installation

The property owner is responsible for complying with state and local permit conditions, laws, and rules regarding all aspects of the system, including installation, location, maintenance, monitoring, operation, reporting and repair. A system meeting at least one of these conditions must be designed by a professional engineer:

  • System handles over 3,000 gallons per day, except as an individual system serving one dwelling unit or as one of several individual systems, each of which serves only one unit.
  • Before disposal, the system needs pretreatment.
  • System uses sewage pumps before the septic tank or pretreatment system, with the exception of systems subject to the state’s plumbing code or those consisting of grinder pumps and pump basins approved and listed according to National Sanitation Foundation standards.
  • System uses more than one siphon or pump in a single pump tank.
  • System has a collection sewer serving two or more buildings before the septic tank or pretreatment system, with the exception of systems subject to the state’s plumbing code.

Exceptions to Engineer Requirements

An engineer is also needed if the system has structures that aren’t pre-engineered and it is designed for industrial process wastewater collection, treatment and disposal. There are exceptions to this rule when:

  • State determines the wastewater’s pollutant strength is lower than, or equal to, domestic sewage, and does not need pretreatment or management.
  • State pre-approves a pretreatment system that has been predesigned, or a process and management method proposed by the owner, giving the wastewater a pollutant strength that is lower than, or equal to, that of domestic sewage.
  • Any other system serving a multi-family dwelling or business is specified by the local health department.

Septic System’s Topography

Uniform slopes less than 15 percent have suitable topography for a septic system, while those between 15 and 30 are provisionally suitable. Slopes that are more than 30 percent are unsuitable, but can be provisionally suitable if they are modified.

However, anything greater than 65 percent cannot be reclassified. Other unsuitable topography are slopes divided by gullies and ravines, and depressions in regard to landscape position, unless the site complies with the slope requirements of the local health department.

Landscaping of Surface Area

The surface area above and around a sewage system must be landscaped with the intent of providing enough drainage if the local health department requests it. The interception of lateral or perched ground-water movement must be provided to stop soil saturation above and around a sewage system.

Designated wetlands are, therefore, unsuitable typography, unless the U.S. Army Corps of Engineers or the state Division of Coastal Management approves their proposed use.

Soil Wetness and Depth

Areas where soil wetness conditions are at least 48 inches below the natural soil surface are suitable, while conditions between 36 and 48 inches below the natural soil surface are provisionally suitable.

Sites where soil wetness conditions are less than 36 inches below the natural soil surface are unsuitable. However, they can be reclassified as provisionally suitable if a modified or alternative system is installed.

Soil depths to rock, parent material, or saprolite that are greater than 48 inches are suitable, while soil depths to rock, parent material or saprolite between 36 and 48 inches below the natural soil surface are provisionally suitable. Anything less than 36 inches is unsuitable, but these depths may be reclassified as provisionally suitable if a modified or alternative system is installed.

Tank Size Related to Home Size

The size of septic tanks for individual homes or units should have a liquid equivalent capacity that is equal to the number of bedrooms. The tank’s flow rate should be 120 gallons a day per bedroom. Each dwelling unit’s minimum sewage volume should be 240 gallons a day, with each bedroom over two increasing the sewage volume by 120 gallons a day. For example:

  • Three bedrooms or less: 900 gallons (minimum liquid capacity) or 300 gallons per bedroom.
  • Four bedrooms or less: 1,000 gallons (minimum liquid capacity) or 250 gallons per bedroom.
  • Five bedrooms or less: 1,250 gallons (minimum liquid capacity) or 250 gallons per bedroom.

To determine the number of bedrooms per dwelling unit, each bedroom and those that could function as bedrooms should be considered when designing the tank. If a unit’s occupancy exceeds two people per bedroom, the sewage volume must be determined by the maximum occupancy of 60 gallons per person a day.

Septic Tank Setback Rules in North Carolina

Septic tanks need to be several feet away from a variety of landmarks. For example, they must be:

  • Minimum of a 100 feet away from private or public water supplies, including streams and reservoirs.
  • Minimum of 50 feet away from canals, coastal waters, lakes, marshes, ponds and storm water retention ponds.
  • Minimum of 20 feet away from a nitrification field with the exception of its repair area.
  • 15 feet of separation between a septic system and basements, drainage system side-slopes, embankments and pools.
  • Minimum of 10 feet from water and property lines and the upslope of a drainage system.
  • Minimum of five feet from a building’s foundation.

Septic systems must have a setback of 25 feet for the downslope of drainage systems and groundwater-lowering ditches and devices. They cannot be installed in areas where flooding occurs less frequently than every 10 years, unless they are watertight and can remain so during storms.

Septic Tank Design in North Carolina

A septic tank must be structurally sound and not subject to excessive corrosion. It must have two compartments, and its inlet compartment must hold between two-thirds and three-fourths of its total capacity.

A dosing siphon or pump that has been properly designed must discharge liquid waste into nitrification lines when their length is more than 750 linear feet in a single system.

Septic Tank Capacity in North Carolina

When a single system’s design daily flow is more than 3,000 gallons per day or when the length of its nitrification lines exceed 2,000 linear feet, alternating pumps or siphons will discharge to separate nitrification fields.

The dose volume of these systems must fill the nitrification lines to between 66 to 75 percent of their capacity at each discharge (unless they are low-pressure distribution systems), and their discharge rate must maximize the distribution of liquid waste throughout the nitrification field.

Manhole and Alarm Requirements

A tank with a top that is lower than 30 inches below a finished grade must have an accessible, covered manhole extending within six inches of that grade and an opening that will accommodate the installation or removal of the septic tank lid.

Any system with sewage flow of more than 3,000 gallons a day must have accessible, covered manholes over each compartment and the outlet tee. These holes must extend to the finished grade, prevent inflow from surface water, and be sized for proper inspection and maintenance.

Dosing tanks must have a high-water alarm that is audible and visible by system users and weatherproofed for the outdoors – their alarm circuit should have a manual disconnect in an outside enclosure that is corrosion resistant.