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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