Principles of anaerobic wastewater treatment and sludge treatment Jan Bartáček ICT Prague...

Principles of anaerobic wastewater treatment and sludge treatment

Jan BartáčekICT PragueDepartment of Water Technology and Environmental EngineeringJan.bartacek@vscht.cz

Anaerobic digestion technology•Wastewater

▫wastewater treatment▫sludge stabilization

•Solid waste▫biogas plants▫landfilling with biogas collection

Sustainable approach to wastewater treatmentNot only to dispose, but to reuse•water •raw materials •energy

Transformation of pollution into biogas

aerobicWWT

BM anaerobicstabilization

WWWWT

BGanaerobic

AD milestones•end of 19th century: beginning

(septic tank, biogas use)•mid-20th century : sludge stabilization •1970s oil crisis: interest in new

energy sources

Anaerobic digestion (AD)• CxHyOz + a H2O b CH4 + c CO2 +

biomass• (S) H2S / S2-

• (N) NH3 / NH4+

Anaerobic conditions

O2

Oxidation-Reduction potential (ORP)•A measure of the tendency of chemical

species to acquire electrons and thereby be reduced

•Nernst equation 

Oxidation-Reduction potential (ORP)•Standard half-cell potential (E0)

▫ V▫F2(g) + 2e- 2F-

(aq) +2.87▫O3(g) + 2H+

(aq) + 2e- O2(g) + H2O(l)

+2.08▫AgCl(s) + e- Ag(s) + Cl-

(aq)

+0.22▫2 H+

(aq) + 2e- H2(g) 0.00

▫Fe2+(aq) + 2e- Fe(s) –0.44

▫Na+(aq) + e- Na(s) –2.71

Oxidation-Reduction potential (ORP)•Standard half-cell potential (E0)

▫ V▫F2(g) + 2e- 2F-

(aq) +2.87▫O3(g) + 2H+

(aq) + 2e- O2(g) + H2O(l)

+2.08▫AgCl(s) + e- Ag(s) + Cl-

(aq) +0.22▫2 H+

(aq) + 2e- H2(g) 0.00▫Fe2+

(aq) + 2e- Fe(s) –0.44▫Na+

(aq) + e- Na(s) –2.71

Oxidation-Reduction potential (ORP)•Standard half-cell potential (E0)

▫ V▫F2(g) + 2e- 2F-

(aq) +2.87▫O3(g) + 2H+

(aq) + 2e- O2(g) + H2O(l)

+2.08▫AgCl(s) + e- Ag(s) + Cl-

(aq) +0.22▫2 H+

(aq) + 2e- H2(g) 0.00▫Fe2+

(aq) + 2e- Fe(s) –0.44▫Na+

(aq) + e- Na(s) –2.71

Processes at Biological WWTP

DenitrificationAnoxic oxidation

Oxic oxidation

Nitrification

Phosphate depolymerisationDesulphatation

Acidogenesis

Acetogenesis

Methanogenesis

ORPH

(mV)

-300

270

170

Processes at Biological WWTP

DenitrificationAnoxic oxidation

Oxic oxidation

Nitrification

Phosphate depolymerisationDesulphatation

Acidogenesis

Acetogenesis

Methanogenesis

ORP’ (mV)

-500

+50

-50

Anaerobic degradation of organic compounds

Proteins Polysaccharides Lipids

Alcohols, VFA

Acetic acids Hydrogen

Methane

Aminoacids Monosaccharides Fatty acidshydrolysis

acidogenesis

acetogenesis

methanogenesis

Hydrolytic bacteria

Synthrophic bacteria

Acidogenic bacteria

Methanogenic bacteria

Hydrolysis•Polymeric substances Oligomers•Products of hydrolysis are suitable for transport into bacterial cells where they can be utilized.

•Extracellular hydrolytic enzymes•Rate-limiting step for solid substrates

•Temperature sensitive

Acidogenesis•Production of

▫volatile fatty acids (VFA) – namely acetic acid, propionic acid, butyric acid, valeric acid etc.)

▫alcohols – ethanol, butanol•Large number of acidogenic bacteria

(~1% of all known species), e.g. Clostridium, Enterobacter or Thermoanaerobacterium

Acetogenesis• Specific functional groups –

▫Syntrophic acetogens ▫Homoacetogens

• Important part of the anaerobic microbial community

• VFA acetic acid, hydrogen and carbon dioxide• Homoacetogens

▫heterogenic group of bacteria▫produce acetic acid from a mixture of low-carbon

(mostly mono-carbon) compounds and hydrogen.▫Carbon dioxide, carbon monoxide and methanol are

the most important substrates.

Methanogenesis•Methanogens - strictly anaerobic Archaea

(Methanococcus, Methanocaldococcus, Methanobacterium, Methanothermus, Methanosarcina, Methanosaeta and Methanopyrus)

▫Hydrogenotrophic m. H2 + CO2 CH4+H2O

▫Acetotrophic m. (Acetoclastic m.) CH3COOH CH4 + CO2

•Extremely sensitive (temperature, pH, toxicity)

Anaerobic degradation of organic compounds

Proteins Polysaccharides Lipids

Alcohols, VFA

Acetic acids Hydrogen

Methane

Aminoacids Monosaccharides Fatty acidshydrolysis

acidogenesis

acetogenesis

methanogenesis

Hydrolytic bacteria

Synthrophic bacteria

Acidogenic bacteria

Methanogenic bacteria

Methanogenesis in nature•Probably the oldest mode of life•Any organics-rich environment with low

ORP▫Sediments (freshwater or marine)▫Wetlands/swamps▫Guts of animals▫Hot springs

•Able to adapt to extreme conditions▫~15 – 100 °C▫pH 3 – 9▫From halophiles to freshwater

Methanogenesis in nature

Methanogens in biofilm

Methanosarcina sp.

Methanosaeta sp.

Anaerobic granular sludge

Sekiguchi et al. 1999 Applied And Environmental Microbiology, 65(3), 1280-1288.

Fernández, et al 2008. Chemosphere, 70(3), 462-474.

Role of Hydrogen•Inhibition –

thermodynamic effect

Role of Hydrogen•Inhibition –

thermodynamic effect▫C6H12O6 + 2H2O 2CH3COOH + 2CO2 +4H2 ▫C6H12O6 CH3CH2CH2COOH + 2CO2 +2H2 ▫C6H12O6 + 2H2 2CH3CH2COOH + 2H2O

Role of Hydrogen•Inhibition –

thermodynamic effect▫C6H12O6 + 2H2O 2CH3COOH + 2CO2 +4H2 ▫C6H12O6 CH3CH2CH2COOH + 2CO2 +2H2 ▫C6H12O6 + 2H2 2CH3CH2COOH + 2H2O

Hard to degrade

Role of Hydrogen

Reaction possible

Reaction impossible

Methanogenicniche

Effect of temperature•Each species has its own optimum

psychrophilicmesophilic

thermophilichyperthermophilic

37 °C 55 °C

Effect of pH•Most vulnerable are methanogens

•Extremely important buffering systems▫H2CO3 HCO3

- + H+ CO32- + 2 H+

▫NH3 ·H2O NH4+ + OH- NH3(aq) + H2O

Optimum pH

Methanogens 6.5 – 7.5Acidogens (e.g. Clostridium sp.)

4.5 – 7.5

Effect of pH – buffering capacity

Effect of pH – buffering capacity

Acidification of anaerobic reactors•Frequent result of process instability

Methanogenic capacity exceeded

VFA increase

pH decreaseUnionized VFA increase

Toxicity increasePropionate increase

H2 pressure increase

COD Balance•organic pollution is measured by the mass

of oxygen needed for its chemical oxidation▫“Chemical Oxygen Demand” (COD)

•COD expresses the amount of energy contained in organic compounds

•Can be used to asses energy flow

COD Balance

Comparison of the COD balance during anaerobic and aerobic treatment of wastewater containing organic pollution

BiogasCH4 60 - 80 %CO2 20 - 40 %

( H2O, H2, H2S, N2, higher hydrocarbons, … )

Heat value 17 – 25 MJ/m3

Biogas composition•Depends on Mean Oxidation State of

Carbon▫CnHaObNd + ¼(4n+1-2b-3d)O2 nCO2 +

(a/2-3d/2)H2O + dNH3

▫Cox.= (2b-a+3d)/n▫COD=8(4n+a-2b-3d)/(12n+a+16b+14d)▫TOC=12n/(12n+a+16b+14d)▫COD/TOC = 8/3+2(a-2b-3d)/3n

= 8/3-2/3Cox.

Advantages of anaerobic WWT( in comparison with aerobic )

low energy consumption low biomass production high biomass concentration high organic loading rate low nutrients demand

Limits of anaerobic WWT( in comparison with aerobic )

longer start-up higher sensitivity to change of conditions minimum nutrients removal need of post-treatment

Principles of anaerobic wastewater treatment and sludge treatment

Jan BartáčekICT PragueDepartment of Water Technology and Environmental EngineeringJan.bartacek@vscht.cz