5 Critical Septic Tank Design Features You Need to Get Right

Installing a properly designed and constructed septic tank is one of the most important aspects of a septic system. Septic tanks that lack critical design features can fail prematurely, leading to costly repairs, groundwater contamination, and even complete system replacement.

Get these key features right in your septic tank design, and you'll be well on your way to decades of effective wastewater treatment.

Size It Right

The liquid volume or capacity of your septic tank directly relates to the number of bedrooms and occupants in the home. An undersized tank won't be able to properly treat wastewater flows, while an oversized tank will suffer from solids building up and failing to decompose.

Most residential septic tank designs range from 1,000 gallons up to 1,500 gallons. The general sizing guidelines are:

• 1-3 bedrooms: 1,000 gallons
• 4 bedrooms: 1,250 gallons
• Each additional bedroom: Add 250 gallons

These sizes assume standard water usage. If your home has features like whirlpool tubs that increase water use, adjust the size accordingly. You should also consider any plans to finish basement areas or add bedrooms later - this would require a larger tank size to accommodate future capacity.

Undersizing a septic tank by even just a few hundred gallons can lead to serious problems. Excess solids may get flushed out into the drain field and clog pipes. You'll also experience sewage backups into the home - a health hazard and extremely unpleasant experience!

Ensure Your Septic Contractor Performs Proper Sizing Calculations

Many homeowners simply go with the minimum 1,000 gallon tank to save on cost. However, only a detailed sizing calculation based on bedrooms, home features, and occupancy will determine the optimal size.

Be sure your septic contractor performs these wastewater flow calculations before installing the tank. This small upfront investment of time and money can prevent major headaches with backups and contamination down the road.

Inlet and Outlet Piping

How wastewater enters and exits the septic tank is critical for proper treatment. Inlet and outlet piping should meet the following design standards:

Inlet Piping

The inlet pipe carries wastewater into the tank from the home. It should:

• Enter 12-18 inches below the liquid level
• Extend at least 5 feet into the tank
• Point downward, not upwards

These specifications prevent turbulence and ensure the incoming waste has time to separate. Turbulence stirs up solids and pushes them out of the tank prematurely.

Outlet Piping

The outlet pipe transfers clarified liquids out to the drain field. Proper design calls for:

• An outlet tee or baffle 1 inch lower than the inlet pipe
• An outlet tee that extends down 50-60% of the tank's depth

This configuration retains and separates solids, while allowing only clarified effluent into the drain field. The lower outlet position takes advantage of gravity.

Secure Fittings and Connections

All inlet and outlet fittings should have watertight seals. Rubber boots around tank penetrations prevent leakage. The pipes must connect securely with the proper adhesive cement.

Also ensure baffles are properly installed and secured. Baffles prevent solids and scum from escaping the tank into the drain field where they can clog and damage pipes.

Inspection and Access

Cleanouts and inspection ports allow access to inlet and outlet pipes. Make sure to include these in the design for maintenance down the road. Having to break into the tank for access is highly undesirable!

Access Risers and Lids

Access risers bring the "manholes" up to ground level for inspection and pumping of a septic tank. Proper placement and construction of these openings is essential.

Include at Least Two Risers

Building codes require at least two access risers - one for the inlet side and one for the outlet side. This allows access to both compartments for pumping and troubleshooting.

Trying to save a few dollars with a single riser is extremely short-sighted. Without inlet and outlet access, pumping the tank and diagnosing issues become difficult or impossible.

Secure, Watertight Lids

The riser lids must seal tightly against the top of the riser with a waterproof gasket. Look for tapered lids designed specifically to form a watertight seal.

Lids that simply rest on top of the riser are prone to shifting and allowing in soil or water. Make sure lids are locked or mechanically fastened as an extra anti-shift measure.

Riser-to-Tank Connection

A common source of leaks is where the riser meets the septic tank. Be sure this critical interface is sealed with a high-grade, flexible caulk or sealant. Don't allow cracks or gaps where water could penetrate.

Also ensure risers are securely connected to the tank with watertight rubber boots or bonding. Any compromise in the riser seal can allow groundwater into the tank, reducing capacity.

Structural Strength

Septic tanks must contain high-strength concrete reinforced with steel bars to withstand years of use. Here are the key structural considerations:

Concrete Specifications

• Minimum 3,000 psi compressive strength
• At least 4 inches thick for walls and floor
• Air-entrained for freeze-thaw resistance

High-quality, cured concrete meeting these specifications will prevent cracks and leakage. Don't cut corners with lower grade concrete to save money.

Steel Reinforcing Bars

Steel rebar size and spacing in the concrete walls and lids provide flexural and shear strength:

• No. 4 rebar max 18 inch spacing for walls
• No. 5 rebar max 12 inch spacing for lids

With proper steel reinforcement, the tank resists the soil pressures and weight loads placed on it. Insufficient rebar spacing risks cracks, wall failures, and deformation over time.

Watertight Construction

Septic tanks must be entirely watertight to contain sewage and prevent groundwater contamination. All joints, seals, and penetrations should be waterproofed.

Joint Sealants

High-performance caulk, mastic, or pliable sealants ensure waterproofing at all joints:

• Between tank sections
• Between risers and tank
• Between access covers and risers

Sealants maintain flexibility to accommodate shifting and settling without compromising the seal. Self-leveling caulks work well for horizontal joints.

Boots and Gaskets

Inlet and outlet pipe penetrations rely on rubber boots and gaskets for waterproofing. The boots should have a stainless steel clamp band for a tight, flexible fit to the pipe.

Construction Material

Using waterproof concrete and proper curing methods eliminates pores and pathways for water. Concrete additives can improve water resistance as well.

Leak Testing

Before burying the tank, fill it with water and check for leaks at all joints and seals. This simple test identifies any compromised areas in workmanship or materials.

We've covered the 5 most important elements of proper septic tank design. Taking the time to get these features right will ensure your tank reliably treats wastewater for decades to come.

Be sure to work with qualified designers and installers who follow all state and local codes. Don't cut corners to save money - a failure down the road will cost far more in repairs and property damage.

Investing in robust design is well worth the peace of mind knowing your septic system was built right the first time. Follow these critical guidelines, and you can look forward to many years of trouble-free wastewater treatment.