Standard Septic Tank Design Fundamentals

For homes and buildings not connected to a municipal sewer system, an onsite septic system is often the best solution for wastewater treatment and disposal. At the heart of most septic systems is the septic tank, which provides preliminary treatment of household wastewater.

Septic tanks are large containers made of concrete, fiberglass, or polyethylene usually buried underground. They hold wastewater long enough for solids to settle out and separate from the liquid, and provide anaerobic digestion of organic matter. Well-designed and properly sized septic tanks are essential for effective wastewater treatment and prevention of groundwater contamination.

Key Components of Standard Septic Tank Design

While septic tank configurations can vary, standard designs include several key components:

• Inlet and outlet tees - Allow wastewater to enter and exit the tank.
• Baffles - Retain solids and prevent them from leaving the tank.
• Access lids - Provide entry to pump out accumulated sludge.
• Adequate capacity - Hold wastewater to allow separation and digestion.

The inlet tee dissipates the energy of the incoming wastewater and reduces disturbance of the settled sludge layer. Baffles and the outlet tee prevent solids from being carried out of the tank. Access lids permit periodic pumping of the accumulated solids and floating scum. Sufficient capacity provides adequate detention time for separation and anaerobic digestion.

Sizing Considerations

Proper sizing is crucial for septic tanks to function effectively. The minimum capacity for a septic tank serving a conventional home is 1000 gallons. Larger homes, additional bedrooms, or the presence of a whirlpool bath may require 1250 gallons or more. Consult your local health department for specific regulations in your area.

In addition to minimum size requirements, the septic tank must have sufficient capacity to accommodate peak usage periods, such as when guests are visiting or when multiple loads of laundry are done in one day. An adequate design allows for peak flows that are at least 50% higher than average daily flows.

Layout and Configuration

Standard septic tank configurations include either a single compartment or two compartments. Two-compartment tanks provide additional solids separation and are strongly recommended. The first compartment is typically 2/3 of the total volume, while the second is 1/3.

The inlet and outlet tees must be properly positioned to promote effective flow patterns through the tank. Baffles are oriented opposite the inlet and outlet to retain solids. Multiple access ports make pumping easier. Risers extend the access lids up to ground level for convenient access.

Materials and Construction

Septic tanks must be watertight and able to withstand corrosion, cracking, and ground stresses. Concrete and fiberglass are common materials, providing strength, durability, and longevity when properly constructed. Polyethylene tanks are lightweight but may not be approved in all areas.

Joints between tank sections must be sealed with gaskets or mastic to prevent leaks. Tank walls, floors, and access lids require adequate thickness and reinforcement. Steel tanks are unacceptable due to corrosion concerns. Only approved materials should be used.

Site Selection and Installation Factors

Choosing an appropriate location is critical for proper functioning of the septic system. Tanks should be installed on flat ground and bedded with sand, gravel, or other approved material to avoid settling.

Septic systems must meet required setback distances from features like wells, streams, steep slopes, and property lines. Soil conditions, depth to groundwater, and other hydrogeological factors also determine suitable sites.

Excavations must provide adequate space for the tank and access to fittings during installation. Backfill should be placed and compacted carefully around the tank. All pipes must slope properly and make watertight connections.

Site Evaluation and Permitting

An in-depth site evaluation examines soil conditions, percolation rates, depth to groundwater, and proximity to sensitive features. This data informs the system design. A permit from the local health department is required before installing a septic system.

Inspections and Approvals

Once installed, the tank and piping must be inspected before backfilling. Afinal inspection checks all system components. Local authorities must approve the installation before wastewater can flow to the septic system.

Operation, Maintenance, and Troubleshooting

Regular septic tank pumping is vital to remove accumulated solids and prevent clogs. Pumping frequency varies based on tank size and household occupancy, but every 3-5 years is typical. Annual inspections check for signs of malfunction.

Common issues include blocked inlets/outlets, leaks, or infiltration of groundwater. Trained technicians can diagnose problems through visual checks, pumping, dye tracing, and other tests. Repairs should address root causes, not just symptoms.

Upgrading inadequate or failing septic systems may involve adding filters or aerators, replacing damaged components, or installing an alternative treatment technology. Continued maintenance maximizes system lifespan.

Innovations in Septic Tank Design

Advanced septic tank designs incorporate features like effluent filters, multiple compartments, and enhanced access. Some systems use air or chemicals to augment solids separation and digestion.

Prefabricated tanks with lightweight plastic or fiberglass construction allow for easier transport and installation. Polyethylene tanks won't corrode, crack, or leach chemicals. Improved diagnostics and remote monitoring enhance preventative maintenance.

While innovations continue, most septic systems still rely on a standard, passively operating septic tank. Proper design, sizing, installation, and maintenance remain critical to effective, long-term performance.

Designing and installing septic systems requires specialized expertise. Work with qualified septic contractors, engineers, and health department officials when planning your onsite wastewater system.

Look for licensed professionals with extensive septic system experience. Get multiple bids and check references. Ask lots of questions to find someone you trust. Investing in professional services results in a code-compliant system designed and built to handle your home's needs.