Lecture 6: Water & Wastewater Treatment Objectives: Objectives: Define primary, secondary, and tertiary treatment Define primary, secondary, and tertiary treatment Define BOD Define BOD Describe the activated sludge process Describe the activated sludge process Setup and solve a mass balance for an activated sludge system Setup and solve a mass balance for an activated sludge system

Settling Settling Settling velocity: Settling velocity: Percent of particles removed: (1-C ss /C ss,o ) x 100% Percent of particles removed: (1-C ss /C ss,o ) x 100% Where, Where, Review Sorption: Sorption: K d =C s /C L K d =C s /C L C T =(1+K d C ss )C L C T =(1+K d C ss )C L Fraction sorbed vs. fraction remaining in water Fraction sorbed vs. fraction remaining in water

Well-Mixed Settling Tank Q, C ss,o Q, C ss C ss vsvs V Suspended solids remaining:Define the Overflow Rate: ~ 20 – 100 m/day in treatment plants

Wastewater Treatment POTW – Publicly Owned Treatment Works POTW – Publicly Owned Treatment Works 0.4 – 0.6 m 3 /person/day 0.4 – 0.6 m 3 /person/day 15 million people in Los Angeles  7.5 x 10 6 m 3 /day or 2000 MGD (million gallons per day) 15 million people in Los Angeles  7.5 x 10 6 m 3 /day or 2000 MGD (million gallons per day) Hyperion – 450 MGD Hyperion – 450 MGD Clean Water Act (CWA) – 1977 – Set effluent (what is released by treatment plants into the environment) standards Clean Water Act (CWA) – 1977 – Set effluent (what is released by treatment plants into the environment) standards

Stages of Water Treatment Primary Primary Contaminants (60% of solids and 35% of BOD removed) Contaminants (60% of solids and 35% of BOD removed) Oil & Grease Oil & Grease Total Suspended Solids (C ss or TSS) – 60% Removed Total Suspended Solids (C ss or TSS) – 60% Removed Pathogens Pathogens BOD – 35% removed BOD – 35% removed Processes Processes Screens Screens Grit Settling Grit Settling Scum Flotation Scum Flotation Primary Settling Primary Settling

Secondary Secondary Contaminants Contaminants BOD – 90% Removed BOD – 90% Removed TSS – 90% Removed TSS – 90% Removed Processes Processes Trickling Filter – rotating disk Trickling Filter – rotating disk Activated Sludge – Suspended and mixed Activated Sludge – Suspended and mixed Oxidation ponds – lagoons Oxidation ponds – lagoons (promote contact between microbes and contaminants) Stages (continued)

Tertiary Tertiary Contaminants Contaminants Nutrients Nutrients Dissolved solids (e.g., salt, other ions, etc.) Dissolved solids (e.g., salt, other ions, etc.) Processes Processes Denitrification – bacteria Denitrification – bacteria Phosphorus removal – precipitation Phosphorus removal – precipitation Other chemicals – adsorption and precipitation Other chemicals – adsorption and precipitation

Primary Sludge (cont’d)

Primary Sludge

Primary Sludge (cont’d) Given: Q = 4000 m 3 /d C ss,o = 200 mg/L and C ss = 100 mg/L Sludge density = 0.05 kg/L Overflow rate of 50 m/d Find Population of town served by this unit Sludge production rate Area of settling tank Settling velocity of particles Cut-off size of particles (find the particle diameter corresponding to this settling velocity. Assume  s = 2600 kg/m 3. All particles larger than this size will settle) Q, C ss,o Q, C ss

Activated Sludge

Activated Sludge Components

Activated Sludge (cont’d)

Activated Sludge Nomenclature Q, S o, X o Q+Q R, S, X Q-Q s, S Q s +Q R, X s Q s, X s Q R, X s S stands for conc. of substrate (organic matter, waste, etc.) or BOD X stands for conc. of microorganisms

Activated Sludge Nomenclature (cont’d) Q, S o, X o Q+Q R, S, X~Q, S Q s +Q R, X s Q s, X s Q R, X s Assumptions: Effluent bacteria concentration is 0 Concentration of substrate or BOD in sludge is 0 Sludge flowrate (Q s ) is much smaller than Q , V

Decay of BOD and growth of 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  :

Activated Sludge Equations The following equations are derived from conducting mass balances over: The following equations are derived from conducting mass balances over: The entire system The entire system The aeration tank The aeration tank The sedimentation tank The sedimentation tank Any good book on wastewater engineering will have the derivations if you are curious! Any good book on wastewater engineering will have the derivations if you are curious!

Activated Sludge Equations Biomass (X) balance over entire system: Biomass (X) balance over entire system: Substrate (S) balance over entire system: Substrate (S) balance over entire system:

More 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 = QS o /XV - Food-to-microbe ratio: 0.3 – 0.7 d -1 F/M = QS o /XV - Food-to-microbe ratio: 0.3 – 0.7 d -1 Q R ~ 0.25 – 0.50 x Q Q R ~ 0.25 – 0.50 x Q X ~ 1000 – 2000 mg/L X ~ 1000 – 2000 mg/L Problem types: Problem types: Given Q, S o, and S (target concentration) Given Q, S o, and S (target concentration) Find Q R, Q s, X, , V, Y Find Q R, Q s, X, , V, Y

Example Find Q s, , V, Y Find Q s, , V, Y Given: Given: Q = 1000 m 3 /d Q = 1000 m 3 /d S o = 150 mg/L S o = 150 mg/L S = 15 mg/L S = 15 mg/L Q R = 240 m 3 /d Q R = 240 m 3 /d F/M = 0.3 d -1 F/M = 0.3 d -1 X = 2000 mg/L X = 2000 mg/L X s = 1% or 10,000 mg/L X s = 1% or 10,000 mg/L

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