Guide for Septic Tank Replacement

How a Septic Tank Works 

Your septic tank is a key part of your home’s wastewater system. It:

• Holds wastewater for at least 24 hours
• Allows heavy solids to settle to the bottom (sludge)
• Allows lighter materials to float to the top (scum)
• Sends the liquid between these layers to the drainfield 

Over time, sludge and scum build up. The tank must be pumped periodically to prevent damage to the drainfield.

After many years — especially with older metal tanks — tanks may leak or become structurally unsound. When this happens, the tank must be replaced. A minor repair permit is required to replace a septic tank. 

Septic Tank Size Requirements

For single-family homes:

• Minimum 1,000 gallons for homes with up to 4 bedrooms
• Minimum 1,500 gallons for homes with more than 4 bedrooms

The tank must be on DEQ’s Approved Tank list.

Note: Some approved tanks may not be suitable for sites with high groundwater.

Tank Placement  

Follow Manufacturer Instructions

Each tank must be installed according to the manufacturer’s installation guide to ensure it remains level, watertight, and structurally sound.

Proper Installation Includes:

• Stable bedding material (such as pea gravel)
• Tank placed level side-to-side and end-to-end
• Proper excavation depth to maintain plumbing code requirements
• Backfilling to the inlet/outlet saddle for safe inspector access, as shown below

Gravity Flow Requirements

• Effluent pipe must have at least 2 inches of fall
• Must maintain 4 inches per 100 feet minimum grade
• Tank outlet must be at least 8 inches higher than the first drainfield trench

If these elevation requirements cannot be met, a pump system is required. 

Setback Requirements

Replacement tanks must meet minimum setback distances from:

• Buildings
• Property lines
• Wells
• Other site features

Service Access Risers

All tanks must have:

• At least one watertight service riser to finished grade
• Minimum 20-inch diameter (if soil cover is 36 inches or less)
• Minimum 30-inch diameter (if soil cover exceeds 36 inches)
• Secure, gasketed lid to prevent odors and unauthorized access

Multi-compartment tanks must have a riser over each compartment.

Anti-Flotation (High Groundwater Areas)

If groundwater is present seasonally, anti-flotation measures may be required.

Some tanks cannot be installed where groundwater rises above the bottom of the tank. Follow manufacturer guidance if anti-flotation is required.

Connecting to an Existing Drainfield

The connection point between the tank and absorption system must be exposed for inspection. This may include:

• Distribution boxes
• Serial system boxes
• Drywell connections
• Transition to absorption trenches

24-Hour Water Tightness Test

After installation, the tank must pass a DEQ-required 24-hour water test:

• Fill tank to 2 inches above the riser-to-tank joint
  • Do not fill higher — this can damage the tank.
• Mark water level, date, time, and initials.
• Check after 24 hours.
• If water drops more than 1 inch, identify and fix the problem.

Do not add or remove water before inspection.

The tank must remain on test for County inspection.

Decommissioning the Old Tank

• The old tank must be properly decommissioned:
• Pumped by a licensed septic pumping service
• Removed from the property or filled with approved material (reject sand or gravel)
• Installer must submit:
  • Tank Decommissioning Certificate
  • Pumping receipt

For site-specific questions, contact us before installation begins.