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Topic VI: Biological Treatment Processes

Learning Objectives:

1. Discuss microbiology and relate it to wastewater treatment.

2. Describe growth kinetics of pure bacterial culture with Monod expression and MATLAB.

3. Analyze wastewater composition and calculate nutrient requirement for biological treatment.

4. Describe activated sludge processes and

their design as well as operation considerations.

5. Discuss kinetic model of activated sludge process and determine parameters using linear regression technique.

Reading Assignments:

Capter 12 (skip 12.16) and supplement

Types and Characteristics of Microorganisms

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Activated Sludge Process Microbilogy

Heterotrophic bacteria

Autotrophic bacteria

Aerobes

Anaerobes

Facultative bacteria

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Aerobic:

Anaerobic:

Metabolism, Energy, and SynthesisEnergy Conversions in Anaerobic Metabolism

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Energy Conversions in Aerobic Metabolism

Growth Kinetics of Pure Bacterial Cultures

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μ

μm

Ks S

Table 12.1 Approximate Composition Of An Average Domestic Wastewater

Before After BiologicalSedimentation Sedimentation Treated

Total solids 800 680 530Total volatile solids 440 340 220Suspended solids 240 120 30Volatile suspended solids 180 100 20BOD 200 130 30Ammonia nitrogen as N 15 15 24Total nitrogen as N 35 30 26Soluble phosphorus as P 7 7 7Total phosphorus as P 10 9 8

Nutrient Requirement for Biological Treatment Mixing Waste Streams

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Example 1: Nutrient RequirementsWastewater

40% coffee wastewater

60% domestic wastewater

Coffee Wastewater : 840 mg/L BOD

6 mg/L Total Nitrogen

2 mg/L Total Phosphorus

Domestic Wastewater : 200 mg/L BOD

35 mg/L Total Nitrogen

7 mg/L Total Phosphorus

If BOD/N/P = 100/6.0/1.5 is required, are the N & P adequate in the combined wastewater ? How much pure NH4NO3 and H3PO4 must be added if the nutrient content is insufficient?

Wastewater Flow and Strength Variations

Example 2: Population Equivalent

Domestic wastewater contains 0.24 lb of suspended solids and 0.20 lb of BOD per 120 gal.

a. Calculate the suspended-solids and BOD concentrations in mg/L.

b. Calculate the BOD equivalent population and the hydraulic equivalent population of an industry that discharges 0.1 mgd of wastewater with an average BOD of 450 mg/L.

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Q, B, S are average annual wastewater flow (mgd), BOD load (lb/day), and SS load (lb/day) respectively.Qm, Bm, Sm = average flow, BOD, and SS values during the peak month.

Qd, Bd, Sd = average flow, BOD, and SS values for the peak day.

Suspended-Growth Treatment Systems

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Return sludgeWaste sludge

Primary

Final Effluent

Conventional activated sludge process

Influent

Fig 12.33 Conventional Activated Sludge Process (a) Long rectangular aeration tank with submerged coarse-bubble diffusers along one side (Santee, CA)

Fig 12.33 Conventional Activated Sludge Process (b) Cross section of typical aeration tank illustrating the spiral flow pattern created by aeration along one side

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Return sludgeWaste sludge

Influent Primary

Final Effluent

Step Aeration Activated Sludge process

Aerationθ=30 ~ 90 min

Reaerationθ=3 ~ 6 hr

Sed.

Return sludgeWaste sludge

Effluent

Influent

Contact Stabilization without Primary Sedimentation

Effluent Final

Return sludge

Influent Aeration

Waste sludge

Extended aeration without Primary Sedimentation

Table 12.3 General Loading and Operational Parameters for Activated-Sludge Process

BOD LOADING AVERAGEPROCESS lb BOD/ lb BOD/ SLUDGE AERATION RETURN

1000 ft3/ day/lb of AGE PERIOD SLUDGEdaya MLSS (days) (hr) RATES (%)

Step aeration 30-50 0.2-0.5 5-15 5.0-7.0 50Conventional(tapered aeration) 30-40 0.2-0.5 5-15 6.0-7.5 30

Contact stabilization 30-50 0.2-0.5 5-15 6.0-9.0 100

Extended aeration 10-30 0.05-0.2 20+ 20-30 100

High-purityoxygen 120+ 0.6-1.5 5-10 1.0-3.0 30

a 1.0 lb/1000 ft3/day = 16 g/m3.d

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Design and Operation ParametersExample 3: Aeration Tank Design

Size a conventional activated sludge process receiving a wastewater flow of 18.2 mgd with 200 mg/L unsettled BOD.

Primary Settling

Secondary Settling

Aeration Tank MLSS

V

QQ, SSe

QR, SSe

Qw, SSw

Sludge Age (Mean Cell Residence Time)

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Example 4: Loading and operational parameters

Giving the data below, calculate the loading and operational parameters:

settled wastewater flow = 3.67 mgd

aeration tank volume = 120,000 ft3 = 0.898 mil gal

return sludge flow = 1.27 mgd

waste sludge flow = 18,900 gpd = 0.0189 mgd

MLSS in aeration tank = 2350 mg/L

SS in waste sludge = 11,000 mg/L

influent wastewater BOD (settled) = 128 mg/L

effluent wastewater BOD = 22 mg/L

effluent SS = 26 mg/L

Kinetic Model of Activated Sludge Process

Example 5: Kinetic Constant Determination

A municipal wastewater was tested to determine the kinetic constants using a bench-scale unit. Determine the values for Y, kd, k, and Ks from the following laboratory data.

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Loading and Operational Parameters

1. Quantity of air supply to the aeration basin.

2. Rate of activated sludge recirculation.

3. Amount of excess sludge withdrawn to control MLSS in the aeration tank, F/M, and sludge age.

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Endogenous phase

Declining growth phase

Exponential growth phase

Poor settling characteristics

Poor BOD removal efficiency

High BOD removal efficiency

Good settling characteri-stics

Range of operation for most activated-sludge-treatment systems

Food/microorganism

Rat

e of

met

abol

ism

Fig. 12.15 Rate of metabolism versus increasing food/microorganism ratio

Sequencing Batch Reactor (SBR)

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Stabilization Ponds, Lagoons, Oxidation PondsFlat-bottomed pond enclosed by an earth dike.

Liquid depth: 2 ~ 5 ft

Treat raw or partially treated wastewater.

Low BOD loading: 0.1 ~ 0.3 lb/1000 ft3/day or 25 ~ 35 lb BOD/acre/day (Northen States), 40 ~ 50 lb BOD/acre/day (Southern States).

Long liquid retention times: 50 ~ 150 days

Process microbiology: symbiotic relationship between bacteria and algae.

O2 CO2, NH3, PO43-

Completely Mixed Aerated Lagoons

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Example 6: Aerated Lagoon

Size an aerated lagoon to treat a wastewater flow of 0.3 mgd with an average BOD of 600 mg/L. The temperature extremes for the lagoon contents range from 10° C in winter to 35° C in summer. Minimum BOD reduction through the lagoon should be 75%.

Fixed–Film Biological Process

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InfluentPrimary Filter

EffluentFinal

Recirculation and sludge return

InfluentPrimary Filter Final

Direct recirculation

Sludge return with or without recirculation

(a)

(b)

Effluent

Influent Primary Filter FinalFilterEffluent

Direct recirculation

Sludge return

Primary Filter FinalFilter

Recirculation

Sludge return

Inter

Recirculation

Influent Effluent

Fig. 12.29 Typical flow diagrams for two stage trickling filters

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Efficiency Equation for Stone–Media Trickling Filters

Example 7: Trickling Filter

Calculate the BOD loading, hydraulic loading, BOD removal efficiency, and effluent BOD concentration of a single-stage trickling filter based on the following data:

average raw wastewater flow = 280 gpm

recirculation ratio = 0.5

settled wastewater BOD = 130 mg/L

diameter of filter = 18.0 m

depth of media = 2.1 m

wastewater temperature = 18° C

Efficiency Equation For Plastic-Media Tricking Filters

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Design Criteria for RBC Example 8: RBC Area Requirement

Calculate the RBC area required for secondary treatment of a raw domestic wastewater having 230 mg/L BOD. The design flow is 2.0 mgd at a temperature of 50° F.