Septic Systems -Design
conventional systems
In Arizona, an on-site septic system is either “conventional” or alternative. Alternative systems are more costly and maintenance intensive and are usually only chosen if the soils conditions or other site limiting conditions require it. Alternative systems have to be designed by a licensed individual. Here we will only focus on conventional systems!
First Step
Site Investigation
The first step in designing an on-site septic system is the so called site investigation. It will determine the SAR (Soils Absorption Rate) and other site conditions that will have an influence on the system. For example a large percentage of rock in the soil may trigger an alternative system. This has to be done by a licensed person and needs to be signed off by the permitting authority (usually County) before continuing with the design.
Second Step
Now it is time to decide what system will be constructed. Possible systems are listen in the Arizona Administrative Code (R18-9 PART E. TYPE 4 GENERAL PERMITS), the most common ones are:
- Chamber
- Trench
- EZ Flow
The Flows
To determine the flows, the number of fixtures in the house (garage, casita, etc.) are listed and fixture units are totaled. A toilet for example accounts for more than a single bathroom sink:
| Fixture Type | Unit |
| Bath Tub | 2 |
| Bidet | 2 |
| Clothes Washer | 2 |
| Dishwasher (separate from kitchen) | 2 |
| Lavatory (bathroom sink), single | 1 |
| Lavatory, double in masterr bedroom | 1 |
| Shower, single stall | 2 |
| Sink, bar | 1 |
| Sink, kitchen (including dishwasher) | 2 |
| Sink, service | 3 |
| Utility Tub or Sink | 2 |
| Water Closet (toilet) | 3 |
The number of bedrooms (The definition is very WIDE) also influence the determination of the flow.
The Size
Many Counties have worksheets available for the most common systems to calculate the size. Links give here are for Maricopa County and may not be updated:
(Find other sizing charts HERE)
The Design Flow, Tank Size, and Soils Absorption Rate are added to the worksheet to calculate the size of the disposal system. Maricopa County (and some other Counties) also have a number of other design documents to help with this process.
For the application a list of materials used for construction needs to be added as well.
For Maricopa County, the site containing all needed documents is HERE.
Conventional System Application
Conventional System Design Instructions
Materials, Components and Equipment List
NOTE: THE INFORMATION GIVEN ON THIS PAGE MAY NOT BE ACCURATE. ALWAYS CHECK WITH THE COUNTY DIRECTLY TO GET THE MOST UP TO DATE INFORMATION!