last lecture Flashcards
(39 cards)
1
Q
Public health aspects of wastewater treatment
- ___ million people, mostly children, die annually from water-related diseases
- ___ million people lack access to basic sanitation include the poorest in the world
- ____ billion people lack access to even improved water sources
- Access to safe water supply and sanitation is fundamental for better health, poverty alleviation and development (WHO data)
A
3.4 million
2.4 million
1.1 billion
2
Q
- Minimum capital cost
- Reliable and economic operation
- Protect public health from contamination of water supplies
- Removal of floating, suspended and soluble matter
- Reduce BOD, COD, pathogenic organisms and nutrient
- Efficient collection system for aerobic conditions
- Maintain aesthetics of natural water bodies, ecology of water systems
A
Wastewater Treatment Goals
3
Q
Treatment Selection
- Wastewater treatment comprises ____, _____, and ____ treatments
- The selection of appropriate treatment processes is dependent upon the nature and strength of pollutants, quantity of flow, and discharge licence conditions
A
primary, secondary and tertiary
4
Q
The first stage of wastewater treatment comprises largely physical processes.
A
Primary Treatment
5
Q
Primary Treatment
- A well-designed primary treatment should remove about 40-75% of TSS and about 25-40% BOD5
- A possible pre-treatment is the injection of air, O2, H2O2 and pre-chlorination if the influent is _____
- Processes include screening, grit removal and primary settling
A
anaerobic
6
Q
The removal of large objects that may damage pumps or block channels is done using
A
screens
7
Q
- Fixed or mechanical
- Velocity in channels about 0.3-0.4 m/s
- Velocity through openings about 0.61 m/s
- All screenings to be removed/buried
- Location of strong odour from decomposition
A
Screens
8
Q
- These are mechanical cutting screens that reduce the size of large objects
- Shredded matter are returned to the flow stream
- A by-pass may be included
A
Comminutors
9
Q
Purpose is to remove inorganic grit/sand 0.2-1 mm size through differential settling
Aim is to prevent damage to pumps, blockage of channels and cementing of sludge in settling tanks
A
Grit Chambers
10
Q
Two types of grit chambers,
A
constant velocity
aerated/spiral flow tanks
11
Q
- Class I settling - horizontal flow
- Uniform velocity at 0.25-0.35 m/s
- Ideal parabolic shape or approximation
- Width:depth ratio 1:1
- Length ≥ 18 x max. depth
A
Constant Velocity Grit Chamber
12
Q
- Flexibility of control; more efficient grit removal and can assist pre-aeration
- Suitable for larger population > 10 000 ep
- HRT of about 3 min at PWWF
A
Aerated or Spiral Flow Grit Chamber
13
Q
Largely class II settling of flocculent matter and natural coalescence or flocculation occurs
A
Primary Sedimentation
14
Q
Primary Sedimentation
- A test column is used to establish settling characteristics and correction factor of 0.65 -0.85 is applied to overflow rate and 1.25-1.5 to detention time values
- Per cent removal = ΣΔΗ (R + R)/(2h)
- The _____ are pumped to an anaerobic digestion tank. The effluent (______) from primary treatment flows to the next stage i.e. secondary treatment
A
settled solids
settled sewage
15
Q
Some Features of Primary Settling
- Design to accept 2 to 3 x ADWF
- Removal of 40-75% suspended solids
- Some incidental BOD, reduction 25-40%
- Hydraulic loading Q/A = 30 m³/m².d
- HRT 1.5 to 3 h; depth 2.5 to 5 m
- Even inlet distribution > 3 m/s
- Sludge scrapers should not cause re-suspension
A
16
Q
Types of Primary Settling tanks
___
- Tanks use less space
- Forward velocity 10-15 mm/s
- Weir loading rate < 300 m³/m.d
- Length:width ratio 3:1
A
Rectangular horizontal-flow
17
Q
Central process is biological in which dissolved organics are utilised by microorganisms
A
Secondary Treatment
18
Q
Types of Primary Settling tanks
___
- Radial-horizontal flow
- Uses radial scrapers to remove sludge
A
Circular radial flow tank
19
Q
Secondary Treatment
- secondary treatment is often known as _____
- The contaminant growth of biomass (cells) and substrate removal must be followed by _____
A
biological treatment; separation
20
Q
Classification of Microorganisms
A
Microorganism
Energy source
Carbon source
21
Q
TYPES OF MICROORGANISM
1. ___
photoautotrophic
chemoautotrophic
- ____
photoheterotrophic
chemoheterotrophic
A
Autotrophic; Heterotrophic
22
Q
TYPES OF ENERGY SOURCE
1. ___
light
inorganic oxidation-reduction reaction
- _____
light
organic oxidation-reduction reaction
A
Autotrophic; Heterotrophic
23
Q
TYPES OF CARBON SOURCE
1. ___
CO2
- _____
organic caarbon
A
Autotrophic; Heterotrophic
24
Q
Biological processes
- ___ condition - presence of free molecular oxygen
- ___ condition - devoid of free molecular oxygen
- ____ absence of free molecular oxygen but presence of nitrate
A
Aerobic
Anaerobic
Anoxic
25
Types of Metabolism
* **aerobic** microorganisms generate energy by **enzyme-mediated electron transport** from an electron donor to an external electron acceptor eg O2
* **anoxic process** uses NO, and SO, as the electron acceptors
Respiratory metabolism
26
Types of Metabolism
* **anaerobic processes** that do not involve an external electron acceptor
* process is **less energy efficient** and is characterised by low growth rates and low cell yield
* **facultative anaerobes** can shift from fermentative to aerobic respiratory metabolism depending on the absence or presence of O2
Fermentative metabolism
27
Some Concepts of Biological Treatment
* Biological growth curve
* Food:microorganism ratio ie F/M
* Fixed-film (attached) system and suspended growth system
28
Lag phase
Log-growth or exponential phase I
Stationary phase
Log-death or endogenous phase
Biological growth curve
29
**F/M ratio**
* Food is the _____ i.e. (Q x S)
* Microorganisms i.e. (reactor volume x biomass conc.)
* F/M is expressed as t¹
* F/M is used as a ____ design criterion
substrate
preliminary
30
* Influent (or **settled sewage** from primary treatment) enters the reactor (**aerator tank**) where contaminants are **biodegraded** by selected microorganisms
* Reaction processes lead to the **reduction of contaminants** and **increase of biomass** (cells)
* In the activated sludge process the **biomass is** often referred as the **mixed liquor volatile suspended solids** (MLVSS)
* MLSS is the **mixed liquor suspended solids** (MLVSS = 0.8 MLSS)
Activated Sludge Process
31
Activated Sludge Process (cont)
* The biomass is separated in a **final sedimentation tank** (____) as settled sludge and recirculated as **return activated sludge (RAS)** to the reactor
* The **clarified effluent** is often of a standard that may be **discharged into receiving waters**
* The ____ **increases the MLVSS** concentration in the reactor
* To maintain a designed MLVSS (at steady state) some **biomass must be wasted**
clarifier; RAS
32
Some Features of the Activated Sludge Process
* Design for PDWF and F/M ratio
* **System is aerobic**; requires 0.5-2 mg/L DO using diffused air, surface aerators, turbines
* **Microorganisms** are mainly ___ & **facultative heterotrophs** and some autotrophs for _____.
* Microorganisms are **kept in suspension** by mixing
aerobic; nitrification
33
Hydraulic Characteristics of Reactor Tanks
____
* Each element has the **same residence time**
* Long and narrow in dimension
* No longitudinal mixing
* BOD highest at inlet
* DO lowest at inlet
* Lower average MLSS
* **Theoretically more efficient** than completely mixed flows
Plug flow system
34
**Land treatment**, trickling and rotating
biological filters are predominantly
**aerobic biological processes**
**Land treatment** i.e. broadcasting of
sewage is one of the **earliest forms of
wastewater treatment**
Fixed-Film Systems
35
* Development of a **biofilm** on an inert surface where macro and microorganisms break down organic matter
* Natural **sloughing** of the biofilm owing to **aerobic growth**, decay and shear stress at the interface
* **Filter medium** voids promote air circulation and aerobic condition
Trickling Filter
36
Trickling Filter (cont)
* Design for PWWF
* **Simplicity** in construction but little control
* **Ease of operation but high initial capital cost**
* **Balance** of hydraulic and organic loading
necessary to prevent clogging of voids
* BOD removal efficiency E = 1/[1 + 0.44√(W/VF)]
* **Speed of distributor is critical**
* Recirculation ratio 0.5 - 3
* **Humus sludge** production 0.3 - 0.5 g/g BOD5 removed
basa
37
* A fixed-film aerobic process comprising of **large number of discs rotating** half submerged in a tank
* Wastewater flows through the tank
* **Development of biofilm on the disc** that interacts with the wastewater
* The rotating biological contact units are **compact with low energy consumption**
Rotating Biological Contact Unit
38
* **Microorganisms are held in suspension** as a high concentration flocculent, bulky matter through agitation, stirring
* The **microorganisms interact with influent wastewater** and biodegrade organic matter into CO2, H₂O and byproducts, releasing energy for growth of new cells
* The **activated sludge** process is an example of an aerobic suspended growth system. The **anaerobic digester** for the break down of waste sludge is an example of an anaerobic suspended growth system
Suspended Growth Systems
39
Hydraulic Characteristics of Reactor
Tanks
___
* Each element may not have the same HRT
* **Continuous** and thorough mixing
* Rectangular tanks, typically 6 -7 m width x 3-5 m depth
* Uniform MLSS and BOD
* Higher MLSS
* **Substrate concentration in tank and effluent are equal**
* **Better resistance** to shock hydraulic and pollutant loads
* Better resistance to toxic loads
Completely-mixed system
