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