ESM 214

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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.
• 4 6 ft deep trench typ.
• 2 ft. wide trench
• 2 3 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
  treatment
• Soils 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) protects waters of
  the state
• CWA administered by most states in CA it is the
  SWRCB
• NPDES (national pollutant discharge elimination
  system) necessitates a permit to discharge 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 or
  Biological processes
• Materials Balance Mass Balance basis for
  analyzing unit processes accounts for mass in
  and out, as well as reactions
• Reactors vessels or tanks where unit processes
  are carried out.

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• Reactors
• Batch
• CMR CFSTR CFR
• Plug flow (open)
• Plug flow (closed)
• CMRs in series
• Packed bed (down flow)
• Packed bed (up flow)
• 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 or
  bacteria 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
Verbal In Out 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
Substitute a rate equation for r e.g. 1st order
decay of C -kC So, -kC
Rearrange, integrate
?
?
?
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Mass Balance Batchexample of exponential
decayC0 100 mg/L, k -0.2/hr
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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 ME)
• Recognize that rate expression is part of overall
  mass 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
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Mass Balance CFSTR
Take limits as ?C and ?t ? 0
Q/V (C0 C) r
?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 ME)
Eqn. 4-102 ME
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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
ME 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 QCx ?t QCx?x ?t ?r V ?t
?C V
Units l3/t m/l3 t m/l3t
t l3 m/l3 l3
?Substitute V A?x ?Divide by A?x and by ?t
Rearrange Q/(A ?x )(Cx Cx?x ) r
?C/?t
?Take limits as ?t and ?x ?? 0
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Mass Balance Plug Flow
?Make SS assumption ?Substitute a 1st order rate
expression
Eq. 4-116 ME
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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 activated
  sludge
• 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 ME)
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Reactor combinations (4-28 ME)
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Mass BalancePacked 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 C
• Table 4-9 ME (reactors for mt operations)
• NOTE mt operations occur in all types of reactors