Lecture 6: Water & Lecture 6: Water & Wastewater TreatmentWastewater Treatment

Objectives:Objectives: Define primary, secondary, and tertiary Define primary, secondary, and tertiary

treatmenttreatment Define BODDefine BOD Describe the activated sludge processDescribe the activated sludge process Setup and solve a mass balance for an Setup and solve a mass balance for an

activated sludge systemactivated sludge system

SettlingSettling Settling velocity:Settling velocity:

Percent of particles removed: (1-CPercent of particles removed: (1-Cssss/C/Css,oss,o) x 100%) x 100% Where, Where,

ReviewReview

Sorption:Sorption: KKdd=C=Css/C/CLL

CCTT=(1+K=(1+KddCCssss)C)CLL

Fraction sorbed vs. fraction remaining in waterFraction sorbed vs. fraction remaining in water

2

18d

gv wss

QAvC

C

soss

ss

1

1

,

Well-Mixed Settling TankWell-Mixed Settling Tank

QAvC

C

soss

ss

1

1

,

Q, Css,o Q, CssCssvs V

Suspended solids remaining: Define the Overflow Rate:

A

Q

~ 20 – 100 m/day in treatment plants

Wastewater TreatmentWastewater Treatment

POTW – Publicly Owned Treatment POTW – Publicly Owned Treatment WorksWorks 0.4 – 0.6 m0.4 – 0.6 m33/person/day/person/day 15 million people in Los Angeles 15 million people in Los Angeles 7.5 x 7.5 x

101066mm33/day or 2000 MGD (million gallons per /day or 2000 MGD (million gallons per day)day)

Hyperion – 450 MGDHyperion – 450 MGD Clean Water Act (CWA) – 1977 – Set Clean Water Act (CWA) – 1977 – Set

effluent (what is released by treatment effluent (what is released by treatment plants into the environment) standardsplants into the environment) standards

Stages of Water Stages of Water TreatmentTreatment

PrimaryPrimary Contaminants Contaminants (60% of solids and 35% of BOD removed)(60% of solids and 35% of BOD removed)

Oil & GreaseOil & Grease Total Suspended Solids (CTotal Suspended Solids (Cssss or TSS) – 60% Removed or TSS) – 60% Removed PathogensPathogens BOD – 35% removedBOD – 35% removed

ProcessesProcesses ScreensScreens Grit SettlingGrit Settling Scum FlotationScum Flotation Primary SettlingPrimary Settling

SecondarySecondary ContaminantsContaminants

BOD – 90% RemovedBOD – 90% Removed TSS – 90% RemovedTSS – 90% Removed

ProcessesProcesses Trickling Filter – rotating diskTrickling Filter – rotating disk Activated Sludge – Suspended and mixedActivated Sludge – Suspended and mixed Oxidation ponds – lagoonsOxidation ponds – lagoons

(promote contact between microbes and (promote contact between microbes and contaminants)contaminants)

Stages (continued)Stages (continued)

Stages (continued)Stages (continued)

TertiaryTertiary ContaminantsContaminants

NutrientsNutrients Dissolved solids (e.g., salt, other ions, etc.)Dissolved solids (e.g., salt, other ions, etc.)

ProcessesProcesses Denitrification – bacteriaDenitrification – bacteria Phosphorus removal – precipitationPhosphorus removal – precipitation Other chemicals – adsorption and Other chemicals – adsorption and

precipitationprecipitation

Primary Sludge (cont’d)Primary Sludge (cont’d)

Primary SludgePrimary Sludge

Primary Primary Sludge Sludge (cont’d)(cont’d)

Given:Given: Q = 4000 mQ = 4000 m33/d/d CCss,oss,o = 200 mg/L and C = 200 mg/L and Cssss = 100 mg/L = 100 mg/L Sludge density = 0.05 kg/LSludge density = 0.05 kg/L Overflow rate of 50 m/dOverflow rate of 50 m/d

FindFind Population of town served by this unitPopulation of town served by this unit Sludge production rateSludge production rate Area of settling tankArea of settling tank Settling velocity of particlesSettling velocity of particles Cut-off size of particles (find the particle diameter Cut-off size of particles (find the particle diameter

corresponding to this settling velocity. Assume corresponding to this settling velocity. Assume ss = = 2600 kg/m2600 kg/m33. All particles larger than this size will . All particles larger than this size will settle)settle)

Q, Css,o Q, Css

rateproductionsludgems

Activated SludgeActivated Sludge

Activated Sludge Activated Sludge ComponentsComponents

Activated Sludge Activated Sludge ComponentsComponents

Activated Sludge (cont’d)Activated Sludge (cont’d)

Activated Sludge Activated Sludge NomenclatureNomenclature

Q, So, Xo

Q+QR

, S, XQ-Qs, S

Qs+QR, Xs

Qs, Xs

QR, Xs

S stands for conc. of substrate (organic matter, waste, etc.) or BODX stands for conc. of microorganisms

Activated Sludge Activated Sludge Nomenclature (cont’d)Nomenclature (cont’d)

Q, So, Xo Q+QR, S, X ~Q, S

Qs+QR, Xs

Qs, Xs

QR, Xs

• Assumptions:• Effluent bacteria concentration is 0• Concentration of substrate or BOD in sludge is 0 • Sludge flowrate (Qs) is much smaller than Q

, V

Decay of BOD and growth Decay of BOD and growth of organismsof organisms

Substrate or BOD (S) decays with rate k:Substrate or BOD (S) decays with rate k:

Microbes (X) grow at rate Microbes (X) grow at rate ::

kSdt

dS

Xdt

dX

Activated Sludge Activated Sludge EquationsEquations

The following equations are derived The following equations are derived from conducting mass balances from conducting mass balances over:over: The entire systemThe entire system The aeration tankThe aeration tank The sedimentation tankThe sedimentation tank

Any good book on wastewater Any good book on wastewater engineering will have the derivations engineering will have the derivations if you are curious!if you are curious!

Activated Sludge Activated Sludge EquationsEquations

Biomass (X) balance over entire system:Biomass (X) balance over entire system:

Substrate (S) balance over entire Substrate (S) balance over entire system:system:

XVXQm sss

kSV

XV

consumedBOD

producedbiomassY

mXVSSYQ so

More AS equationsMore AS equations Mass balance over sedimentation tank:Mass balance over sedimentation tank:

Other equation(s)/rules of thumb:Other equation(s)/rules of thumb: F/M = QSF/M = QSoo/XV - Food-to-microbe ratio: 0.3 – 0.7 d/XV - Food-to-microbe ratio: 0.3 – 0.7 d-1-1

QQRR ~ 0.25 – 0.50 x Q ~ 0.25 – 0.50 x Q X ~ 1000 – 2000 mg/LX ~ 1000 – 2000 mg/L

Problem types:Problem types: Given Q, SGiven Q, Soo, and S (target concentration), and S (target concentration)

Find QFind QRR, Q, Qss, X, , X, , V, Y, V, Y

sRR XQXVXQQ

ExampleExample

Find QFind Qss, , , V, Y, V, Y Given:Given:

Q = 1000 mQ = 1000 m33/d/d SSoo = 150 mg/L = 150 mg/L S = 15 mg/LS = 15 mg/L QQRR = 240 m = 240 m33/d/d F/M = 0.3 dF/M = 0.3 d-1-1

X = 2000 mg/LX = 2000 mg/L XXss = 1% or 10,000 mg/L = 1% or 10,000 mg/L

Definition of BODDefinition of BOD Microorganisms (e.g., bacteria) are responsible for Microorganisms (e.g., bacteria) are responsible for

decomposing organic waste. When organic matter such decomposing organic waste. When organic matter such as dead plants, leaves, grass clippings, manure, as dead plants, leaves, grass clippings, manure, sewage, or even food waste is present in a water sewage, or even food waste is present in a water supply, the bacteria will begin the process of breaking supply, the bacteria will begin the process of breaking down this waste. When this happens, much of the down this waste. When this happens, much of the available dissolved oxygen is consumed by aerobic available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen bacteria, robbing other aquatic organisms of the oxygen they need to live. Biological Oxygen Demand (BOD) is a they need to live. Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to measure of the oxygen used by microorganisms to decompose this waste. If there is a large quantity of decompose this waste. If there is a large quantity of organic waste in the water supply, there will also be a organic waste in the water supply, there will also be a lot of bacteria present working to decompose this lot of bacteria present working to decompose this waste. In this case, the demand for oxygen will be high waste. In this case, the demand for oxygen will be high (due to all the bacteria) so the BOD level will be high. (due to all the bacteria) so the BOD level will be high. As the waste is consumed or dispersed through the As the waste is consumed or dispersed through the water, BOD levels will begin to decline. water, BOD levels will begin to decline.