CEE484Decentralized and Onsite

Wastewater Management and Reuse

Septic TanksApril 4, 2007

H. David Stensel

Typical Septic System

evapotranspiration

septic tank

basement

soil absorption

well

treatment

ground water

streams, lakes

House

trenches

effluentbaffle

HISTORY

M. MOURAS OF FRANCEDEVELOPED AND OBTAINEDPATENT IN 1881

Mouras Automatic Scavenger“A mysterious contrivance consisting of a vault hermetically closed by a hydraulic seal. By a mysterious operation, and one which reveals an entirely novel principle, it rapidly transforms all the excrementitiousmatters it receives into a homogeneous fluid, only slightly turbid, and holding all the solid matter in suspension in the form of scarcely visible filaments. The vault is self emptying and continuous in its workings.” (1)

DONALD CAMERON (1895) OFGREAT BRITAIN CORRECTLYDESCRIBED SEPTIC ACTIONSAND NAMED IT SEPTIC TANK

FIRST USE IN USA, LATE 1880s

Septic Tank

Baffle

Effective Volume

Riser

Cleanout Pipe

What are treatment mechanisms and typical performance?

Septic Tank Functions• Solids removal by settling & flotation

– 60-80% solids removal– 1-2 days liquid detention time– Quiescent settling – Grease and scum floats

• Anaerobic digestion of settled solids– Solids hydrolysis– Fermentation to organic acids (acetate etc)– Conversion of acetate and hydrogen to methane– Sulfate reduction to H2S

• Storage of solids

Septic Tank Zones

IN OUT

SLUDGE

SCUM

SETTLING ZONE

Half of the volume may be occupied by sludge and scum, so detention time for settling based on settling zone volume

Treatment performance is a little better than primary sedimentation performance at

wastewater treatment plants40-60% BODr

60-78% TSS removal

Materials of construction• Concrete• Fiberglass• Polyethylene

• Must be Watertight and Structurally Sound– Support a 2500# wheel load in addition to soil load– Tanks must be properly reinforced according to a standard

• ASTM• NPCA

– Want to avoid LUST– Watertightness test

• Hydrostatic• Vacuum

Tank Materials

Fiberglas

Polyethylene

Concrete

Joint Design for Concrete Tanks

Mid-seam

Top seam

“Monolithic” Concrete Tanks

Concrete Tank Seams (cont.)• Achieving a watertight joint:

– Extra butyl rubber wrap around joint– Joint must be tested to be sure it does not leak

Tank Configuration and geometry• Length to width ratios = 3:1• Inlet to outlet drop ~ 2 in.

• Example of dimensions for 1500 gal tank-height = 6ft, width = 4.5 ft, length = 8.0 ft-soil depth above tank = 4 in. to 4 ft (local conditions)

Orenco

14. TYPICAL GRAVITY SEPTIC TANK(SINGLE COMPARTMENT)

15. DOSING SEPTIC TANK*(SINGLE COMPARTMENT)

Orenco

*Installation may vary with centrifugal pump set on tank bottom.Single Compartment with and without dosing chamber

Two Compartment Tank(need air vent between compartments)

2/3 total volume

1/3 total volume

Advantages: reported performance results variablelose settling distance2nd compartment good

if influent pumping used

Meander Tank ExampleINLET

OUTLET

Baffles

Advantages of a meander tank• Longer flow path• Opportunities to drop solids as flow turns• Most solids are removed in first chamber

Tank Sizing• Generally prescribed by the permitting agency for

individual homes based on home size• Typical Tank Sizes

– 1-2 bedroom 1000 gal– 3 bedroom 1500 gal– 4 bedroom 2000 gal

• Crites and Tchobanoglous for large tanks– Function of pump out interval, Qavg, and peaking factor– Rule of thumb – about volume = about 5 times avg gallons/d at

3-4 year pumping interval• Criteria: Hydraulic detention time plus solids storage

– 1 to 2 days detention of design flow– Add solids storage volume equal to 1/3 – 1/2 of the above

hydraulic detention

Tank Appurtenances

• Tees and baffles• Effluent screens• Access risers

Inlet and Outlet Baffles/Tees

• Inlet baffle – Directs the flow– Minimizes turbulence and

short circuiting

Sludge

Clear Zone

Scum

Dual Chamber Septic Tank

• Outlet baffle– Assures outflow comes

from clear zone– Holds floating scum in the

tank

Tee-type baffle outlet

• Baffle made from sanitary tee and 4-in pipe nipples

• Positioned directly under tank opening for access

• Some older tanks have (or had) tee-type baffles made of clay or concrete pipe– These deteriorate and fall off

in time– Should be replaced when

tank serviced

Inlet baffle of concrete cast into tank

• Curtain baffle penetrates to well below liquid depth

• Outlet ports are made like this too

• Groove at top allows gas transfer across tank and up sewer to roof vent

Effluent Screens

• Designed to keep larger suspended solids in the tank

• Control outflow rate• Protect the downstream components• Typically replace the outlet baffle• Require riser to grade for access to screen

Installation issues• Location

– Tank– Sump– Pump vault

• Can be equipped with alarm• Screen in second compartment of a two

compartment tank will require less service• Should be secure in place • No bypass flow if clogging occurs• Housing should not interfere with normal tank

cleaning

Choosing an Effluent Screen

• Ease of serviceability• Size appropriately for the flow• Openings of 1/16 – 1/8 inch• Designed to prevent solids bypass during

cleaning• Locate so that access for pumping is not

hampered

Proprietary effluent screens

Effluent Screen installed to replace outlet baffle

Location of Effluent Screen

Access risers

• Provide easy access to tank and components

• A must for tanks containing effluent filters or pumps

• Shallow tanks and short risers – the preferred situation

Riser Design

Operation and Maintenance

what is key issue for homeowner?

Operation and Maintenance of Septic Tanks

• Solids accumulate in septic tanks– sludge in the bottom – scum on top

• Pump before solids begin to increase in the effluent

Frequency Of Pumping• Calendar recommendation

– Every 3-5 years

• As needed – Measurement of sludge and scum

• Sludge accumulation– 50 – 125 gal/capita every 4 years

Determining Need for Pumping

Pump when:• scum clear space is <3” or• sludge clear space is <9”

Sludge

Total Clear Space

Liquid Level

Scum

Outlet

Sludge Clear Space (9" min.)

Scum Clear Space (3" min.)

Measuring Scum

Devices for Measuring Sludge

???? Infrared eye on a pole Clear plastic tube with a foot valve on the end

Pumping as Needed According to Actual Measurement

• Not done much– Most tanks are buried – no riser to grade– “If you have to dig up the tank, you might as

well just pump it”– Lack of skill

• Opinions vary on frequency• May be dictated by state regulation• Recommendations here are based on actual

measurements of when accumulated solids affect effluent quality –i.e. science

46912

Pump-out interval (yrs)4579

Pump-out interval (yrs)

8765Number of occupants 8765

Number of occupants

1500Capacity (Gallons)1500

Capacity (Gallons)

591425

Pump-out interval (yrs)4711

22

Pump-out interval (yrs)

5432Number of occupants 5432

Number of occupants

1000Capacity (Gallons)1000

Capacity (Gallons)

US Public Health Service (1955)

Glide Effluent Sewer (1987)

Servicing Effluent Screens

• Clean screen when pumping tank

• Wash material back into tank

• If cleaned at other times:– Remove and clean

elsewhere– Clean over inlet end– No solids bypass

Effluent Screen Cleaning

• Wear gloves!• Wash off directly into

the inlet end• Solids bypass

protection

Myths and Additives• Tanks typically do not require additives

– No need to “start” a tank with a dead chicken– Adding yeast, while harmless, is not needed– Commercial additives are normally not needed

• Beware of any additive that suggests it will reduce pumping frequency– Normal function means some accumulation

• Nonbiodegradables – e.g. synthetic fabric lint– Solids may be washed out to next downstream

treatment component– Independent research shows no benefit

• Adding things to tank that are not degraded well or cause clogging or build up of solids, e.g.– Cooking fats, disposable diapers, female sanitary products,

coffee grounds, garbage grinders• Adding toxic chemicals, e.g.

– Paints, paint thinner, pesticides, drano, gasoline• Laundry

– Use lint filter• Higher flows

– Space water use, increase occupancy, sump pump discharge, water softener

• Other

What can negatively effect septic tank and drain field perfomance?

Operation and Maintenance

• In most cases, the homeowner is the operator

• Homeowners need basic information on operation– How the system works– How to use the system– What should not be put into septic systems

• Homeowner must be encouraged to– Have the system inspected periodically– Pump the tank as needed