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Onsite Wastewater Systems

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1. How do Onsite Wastewater Treatment Facilities (OSWTFs) work?

Most OSWTFs include a septic tank, which digests organic matter and separates floatable matter, such as grease and oil, and solids that settle in the tank, from the wastewater. Soil-based systems discharge septic tank effluent (liquids) into a series of perforated pipes or chambers buried in a disposal field or pit designed to slowly release effluent into the soil.

Alternative systems may use pumps or gravity to trickle septic tank effluent through sand, organic matter (e.g. peat, sawdust), constructed wetlands, or other media to remove or neutralize pollutants, such as disease-causing pathogens, nitrogen, phosphorous, and other contaminants. Some alternative systems are designed to evaporate wastewater or disinfect it prior to discharging it into the soil.

2. Why do Onsite Wastewater Treatment Facilities fail?

Most system failures are related to inappropriate design and poor maintenance. Soil-based systems (with a drain field) may have been installed at sites with inadequate or inappropriate soils, excessive slopes, or high ground water tables. These conditions can cause hydraulic failures and water resource contamination. Failure to perform routine maintenance can cause solids or greases and oils in the tank to migrate into the drain field and clog the system.

The easiest way to ensure the longevity of your septic system is to routinely pump the tank every three to five years, not dump unapproved products into the system, and clean the effluent filter inside the tank every six to twelve months.

3. What items should not be put down the drain?

The following items should not be put into the septic disposal system:
- Hair Combings
- Cleansers
- Cigarette Butts
- Coffee Grounds
- Condoms
- Dental Floss
- Disposable Diapers
- Fat, Grease, Oil
- Gauze Bandages
- Kitty Litter
- Paints
- Paper Towels
- Pesticides
- Photographic Solutions
- Sanitary Napkins
- Solvents
- Tampons
- Thinner
- Varnishes
- Waste Oils

Do not dispose of non-biodegradable materials with wastewater through drains and toilets as they displace storage volume of the septic tank and will upset the biological activity occurring in the tank.

4. Do Onsite Wastewater Treatment Facilities (OSWTFs) cause health or water quality problems?

OSWTFs that are properly planned, designed, sized, installed, operated and maintained can provide excellent wastewater treatment. However, systems that are sited in densities that exceed the treatment capacity of regional soils or are poorly designed, installed, operated, or maintained may cause problems. The most-documented problems involve contamination of surface and ground waters with disease-causing pathogens and nitrates.

5. Can I apply for a permit to drill a well at the same time I apply for the Onsite Wastewater Treatment Facilities (OSWTF) permit?

Yes. Private water well Notice of Intent (NOI) permit applications, for properties up to five acres, are available at our office, through Arizona Department of Water Resources (ADWR), or through your drilling company. The application must be filled out completely and accurately, including well driller information.

All required documents and fees must be submitted to Maricopa County Environmental Services Division. Once the well site inspection is completed, all documents will be forwarded to ADWR for final processing of the well permit.

Arizona Department of Water Resources website

6. My neighbor’s OSWTF is located on my property. What do I do?

You and your neighbor must contact the Maricopa County Environmental Services Department to obtain the necessary permits and instructions to install a new system on your own lot. If a lot split occurs and the OSWTF no longer meets any minimum setback requirement, the system must be relocated. Additional inspections, plan reviews and fees may apply.

If a lot split had taken place prior to your ownership and the property line now cuts through the existing OSWTF (or the neighbor’s OSWTF), splitting the system between the two lots, the issue then becomes a legal matter. In these cases, it is recommended that you contact legal counsel or your real estate agent to obtain advice on how to rectify the situation. In most cases, each lot will be required to install an OSWTF.

7. What is an alternative system?

An alternative system is any type of septic system other than a septic tank using gravity to convey effluent for disposal into trenches, beds, chambers, or pits. Typical alternative systems include aerobic system, cap system, sand lined trench, intermittent sand filter, Wisconsin mound, composting toilet, peat filter, textile filter, pressure distribution, subsurface drip irrigation, and Ruck system. Alternative OSWTFs must be designed by and stamped with the seal of a Professional Engineer (P.E.), registered to work in the state of Arizona and with expertise in the on-site wastewater field.

8. What are the different types of alternative systems allowed in Maricopa County?

See for the current approved list of alternative components allowed in the state of Arizona. These components may be used alone or with other components in an alternative septic disposal system.

This list is provided by the Arizona Department of Environmental Quality (ADEQ). Maricopa County may have additional requirements or restrictions. Approved Proprietary Products (PDF)

9. Do I have to hire an engineer to design my system and draft my plans?

No. Only an alternative system requires an engineer to properly design, select and lay out the onsite system. Conventional systems may be designed by the homeowner or any other party authorized by the owner provided all requirements listed in the Arizona Aquifer Protection Permit Rule and the Maricopa County Onsite Wastewater guidelines are observed and satisfied.

10. Can I make changes after I have submitted the NOID packet?

Yes. All changes must be identified on the newly-submitted, revised site plans with the date of the revision clearly marked on all submittals. Recalculations of the size of the septic system, site plan revisions and additional soils testing may be required, at which time a second plan review must take place before the permit is issued.

11. Can I make changes to an approved plan?

Yes. Any proposed changes to the septic system that occur after permit issuance will require an additional plan review. New or revised site plans and/or recalculations must be submitted and approved prior to construction. Additional fees will apply.

Undocumented changes observed during the construction inspection will result in costly delays, additional procedures, and, potentially, additional fees. The Maricopa County Environmental Services Division encourages all applicants to spend a little extra time planning the project, keep up to date on any changes that do occur throughout the project, and notify the proper authorities of such changes as soon as possible.

12. Can I request a partial construction inspection?

Yes. The same procedure applies as for a construction inspection. You may designate that it is a partial inspection on your requests. Indicate which components of the system are to be inspected.

13. What type of rock should I use in the trenches, beds, or pits?

Leach rock for Onsite Wastewater Treatment Facilities must be cleaned, washed aggregate with a screened diameter of 0.75 to 2.5 inches and a void space of 30%.

14. What are the requirements for installing a septic tank?

The tank should be installed on a six inch bed of pea gravel or sand to maintain stability. This is required when the tank is installed where jagged or large rocks are present. Access openings on the tank must be brought to within six inches of finished grade. If tanks are installed such that the top of the tank is greater than twelve inches below finished grade, MCESD-approved inspection risers (concrete or dual-wall corrugated polyethylene) are required. Tanks deeper than twenty-four inches are required to have a reinforced lid. The top of the tank shall not be greater than sixty inches below finished grade.

Native soil can be used to backfill the tank excavation as long as there are no jagged or large rocks in the mix that could damage the tank during or after backfilling. If jagged or large rocks are present, the soil must be screened to remove the potentially damaging rocks prior to backfilling.

The inlet t-baffle and in-line effluent filter at the outlet end of the tank must be easily accessible for maintenance and cleaning. Ensure that the inlet t-baffle can be easily reached and the effluent filter can be pulled up through the smaller inspection port on the top of the tank. If not properly aligned, the larger inspection plugs will need to be removed for routine maintenance.

15. What are the requirements for installing the distribution box?

Distribution boxes (d-boxes) must be level, set on a firm foundation and placed in the correct direction. One opening is one inch higher than the remaining openings; this is the inlet side of the d-box. Attach the inlet end of the d-box to the septic tank by a solid pipe at least two feet in length.

Pipes must extend into the d-box one inch and be level, secure, and watertight. Once approval to backfill has been granted, seal the d-box lid so that it is watertight. All connections outside of the disposal field must be watertight.

16. What are the requirements for installing the disposal field?

Disposal field risers shall be brought to within six inches of finished grade or higher. Disposal pipes that are deeper than twenty-four inches will require a minimum of SDR 35 or equivalent pipe. Trench bottoms and disposal pipes must be level.

17. My system is failing. What do I do?

If your septic system is not functioning properly, contact a plumbing or septic specialist. If one component of the system (tank or disposal) has failed, an Alteration permit to replace that component will be required. If both the tank and the disposal field have failed, a new Onsite Wastewater Treatment Facility must be installed.

18. Can I use the existing septic system for my new house?

It may be possible to use the existing septic system with a new house. For more information, view the Minor Plan Review

19. When can you release my building permit?

Environmental Services can only release building permits for unincorporated areas. The building permit will be released when the OSWTF is approved for construction. Building permits will be released after plan review has been completed.

Construction in incorporated areas may have different requirements. Contact the appropriate building department for more information.