Biological Treatment Flashcards
(120 cards)
1
Q
What is biological treatment used for?
A
Wastewater and sewage only.
2
Q
Give the main stages in preliminary and primary in a WWTP.
A
- Screening
- Pumping
- Grit Removal
- Primary settling
3
Q
Give the main stages in secondary in a WWTP.
A
- Biological treatment
- Secondary settling
4
Q
Give the main stages in solids treatment in a WWTP.
A
Waste thickening sludge
Sludge digestion
Solids dewatering
Biosolids disposal
5
Q
What is the role of preliminary treatment in WWTPs?
A
Removes large solids like rags, sticks, floatables, grit, and grease
To prevent maintenance and operational problems in later processes.
6
Q
What is the purpose of primary treatment?
A
Removes a portion of SS through sedimentation, reducing BOD.
60-75% TSS removed
1/3 biological removed
7
Q
What does secondary treatment achieve?
A
Removes biodegradable organic matter in solution or suspension, reducing oxygen demand.
8
Q
What is tertiary treatment, and when is it used?
A
Provides ADDITIONAL POLLUTANT REMOVAL (e.g., nutrients, bioactive chemicals, xenobiotics) before effluent discharge.
It’s increasingly used for nutrient reduction.
9
Q
What does p.e. stand for in WWTP classification?
A
population equivalent
9
Q
How are WWTPs in England and Wales classified by size?
A
Small works: 2,000 p.e.
Medium works: 10,000 p.e.
Large works: 100,000 p.e.
9
Q
Compare treatment processes are in small and large WWTPs in England and Wales?
A
Both have primary treatment.
Small = followed by trickling filters.
Large = followed by activated sludge.
10
Q
Why might a pumping station be required in a WWTP?
A
Raw wastewater is carried to the WWTP by gravity.
A pumping station raises the wastewater above ground level, enabling gravity flow through the treatment process.
Cheap.
11
Q
What factors result in the expansion of WWTP?
A
Peak upstream flows (e.g., heavy rainfall and storms)
Seasonal variations (tourism, summer months 20–30% higher flows)
Diurnal variations (low flows from 23:00–05:00)
Future increases in population and industry.
12
Q
How does seasonal variation affect WWTP operation?
A
Increased tourism, lead to fluctuating water consumption and higher wastewater flows, requiring the plant to handle peak conditions efficiently.
13
Q
How might diurnal variations impact WWTP performance?
A
Low flows at night, high flows during the day require flexible treatment processes that can adjust to changing loads, ensuring consistent treatment performance.
14
Q
What design considerations are needed for WWTPs to handle peak flow conditions?
A
Pumping stations for flow equalisation.
Flexible capacity to manage variable flow rates.
Storage or stormwater tanks to buffer surges during heavy rainfall or storms.
Scalable infrastructure to accommodate future growth.
15
Q
Why is it important to dampen variations in wastewater flow?
A
Treatment can occur at a near-constant flowrate.
Improves performance and capacity in existing WWTPs
Reducing size and cost in new WWTPs.
16
Q
What is damping?
A
Storing excess wastewater and releasing it during low-flow periods to maintain a steady flowrate.
17
Q
What is the role of hydraulic controls in large WWTPs during peak flows?
A
Hydraulic controls direct flows above plant capacity to storm systems, preventing overloads.
18
Q
What is Dry Weather Flow (DWF)?
A
Daily flow rate during dry weather conditions
19
Q
How is DWF calculated?
A
DWF = P*G + I + E
P = Population
G = Average daily consumption per head (L/h/d)
I = Infiltration (in dry weather) (L/d)
E = Trade discharges (L/d) (industrial & commercial)
20
Q
What is the average daily consumption per head?
A
G ~ 175 L/d
21
Q
What is Average Dry Weather Flow (ADWF), and why is it used?
A
Average daily dry weather flow adjusted for variability, peak loads and rainfall.
22
Q
How to calculate ADWF?
A
ADWF = 1.25 DWF
23
What is Formula 'A' used for in WWTP design?
Calculates the maximum flow to a WWTP during rainfall or storms to prevent pollution from untreated overflow.
24
What is formula A?
Formula A = (P*G + I + E) + 1360P + 2E
25
What is Flow to Full Treatment (FFT)?
Max flow rate accepted for clarification and biological treatment.
FFT used for hydraulic process design
26
What is the equation for FFT?
FFT = 3*DWF = 3*P*G + I + 3E
Can range between 3 - 6 times the DWF
27
What happens to flows greater than FFT during storms?
Excess flows are:
Stored on-site in storm tanks until flows return to ADWF.
Overflowed (without treatment) if storm flows exceed 2 hours.
28
What are the 3 main functions of preliminary treatment in WWTPs?
Removal of untreatable solid materials.
Protection and improvement of following treatment units.
29
What is the purpose of pre-treatment in WWTPs?
Removes objects larger than 0.5-1 inch to prevent damage and clogging of pumps and skimmers in primary treatment clarifiers.
30
How is pre-treatment cleaning performed?
Large / modern = mechanically cleaned.
Small / old = by hand
31
What are the key equipment and stages of preliminary treatment?
Screenings – Remove large objects like rags, sticks, and debris.
Comminutors – Grind up solids to smaller, manageable sizes.
Grit Chambers – Remove heavy inorganic particles like sand and gravel.
32
What is the difference between unit operations and unit processes?
Operations = physical forces
Processes = biological and chemical reactions
32
Why is preliminary treatment critical for protecting downstream processes?
Preliminary treatment removes large and untreatable solids, preventing damage to pumps and equipment, reducing maintenance costs, and ensuring efficient operation of subsequent units.
32
List examples of unit operations.
Physical
Screening
Sedimentation
Filtration
33
List examples of biological unit operations.
Aerobic
Anaerobic
33
List examples of chemical unit operations.
Precipitation
Chlorination
34
What is saponification and why is it used in water treatment?
Converts recovered grease and oil into soap by reacting them with an alkali.
Repurposes oil and grease skimmed from tank.
35
What is sludge digestion?
Digesting sludge from primary and secondary stages
To produce nutrient-rich product which can be incinerated,
disposed in landfills, used as fertiliser or fuel.
36
How can sludge be utilised?
Bioenergy via anaerobic digestion
37
How are biological processes used in wastewater treatment?
Use microorganisms to degrade and remove pollutants
Environmentally friendly, simple, and cost-effective alternative to physical and chemical clean-up methods.
38
How do microbes contribute to wastewater treatment?
- Microbes convert carbon into cell tissue and oxidized end-products like CO2 and H2O.
- Some microbes oxidize ammonia to nitrate through nitrification.
39
What are biofilms, and why are they important in wastewater treatment?
Microorganisms living in communities suspended in
liquid environments attached to surfaces (e.g gravel)
They enhance oxidation and pollutant removal, improving treatment efficiency.
40
How much BOD can biological processes remove?
up to 95%
41
Why is maintaining aerobic conditions important in biological wastewater treatment?
- support microbial oxidation of organic material
- enhance biofilm formation
- suppress the growth of pathogenic microbes.
42
What role does activated sludge play in wastewater treatment?
Kill pathogens.
Activated sludge returns 20–50% of digested sludge containing bacteria that compete with pathogens, reducing pathogen levels by 90–99%.
43
List types of microorganisms are typically found in wastewater treatment systems?
Fungi
Protozoa (e.g rotifers)
Viruses
Bacteria
44
What does the presence of protozoa indicate about activated sludge?
- Healthy aerobic environment
- Good oxygen availability
- Low toxicity levels
45
Compare aerobic and anaerobic bacteria in wastewater treatment in terms of metabolism, efficiency, and by-products.
Metabolism
Aerobic = higher metabolism therefore faster degradation.
Anaerobic = slower metabolism as they utilize sulfates and nitrates for energy instead of free oxygen.
Efficiency:
Aerobic = Higher organic removal. Less than 90% organism required.
Anaerobic = Require longer retention times (several days) to achieve a 50% reduction in organic material therefore less efficient.
By-Products:
Aerobic = CO2 and H2O
Anaerobic = CH4, H2S and partially degraded organic matter.
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47
What is co-metabolism, and how can it be used in wastewater treatment?
When microbes produce enzymes to degrade a primary substrate, but the enzymes also break down other pollutants unintentionally.
47
48
How to improve treatment efficiency when co-metabolism takes place?
Add more primary substrate
Speeds up the breakdown of these pollutants.
49
What is the purpose of secondary treatment in wastewater treatment?
- Oxidizes biodegradable BOD that escapes primary treatment
- Removes SS.
50
What are the two main types of secondary treatment processes?
Attached / Fixed Growth - Microorganisms grow on an inert media (e.g., trickling filters).
Suspended Growth - Microorganisms are suspended in the wastewater (e.g., Activated Sludge Process (ASP), stabilization ponds).
51
How does a trickling filter work?
Wastewater is trickled over a filter bed with attached biomass.
Treated effluent exits through an underdrain before entering a sedimentation tank.
52
What waste byproducts are produced in a trickling filter system?
Biomass eventually sloughs off the filter media, forming 'humus sludge.'
This sludge is collected in the sedimentation tank and treated (e.g., in anaerobic digesters) before disposal or reuse.
53
What factor impacts effluent quality in trickling filters?
Seasonal temperature change
In winter, microorganisms move closer to centre of filter, away from cold.
Process volume decreases.
Efficiency decreases
54
What is the primary mechanistic difference between fixed/attached growth and suspended growth reactors?
Mass transfer
Suspended = kinetically limited, relying on the availability of BIOMASS for treatment.
Fixed/attached = diffusion limited, depending on SURFACE AREA for mass transfer through biofilms.
55
List advantages of trickling filters.
- less energy needed
- simpler aeration
-no bulking sludge problem
(better sludge thickening)
- less operating and maintenance costs,
- withstands shock toxic loads
56
List disadvantages of trickling filters.
- poorer effluent quality
- sensitive to low temperatures
- produces odour
- sloughing will create lots of sludge in short time making nitrogen removal difficult
57
Why is recirculation used in trickling filters during low flow periods?
prevents biofilms from drying out
by circulating flow from secondary sedimentation through primary sedimentation and back to the trickling filter.
58
What happens to secondary clarifier sludge in a trickling filter system?
Humus sent to primary treatment for resettling and disposal.
59
What are the different types of trickling filters?
Low rate = 1-3 m rock
High rate = 1-2 m rock or plastic
Super rate = synethetic
Different media have different loading rates so must ensure that there is enough food for microorganisms to function.
60
Which type of trickling filter has the highest efficiency?
Low rate = 90-95%
High = 85-90%
Super = used as roughing filter
61
What is activated sludge?
Treat wastewater using suspended biomass through biological breakdown and adsorption.
Mixed liquor (influent wastewater and return activated sludge) is aerated and passed to a settlement tank.
62
What are the waste byproducts of activated sludge systems?
settled solids and bacterial cells.
This sludge is treated (e.g., anaerobic digesters) before disposal or reuse.
63
How common is the use of activated sludge in WWTPs?
used in approximately 20% of WWTPs with secondary treatment
to meet higher removal efficiency demands.
64
How does activated sludge impact effluent quality?
achieves higher ammonia and nutrient removal than trickling filters
depending on aeration rates, better BOD removal rates
65
What is MLSS?
Mixed liquor suspended solids
refers to the conc of SS, including microorganisms and organic/inorganic particles, in the aeration tank of an activated sludge process.
66
What is MLVSS?
Mixed Liquor Volatile Suspended Solids- organic
67
What factors influence MLSS performance in activated sludge systems?
- Hydraulic retention time (HRT)
- solids retention time (SRT/sludge age)
- sludge loading rate (SLR or F/M ratio)
- microbial activity
68
What is the formula for MLSS?
MLSS = (RAS * S) / (Q + RAS)
RAS = returned activated sludge (recycle)
S = RAS SS
69
What does the F/M ratio dictate?
amount of organic material (food) available per unit of microorganism mass in a biological treatment process, such as activated sludge.
70
Which is preferable: a high or low F/M ratio?
low F/M ratio is preferable
results in better settling flocs and higher treatment efficiency in the secondary clarifier.
71
What happens at a high F/M ratio?
Excess food causes bacteria to grow fast.
Bacteria dominate due to plentiful food
Small flocs that don't settle
Excess food carries into the effluent, reducing treatment efficiency.
72
What happens at a low F/M ratio?
Cells are starved.
Slower growth of bacteria.
Nearly all substrate is consumed, resulting in high treatment efficiency.
Cells are mostly attached to flocs
Better settling of flocs
73
What is the SRT (Sludge Retention Time)?
SRT measures average time biological solids remain in system
SRT = X / X w
X = Biological solids in the reactor (MLSS × Volume)
Xw = Solids in WAS flow (WAS × concentration).Typical values: 5–15 days (BOD removal: 4–8 days; nitrification: 6–12 days).
74
How does SRT affect process performance?
High F/M, Low θc: Promotes growth of bacteria and fungi with poor settling characteristics.
Low F/M, High θc: May cause excessive breaking down of cells and cell dispersion, reducing efficiency.
75
What are the two main pathways for reducing BOD in activated sludge systems?
Oxidation of organic matter:
consumes about 40% of the biomass and involves the breakdown of organic material (e.g., CH2O) into CO2, H2O, and energy.
Formation of new bacterial cells:
consumes about 60% of the biomass, where organic matter, nitrogen (N), phosphorus (P), and trace metals are used to form new bacterial cells.
76
How do bacteria remove nitrogen, phosphorus, and heavy metals in activated sludge?
Nitrogen: Removed via nitrification (converts ammonia to nitrate) and denitrification (converts nitrate to nitrogen gas).
Phosphorus: Removed by precipitation at high pH (10.5–11).
Heavy metals: Incorporated into new bacterial cells.
77
What happens in nitrification?
Ammonia oxidation and nitrite oxidation
78
What are the characteristics of stabilisation (oxidation) ponds?
Massive blue pools for low maintenance approach at removing waste
depth = 1 m
liquid retention time = over 90 days
Symbiotic relationship
79
What is the symbiotic relationship in stabilisation ponds?
Bacteria decompose organic matter, releasing nitrogen, phosphates, and CO2.
Algae use these for photosynthesis and release O2, which bacteria consumes
80
What waste and byproducts are produced in stabilization ponds?
Suspended algae.
Excess bacterial decomposition products.
81
How do evaporation rates affect stabilisation ponds in dry climates?
evaporation rate ≥ liquid loading
ensuring complete wastewater retention.
82
Where are stabilisation ponds typically used?
Rural areas and warm climates.
Simple, cheap, but requires space.
83
What is the impact of stabilization ponds on effluent quality?
Temperature-dependent: BOD removal >95% in warm conditions.
Do not provide biological nutrient removal.
Effluent can be used for irrigation or overflow to streams, with algae screened out to meet discharge requirements.
84
How are the three categories of stabilisation ponds arranged?
They can be arranged in series:
Anaerobic ponds
Facultative lagoons
Maturation ponds
85
What is the function of anaerobic ponds in stabilisation systems?
Remove BOD by sedimentation.
Sludge is digested in the bottom layer.
86
What occurs in facultative lagoons?
Bacteria degrade waste, using O2 and generating CO2.
Algae use CO2 for photosynthesis and generate O2.
87
What is the role of maturation ponds?
Polish effluent from other biological processes.
88
When is tertiary treatment required?
If treated water after secondary stage does not meet legislation quota.
89
Name two methods phosphorus can be removed in tertiary treatment.
Biological
Chemical precipitation
90
How is phosphorus removed biologically in tertiary treatment?
using bacteria (polyphosphate-accumulating organisms)
These organisms store phosphorus in cells (<20% biomass).
High fertiliser value after separation from wastewater.
91
What chemicals are used for phosphorus removal by precipitation?
Ferric chloride, alum, or lime.
92
List disadvantages of using chemical precipitation to remove phosphorus in tertiary treatment.
May cause excess sludge
Expensive chemicals.
93
What are advanced oxidation processes used for?
To oxidise materials resistant to biological oxidation.
Breaks larger molecules into smaller, biodegradable forms.
94
What is used in advanced oxidation processes to breakdown resistant molecules?
UV
Hydrogen peroxide
Ozone
95
What does adsorption do in tertiary treatment?
Uses granular activated carbon.
Removes organics and metals that cause taste and odour problems.
96
What do membrane biological reactors (MBRs) combine?
Activated sludge treatment (aerated)
with membrane liquid-solid separation
using microfiltration or ultrafiltration.
97
What is a key advantage of MBRs over conventional activated sludge processes (ASP)?
No need for sedimentation.
Overcomes poor sludge settling issues in conventional ASP.
98
What is the typical MLSS (mixed liquor suspended solids) range in MBRs?
8,000–12,000 mg/L
(higher than 2,000–3,000 mg/L in conventional ASP).
99
What is the typical SRT (solids retention time) in MBRs?
>15 days
Ensuring complete nitrification, even in cold weather.
100
What waste/byproducts are produced in MBRs?
Sludge
Sludge is scoured from membranes to prevent clogging
Must be treated before disposal.
101
Where are MBRs most useful?
Suitable for areas with limited space.
Water reuse applications due to high-quality effluent.
102
What are the limitations of MBRs?
MBRs prone to fouling (blinded by grease or damaged by grit)
Less flexible than clarifiers for peak flows.
Higher CAPEX and OPEX
103
How do MBRs affect effluent quality?
High biomass concentration in MBR
improves removal of soluble and particulate biodegradable materials at high loading rates.
Efficient at removing emerging pollutants
except very soluble metals (e.g., nickel).
104
What are reed beds or constructed wetlands?
Specially constructed areas with gravel medium and impermeable base.
Aquatic plants vibing.
105
How do reed beds improve water quality?
Reeds absorb and transport oxygen through their stems to the root zone.
Oxygen supports aerobic bacteria in breaking down organic matter in the wastewater.
The gravel medium promotes oxygen flow and provides a surface for biofilm growth, enhancing treatment.
106
Where are reed beds commonly used?
Secondary treatment for small or rural communities.
e.g Chester Zoo have reed beds to clean elephant enclosure drainage
107
What is a constructed wetland?
An area where land is saturated with water, creating conditions for:
Poorly drained soils.
Water-loving vegetation.
Biological processes suited to wet areas.
108
What are the benefits of constructed wetlands?
SS settle in slow waters.
Nitrogen removal by natural and bacterial processes.
Phosphorus uptake by plants.
Organic matter breakdown by algae and bacteria.
Habitat for microorganisms and wildlife.
Tourism opportunities from attracting waterfowl.
Low maintenance than effluent treatment.
109
What is the size and cost of the Lamesley Wetlands project?
£3.1M project.
8 hectares.
Completed in 2005.
110
What motivated the creation of Lamesley Wetlands?
Improve PHOSPHATE and AMMONIA-rich effluent from WWTP.
Treat IRON-contaminated minewater before river discharge.
111
How does combined treatment create synergy at Lamesley Wetlands?
Aerated minewater promotes oxidation.
SS in effluent help form iron flocs that precipitate.
Phosphate is removed by sorption onto iron oxyhydroxide precipitates.
112
What infrastructure was installed at Lamesley Wetlands?
Rising mains, gravity pipes, and cascade aeration structures.
Bentonite-lined ponds with topsoil for 200,000 reeds.
Footpaths, viewing platforms, and information boards
