fundamentals of onsite design - onsite.tennessee.eduonsite.tennessee.edu/fundamentals of onsite...

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

• let’s 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

Phosphorus

Typical Raw Residential Sewage Characteristics

60 mg/L26 – 75 mg/LTotal Nitrogen

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

109 CFU/100mL108 – 1010

CFU/100mLTotal Coliform Bacteria

10 mg/L6 - 12 mg/LTotal Phosphorus

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

107 CFU/100mL106 – 108

CFU/100mLFecal 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

Component

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 parentheses••ccBasedBased 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

Sludge

Clear Zone

Scum

Dual Chamber Septic Tank

Risers to gradeOutlet tee

with effluent screen

Inlet tee

Goal is Near Zero Velocity in Tank for Optimum Solids

Removal

Maximize distance between inlet and outletLength:Width ratio

at least 3:1

Inlet to outlet drop~ 2”

Sludge

Clear Zone

Scum

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

Component

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 soil’s ability to renovate wastewater

Final Treatment & Dispersal

Groundwater

Well

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

33

0.400.01 – 3.8

35

8.67.2 – 17.0

11

MeanRange

#samples

TP(mg/L)

13.02.0 – 29.0

32

21.61.7 – 39.0

35

0.040.01 – 0.16

11

MeanRange

#samples

NO3-N(mg/L)

0.770.25 – 2.10

0.770.4 – 1.40

35

44.219 – 53

11

MeanRange

#samples

TKN(mg/L)

8.03.1 – 25.0

33

7.83.7 – 17.0

34

47.431 – 68

11

MeanRange

#samples

TOC(mg/L)

<1<16

<1<16

93.546 -156

11

MeanRange

#samples

BOD(mg/L)

Soil Water Quality at 1.2 m

Soil Water Quality at 0.6 m

Septic Tank Effluent Quality

StatisticsParameter(units)

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

nd<120

nd<123

3.601.9 – 5.3

11

MeanRange

#samples

F. strep.(log # per 100 mL)

nd<121

ndc

<124

4.573.6 – 5.5

11

MeanRange

#samples

F. Coli(log # per 100 mL)

299 – 49

31

419 – 65

34

7037 – 110

11

MeanRange

#samples

Cl(mg/L)

355200 – 592

32

448184 – 620

34

497354 – 610

11

MeanRange

#samples

TDS(mg/L)

Case Study: Septic Tank Effluent and Soil Water Quality

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

2

4

6

8

10

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