Introduction to Septic Tanks John R. Buchanan, Ph.D., P.E. University of Tennessee

Introduction to Septic Tanks More than just a box in the ground –provides pretreatment of household wastewater –the soil provides final treatment

Typical Septic System

Agenda Function Tank geometry and volume Water movement through the tank Inlets and Outlets Effluent screens Watertighness Other tanks in septic systems

Manual of Septic Tank Practice First printed in 1957 Indicated that the primary purpose of the septic tank was to protect the drainfield

Pretreatment Fundaments The soil is a powerful wastewater treatment plant –but it has its limits Pretreatment protects the soil system –from waste materials that will not degrade –from solids that would plug the infiltrative surface within the trench

Separate the Solids Septic tanks –supposedly first tank was developed about 1860 –and after 10 years of use, it was expected to full of fecal solids found that the solids had liquefied and the rest is history

Septic Tank Function Primary Treatment –Designed to skim off floating material retain sinking material provide a minimum amount of anaerobic digestion have two or three design-flow days volume dampen inflow rate

Average Removal of BOD, TSS, and Grease in Septic Tank Parameter Average Raw Sewage Influent Average Septic Tank Effluent % Removal BOD (mg/L) TSS (mg/L) Grease (mg/L) Seabloom, R.W., T.R. Bounds, and T.L. Loudon

Typical Cross Section Risers to grade Outlet tee with effluent screen Inlet tee

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

ORGANIC MATTER GASES + HUMUS + H 2 0 CO 2 CH 4 H 2 S NH 3 Anaerobic Digestion

Factors that Influence Anaerobic Digestion pH Chemicals Highly variable flow patterns Pharmaceuticals Process wastewaters Lack of tank maintenance Fat, oils, and grease (FOG) Non-biodegradable items

Suspended Solids Need to separate the solids much of the septic solids are near to or less than the density of water therefore they do not sink –paper products and FOG tend to accumulate as scum difficult to degrade –Sludge is mostly dead bacteria (biomass) and heavier solids

Septic Tank Effluent Discharge from septic tank –Much of the solids have been removed still have suspended particles –Some pathogen reduction cooler temperature than human body different oxygen states –Not much conversion of nutrients

Septic Tank Design Sizing Geometry Compartments

Goal is Near Zero Velocity across the Tank for Optimum Solids Removal Maximize distance between inlet and outlet Length:Width ratio – at least 2:1 –3 to 1 is better Inlet to outlet drop – ~ 2”

Attenuation of Flow Quick example –Washing machine discharge 20 gallons in two minutes (2.67ft 3 ) 10 gpm –septic tank is 4’ wide by 8’ long (32 ft 2 ) 20 gallons will raise depth by 1 inch –septic tank outlet 4” diameter, peak outflow is about 3 gpm roughly speaking –2 min inflow and 8 min outflow

Velocity Reduction Inflow velocity –assuming a 2% pipe slope and PVC pipe 3.4 fpm peak Tank cross section –4’ wide by 5’ tall peak outflow rate 3 gpm ft/min horizontal velocity in tank 7.5 hours to go across tank particles are settling as water moves across

Previous Example Assumed No Solids in Tank Solids reduce the cross section area of tank –with 12” of sludge on bottom –and 6” of scum on top –cross sectional area is 3.5’ tall and 4’ wide Effective velocity across tank increases –0.029 ft/min –61% velocity increase

Determining Need for Pumping Pump when –scum clear space is <3” or –sludge clear space is <9”

Texas Septic Tank Rules Daily FlowSeptic Tank Volume 250 gpd750 gallons 350 gpd1000 gallons 500 gpd1250 gallons

Texas Design Flows SourceDesign Flow per Home 1 or 2 bedroom < 1500 ft gpd 3 bedroom < 2500 ft gpd 4 bedroom < 3500 ft gpd

So, A typical three-bedroom home –will have a 1,000 gallon tank –volume measured from inside bottom of tank to outlet invert –must be at least 30”

Regulations Regulations are the minimum standard –these are the rules –they are prescriptive However –sometimes we need to go beyond the minimum to ensure performance –the regulations do not ensure a system will work

General Tank Sizing Generally prescribed for individual homes based on home size (bedrooms) 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

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

Septic Tank Sizing Example Add solids storage –1/3 of the above = 1/3 x 900 = 300 gal Total tank volume = = 1200 gal

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.

Other Factors that Affect Tank Size Garbage grinders –adds to solids accumulation rate and organic load –may add fat and oil –increase hydraulic load some Though not recommended with septic systems –they will be used in many homes.

Other Factors that Affect Tank Size Sewage (grinder) lift pumps –When fixtures are lower than tank inlet 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

Tank Compartments Advantages of multiple compartments –More complete solids removal –Improved effluent quality –Protect against solids discharge due to lack of maintenance

Two Compartment Septic Tank 2/3 total volume 1/3 total volume

Tank Compartments

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

Tank Appurtenances Tees and baffles Effluent screens Access risers

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

Tee-Type Outlet Baffle Baffle made from sanitary tee and 4-in pipe nipples Positioned directly under tank opening for access

Curtain Baffle Penetrates into clear zone Groove at top allows gas transfer across tank and up sewer to roof vent

Baffle Fastened to Tank Wall

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

Effluent Screens Riser must be installed for maintenance

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

Effluent Screen Installation Issues Must 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

Access Risers Provide easy access to tank & components A must for tanks with effluent screens or pumps Shallow tanks and short risers preferred

Riser Design

More Risers

Cast-in-Place Concrete Risers Cold joint –riser is casted separately and then placed on form when pouring the lid –may require additional sealant

Cast-in-Place Poly Risers Concrete does not bond well to poly –may still need to run a bead of sealant along boarder with concrete and poly

Adding Concrete Risers Mastic provides a better seal than mortar

Adding Concrete Risers (cont.)

Adding Poly Risers to Concrete Tanks Adapter rings

Adding Poly Risers to Concrete Tanks Riser attached to adapter using adhesive and stainless bolts

Child Proofing is Paramount

Tank Construction

Tank Materials Fiberglass- reinforced plastic (FRP) Polyethylene/Poly- propylene Reinforced concrete

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

Seam Location for Concrete Tanks Mid-seam Top seam

Sealing Materials for Pre-Cast Tanks Blended sealant compounds –Butyl-rubber based –Asphalt-based (bituminous)

Mastic Rules of thumb –does not compress much between thumb and forefinger –extra care during cold-weather installations –does not shred or snap when hand-stretched –higher is better than wider 50% compression is desirable –knead joined ropes prior to placement

Achieving a Watertight Joint High quality mastics, seal gaskets Seams must be smooth, clean and dry Proper placement of mastic Standard ASTM C990 Standard specification for joints for concrete pipe, manholes, and precast box sections using preformed flexible sealants

Extra Measures Butyl rubber wrap around joint

National Precast Concrete Association Recommends minimum three-inch wall thickness 28-day compressive strength at least 4,000 psi water to cementitious ratio no higher than 0.45 vibration without segregation structural reinforcement cages controlled curing flexible sealants quality control: manufacturing, delivery, & installation

ASTM Standards ASTM C1227 –Standard Specification for Precast Concrete Septic Tanks ASTM C805 –Standard test method for rebound number of hardened concrete rebound hammer

Polyethylene Tanks Watertightness is not a Given –Material is inherently watertight, but not structurally sound that’s why they have ribs and flanges –Must bed in sand or fine gravel –No seasonal high water table especially for pump tanks –No placement in shrink-swell clays

Fiberglass Tanks Again, watertightness is not a Given –watch that seam !! Success is very dependent on bedding and backfill –many tanks are damaged during backfill –seek manufacturer’s recommendation for ballast –make sure “low water level” in tank will provide sufficient weight to prevent floating

Pipe Penetrations in Concrete Tanks

Overall Quality of Tanks: Looks are not Everything Cosmetic deficiencies may not affect performance Good-looking tanks may have structural deficiencies

Air Voids – Will they Leak?

Clean Connection between Halves

Will be Difficult to Seal

Honeycombing – Form Leak

Aggregate without Cement

Exposed Reinforcement Wire

Problems with Non-Concrete Tanks

What Makes a Good Concrete Tank? Reasonably smooth surface –No honeycombing or cracks –No exposed rebar or wire Smooth, well-made joint with mastic –Flexible, watertight pipe seals at all pipe penetrations –Cast-in-place or mechanically-attached riser with tight fitting lid

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.

The Necessity of Watertight Tanks All water that comes in contact with wastewater becomes wastewater and must be treated as such… Watertightness is also defined as preventing water from the external environment from entering the tank

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

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

Watertightness Watertight seals –All joints –Pipe penetrations –Riser and lid

Testing for Watertightness Hydrostatic (water) testing Vacuum testing

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 per 1000 gallon tank Be careful not to the float lid

Vacuum Testing Equipment Pipe seal Gage to measure vacuum Plate seal on top of riser or tank Vacuum pump

Existing Tank Watertightness Indicators Root intrusion High water table area: –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.

Other Tanks Grease traps/interceptors Pre-treatment or trash tanks ATU tanks STEP/STEG tanks Pump tanks Recirculation/process tanks Holding tanks

Grease Trap Collects fats, oils, and grease Usually connected to commercial kitchens Provides treatment –Fats and grease settle –Oils float Temperature is a key factor

Grease Trap Maintenance No matter how big or well built, maintenance is the KEY for performance Serviced when solids reach 25% Inspection and cleaning of tees and screens Record keeping Pump it ALL!

Grease Trap Flexibility –Mostly static with limited flexibility –Compartmentalization with piping allowing for diversion Interferences –Chemicals cause emulsification –High temperature prevents separation Sampling location –Profile collected from tank –Effluent sample from outlet baffle

Pump Tank A concrete, fiberglass, or polyethylene container that collects and stores septic tank or aerobically treated effluent. Sized to hold the dose volume, minimum storage volume, and alarm volume. Considered a storage component more than a treatment unit.

Pump Tank Controls –Timer controlled or demand dosed Flow equalization tank timer controlled only –Sensor or floats used to detect water level –Alarms used for user notification

Trash Tank A smaller septic tank –75 – 100% daily flow Designed to deal with big pieces (trash) Some Aerobic Treatment Units (ATU’s) use a trash tank or trash compartment as part of their treatment unit

Processing Tank Anaerobic component of a treatment train Aerobically treated effluent is recirculated into the anaerobic tank Used in Nitrogen reduction systems NH 4 +  NO 3 -  N 2 Textile filters are typical units that use processing tanks

Tanks Built for ATUs Specifications come from ATU manufacturer

Tanks for Package Plants Includes additional unit processes –anaerobic digestion with denitrification –disinfection –sludge recycling Specifications come from the Plant Manufacturer

Holding Tanks Function as a collection and storage system Water tightness is imperative Pump when the tank is full Remote telemetry may help with management No treatment

Septic and Other Tanks Serve a valuable purpose –separates solids from liquid wastewater holds the solids until pump out –stores effluent between pump cycles makes the pump more efficient –serves to process wastewater BOD removal nitrogen conversion

Discussion and Questions?