This document provides information on various stages of municipal wastewater treatment processes. It discusses preliminary treatment including bar racks and grit chambers. Primary treatment includes primary sedimentation to remove settleable solids. Secondary treatment processes like activated sludge and trickling filters are used to further remove suspended solids and nutrients. Final treatment includes disinfection using chlorine or UV to inactivate pathogens before discharge or reuse of treated effluent. The overall goal of wastewater treatment is to reduce BOD, suspended solids and fecal coliforms to levels safe for discharge.
2. Key points
Purpose of the individual unit processes
The typical operating conditions
The outcome of the processes
Microbial reduction in the processes
3.
4. How much wastewater do we produce each day?
Wastewater Characteristics
Source Average Daily Flow
Domestic sewage 60-120 gal/capita
Shopping centers 60-120 gal/1000 ft2
total floor
area
Hospitals 240-480 gal/bed
Schools 18-36 gal/student
Travel trailer parks
Without individual
hookups
90 gal/site
With individual
hookups
210 gal/site
Campgrounds 60-150 gal/campsite
Mobile home parks 265 gal/unit
Motels 40-53 gal/bed
Hotels 60 gal/bed
Industrial areas
Light industrial area 3750 gal/acre
Heavy industrial 5350 gal/acre
Source: Droste, R.L., 1997. Theory and Practice of
Water and Wastewater Treatment
These values are
rough estimates only
and vary greatly by
locale.
5. Wastewater treatment systems
Decentralized
Septic tank
Waste stabilization ponds
Facultative lagoon
Maturation lagoon
Land treatment
Centralized
10. Conventional Community (Centralized) Sewage
Treatment
Pathogen Reductions Vary from:
low (<90%) to Very High
(>99.99+%)
Secondary Treatment Using Activated Sludge Process
Sludge drying bed or
mechanical dewatering
process
11. Typical Municipal Wastewater Treatment System
Preliminary or Pre-
Treatment
Primary
Treatment
Secondary
Treatment
Disinfection
Sludge Treatment
& Disposal
13. Preliminary Treatment Facilities
Preliminary Treatment - Bar Racks
Bar Racks: are used to remove large objects that
could potentially damage downstream
treatment/pumping facilities.
Ref: Metcalf & Eddy, 1991
14. Preliminary Treatment - Grit chamber
Grit chamber: used to remove small to medium sized, dense
objects such as sand, broken glass, bone fragments,
pebbles, etc.
22. The Activated Sludge Process
Aerobic microbes utilities carbon
and other nutrients to form a
healthy activated sludge (AS)
biomass (floc)
The biomass floc is allowed to
settle out in the next reactor;
some of the AS is recycled
Secondary Treatment
Simplified Activated Sludge Description
23. Activated sludge process
To remove suspended solids, nitrogen, and phosphate
Food to microorganism ratio (F:M ratio): 0.25 kg BOD5
per kg MLSS (mixed liquor suspended solids) per day at
10 oC or 0.4 kg BOD5 per kg MLSS per day at 20 oC
Residence time: 2 days for high F:M ratio, 10 days or
more for low F:M ratio
Optimum nutrient ratio: BOD5:N:P =>100:5:1
90 % removal of BOD5 and SS
~20 % removal of phosphate
>90 % removal of viruses and protozoa and 45 - 95 %
removal of bacteria
24. Secondary Treatment Using Trickling Filter Process
Secondary
Treatment
Secondary Treatment
Trickling
Filter
27. Tricking filter process
To remove suspended solids, nitrogen, and
phosphate
Organic loading (BOD5 X flow/volume of filter):
0.1 kg BOD5 per m3 per day
Hydraulic loading: 0.4 m3 per day per m3 of plan
area
90 % removal of BOD5 and SS
~20 % removal of phosphate
Variable removal levels of viruses, 20-80 %
removal of bacteria and >90 % removal of
protozoa
39. Barrier Approach to Protect Public
Health in Drinking Water
Source Water Protection
Treatment Technology
Disinfection
Disinfectant residual in distribution system
49. Sedimentation Basin ExampleWater coming from
flocculation basin.
Water goes to
filter.
Floc (sludge) collected
in hopperSludge to solids
treatment
50. Coagulation/flocculation/and
sedimentation
To remove particulates and natural organic materials in water
Coagulation
20 -50 mg/L of Alum at pH 5.5-6.5 (sweep coagulation)
rapid mixing: G values = 300-800/second
Flocculation:
Slow mixing: G values = 30-70/second
Residence time:10 -30 minutes
Sedimentation
Surface loading: 0.3 -1.0 gpm/ft2
Residence time: 1 – 2 hours
Removal of suspended solids and turbidity: 60-80 %
Reduction of microbes
74-97 % Total coliform
76-83 % of fecal coliform
88-95 % of Enteric viruses
58-99 % of Giardia
90 % of Cryptosporidium
51. Filtration
To remove particles and floc that do not settle by
gravity in sedimentation process
Types of granular media
Sand
Sand + anthracite
Granular activated carbon
Media depth ranges from 24 to 72 inches
52. Filter Example
Water coming from
sedimentation
basin.
Anthracite
Sand
Gravel (support
media)
Water going to disinfection
53. Mechanisms Involved in Filtration
Interception: hits
& sticks
Sedimentation:
quiescent, settles,
& attaches
Flocculation:
Floc gets
larger within
filter
Entrapment:
large floc gets
trapped in
space between
particles
Floc particles
Granular media,
e.g., grain of sand
Removal of bacteria, viruses and protozoa by a
granular media filter requires water to be coagulated
54. Rapid filtration
To remove particulates in water
Flow rate: 2-4 gpm/ft2
Turbidity: < 0.5 NTU (often times < 0.1 NTU)
Reduction of microbes
50-98 % Total coliform
50-98 % of fecal coliform
10-99 % of enteric viruses
97-99.9 % of Giardia
99 % of Cryptosporidium
55. Disinfection in water
To inactivate pathogens in water
Various types
Free chlorine
Chloramines
Chlorine dioxide
Ozone
UV
57. Comparison of major disinfectants
Consideration Disinfect ants
Cl2 ClO2 O3 NH2Cl
Oxidation
potential
Strong Stronger? Strongest Weak
Residuals Yes No No Yes
Mode of
action
Proteins/
NA
Proteins/
NA
Proteins/
NA
Proteins
Disinfecting
efficacy
Good Very good Excellent Moderate
By-products Yes Yes Yes? No