CEE 370 Environmental Engineering Principles

Transcription

1 Updated: 19 November 2015 Print version CEE 370 Environmental Engineering Principles Lecture #32 Wastewater Treatment III: Process Modeling & Residuals Reading M&Z: Chapter 9 Reading: Davis & Cornwall, Chapt 6-1 to 6-8 Reading: Davis & Masten, Chapter to David Reckhow CEE 370 L#32 1

2 Updated: 19 November 2015 Print version CEE 370 Environmental Engineering Principles Lecture #31 Wastewater Treatment II: Growth and Process Modeling Reading: Davis & Cornwall, Chapt 4-8 to 4-10 Reading: Davis & Masten, Chapter 11-8 to David Reckhow CEE 370 L#32 2

3 Microbial Biomass in a CMFR General Reactor mass balance dm A n (C Q ) (C Q ) r V i1 Ai i in n j1 Aj j out A But with CMFRs we have a single outlet concentration (C A ) and usually a single inlet flow as well C A0 Q 0 C A V C A Q 0 David Reckhow CEE 370 L#32 3

4 Batch Microbial Growth General Reactor mass balance dm A 0 0 n n (C Ai Q ) i in (C Aj Q ) j out - r AV i1 j1 Batch reactors are usually filled, allowed to react, then emptied for the next batch Because there isn t any flow in a batch reactor: And: 1 V dc dm A A - r - r A A k For 1 st order biomass growth C A David Reckhow CEE 370 L#32 4 V

5 Batch Microbial Growth Observed behavior tationary Covered in lecture #17 Lag Exponential Growth Death Time David Reckhow CEE 370 L#32 5

6 Exponential Growth model D&M Text d gr µ Covered in lecture #17 N t r dn/ where, concentration of microorganisms at time t t time µ proportionality constant or specific growth rate, [time 1 ] d/ microbial growth rate, [mass per volume-time] David Reckhow CEE 370 L#32 6

7 Exp. Growth (cont.) Covered in lecture #17 d gr µ or d gr µ ln o µ t µt o e David Reckhow CEE 370 L#32 7

8 ubstrate-limited Growth Also known as resource-limited growth THE MONOD MODEL µ µ max and K + d K µ µ max gr + where, µ max maximum specific growth rate, [day -1 ] concentration of limiting substrate, [mg/l] K s Monod or half-velocity constant, or half saturation coefficient, [mg/l] David Reckhow CEE 370 L#32 8

9 Monod Kinetics Covered in lecture #17 0.5*µ m K David Reckhow CEE 370 L#32 9

10 ubstrate Utilization & Yield Related to growth by Y, the yield coefficient Mass of cells produced per mass of substrate utilized Y d d Just pertains to cell growth H&H, Fig 11-38, pp.406 d gr Y d David Reckhow CEE 370 L#32 10

11 Microbial Growth d gr Y d d Monod kinetics in a chemostat (batch reactor) µ K µ max gr + Where ubstitute for d & Divide by Y d/ r su actual substrate utilization rate k maximum substrate utilization rate μ max /Y concentration of substrate ( e in H&H) K half-saturation constant Y cell yield d/d d µ max Y K + r su k K + e e David Reckhow CEE 370 L#32 11

12 Death Bacterial cells also die at a characteristic first order rate with a rate constant, k d d k d This occurs at all times, and is independent of the substrate concentration David Reckhow CEE 370 L#32 12

13 Overall model: chemostat Combining growth and death, we have: d net d µ max gr K + k And in terms of substrate utilization + d d d ee: M&Z equ 9.3 Y d gr d d net Y d k d David Reckhow CEE 370 L#32 13

14 Activated ludge Flow chematic Conventional o Q o Influent Aeration Basin V, ettling Tank e Effluent Q r r Return activated sludge Q w r Waste activated 14 sludge David Reckhow CEE 370 L#32

15 Efficiency & HRT Efficiency of BOD removal E ( ) Hydraulic Retention Time, HRT (Aeration Time) ame as retention time in DWT (t R ) Actual HRT is a bit different Isn t used as much in design o o 100% θ V Q θ act Q V + Q R David Reckhow CEE 370 L#32 15

16 RT solids retention time & R RT: Primary operation and design parameter How long does biomass stay in system θ c V V Q ( Q Qw ) e + Qw r w r ee: M&Z equ 9.10 Typically equals 5-15 days Recycle Ratio Values of are typical R Qr Q David Reckhow CEE 370 L#32 16

17 F:M Ratio and volumetric loading Food-to-Microorganism Ratio (F/M) Typical values are in complete mixed A BOD volumetric Loading Loading Typically lb BOD/day/1000ft 3 tank volume F M Q V o Q V o F M M&Z equ 9.16 Q BOD V David Reckhow CEE 370 L#32 17

18 Act. ludge: Biomass Model teady tate mass balance on biomass V d d 0 Q o Qe e Qw r + V µ max kd K + Incorporating the chemostat model gets: batch d net d gr + d From chemostat model d V d 0 Q o Qe e Qw r + V µ max kd K + And simplifying Q o + Q e e + Q w r V µ max K + k Finally, we recognize that the amount of solids entering with the WW (i.e., o ) and leaving in the treated effluent (i.e., e ) is quite small and can be neglected d David Reckhow CEE 370 L#32 18

19 Biomass Model II o it becomes Q And rearranging 1 θ c w r V µ max kd K + Qw V r µ max K + k d Earlier equation for RT θ c V V ( Q Qw ) e + Qw r Qw r David Reckhow CEE 370 L#32 19

20 Act. ludge: ubstrate Model teady state mass balance on substrate V d 0 Q ubstituting and noting that Q e Q-Q w Q o Q And further simplifying Q o Q w Q Q e + Q w w V + V Y µ max d ( ) V Y K + o µ max batch K + d µ max Y K + From chemostat model David Reckhow CEE 370 L#32 20

21 Merging the biomass & substrate models If we divide the previous equation by V and Q ( ) o V µ max Y K + Q ( ) V Y K + o µ max Multiply both sides by Y YQ( o ) µ max V K + Now insert the LH term into the earlier equation based on biomass 1 θ c Qw V r YQ o V ( ) k d M&Z equ θ c Q V w r µ max M&Z equ 9.9 K + k d David Reckhow CEE 370 L#32 21

22 Combined model II Now recognize that Q/V is the reciprocal of the HRT 1 Y ( ) 1 θ θ c o k d David Reckhow CEE 370 L#32 22

23 Question All else being equal, as RT goes up: 1. ettleability goes down 2. F/M goes down 3. Waste sludge return ratio must go down 4. Endogenous respiration becomes less important 5. ludge yield increases David Reckhow CEE 370 L#32 23

24 Aeration: Loadings Food-to-Microorganism Ratio (F/M) F M Q BOD V ludge Age or mean cell residence time (ɵ c ) θ c ( Q ) + ( Q ) W e V Q e W V W W Where QWW flow Vvolume of aeration tank MLVmixed liquor volatile suspended solids (biomass concentration) e V e suspended solids in wastewater effluent W V w suspended solids in waste sludge Q w flow of waste sludge is sometimes used instead of V David Reckhow CEE 370 L#32 24

25 Operating Criteria Loading, biomass, retention time, etc H&H, Table11-4, pp.395 David Reckhow CEE 370 L#32 25

26 Activated ludge Mixed liquor Return Activated sludge 1. urface aerators 2. Bubble diffusers David Reckhow CEE 370 L#32 26

27 Updated: 19 November 2015 Print version CEE 370 Environmental Engineering Principles Lecture #32 Wastewater Treatment III: Process Modeling & Residuals Reading: Davis & Cornwall, Chapt 6-1 to 6-8 Reading: Davis & Masten, Chapter to David Reckhow CEE 370 L#32 27

28 Anaerobic Digester Problem Anaerobic digesters are commonly used in wastewater treatment. The biological process produces both carbon dioxide and methane gases. A laboratory worker plans to make a "synthetic" digester gas. There is currently 2 L of methane gas at 1.5 atm and 1 L of carbon dioxide gas at 1 atm in the lab. If these two samples are mixed in a 4 L tank, what will be the partial pressures of the individual gases? The total pressure? Example 4.4 from Ray David Reckhow CEE 370 L#32 28

29 olution to Anaerobic Digester Problem First, we must find the partial pressures of the individual gases using the ideal gas law: 1 P P V V 2 P1 V 1 nrt P2 V2 or 2 1 For methane gas For carbon dioxide gas: P atm P 2 1 atm 2 L 4 L 1 L 4 L 0.75 atm 0.25 atm And the total is: P t P CH + P CO 1 atm 4 2 David Reckhow CEE 370 L#32 29

30 RTsolids retention time olids Balance RT V Q w u mass of organisms in tank mass of organisms removed per day Q 0 0 Aeration Tank V, econdary Clarifier Q 0 -Q w e Q R Return Activated ludge (RA) u ludge HRT V Q Q w Waste Activated ludge (WA) David Reckhow CEE 370 L#32 30

31 olids Mass Balance We will cover this in CEE 371 Consider aeration tank and clarifier together Biomass in + biomass produced due to growth biomass out d Q V 0 + ( Q Qw ) e Qw w Now using the combined growth equation without limitation to carrying capacity: d µ max K s + k d Combining and assuming 0 and e to be negligible: µ max K s + Qw V David Reckhow CEE 370 L#32 31 w + k d

32 ubstrate Mass Balance We cover this in detail in CEE 471 Consider aeration tank and clarifier together ubstrate in + substrate consumed by biomass substrate out Q d + V 0 Now using the combined substrate utilization equation without limitation to carrying capacity: Combining and rearranging: 0 0 d µ max K 1 µ Y max s + ( Q Q ) Q Q0Y V w + ( ) David Reckhow CEE 370 L#32 32 K s + 0 k w d Note that effluent and waste sludge substrate concentrations are considered the same

33 Combined Mass Balances We cover this in CEE 471 In summary the solids and substrate mass balance equations are: µ Qw µ max w max Q0Y + k ( ) d 0 K s + V K s + V These can be easily combined (left hand terms are the same): 1 Θ c Qw V w Q0Y V 0 ( ) kd The mean cell residence time, or sludge age David Reckhow CEE 370 L#32 33

34 ludge Treatment Depends on type of sludge Typical process train Thickening or dewatering Conditioning tabilization (usually for wastewater) Disposal Nonmechanical methods Lagoons and-drying beds Freeze treatment Mechanical methods Centrifugation Vacuum filtration Belt filter press Plate filters David Reckhow CEE 370 L#32 34

35 Centrifuge David Reckhow CEE 370 L#32 35

36 Vacuum Filter David Reckhow CEE 370 L#32 36

37 Belt Filter Press David Reckhow CEE 370 L#32 37

38 To next lecture David Reckhow CEE 370 L#32 38