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ESM 214

Lecture 4: Wastewater Treatment

Overview, Reactors

W05

T. Holden

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Onsite Wastewater Treatment

25% of US population relies on it

On site treatment typically (95%) is by

septic system and associated leachfield

Regulations and design are somewhat

standardized, but public health authority

(county) regulates and manages.

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Septic system schematic

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Septic system schematic

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Septic Tank

1000 gal typ.Concrete

Gravity in/out typ.

dosing alternative

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Leach (absorption) field

tank 10 ft from house field 100 ft from well or pond

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Leach field cross section

4 inch diameter drain pipe typ. 46 ft deep trench typ.

2 ft. wide trench

23 ft. gravel depth

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Good leach field performance

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Bad leach field performance

Shallow clay

lense.

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Factors effecting failure of the

septic system bad soil

Clogging (biofouling) High water table

Roots

Physical damage

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Percolation in leach field design Consult your local authority for the real facts !!

Example:

Measure percolation, pick application rate from table,

choose allowed flow rate calculate area (sf)

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Centralized wastewater treatment

Required where:

Population density is too high to support onsite

treatmentSoils are unsuitable for onsite treatment

Consists of:

Collection and conveyance system

Wastewater Treatment Plant (WTP, WWTP)owned by community or sewerage agency(a.k.a. treatment district)

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Regulation

Federal Clean Water Act (CWA) protectswaters of the state

CWA administered by most states; in CA itis the SWRCB

NPDES (national pollutant discharge

elimination system) necessitates a permit todischarge to surface water (river, ocean,etc.)

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Regulatory Linkages

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WWTPs in the US

Total ca. 16,000 in 1997

Degree of treatment (as of 1997)

1 (primary) 176

2 (secondary) 9388

Better than secondary 4428

No discharge (land application) 2032

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Process flow diagram

Schematic of waste treatment train

Shows unit processes and their linkages

Can provide extended information

Vessel size

Target operating conditions, etc.

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Process flow diagram (ex.)

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El Estero WWTP Schematic

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WWT System Concepts

Unit Processes = Unit Operations:treatment methods that employ Physical,

Chemical orBiological processes Materials Balance =Mass Balance: basis

for analyzing unit processes; accounts formass in and out, as well as reactions

Reactors: vessels or tanks where unitprocesses are carried out.

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Reactors

a) Batch

b) CMR; CFSTR; CFR

c) Plug flow (open)

d) Plug flow (closed)

e) CMRs in series

f) Packed bed (down flow)

g) Packed bed (up flow)

h) Fluidized packed bed

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Batch Reactors

Closed: no inflow and no outflow

Constant volume, V

Well-mixed

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CFSTRs

Complete, instantaneous mix

Concentration, C, inside is same as effluent

Continuous flow in and out

Constant volume (so Q0 = Q)

Infinite series approaches plug flow

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Plug Flow

Continuous flow in and out

Concentration changes progressively along

the flow direction

Well-mixed contents perpendicular to flow

No longitudinal mixing (ideal)

Under idealized conditions, all particles

reside for same amount of time in reactor

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Abbreviations

L = length (generically, L)

V = volume (L3)

M = mass (m)

C = concentration (m/ L3)

t = time

t = increment of time

Q = volumetric flow rate (L3/t)

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Mass Balance Analysis Steps

Choose mass of interest (e.g. water or BOD5 orbacteria or solids)

Draw system; label inflows and outflows Define control volume (CV)

Write verbal expression

Substitute words with mathematical phrases

Check units

Recognize and state assumptions

Rearrange and solve

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Mass Balance: Batch example

CV

Verbal: InOut + Reaction = Accumulation

Math: 0 0 rV t C V

Units: m/l3-t l3 t m/l3 l3

Rearrange: r V = C/t V

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Mass Balance: Batch example

Take limits as C and t 0

r =dt

dC

Substitute a rate equation for r

e.g. 1st order decay of C: -kC

So, -kC = dt

dC

t

0

C

CtkC

dCt

0Rearrange, integrate:

ktClnC

Ct

0

kt

C0

Cte kt

C0Ct e

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Mass Balance: Batch

example of exponential decayC0 = 100 mg/L, k =-0.2/hrConcentration versus time

0

20

40

60

80

100

120

0 1 2 3 4 5 6

time, hours

Concentration(mg/L)

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Comments on Rate Expressions

Note assumptions for units of r (e.g. m/t or m/V-t or m/V-#-t)

Choose rate expression carefully (Table 4-6 M&E)

Recognize that rate expression is part of overallmass balance

Dont know order or rate equation? Analyze

concentration versus time data (Fig. 4-21): Linear C vs t is 0-order

Linear log C vs t is 1st order

Linear 1/C vs t is 2nd order

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Mass Balance: CFSTR

CV

Verbal: In Out + Reaction = Accumulation

Math: QC0t - QC t + r V t = C V

Units: l3/t m/l3 t m/l3t t l3 m/l3 l3

Rearrange: Q/V (C0C) + r = C/t

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Mass Balance: CFSTR

Take limits as C and t 0

dt

dC

Substitute a rate equation for r

e.g. 1st order decay of C: -kC

Make steady state (SS) assumption

(no net accumulation or depletion:Rearrange (see pg 270 M&E)

0

dt

dC

Q/V (C0C) + r =

)Q/V(k1C0C

Eqn. 4-102 M&E

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CFSTR example: aeration basin

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A few words more about

CFSTRs SS solution typical

Key is that concentration inside = effluent

concentration (instantaneous mixing)

Include terms for all streams cut by CV

(e.g. recycle if present)

When mass balance is applied to biological

system, kinetics are more complex

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CFSTRs in series

nQ/kV1 n

C0Cn

M&E 4-107

n = number of reactors and Cn

is the concentration

exiting the nth reactor.

V = total volume of all the reactors in series

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Mass Balance: Plug Flow

(Fig. 4-5)

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Mass balance: Plug Flow

Verbal: In Out + Reaction = Accumulation

Math: QCxtQCx+xt + r V t = C V

Units: l3/t m/l3 t m/l3t t l3 m/l3 l3

Rearrange: [Q/(A

x )](CxCx+x ) + r =

C/

t

Substitute V = Ax

Divide by Ax and by t

Take limits as t and x 0

rc

x

C

A

Q

t

C

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Mass Balance: Plug Flow

Make SS assumption

Substitute a 1st order rate expression

e QkV

C0

C Eq. 4-116 M&E

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Plug Flow Reactor Examples:

Chlorine Contact & Aeration Basins

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Reactor Applications (examples)

Batch: SBRs (fill & draw), chemical (e.g.polymer) dilution

CFSTR with recycle: aeration basins for activatedsludge

CFSTRs w/o recycle: aerobic digestion, lagoons

Plug flow: chlorine contact basins

Plug flow with recycle: activated sludge Packed bed: trickling filters; effluent filtration

Upflow packed bed: air stripping

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Flow regimes in WWT

(4-27 M&E)

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Reactor combinations

(4-28 M&E)

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Mass Balance:Packed Bed

Reactors Model using plug flow approach

Reaction rate for gas-liquid m.t.:

r = KLa(Cs-Cl)where KLa = mass transfer coeff.

Cs = gas phase C

Cl = liquid phase CTable 4-9 M&E (reactors for mt operations)

NOTE: mt operations occur in all types of reactors