Activated Sludge Process and Its Design, Operation and Control

Activated Sludge Process

and its Components

Activated Sludge Process

Most commonly used aerobic biological treatment process • Microorganisms (mostly bacteria including nitrifiers,

denitrifiers and phosophate assimilators) are involved in the treatment process

Used for secondary treatment of medium and low strength industrial and municipal wastewaters

• Designed and used mainly for the removal of biodegradable organic matter

• Often designed and used even for the removal of nutrients (nitrogen and phosphorus)

• Removal of nutrients, TSS, pathogens and heavy metals can be coincidental

Activated Sludge Process

Usually employed in conjunction with physical and

chemical treatment processes meant for

• Preliminary & primary treatment (primary clarifier/ clari-flocculator)

• Post/ advanced/ tertiary treatment (disinfection, filtration, etc.)

Usually receives clarified wastewaters

• Primary clarification is often omitted (in case of

small communities for small flows and low TSS levels and in hot climates for avoiding/controlling odour problems

• Certain modifications of ASP (sequencing batch reactors, oxidation ditches, aerated lagoons, contact-stabilization process) do not require primary clarification of wastewater

Grit chamber Primary clarifier Aeration tank Secondary clarifier Stabilization

tank drying bedsSludge Sewage Sump & pumping Bar screen Biogas flaring unit Dried sludge for disposal Exhaust gases (CO₂ and H₂O) Filtrate to sewage sump Clarified effluent to sewage sump Treated effluent Raw sewage Screenings Biogas if not flared

(supplied as fuel or emitted) Grit

Sewage Treatment Plant Incorporating Activated Sludge Process

Equalization Tank air air scum

ASP

Aeration basin

Secondary clarifier

Influent Effluent

Wasted activated sludge Sludge recycling

Nutrients and Alkalinity (if needed)

Air or oxygen supply

Components of ASP

Facilities and equipment of an ASP

• Aeration basin(s)

• Air/Oxygen supply/transfer system(s) • Secondary clarifier(s)

• Return activated sludge system(s)

• Waste activated sludge handling facilities/system • Chemical (nutrients and/or alkalinity) feed systems

Components of ASP

Aeration basin

• Wastewater is brought in contact with active microbial biomass for treatment (through bioflocculation,

biosoprtion, and biooxidation including nitrification) • Favourable conditions for biological treatment are

maintained in the aeration basin through aeration (for oxygen supply and mixing) and chemicals addition • Aeration basin may often include

– An anoxic section (for denitrification)

– A selector section for tackling bulking (often along with denitrification) and even for phosphate removal

Components of ASP

Air/Oxygen supply and transfer systems

• Mainly two types: diffused aeration and mechanical aeration systems

• Diffused air system includes diffusers, air headers, air mains, and other piping and fixers, and blowers

• Mechanical aerators

– Surface mechanical aerators (fixed and floating aerators) with or without draft tubes

– Submerged turbine aerators

– Horizontal axis aerators (brush aerators)

• Aeration system should be capable of

– Supplying enough oxygen to meet the demands

Components of ASP

Secondary Clarifier/ Secondary Settling Tank

• Meant to remove biological flocs from mixed liquor and allow clarified secondary effluent out

– Sludge thickening to desired level to facilitate both sludge recycling and wasting

• These are center-feed circular tanks of side wall liquid depth of 3.7 to 6 m and radius of < 5 times liquid depth

– Rim feed circular clarifiers & rectangular clarif. are also used

Secondary clarifier has

• Cylindrical baffle of diameter 30-35% of tank diameter • A central well (or mixed liquor inlet section) designed to

– dissipate the influent energy – evenly distribute flow

Secondary clarifier has

• Revolving mechanism for scrapping (transport & remove) the settled sludge and for the removal of floating scum

– The sludge is either plowed to the central hopper for removal or it is removed directly from the tank bottom by suction

orifices either hydrostatically or by pumping

– Very little scum is usually formed - removal becomes necessary when primary clarifier is not used

• Overflow weirs and collection troughs

– placed at 2/3rd _{to 3/4}th _{radial distance from the center in larger} tanks and at the perimeter in the smaller tanks

– baffles may be provided to deflect density currents and to avoid scum overflow

Components of ASP

Return activated sludge system

• Underdrain of the secondary clarifier • Reliable pumping and piping

• Appurtenances for regulating return sludge pumping rate • Return sludge may pass through a selector (aerobic,

anaerobic or anoxic)

• Return sludge (bioflocculated organic matter!) stabilization prior to mixing with the influent

Components of ASP

Waste activated sludge system

• Wasting can be either from the secondary clarifier or from the aeration basin directly

• Better regulation if wasted directly from the aeration basin – but volume wasted is higher

• Wasted sludge needs handling and disposal – stabilization, thickening, dewatering and drying

– Aerobic or anaerobic stabilization – Chemical or thermal stabilization – Thickeners and sludge drying beds

– Simultaneous thickening and aerobic or anaerobic stabilization

Components of ASP

Chemical feed systems

• Nutrients and alkalinity addition may be required if the influent is deficient in them

– Urea and Diammonium phosphate are usually used –

phosphoric acid/phosphate rock (can these be used in place of the DAP?)

– Nitrification and denitrification levels can influence the alkalinity addition required

• Chemical addition may also be made for the chemical precipitation removal of phosphorus from wastewater • Polymers may often be added for improving the settling

characteristics of the mixed liquor solids

• Chemical dosing often for temporary tackling of bulking sludge problem

Mechanisms of treatment:

Organic matter removal

Aerobic microorganisms (activated sludge), specially bacteria, are responsible and treatment involves bioflocculation, biosorption and biooxidation

• Suspended & colloidal organic matter becomes integral part of biological sludge by bioflocculation and biosorption • Soluble organic matter is removed by biosorption

(adsorption and absorption)

• Bioflocculated & biosorbed organic matter is solubilized through hydrolysis and absorbed by microbes as food

• Absorbed matter is biooxidized (partly respired and rest is used in synthesizing new microbial biomass)

• Through wasting excess activated sludge (at secondary clarifier) organic matter is removed as biological flocs

Suspended organic matter Soluble organic matter Colloidal organic matter Activated sludge Recycled sludge

Inorganic end products

(CO₂, H₂O, NH₃, Energy, etc.) Residualsludge New microbialbiomass Mixed liquor solids bio-ox idation respira tion bio -ox ida tio n bio sy nth es is bios orpt ion bi o so rp tio n bi o flo ccul at io n bio flo cc ula tio n

Bioflocculation

• Microbes of the aeration basin produce biopolymers

that bring about flocculation and form biological flocs

• Biological flocs are constituted of

– Microorganisms – Cell debris

– Suspended and colloidal organic and inorganic constituents of the wastewater

• Bioflocculated organic material can be hydrolysed into

soluble organic matter and biosorbed

Mechanisms of treatment:

Organic matter removal

Biosorption

• More rapid than biooxidation and involves both adsorption and absorption by microbes

• Adsorbed matter can be solubilized by hydrolysis and partly absorbed by microbes and rest is bled into effluent

Biooxidation

• Represents actual removal of biodegradable org. matter • Involves both aerobic respiration (including autooxidation)

and biosynthesis

• Respiration requires O₂ (DO - 0.5 to 1.5 mg/L, 1.07 g/g) and produces H₂O, CO₂, NH₃, etc. and energy

• Biosynthesis produces new microbial biomass (1.42 g/g, yield coefficient) – requires nutrients (N and P)

Mechanisms of treatment:

Organic matter removal

Mechanisms of treatment: Nitrogen removal

Organic-N Ammonical-N Nitrite-N Nitrate-N Organic-N Nitrogen gas Nitrous oxide gas

Organic-N (net growth) decomposition and hydrolysis oxygen oxygen assimilation

lysis & auto-oxidation

denitrification

Organic carbon nitrification

nitrification

Mechanisms of treatment:

Nitrogen removal

• Biological nitrogen removal occurs through

nitrification and denitrification

• Nitrification is aerobic 2-step process (NH

₃

-N to NO

₂

-N to -NO

₃

-N) by aerobic autotrophic bacteria

• Nitroso bacteria (Nitrosomonas, Nitrosococcus,

Nitrosospira, Nitrosolobus, Nitrosorobrio, etc.) are

responsible for step-1

• Nitro bacteria (Nitrobacter, Nitrococcus, Nitrosdpira,

Nitrospina, Nitroeystis, etc.) are responsible for step-2

• Can be achieved along with BOD removal in the

same biological treatment unit

Mechanisms of treatment:

Nitrogen removal - Nitrification

Nitrification

BOD removal _{Clarifier Clarifier}

Wasted

activated sludge

Wasted

activated sludge

When concentration of potentially toxic and inhibitory

substances is high, two-sludge systems, each with an

aeration tank and a clarifier, in series are used

• 1st system is for BOD removal

• Nitrification occurs in the 2nd system

• Raw influent is partially bypassed into the 2nd system to facilitate sufficient flocculation & clarification

Effluent Influent

Mechanisms of treatment:

Nitrogen removal - Nitrification

• Oxygen is required - 4.57 g/g of ammonical nitrogen

(3.43 for nitrite and 1.14 g for nitrate)

• Alkalinity is required - 7.14 g/g as CaCO

₃

• Nitrification is inhibited by

– Low DO levels (<0.5 mg/L is inhibitory - rate increases with DO upto 3 to 4 mg/L - >2 mg/L is favourable)

– pH below 6 is inhibitory and 7.5 to 8 is optimal

– Sensitive to a multitude of organic toxicants (solvents, amines, proteins, tannins, phenols, alcohols, cyanates, ethers, carbamates, benzene, etc.)

– Metals are inhibitory (complete inhibition at 0.25 mg/L for nickel and chromium, and 0.1 mg/L for copper)

Mechanisms of treatment:

Nitrogen removal - Denitrification

• Denitrification involves reduction of nitrate by

heterotrophic bacteria to nitrous oxide, and nitrogen

• Wide range of bacteria (but not algae and fungi) are capable

• Coupled with respiratory electron transport chain –

under anoxic conditions nitrate replaces oxygen

• O₂ equivalence of using nitrate or nitrite in place of oxygen is 2.86 g/g and 1.71 g/g respectively

• BOD demand is 4 g/g NO₃ reduced

• Alkalinity is produced in the process - 3.57 g (as

CaCO

₃

) per gram of nitrate reduced

• Higher DO levels (>0.2 mg/l for pseudomonas &

Mechanisms of treatment:

Nitrogen removal - Denitrification

• Two basic schemes, pre-anoxic (Substrate) and

post-anoxic, are used for the denitrification

• In the pre-anoxic scheme the anoxic tank is followed

by the aeration basin of the ASP and organic matter

of the influent acts as an electron donor

• In the post-anoxic scheme endogenous decay of

microbial mass (also exogenous sources like

methanol/acetate) provides electron donor

• Simultaneous nitrification & denitrification is possible

– Nitrification on the floc surface (if DO in the bulk liquid is high enough) and denitrification in the floc interior (if DO in the interior is low enough)

– Depending on the mixing and aeration conditions,

Mechanisms of treatment:

Nitrogen removal - Denitrification

Aerobic section Anoxic section Aerobic section Anoxic section

Pre-anoxic Denitrification

Post-anoxic Denitrification

Sludge recycling Sludge recycling Mixed liquor recycling

Influent Treated effluent

Influent Treated effluent Wasted sludge

Mechanisms of treatment:

Phosphorus Removal

Phosphorus Accumulating Organisms (PAO) bring

about the removal in an anaerobic – aerobic system

• Phosphorus is incorporated into sludge (as polyphosphate) in volutin granules and removed through sludge wastage

In the anaerobic tank of the system

• Proliferation of PAOs occur and assimilate fermentation products (specially acetate) into storage products

(polyhydroxybutyrate-PHB)

• Concomitantly the stored polyphosphate is released as orthro phosphate

• Acetate is essential for forming PHB and for providing competitive advantage to the PAOs

• Presence of nitrate can be inhibitory (acetate can be depleted and become not available to PAOs)

Mechanisms of treatment:

Phosphorus Removal

In the aerobic tank of the system

• Stored products (PHB) are oxidized to release energy and concomitantly phosphate of the effluent is stored within the cell as polyphosphate

– Mg, K and Ca ratios of 0.71, 0.5 and 0.25 to phosphorus respectively are believed to facilitate polyphosphate storage – pH and DO should be >6.5 and >1.0 mg/L respectively

Typical microbial biomass has 1.5 to 2% phosphorus - in PAOs phosphorus content can be as high as 20-30%

Stoichiometrically about 10 grams of bCOD is needed for the removal of one gram of phosphate from wastewater

Mechanisms of treatment:

Phosphorus Removal

Anaerobic system Aerobic system Clarifier Treated effluent Influent Recycled sludge Wasted sludge containing phosphorus

Reactor for phosphorus removal includes an anaerobic tank

with HRT 0.5 to 1 hour and placed ahead of the aeration tank

Return activated sludge and influent are brought in contact in the anaerobic tank

Plug flow reactor

Long narrow aeration basins (length:width = >10:1) with plug flow regime

True plug flow does not exist – extent of longitudinal mixing depends on the type of aeration system used

Degree of longitudinal mixing can be described by Dispersion Number (N_D) as D/(UL) or Dt/L2

D – coefficient of axial dispersion (m2_{/sec.) – for diffused} aeration system it increases by a factor of 2 with air flow increase from 20 to 100 ft3_{/min.1000 ft}3 _{tank volume}

U – mean velocity of flow (m/Sec.) L – length of the tank

t – HRT of the tank (L/U) for Q+Q_R flow

For good plug flow condition N_D value should be <0.1 A plug flow reactor can be better described by a series of

Plug flow reactor

Substrate concentration varies along the reactor length

O₂ utilization is highest at inlet end and decreases towards lowest at the outlet end

Affected by toxic or inhibiting organics (problematic for industrial wastewater with toxic constituents)

A baffled inlet section can ensure better sorption in case of readily degradable wastewaters

A separate inlet zone (15% of total volume) with only mixing but no aeration can facilitate denitrification of recycled sludge

If designed for nitrification, an anoxic zone at outlet end can bring about denitrification

Plug flow reactor

Typical design values

SRT: 3-15 days

F/M ratio: 0.2 to 0.4/day

BOD loading: 0.3 to 0.7 kg/m3_.day MLSS: 1000 to 3000 mg/l

HRT: 4 to 8 hours

Sludge recycling ratio: 0.25 to 0.75

Design and operation are relatively more complicated – matching oxygen demand and supply is difficult

Early designs had uniform air application throughout the tank length but modern designs have tapered aeration

Plug flow Aeration tank Wasted sludge R ec yc le d sl ud ge Clarifier Effluent Influent

Plug flow reactor: High rate aeration

Conventional plug flow reactor with lower MLSS and higher

BOD loading

Characterized by shorter HRT, higher sludge recycle ratio and higher F/M ratio

Advantages

Requires less aeration tank volume

Aeration energy requirements are relatively low

Disadvantages

Has lower effluent quality (in terms of BOD and TSS)

Relatively less stable and needs extra care for stable operation

Flow variations can disrupt operation (sludge washout occur) Adequate mixing and aeration are important

Plug flow reactor: Step Feed

Plug flow reactor with multiple passes and wastewater

introduction at 3 or 4 feed points (equalizes F/M ratio) MLSS is highest (5000 to 9000 mg/L) in the first pass, and

decreases with each subsequent pass Establishes more uniform oxygen demand Flexible operation

Wet weather flows can be bypassed to the last pass Adaptable to many operating schemes (anoxic/aerobic

processes)

If needed can also be operated in contact stabilization mode by feeding only the last pass

Plug flow reactor: Step feed

More complicated design and complex operation Typical design and operation values

SRT: 3-15 days

F/M ratio: 0.2 to 0.4/day

BOD loading: 0.7-1.0 kg/m3_.day MLSS: 1500-4000 mg/l

HRT: 3-5 hours

Plug flow Aeration tank Effluent Influent Recycled sludge Clarifier

Contact Stabilization process

Has two separate tanks or compartments one for wastewater treatment and the other for sludge stabilization

Contact time: relatively shorter (30 to 60 min.)

Stabilization time: 2 to 4 hours (with respect to inflow) SRT: 5-10 days

F/M ratio: 0.2 to 0.6 per day

Volumetric loading of BOD: 1 to 1.3 kg/m3_day

MLSS: 1000-3000 mg/L (contact) & 6000-10000 (stabilization) Sludge recycling ratio: 0.5 to 1.5

Requires smaller aeration volume and good for low solubility index wastewaters

Wet weather flows can be handled without loss of MLSS Has little or no nitrification capacity and operation is

Contact tank

Stabilization tank _Clarifier Effluent

Wasted sludge Influent

Complete mix reactor (CFSTR)

Most used in India – simple design - suitable for all types of aeration equipment

Uniform and low levels of substrate, and uniform MLSS and constant oxygen demand throughout the basin

Resistant to shock loads and toxic loads

Hydraulic and organic load variations are dampened better Toxic discharges are mitigated through greater dilution

Filamentous bulking from exposure of recycled sludge to relatively low levels of substrate

A pre-contacting zone (of 15 min. HRT) can avoid this!

Typical design and operational conditions

SRT: 3-15 days MLSS: 1500 to 4000 mg/L

F/M ratio: 0.2 to 0.6/day BOD loading: 0.3 to 1.6 kg/m3_.day HRT: 3 to 5 hours Sludge recycle ratio: 0.25 to 1.0

Extended Aeration Process

Well stabilized and low bio-solids sludge is generated – the sludge is mainly of cell debris and sludge contributed by the influent

Primary clarification is usually not used Considered suitable for smaller flows

Aeration tanks are larger, and oxygen demand and aeration energy requirement are higher

Aeration equipment design is controlled by mixing needs (mostly not by oxygen demand)

Sensitive to hydraulic overloads (clarifier can be overloaded by solids) and insensitive to concentration shock loads Typical design and operational parameters

F/M ratio is 0.04-0.10/day BOD loading: 0.1-0.3 kg/m3_.day SRT: 20-40 days MLSS: 2000-5000 g/m3

Extended Aeration: Oxidation Ditch

Ring or oval shaped loop reactor system with unidirectional flow (velocity: 0.25-0.3 m/sec. and cycling time: 5-15 min.) Brush type/surface type mechanical aerators power horizontal

flow and bring about aeration/mixing

Screened wastewater is mixed with recycled sludge and allowed into the tank of 20-30 hour HRT

Intra-channel clarifiers can be used (for secondary clarifiers) Advantages

Highly reliable process and simple operation Amenable for both BOD and nitrogen removal

Uses less energy and adaptable for nutrient removal Disadvantages

Space requirement is higher

Plant capacity expansion is more difficult Low F/M bulking is possible

Effluent Clarifier Wasted sludge Oxidation ditch Mixed liquor Recycled sludge Influent

Extended Aeration Process:

Counter-current aeration system

A circular tank with revolving bridge is used

Air diffusers mounted at the bottom of the revolving bridge supply oxygen

Turning off air but revolving the bridge keeps the tank contents in suspension and facilitate denitrification Typical design and operational parameters

SRT: 10-30 days F/M ratio: 0.04 to 0.1/day

HRT: 15-40 hours BOD loading: 0.1-0.3 kg/m3_.day

MLSS: 2000-4000 mg/L Sludge recycle ratio: 0.25-0.75

Oxygen transfer efficiency is higher but diffuser fouling can be problem (fine screening of wastewater can prevent)

Complicated operation requiring good operator skills Down time for maintenance is relatively higher

Extended Aeration: Other

modifications

Orbal process

A modified oxidation ditch using a series of concentric channels of depth upto 4.3 m

Wastewater enters the outer channel and flows towards the center before entering an internal/external clarifier Nitrification and denitrification are facilitated by regulating

aeration rates

Biolac process

Earthen tanks of 2.4-4.6 m depth with submerged

aeration and with either internal or external clarifiers Fine bubble diffusers attached to floating aeration chains

move across the basin by air released from diffusers Use of timers to cycle air flow through each aeration

Staged Reactor Systems

Consist of 2 or more complete mix reactors in series

– Aeration may be avoided in some of the stages – Internal recycling of flows may be used

– Clarifiers may be used between the stages

– Employed for nitrogen removal (or just nitrification) and for phosphorus removal

System employed for nitrification applications

• Two stages: Stage-1 for BOD removal and stage-2 for nitrification

– Each stage has a clarifier of its own

– Portion of wastewater is bypassed stage-1 and taken into stage-2 to facilitate nitrification

– Stage-2 is operated at a longer SRT

• Sensitive nitrifying bacteria are protected from the

toxic substances of the incoming wastewater

Aeration reactor Nitrification reactor Influent Effluent Wasted sludge clarifier Return sludge clarifier Influent bypass Return sludge Wasted sludge

Staged Reactor Systems

System for the phosphorus removal

Two stage system with a single clarifier after stage-2 Stage-1 is anaerobic where as stage-2 aerobic

Wasted sludge (phosphate accumulating organisms) contains the removed phosphorus in the form of polyphosphates

Kraus process (variation of step feed process)

Suitable for nitrogen deficient industrial wastewater Sludge digester supernatant is added with underflow

sludge and nitrified in a separate aeration tank

Output of the sludge aerator is added to the main reactor to provide nitrogen

Anaerobic reactor Aeration reactor Influent Effluent Wasted sludge clarifier Return sludge Aeration reactor Aeration reactor

Digester Stabilized_sludge

Supernatant Return sludge

Influent Effluent

Return sludge clarifier

Sequencing Batch Reactor (SBR)

More popular specially for smaller communities and

industrial installations with intermittent flows

– Better option for avoiding filamentous bulking associated with readily degradable wastewaters

Single vessel accommodates all the unit processes and

operations normally associated with ASP

– The processes and operations are accomplished as timed sequences: Fill (3 hr) → React (2-20 hr) → Settle (0.5-1hr) → Decant (0.5-1 hr) → Idle

– Mixed liquor remains in the reactor throughout and this eliminates need for a separate clarifier

– Even nitrification, denitrification and even sludge stabilization can also be accommodated within

– Sludge wasting occurs during the aeration stage

– Just mixing without aeration during the fill stage ensures anoxic conditions needed for denitrification

Typical design and operational parameters

SRT: 10-30 days

F/M ratio: 0.04-0.1/day

BOD loading: 0.1-0.3 kg/m3.day MLSS: 2000-5000 mg/L

A compact facility with operational flexibility

Complicated process control and require higher maintenance skills for the equipment used

Batch discharge may necessitate equalization for down stream processing of the effluent

SBR Modifications

Batch decant reactor, intermittent extended aeration

system:

– Wastewater feeding is continuous, but effluent removal is on a batch basis

– A pre-react (baffled) chamber facilitates continuous feeding without disturbing the settling/decanting contents of the tank – Treatment steps of the reactor include reaction, settling and

decanting

Cyclic activated sludge system:

– Wastewater feeding is continuous, but effluent removal is in batches

– Reactor has three baffled zones of 1:2:20 volumes and is fed continuously but the effluent is removed on a batch basis – Mixed liquor is recycled from 3rd _{zone to 1}st _{zone of reactor}

Pure Oxygen Activated Sludge Process

A series of well-mixed covered aeration tank with co-current gas-liquid contact

Influent, recycled sludge & O₂ are introduced at stage-1 (O₂ can also be mixed with the influent under pressure)

Restricted exhaust is allowed from the last stage (O₂ in the exhaust is ~50% and O₂ utilization rate ~90%)

Designed for

DO >6 mg/L F/M ratio 0.6-1/day SRT: 1-4 days MLSS

2000-9000 mg/L BOD loading: 1.3-3.2 kg/m3.day HRT: 1-3 hours Sludge recycling ratio: 0.25-0.5

Disadvantages

More complicated equipment and complex installation, operation and maintenance

Peak flows can disrupt operation by sludge washout Has limited capacity for nitrification

Activated Sludge Process: Other

Modifications

Deep shaft aeration

• Shaft depths can be as high as 400 feet

• F/M ratio of 1 to 2/day, mixed liquor DO level of 10-20 mg/L and MLSS of 8000 to 12000 mg/L are possible

• Solid liquid separation may be by dissolved air flotation or by vacuum degasification and conventional gravity

clarification

Integrated fixed film activated sludge process

• Can enhance nitrification specially at low temperatures • SVI and solids loading to clarifier are decreased

Activated Sludge Process: Other

Modifications

Thermophilic aerobic activated sludge

• Optimum temperatures are 55-60°C (>45°C)

• Autoheating to maintain temperature may be possible at 20,000-40,000 mg/L COD removal and 10-20% O₂ transfer efficiency • Advantages: Rapid degradation rates (3-10 times greater and

10 times greater autooxidation) and low sludge yield

• Thermophilic bacteria fail to flocculate - hence difficult to settle

Membrane filtration

• Hollow fiber membranes of 0.1 micron pore size and 13kPa (1.3 m water column) suction is possible

• Permits operation at high MLSS levels (10,000-40,000 mg/L) and make sludge quality unimportant

• High quality effluent (directly disinfectable) is possible • Membrane clogging is however inevitable

Soluble organic matter Nb soluble organic matter Nb. suspended organic matter Oxygen (1-1.42Y) CO₂, H₂O, NH₃, Energy, etc. New heterotrophic Microbial biomass Auto-oxidation k_d CO₂, H₂O, NH₃, Energy, etc. ammonia Oxygen (3,43 g/g) nitrite nitrate Oxygen (1.14 g)

Carbonaceous BOD is the sum of oxygen utilized during biooxidation of the organic matter and during autooxidation of the microbial biomass

Oxygen (1.42K_d) Residual biomass Bi o-oxid atio n Bio -sy nth es_is Y Suspended organic matter Hydrolysis Residual biodegradable organic matter

Nitrogenous BOD is the sum oxygen utilized during nitrification of Ammonical-N to nitrite-N and nitrite-N to Nitrate-N

ASP kinetics

Rate of utilization of soluble substrate

r_su is organic matter utilization rate (g/m3_.day)

q_max is maximum specific organic matter utilization rate (g/g microbial mass) X_a is microbial biomass concentration (g/m3₎

S_e is organic matter concentration (g/m3_{) in the ASP}

K_s is half-velocity constant (organic matter concentration in g/m3 _{at which} organic matter utilization rate is q_max./2 )

Aeration

tank

e s e a e i su

S

K

S

q

x

V

S

S

Q

r

+

−

=

−

=

(

)

max.

V & X

_a

Q & S

_i

QS

_e

S or S

_e

q

_max.

q

_max.

/2

K

_s

Biomass growth rate

• Microbial biomass growth rate is proportional to the organic matter utilization rate and biomass decay rate

r_g is net biomass production rate (g VSS/m3_.day)

K_d is endogenous decay coefficient (g VSS/g VSS. Day) Y is yield coefficient

• Can also be shown as

ASP kinetics

a d su g

Yr

k

x

r

=

−

d

k

x

S

K

S

q

x

Y

r

_a e s e a g

.

)

(

.

.

.

_max.

−

+

=

d a e i g

x

k

V

S

S

Q

Y

r

=

.

(

−

)

−

.

Oxygen utilization rate

• Oxygen utilization rate is sum of oxygen utilization for bio-oxidation of organic matter and for autobio-oxidation of biomass

– (1-1.42Y) is the fraction of utilized organic matter bio-oxidized – 1.42k_d is auto-oxidation rate in terms of oxygen or bCOD

• Oxygen utilization rate can also be expressed as

ASP kinetics

a d su O

Y

r

k

x

r

(

1

1

.

42

)

1

.

42

.

2

=

−

+

d

k

x

S

K

S

q

x

Y

r

_a e s e a O

1

.

42

.

)

(

.

.

)

42

.

1

1

(

_max. 2

+

+

−

=

d a e i O

x

k

V

S

S

Q

Y

r

(

1

1

.

42

)

(

)

1

.

42

.

2

+

−

−

=

g su O

r

r

r

1

.

42

2

=

−

ASP kinetic parameters

q_max. (2-10 g of bCOD per g VSS day, 5 is typical) K_s (10-60 mg/l of bCOD, 40 is typical)

Y (0.3 to 0.6 mg VSS per mg bCOD, 0.4 is typical) k_d (0.06 to 0.15 g VSS per g VSS.day, 0.1 is typical) Values in parentheses are for domestic sewage

Kinetic coefficient values vary with the wastewater, with the Microbial population and with Temperature

Kinetic coefficient values can be determined from bench scale testing or full-scale plant test results

Temperature correction to the kinetic coefficients is done by

θ is temperature activity coefficient (typical value 1.02 to 1.25) k_T and k₂₀ are k values at T°C and 20°C respectively

ASP kinetics

) 20 ( 20 −

=

T T

k

k

θ

ASP design: Inputs

Quantities and characteristics (and their diurnal, seasonal and

wet-weather variations) of the wastewater to be treated

– Carbonaceous substrates (bCOD, sbCOD, nb suspended COD)

– Nutrients: Nitrogen (TKN and nitrate-N (plus nitrite-N)) and Phosphorous (total and orthro phosphorus)

– Suspended solids (Total, volatile, biodegradable volatile and non-biodegradable volatile)

– Alkalinity

– Flow rates and variations (average flow and peaking factor) – Temperature (winter and summer critical temperatures of

ASP design: Inputs

• Purposes to be served by the ASP – removal of bCOD

– bCOD removal and nitrification

– Removal of bCOD and nitrogen (nitrification-denitrification) – bCOD and phosphorus removal

• Treated effluent characteristics required (only bCOD or both bCOD and nutrient levels desired)

• ASP kinetics parameters (q_max. , K_s, Y and k_d)

• Settling characteristics of bio-solids (SVI and zone settling velocities of the mixed liquor solids)

• Solids retention time (SRT) and loading criteria (F/M ratio and volumetric organic loading) to be used for good sludge settling properties

• SRT for BOD removal is typically 3 to 5 days – shorter SRT discourages nitrification.

• Typical F/M ratio may range from 0.04/day for extended aeration units to 1.0/day for high rate process.

• Volumetric organic loading rate typically varies from 0.3 to 3.0 kg/m3_.day.

• Expansion to meet the future treatment needs is an important consideration in the design.

• Type and size of reactors and solid separation facilities influence both construction and operation costs.

• Selectors may be needed for nutrient removal and for limiting the filamentous growth.

• Staged reactor or plug flow reactor may be appropriate for nitrification – toxic or inhibitory substances can depress the nitrification rates.

Design of ASP requires determination of

• Aeration basin volume • Aeration requirements

• Chemical (nutrients and alkalinity) dosing requirements • _{Sizing and detailing of the secondary clarifier}

• Sludge recycling requirements

• Activated sludge wastage rates required • Treated effluent characterization

Aeration tank S_e,X_a,V Settling tank Q,S_i,X_i Q_r,X_r,S_e Q_w,X_r,S_e Q_e or (Q-Q_w) X_e,S_e Aeration tank S_e,X_a,V Settling tank Q,S_i,X_i Q_r,X_r,S_e Q_w,X_a,S_e Q_e or (Q-Q_w) X_e,S_e X_i is considered negligible

All biodegradable suspended organic solids of influent are hydrolyzed into soluble organic matter

Inorganic and non-biodegradable organic SS remain unaffected and no new SS of these categories formed Nothing except settling & thickening occurs in clarifier

Treated Effluent Soluble bCOD

Use of this equation requires

– Primary variable SRT (assumed)

– ASP kinetic parameters K_s, k_d, q_max and Y

Obtained from the following through solving for S

_e

Independent of the influent bCOD

[

]

(

.

)

1

)

(

1

. max

−

−

+

=

d d s e

k

Y

q

SRT

SRT

k

K

S

d e s e

_k

S

K

S

q

Y

SRT

=

+

−

.

.

1

_max

Specific substrate use for ASP

Specific substrate utilization rate according to Michaelis-Menten

equation

e s e

S

K

S

q

q

+

=

max

τ

a e i a e i

x

S

S

V

x

S

S

Q

q

=

(

−

)

=

−

e s e a e i

S

K

S

q

x

S

S

+

=

−

_max.

τ

d a e i

_k

x

Y

S

S

SRT

−

−

=

τ

.

).

(

1

Treated Effluent Soluble bCOD

Q

V

=

τ

d e s e

_k

S

K

S

q

Y

SRT

=

+

−

.

.

1

_max.

Mixed Liquor Active Biomass Concentration

Use of this equation requires

– Primary variables SRT and τ

– ASP kinetics parameters Y and k_d

Obtained from the following basic equation through solving for x

_a

Here x

_a

depends on k

_d

, Y, SRT,

τ

and bCOD removal

(

)

)

(

1

k

SRT

Y

S

S

SRT

x

d e i a

+

−

=

τ

(

)

d a e i

_k

x

S

S

Y

SRT

−

−

=

.

1

τ

Net activated sludge synthesis rate is equal to activated sludge

wastage rate

Sludge age or mean cell residence time is

x

V

x

k

Y

S

S

Q

(

_i

−

_e

)

−

_d

.

_a

.

=

∆

)

(

)

(

)

(

rate

wastage

sludge

or

rate

generation

sludge

net

system

the

of

sludge

total

SRT

=

a d e i a

x

V

k

Y

S

S

Q

Vx

SRT

.

.

)

(

−

−

=

d a e i

_k

x

Y

S

S

SRT

−

−

=

τ

)

(

1

Net Biomass Synthesis Rate

Net biomass synthesis rate (NBSR) is estimated by

Use of this equation requires

– Primary variable SRT

– ASP kinetics parameters Y and k_d

Obtained through simplification of the following material balance equation

Here V is replaced by Q.τ and the expression for x_a is used

)

(

1

)

(

.

SRT

k

S

S

Q

Y

NBSR

d e i

+

−

=













−













=













rate

ion

autooxidat

Biomass

rate

synthesis

biomass

Gross

rate

synthesis

biomass

Net

d a e i

S

x

V

k

S

Q

Y

NBSR

=

.

(

−

)

−

.

.

In the above by replacing

And on simplification

Here net synthesis of nitrifying microbes is not considered

Depends on the TKN nitrified (Influent and effluent TKN

difference minus nitrogen assimilated into biomass)

d a e i

S

x

Vk

S

YQ

NBSR

=

(

−

)

−

(

)

a d e i

_for

_x

SRT

k

Y

S

S

SRT

and

V

for

Q

)

(

1

+

−

τ

τ

)

(

1

)

(

SRT

k

S

S

YQ

NBSR

d e i

+

−

=

Cell Debris Generation Rate

Cell debris generation rate (CDGR) is estimated by

Use of this equation requires

– Primary variable SRT

– ASP kinetics parameters Y and k_d – other constant f_d

Obtained from multiplication of the expression for x

_a

with V, k

_d

and f

_d

(V is replaced by Q.

τ

)

– Here x_a.V.k_d indicates the biomass autooxidation rate

SRT

k

SRT

k

S

S

Q

Y

f

CDGR

d d e i d

.

1

.

).

(

.

+

−

=

Secondary Sludge Generation Rate

Secondary sludge generation rate is comprised of

– Net biomass synthesis rate

– Cell debris generation rate from biomass autooxidation

– Rate of contribution of Nonbiodegradable VSS by the influent (Nb.VSS)

– Rate of contribution of Inorganic suspended solids by the influent (In.SS)

Secondary sludge generation rate (SSGR) is

Here

GR SS In GR VSS Nb CDGR NBSR SSGR = + + . . + . .

)

.

.(

.

.

VSS

GR

Q

Nb

VSS

Nb

=

)

.

.(

.

.

SS

GR

Q

In

SS

In

=

Active biomass, MLSS and MLVSS

Active biomass to MLSS ratio

MLVSS to MLSS ratio

MLSS value

MLVSS value

GR SS In GR VSS Nb CDGR NBSR NBSR . . . . + + + GR SS In GR VSS Nb CDGR NBSR GR VSS Nb CDGR NBSR . . . . . . + + + + + NBSR GR SS In GR VSS Nb CDGR NBSR x_a. + + . . + . . GR SS In GR VSS Nb CDSR NBSR GR VSS Nb CDGR NBSR MLSS . . . . . . . + + + + +

Sludge Wastage Rate

Sludge wastage can be from the

– Secondary clarifier under flow line

– Aeration tank or its outlet prior to sec. clarifier as mixed liquor

• Rate of wastage depends on secondary sludge generation rate (SSGR) minus secondary sludge washout rate (SWOR)

Where SWOR is Q.TSS_e

SWOR

SSGR

Sludge Wastage Rate

Volumetric sludge wastage rate is

– SSWR/MLSS_u (when wasted from the secondary clarifier underflow) – SSWR/MLSS_a (when wasted from the aeration tank or its outlet prior

to the secondary clarifier)

Observed SRT is (V.MLSS

_a

)/SSWR

SRT chosen as the primary variable is (V.MLSS_a)/SSGR

Observed SRT is greater than the SRT chosen as the primary

variable

– Difference between the two will depend on the TSS of the clarified secondary effluent

Oxygen Demand Rate

Here ‘n’ is oxygen equivalence of microbial biomass(1.42!)

The oxygen demanded is supplied by

– Surface (floating or fixed) aerators

– Diffused aeration systems (introduce oxygen/air into liquid)

• Turbine mixers can disperse introduced air bubbles • Hydraulic shear devices can reduce bubble size

Suppliers of aeration systems indicate oxygen transfer rates of

their equipment at standard conditions

– Rates require correction to actual operating conditions

      −       =       CDGR plus NBSR of equivalent Oxygen substrate loaded of equivalent Oxygen demand Oxygen

(

S S

)

n

[

NBSR CDGR

]

Q demand O₂ = _i − _e − +

Actual Oxygen Transfer Rate

• AOTR is actual oxygen transfer rate under field conditions – Salinity-surface tension of the wastewater (_β )

– Operating temperature of the wastewater – Atmospheric pressure (related to altitude)

– Average depth of aeration (diffused aeration system) – Operating DO of the aeration tank

– Oxygen transfer coefficient of wastewater compared to that of clean tap water (α )

– Degree of fouling of the diffusers (diffused aeration system)

• SOTR is standard oxygen transfer rate in tap water at 20_°C and zero dissolved oxygen level

• Applicable even for oxygen transfer efficiencies

(

)

F

C

C

C

SOTR

AOTR

T s L TH s

₁

_.

₀₂₄

_.

_.

.

₂₀ 20 ,

α

β

−









₋

=

Actual Oxygen Transfer Rate

β

is salinity – surface tension factor

• Taken as ratio of saturation DO wastewater to clean water • Typical value is 0.92 to 0.98 (0.95 is commonly used)

α

is oxygen transfer correction factor for the wastewater

• Typical range for diffused aeration systems is 0.4-0.8 • Typical range for mechanical aerators is 0.6-1.2

F is fouling factor accounting for both internal and external

fouling of diffusers

• Impurities of compressed air cause internal fouling

• Biological slimes and inorganic precipitants cause external fouling

Actual Oxygen Transfer Rate

C_sֿ,T,H is average saturation of clean water at the operating temperature, altitude and aerator depth

• For surface aerators

• Can be obtained from literature (for the atmospheric pressure at the altitude in question)

• For diffused aerators it can be obtained by

• Applicable if biological oxygen uptake is not considered

• O_t is % O₂ in air leaving aeration basin (typically 18-20%)

H T s H T s

C

C

_{, ,}

=

_{, ,}     + = H atm depth mid w H atm H T s H T s P P P C C , , , , , , ,     + = 21 2 1 . , , , , t H atm d H T s H T s O P P C C

(

)

(

)

_    + − × − = T H P P atm H atm 15 . 273 8314 0 97 . 28 81 . 9 exp 0 , ,

Air Requirements of Diffused Aeration

Expressed in kg/hr. and Nm3/hr

Actual temperature depends on the level of compression

(Ambient temperature + pressure (in kg/cm2 _{gauge) X 10}°_C)!

Filtered air can minimize internal fouling

Consider air flow velocity and temperature while sizing ducting Relate air delivery pressure to the water column over the diffuser

Consider head losses in the diffused aeration system in estimating the air pressure required

Find the number of diffusers on the basis of typical air delivery per diffuser (consider internal and external fouling)

{

}

      ×       =       air the in fraction oxygen efficiency transfer oxygen Actual demand Oxygen required Air

Nutrient Requirements

Inflow of nitrogen

Influent may have TKN (organic-N+ammonical-N) and nitrate-N (nitrate+nitrite)

Nutrient addition (in the form of Urea and DAP)

Fate of nitrogen in the ASP

Organic-N is converted into ammonical-N Ammonical-N can nitrified into nitrate-N

Nitrate-N can be denitrified and lost in the gaseous from (as N2O and N2) Ammonical-N and Nitrate-N can be assimilation by active biomass and

stored within as organic-N

Outflow of nitrogen

Loss in the treated effluent either as TKN or as nitrate-N or as both Loss as organic-N in wasted activated sludge

Nutrient Requirements

Nitrate-N in the influent is usually negligible influent mainly has TKN Nitrogen in the treated effluent can be ammonical-N or nitrate-N or

organic-N (in the TSS_e)

Nitrogen in the wasted activated sludge is 12.23% - obtained from empirical formula of the activated sludge (C₆₀H₈₇O₂₃N₁₂P)

Denitrification loss of nitrogen can be significant if the ASP is designed for nitrification and denitrification to occur

When concentration is <0.3 mg/L nitrogen is believed to be limiting for the biooxidation removal of substrate

      −       +       +      

= _effluentN in the _wastedN in the_{sludge denitrific}N lost through_ation N in_luentthe

t requiremen N inf

(

TKN Nitrate N

)

Q x Q MLSS x TSS Q t requiremen N a _{w u} e + − +      ₊ = 0.3 0.1223 0.1223

Phosphorus requirement can be assessed in a manner similar to

the nitrogen requirement by

N and P required can also be conservatively estimated as

Here bCOD is in g/m3

Y is yield coefficient (0.4!)

Nutrient requirement can also be expressed as the required

bCOD:N:P ratio of the influent

Nutrient Requirements

)

3

.

0

.

1223

.

0

(

bCOD

_i

Y

TKN

_i

Nitrate

N

_i

Q

t

requiremen

N

=

+

−

−

)

3

.

0

.

0226

.

0

(

bCOD

_i

Y

Total

P

_i

Q

t

requiremen

P

=

+

−

2

.

1

:

2

.

5

:

100

:

:

N

P

=

bCOD

(

i

)

u w a e Q x Q Total P MLSS x TSS Q t requiremen P _+ −      ₊ = 0.3 0.0226 0.02263

Alkalinity Requirements

• 70-80 mg/L as CaCO₃ for maintaining the pH at 6.8 to 7.4

• Nitrification if occurring requires 7.07 g as CaCO₃ per g of NH₃-N nitrified

• Denitrification if occurring produces 3.57 g as CaCO₃ per g of nitrate reduced

Treated effluent quality

• Characterized by soluble bCOD, TSS and VSS, and nutrients • Soluble bCOD for SRT >4days is 2 to 4 mg/L

• Ammonical nitrogen and total phosphorus (soluble form) are >0.1 and >0.3 mg/L respectively

• For properly functioning secondary clarifier in case of mixed liquor solids with good settling characteristics TSS is 5-15 mg/L

F/M Ratio, BOD Volumetric Loading

V

x

QS

M

F

a i

=

V

QS

loading

BOD

=

i

Q

HRT

V

=

.

V

MLVSS

QS

M

F

_i

.

=

Aeration tank volume

Food to microorganisms ratio

• In terms of active biomass • In terms of MLVSS

BOD loading

Total bCOD of the effluent

MLSS

x

TSS

S

a e e

+

1

.

42

×

×

Design of Secondary Clarifier

Secondary clarifier includes

– Inlet section or central well – Sludge settling zone

– Sludge thickening and storage zone

– Clarified effluent overflow weir and collection trough

Requires sludge settling zone surface area,

π

(D2-d2)/4

• Area required for clarification and area required for thickening are found out and the larger of the two is used

• Either of the following two design approaches can be followed – Talmadge and Fitch method - uses data derived from a single batch

settling test

– Solids flux method - uses data obtained from a series settling tests conducted at different solids concentration

All other details of the clarifier are either assumed or obtained

through hydraulic and mechanical design

Secondary Clarifier: Talmadge and Fitch

method

Final overflow rate for a secondary clarifier is selected based on the consideration of

– Area for clarification – Area for thickening

– Rate of sludge withdrawal

Data from a single settling test is used for finding both area

required for thickening and for clarification and greater of the two is considered for design Area required for clarification is

usually greater than the area required for thickening

Area required for thickening

• T_u corresponds to H_u and obtained through • C_o is initial TSS and H_o column height

• C_u is underflow sludge concentration

Critical concentration controlling sludge handling capability

– Draw tangents to initial and final legs of settling curve

– Bisect the angle of intersection and extend to settling curve to get C_c

Find t_u (time at which sludge concentration is C_u)

• Draw tangent through C_c

• Locate H_u on y-axis, extend horizontal line to the tangent through C_c - draw vertical from the intersection to obtain T_u

o u t

H

Qt

A

=

u o o u

C

C

H

H

=

Secondary Clarifier:

Secondary Clarifier:

Talmadge and Fitch method

Area for clarification

– Here Q_c is clarification rate

– V is interface subsidence velocity

Interface subsidence velocity

• Slope of the tangent on the initial leg of the settling curve

is taken as subsidence velocity

Clarification rate

• Taken as proportional to the liquid volume above H

_u

and computed as

– Here H_u is sludge depth curresponding to t_u – Q is flow rate of mixed liquor into the clarifier

v

Q

A

c c

=

o u c

H

H

H

Q

Q

=

0

−

Secondary Clarifier: Solids flux method

Area required for thickening depends on the limiting solids flux that can be transported to the bottom of the settling tank

Data obtained from a series of

column settling tests conducted at different solids concentration is used

Solids flux depends on the

characteristics of the sludge (relationship between sludge

concentration and settling rate and solids flux)

Downward flux of solids in a settling tank occurs due

– gravity settling

– bulk transport from sludge withdrawal – Here SF_g is solids flux due to gravity – SF_u is solids flux by bulk transport

Solids flux due to gravity

– Ci is concentration of solids at the point in question – Vi is settling velocity of the solids at Ci concentration

– V_i of sludge at different concentrations is obtained from multiple settling tests - Slope of the initial portion of the curve is Vi

Secondary Clarifier: Solids flux method

u g t

SF

SF

SF

=

+

i i g

C

V

SF

=

Solids flux by bulk transport

– U_b is bulk underflow velocity – Q_u is underflow rate of sludge

– A cross sectional area of the sludge

– Flux by bulk transport linearly increases with increasing withdrawal rate

Total flux increases initially, then drops to limiting solids flux (SF_L)and then increases with increasing withdrawal rate

Secondary Clarifier: Solids flux method

A

Q

C

U

C

SF

i u b i u

=

=

Alternative graphical method for limiting solids flux (SF_L)

• Uses only the gravity flux curve

• Decide the underflow sludge concentration and draw tangent to gravity flux curve through Cu on X-axis and extend to Y-axis • Point of intersection on Y-axis gives SF_L

Secondary Clarifier: Solids flux method

Area for thickening

• Area required for thickening will that area at which actual solids is lower than equal to limiting solids flux (SF_L)

– If solids loading is greater than limiting solids flux then solids will build up in the settling basin and ultimately overflow

• Area required for thickening

• For a desired underflow concentration one can increase or decrease the slope of the underflow flux line

(

)

L u u

SF

C

Q

Q

A

=

+

Q is overflow _{Qu is underflow}

Settling and thickening characteristics of the mixed liquor measured by either SVI or ZSV can be used as basis

SVI below 100 is desired and above 150 typically indicates filamentous growth

Surface over flow rate for a secondary clarifier is related to zone settling velocity as shown below

ZSV (V_i) can be estimated by

Here V_i is zone settling velocity (SVI) SF is safety factor and taken as 1.75 to 2.5

V_max is maximum zone settling velocity taken as 7 m/h K is a constant with value 600 l/mg for ML with SVI 150 X if MLSS concentration

Design of Secondary clarifier on the basis

of SVI and ZSV

SF

V

rate

overflow

Surface

=

i

x

K

V

V

_i

=

_max

exp(

−

)