This presentation provides an overview of domestic wastewater treatment. It discusses preliminary treatment including screening and grit removal. Primary treatment involves settling suspended solids. Secondary treatment uses biological processes like activated sludge or trickling filters. Sludge is treated through thickening, stabilization via aerobic or anaerobic digestion, dewatering, and ultimate disposal. Nitrogen and phosphorus can be removed through additional processes.

1. Welcome to the presentation on
Domestic Waste Water Treatment
by
Pritilata Sarker
Bhabi Mazumder
Ranjan Kumar Das
Sharmin Akhter

2. Objectives
• Domestic waste water treatment and major ways
of classifying wastewater treatment plants
• How to design major unit operations and
processes for treating wastewater
• Microbial growth in heterogeneous cultures
• Sludge volume and weight relationships
• How to design sludge thickening systems
• How to design aerobic and anaerobic sludge
digestion systems

3. Introduction
• Wastewater is water that has been
contaminated to the degree that it is no
longer beneficial, and therefore must be
treated before it can be used or released back
into the environment.
• Four major types of wastewater are 1.
Domestic/Municipal wastewater 2. Industrial
3. Urban runoff and 4. Agricultural runoff

4. Wastewater Treatment
Categorization

5. Secondary Wastewater Treatment Plant
• TSS
• BOD

6. Advanced Wastewater Treatment Plant
• Remove N & P
• Remove additional BOD & TSS that secondary
treatment cannot remove
• AWT utilizes chemical, biological & physical

7. N removal by AWT
• Nitrification
• Denitrification

8. Nitrification
• Nitrification is an aerobic process that
transforms ammonium nitrogen (reduced
state) into an oxidized form (NO-3)
• This process is carried out by autotropic
bacteria of the genera Nitrosomonas &
Nitrobacter
• The overall reaction
• NH+4 + 2O2 NO-3 +2H+ + H2O

9. Denitrification
• To remove the nitrate that formed during nitrification , a
second biological process known as denitrification must
be used in an anoxic environment (void of dissolved
oxygen)
• Denitrifying , heterotrophic microorganisms reduce
nitrate into nitrogen gas, which is released into the
atmosphere
• The equation is
• 6NO-3 + 5CH3OH 3N2 + 6OH- + 5CO2 + 7H2O

10. P removal by AWT- EBPR
• Biological phosphorus removal may be
accomplished by providing alternating
anaerobic/aerobic treatment of the waste water.
• Such systems promote the growth of
phosphorus accumulating organisms (PAOs) such
as Acinetobacter.
• PAOs takes up excess amount of P from
wastewater during the aerobic phase of
biological treatment.
• These process is known as enhanced biological
phosphorus removal (EBPR)

11. Overview of Wastewater Treatment System
Influent Secondary
Grit Flow Aeration Chlorine Effluent
clarifier
meter basin contact
removal basin

12. Preliminary Treatment
• Bar racks and screens
• Grit removal
• Pumping
• Flow measurement
• Equalization
• Pre-aeration
• Pre-disinfection
• Shredding
• Flotation

13. Screening
• Mechanically cleaned bar racks-
-clear opening 1.5 -6 inch
-remove large objects such as logs or other
debris
• Bar screen
-opening 1-2 inch
-remove rags, paper & other debris

14. Grit Removal
• Grit consists of sand, silt, small gravel, cinder,
coffee grounds, eggshells, and other inert
materials, which typically have specific gravity
around 2.65
• These material are abrasive and will cause pump
impellers to wear excessively, and they will
accumulate in tanks, digester and pipes.
• Three major types of grit removal are aerated
chamber, horizontal flow through basin and
vortex removal system

15. Primary Treatment
• Primary treatment is clarification, or separation of
suspended solids from the wastewater.
• Primary Treatment follows preliminary treatment,
settling of SS & removal of oil, grease, and scum that
floats on surface of wastewater.
• Since these species contain organic matter, BOD
removal is accomplished.
• Primary Treatment does not remove organics soluble
or colloidal organic materials. Light weight organics
that floats to the surface are skimmed off & pumped
to the digesters for treatment

16. Primary Treatment (continued)
• The sludge that accumulates at the bottom of
a primary clarifier is normally stabilized by
anaerobic digestion before disposal.
• Primary sludge is is called “raw” sludge and
contain pathogens and organics that produces
odors

17. Primary Treatment (continued)
• Primary clarifier:
Sl Particulars Specifications
1 Size Circular/ Rectangular
2 Depth 10-16 ft
3 Length 50-300 ft
4 Width 10-80 ft
5 Dia 10-200 ft
6 Detention time 1.5-2.5 hrs
7 Avg. over flow rate 800-1200 gpd/ft2
8 Weir loading rate 10000-40000 gpd/ft2

18. Primary Treatment (continued)
• Design of Primary clarifier:
Design of Primary clarifier is based on:
-Avg. over flow rate
-Detention time
-Weir loading rate

19. Design of Primary clarifier (continued)
• The overflow rate (Vo) is defined as the flow rate divided by
the surface area of the clarifier
where Vo= over flow rate, gpd/ft2
Q= design flow rate, MGD,
As= surface area of the clarifier, ft2
• The surface area of the clarifier is determined by dividing
the design flow rate by the overflow rate. This area
calculated is then converted into either a circular or a
rectangular area
• Selecting detention time the total volume is calculated and
clarifier depth is determined.

20. Design of Primary clarifier (continued)
• Detention time (θ) is the average unit of time
that the wastewater remains in the clarifier & is
determined as follows:
where V= volume of the primary
clarifier, Q= Design flow rate
• Weir loading rate (q) is the third parameter that
must be determined when designing primary
clarifiers.
q= weir loading rate, gpd
weir length= length of primary clarifier effluent
weir

21. Secondary Treatment
• Secondary wastewater treatment implies that a
biological process is being used for treating the
wastewater.
• Microorganisms indigenous to the wastewater
use organic carbon, along with N & P, to grow
more microorganisms, primarily bacteria.
• Bacteria use the organic matter as measured by
BOD & COD for their energy and carbon source.
• Oxidation of the organic matter produces energy
that is captured in the microbe’s biochemical
pathways, while a portion of a organic material is
used in the synthesis of biomass.

22. Secondary Treatment (continued)
• The following equations shows the organic
materials is being oxidized for energy and
organic materials is being synthesized into
new microbial cells respectively-
organics + O2 CO2+ H2O + energy
organics + O2 + N + P C60H87O23N42P

23. Activated Sludge
• It is an aerobic, suspended growth, biological
process characterized by two major steps-
1. Substrate adsorption and utilization in
the aeration basin.
2. Solids/liquid separation in the secondary
clarifier

24. Activated Sludge
• Wastewater flows into the aeration basin,
where it is brought into contact with a
heterogeneous culture of microbes, consisting
primarily of heterotrophic bacteria. The liquid
inside the aeration basin is called “mixed
liquor” or “Activated sludge”. A schematic of
the process is presented in the next slide

25. System boundary
Air Alternative sludge wasting
Q + Qr Secondary Effluent
clarifier
So Aeration basin X, V, S e
Q
(Q+Qr) (Q-Qr)
Xi
Xe, Se Xe, Se
Si
RAS/WAS
WAS
Pumping
Return activated Q, X, Se
station
sludge, RAS
Qw Xr
Schematic of Activated Sludge

26. Design and operational parameters
• The main Design and operational parameters
for the activated sludge process is the mean
cell residence time (MCTR).MCTR represents
the average time that the microorganisms or
biomass remain in the system. MCTR is also
called sludge age, solid retention time (SRT) or
θc. Mathematically

27. Design and operational parameters
• Where :
• X= biomass or microorganism concentration in aeration basin
expressed as TSS or VSS, mg/L,
• Xe= secondary effluent TSS or VSS concentration, mg/L,
• Xr= TSS or VSS concentration in return activated sludge, mg/L,
• V= volume of the aeration basin, mg/L, (m3)
• Q= influent wastewater flow rate, MGD (m3 /d)
• Qr= return activated sludge flow rate, MGD (m3 /d)
• Qw= sludge wastage flow rate, MDG (m3 /d)
• Qe= Q-Qw= effluent wastewater flow rate, MDG, (m3 /d)

28. Design and operational parameters
• MCTR typically varies from 5 to 30 days and
determines the overall removal efficiency of the
process. Generally, the longer the MCTR the lower
the effluent substrate concentration as measured by
BOD or COD. MCTR is longer than the hydraulic
detention time θ. The hydraulic detention time θ is
defined as the volume of the reactor or the basin
divided by the volumetric flow rate.

29. Design and operational parameters
• θ= detention time based on influent wastewater flow
rate, d or h and
• θ’= actual detention time for recycle system, d or h.
• Another design and operational parameter is the
food-to-microorganism ratio which is defined as
• F:M= food-to-microorganism ratio
• So=influent substrate concentration to aeration basin
expressed as BOD, COD or TOC, mg/L

30. Biochemical kinematics of activated sludge in
completely mixed system
The relationship between the MCRT and net microbial
cultures growth rate is
Substrate utilization can be modeled by a Michaelis-Minton
type equation as proposed by Lawrence and McCarty (1970)
where k= maximum specific substrate utilization rate, d-1.
The effluent soluble substrate concentration Se

31. Biochemical kinematics of activated sludge in
completely mixed system
• The microorganism concentration (X)
• where So and Se= influent and effluent soluble organic
concentration, mass/volume. The volume of the aeration
basin, V
• The total quantity of biomass produced daily(Px)

32. Oxygen requirement
• Oxygen serves as electron acceptor in the activated sludge
process, so sufficient quantities of oxygen must be provided.
The following equation is used for estimating the quantity of
oxygen required to meet the total oxygen demand. The
nitrogenous demand (NOD) is the amount of oxygen required
for the nitrifying bacteria.
where O2 is the total oxygen required to meet the
carbonaceous and nitrogenous oxygen demand.

33. Aerator System
• There are two types of Aerator System 1.
Mechanical aerator 2. Diffuser
• Mechanical aerator consists of mixers or
brush rotors that transfer oxygen into the
wastewater by spraying the wastewater into
the air. Diffused aerator systems are similar to
fish aquarium tanks in which oxygen diffuses
through a diffuser stone or membrane,
thereby transfer oxygen into the wastewater.

34. Trickling Filters
• Alternate secondary wastewater treatment
process.
• Wastewater is applied to some type of filter
medium.
• Media:- Past -Rock or Slag.
• Media:- Present- Plastic.
• Biomass growing on the media uses the organic
matter along with a portion of the nitrogen &
phosphorus to grow new microorganisms.

35. Trickling Filters
Recirculation Pump
Primary Secondary
Trickling filter humus sludge
Primary & Secondary humus sludge

36. SECONDARY CLARIFICATION
• -It must separate the suspended & biological
solids from the liquid wastewater.
• - The design requires calculating the area of
the clarifier based on clarification is
determined by dividing the design flow rate by
the overflow rate according to the following
equation:
Ac =

37. SECONDARY CLARIFICATION
• Where:
• Ac = the area of clarifier based on
clarification,ft2 (m2)
• Q = Design flow rate, excluding sludge return
flow applied to the secondary clarifier, MGD
(m3/d), and
• Vo = Overflow rate or surface loading rate,
gpd/ft2 [m3/d.m2]

38. SECONDARY CLARIFICATION
• Based on thickening consideration, the following equation
is used for determining the secondary clarifier area.
AT=
• Where:
• AT= secondary clarifier surface area.
• Q = wastewater design flow rate applied to the secondary
clarifier excluding the return activated sludge flow rate,
MGD (m3/d)
• QR= return activated sludge flow rate, MGD (m3/d)
• MLSS= suspended solids concentration in the aeration
basin, and
• SLR = solids loading rate design criteria, typically 25 ppd/ft2
at average daily flow.

39. Disinfectants used in Wastewater Treatment
• -Disinfectants-Cl2, O3 & ultraviolet (UV)
radiations.
• -These disinfectants offer the advantage of
eliminating the production of trihalmethanes
(THMs), which are chlorinated organic species
and suspected carcinogens that result when
chlorine is added to water containing organic
compounds.

40. Chlorination of Wastewater
• -Chlorine is delivered to wastewater treatment
facilities in pressurized containers, which range in
size from 150-lb to 1-ton cylinders.
• -Chlorine is liquefied under high pressure &
withdrawn as a gas or liquid, depending on the
withdrawal rate.
• - At large waste water treatment plant, chlorine
is withdrawn as a liquid & must pass through an
evaporator to be converted to a gas.

41. Chlorine Contact Basin
• -When chlorine is added to secondary
effluent, it must have sufficient contact with
the wastewater to kill pathogens.
• -Long, rectangular serpentine channels
simulating the plug flow regime are used in
the design of chlorine contact basins.

42. Sludge Treatment & Disposal
• Sludge or bio-solids and other residuals are generated
at wastewater treatment facilities.
• Screening & grit from preliminary treatment unit are
normally collected in dumpsters; volume is reduced &
hauled off to sanitary landfills for ultimate disposal.
• Primary sludge contains organic solids & pathogens.
• Secondary sludge consists of biological organisms,i.e.
biomass & is stabilized using aerobic or anaerobic
digestion.
• Aerobic & anaerobic digestion decrease the volume &
quantity of sludge

43. 4 Steps of Sludge treatment & disposal
• Sludge volume is reduced through thickening
operation.
• Stabilization with aerobic or anaerobic
digestion.
• Dewatering operations to increase the
concentration of solids.
• Ultimate disposal.

44. Sludge Weight & Volume Relationships

45. 4 Steps of Sludge treatment & disposal
• The volume (V) occupiedby wet sludge (ft3 or
m3) is determined using the following
equation:
V=
• Where:
• S= Solids content,%
• ϒ= Specific weight of water, 62.4 lb/ft3

46. Thickening Operations
• Important because
• It reduces the volume of the sludge to be handled,
thereby reducing the size of subsequent solids
handling processes such as sludge stabilization.
• Sludge thickening operations & the approximate solid
concentration associated with each include
• a) gravity thickening (2% -10% solids)
• b)gravity belt thickeners (3%-6%)
• c)dissolved air flotation (3%-6%)
• d)thickening centrifuges (4%-8%)

47. Stabilization
• Anaerobic Digestion
• -A biological process that takes place in an enclosed
reactor with no oxygen present.
• -It produces a stable sludge & a valuable by-product,
methane gas,which can be combustedto provide heat
for the digesters or used for generating electricity.
• -It is sensitive to operate & prone to biological upsets.
• -Anaerobically digested sludge is usually difficult to
dewater by mechanical means.
• - 3 step process- Hydrolysis
Acidogenesis
Methanogenesis

48. Anaerobic Digestion
• Hydrolysis:
• -Complex organic solids are hydrolyzed by bacteria.
• -Carbohydrates,proteins & fats are converted to simple
carbohydrates, amino acids and fatty acids.
•
• Acidogenesis
• -Involves the conversion of soluble carbon formed during
hydrolysis into organic acids & H2.
•
• Methanogenesis
• -Bacterial conversion of fatty acids & hydrogen into CH4 &
Co2 by strict anaerobes.

49. Anaerobic Digestion
• Low or standard-rate anaerobic digester volume can
be estimated as follows:
• V= T1+ Q2T2
• Where:
• V =total digester capacity, ft3 (m3)
• Q1 =volume of raw sludge fed daily, cfd (m3/d)
• T1 =period required for digestion, approximately 25 d
• Q2 =volume of daily digested sludge accumulation in
tank, cfd (m3/d)
• T2 =period of digested storage, 20 to 120 days.

50. Anaerobic Digestion
• For determining the volume of a high-rate
anaerobic digester.
• V1= Q1xT
• V1=digester capacity required for 1st-stage or
high-rate digestion, ft3 (m3)
• T=period required for digestion, d.
• The volume of a 2nd-stage,
VH= T1+ Q2T2

51. Anaerobic Digestion
• Where:
• VH=digester capacity required for 2nd-stage, ft3 (m3)
• Q1=volume of digested sludge feed=volume of
average daily raw sludge feed cfd (m3/d)
• Q2=volume of daily digested sludge accumulation
in tank, cfd (m3/d)
• T1=period required for thickening, days
• T2= period required for digested sludge storage,
days

52. Aerobic Digestion
• Similar to the activated sludge process.
• -Organic sludge is aerated for extended
periods to oxidized organic material.
• -Works best on waste activated sludge.
• -Reduces significant pathogens.
Organic matter+O2 new cells+energy+CO2+H2O+end products

53. Dewatering
• -Consists of reducing the sludge volume &
increasing the solids content.
• -Concentration includes-
• * Centrifuges (5% to 40% solids)
• * Belt filter presses (12% to 50%)
• * Vacuum filtration (20% to 30%)
• * Plate & frame presses (34% to 60%)
• * Sand drying beds (up to 50%)

54. Sludge Disposal
• Various options are available for proper
disposal.
• Incineration of sludge is an option that is
becoming less attractive because of the high
cost of building & operating incinerators, plus
the creation of air pollution & need to landfill
the ash.

55. THANKS