waste water treatment

1. 1
CSEN 514 – Sewerage and Wastewater Treatment
LESSON 1 – WASTEWATER TREATMENT
P.M. Mawioo, PhD

2. 2
1. Conservancy system
 Dry system of sanitation
 Refuse, garbage, storm water are collected, conveyed and
disposed of separately
 Human excreta collected in separately in conservancy latrines
2. Water carriage system
 Water used for collection and conveyance of human excreta
 99.9% water and 0.1% solids.
Sanitation Systems

3. 3
Types of Sewerage Systems
1. Separate sewerage system

4. 4
Types of Sewerage Systems
2. Combined sewerage system

5. 5
Types of Sewerage Systems
3. Partially combined/separated sewerage system

6. 6
1. Residential/Domestic
2. Institutional
3. Commercial
4. Industrial
5. Inflows & Infiltration (I&I)
Sources of Sanitary Sewage

7. 7
 Characteristics & compositions depends on sources
 Organic & inorganic matters (dissolved, suspended and
in colloidal state, etc.)
 Microorganism e.g. bacteria, virus, protozoa, etc.
 Toxic or other similar materials e.g. from industrial
discharges
 Before the design of WWTP the knowledge of the nature &
characteristics of sewage is essential
Characteristics of Sanitary Sewage

8. 8
 Sewage Q important for;
 To design the sewerage system e.g. size, shape and depth of
sewers; size of pumping unit
 To design WWTP
 Efficient disposal
Quantity of Sewage

9. 9
Factors affecting the quantity of sanitary sewage flow
 Population
 Type of area
 Rate of water supply
 Infiltration and exfiltration

10. 10
Design discharge of sanitary sewage
 Forecast population x per capita sewage generation x peak factor
 Per capita sewage generation = 75-80% of per capita water
supply

11. 11
Variations in sewage flow
 Seasonal or monthly, daily and hourly fluctuations
 Max & min flows control factors in sewer design;
 Capacity to carry max Q
 Capacity to ensure self cleansing velocity during min Q

12. 12
Factors affecting the quantity of storm sewage
 Rainfall intensity and duration
 Area of the catchment
 Slope and shape of the catchment area
 Nature of the soil and the degree of porosity
 Initial state of the catchment with respect to wetness
Estimating quantity of storm sewage
 2 methods used to calculate quantity of storm water;
 Rational method – up to 50 ha
 Empirical formulae method – for large areas
 The quantity is a function of the A, I & C

13. 13
Estimate of sanitary sewage
Sanitary sewage
 Mostly spent water of the community
 Some ground water
 A fraction of the storm runoff from the area
 Estimate based on population and the per-capita flow of sewage
• Both factors are guided by the design period
Design of sewers
 The hydraulic design of sewers and drains
 Finding out their sections and gradients

14. 14
• Generally 30 years (check for Kenya)
• The design period depends upon the following:
 Ease and difficulty in expansion, amount and availability of investment
 Expected rate of pop. growth, communities shifts, industries and
commercial inv.
 Hydraulic constraints of the systems designed
 Life of the material and equipment
• Design period for different components;
 Laterals less than 15 cm diameter: 40 to 50 years
 Trunk or main sewers: 40 to 50 year
 Treatment Units: 15 to 20 year
 Pumping plant: 5 to 10 years
Design period

15. 15
• Several methods to forecast population exist
 Most suitable approach is to base the estimation on pop. ultimate density
 If desired info. not available you can adopt the following densities;
Population estimates
Size of town (population) Density of population/hectare
Up to 5000 75 – 150
5,000 – 20,000 150 – 250
20,000 – 50,000 250 – 300
50,000 – 100,000 300 – 350
More than 100,000 350 – 1,000

16. 16
 The tributary area for any section under consideration need to be marked
on key plan.
 The topography, layout of buildings, legal limitations etc., determine the
tributary area draining to a sewer section.
 The area is to be measured from the map.
Area

17. 17
 A small portion of spent water is lost in evaporation, seepage in ground,
leakage etc.
 75-80% of the WS expected to reach the sewers
Per capita sewage flow

18. 18
 Infiltration of ground water through joints, etc
 Qinf depends on the workmanship, ground water table, the material
of sewer, nature of soil etc
 Following values may be assumed
 5000 to 50000 liters/day/hectare.
 500 to 5000 liters/day/km of sewers/cm of diameter.
Ground water infiltration
 Considering the particle size and specific weight of the suspended
solids in sewage.
 Can be found Shield’s formula
V =
1
(
8𝑘
𝑓
)(
𝑆𝑠−𝑆
𝑆
)𝑔𝑑
Self-cleansing velocity
Diameter of sewers (mm) Self-cleansing velocity(m/s)
150 – 300 1.00
300 – 600 0.75
>600 0.60

19. 19
Non-scouring velocity
Material of sewer Non-scouring velocity (cm/s)
Earth channels 60 – 120
Ordinary brick lined sewer 150 – 250
Cement concrete sewers 250 – 300
Stone ware sewers 300 – 350
Cast iron sewer pipes 300 -450
Vitrified tile and glazed bricks 450 – 500

20. 20
1. Chezy’s formula
2. Hazen William formula
3. Manning's formula
4. Basin's formula
5. Crimp and Burge’s formula
6. Kutter’s formula
7. Colebrook-White’s formula
Empirical formula for design of sewers

21. 21
Hydraulic characteristics of circular sewer partially full
V=
𝑹
𝟐
𝟑 𝑺
𝟏
𝟐
𝒏

22. 22
1. Max V tends develop at d/D of 0.8
2. Max Q tends at d/D of 0.95
Proportionate elements of partial flow
See separately;
a. Example 1
b. Example 2

23. 23
 Structures and devices of a sewerage system which are
constructed at suitable intervals along a sewer line to assist in the
efficient operation and maintenance of the system
Examples
1. Manholes
2. Street or storm water Inlets (curb, gutter, combination, etc.)
3. Oil and grease traps
4. Catch basins
5. Inverted siphons
6. Lamp holes
7. Flow regulators e.g. storm water regulators
Sewer Appurtenances

24. 24
Installed at change of;
 Direction
 Gradient
 Sewer size
 Even in straight stretches!
Sewer Appurtenances
 Even in straight stretches!
See recommended intervals in notes

25. 25
Sewer Appurtenances
 Combination inlet
 Gutter inlet

26. 26
 Wastewater characteristics can be grouped in 3No. categories:
1. Physical Characteristics
2. Chemical Characteristics
3. Biological Characteristics
- Standard Methods for the Examination of Water and Wastewater
(APHA/AWWA) – mostly applied in the characterization of WW
Characterization of Wastewater

27. 27
Turbidity
• Presence of Solids
• Classified into three main types:
 Total Solids (TS): All the matter that remains as residue upon
evaporation at 103oC to 105oC.
 Settleable solids: Settleable solids are measured as ml/L, which is
an approximate measure of the sludge that can be removed by
primary sedimentation.
 Total suspended solids (TSS) and Filterable solids (FS).
Characterization of Wastewater
Physical Characteristics

28. 28
Odor
 Gas due to decomposition of organic matter e.g. H2S
 Substances added to the wastewater.
Characterization of Wastewater
Physical Characteristics

29. 29
Temperature
 Commonly higher than that of water supply.
 Ranges 10 to 21oC with an average of 16oC.
 Importance of temperature: -
 Affects biological activity of bacteria;
 Optimum Temp. range for bacterial activity - 25°C - 35oC
 Solubility of gases e.g. O2 solubility is less in warm water than
cold water;
 Affects aquatic life;
Characterization of Wastewater
Physical Characteristics

30. 30
Color
 Fresh wastewater- light greyish brownish
 With time - dark gray
 More time - black (septic)
 Sometimes - pink due to algae or due to industrial color
Characterization of Wastewater
Physical Characteristics

31. 31
Organic matter
 Parameters:
o Biochemical oxygen demand (BOD)
o Chemical oxygen demand (COD)
o Total organic carbon (TOC)
o Total oxygen demand (TOD)
Characterization of Wastewater
Chemical Characteristics

32. 32
Inorganic matter
 Parameters :
o Salinity,
o Hardness,
o pH,
o Acidity and alkalinity
o Concentrations of ionized metals such as iron and
manganese
o Concentration of anionic entities such as chlorides,
sulfates, sulfides, nitrates and phosphates
Characterization of Wastewater
Chemical Characteristics

33. 33
Bacteria:
 Two types of bacteria are found in sewage
 Intestinal bacteria:
• Non-pathogenic & pathogenic intestinal bacteria
 Real sewage bacteria
• Both aerobic as well as anaerobic are found in sewage
• Up-flow Anaerobic Sludge Blanket Reactor (UASB)
• CAS
• TF
Characterization of Wastewater
Biological Characteristics

34. 34
Algae
 Some algae found in sewage includes Chlorella phormidum,
Ulothrix etc
 Algae are used in trickling filter in sewage treatment plant
Fungi
Fungi like Fusarium and Sporotricum are found in sewage
which play important role in trickling filter.
Characterization of Wastewater
Biological Characteristics

35. 35
Virus:
• Some viruses causing human disease such as Poliovirus,
Rotavirus, Hepatitis A and E etc are found in sewage which get
access through stool of patients
Protozoa:
• Some protozoa that cause disease of intestinal tract enter into
sewage together with stool of patient
• Examples: Entamoeba histolytica, Giardia, Balantidium coli etc
are pathogenic protozoa
• Few protozoa such as Vorticella and Opercularia are found in
trickling filter
Characterization of Wastewater
Biological Characteristics