Introduction to Septic Tanks

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

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

3. Typical Septic System

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

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

6. 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

7. 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

8. 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

9. Average Removal of BOD, TSS, and Grease in Septic Tank Seabloom, R.W., T.R. Bounds, and T.L. Loudon. 2005.

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

11. 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

12. Anaerobic Digestion ORGANIC MATTER GASES + HUMUS + H20 CO2 CH4 H2S NH3

13. 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

14. 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

15. 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

16. Septic Tank Design • Sizing • Geometry • Compartments

17. 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”

18. Attenuation of Flow • Quick example • Washing machine discharge • 20 gallons in two minutes (2.67ft3) • 10 gpm • septic tank is 4’ wide by 8’ long (32 ft2) • 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

19. 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 • 0.018 ft/min horizontal velocity in tank • 7.5 hours to go across tank • particles are settling as water moves across

20. 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

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

22. Texas Septic Tank Rules

23. Texas Design Flows

24. 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”

25. 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

26. 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

27. 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

28. 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

29. 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.

30. 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.

31. 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

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

33. 2/3 total volume 1/3 total volume Two Compartment Septic Tank

34. Tank Compartments

35. 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

36. Tank Appurtenances • Tees and baffles • Effluent screens • Access risers

37. 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

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

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

40. Baffle Fastened to Tank Wall

41. 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

42. Effluent Screens Riser must be installed for maintenance

43. 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

44. Effluent Screen Installation Issues • Must be secure in place • No bypass flow if clogging occurs • Housing should not interfere with normal tank cleaning

45. 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

46. Proprietary Effluent Screens

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

48. Riser Design

49. More Risers

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