+a -2b -3d. Oxidation state: (2b + 3d - a)/n. 3. COD conversion to CH 4. What is Chemical Oxygen Demand (COD)?? C n H a O b N d

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1 3. COD conversion to CH 4 What is Chemical Oxygen Demand (COD)?? The theoretical COD calculation of organic compound C n H a O b N d is based on a complete oxidation. The amount of required O depends on the oxidation state of C: C n H a O b N d Oxidation state: (b + 3d - a)/n +a -b -3d

2 Chemical Oxygen Demand (COD) The number of electrons made free per atom C in the complete oxidation of C n H a O b N d amounts to: 4 (b+3d-a)/n (or 4n + a - b - 3d) as +4 is the most oxidized form of C (CO ) The required number of O molecules for the complete oxidation is: C n H a O b N d + ¼ (4n+a-b-3d) O n CO + ½(a-3d) H O + d NH 3 1 O accepts max. 4 electrons Chemical Oxygen Demand (COD) In the standardized COD test with bichromate as oxidizer (150 C) almost all organic compounds C n H a O b N d are completely converted in CO and H O But: organic N stays reduced and is converted in NH 3 The required number of O molecules for the complete oxidation is: C n H a O b + ¼ (4n+a-b) O n CO + ½a H O

3 That is Chemical Oxygen Demand (COD)!! 1 mol of organic matter demands: ¼ (4n+a-b) moles O or 8(4n+a-b) g O Theoretical COD calculation: COD t = 8(4n+a-b)/(1n+a+16b) mg COD/mg C n H a O b C n H a O b + ¼ (4n+a-b) O n CO + ½a H O What is Total Organic Carbon (TOC)?? Organic matter measured as CO after incineration (corrections needed for inorganic carbon in waste sample) TOC t = 1n / (1n + a + 16b) g TOC/gC n H a O b

4 g COD and g TOC per g organic compound Ratio COD / TOC: 8(4n+a-b)/(1n) = 8/3 + (a-b)/(3n) Compound n a b g COD (g C nh ao b) g TOC (g C nh ao b) COD/TOC ratio Oxalic acid Formic acid Citric acid Glucose Lactic acid Acetic acid Glycerine Phenol Ethylene glycol Benzene Acetone Palmitic acid Cyclohexane Ethylene Ethanol Methanol Ethane Methane Calculating the Theoretical Methane Production Theoretical methane production depends on: - the biodegradability of the substrate - the oxidation state of the organic compound Oxidation state of biodegradable compound: C n H a O b N d + a -b -3d Oxidation state: b + 3d -a n

5 Theoretical CH 4 / CO production of various organic compounds GAS COMPOSITION OF THE BIOG 100 CH 4 0 CO CH 4 50 CO Methanol, Methylamine Fats Algae, Bacteria Proteins Carbohydrates, Acetic acid Citric acid Formic acid, Carbon monoxide Oxalic acid 0 CH CO MEAN OXIDATION STATE OF CARBON Urea Methane Production from C n H a O b N d Basic principles: - part of C will be completely oxidized - the other part of the C will be completely reduced - N and O stay completely reduced - the average oxidation state of C stays the same

6 Methane Production from C n H a O b N d Assumption: - fraction X of C goes to CH 4 (oxidation state C = -4) - fraction (1-X) of C goes to CO (oxidation state C = +4) Since the oxidation state does not change: -4X + 4(1-X) = b + 3d - a n x = (a - b - 3d)/8n + 4/8 Methane Production from C n H a O b N d so C n H a O b N d is converted in: n.x.ch 4 n. (1-x).CO d NH 3 substitution of x gives:... x = (a - b - 3d)/8n + 4/8

7 Methane Production from C n H a O b N d C n H a O b N d n X CH 4 + n (1-X) CO + d NH 3 (n/ + a/8 b/4 3d/8) CH 4 (n/ a/8 + b/4 + 3d/8) CO Buswell s formula x = (a - b - 3d)/8n + 4/8 Buswell s formula Theoretical CH 4 -yield for a compound C n H a O b N d follows from: C n H a O b N d + (n a/4 b/ + 3d/4) H O (n/ + a/8 b/4 3d/8) CH 4 + (n/ a/8 + b/4 + 3d/8) CO + d NH 3 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 CO / HCO 3- in the water fraction

8 COD consumed by alternative electron acceptors Required COD to be calculated from complete reduction reaction 1. COD oxidation with sulphate. COD oxidation with sulphite 3. COD oxidation with nitrate COD + SO e + S S COD + SO e + S S COD + NO3 5e + N 5+ N H H 0 N S + CO S + CO + CO Stochiometric calc.: 1 mol SO - 4 ~ mol O 1g SO - 4 => 0.67 g COD 1 g SO - 3 => 0.6 g COD 1 g NO 3 N => 0/7 g COD =.86 g COD (1 g NO 3 => 0.65 g COD) The ratio of unionised VFA to total VFA as a function of ph CH 3 COOH CH 3 COO - + H + Propionate Butyrate Acetate At neutral ph: CH 3 COO - + H O CH 4 + HCO 3 - At low ph: CH 3 COOH CH 4 +CO ph 9

9 Metabolism generated alkalinity from protein degradation: decrease in biogas CO C n H a O b N d + (n - a/4 - b/ + 3d/4) H O (n/ + a/8 - b/4-3d/8) CH 4 + (n/ - a/8 + b/4 + 3d/8) CO + d NH 3 H O OH - + H + + d NH 4 + CO + H O H CO 3 H + + HCO 3 - Theoretical CH 4 / CO production of various organic compounds COMPOSITION OF THE BIOG GAS 100 CH 4 0 CO CH 4 50 CO Methanol, Methylamine Fats Actual production depends on overall wastewater characteristics! Algae, Bacteria Proteins Carbohydrates, Acetic acid Citric acid Formic acid, Carbon monoxide Oxalic acid 0 CH CO MEAN OXIDATION STATE OF CARBON Urea

10 Conversion CH 4 production - COD The oxidation of CH 4 requires moles of O : CH 4 + O CO + H O x = (a - b - 3d)/8n + 4/8 Since the fraction X of the compound C n H a O b N d will go to CH 4, The COD of compound C n H a O b N d is fraction X mol O /l or: (n/ + a/8 b/4 3d/8) mol O /l The overall oxidation state of organic compounds will not change during anaerobic conversion: A COD balance can be made: COD-in = COD-out COD-Balance COD gas COD influent Anaerobic reactor COD effluent COD sludge COD influent = COD effluent + COD gas + COD sludge Balance always fits!!!

11 Measurable COD fractions in various compartments COD influent: COD effluent: - COD soluble - COD solids - COD colloidal - COD soluble organic - COD soluble inorganic (e.g. H S) - COD solids - COD dissolved reduced gases COD gas: - COD CH 4 -COD H S - COD H COD sludge: - COD entrapped solids - COD newly grown biomass - COD entrapped Working with the COD Balance COD equivalents in produced gas: 1 mol CH 4 = mol O.4 l (STP) CH 4 = 64 g O or 64 g COD 1 l CH 4 (STP) = 64/.4 =.86 g COD Or: 1 g COD = 0.35 l CH 4 (STP) COD equivalents in sludge: 1 g sludge - VSS = 1.4 g COD (based on heterotrophic biomass: C 5H 7O N 113 g VSS per mol X) Question: what is the biogas and sludge production in a UASB treating sugar mill wastewater: - Q = 500 m 3 /day - COD = 3.5 kg/m 3 - Sludge Yield = 10% - Efficiency = 90%

12 4. Impacts of SO 4 --

13 1. Introduction: bio-chemical properties of C and S Similarities: Appearance in various states of oxidation, C: (-4) (+ 4) CH 4 CO S: (-) (+ 6) H S H SO 4 Can both act as electron acceptor / donor in anaerobic systems Reduced end-product is gaseous Possible formation of precipitates: MeS, CaCO 3 Bacteria using either S or C as electron acceptor may compete for the same substrate 1. Introduction: Sulphate, sulphite and nitrate as electron acceptors 1. Oxidation with oxygen. Oxidation with sulphate 3. Oxidation with sulphite 4. Oxidation with nitrate COD + O 4e + O COD 8e + S 6e + S H O + CO O + SO4 HS + CO 6 + COD + SO 4+ S 3 S H S + CO COD + NO3 N + CO e + N N Conclusions: 1 mol SO - 4 ~ mol O 1g SO - 4 => 0.67 g COD 1 g SO - 3 => 0.6 g COD 1 g NO 3 N => 0/7 g COD =.86 g COD (1 g NO 3 => 0.65 g COD)

14 Anaerobic Conversion of Organic Matter Organic Polymers proteins carbohydrates lipids Mono- o and oligomers amino acids, sugars, fatty acids Hydrolysis Acidogenesis Volatile Fatty Acids Lactate Ethanol Acetogenesis H / CO Acetate Methanogenesis CH 4 / CO Anaerobic Conversion of Organic Matter with Sulfate Organic Polymers proteins carbohydrates lipids Mono- o and oligomers amino acids, sugars, fatty acids Volatile Fatty Acids Lactate Ethanol H / CO Acetate CH 4 / CO Sulfate reduction H S / CO

15 Problems Associated With Sulphate Reduction - Negative effect on energy balance of anaerobic reactor => Lower methane yield per unit organic waste => Biogas treatment required (H S removal) - Maintenance Costs Increase => Malodor => Corrosion to engines, boilers, reactor parts, pipes,.. - Effluent Polishing Required => S - increases effluent COD (1 mol S - = mol O ) => Bulking of activated sludge (successive post-treatment) Odour and human toxicity H S is a colorless, very flammable, toxic gas. It is heavier than air. In low concentrations it smells like "rotten eggs. Low-level exposures usually produce local eye and mucous membrane irritation At levels of 300 ppm the olfactory nerve loses sensitivity. At first the "rotten egg" odor is detected but on the second or third breath, the odor is no longer noticed. At 600 ppm, breathing is inhibited, as the lungs fill with the gas. Exposures of ppm or greater usually result in death The Netherlands: MAC-value = 10 ppm (15 mg/m 3 )

16 Corrosion above the water level sulfuric acid attack Concrete sewer pipe Iron influent distribution box of a UASB treating domestic sewage Corrosion due to microbiological and chemical oxidation of H S to H SO 4 H S reduction of oxidized S-compounds to H S by SRB uncorroded concrete Inhibition Phenomena Sulfide toxicity - Only undissociated H S is toxic - diffuses freely through cell membrane - Denatures proteins and enzymes (sulfide cross-linking) - Effect internal cell ph - Inhibition of methane producing bacteria

17 ph dependent toxicity H S gas H S aq H S liq = K H * ph S H S HS + H Fraction of total dissolved sulfide present as undissociated H S: f HS, (30 C). + f HS = H S/(H S + HS - ) = 1/(10 ph-pka + 1) Sulfide inhibition at various ph, temp., and different sludge types H S and Total-sulfide (TS) concentrations causing a 50 % inhibition of acetotrophic methanogenesis. Sludge type ph T H S TS Ref C mg.l -1 suspended granular Oleskiewicz et al. 1989,. Visser et al. 1993e, 3. Koster et al

18 Competition between SRB MPB: Hydrogen Clear kinetic and thermodynamic advantage of SRB over MB Competition between SRB MPB: Acetate General Observations: - Without the presence of SO -- 4 : 70% of COD is degraded via acetate - In presence of SO -- 4 : Acetate is still converted into CH 4, even ee in ecessosuate excess of sulfate Competition is affected by: Influent composition: - Acetate concentration - Sulfate concentration (mass transfer limitation) - COD/sulfate ratio - Iron concentration - Toxic compounds

19 Competition of SRB and MB for Acetate Gradual conversion of a methanogenic bioreactor in a sulfidogenic reactor Treatment of sulfate rich wastewaters - Removal of sulfate COD/S0-4 = Enough sulfate for COD removal by SRB Enough COD for sulfate removal by SRB COD/ S0-4 < 0.67 Extra COD should be added for complete S0-4 removal COD/ S0-4 > 0.67 Complete COD removal only if also Methanogenesis takes place Objectives of treatment should be set: - Removal of organic matter and/or sulfate - Removal of organic matter

20 COD/SO 4 ratio as guidance for successful treatment Literature results of anaerobic treatment systems treating sulfate containing wastewater. t o successful treatment, no successful treatment (from Rinzema and Lettinga, 1988) Toxicity relieve through sulfide stripping by produced biogas H S g H S l HS - + H + CH 4 COD + SO - 4 H S-stripping by biogas at high COD/SO 4 -ratio s

21 Toxicity relieve through sulfide stripping by produced biogas H S g H S l HS - + H + CH 4 COD + SO - 4 H S-stripping by biogas at high COD/SO 4 -ratio s Toxicity relieve through sulfide stripping by produced biogas H S g H S l HS - + H + CH 4 COD + SO - 4 H S-stripping by biogas at high COD/SO 4 -ratio s

22 Toxicity relieve through sulfide stripping by produced biogas Reactor hydrogen sulfide concentration as a function of the COD-influent at various COD/SO 4 ratios (from Rinzema and Lettinga 1988) Methods for reducing H S concentration in UASB 1. Stripping of H S from the solution. Dilution of wastewater 3. Increase of the ph of the reactor

23 Toxicity relieve by H S stripping / removal - addition of Fe-salts in the reactor - stripping H S by biogas recirculation and biogas scrubbing Biogas Gas recycle Waste water Methanogenesis sulfate reduction effluent Gas scrubbing Biological biogas scrubber H S gas + OH - HS - aq + H O HS - + ½ O S 0 + OH - THIOPAQ - Bioscrubber for H S removal from biogas produced in papermill UASB, The Netherlands Low chemical consumption (90% chemical savings High efficiency (99%) S production

24 Toxicity relieve by dilution with H S-free effluent Effluent Recirculation After Sulfide Removal Effluent recycle Waste water Methanogenesis sulfate reduction Sulfide removal effluent Stripping adsorption biological oxidation to S 0 Preventive measure: separation of sulfide production and methanogenesis Waste water Acidification sulfate reduction Sulfide removal methanogenesis effluent