5.2 Septic System Design & Inspection

5.2 Septic System Design & Inspection

Once a site’s soil and percolation characteristics are established, the appropriate onsite wastewater treatment system (OWTS) can be designed. An Environmental Health Specialist must be proficient in evaluating system sizing, components, alternative technologies, and inspection protocols to safeguard public health.

Conventional Septic System Components

A conventional gravity-fed septic system relies on two main components: the septic tank for primary treatment and the soil absorption field (leach field) for secondary treatment and dispersal.

The Septic Tank

The septic tank is a watertight, underground receptacle designed to condition wastewater before it enters the absorption field. Its primary functions are settling, flotation, and anaerobic digestion.

Wastewater enters the tank through an inlet pipe. Inside, the velocity of the water slows down dramatically, allowing separation into three distinct layers:

1. Scum Layer: Lighter solids, such as fats, oils, and greases (FOG), float to the top.
2. Clear Zone (Effluent): The relatively clear liquid in the middle of the tank.
3. Sludge Layer: Heavy organic and inorganic solids sink to the bottom.

Modern septic tanks require two compartments. The first compartment handles roughly two-thirds of the tank's volume and does the heavy lifting of separation. The second compartment provides further settling before the effluent exits.

Baffles or sanitary tees are critical components located at the inlet and outlet. The inlet baffle directs incoming flow downward, preventing the disruption of the scum layer. The outlet baffle draws liquid from the clear zone in the middle of the tank, preventing floating scum or settled sludge from escaping and clogging the absorption field. Within the sludge layer, anaerobic bacteria (organisms that live without oxygen) slowly digest the organic matter, reducing its volume. However, the sludge and scum layers will inevitably build up over time, necessitating periodic pumping.

The Soil Absorption Field

Effluent leaving the tank flows to the absorption field, a network of shallow trenches excavated into native soil.

• Distribution Box (D-Box): Effluent often flows first into a small concrete or plastic distribution box. The D-Box ensures an equal flow of wastewater to each trench in the field.
• Trenches: Trenches are typically 18 to 36 inches wide and filled with a layer of washed gravel. A perforated PVC pipe runs along the top of the gravel.
• Biomat: As effluent trickles out of the pipe and through the gravel, it enters the soil. At the soil-gravel interface, a biological layer called the biomat forms. This slimy layer of anaerobic bacteria acts as an intense biological filter, consuming pathogens and organic matter before the water percolates deeper into the aerated soil below.

Sizing Calculations

System sizing depends on two factors: the anticipated daily wastewater flow and the soil's acceptance rate.

1. Design Flow: For residential systems, flow is estimated based on the number of bedrooms, not the number of bathrooms or current occupants. A common standard is 150 gallons per day (gpd) per bedroom. A 4-bedroom house has a design flow of 600 gpd.
2. Required Trench Area: The size of the absorption field is calculated by dividing the design flow by the soil loading rate (determined from the percolation test).
   • Formula: Trench Bottom Area (ft²) = Design Flow (gpd) / Soil Loading Rate (gpd/ft²)
   • Example: For a 600 gpd design flow and a soil loading rate of 0.5 gpd/ft², the required trench bottom area is 1,200 ft².

Alternative Treatment Systems

When site constraints—such as high groundwater, shallow bedrock, or poor soils—prevent the use of a conventional system, alternative systems are utilized.

• Sand Filters: Effluent from the septic tank is pumped intermittently over a bed of specialized sand. As water trickles through the sand, it receives highly effective aerobic (oxygen-rich) treatment before being collected and discharged to a smaller absorption field.
• Mound Systems: Used for high water tables or shallow bedrock. A mound of carefully specified sand is built above the natural grade. Effluent is pressure-dosed into a gravel bed within the mound, filtering through the sand before reaching the native soil.
• Aerobic Treatment Units (ATUs): Basically a miniaturized municipal sewage plant. An ATU injects air into the wastewater, fostering aerobic bacteria that digest organics much faster and more thoroughly than the anaerobic bacteria in a standard tank. The resulting effluent is highly treated.
• Pressure Distribution: Instead of relying on gravity, a pump tank doses the entire absorption field simultaneously and evenly under low pressure. This prevents localized overloading (creeping failure) at the front of the trenches and is often used in highly permeable soils.

System Inspection Techniques

Routine inspections and failure investigations are a core duty of the sanitarian.

Tank Inspection

• Levels and Baffles: The liquid level should be exactly at the invert of the outlet pipe. If it is higher, the field is failing or a pipe is clogged. If it is lower, the tank has a structural leak. The presence and integrity of baffles must be verified.
• Measuring Sludge and Scum: A specialized hollow tube called a Sludge Judge is lowered into the tank to capture a vertical core sample of the liquid. This reveals the thickness of the sludge and scum layers. Generally, a tank must be pumped when the sludge and scum occupy more than 25% to 33% of the tank's liquid depth.

Field Inspection and Failure Indicators

Hydraulic failure of the absorption field occurs when the soil can no longer accept effluent. Signs include:

• Surfacing Effluent: Black, odorous water pooling on the ground surface over the trenches.
• Lush Vegetation: Unusually green, rapid-growing grass over the drainfield during dry periods.
• Plumbing Backups: Slow drains or sewage backing up into the house.
• Dye Testing: If a failure is suspected but not obvious (e.g., intermittent wet spots or a nearby contaminated stream), a non-toxic fluorescent dye is flushed down the toilet. If the dye appears on the surface or in a nearby waterbody days later, a failure is confirmed.

Minimum Setback Distances

To prevent cross-contamination, strict horizontal setback distances must be maintained during design and verified during inspection. While local codes vary, standard minimums typically mandate:

• Septic Tank: 50 feet from private wells; 10 feet from property lines; 50 feet from surface waters.
• Absorption Field: 100 feet from private wells (to ensure adequate soil filtration time); 10 to 20 feet from property lines; 100 feet from surface waters (lakes, streams).