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Wastewater Treatment Systems

Biological Wastewater Treatment

Biological Treatment Systems

  Attached Growth Systems   Trickling Filter   Rotating Biological Contactor

  Suspended Growth System   Activated Sludge System

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Trickling Filters

  See Fig 12.18   General description of trickling filter

  growth of microorganisms on media in trickling filter   aerobic zone on outer portion of microbial film (Fig 12.17)   anaerobic zone on inner portion of microbial film (Fig 12.17)   oxygen supply is provided by air flowing through bed   wastewater is treated as it flows over media

  soluble organics metabolized   colloidal organic matter absorbed onto bacterial layer

  Filter flies (Psychoda) can be nuisance in warm weather

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Trickling Filter Secondary Treatment Systems   As microorganisms grow, the microbial growth sloughs off the

media   Microbial solids are captured in a final clarifier   Recirculation

  dilutes raw wastewater and reduces wastewater strength   wastewater passes through trickling filter more than once   maintains a minimum hydraulic loading on filter   common range for QR/Q: 0.5 - 3.0   see other recirculation patterns (Fig 12.22, Fig. 12.23)

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Trickling Filter Media

  Stone, slag, crushed rock   durable media   disadvantages

  low percentage of void space for air passage   limited surface area per unit volume for microbial growth   bed depths limited

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Trickling Filter Media

  Plastic   high void space for air flow   high surface area for microbial growth   deep beds (10 - 20 ft)   types include cylinders (Fig 12.19) and modules

(Fig. 12.20)

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Rotating Biological Contactor

  Fig 12.30

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Activated Sludge Systems

  Fig 12.33   conventional activated sludge process: plug flow   air bubble diffusers:

  keep tank contents suspended   provide microorganisms with oxygen

  contents of aeration tank:   mixed liquor suspended solids (MLSS)

Activated Sludge Systems

  Fig 12.34   aeration tank with fine bubble diffusers

  Fig 12.32 (a)   Flow schematic of conventional activated sludge

process

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Description of Activated Sludge

  Wastewater flows into aerated tank   Microorganisms metabolize organic substrate   Biological flocs of microorganisms (activated

sludge) settle out in secondary clarifier

Description of Activated Sludge

  Settled microorganisms are recycled to the aeration tank

  Clear supernatant from the secondary clarifier can be disinfected and discharged from the plant if no additional treatment is required

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Description of Activated Sludge

  When organic substrate is utilized in the synthesis of new cellular material, microbial mass accumulates in the system

  The excess microbial mass (sludge) is wasted from the system

  The sludge wasted from the system is treated in the sludge processing units of the plant before returning to the environment

Activated Sludge Systems Problems   Treatment Efficiency - Example 1   Activated Sludge Mass Balance - Example 1

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F:M Ratio

  This text: BOD Loading or F:M

  Other authors:

  When comparing to published values, note whether the values are expressed in terms of MLSS or MLVSS

Sludge Age (also called mean cell residence time)

  This text in Section 12.19

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Sludge Age (also called mean cell residence time)

  Other sources and this text beginning in Section 12.22

  For a system such as that shown in Fig 12.40, MLVSS and VSSw are usually represented by X and VSSe by Xe

  Explanation: the equation for sludge age expresses mean cell residence time more realistically when calculated using the volatile portion of suspended solids, since the volatile portion is more representative of microorganisms.

Activated Sludge Systems Problems

  Example problem with BOD loadings, aeration period, and sludge age   Activated Sludge - Example 1

  See also Example 12.9 in text

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Activated Sludge Processes   Conventional activated sludge process

  plug flow: Fig 12.32(a)

  Step-aeration activated sludge process   also called “step-feed”   influent wastewater introduced at several points helps

provide a more uniform oxygen demand throughout the flow path

  Fig 12.32(b)

  Completely-mixed activated sludge process   Fig 12.40

Activated Sludge Processes: Extended Aeration   Extended aeration activated sludge process

  Fig 12.32(d): note that there is no primary clarifier   residential communities, small towns, etc.   aeration period: 24 hours or longer   operates in endogenous growth phase   final clarifiers

  long detention time   overflow rate: 200 - 600 gpd/ft2

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Activated Sludge Processes: Extended Aeration   Types of extended aeration systems

  oxidation ditch (Fig 12.36)   Carrousel® system (Fig 12.37)   field erected plant (Fig 12.35)

Design of Activated Sludge Systems   Using an approach that relies on data such as that

presented in Table 12.3 and/or state standards   Using a kinetics model, as presented in Section

12.22, assuming certain wastewater characteristics   Using kinetic constants determined in a laboratory or

pilot plant study as described in Section 12.23

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Design of Activated Sludge Systems   Design of Step Aeration System

  Activated Sludge - Example 2   see also Example 12.10, text

  Design of Extended Aeration System   Activated Sludge - Example 3   see also Example 12.12

Design of Activated Sludge Systems   Design of a completely-mixed activated

sludge process using kinetics approach   Example 12.15, text