Septic System Basics

Kitt Farrell-PoeExtension Water Quality Specialist

Ag & Biosystems Engineering Dept.

Today’s Presentation:

• What is an “onsite wastewater treatment” system?

• Components of a septic system• Treatment & dispersal• Management

Common Domestic Pollutants• Microorganisms

– Bacteria, viruses, protozoa, etc.• Nutrients

– Ammonia, nitrate, phosphorus• Suspended Solids• Toxics

– Heavy metals, pharmaceuticals• Organics

– pesticides

What is an Onsite Wastewater Treatment System?

Source

Pretreatment

Final treatment & dispersal

What should an onsite wastewater treatment system do?

• Treat sewage– Public health concerns– Environmental quality concerns

• Disperse sewage• Provide acceptable level of risk – put the

“odds” on our side

Goals of Treatment• Separate solids *

• Reduce organic materials (BOD)* †• Reduce nutrients†• Reduce pathogens†• Reduce toxic discharges†

* Septic tank† Soil treatment area

Goals of Dispersal

• Disperse effluent evenly across soil treatment area to keep soil unsaturated and aerobic.

• Provide enough time in the soil for treatment to take place.

• Percolate to groundwater so original water can be recycled back into the water cycle.

Components

Pretreatment Final Treatment & Dispersal

Application-Distribution

OWTS Components

Source

EXCEPT: Place all water softener backwash into a separate component if possible.

Source: All Sewage!

Conserving water helps -• Maintain retention time in septic tank• Promote oxygen levels in drainfield

Properly dispose of cooking oils

Chemicals & Other Materials

* NOTE: Septic tanks should only receive pre-digested foods

Garbage Disposals

• Higher risk of solids carryover to drainfield• Recommend composting kitchen scraps • If used, then increase septic tank size (2X)• Expect to pump solids more often (2X)• Use an effluent screen

Pretreatment Final Treatment & Dispersal

Application-Distribution

OWTS Components

Pretreatment – Purpose: “Pretreat” wastewater so downstream component(s) can function more reliably for longer terms.

Source

Pretreatment: Septic Tank• Functions

– Separate solids, oils, and greases from liquid– Anaerobic decomposition (some, not a lot)– Keep sewage in tank at least 24-36 hours– Store solids– Dissipate energy of wastewater

Screen

As scum and sludge layers build and come

closer together, the 2-day detention time becomes less and solids removal

efficiency drops.

Pretreatment: Septic Tank

• Components– Sanitary tees– Compartments– Effluent screen– Inspection port– Manhole

Screen

screen

Septic Tank - Materials

Tanks should betested for water

tightness

Septic Tank Outlet Screens

• Functions– Helps keep solids out of

drainfield– Helps lessen energy of surge

flows

• Goes in outlet of tank or replaces it

• Variety of types of models• Will plug periodically• Needs careful periodic

cleaning

Septic Tank• Management

– Pump -- sludge & scum accumulations• 25-33% = need to pump

– Inspect• Structural soundness• All baffles/inlets/outlets, etc. are intact

Septic Tank Additives

No published scientific evidence showing additives increase system lifespan.

“Never pump again” works against the whole purpose of the septic tank, which is to trap solids and prevent them from getting to the drainfield.

OWTS Components

Pretreatment Final Treatment & Dispersal

Application-Distribution

Source

Soil Treatment Area

• Receives effluent & transmits to soil• Soil underneath provides treatment

– physical filtration– biological activity– chemical reactions

• Soil should remove disease-causing organisms and some chemicals of concern

Courtesy of National Assn of Wastewater Transporters

Pathogen capture

Courtesy of National Assn of Wastewater Transporters

Pathogen removal

How effluent moves

“Conventional” Soil Treatment Systems

• Drainfield [Soil treatment area]– Network of trenches, beds, chambers, seepage pits– Gravity distribution

Trench

Bed

Seepage Pit

Chamber Technology

“Alternative” Soil Treatment Systems

– Usually pressure distribution– Examples include mound and drip systems

Alternative Soil Treatment Systems

• Allows use of septic systems in areas with poor soils, shallow bedrock, or high water table

• Operates in all climates• Higher installation cost than conventional

drainfields

• Greater space needed

• Limits on slopes

• Requires pump O&M

Distribution Media

How are the pieces connected?

• Parallel • Serial

Distribution boxDrop box

Construction Concerns

• Compaction• Material selection

– Clean?• Soil conditions• Divert run-on

away

Drainfield Management

• Traffic control• Surface water• Inspection pipes

– 1-½” to 4”– At the end of the

system– Connected into the

system

REMEMBER:Conventional Septic System

• Effective where lots are appropriate• Must have well-drained soils with limited

slopes• Requires regular inspection and periodic

pumping• Alternatives broaden applications and can

improve effectiveness• Avoid garbage grinders• Groundwater problems

are possible• Systems fail if forgotten

6.22

IN SUMMARY:What conditions are important to

maximize system function?• An environment that allows bacteria to thrive

– proper temperature and pH, no deterrent to growth– anaerobic conditions in septic tank

• Watertight and structurally sound septic tank• Sufficient time in tanks and soil• 2-5 feet of slow, downward flow through dry,

aerobic soil – but not too coarse• Certain horizontal distances from wells, surface

water, etc. to soil treatment area

QUESTIONS??

http://ag.arizona.edu/waterquality/onsiteOr

http://extension.arizona.edu/water-portal/onsite-wastewater-education-program