tank functions - neiwpccneiwpcc.org/onsiteshortcourseold/onsite_docs/2016/septic tank... ·...
TRANSCRIPT
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Tank Functions
• Solids removal by settling & floatation
– 60-80% solids removal
• Anaerobic digestion
• Storage of solids
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Parameter
Average
Raw
Sewage
Influent
Average
Septic
Tank Effluent
% Removal
BOD (mg/L) 308 122 60
TSS (mg/L) 316 72 77
Grease (mg/L) 102 21 79
Seabloom, R.W., T.R. Bounds, and T.L. Loudon. 2005.
Average Removal of BOD,
TSS, and Grease in Septic Tank
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ORGANIC MATTER
GASES + HUMUS
CO2
CH4
H2S NH3
Anaerobic Digestion
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Biological Activity in the Septic Tank
• Anaerobic (without Oxygen)
– Incomplete
– Cheap and easy
– Reliable
• Gases produced are odoriferous
• Not all solids in tank are biodegradable
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Factors that Influence
Anaerobic Digestion
• pH
• Chemicals
• Highly variable flow patterns
• Pharmaceuticals
• Process wastewaters
• Lack of tank maintenance
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Factors that Influence
Wastewater Strength
• FOGs
• Flow pattern
• Flow rates
• Nonbiodegradable items
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Septic Tank Design
• Sizing
• Geometry
• Compartments
• Vehicular traffic
• Appurtenances
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Effective Volume (new tank)
Baffle
Effective
Volume
Riser
Cleanout
Pipe
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Tank Sizing
• Generally prescribed for individual homes
based on home size
• 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
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Septic Tank Sizing Example
• Consider a 3-bedroom home
• Design flow: 3 br, 2 people/br, 75
gpd/person
– Flow = 3 x 2 x 75 gpd = 450 gpd
– Provide for 2 day detention => 2 x 450 = 900
gal
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Septic Tank Sizing Example
• Add solids storage
– 1/3 of the above = 1/3 x 900 = 300 gal
• Total tank volume = 900 + 300 = 1200 gal
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Septic Tank Sizing Example
• This is the minimum recommended tank
size
– The tank should have two compartments
• Many regulatory agencies now require
1,500 gal tank for a 3-br home, but sizing
starts with a procedure like this.
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Goal: Near Zero Velocity for
Optimum Solids Removal • Maximize distance between inlet and outlet
• Length:Width ratio at least 3:1
• Inlet to outlet drop ~ 2"
Sludge
Clear Zone
Scum
Dual Chamber Septic Tank
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Other Factors that Affect Tank
Size • Garbage grinders
– Add to solids accumulation rate and organic
load
– May add grease and oil
– Increase hydraulic load some
• Though not recommended with septic
systems
– they will be used in many homes.
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Other Factors that Affect Tank
Size • Sewage (grinder) lift pumps
– Increase turbulence in the septic tank
– Should discharge into sewer line – not directly
to tank
– Two compartment tanks highly recommended
with pumps
– Set pumps for minimum discharge volumes
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Tank Compartments
• Advantages of multiple compartments
– More complete solids removal
– Improved effluent quality
– Protect against solids discharge due to lack of
maintenance
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Two Compartment Septic Tank
2/3 total
volume 1/3 total
volume
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Vehicular Traffic
• Standard concrete tanks are not designed
to handle traffic loads
– ASTM Standard C-857 provides information
on these design issues
• Use other tanks in areas subject to traffic
only with manufacturer guidance and
engineer approval
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Tank Appurtenances
• Tees and baffles
• Effluent screens
• Access risers
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Inlet and Outlet Baffles/Tees
• Inlet baffle
– Directs flow
– Minimizes
turbulence and
short circuiting
• Outlet baffle
– Assures outflow
comes from clear
zone
– Holds floating scum
in tank
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Inlet and Outlet Baffles/Tees
Sludge
Clear Zone
Scum
Dual Chamber Septic Tank
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Tee-Type Outlet Baffle
• Baffle made from
sanitary tee and 4-
in pipe nipples
• Positioned directly
under tank opening
for access
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Curtain Baffle
• Penetrates into clear zone
• Groove at top allows gas transfer across tank and up sewer to roof vent
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Baffle Fastened to Tank Wall
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Effluent Screens
• Designed to keep larger suspended solids
in the tank
• Control outflow rate
• Protect downstream components
• Typically replace the outlet baffle
• Require riser to grade for access to screen
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Effluent Screen Installation
Issues • Location
– Tank
– Sump
– Pump vault
• Can be equipped with alarm
• Screen in second compartment of a two-
compartment tank requires less service
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Effluent Screen Installation
Issues
• Must be secure in place
• No bypass flow if clogging occurs
• Housing should not interfere with normal
tank cleaning
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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
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Proprietary Effluent Screens
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Location of Effluent Screen
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Access Risers
• Provide easy access to tank &
components
• A must for tanks with effluent screens or
pumps
• Shallow tanks and short risers preferred
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Riser Design
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More Risers
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Safety
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Tank Construction
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Tank Materials
Fiberglass-
reinforced
plastic (FRP)
Polyethylene/Poly-
propylene
Reinforced
concrete
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Structural Soundness
• Withstand handling and transport
• Not susceptible to damage during
installation
• Resist external and internal pressures
• Properly reinforced according to a
standard
– ASTM
– NPCA
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Seam Location for Concrete
Tanks
Mid-seam
Top seam
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Sealing Materials for Pre-Cast
Tanks • Blended sealant compounds
– Butyl-rubber based
– Asphalt-based (bituminous)
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Achieving a Watertight Joint
• High quality
mastics, seal
gaskets
• Seams must be
smooth, clean and
dry
• Proper placement
of mastic
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Extra Measures
• Butyl rubber wrap around joint
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Proof Testing Concrete Tanks
for Structural Soundness
• Tanks should reach 4,000 psi before
delivery to site
• Should comply with ASTM and NPCA
standards
• Other engineering tests also available
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Access Risers for Pre-cast
Tanks
• Made from various materials
• Cast-in-place or added after tank
construction
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Cast-in-Place Concrete Risers
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Cast-in-Place Poly Risers
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Overall Quality of Tanks: Looks
are not Everything • Cosmetic deficiencies may not affect
performance
• Good-looking tanks may have structural
deficiencies
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Ultimately, it is Essential to
TEST • Investigate irregularities in tank of any
material thoroughly
• If unsure, consult with manufacturer or
engineer
• Testing will ensure quality, watertight
installations.
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Why do we Care?
• Exfiltration could release untreated
sewage deep in the soil
• Infiltration may occur
– Disrupt settling processes in tanks
– Overload drainfield or downstream
components
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Possible Points of Leakage
• Weep holes at the base of the tank
• Mid-seam or top seam joint
• Inlet/outlet pipe penetrations
• Tank top/access riser joint
• Access riser/lid joint
• Any damaged, improperly-formed location
or area where material is too thin
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Watertightness
• Watertight seals
– All joints
– Pipe
penetrations
– Riser and lid
Dual Chamber Septic Tank:
Points of Infiltration
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Testing for Watertightness
• Hydrostatic (water) testing
• Vacuum testing
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Hydrostatic Testing New Tanks
• Prior to backfilling
– Cap pipes
– Fill 2" into riser
– Soak for 24 hrs
– Refill if concrete
– Check in 24 hrs
– Allowable loss is
less than one
gallon
Water Test
Less
Than 2"
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Vacuum Testing Equipment
Pipe seal
Gage to
measure
vacuum
Plate seal on
top of riser or
tank
Vacuum pump
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Checking Existing Tanks
for Watertightness
• Plug inlet and outlet
• Ensure no flow
• Water or vacuum test
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Watertightness Indicators
• Root intrusion
• High water table area:
– Pump during wet season and look for
infiltration
– Outflow when there is no inflow
– Beware of floatation
• Excavate outside of tank and look for
evidence of exfiltration – blackness, odor,
etc.
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Tanks Operation &
Maintenance
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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
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Frequency Of Pumping
• Calendar recommendation
– Every 3-5 years
• As needed
– Measurement of sludge and scum
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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.)
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Pumping Frequency
• Opinions vary on frequency
– Most tanks are buried – no riser to grade
• “If you have to dig up the tank, you might as well just
pump it”
– May be dictated by state regulation
• Should be based on actual measurements
– when accumulated solids affect effluent quality –
i.e. science
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Tank Inspection at Time of
Pumping • Tank should be thoroughly inspected
– Damaged or missing baffles
– General tank deterioration
• especially in the head space above the water level
– Honeycomb in concrete surface
– Pin holes or non-uniform wall thickness in
fiberglass or plastic
– Root intrusion
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Root Intrusion
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Tank Inspection at Time of
Pumping • Other indications of leaks
– Fluctuating tank levels
– Damaged seams
– Cracks
• Consider adding an effluent screen
• Inspect risers and lids for leakage or
damage
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Additional Considerations
• Two compartment tanks: open and inspect
both chambers
• Pumping too often may be detrimental
– Normal development of scum and sludge layers
– Normal population of beneficial microbes
– Increased the burden of septage disposal
– Unnecessarily adds to cost for owner
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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
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Servicing Effluent Screens
• Wear gloves! – (and long pants)
• Wash off directly into the inlet end
• Solids bypass protection
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Servicing Effluent Screens
• Often, a tool is
needed to remove
the screen for
cleaning and then to
replace it into the
housing
• Note that riser
opening allows easy
access to filter
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Factors that Increase Screen
Cleaning Frequency
• High fat, oil and grease in sewage
• Extensive hair or laundry lint
• High water usage or high peak flows
• Screen too small for the application
• Backwash from water softener
– hotly debated topic
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Excessive Screen Clogging
May Indicate
• Lack of proper biological activity in the
tank
• Excessive flows
– Infiltration
– Leaky plumbing fixtures
• Neglecting to pump the tank when needed
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Myths and Additives
• Tanks typically do not require additives
– No need to “start” a tank with a dead chicken
(or possum)
– Adding yeast, while harmless, is not needed
– Commercial additives are normally not
needed
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Myths and Additives
• Beware of any additive that suggests it will
reduce pumping frequency
– Normal function means some accumulation
– Solids may be washed out to next
downstream treatment component
– Independent research shows no benefit
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