Biological Treatment
• Objectives
• Transform dissolved and particulate biodegradable
constituents into acceptable end products
• Capture and incorporate suspended and non-settleable
colloidal solids into biological floc or biofilm
Biological Treatment
• Transform or remove nutrients such as nitrogen and
phosphorus
CHONPS + O2 CO2 + H2O+ NO2 + PO4 + SO4
BOD Terminologies
• Oxidation of biodegradable organic matter • Aerobic process
• Biological treatment process occurs in the presence of oxygen
O2 Temperature • Anaerobic process
CO2 + • Biological treatment process occurs in the absence of oxygen
H2O
• Anoxic process
Biodegradable
OM + Oxidation
+ NO2
+ PO4
+ SO4
• The process by which nitrate nitrogen is converted biologically to
nitrogen gas in the absence of oxygen. This process is also known
as de-nitrification
• Facultative process
• Both aerobic and anaerobic process occurs simultaneously
Nutrients
Terminologies Bacterial Growth
• Suspended growth process
• Biological treatment process in which the microorganism
responsible for the conversion of the organic matter or other
constituents in the wastewater to gases and cell tissues are
maintained in suspension within the liquid
• Attached growth process
• Biological treatment process in which the microorganisms
responsible for the conversion of the organic matter or other
constituents in the wastewater to gases or cell tissues are
attached to some inert medium, such as rocks, slag or specially
designed ceramic or plastic materials. Attached growth treatment
process are also known as fixed-film process
1
Bacterial Growth Activated Sludge Process
• Segment _1 lag phase • In 1880s, Dr. Smith investigated the aeration of wastewater
• Bacteria first acclimated to their surrounding environment and tanks and oxidation of organic matter
to the food provided • 1910 Black and Phelps reported that a considerable
• The acclimation period is called the lag phase reduction in the organic matter by forcing air into wastewater
• Represented by the segment_1 • 1913 Clark and Gage
• Vary in length depending on the history of the seed organisms. • The process was named activated sludge by Adren and
Organism accustomed to similar environment and food Locklett because it involved the production of an activated
• mass of micro-organism capable of aerobic stabilization of
organic material in wastewater
Design of Activated Sludge
Activated Sludge Process
Process
• The hydraulic retention time (HRT), also known as hydraulic
residence time, is a measure of the average length
of time that a compound (ex. water) remains in a storage unit
(ex. lake, pond, ocean).
• Hydraulic retention time is the volume of the storage unit
divided by the influent flowrate.
ɵ=V
Q
V= volume of aeration tank, m3, and
Q= sewage inflow, m3/d
Design of Activated Sludge Design of Activated Sludge
Process Process
• Solids retention time /sludge retention time / mean cell • The concentration of biomass in reactor is often called as
residence time mixed-liquor suspended solids (MLSS)
• The average time that micro-organisms spend in the reactor is • MLSS, HRT and SRT
called as SRT or MCRT
• Generally SRT is greater than HRT
• The ratio of the total biomass in the reactor to the biomass
wasted per given time
Xu
2
Design of Activated Sludge Design of Activated Sludge
Process Process
• Volumetric loading rate • F/M ratio
• The volumetric loading rate VL is the mass of BOD in the • The F/M ratio is the mass of BOD removed divided by the
influent divided by the volume of the reactor biomass in the reactor
QSo Q( S o S )
VL F/M
VX
V
Design of ASP Problem
• An activated-sludge system is to be used for secondary
treatment of 10000 m3/day of municipal wastewater. After
primary clarification, the BOD is 150 mg/L, and it is desired to
have not more than 5 mg/L of soluble BOD in the effluent. A
completely mixed reactor is to be used, and piolet-plant
analysis has established the following kinetic values: Y= 0.5
kg/kg, kd= 0.05 /day. Assuming MLSS concentration of 3000
mg/L and an underflow concentration of 10000 mg/L from the
secondary clarifier. Determine
• Volume of the reactor
• Mass and volume of solids that must be wasted each day
Solution Solution
• Assume SRT • Now apply mass balance to secondary clarifier and assume
that solids in the effluent are negligible compared to the
influent and underflow;
• Qr = (QX-QwXu) / Xu-X
• Calculate Volume • Then estimate the recirculation ratio
• Check for HRT • Qr/Q =?
• Mass and volume of solid wasted per day QwXu
Xu
• Xu = 3000 mg/L
• Calculate Qw
3
Aeration of ASP Aeration of ASP
• Oxygen utilization rate • Aerators
• The rate at which oxygen consumed by microorganisms in the 1. Diffused aerators
biological reactor is called the oxygen utilization rate 2. Mechanical aerators
• ASP Oxygen utilization rate will always exceed the rate of • Air diffusers
• Fine bubble diffusers
natural replenishment
• Bubbles 2.0 to 2.5 mm in diameter
• Thus some artificial means of adding oxygen is supplied by • Coarse bubble diffusers
aerating the mixed liquor in the biological reactor • Diameter up to 25 mm
• Treatment of ordinary municipal waste by conventional ASP • Finer bubbles more efficient Larger surface area per unit
results in an Oxygen utilization rate of 30 mg/L.h and up to volume of air
100 mg/L.h • But head loss through the small pores necessitates greater
compression of air greater energy requirements
• Compressed air must be filtered to avoid clogging of diffuser
opening
Aeration of ASP Aeration of ASP
• Coarse bubble diffusers • Mechanical aerators
• Less maintenance and lower head loss • Causes turbulence at the air-liquid interface and this
• Poor oxygen transfer efficiencies turbulence entrains air into the liquid.
• High speed impellers that add large quantities of air to
relatively small quantities of water
• Aerated water is then mixed with the reactor contents
through velocity gradients
• Smaller high speed units extended aeration
• Slow-speed units conventional ASP
4
Based on rates and points of air or waste
water applications
Detentions times
Use of pure oxygen rather than air
Step aeration
Tapered aeration
Contact Stabilization
Pure-oxygen activated sludge
Oxidation ditch Influent addition at intermediate points provides more uniform
BOD removal throughout tank
High rate
Extended aeration
Air is added in proportion to BOD exerted
Biomass absorb organics in contact basins
Settles out in secondary clarifier
The thickened sludge is aerated before being returned
to the contact basin
1
Oxygen added under pressure keeps dissolved
oxygen level high
Long detention time and low F/M ratio in aerator to
Short detention time and high F/M Ratio in aerator to maintain the culture in
maintain culture in Log-growth phase Endogenous Phase
2
Reactor
The containers, vessels or tanks in which
chemical and biological reactions are
carried out are commonly called reactors
Types of Reactors
Types of Reactors Batch Reactor
• Flow in neither entering nor leaving the reactor
1. Batch • Flow enters, is treated and then is discharged and the cycle
repeats
2. Plug-flow/ Tubular flow • The liquid contents of the reactor are mixed completely
3. Complete mix
4. Arbitrary –flow
5. Packed-bed
6. Fluidized-bed
Complete-Mix Reactor Plug-flow Reactor
• In this reactor it is assumed that complete mixing occurs • Fluid particles pass through the reactor with little or no
instantaneously and uniformly throughout the reactor as fluid longitudinal mixing and exit from the reactor in the same
particles enter the reactor sequence in which they entered
• Particles retain their identity and remain in the reactor for a
time equal to the theoretical detention time
• Mainly occurs in long open tanks with a high length-to width
ratio in which longitudinal dispersion is minimal or absent
1
Packed-Bed Reactor Fluidized-Bed Reactor
• The packed bed reactor is filled with some type of packing • Similar to packed bed reactor in many aspects, but the packing
material, such as rock, slag, ceramic or now commonly plastic material is expanded by the upward movement of fluid
• Packing material can be continuous or arranged in stages through the bed
• The expanded porosity of the fluidized-bed packing material
can be varied by controlling the flow rate of the fluid.
