Module 8: Wastewater Treatment Flashcards
(91 cards)
1
Q
Public health aspects of wastewater treatment:
* _______ people, mostly children, die annually from water-related diseases.
* _______ people lack access to basic sanitation include the poorest in the world.
* _______ people lack access to even improved water sources.
A
- 3.4 million people
- 2.4 million people
- 1.1 billion people
2
Q
Wastewater Treatment Goals
A
- 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
3
Q
Wastewater treatment comprises: (3)
A
primary, secondary and tertiary treatments
4
Q
The __________ processes is dependent upon the nature and strength of pollutants, quantity of flow, and discharge licence conditions
A
selection of appropriate treatment processes
4
Q
- A well-designed primary treatment should remove about ______ of TSS and about ______ BOD5
- A possible pre-treatment is the injection of ______________ if the influent is anaerobic
- Processes include screening, grit removal and primary settling
A
- 40 - 75%
- 25 - 40%
- air, O2, H2O2 and pre-chlorination
4
Q
__________
* The first stage of wastewater treatment comprises largely physical processes.
A
Primary Treatment
4
Q
The removal of large objects that may damage pumps or block channels
A
Screens
5
Q
- Fixed or mechanical
- Velocity in channels about 0.3 - 0.4 m/s
- Velocity through openings about 0.6 - 1 m/s
- All screenings to be removed/buried
- Location of strong odour from decomposition
A
Screens
6
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
7
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
7
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
7
Q
Two types of grit chambers, namely:
A
- constant velocity
- aerated/spiral flow tanks
8
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
9
Q
___________
* Largely class II settling of flocculent matter and natural coalescence or flocculation occurs
* A ______ 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
* The settled solids are pumped to an ________ tank. The effluent (________) from primary treatment flows to the next stage i.e. secondary treatment
A
Primary Sedimentation
* test column
* anaerobic digestion tank
* settled sewage
10
Q
Some Features of ________
* Design to accept 2 to 3 x ADWF
* Removal of 40 - 75% suspended solids
* Some incidental BOD5 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
Primary Settling
11
Q
Types of Primary Settling Tanks (3)
A
- Rectangular horizontal-flow
- Up-flow tank
- Circular radial flow tank
11
Q
Type of Primary Settling Tank:
- 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
11
Q
Type of Primary Settling Tank:
- Square with 60° sludge hopper
- No moving parts as sludge is removed hydrostatically
- Some possible particle carry over
A
Up-flow tank
12
Q
Type of Primary Settling Tank:
- Radial-horizontal flow
- Uses radial scrapers to remove sludge
A
Circular radial flow tank
13
Q
Central process is biological in which dissolved organics are utilised by microorganisms
A
Secondary Treatment
14
Q
- secondary treatment is often known as __________
- The concomitant growth of biomass (cells) and substrate removal must be followed by separation
A
biological treatment
15
Q
- Heterotrophic
* ________
Energy Source: light
Organic Carbon
* ________
* Organic oxidation-reduction reaction
* Carbon Source: Organic Carbon
A
- Photoheterotrophic
- Chemoheterotrophic
15
Q
Classification of Microorganisms:
1. Autotrophic
* _______
Energy Source: light
Carbon Source: CO2
* _______
Energy Source: inorganic oxidation-reduction reaction
Carbon Source: CO2
A
- Photoautotrophic
- Chemoautotrophic
15
Q
Types of Microorganism: (2)
A
- autotrophic
- Heterotrophic
16
Biological Processes
* Aerobic Condition
* Anaerobic Condition
* Anoxic
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Presence of free molecular oxygen
Aerobic Condition
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devoid of free molecular oxygen
Anaerobic Condition
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Absence of free molecular oxygen but presence of nitrate
Anoxic
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Types of Metabolism (2)
* Respiratory Metabolism
* Fermentative Metabolism
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* aerobic microorganisms generate energy by enzyme-mediated electron transport from an electron donor to an external electron acceptor eg O2
* ________ uses *NO3 and SO4^2-* as the **electron acceptors**
Respiratory metabolism
* **anoxic process**
20
* 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
* _____________ can shift from fermentative to aerobic respiratory metabolism depending on the absence or presence of O2
Fermentative metabolism
* facultative anaerobes
20
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Biological growth curve
* Lag phase
* Log-growth or exponential phase
* Stationary phase
* Log-death or endogenous phase
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F/M ratio
* Food is the ______ i.e. (Q x S)
* Microorganisms i.e. (reactor volume x biomass conc.)
* F/M is expressed as t^-1
* F/M is used as a ________
* substrate
* preliminary design
21
* 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
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* 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
Trickling Filter
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___________
* 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
* ________ promote air circulation and aerobic condition
Trickling Filter
* Filter medium voids
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The ___________ are compact with low energy consumption
rotating biological contact units
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* 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
Rotating Biological Contact Unit
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* 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 by-products, releasing energy for growth of new cells
Suspended Growth Systems
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The __________ 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
activated sludge process
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* 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)
Activated Sludge Process
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* In the activated sludge process the ________ is often referred as the **mixed liquor volatile suspended solids** (MLVSS)
* MLSS is the mixed liquor suspended solids (MLVSS ≈ 0.8 MLSS)
biomass
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Activated Sludge Process:
* The biomass is separated in a final sedimentation tank (clarifier) as ______ and recirculated as __________ to the reactor
* The ________ is often of a standard that may be discharged into receiving waters
* The RAS increases the MLVSS concentration in the reactor
* To maintain a designed MLVSS (at steady state) some biomass must be wasted
* settled sludge
* return activated sludge (RAS)
* clarified effluent
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SRT
Solids Retention Time or sludge age
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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 **aerobic & facultative heterotrophs** and some **autotrophs** for *nitrification*.
* Microorganisms are kept in suspension by mixing
* **Sludge recycle** (RAS) is an essential part of the process.
* Owing to recycle the HRT is not the same as the **solids retention time** (SRT) or sludge age
* **Sludge age** is controlled by wasting the correct mass of sludge daily
basa na lang
26
is controlled by wasting the correct mass of sludge daily
**Sludge age**
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* is a mixture of microorganisms and particulate matter
* serves as a quantitative measure of activated sludge concentration
Mixed liquor suspended solids (MLSS)
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* Final_____ separate the MLSS from the treated wastewater using class III and IV for settling and thickening sludge
* _______ tanks are usually circular 10-30 m dia. and depth is important 4 - 4.5 m
Clarifier
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* Mixing regimes in **reactor tanks** may be __________ or ___________
* Several variations of activated sludge processes are possible
* These range from the conventional systems with high F/M to extended aeration plants with low F/M
* Better effluent quality from activated sludge plants compared with trickling filters
plug flow or completely mixed system
28
# 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
29
# 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
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* For the transfer of gas molecules from the gas phase to the liquid phase, slightly soluble gases encounter the primary resistance from the liquid film
* Very soluble gases encounter the primary resistance to transfer from the gaseous film
Aeration
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____________ - a physical mass transport across gas film and liquid film
Two-film theory
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Aim of the ______ - to increase O2 transfer from liquid film to the bulk liquid at a rate sufficient to meet the O₂ demands of metabolism
aerator
32
# Aeration
* Function of ____ in activated sludge is a two stage process
* Aeration provides the ____ (electron acceptor) for aerobic metabolism
* DO of ______ mg/L is necessary for aerobic condition
* Aeration must balance the ________ by the microorganisms
* O2
* DO
* 0.5-2 mg/L
* oxygen uptake
32
A major energy consuming process
Aeration
33
Types of Aerators (3)
* Surface Brushes
* Surface Aerators
* Floating Surface Aerator
34
* A physical separation process to **settle the solids** (microorganisms, particulate) from the clarified effluent
* **Thicken sludge** is returned to the reactor tank
Final Sedimentation Tank
34
* Occurs after the biological or transformation process
* A physical process of settling generally in a separate tank
* In some processes this removal of solids can also occur in the same tank but separated in time
Separation of treated wastewater from the solids
34
# Final Sedimentation Tank
* Involves 2 imporatnt functions
* Thickening capacity is based on the ___________
* A concept of maximum quantity of solids that can be handled by a _______ at a given underflow removal rate without affecting performance. It involves the solids loading rate
* Solid Flux Theory
* settling tank
35
* A criterion for measuring the settleability of sludge
* It is related to the recycling of activated sludge
* defined as the settled volume of sludge (mL/L) in 30 minutes per unit MLSS (mg/L
Sludge Volume Index (SVI)
36
* SVI of 50-100 mL/g indicate ________
* SVI > 150 mL/g are light, _______ (bulking sludge)
* good dense sludge
* poorly compacting
37
Factors affecting SVI
* __________; relationship between zoogloeal and filamentous growth are dependent on industrial wastes, carbohydrates etc
* Degree of ________ in reactor tank; plug flow is less prone to bulking
* ________ conditions and ______ systems result in low SVI
* Sewage composition
* longitudinal mixing
* Anoxic conditions and nitrifying systems
38
RAS
Return Activated Sludge
39
Types of Activated Sludge Systems (2)
* Conventional Activated Sludge
* Continuous Extended Aeration Process
39
* Systems operate with low organic loadings (F/M); high 0, and high HRT
* Process minimises sludge handling, consequently have no primary sedimentation tanks
* Increased endogenous respiration results in less sludge, but increase O2 demand
* Exhibits **completely mixed**; hence more stable to fluctuations in flow and loading (organic); requires less stringent recycle
* Examples are continuous oxidation ditches eg. carousels
Continuous extended aeration process
39
* Operates at F/M ratios of 0.2 to 0.5
* Design to **remove BOD** and may also nitrify
* **Plug flow**, limited longitudinal mixing, spiral flow along tank through diffusers
* Reactor tanks are long, narrow up to 150 m length; W:D = 1:1 to 2.2:1; D = 3 to 5 m; W = 6 to 12 m
* Limited resistance to shock and toxic loads
Conventional activated sludge
39
* Biological oxidation and final clarification occur in the same tank: functions are only separated in time
* Primary treatment is not necessary
Intermittent Decanting Extended Aeration (IDEA)
40
Intermittent Decanting Extended Aeration (IDEA)
* Treated and clarified water is ________ but raw sewage is fed _______
* _______ is wasted during the aeration cycle to maintain a constant MLSS of 3500 - 5000 mg/L
* decanted intermittently
* continuously
* Sludge
41
* An example of the intermittent decanting extended aeration process serving 500 to 2000 ep
* 4 hours operating cycle for normal operation; 3 phases per cycle controlled by an automatic timing device
Pasveer Oxidation Ditch
42
______
Secondary treatment will remove up to 98% of microorganisms and 105 - 107/100 mL of coliform remains
* ______ remains the common disinfection agent
* A contact time of 20 - 30 minutes is required
Disinfection
* Chlorine
42
Major advantages of the IDEA process
* Cheaper than continuous activated sludge systems
* Easily modified to remove nutrients
* Easy operation and minimum attendance
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Disadvantages of the IDEA process
* High operating energy requirements
* Sludges are often difficult to settle
* Not suitable for large flows
42
Disinfection:
* Other environmentally friendly methods are preferred such as: (4)
* UVL,
* ozone,
* membrane filtration,
* artificial wetlands
43
The major components of nutrients in wastewater are ________. They contribute to the eutrophication of receiving water
nitrates and phosphates
44
In the nitrogen cycle, organic and ammonium nitrogen are converted first to nitrite and then to nitrate
Nitrification
44
Nitrification
* ________ in wastewater is toxic to fish; it has a high O2 demand; it increases Cl2 demand during disinfection
* Primary treatment removes ______ influent nitrogen
* Secondary treatment removes about ____ cumulative
* Limit for ammonium-N in treated effluent < 2 mg/L
* Ammonia
* < 20%
* 30%
44
_______ is a 2-stage process by different types of aerobic autotrophic bacteria
Nitrification
45
are sensitive to toxic substances; grow more slowly (high 0); optimum temp 28-39°C and decreases with low temp
Nitrifying bacteria
45
Denitrification
* DO inhibits denitrification
* A ________ source must be available (external or recycled endogenous carbon)
* Some ______ is removed but more slowly than aerobic respiration
* Denitrification can be induced in the ______ part of fixed growth systems by making the filter bed deeper (2.5 – 3 m) but use of __________ is the normal process
* carbon
* BOD
* anoxic
* activated sludge
46
* Conversion of nitrates (derived from nitrification) to nitrogen gas
* is a type of respiration carried out by facultative heterotrophs; a process known as anoxic as NO3 is the terminal electron acceptor
Denitrification
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In ___________, the anoxic zone within the reactor tank may be 30 - 40% of volume and precedes the aerobic zone
conventional activated sludge
48
In ____________, the establishment of sequential aerobic zones coupled with long HRT and high 0 promote endogenous denitrification
carousel systems
48
Denitrification can be achieved in separate reactors using suitable ________
organic source
49
__________
Sources are from domestic wastewater, trade and agricultural wastes; usually present in 3 forms:
Phosphorus Removal
* Orthophosphate (removed by chemical/or biological processes)
* Polyphosphate
* Organic phosphorus
50
are less easily removed until transformed to orthophosphate after secondary treatment
Polyphosphate and organic phosphorus
51
About 10% of insoluble phosphorus can be removed by ______
primary settling
52
_____________ removes a further 15 -35% of insoluble phosphorus by assimilation during biomass growth, but a well designed _____________ plant can remove up to 95% of P
* Conventional biological treatment
* BNR (biological nutrient removal)
53
Almost all soluble phosphorus can be removed by ________
chemical precipitation
54
Phosphorus Removal:
A more efficient process is _______
biological phosphorus removal
54
Biological Phosphorus Removal:
* One example is the modified ____________ for biological nutrient removal
* Denitrifying plants can be modified by a fermentation zone at the head of the aeration tank
* Selective growth of bacteria (________) absorbs the phosphorus
* Daily wasting of activated sludge removes the stored phosphorus
* University of Cape Town model (UCT)
* acinetobacter
