Activated sludge process
UEE201 Introduction to Environmental Engineering
Wastewater treatment flow
Wastewater
How to monitor?
Physical Chemical
Biological Processes
Processes Processes
Screening, sedimentation, Biological activity!!! Precipitation, oxidation,
flocculation, floatation, adsorption, disinfection,
filtration, etc. etc.
Clean Water
Aerobic biological treatment
Wastewater
Black box
Engineering
HRT, SRT, DO,
Treated water
F/M ratio, etc.
Biodegradation
“Biodegradation is the breakdown of organic contaminants by microbial
organisms into smaller compounds. The microbial organisms transform
the contaminants through metabolic or enzymatic processes.
Biodegradation processes vary greatly, but frequently the final product
of the degradation is CO2 or CH4. Biodegradation is a key process in
the natural attenuation of contaminants at hazardous waste sites.”
– US Geological Survey
CO2 (or CH4) and wastes
Bacterial growth
Conventional activated sludge (AS) process
Aeration & Mixing
Return activated sludge (RAS)
• The liquid suspension of MOs in an aeration basin is referred to as mixed liquor (ML),
and its suspended solids (SS) portion is called MLSS.
• The MOs in RAS are observed to be very “active” in removing organics.
• Typical reactor type is continuously mixed tank reactor (CSTR).
Aeration devices
Surface
aeration
Bottom sparging
Full-scale AS process
Mixed liquor
Gulhwa WWTP
Aeration is a BIG deal!
Influent pump station, 4.5%
Post-aeration and
chlorine mixing, 3.1%
Headworks, 0.4%
Lighting, 2.2%
Heating, 7.1% Primary clarifier and sludge pump, 10.3%
Solid dewatering, 7%
Process water, 3.6%
Effluent filters, 0.9%
Thickener and
Activated sludge
sludge pump, 1.6% aeration, 55.6%
Secondary clarifier and
recycling activated sludge, 3.7%
(Metcalf & Eddy, 2003)
Why do we still use “aerobic” treatment?
AS process operating parameters (1)
1 Clarifier 3
2
1. Influent
2. Aeration basin
Aeration basin 3. Effluent
4. WAS
5 4 5. RAS
MLSS is typically about 2500 to 3500 mg/L.
MLVSS typically represents 65 to 75% of MLSS.
Food-to-Microorganism ratio (F/M ratio) is a measure of the organic load to an
AS process: Mass of substrate (BOD) per unit mass of biomass per day = kg
BOD/kg MLSS (or MLVSS)/day.
???
Q1 × BOD1
F/M =
MLSS2 × V2
For conventional aeration tanks, F/M is 0.2 to 0.5 per day, but it can be higher (up
to 1.5 per day) when using high-purity oxygen.
AS process operating parameters (2)
Hydraulic Retention Time (HRT) is the average time the influent liquid stay in the
aeration basin. It is the reciprocal of the dilution rate.
V2 1
HRT = =
Q1 D
Solids (or sludge) retention time (SRT, θ), aka sludge age, is the average time
for AS microorganisms (i.e., sludge or MLSS) to reside in the system.
MLSS2 × V2
SRT =
MLSS3 × Q 3 + MLSS 4 × Q 4
While HRT may vary from 3 to 30 hours, SRT is much greater to be 5 to 15 days
in conventional an AS process. Why?
Sludge flocs
Aggregation of individual cells
• MOs exist in the form of freely suspended flocs that are the basic units of activity.
• Macroscopically visible and composed of several million microbial cells.
Floc structure
• Good for what?
• Key factor?
O2 • How to control?
Nutrients
Formation of sludge floc
• Extracellular polymeric matrix
– Macromolecular biopolymers (extracellular polymeric substances, EPS) secreted out of cells in
the form of capsule or slime layer
– Cell deaths/debris and cell lysis products
– Viscous and fibrous compounds
• Multivalent metal ions
– Bind to the negatively charged functional groups of extracellular polymers (e.g. carboxyl groups)
to form “bridges”.
– Charge neutralization: Ca2+ and Mg 2+
• Floc size
– <1 µm to >1 mm
Microorganisms in activated sludge
• Bacteria: major component
• Fungi: tolerable to low pH, toxicity, N deficient wastewater
• Protozoa: prey on bacteria
• Rotifers: multicellular organisms (help floc formation)
• Organic/inorganic particles
Secondary sludge settling
Aeration & Mixing
• Separation of sludge (RAS + WAS) from the treated effluent
• Floc formation/density is important for efficient settling in the clarifier.
• Cell aggregation is generally a response of MOs to nutritional conditions.
• Sludge settling efficiency highly depends on F/M ratio
– Good settling at a low F/M ratio, BUT not too low!
– The optimum value for municipal wastewater: 0.2 to 0.5.
Floc structures
Ideal sludge floc
• Filamentous and floc-forming bacteria in balance
• Strong, large floc
• Clear supernatant
• Good settling
Filamentous bulking floc
• Filamentous bacteria dominate
• Strong, large floc
• Clear supernatant
• Poor settling
Pinpoint floc
• Low filamentous bacteria
• Weak, small floc
• Turbid supernatant
• Good & poor settling
Sludge bulking and foaming
Excessive foaming
Hydrophobic filamentous bacteria
nocardioform actinomycetes Nocardia limicola Microthrix parvicella
HFB-induced foaming
1. HFBs produce and release biosurfactants that form micelles. Then?
2. Modified surface activity, together with aeration, initiates foaming. Anything else?
3. HFBs tend to attach to gas bubbles and form bacteria-bubble complexes.
4. B-B complexes rise to the surface of mixed liquor and stabilize foam structure.
Permanent foaming problem
Sludge volume index (SVI)
The volume in mL occupied by 1 g of sludge suspension after 30-min settling
SV × 1,000
SVI (mL/g) =
MLSS × Test volume
SV = volume of the settled sludge in the graduated cylinder (mL)
MLSS = mixed liquor suspended solids (mg/L)
SVI Settleability
< 80 Excellent
80 – 150 Normal
> 150 Poor
• Normal range: 50 to 150
1L Imhoff cone; for 30 min • SVI > 150 typically indicates filamentous bulking
Time capsule
History of the Hyperion Plant, LA
https://www.lacitysan.org/san/faces/wcnav_externalId/s-lsh-wwd-cw-p-hwrp?_adf.ctrl-state=124sjnadqx_5&_afrLoop=3705421539654214#!
1925 1950 1998
Anaerobic digestion
UEE201 Introduction to Environmental Engineering
Waste activated sludge
Anaerobic digestion (AD)
Hydrolysis
Acidogenesis
Methanogenesis
• No need for aeration
• Low sludge production
• Biogas (60% CH4 + 40% CO2) production; energy content: 23 MJ/m3
Aerobic vs Anaerobic
O2 > NO3- > SO42- > CO2
Metabolic modes e- acceptor e- acceptor source
Aerobic respiration O2 External
Anaerobic respiration SO42-, NO3-, CO2, etc. External
Anaerobic fermentation Organic compounds Internal
Oxidation and reduction
Aerobic
• Fully oxidized by oxygen, a strong oxidant
• CH3COOH + 2O2 2CO2 + 2H2O
• No energy compound
Anaerobic
• Oxidation (using endogenous electron acceptor) & reduction
• CH3COOH CH4 + CO2
• Energy conserved
ENERGY RECOVERED!
Energy balance (simplified)
Heat (CO2 + H2O) CH4 (biogas)
50% 90%
Biodegradable Biodegradable
COD COD
Biomass (sludge) Biomass (sludge)
50% 10%
Aerobic Anaerobic
Biogas
• 1 kg COD 0.35 m3 CH4; heating value: 35.8 MJ/m3
• 60-70% CH4 and 30-40% CO2; gas impurities (H2S, H2, N2, etc)
• Energy content, 23 MJ/m3 (60% CH4): ~60% of pipeline natural gas
• Sewage sludge, food(-processing) waste, animal manure, etc.
• Heating, electricity generation, and gas fuel
Full-scale anaerobic digesters
SBK biogas plant
Potential net energy producer
Energy (106 kJ/d) Aerobic Anaerobic
Aeration -1.9
Biogas 12.5
Heating to 30oC -4.2
Net Energy -1.9 8.3
Wastewater flow: 100 m3/d; Influent organic load: 10 kg COD/d. (Metcalf and Eddy, 2002)
In practice, wastewaters with 30-300 g COD/L are suitable for anaerobic treatment
and influents with less than 1,300 mg COD/L are typically treated aerobically.
Why better for high-COD wastes?
However, there are many cases of anaerobic treatment at lower influent strengths!
Operating temperature
Psychrophilic Mesophilic Thermophilic
Temperature < 15oC 30 – 40oC 50 – 60oC
Reaction rate Slow Medium Fast
Energy consumption Low Medium High
Biogas production rate Low Medium High
Pathogen reduction Fair Good Very good
Remarks • Advantageous in cold • Most widely used • High-rate processes
climate regions • Suspended, fixed, or • Suspended or fixed
• Maintaining enough granulated systems • Energy consumption
amount of active • Process technology is for heating is very high
biomass is important well established • Easy to be net energy-
• Fixed or granulated • Often be net energy- consuming
systems are desired producing
Organic waste, the largest stream
• “In 2017, the UN estimated that almost a third of all food that is produced is discarded.
Edible food makes up approximately 1.3 gigatonnes of this (one gigatonne is a billion
tonnes). For comparison, one tonne of wasted food is about the equivalent of 127
large plastic bin bags. This not only represents a phenomenal loss in terms of food
that could feed people, but also a loss in resources such as water, labour power, soil
nutrients, transportation energy and so forth.” – The Conversation (5 Sep 2018)
https://theconversation.com/enormous-amounts-of-food-are-wasted-during-manufacturing-heres-where-it-occurs-102310
• ~1.3 billion cows and ~ 2 billion pigs in the world; livestock manure: ~6.3 Gton/year
(~113 Mton N/year) – UN FAO (2018)
• Sewage sludge: 25-30 million dry tons per year
Environ. Res. Lett.,
15: 074021 (2020)
EU’s vision for biogas
Biogas/methane in Europe
40
Biomethane production in Europe
41
Biogas plants in the US
Over 2,200 sites in all 50 states
: 250 on farms, 1,269 in sewage treatment facilities (~860 use biogas on site), 66
stand-alone systems digesting food waste, and 652 landfill gas projects.
EBA-GiE European Biomethane Map
43
Biogas as transportation fuel
Kobe biogas station
• Opened in Apr 2008
• Produces 6,000,000 m3 methane/year from swage sludge
• Sewage treatment plant serves 1,100,000 people
Sudokwon landfill site biogas station
• Opened in Jun 2011
• Produces 2,400,000 m3 methane/yr from 300,000 tonnes
food waste leachate
• Fuels 300 busses and dust carts
47
SUDOKWON landfill site
AD plant
• Opened in Jun 2011
• 2,400,000 m3 methane/yr from 300,000 tons food wastewater
• Fuels 300 busses and dust carts
Biogas station
Full-scale biogas plants in Korea (~110 sites)
Asan, Chungnam Yangsan, Gyeongnam
Yongyeon, Ulsan Dongdaemun, Seoul
Microbiology of AD
5% Complex Organic 20%
Compounds
(Carbohydrates, Proteins, Lipids) Hydrolytic bacteria
Hydrolysis
10% 35%
Simple Organic Compounds
(Sugars, Aminoacids, Peptides)
Acidogenic bacteria (acidogens)
Acidogenesis
Long Chain Fatty Acids
(Propionate, Butyrate, ETC)
13% 17%
H2, CO2 Acetate
Methanogenic archaea (methanogens)
Methanogenesis
28% 72%
CH4, CO2
Rate-limiting step?
Phylogenetic tree: Tree of life
Domain – (Kingdom) – Phylum – Class – Order – Family – Genus – Species
Methanogens (old)
Methanogenic orders Aceticlastic families
Methanogens Methanobacteriales
Methanococcales
Methanomicrobiales
Methanopyrales
Methanosarcinales Methanosarcinaceae
Methanosaetaceae
Hydrogenotrophic; Aceticlastic (acetoclastic, acetotrophic)
Aceticlastic families Substrate Maximum Substrate
specific affinity
growth rate
Methanosarcinaceae Acetate, H2/CO2, Methanol High Low
Methanosaetaceae Acetate Low High
Methanogens
Aceticlastic methanogens
(acetoclastic, acetotrophic)
Aceticlastic families Maximum specific Substrate
growth rate affinity
Methanosarcinaceae High Low
Methanotrichaceae Low High
Source: Evans et al (2019) Nat Rev Microbiol 17: 219-232
Morphology of methanogens
(a) Methanospirillum hungatei (×2000) (a) Bacterial (green) and archaeal (red) cells
(b) Methanobrevibacter smithii (scale bar = 1 μm) (b) Methanosaeta (red) and Desulfotomaculum (green)
(c) Methanosarcina barkeri from sewage digester (×6000) Scale bars, 20 µm (a), 10 µm (b).
(d) Methanosarcina mazei (scale bar = 5 μm)
(e) Methanobacterium bryantii (×2000)
(f) Methanogenium marisnigri (×45,000)
Facts about you farts
https://visual.ly/your-farts-facts
Planet Mechanics Ep. 2: Cow Power
Dick Strawbridge
Jem Stansfield
https://www.youtube.com/watch?v=Tosvw-L8RaY
More about Planet Mechanics?
https://en.wikipedia.org/wiki/Planet_Mechanics 55
