Phosphrus Removal

11. 0 Phosphorus Removal

• Phosphorus

- essential macronutrient - spur the growth of photosynthetic algae and cyanobacteria

• Phosphorous removal from wastewater:I) prior to biological treatment

II) as part of biological treatmentII) as part of biological treatmentIII) following biological treatment

I) & III) depends on chemical precipitation :

• II) Removal of P as part of Microbiological treatment process:

I) & III) depends on chemical precipitation :Ca+2 (or Al3+ or Fe3+) + PO4

3- Ca3 (PO4)2

-

II) Removal of P as part of Microbiological treatment process:

1) Normal phosphorus uptake into biomass2) P i it ti b t l lt dditi t i bi l i l2) Precipitation by metal-salts addition to a microbiological process3) Enhanced biological phosphorus uptake into biomass

11.1 Normal Phosphorus Uptake into Biomass

- The normal biomass contains 2 to 3% in its dry weighty gThe modified formula = C5H7O2NP0.1 (116 g/mol, P is 2.67%)

- Sludge wasted from the process removes P in proportion to ww XQthe mass rate of sludge VSS wasted ( )

- A steady-state mass balance on total P

wv

w XQ

- A steady-state mass balance on total P,

]1.11[)/0267.0(0 0 VSSgPgXQQPQP wv

w−−=

P0 = the influent total P concentrationP = the effluent total P concentrationQ = the influent flow rate

- The effluent total P concentration,

]2.11[)0267.0()0267.0( 0

0

QXQ

PQXQQP

Pwv

wwv

w

−=−

=

11.1 Normal Phosphorus Uptake into Biomass

]2.11[)0267.0()0267.0( 0

0

QXQ

PQXQQP

Pwv

wwv

w

−=−

=

- The rate of sludge wasting is proportional to the BOD removal and the observed yield (Yn) :

]3.11[)( Lnwv

w BODQYXQ Δ=

- The Net yield (same as Equation 3.32),

)1(1 bf θ+]4.11[

1)1(1

x

xdn b

bfYY

θθ

+−+

=

- Combining Equations 11.2 to 11.4

]511[))()1(1()0267.0(0 Lxd BODbfY

PPθ Δ−+

]5.11[1

))()(()(0

x

Lxd

bf

PPθ+

−=

11. 1 Normal Phosphorus Uptake into Biomass

-Table 11.1 compiles effluent P concentrations for a range of SRTs and BOD removals

i) O l i i ld ffl t P t ti l th 1 /Li) Only one scenario yields an effluent P concentration less than 1 mg/L- Ii) Increasing SRT and decreasing BOD removal result in less P removal

iii) The large SRTs needed for nitrification reduce sludge wasting and normal P removal

11.2 Precipitation by metal-salts addition to a biological process

• Precipitation of aluminum and ferric cations

- Al3+ and Fe3+ precipitate with orthophosphate anion at pH valuesAl and Fe precipitate with orthophosphate anion at pH values that are compatible with microbiological treatment. Precipitate form, are

incorporated into sludge, and are removed from the system by sludge wastingwasting.

2133 −+ KPOAlAlPO- The key precipitates are AlPO4(s) and FePO4(s)

2134

3)(4 =+= +

sos pKPOAlAlPO

239.2134

3)(4 topKPOFeFePO sos =+= −+

4)(4 p sos

Although the solubility products are very small the conditional solubility- Although the solubility products are very small, the conditional solubility of phosphates which is dependent on pH, are not necessarily miniscule,because Al3+, Fe3+, and PO4

3- undergo competing acid/base and l ti ticomplexation reactions.


- For phosphate, the key competing reactions are acid/base ones that result in the formation of protonated species.

3.123,34

24 =+= −+−

apKPOHHPO272−+− KHPOHPOH 2.72,

2442 =+= +

apKHPOHPOH1.21,

34243 =+= −+

apKPOHHPOH

- The dominant species of near neutral pH are HPO42-, H2PO4

-, not PO43-


- For aluminum and ferric cations, the hydroxy complexes are very important. Key complex-formation reactions and stability constants (pK1-4):y p y (p 1-4)

8.11123 ==+ +−+ pKFeOHOHFe 0.91

23 ==+ +−+ pKAlOHOHAl

8.10)( 222 ==+ +−+ pKOHFeOHFeOH

0

7.9)( 222 ==+ +−+ pKOHAlOHAlOH

38)()( 0 ==+ −+ pKOHAlOHOHAl7.7)()( 3032 ==+ −+ pKOHFeOHOHFe

4.4)()( 443 ==+ −− pKOHFeOHOHFe

3.8)()( 332 ==+ pKOHAlOHOHAl

0.6)()( 443 ==+ −− pKOHAlOHOHAl)()( 443 p

- At neutral pH : the dominant species Fe(OH)2+, Fe(OH)30, Al(OH)3

0

At lower pH : the dominant species Al3+ Fe3+-At lower pH : the dominant species Al3+, Fe3+

- Due to the opposing pH trends for the presipitating anion (PO43-) versus

the precipitating cations ( Al3+ or Fe3+), an optimal pH exists.- The pHs of minimum solubility : ~ 6 for AlPO4 (s) and ~5 for FePO4(s)

11. 2 Precipitation by metal-salts addition to a biological process

- Near these optimal pHs, stoichiometric (theoretical) additions of cations can drive the total phosphate concentrations to well below 1 mg P /L .

- However, the best dosage in practice always exceeds the stochimetric amount determined by the above equations.

Typically, the metal-salts dosage is 1.5 to 2.5 times the stoichiometric amount.

** Several factors act to complicate the chemistry andincrease the metal-salts

1. Phosphate forms competing complexes, such as CaHPO4, MgHPO4, and FeHPO4+, Thus the fraction of total phosphate that is present as PO43- is less than predicted by acid-base chemistry alone

2. Aluminum and iron form other complexes, particularly with organic ligands, orprecipitated as Al(OH)3(s) These reaction reduce the available Al3+precipitated as Al(OH)3(s). These reaction reduce the available Al3+


3. Some of the total phosphorus is not orthophosphate, but is tied up in organic compoundscompounds

4. The optimal pH for precipitation may not be compatible with the optimal microbiological activity. The pH cannot be changed so much that metabolicactivity is significantly inhibited

5 Th i it ti ti b ki ti ll t ll d d t h it5. The precipitation reaction may be kinetically controlled and not reach itsmaximum extent, which occurs at equilibrium


• An important consideration for metal-salts addition

Th dd d t l b h id d lk li it th- The added metals behave as acids and consume alkalinity as theyprecipitate with PO4

3- or complex with hydroxide

- In such cases, a base, such as lime(CaO), can be added to supplementthe alkalinity

−+ ++=+ OHOHAlNaOHNaAlO 322 )(2

- Another alternative is to dose sodium aluminate(NaAlO2)

+++ OHOHAlNaOHNaAlO 322 )(2

- Another alternative is to dose ferrous iron(Fe2+), which is oxidized to Fe3+

−++ +=++ OHFeOHOFe 322

2 5.025.0

( ),


- Precipitation of AlPO4(s), or FePO4(s) creates significant quantities of inorganic sludge that is retained within the biological process and ultimately wastedwasted.

- The stoichiometric production ratios : 3.9g AlPO4(s)/gP, 4.9gFePO4(s)/gP

- Removal of 8 mg P/L produces 39 mg FePO4(s) /L of plant flow.

If HRT 0 25 d SRT 6 d th t ti f t 24 dIf HRT = 0.25 d, SRT = 6 d, then concentration factor = 24 and the concentration of FeSO4(s) built up in the mixed liquor is 935 mg/L.

- The metal salt can be added with the influent wastewater, directly to the aeration basin, or to the mixed liquor before it goes to the settler

11. 3 Enhanced Biological Phosphorus Removal

- Certain heterotrophic bacteria are capable of sequestering high levels of P as intracellular polyphosphate (poly P), which is an energy storage material.

- The goal of enhanced biological phosphorus removal is to create a process environment that attains each of following three goals:

1) Selection of such microorganisms2) Inducing them to store poly P3) Wasting them when rich in poly p

- The biomass carrying out enhanced biological phosphorus removal (EBPR)contains 2 to 5 times the P content of normal biomass.

]611[))()1(1()0801.0(0 Lxd BODbfY

PPθ Δ−+

−= ]6.11[1 xb

PPθ+

−=

Assume three-fold enrichment(=3*0.0267)


- Eq. 11-5 can be revised to eq. 11-6 if three fold enrichment is attained :

]5.11[1

))()1(1()0267.0(0

x

Lxd

bBODbfY

PPθ

θ+

Δ−+−=

]6.11[1

))()1(1()0801.0(0

x

Lxd

bBODbfY

PPθ

θ+

Δ−+−=

Assume three fold enrichment(=3*0 0267)

* Increased poly-P enrichment or a higher BODL : P ratio in the influent

Assume three-fold enrichment(=3 0.0267)

p y g L

lower P in the effluent

11. 3 Schematic diagram for EBPR process

11. 3 Biochemical mechanisms operating in the anaerobic reactor

- Bio-P bacteria take up simple organic molecules.

- Because no e- acceptors are available, the bio-p bacteria sequester e- and carbon and polymerize them in the form of insolubleand polymerize them in the form of insolubleintracellular solids, polyhydroxybutyrate (PHB).

- The bio-p bacteria contain and hydrolyze poly P to obtainenergy necessary for the formation of PHB.

- The hydrolysis of PHB releases energy and phosphate (Pi)

11. 3 Biochemical mechanisms operating in the anaerobic reactor

- To make PHB via polymerization, asn activated chemical form, Acetyl –coenzyme A (Acetyl-CoA or HSCoA).F ti f HSC A i i t- Formation of HSCoA is an energy–consuming step.

- The energy which is transported as ATP comes from the hydrolysis of poly P.

OutsideCell

AcetateAcetate

InsideCell A. AnaerobicOutside

Cell

AcetateAcetate

InsideCell

OutsideCell

AcetateAcetate

InsideCell A. AnaerobicA. Anaerobic

ATP

ADP

Poly-Pn-1

Poly-Pn+

ATP

ADP

Poly-Pn-1

Poly-Pn+

ATP

ADP

Poly-Pn-1

Poly-Pn+Pi

PiPi Acetyl-CoA TCACycle

Pi

PiPi

Pi

PiPi Acetyl-CoA TCACycle

Acetyl-CoA TCACycle

Acetyl-CoA TCACycleTCACycle

NADH + H+

NAD+H+ + e-

NADH + H+

NAD+H+ + e-

NADH + H+

NAD+H+ + e-

NADH + H+

NAD+H+ + e-

PHBPHBPHBPHB

11.3 Biochemical mechanisms operating in the main (or aerobic) reactor

- When the Bio-P bacteria move to the main (aerobic reactor) where lots of e- acceptors(O2 ) are supplied, the biochemical reactions works in the opposite direction.

-The Bio- P bacteria synthesize poly P from inorganic phosphate, Pi .

-The Bio- P bacteria are significant sinks for environmental Pi.The Bio P bacteria are significant sinks for environmental Pi.

11.3 Biochemical mechanisms operating in the main (or aerobic) reactor

- PHB (e- storage material) is hydrolyzed to HSCoA, which is then oxidized in the TCA cycle. The released e- are used for the synthesis of ATP via respiration with NO3

- or O2.

- Some of ATP and e- generated are invested in the synthesis of poly P.

OutsideCell

Anabolism

InsideCell

H2O

B. Aerobic

PHB

OutsideCell

Anabolism

InsideCell

H2O

OutsideCell

Anabolism

InsideCell

H2O

B. Aerobic

PHB

B. Aerobic

PHB

ATP

ADP

Poly-Pn-1

Poly-P

Anabolism O2

NADH + H+

NAD+

ATP

ADP

Poly-Pn-1

Poly-P

Anabolism

ATP

ADP

Poly-Pn-1

Poly-P

Anabolism O2

NADH + H+

NAD+O2

NADH + H+

NAD+

ADPPoly Pn

PP

Acetyl-CoA

TCA

ADPPoly Pn

PP

ADPPoly Pn

PP

Acetyl-CoA

TCA

Acetyl-CoA

TCATCAPiPi TCACycle

NAD+ H+ + e-

H+ + e-

PiPi PiPi TCACycle

NAD+ H+ + e-

H+ + e-

TCACycleTCACycle

NAD+ H+ + e-

H+ + e-

NADH + H+AnabolismNADH + H+AnabolismNADH + H+Anabolism

11.3 Four essential components for EBPR

1. -The influent wastewater and recycle sludge must mix and first enter an anaerobic reactor. Electron acceptor(O2, NO3-) must be excluded to the maximum degree possible so that BOD oxidation is insignificant in this reactormaximum degree possible so that BOD oxidation is insignificant in this reactor. - The e- equivalents and carbon source in the influent BOD are not transferred to a terminal e- acceptor, but used for the synthesis of and stored in PHB.

2. The mixed liquor flows from the anaerobic tank to the main activated sludge bioreactor. Due to the ample O2, the Bio-P bacteria gain energy and grow.

11.3 Four essential components for EBPR

3. - The mixed liquor exiting the main bioreactor is settled, and most of it is recycled to the head of the process.Thi li th t ll f th bi i lt ti bi- This recycling ensures that all of the biomass experiences alternating anaerobic and aerobic conditions.

4. The sludge is wasted to control the SRT and to remove the biomass4. The sludge is wasted to control the SRT and to remove the biomass when it is enriched in poly P.


-In order to select the bio-P bacteria, the process must exert a strong ecologicalpressure to favor them.

*The cycling between the anaerobic and respiratory phases creates that ecological press

1. Anaerobic phase

- The Bio-P bacteria take-up and sequester the available electron donorsp qand carbon sources

2. Aerobic phase

- The Bio-P bacteria have rich reserves of electrons and carbon in PHB.- Hydrolysis and oxidation of PHB which they keep in their cells gives them a major

energy source for growth although the aqueous environment in the main reactor is oligotrophic.

- Generating energy from PHB allows the Bio-P bacteria to outgrow other heterotrophic- Generating energy from PHB allows the Bio-P bacteria to outgrow other heterotrophic bacteria and gradually establish them selves as a major fraction of the biomass.


- Genus of Bio-P bacteria

- Acinetobactor (In the past),Acinetobactor (In the past),

-Pseudomonas, Arthrobactor, Nocardia, Beyerinkia, O t b t A l Mi l t d thOzotobactor, Aeromonal, Microlunatus, and others

- In some instances, Chemical precipitation plays a role togetherwith EBPR in the removal of P.with EBPR in the removal of P.

* Chemical precipitations: i) Ca5(OH)(PO4)3(s) ( pKso = 55.9), p p ) 5( )( 4)3(s) ( p so ),ii) MgNH4PO4 (Struvite)

* Chemical precipitation as part of EBPR is accentuated by low alkalinity,which minimizes the buffering against theses pH increase.

11.3 Predenitrification + EBPR

1) Phoredox

- adds the initial anaerobic tank to a classical predenitrification system.


2) Bardenpho

- adds the anaerobic tank to the Barnard process (next slide), which has anoxic cell decay and aerobic polishing steps to effect added N removalcell decay and aerobic polishing steps to effect added N removal.

- Burdick report (1983):Burdick report (1983):N removal : 93%, P removal : 65%, Containing P of sludge : 4 to 5 % (twice the normal)the effluent total P conc. : 1.9 ~ 2.3 mg/L

10.3.2 Variations on the Basic One-Sludge processes

Barnard process (1975) by Dr. J. Barnard of South Africa50 mg/L TKN

• Influent TKN : 50 mg/L• recycle ratio (Qr2 ) from 2 to 1

: 400 %d it ifi ti t ( t 1)

g

• denitrification rate (reactor 1) : 100% of recycled input: 80 % of the influent TKN (Qr2=400%) 10 mg/L (20%)80 % o e ue (Q 00%)

• NO3--N/L leaving reactor 2

: 10 mg/L (20%) •Reactor 3 :endogenous decay of cells

3 mg NH4+-N /L

g y:10 mg/L of NO3—N converted to N2 gas: 0.3 mg NH4

+-N/mg NO3—N generated

due to endogenous cell decay 3 mg NO3--N/L due to endogenous cell decay

•Reactor 3 releases 3mg NH3-N/L•Reactor 4 releases 3 mg NO3

-N/L•

(6 %)

Final effluent TKN : 3 mg NO3--N/L

Total removal rate : 94 % Fig.10.2 Schematic of the Barnard process


- One possible drawback of the modification of predenitrification is that the sludge

3) Two UCT processesp p g

recycle returns NO3- e- acceptor is applied to the anaerobic reactor.

i) UCT (University of Cape Town )process ) UC (U e s y o Cape o )p ocess- It eliminates NO3

- from the return sludge- The sludge recycle goes directly to the anoxic predenitrification tank,

h ti d it ifi ti h ld k th NO t ti lwhere active denitrification should keep the NO3- concentration very low.

- Biomass is recycled to the anaerobic tank from the anoxic tank.

The sludge recycle goes directly to the anoxic predenitrification tank


3) Two UCT processes

ii) Modified UCT process- The anoxic tank is partitioned into two zones:

The first zone : receives only the small NO3- input from sludge recycle andThe first zone : receives only the small NO3 input from sludge recycle and

the source of mixed-liquor sent the anaerobic tank

The second zone: receives the main mixed-liquor NO3- recycleThe second zone: receives the main mixed liquor NO3 recycle

Partitioning the anoxic tanking into two zones


• SBR (Sequencing batch reactors)- They can be used for EBPR, as well as nitrogen removal.- Carryover of settled sludge from the previous cycle always contains some NO3

-.Once the NO is denitrified during an unaerated fill period anaerobic conditions- Once the NO3

- is denitrified during an unaerated fill period, anaerobic conditionscan be established and lead to poly P and PHB recycling.

• Bilfilm processes

- They can be used for EBPR if the biofilm is exposed to alternating anaerobic and respiratory periodsy

- Cycle times of around 6h are workable, but further research is needed to optimize process for N and P removal

TMP profile of (a) MSBR with anoxic phase of 10TMP profile of (a) MSBR with anoxic phase of 10 min.

and (b) MSBR without anoxic phase.

25

(a) (b)

15

20(k

Pa)

( ) (b)

5

10TMP

(

00.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

Ti (d )Time (day)

anoxic fill aerobic filtrationCycle format

0 10min 3hrs 50min10min 10min 2hrs 50min 50min

(a)(b)


• PhoStrip- It combines Bio-P bacteria with chemical precipitation.It combines Bio P bacteria with chemical precipitation.- is unique in that it does not rely on direct wasting of biomass enriched in poly-P

i) Anaerobic thickener and P stripper- The thickened sludge is returned to the

aeration basin after having hydrolyzed and released its soluble phosphate to the supernatantsupernatant.

ii) Precipitation reactor- addition of lime raises the pH to around 11and precipitates Ca3(PO4)2(s).

- Ca3(PO4)2(s) is wasted , while the supernatant is discharged to the effluent.

Documents

Phosphrus Removal