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Fundamentals of Onsite Fundamentals of Onsite Wastewater System Wastewater System Design Design John R. Buchanan, Ph.D., P. E. Associate Professor Biosystems Engineering & Soil Science University of Tennessee

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  • Fundamentals of Onsite Fundamentals of Onsite Wastewater System Wastewater System

    DesignDesignJohn R. Buchanan, Ph.D., P. E.

    Associate ProfessorBiosystems Engineering & Soil Science

    University of Tennessee

  • Fundamentals of Onsite Fundamentals of Onsite Wastewater System DesignWastewater System Design

    IntroductionMuch of this presentation was originally developed by the members of the Consortium of Institutes for Decentralized Wastewater Treatment

  • Fundamentals of Onsite Wastewater System Design

    What are the Fundamentals?Understanding the Wastewater sourceUnderstanding the Wastewater strengthProviding Renovation Onsite (or near site)Returning water back to the Hydrologic Cycle

  • Wastewater Defined

    Wastewater is water that has been used to collect and transport waste

    Water that has stuff* suspended dissolved floating sinking

    * stuff is a more polite s word

  • So, Design Fundamentals

    Have to include:Understanding the stuff in the wastewater

    lets call it wastewater strengthUnderstanding how much wastewater we are working with

    wastewater volumeUnderstanding how to remove the wastes from water

    wastewater treatment

  • Wastewater Sources

    Residentialsingle family homesapartmentssubdivisions

    Commercialrestaurantsfuel stationsbakeriesschools and day care

    These are the most common wastewater sources outside of sewage service areas

    Onsite (or near site) wastewater renovation is the most efficient and economical means of managing the source water

  • Wastewater Strength

    Typical wastewater constituentsSolidsSuspended and dissolved organic matterPathogensNutrientsPersonal care products and pharmaceuticals

  • Wastewater Strength

    SolidsOrganic and inorganic

    Fecal solids personal hygiene items

    Floaters and sinkers fats, oils and grease (FOG)

    Suspended and dissolved dissolved minerals from source water toilet paper

  • Wastewater Strength

    Suspended and dissolved organic matterThe stuff that bacteria, fungi, and other microbes can breakdown

    some organic matter is very difficult to breakdown recalcitrant

    microbes produce enzymes that breakdown organic matter into bio-available forms

    One measure of bio-availability is BOD5 Biochemical Oxygen Demand in Five Days aerobic microbes consume dissolved oxygen out

    of water

  • Breakdown of Organics

    Organic carbon is an energy source to most microorganisms

    2 2 2aerobic

    microorganismsOrganic Carbon + O Energy + CO + H O + Residue

    2 2 2new aerobic

    microorganisms+ O Energy + CO + H O + Residue

    2 2 2new aerobic

    microorganisms+ O Energy + CO + H O + Residue

  • Wastewater StrengthPathogens

    Disease causing microorganisms a disease-carrying person sheds pathogens into

    the wastewater our goal is to minimize the risk of that disease

    being transmittedIndicators of the potential for pathogens

    fecal coliforms and E. coli found in digestive track of all warm-blooded

    animals E. coli O157:H7 is a true pathogen

  • Wastewater Strength

    NutrientsChemicals required for growth, when available in excess, too much growth can occurNitrogen compounds

    ammonia nitrate


  • Typical Raw Residential Sewage Characteristics

    60 mg/L26 75 mg/LTotal Nitrogen

    10 mg/L4 - 13 mg/LAmmonium-Nitrogen, NH4-N

    109 CFU/100mL108 1010CFU/100mL

    Total Coliform Bacteria

    10 mg/L6 - 12 mg/LTotal Phosphorus

    Less than 1 mg/LLess than 1 mg/LNitrate-Nitrogen, NO3-N

    107 CFU/100mL106 108CFU/100mL

    Fecal Coliform Bacteria

    6.5 s.u.6 -9 s.u.pH250 mg/L155 286 mg/LBOD5

    250 mg/L155 330 mg/LTotal Suspended Solids, TSS

    Typical ConcentrationRange of Concentrations


  • Other Wastewater Sources will have Other Strengths

    RestaurantsMore FOGHigher temperaturesSanitizers

    SchoolsHigher nitrogenMore trash

    LaundryLints and detergents

  • Wastewater VolumeTypically

    the daily production of wastewater gallons per day design for high flows (worse case)

    Daily flow versus Instantaneous flowDaily is how much is generated during a dayInstantaneous can be the flow discharged by a device in the system

    dishwasher washing machine

  • Residential Wastewater Flows

    68.6 (259.7)153Weighted Average

    57.1 83.5(216.1 316.1)

    69.3 (252.3)1c1188Mayer et al. (1999)

    26.1 85.2(98.9 322.5)

    50.7 (191.9)225Anderson, et al. (1983)

    65.9 75.6(249.4 289.9)

    70.8 (268.0)390Anderson & Siegrist (1989)

    57.3 73.0(216.9 276.3)b

    66.2 (250.6)a210Brown & Caldwell (1984)

    Study Number of Study Duration Study Average Study range Residences (months) (gal/person/day) (gal/person/day)

    aaBasedBased on indoor water use monitoring and not wastewater flow monitorion indoor water use monitoring and not wastewater flow monitoringngbbLitersLiters per person per day in parenthesesper person per day in parenthesesccBasedBased on two weeks of continuous monitoring in each of two seasons aton two weeks of continuous monitoring in each of two seasons at each homeeach home

  • Remember, We Do Not Design for Average

    We design for high water-use daysthe septic system must be able to handle

    Saturday night heavy laundry day

    a single-family system must be able to handle all the water that could be generated in a day at that house

  • Concept of Load

    Wastewater Strengthis typically measured as a concentrationmass per unit volume (mg/L)

    Wastewater volumeis typically given in volumegallons or liters

    Load is the mass of the constituent(volume) times (concentration) equals mass

  • For Example

    Total Nitrogentypical concentration 50 mg/Ltypical residential daily volume 300 gallonsmust have consistent units

    3.79 L per gallon (1136 L per day)

    (1136 L per day)(50 mg/L) = 56,800 mg/d total Nor 57 grams of total N per day

  • Onsite Wastewater Treatment

    Liquid-Solid separationOrganic carbon conversionPathogen destructionNutrient removal

  • Liquid-Solid Separation

    Primary TreatmentSeptic tanksDesigned 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

  • Typical Cross Section


    Clear Zone


    Dual Chamber Septic Tank

    Risers to gradeOutlet tee

    with effluent screen

    Inlet tee

  • Goal is Near Zero Velocity in Tank for Optimum Solids


    Maximize distance between inlet and outletLength:Width ratio

    at least 3:1

    Inlet to outlet drop~ 2


    Clear Zone


    Dual Chamber Septic Tank

  • Septic Tank Effluent

    Discharge from septic tankMuch stuff has been removed from the water

    still have more to removeMuch of the solids have been removed

    we do not want solids carry-over into the drainfield this material will have to be pumped out of tank

    Some pathogen reduction cooler temperature than human body different oxygen states

    Not much conversion of nutrients

  • Typical Domestic Septic Tank Effluent Characteristics

    8.1 mg/L8.1 8.2 mg/LTotal Phosphorus60 mg/L29.5 63.4 mg/LTotal Nitrogen0 mg/L0 10 mg/LNitrate-Nitrogen, NO3-N

    40 mg/L30 50 mg/LAmmonium-Nitrogen, NH4-N

    106 CFU/100mL106 107 CFU/100mLFecal Coliform Bacteria6.5 s.u.6.4 7.8 s.u.pH

    120 mg/L118 - 189 mg/L5-Day Biochemical Oxygen Demand, BOD5

    60 mg/L36 - 85 mg/LTotal Suspended Solids, TSS

    Typical Concentration

    Concentration Range


  • Question

    What if your wastewater source was not residential

    Will you get the same quality of effluent?Answer - Not Likely

    What comes out depends on what went inHigher strength influent will product higher strength effluenteffluent may require more pretreatment before application to the soil.

  • Additional Treatment OptionsLarger primary tanks

    more separation and more anaerobic digestion

    Grease trapsCool and separate FOG before mixing with black water

    Aerobic treatmentMore BOD5 removal and nitrogen conversion

    aerobic treatment units packed-bed media filters

  • The Soil as Final Treatment

    Soil is a tremendous wastewater treatment media

    Biological propertiesChemical propertiesPhysical properties

    However, the soil has its limitsPretreatment must bring the strength down to a level the soil can assimilate

  • From a Design Perspective

    We have to distribute the effluent to the soil interface such to maximum the soils ability to renovate wastewater

  • Final Treatment & Dispersal



    Aerobic soil

    Returning the water to the hydrologic cycle

  • Organic BiodegradationFixed Film treatment on soil particle surfaces

    Septic Tank EffluentSeptic Tank Effluent

    Microbial filmMicrobial filmSoil ParticleSoil Particle

    Air in unsaturated Air in unsaturated pore spacepore space

  • Soil Based Pathogen Removal

    Soil can hold some pathogens

    many soils have a negative chargemicrobes with a positive charge will bond to the soil

    Predationsome pathogens provide a snack to natural soil biota

  • Case Study: Septic Tank Effluent and Soil Water Quality

    0.180.02 1.80


    0.400.01 3.8


    8.67.2 17.0





    13.02.0 29.0


    21.61.7 39.0


    0.040.01 0.16





    0.770.25 2.10

    0.770.4 1.40


    44.219 53





    8.03.1 25.0


    7.83.7 17.0


    47.431 68





  • Source: Adapted from Anderson, 1994

    cnd = none detected

    bSoil water quality measured in pan lysimeters at unsaturated soil depths of 2 feet (0.6 meters) and 4 feet (1.2 meters)

    aThe soil matrix consisted of a fine sand; the wastewater loading rate was 3.1 cm per day over 9 months. TOC = Total organic carbon; TKN = Total Kjeldahl nitrogen; TDS = Total dissolved solids; Cl = Chlorides; F. Coli = Fecal Coliform; F. strep = Fecal Streptococci


  • Soil Treatment Area Sizing

    Sewage effluent characteristics Soil properties

    TextureStructureConsistence/ Mineralogy

    The biomatHydraulic conductivity?

  • Biomat & Sidewalls

    Biomat develops along the bottom and then around the trenchExcessive ponding depths may create saturated flowNarrower trenches allows more surface areaNarrower trenches allows better O2 transfer

  • Flow Pattern in a Gravity Trench

    Biomat Growth (t = 0 = start )

  • Flow Pattern in a Gravity Trench

    Biomat Growth (t = growth)

  • Flow Pattern in a Gravity Trench Biomat Growth (t=mature)

  • Flow Pattern with Pressure Distribution

  • Pressure Distribution

  • Long Term Acceptance Rate LTAR

    Eventuallythe biomat controls the ability of the soil to accept effluentthis is the LTAR

    Generally, State codes dictate LTARsgallons per day per square foot of trench bottom

  • LTAR

    Texture/ StructureOther tests-Saturated conductivityPercolation rates

    Soil Characteristics and Soil Sizing Factor (> 3' separation)Percolation Rate Soil Sizing Factor

    minutes per inch Soil Texture square feet/gallon(mpi) per day(sqft/gpd)

    faster than 0.1* Coarse sand 0.830.1 to 5 Medium sand 0.83

    Loamy sand0.1 to 5** Fine sand 1.676 to 15 Sandy loam 1.2716 to 30 Loam 1.6731 to 45 Silt loam 2.00

    Silt46 to 60 Clay loam 2.20

    Sandy clay Silty clay

    over 61 to 120*** Clay 4.20 Sandy clay Silty clay

    slower than 120****

    *Use systems for rapidly permeable soils:pressure distribution or serial distribution withno trench >25% of the total system.**Soil having 50% or more fine sand plus very fine sand.***A mound must be used.****An other or performance system must be used

  • Influencing the Biomat

    Good InfluenceDesignLoading

    Hydraulic Organic

    Resting Depth of cover

    Oxygen availability

    Bad InfluencePeroxideAcid

  • Hourly Variation in Wastewater Flow

    Restaurantsdishwashing & food prep

    Schoolslunch time & ballgames

    Grocery Storesnighttime cleaning activities, monthly floor stripping and cleaning activities

  • Hourly Variation in Wastewater Flow

    2 4 6 8 10 Noon 2 4 6 8 10 12






    Time of Day

    Peak Demands

  • With Flow Variation..

    You have to have storageStorage accepts the effluent while waiting for the water to infiltrate into the soilSome storage in septic tank

    but not muchMost of storage is in the trenches

    porosity of trench media typically want two-days water volume in trenches

  • So, Back to the Big Picture

    Fundamentally speakingour business is the protection of public healthand our business is the protection of environmental health

    Both are equally important

  • Presentation can be found at:http://onsite.tennessee.edu