ERT 417/4WASTE TREATMENT IN BIOPROCESS INDUSTRY

CH 8 - Suspended Growth Biological Treatment Process

Prepared by:Pn. Hairul Nazirah Abdul Halim

Activated-Sludge Process

• Activated sludge process is a process for treating sewage and industrial waste water using air and a biological floc

composed of bacteria and protozoans.

• Purpose: to develop a biological floc which reduces the organic content of the sewage

• Formation of flocculent settleable solids that can be removed by gravity settling in sedimentation tanks.

• General types of ASP: plug flow, complete mix and sequencing batch reactor – Fig. 8-1.

• Operated under aerobic/anaerobic.

Activated-Sludge Process

Description of Basic Process

• 3 basic components

1. A reactor in which the microbs responsible for treatment are kept in suspension and aerated.

2. Liquid solid separation – usually sedimentation tank

3. A recycle system for returning solid removed from the liquid-solid separation unit back to the reactor.

Figure 8-1 Typical activated-sludge processes with different types of reactors: (a) schematic flow diagram of plug-flow process and view of plug-flow reactor

Figure 8-1 Typical activated-sludge processes with different types of reactors: b) schematic flow diagram of complete-mix process and view of complete-mix activated-sludge reactor

Figure 8-1 Typical activated-sludge processes with different types of reactors: (c) schematic diagram of sequencing batch reactor process and view of sequencing batch reactor. (From H. D. Stensel.)

Wastewater Characterization

• W/w characterization is important to design an activated sludge treatment process

• Characterization categories:

1. Carbonaceous substrates

2. Nitrogenous compounds

3. Phosphorus compounds

4. Total and volatile organic compounds (TSS and VSS)

5. Alkalinity

Wastewater Characterization

• Activated Sludge Process Design

1. The aeration basin volume

2. The amount of sludge production

3. The amount of oxygen needed

4. The effluent concentration of important parameters

The amount of sludge production

Where;

PX = sludge production rate

XT = Mixed Liquor Suspended Solid (MLSS) concentration

V = reactor volume

SRT = Solid Retention Time

SRT

VXP TX

SOLUTION

ExampleAn activated sludge plant is operated at SRT value of 10 day. The reactor volume is 10 000m3 and MLSS concentration is 400 g/m3. Determine the sludge production rate.

daykgor

daygExP

day

mXmgP

SRT

VXP

x

x

TX

/400_

/0310400

10

)10000(/400 33

Complete Mix Activated Sludge Process• Effluent form the primary sedimentation tank and recycles

return activated sludge are introduced typically at several points in the reactor.

• Tank contents are thoroughly mixed – the organic load, oxygen demands and substrate concentration are uniform through out the entire aeration tank and F/M ratio is low.

• Fig 8-15

Complete-mix activated-sludge process: (a) schematic diagram

Complete-mix activated-sludge process:

(b) view of a typical complete-mix reactor.

Sequencing Batch Reactor Process• SBR have 5 steps in common:

1. Fill

2. React (aeration)

3. Settle (sedimentation/clarification)

4. Draw (decant)

5. Idle

As illustrated in Fig. 8-16 and describe in Table 8-13.

Fig 8-16 Sequencing batch reactor (SBR) activated- sludge process: (a) schematic diagram

Fill• Raw waste water (substrate) are added to the reactor

React• Biomass consumes the substrate under controlled

environmental conditions

Settle• Solid separates from liquid• Clarified supernatant can be discharged as effluent

Decant• Clarified effluent is removed during the decant period

Idle• Idle period to provide time to fill a reactor before switching to

another unit.

(b) view of a typical SBR reactor

c) view of movable weir used to decant contents of SBR reactor. Weir is located on the far side of the second dividing wall shown in (b).

Selection & Design of Physical Facilities For Activated-Sludge Processes

• Physical facilities used in the design of activated sludge treatment systems:

1. Aeration system

2. Aeration tanks and appurtenances

3. Solid separation

4. Solid separation facilities

Aeration System

• Aeration system design must be adequate to:

1. Satisfy bCOD of the waste

2. Satisfy the endogeneous respiration by the biomass

3. Satisfy the oxygen demand for nitrification

4. Provide adequate mixing

5. Maintain minimum dissolved oxygen conc. throughout the aeration tank

Aeration System

• Two basic methods of aerating w/w:

1. To introduce air or pure oxygen into the w/w with submerged diffusers or other aeration devices

2. To agitate the w/w mechanically so as to promote solution of air from the atmosphere.

• Types of aeration system:

1. Diffused-air systems

2. Mechanical aeration

3. High purity oxygen system

1. Diffused Air System• Consist of diffusers that are submerged in the w/w.

Diffusers• 3 categories:

1. Porous or fine-pore diffusers

2. Nonporous diffusers

3. Other diffusion devices – jet aerators, aspirating aerators.

Mechanical Aerators• Aerator with vertical axis and horizontal axis

Oxygen Transfer

• Sufficient oxygen (from air or pure oxygen bubbles) is needed for aerobic process.

• Oxygen is transferred from gaseous to the liquid phase• Function in aerobic process such as activated sludge,

biological filtration and aerobic digestion.

• How to transfer oxygen to water?

1. Submerged bubble aeration: by dispersing air bubbles in the liquid.

2. Hydraulic shear devices – to create small bubbles

3. Turbine mixers – mix liquid in the basin and expose liquid to the atmosphere in the form of small liquid droplets.

Oxygen Transfer in Clean Water

1. Remove the dissolved oxygen (DO) from water by the addition of sodium sulfide.

2. Measure the DO concentration until saturation level (Reoxygenation).

3. Analyze data at each point by simplified mass transfer model:

Where:

KLa = overall liquid film coefficient

Ct = conc. In liquid bulk phase at time t (mg/L)

Cs = conc. In equilibrium with gas as given by Henry’s Law

C0 = initial concentration

taK

s

ts LeCC

CC )(

0

Secondary Clarification

• Biomass generated by secondary treatment represents substantial organic load.

• Must be removed to meet acceptable standards.

• In activated sludge process, solid are removed in secondary clarifiers.

Activated-Sludge Clarifiers

• Secondary clarifiers for activated sludge must accomplish 2 objectives:

1. Produce an effluent sufficiently clarified to meet discharge standards

2. Concentrate the biological solids to minimize the quantity of sludge that must be handled.

Secondary Clarifier Design• Graphical approach.

Example

A column analysis was run to determine the settling characteristics of an activated-sludge suspension. The results of the analysis are shown in the table below:

The effluent concentration of MLSS is 3000 mg/L, and the flow rate is 8000 m3/d. Determine the size of the clarifier that will thicken the solids to 10,000 mg/L.

Concentration (mg/L)

1400 2200 3000 3700 4500 5200 6500 8200

Settling velocity (m/h)

3.0 1.85 1.21 0.76 0.45 0.28 0.13 0.089

2. Plot solids flux vs. MLSS concentration as shown in the accompanying figure. • Draw a line from the desired underfiow concentration, 10,000 mg/L, tangent to the curve and intersecting the ordinate. The value of G at the intersection, 2.4 kg/m2.h, is the limiting flux rate and governs the thickening function.

5. Check clarification function:

Suspended Growth Aerated Lagoons• Shallow earthen basins varying in depth from 2 to 5 m,

provided with mechanical aerators on floats or fixed platforms.

• Types of Suspended Growth Aerated Lagoons

1. Facultative partially mixed

2. Aerobic flow through with partial mixing

3. Aerobic with solids recycle and nominal complete mixing

• Differences in manner in which solids are handled will affect the treatment efficiency, power requirements, hydraulic and solid retention time (SRT), sludge disposal, and environmental considerations.

Figure 8-47 View of an aerobic flow-through aerated lagoon with slow-speed surface aerators mounted

on floats.