ERT 417/4
WASTE 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
•
1.
2.
3.
4.
Activated Sludge Process Design
The aeration basin volume
The amount of sludge production
The amount of oxygen needed
The effluent concentration of important parameters
The amount of sludge production
X TV
PX 
SRT
Where;
PX = sludge production rate
XT = Mixed Liquor Suspended Solid (MLSS) concentration
V = reactor volume
SRT = Solid Retention Time
Example
An 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.
SOLUTION
X TV
PX 
SRT
3
3
400 g / m X (10000m )
Px 
10day
Px  400 x10 E 03g / day


or _ 400kg / day
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
•
1.
2.
3.
4.
5.
SBR have 5 steps in common:
Fill
React (aeration)
Settle (sedimentation/clarification)
Draw (decant)
Idle
As illustrated in Fig. 8-16 and describe in Table 8-13.
Fig 8-16
Sequencing batch reactor (SBR) activatedsludge 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
•
1.
2.
3.
4.
Physical facilities used in the design of activated sludge
treatment systems:
Aeration system
Aeration tanks and appurtenances
Solid separation
Solid separation facilities
Aeration System
•
1.
2.
3.
4.
5.
Aeration system design must be adequate to:
Satisfy bCOD of the waste
Satisfy the endogeneous respiration by the biomass
Satisfy the oxygen demand for nitrification
Provide adequate mixing
Maintain minimum dissolved oxygen conc. throughout the
aeration tank
Aeration System
•
•
1.
2.
3.
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:
Diffused-air systems
Mechanical aeration
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:
C s  Ct
( K L a ) t
e
C s  C0
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
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
Concentrate the biological solids to minimize the quantity
of sludge that must be handled.
2.
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:
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
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.
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.
•
1.
2.
3.
Types of Suspended Growth Aerated Lagoons
Facultative partially mixed
Aerobic flow through with partial mixing
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.