Principles of anaerobic wastewater treatment and sludge treatment Jan Bartáček ICT Prague Department of Water Technology and Environmental Engineering

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 WW WWT BG anaerobic

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) C x H y O z + a H 2 O  b CH 4 + c CO 2 + biomass (S)  H 2 S / S 2- (N)  NH 3 / NH 4 +

Anaerobic conditions O2O2

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 (E 0 ) ▫ V ▫F 2(g) + 2e -  2F - (aq) ▫O 3(g) + 2H + (aq) + 2e -  O 2(g) + H 2 O (l) ▫AgCl (s) + e -  Ag (s) + Cl - (aq) ▫2 H + (aq) + 2e -  H 2(g) 0.00 ▫Fe 2+ (aq) + 2e -  Fe (s) –0.44 ▫Na + (aq) + e -  Na (s) –2.71

Oxidation-Reduction potential (ORP) Standard half-cell potential (E 0 ) ▫ V ▫F 2(g) + 2e -  2F - (aq) ▫O 3(g) + 2H + (aq) + 2e -  O 2(g) + H 2 O (l) ▫AgCl (s) + e -  Ag (s) + Cl - (aq) ▫2 H + (aq) + 2e -  H 2(g) 0.00 ▫Fe 2+ (aq) + 2e -  Fe (s) –0.44 ▫Na + (aq) + e -  Na (s) –2.71

Oxidation-Reduction potential (ORP) Standard half-cell potential (E 0 ) ▫ V ▫F 2(g) + 2e -  2F - (aq) ▫O 3(g) + 2H + (aq) + 2e -  O 2(g) + H 2 O (l) ▫AgCl (s) + e -  Ag (s) + Cl - (aq) ▫2 H + (aq) + 2e -  H 2(g) 0.00 ▫Fe 2+ (aq) + 2e -  Fe (s) –0.44 ▫Na + (aq) + e -  Na (s) –2.71

Processes at Biological WWTP Denitrification Anoxic oxidation Oxic oxidation Nitrification Phosphate depolymerisation Desulphatation Acidogenesis Acetogenesis Methanogenesis ORP H (mV)

Processes at Biological WWTP Denitrification Anoxic oxidation Oxic oxidation Nitrification Phosphate depolymerisation Desulphatation Acidogenesis Acetogenesis Methanogenesis ORP ’ (mV)

Anaerobic degradation of organic compounds Proteins Polysaccharides Lipids Alcohols, VFA Acetic acidsHydrogen Methane Aminoacids Monosaccharides Fatty acids hydrolysis 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 acidsHydrogen Methane Aminoacids Monosaccharides Fatty acids hydrolysis 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 Applied And Environmental Microbiology, 65(3), Fernández, et al Chemosphere, 70(3),

Role of Hydrogen Inhibition – thermodynamic effect

Role of Hydrogen Inhibition – thermodynamic effect ▫C 6 H 12 O H 2 O  2CH 3 COOH + 2CO 2 +4H 2 ▫C 6 H 12 O 6  CH 3 CH 2 CH 2 COOH + 2CO 2 +2H 2 ▫C 6 H 12 O 6 + 2H 2  2CH 3 CH 2 COOH + 2H 2 O

Role of Hydrogen Inhibition – thermodynamic effect ▫C 6 H 12 O H 2 O  2CH 3 COOH + 2CO 2 +4H 2 ▫C 6 H 12 O 6  CH 3 CH 2 CH 2 COOH + 2CO 2 +2H 2 ▫C 6 H 12 O 6 + 2H 2  2CH 3 CH 2 COOH + 2H 2 O Hard to degrade

Role of Hydrogen Reaction possible Reaction impossible Methanogenic niche

Effect of temperature Each species has its own optimum 37 °C55 °C

Effect of pH Most vulnerable are methanogens Extremely important buffering systems ▫H 2 CO 3  HCO H+  CO H + ▫NH 3 ­·H 2 O  NH OH -  NH 3(aq) + H 2 O Optimum pH Methanogens6.5 – 7.5 Acidogens (e.g. Clostridium sp.)4.5 – 7.5

Effect of pH – buffering capacity

Acidification of anaerobic reactors Frequent result of process instability Methanogenic capacity exceeded VFA increase pH decrease Unionized VFA increase Toxicity increase Propionate increase H 2 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

Biogas CH % CO % ( H2O, H2, H2S, N2, higher hydrocarbons, … ) Heat value17 – 25 MJ/m3

Biogas composition Depends on Mean Oxidation State of Carbon ▫C n H a O b N d + ¼(4n+1-2b-3d)O 2  nCO 2 + (a/2- 3d/2)H 2 O + dNH 3 ▫C ox. = (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 = 8/3-2/3C ox.

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