Principles of anaerobic
wastewater treatment and
sludge treatment
Jan Bartáček
ICT Prague
Department of Water Technology and
Environmental Engineering
Jan.bartacek@vscht.cz
Anaerobic digestion technology
• Wastewater
▫ wastewater treatment
▫ sludge stabilization
• Solid waste
▫ biogas plants
▫ landfilling with biogas collection
Sustainable approach to wastewater
treatment
Not only to dispose, but to reuse
• water
• raw materials
• energy
Transformation of pollution into biogas
aerobic
WWT
BM
anaerobic
stabilization
anaerobic
WW
WWT
BG
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)
▫
▫
▫
▫
▫
▫
▫
F2(g) + 2eO3(g) + 2H+(aq) + 2eAgCl(s) + e2 H+(aq) + 2eFe2+(aq) + 2eNa+(aq) + e-
 2F-(aq)
 O2(g) + H2O(l)
 Ag(s) + Cl-(aq)
 H2(g)
 Fe(s)
 Na(s)
V
+2.87
+2.08
+0.22
0.00
–0.44
–2.71
Oxidation-Reduction potential (ORP)
• Standard half-cell potential (E0)
▫
▫
▫
▫
▫
▫
▫
F2(g) + 2eO3(g) + 2H+(aq) + 2eAgCl(s) + e2 H+(aq) + 2eFe2+(aq) + 2eNa+(aq) + e-
 2F-(aq)
 O2(g) + H2O(l)
 Ag(s) + Cl-(aq)
 H2(g)
 Fe(s)
 Na(s)
V
+2.87
+2.08
+0.22
0.00
–0.44
–2.71
Oxidation-Reduction potential (ORP)
• Standard half-cell potential (E0)
▫
▫
▫
▫
▫
▫
▫
F2(g) + 2eO3(g) + 2H+(aq) + 2eAgCl(s) + e2 H+(aq) + 2eFe2+(aq) + 2eNa+(aq) + e-
 2F-(aq)
 O2(g) + H2O(l)
 Ag(s) + Cl-(aq)
 H2(g)
 Fe(s)
 Na(s)
V
+2.87
+2.08
+0.22
0.00
–0.44
–2.71
Processes at Biological WWTP
ORPH
(mV)
Nitrification
Oxic oxidation
270
Anoxic oxidation
Denitrification
170
Acidogenesis
Acetogenesis
-300
Methanogenesis
Phosphate depolymerisation
Desulphatation
Processes at Biological WWTP
ORP’
(mV)
Nitrification
Oxic oxidation
+50
Anoxic oxidation
Denitrification
-50
Acidogenesis
Acetogenesis
-500
Methanogenesis
Phosphate depolymerisation
Desulphatation
Anaerobic degradation of organic compounds
Proteins
hydrolysis
Aminoacids
acidogenesis
Polysaccharides
Monosaccharides
Lipids
Hydrolytic bacteria
Fatty acids
Synthrophic bacteria
Alcohols, VFA
acetogenesis
Acetic acids
Acidogenic bacteria
Hydrogen
methanogenesis
Methanogenic bacteria
Methane
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
hydrolysis
Aminoacids
acidogenesis
Polysaccharides
Monosaccharides
Lipids
Hydrolytic bacteria
Fatty acids
Synthrophic bacteria
Alcohols, VFA
acetogenesis
Acetic acids
Acidogenic bacteria
Hydrogen
methanogenesis
Methanogenic bacteria
Methane
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
Methanosaeta sp.
Methanogens in
biofilm
Methanosarcina 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
Methanogenic
niche
Reaction
possible
Reaction
impossible
Effect of temperature
• Each species has its own optimum
37 °C
55 °C
thermophilic
hyperthermophilic
mesophilic
Effect of pH
• Most vulnerable are methanogens
Optimum pH
Methanogens
6.5 – 7.5
Acidogens (e.g. Clostridium sp.)
4.5 – 7.5
• Extremely important buffering systems
▫ H2CO3 HCO3- + H+ CO32- + 2 H+
▫ NH3·H2O NH4+ + OH- NH3(aq) + H2O
Effect of pH – buffering capacity
Effect of pH – buffering capacity
Acidification of anaerobic reactors
• Frequent result of process instability
Methanogenic capacity exceeded
H2 pressure increase
VFA increase
Propionate increase
Toxicity increase
pH decrease
Unionized VFA
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
Biogas
CH4
CO2
60 - 80 %
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/23d/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 Bartáček
ICT Prague
Department of Water Technology and
Environmental Engineering
Jan.bartacek@vscht.cz