anaerobic fundamentals cod balance

1

CIE4485

Wastewater Treatment

Prof.dr.ir. Jules van Lier

11. Anaerobic Treatment Fundamentals COD Balance

1

13 December 2012

Challenge the future

DelftUniversity ofTechnology

CT4485 Wastewater Treatment

Lecture 5a: Anaerobic Treatment Fundamentals COD balanceProf.dr.ir. Jules van Lier

2Anaerobic Wastewater Treatment

Learning objectives

• What does COD actually means?• Making A COD balance over an anaerobic reactor• How to use the COD balance

IWA: Chapter 16Metcalf & Eddy: Chapt.10

2


Anaerobic Conversion of Organic Matter

CH4 / CO2

Methanogenesis

Acetogenesis

Mono- and oligomersamino acids, sugars, fatty acids

Organic Polymersproteins carbohydrates lipids

Hydrolysis

Acidogenesis

Volatile Fatty AcidsLactate Ethanol

AcetateH2 / CO2

What happens with the COD?


What is Chemical Oxygen Demand (COD) ??

The “theoretical COD” calculation of organic compound

CnHaObNd

is based on a complete oxidation.The amount of required O2 depends on the oxidation state of C:

CnHaObNd

+a-2b

-3d

Oxidation state:

(2b + 3d - a)/n

3


Chemical Oxygen Demand (COD)

The required number of O2 molecules for the complete oxidation is:

CnHaObNd + ¼ (4n+a-2b-3d) O2 n CO2 + ½(a-3d) H2O + d NH3

The number of electrons made free per atom C in the complete oxidation of CnHaObNd amounts to:

4 – (2b+3d-a)/n (or 4n + a - 2b - 3d)

as +4 is the most oxidized form of C (CO2)

1 O2 accepts max. 4 electrons


Chemical Oxygen Demand (COD)

In the standardized COD test with bichromate as oxidizer (150°C) almost all organic compounds

CnHaObNd

are completely converted in CO2 and H2O

But: organic N stays reduced and is converted in NH3

(similar to ‘organic O’)

The required number of O2 molecules for the complete oxidation is (no NOx produced, in contrast to BOD test):


4


That is Chemical Oxygen Demand (COD) !!

1 “mol” of organic matter demands: ¼ (4n+a-2b-3d) moles O2 or 8(4n+a-2b-3d) g O2

Theoretical COD calculation:CODt = 8(4n+a-2b-3d)/(12n+a+16b +14d)

mg COD/mg CnHaObNd



5


What is Total Organic Carbon (TOC) ??

Organic matter measured as CO2 after incineration(corrections needed for inorganic carbon in waste sample)

TOCt = 12n / (12n + a + 16b + 14d) g TOC/gCnHaObNd


g COD and g TOC per g organic compound (no ‘N’)

Compound n a b g COD (g CnHaOb)

g TOC (g CnHaOb)

COD/TOC ratio

Oxalic acid 2 2 4 0.18 0.27 0.67 Formic acid 1 2 2 0.35 0.26 1.33 Citric acid 6 8 7 0.75 0.38 2.00 Glucose 6 12 6 1.07 0.40 2.67 Lactic acid 3 6 3 1.07 0.40 2.67 Acetic acid 2 4 2 1.07 0.40 2.67 Glycerine 3 8 3 1.22 0.39 3.11 Phenol 6 6 1 2.38 0.77 3.11 Ethylene glycol 2 6 2 1.29 0.39 3.33 Benzene 6 6 0 3.08 0.92 3.33

Acetone 3 6 1 2.21 0.62 3.56 Palmitic acid 16 32 2 3.43 0.75 3.83 Cyclohexane 6 12 0 3.43 0.86 4.00 Ethylene 2 4 0 3.43 0.86 4.00 Ethanol 2 6 1 2.09 0.52 4.00 Methanol 1 4 1 1.50 0.38 4.00 Ethane 2 6 0 3.73 0.80 4.67 Methane 1 4 0 4.00 0.75 5.33

Ratio COD / TOC: 8(4n+a-2b-3d)/(12n) = 8/3 + 2(a-2b-3d)/(3n)

6



Calculating the Theoretical Methane Production

Oxidation state of biodegradable compound:

CnHaObNd

+ a

- 2b

- 3d

Oxidation state:

2b + 3d - an

Theoretical methane production depends on: - the biodegradability of the substrate - the oxidation state of the organic compound

7


-4 -2 0 2 4

MEAN OXIDATION STATE OF CARBON

CO

MP

OS

ITIO

N O

F T

HE

BIO

GA

S

0 CH4

100 CO

2

CH450 CO

2

100 CH4

0 CO

2

Urea

Oxalic acid

Citric acid

Carbohydrates, Acetic acid

Proteins

Algae, Bacteria

Fats

Methanol, Methylamine

Formic acid, Carbon monoxide

Theoretical CH4 / CO2 production of various organic compounds


Methane Production from CnHaObNd

Basic principles:

- part of C will be completely reduced (CH4)- the other part of the C will be completely oxidized (CO2)- N and O stay completely reduced- the average oxidation state of C stays the same

8



Assumption:

- fraction X of C goes to reduced CH4 (oxidation state C = -4)

- fraction (1-X) of C goes to oxidised CO2 (oxidation state C = +4)

Since the oxidation state does not change:

-4X + 4(1-X) = 2b + 3d - an

x = (a - 2b - 3d)/8n + 4/8



x = (a - 2b - 3d)/8n + 4/8

so CnHaObNd is converted in:

substitution of x gives: …...

n.x.CH4

n. (1-x).CO2

d NH3

9



CnHaObNd n • X CH4 + n • (1-X) CO2 + d NH3

(n/2 + a/8 – b/4 – 3d/8) CH4 (n/2 – a/8 + b/4 + 3d/8) CO2

Buswell’s formula

x = (a - 2b - 3d)/8n + 4/8


Buswell’s formula

Theoretical CH4-yield for a compound CnHaObNd follows from:

CnHaObNd + (n – a/4 – b/2 + 3d/4) H2O

(n/2 + a/8 – b/4 – 3d/8) CH4 + (n/2 – a/8 + b/4 + 3d/8) CO2 + d NH3

Actual methane production differs owing to:

- Limited biodegradability of compounds- Part of organic matter is used for cell growth (bacterial yield) - Possible presence of alternative electron acceptors- High solubility of CO2 / HCO3

- in the water fraction (influences ratio CO2/CH4, not CH4 production)

10



COD ‘consumed’ by alternative electron acceptors

1. COD oxidation with sulphate

2. COD oxidation with sulphite

3. COD oxidation with nitrate

Stochiometric calc.: 1 mol SO42- ~2 mol O2

1g SO42- => 0.67 g COD

1 g SO32- => 0.6 g COD

1 g NO3- => 0.65 g COD

1 g NO3- -N => 2.86 g COD

Required COD to be calculated from complete reduction reaction

26

222

4

8 SSe

COSHSOCOD

24

222

3

6 SSe

COSHSOCOD

05

22

5

32

NNe

CONNOCOD

11


964 5 7 8

pH

1.0

0.4

0.6

0.9

0.8

0.7

0.5

0.3

0.2

0.1

0

Propionate

ButyrateAcetate

pH and the CO2 fraction in the biogas

At low pH:

CH3COOH CH4 + CO2

At neutral pH:

CH3COO- + H2O CH4 + HCO3

-

CH3COOH CH3COO- + H+


Metabolism generated alkalinity from protein degradation: decrease in biogas CO2

CnHaObNd + (n - a/4 - b/2 + 3d/4) H2O (n/2 + a/8 - b/4 - 3d/8) CH4 + (n/2 - a/8 + b/4 + 3d/8) CO2 + d NH3

H2O OH- + H+

+

d NH4+

CO2 + H2O H2CO3 H+ + HCO3-

12


-4 -2 0 2 4

MEAN OXIDATION STATE OF CARBON

CO

MP

OS

ITIO

N O

F T

HE

BIO

GA

S

0 CH4

100 CO

2

CH450 CO

2

100 CH4

0 CO

2

Urea

Oxalic acid

Citric acid

Carbohydrates, Acetic acid

Proteins

Algae, Bacteria

Fats

Methanol, Methylamine

Formic acid, Carbon monoxide

Theoretical CH4 / CO2 production of various organic compounds

Actual production depends on overall wastewater characteristics !


0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00

COD/TOC ratio

CH

4 in

bio

gas

(%)

+4 0 -4-2+2

CH4

Ethane

Methanol, ethanolPalmitic acid

AcetoneBenzene, ethylene glycolGlycerine, phenol

Acetate, glucose, lactic acid

Citric acid, Glycine

Formic acid

Oxalic acid

CO2

Betaine (trimethyl glycine)

Insuline

Phenyl alanine

“C” mean oxidation state

Theoretical CH4 / CO2 production based on COD/TOC ratio

Expected CH4 %in biogas: 18.75 x COD/TOC

13



Conversion CH4 production - CODThe oxidation of CH4 requires 2 moles of O2:

CH4 + 2 O2 CO2 + 2H2O

Since “the fraction X” of the compound CnHaObNd that will go to CH4,

The COD of compound CnHaObNd is 2 • “fraction X” mol O2/l or:

2 • (n/2 + a/8 – b/4 – 3d/8) mol O2/l

The “overall” oxidation state of organic compounds will not change during anaerobic conversion:

A COD balance can be made: COD-in = COD-out

x = (a - 2b - 3d)/8n + 4/8

14


COD-Balance

COD influent COD effluent

COD sludge

COD gas

Anaerobic reactor

CODinfluent = CODeffluent + CODgas + CODsludge

Balance always fits !!!


COD influent: - COD soluble- COD solids- COD colloidal

COD effluent: - COD soluble organic- COD soluble inorganic (e.g. H2S) - COD solids- COD dissolved reduced gases

COD gas: - COD CH4

- COD H2S- COD H2

Measurable COD fractions in various compartments

COD sludge: - COD entrapped solids- COD newly grown biomass - COD entrapped

15


1 mol CH4 = 2 mol O2

22.4 l (STP) CH4 = 64 g O2 or 64 g COD1 l CH4 (STP) = 64/22.4 = 2.86 g COD

Or: 1 g COD = 0.35 l CH4 (STP)

Working with the COD BalanceCOD equivalents in produced gas:

COD equivalents in sludge:

1 g sludge - VSS = 1.42 g COD(based on heterotrophic biomass:

C5H7O2N 113 g VSS per mol X)

Question: what is the biogas and sludge production in a UASB treating sugar mill wastewater: - Q = 500 m3/day

- COD = 3.5 kg/m3

- Sludge Yield = 10%- Efficiency = 90%


Set up COD Balance

3.5 * 500 = 1750 kg COD/day

90% efficiency = 10% in effluent= 175 kg COD/day

10% sludge Yield:175 kg sludge COD/day123 kg VSS/day

Balance: 1750 - 175 -175 = 1400 kg COD/dag

Anaerobic reactor

= 1400/2.86 = 489,5 m3 CH4/dag

16



CH4/CO2 ratio

What kind of biogas composition do you expect?(Consider sucrose as main component C12H22O11)

X = (a – 2b – 3d)/8n + 4/8 = (22-2*11-0)/(8*12) + 4/8 = 0.5

The composition is 50% CH4 and 50% CO2

• Why CH4 content will be higher in practice?

17


DimensioningWhat should be the size of the UASB reactor to treat the water?

Use the following equations and assumptions:- reactor volume: Vr=C·Qi/Rv - Volumetric loading rate Rv=10 kg COD/m3/day- Minimum hydraulic retention time Θmin = 8 h = Vr/Qi

- reactor volume: Vr=C·Qi/Rv = 3.5*500/10 = 175 m3

- Minimum hydraulic retention time Θmin = 8 h Θ = 175/500 = 0,35 days = 8.4 hrs > Θmin

so Vr is 175 m3


18


Factors that affect the applicability of anaerobic treatment

• Composition and fluctuations of organic compounds (SS,biodegradability)

• Waste water strength + fluctuations• Temperature + fluctuations• Availability of nutrients (N, P, micro-nutrients)• Buffer capacity / pH• Presence of alternative electron acceptors (SO4

--, NO3-, etc.)

• Risk of formation of inorganic precipitates• Risk of formation of scum layers and/or flotation layers• Presence of toxic compounds • Odor problems





--, NO3-, etc.)


19


The same effluent also contains 0.2 kg SO42-/m3

.What will be the biogas production under these conditions?

Impact of SO42- on the COD Balance

Per g SO42-, 0.67 g COD will be oxidized:

0.2 kg/m3 * 500 m3/day = 100 kg SO42- / day and thus

67 kg COD consumption

This gives 1400-67=1333 kg CH4-COD/day produced = 466 m3 CH4/day (and 22,4 m3 of H2S if solubility is 0.015 g/L)

26

222

4

8 SSe

COSHSOCOD 1 mol SO42- ~ 2 mol O2

2 mol of COD (64 g)





--, NO3-, etc.)


20


Effects of pH on anaerobic digestionDirect

• effect on enzyme activity

(charge / tertiary structure of proteins)

• Indirect

• affecting toxicity of various compounds (H2S, NH3, VFA)

• affecting the availability of nutrients

(e.g. by affecting the solubility)

• affecting the availability of substrates (e.g. at pH<6.1, milk

proteins coagulate which makes them less available)

• affecting the availability of CO2

(very low pCO2 at pH>8.0.)


4 5 6 7 8 9

Hydrolysis

Acidogenesis

Acetogenesis

Methanogenesis

acetate

hydrogen

pH range methane digestion

21


Buffer systems

Buffering capacity: capacity of solution to resist pH changes (e.g. added H+ will be neutralised by buffer system)

Buffer system is usually brought by: mixture of weak acid and its conjugate base. The optimum pH of a buffer depends on the pKa/pKb value of the acid/base applied

N.B.:term for buffering capacity with respect to acids: alkalinity, Especially brought by bicarbonate


Buffers in AWWT

“Useful” buffers are active in the neutral pH range (6 – 8).

23332 // COHCOCOH HAcNaAc /

2442 / HPOPOH

34 / NHNH

pKa = 6.3 pKa = 10.3 pKa = 4.8

pKa = 7.2 pKa = 9.3

pH = pKa + log (A-/ HA) HSSH /2

pKa = 7.1

22


Bicarbonate alkalinity

CH3COO- Na+ CH4 + HCO3-

Bicarbonate is generated in the digestion process itself (metabolism generated alkalinity)

If a solution of 5 g NaAc is digested the produced alkalinity is 5/82 = 60 mmol or 60 meq HCO3

-.


Metabolism Generated Alkalinity

(digestion of protein)

Assume wastewater contains 2 g/l proteins (~C3H7O2N)

MW= 89 g

1 mole C3H7O2N => 1 mole NH4+-N

2/89 mole =>2/89 mole NH4+-N, or 22 mmole or 22 meq NH4

+-N

This will “bind” 22 meq HCO3-

3242273 NHCO2CHOH2NOHC

43242273 NHHCOCOCHOH2NOHC (pH = 7)

23


HENRY’S LAW

H2CO3 HCO3- +

H+

CO3- + 2

H+H2O

+CO2 (l)

CO2 (g)

CCO2(l) = H . PCO2


PREDICTING THE pH

Where; KH=(H*R*T)-1= henry’s constant (M atm-1)H=henry’s factor =[CO2]g/[H2CO3]aq=1.2R=0.082057 atm deg-1M-1

T= degrees Kelvin (°C+273)PCO2= partial pressure CO2 atm= (%CO2)/100Ka= dissociation constant H2CO3=10-6.3

[HCO3-]=bicarbonate alkalinity (M)=[ALK]-[VFA]

2COH32 PK*]COH[

*]COH[

]HCO[]H[Ka

32

3

2COH3

32 PKKa

]HCO[]H[*]COH[

]HCO[

KaPK]H[

3

COH 2

24


][][ 2

3

H

KaPKHCO COH





--, NO3-, etc.)


25


Temperature

• Affects metabolic activity of bacteria

• Affects transfer and solubility of gases

• Affects settleability of biological solids

• Reaction decreases with decreasing temperature (Arrhenius)

• Final degradation extent is proportional to temperature


Bacterial Classification on Growth Temperature

I psychrophilic <20°CII mesophilic 20-40 °CIII thermophilic >45°C

(arbitrary borders)

26


Digestion rate at mesophilic temperatures


Differences in growth rate mesophiles -thermophiles

27


Temperature effects on immobilized sludge systems

S

d

35ºC30ºC25ºC

- Maximum activity determined by mass transfer limitation

- Under optimum conditions only outer layer is active

- Decrease in temperature affects bacterial conversion rate but ‘granule activity’ is not affected


anaerobic fundamentals cod balance

Documents