Modelling of gas pressure effects on anaerobic digestion

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  • ELSEVIER 0960-8524(94)00148-0

    Bioresource Technology 52 (1995) 25-32 1995 Elsevier Science Limited

    Printed in Great Britain. All rights reserved 0960-8524/95/$9.5(I

    MODELLING OF GAS PRESSURE EFFECTS ON ANAEROBIC DIGESTION

    V. A. Vavilin,* V. B. Vasiliev & S. V. Rytov

    Water Problems Institute of Russian Academy of Sciences, Novo-Basmannaja str. 10, PO Box 524, 107078 Moscow, Russia

    (Received 25 July 1994; revised version received 30 November 1994; accepted 5 December 1994)

    Abstract The model of anaerobic digestion described earlier by the authors was used for analysis of a pressure change as a method of avoiding a gas component toxicity. The model, cafibrated for laboratory-scale reactors, showed that an increase in pressure reduced ammonia inhibi- tion during simulated cattle-manure digestion. A decrease in pressure reduced free hydrogen sulfide inhibition during a sulfate overload of an anaerobic digestion of a synthetic feedstock.

    Key words: Anaerobic digestion, gas component tox- icity, pressure effect, simulation model.

    INTRODUCTION

    Gas production and composition are the parameters sensitive to unbalance of anaerobic digesters. The response time depends on the head-space volume, rate of gas production and the gas solubility. The properties of the gas phase show more variation with respect to time than do those of the liquid phase (Andrews & Graef, 1971; Hawkes et al., 1994). The liquid phase has more buffering capacity than the gas phase.

    Three causes of digester imbalance can be sug- gested: overloading, toxic substrates and sudden changes in the digester environment. Andrews and Graef (1971) suggested raising the digester pH by scrubbing some of the carbon dioxide from recircu- iated gas to avoid the effect of an overload. To avoid hydrogen sulfide toxicity, Olsson and Schleischer (1994) removed sulfide outside the reactor using the biogas. Kapp (1992) showed that in thermophilic sludge-digestion increasing the total pressure avoi- ded free-ammonia toxicity.

    Two types of model of anaerobic digestion can be mentioned, one where the processes are controlled by the hydrogen partial-pressure in the reactor (McCarty & Mosey, 1991), and one where the vola- tile fatty acids in the liquid phase of the reactor control the process (Angelidaki et al., 1993). It is

    * Author to whom correspondence should be addressed. 25

    usually assumed that the total gas pressure in the reactor is equal to 1 atm (Costello et al., 1993).

    A universal basic model of anaerobic conversion of complex organic material, (METHANE), was developed by the authors (Vasiliev et al., 1993; Vavi- lin et al., 1994a). This model was used for descriptions of the reactions focused on ammonia and hydrogen sulfide inhibition, with cattle manure and an acetate-sulfate synthetic substrate (Vavilin et al., 1994b; 1995). In the present paper, to clarify pressure change as a method of avoiding gas-compo- nent toxicity, the (METHANE) model is used for descriptions of the manure and synthetic acetate- sulfate substrate digestion.

    METHODS

    A system of differential equations for three groups of variables is written in accordance with the mate- rial balances.

    1. Equations describing the dynamics of the sus- pended organic matter (Bi bacteria; Xk suspended solids) in the anaerobic reactor:

    V'I~i=qFBFi--qBxBi + V" PBi (1) V'Xk =qFX~k --qBxXk + V" p~ (2)

    where Bvi (i=1,2,...,7) and XFk (k=l,2,3)=influent concentrations of active bacteria and suspended sol- ids; qF and qn=rates of input and discharge of the waste and excess-sludge mixture; Psi and pxk=rates of biomass growth and substrate transformation, respectively, and V=reactor volume.

    2. Equations describing the dynamics of soluble components in the anaerobic system:

    V'S j=qv' (SF j -S j )+V'ps i+Qj (3)

    where j=1,2,...,12; psi=rate of soluble substrate transformation, Qj=rate of mass exchange between the gaseous phase and the liquid phase.

    3. The balance equation for the gaseous phase:

    Vg ~PF,- -Q,- - ~PF , - -~O, "~T (4) 2

  • 26 V. A. Vavilin, V. B. Vasiliev, S. V. Rytov

    where n = 1, 2 ..... 7 (methane, carbon dioxide, hydro- gen sulfide, hydrogen, ammonia, water vapour, inert gas); Vg=gaseous phase volume; R=universal gas constant; Tk=absolute temperature; q~=gas flux coming into the gaseous phase; PF,,=partial pressure of gas components in the influent wastewater; Pv=total gas pressure in the gaseous phase; Q,,=mass exchange function between liquid and gas for each gaseous component. The rate of mass exchange between the liquid and gaseous phases was described according to Andrews and Graef (1971).

    Detailed information about the model has been reported (Vasiliev et al., 1993; Vavilin et al., 1994a) and the main features of the new version, (METHANE 2.2), are as follows.

    1. Hydrolysis, acidogenesis and methanogenesis are included. The total number of particulate, solu- ble and gaseous variables of the model is about 30. Rates of changes in the biomass concentration (acid- ogens, syntrophs and methanogens) take into account bacterial growth, death and mutual trans- formations.

    2. Four versions of hydrolysis kinetics are con- sidered for suspended solids degradation, including the simplest first-order kinetics.

    3. Dissolved and particulate organic matter is supposed to be a mixture of carbohydrates, proteins and lipids which have limits to fermentation. Lysis and hydrolysis of cell biomass are taken into account.

    4. The rate of the main limiting-substrate trans- formation by bacterial groups is presened as a product of substrate limitation- and inhibition-func- tions and temperature dependence. Inhibition effects of the molecular-hydrogen pressure, pH, con- centration of non-ionized molecules of ammonia, of hydrogen sulfide and of propionate are considered. It is supposed that propionate represents the whole spectrum of volatile fatty acids other than acetate. Thus, the acetate or the propionate paths of fermen- tation carried out by acidogens A or acidogens P are included. The pathway of fermentation is dependent on the hydrogen partial-pressure.

    5. A physico-chemical reaction system of pH level with ammonia, hydrogen sulfide, carbonic acid and volatile fatty acid buffer systems is considered. The buffer capacity of the system can be evaluated using bicarbonate alkalinity. The pH level can be fixed.

    6. The flow-rate of biological solids through the reactor is considered to be independent of the flow- rate of the bulk liquid phase. Thus, the model can be useful for analysis of high-rate digesters with sol- ids retention.

    7. Batch and continuous-flow reactors can be analysed using the first and the second forward-inte- gration methods with variable time-step. The user can change the integration accuracy. In continuous- flow system the carbon balance is calculated to check the integration accuracy.

    8. The dependence of different model variables on such parameters as SRT, HRT and temperature can be analysed for the steady-states of a continu- ous-flow reactor.

    9. The model was realized in a form of a user- friendly computer program for IBM PC AT and it allows the user, in 1 min of computer time, to see the complex dynamic behaviour of an anaerobic system over many days.

    10. The procedure of visual model calibration is developed. The computer program includes a library of experimental data with different organic wastes being treated and corresponding values of constants and parameters. The model was able to provide a good comparison with data examined.

    11. The current values of all variables can be dis- played on the screen in text and in graphic forms and also printed out. Using these data it is easy to analyse the input of particular processes like defined buffer systems used to keep a constant pH level.

    12. (METHANE 2.2) contains a (HELP) proce- dure for a description of the program features in text and in graphic forms.

    RESULTS AND DISCUSSION

    Manure digestion: ammonia toxicity To reduce ammonia concentration to a non-critical level, Kapp (1992) suggested raising the pressure in a thermophilic sludge digester. Using a half-scale pilot-plant he showed that at pressures from 2.5-3 bar the digestion succeeded. An increase in CO2 partial pressure decreased the pH value, which low- ered the concentration of NH3.

    We analysed Kapp's proposals in the case of manure digestion using the (METHANE) model. A continuous-flow anaerobic system, fed on cattle manure and adapted to a particular level of ammo- nia, was used by Angelidaki et al. (1993) to study the effect of a step increase in the concentration of ammonia in the influent water. High pH levels (7.6-8.1), together with high VFA concentrations, were observed in the steady-state.

    For the model a manure composition was repre- sented by a mixture of soluble and insoluble carbohydrates, proteins and lipids (70:29:1) to obtain the suspended and dissolved solids composi- tion and the level of ammonia equal to those in the cattle manure digestion mentioned above. The degradable part of the suspended solids was such as to obtain the methane flow-rate required. The quasi- steady-state of the model was obtained under the conditions presented in Table 1. Some model coeffi- cients are shown in Table 2. Following this, after a sharp increase in influent ammonia concentration from 2.5 to 6.0 g N/I at 0 days the system adapted to the new conditions and reached a new steady-state (Fig. 1). The graphs in Fig. 1 are the computer curves, thus the scales are rough. Only the maximum

  • Modelling of gas pressure effects on anaerobic digestion 27

    Table 1. Simulation conditions for the cattle-manure system

    Chemical composition of the influent water:

    suspended solids 19300.00 mg/l dissolved OM 5400.00 mg/l acetate 4500-00 mg/l propionate 2500-00 mg/l sulfate 5"00 mgS/l carbonate 50.00 mgC/l sulfide 0.00 mgS/1 ammonia nitrogen 2500"00 mgN/l calcium 10.00 mg/l sodium 3200.00 mg/l phosphate 555.00 mg P/I chloride 0.00 mg/l

    Parameters of reactor: volume of reactor 1.00 1 volume of gaseous phase 0.50 1 temperature 33"00C pressure 1.00 bar feed flow-rate 0.07 l/day

    Table 2. Inhibition constants used in the model simula- tions

    Inhibitor Organisms Substrate

    Manure Acetate/sulfate

    Ammonium Methanogens A 1250/1800" (mg N/l) Syntrophs 1210/1800 - - Hydrogen Methanogens A - - 29.7/65.0 sulfide Sulfate-reducers - - 35-0/70.0 (rag S/l) pH Methanogens A - - 5.15/4.85 (acid zone) Sulfate-reducers - - 5.10/4.80 pH Methanogens A 8.10/8.30 - - (alkaline Syntrophs 8"10/8-30 - -

    zone)

    "For ammonia and hydrogen sulfide inhibition two con- stants are used. The first constant (K~I) means the concentration at which the growth rate of the microorgan- isms is reduced to half of the maximum value. The second constant (KI2) corresponds to reduction of the growth rate to one-hundredth of the maximum (see Fig. 3). The pH inihibition functions have 2 2 constants (acid and alka- line zones). For the manure system an ammonia inhibition occurs in the alkaline zone, but for the acetate/sulfate system hydrogen sulfide inhibition occurs in the acid zone. - - Not applicable.

    value of the variable measured or computed is screened. Table 3 contains the accurate data com- puted at the stated times.

    The (METHANE) model was able to provide a good comparison with available data from a digester at 1 bar pressure. During this period, the concentra- tion of volatile fatty acids increased, pH dropped from 7.98 to 7.75 and methane yield decreased because a high non-ionized NH3-concentration of 990 mg N/I inhibited the methanogens. The toxic influence of NH3 on methanogens causes an accu- mulation of volatile fatty acids, thus decreasing the

    1400

    0 3800

    0 2500

    0 8.2

    4 1100

    0 0.810

    0 0.720 g - .

    0 0.270

    0

    , , ,p

    "-,z_._.

    Methanogens A, mg/I

    Acetate, mg/I

    Propionate, mg/I

    pH

    Non-ionized NH 3, mgN/I

    Methane f low rate, Ill*day

    *l *s s ~- Methane partial pressure, bar

    Carbon dioxide partial pressure, bar

    120 days

    Fig. 1. Time-profiles of the main model variables on computer display for the reactor after a step-wise increase at 0 days in the influent ammonium concentration from 2.5 to 6.0 g N/I and with a total gas pressure of 1 bar. Symbols: experimental data (Angelidaki et al., 1993);

    lines: model prediction.

    pH, which, in turn, leads to a decrease of the non- ionized ammonia inhibition. Thus, in the manure system a high level of volatile fatty acids and a high pH occur in the steady-state conditions.

    The simulation was carried out under 3 bar of total gas-pressure conditions (Fig. 2). The accurate data computed at particular times are shown in Table 3. According to the model, increasing the par- tial pressure of CO2 to 0.63 bar decreases the pH value from 7.98 to 7.60, which lowers the non-ion- ized NH3-concentration to 735.5 mg N/I and decreases the inhibition effect substantially (Fig. 3).

    Using the simulations presented in Figs 1 and 2 as the controls, we carried out some tests of the model for analysis of various anaerobic-system dynamics, with the following results.

    1. When the pH level of 7-98, corresponding to the control at 0 days, is kept constant the non-ion- ized NH3-concentration increases to a high level and the concentration of methanogens decreases to zero because of NH3 toxicity (Fig. 4).

    2. When the pH levels of 7.75 (1 bar) and 7.60 (3 bar) corresponding to the earlier models at 120 days are kept constant, the simulations show (Fig. 5) that

  • 28

    Table 3. Computed

    V.A. Vavilin, V.B. Vasiliev, S. V. Rytov

    values of some model variables for the manure continous-flow system at particular times after an initial sharp increase in influent ammonia concentration a

    Pressure Variables t=0 days t= 120 days Steady-state (bar)

    1 pH 7.98 7-75 7.75 Acidogens A, mg/I 2019.68 1546.29 1547.28 Acidogens P, mg/l 96.97 72.89 73.05 Methanogens A, mg/l 1321.77 787.03 787.37 Syntrophs, mg/1 148.28 29.92 1.03 Methanogens H, mg/I 41.04 26-30 24.54 Degradable suspended solids, mg/l 4980.43 9489.91 9472.72 Dissolved organic substance, mg/l 23.12 22.14 22-37 Acetate, mg/l 412.46 3269.38 2835.36 Propionate, mg/l 253.28 2437.24 3016.77 Carbonate, mg/I 15327.98 9247.76 9240.74 Ammonium nitrogen, mg N/! 2205.61 5784.08 5788.21 Non-ionized ammonia, mg N/I 570.60 989.48 986.85 Alkalinity, mg HCO3/I 15123-33 12422.94 8876.96 Methane partial pressure, bar 0-67 0-63 0.63 Carbon dioxide partial pressure, bar 0.25 0.26 0.26 Methane flow rate, I/(1 day) 0.80 0.45 0.44 Carbon dioxide flow rate, 1/(1 day) 0.30 0.19 0.18

    pH Acidogens A, mg/l Acidogens P, mg/l Methanogens A, mg/I Syntrophs, mg/! Methanogens H, mg/I Degradable suspended solids, mg/l Dissolved organic substance, mg/I Acetate, mg/l Propionate, mg/l Carbonate, mg/! Ammonium nitrogen, mg N/! Non-ionized ammonia, mg N/I Alkalinity, mg HCO3/I Methane partial pressure, bar Carbon dioxide partial pressure, bar Methane flow rate, I/(1 day) Carbon dioxide flow rate, 1/(1 day)

    7.98 7.60 7.60 2019.68 1705-31 1705.17

    96.97 80-45 80.46 1321.77 1203.78 1204.29 148.28 149.46 149.51 41-04 36.17 36.18

    4980-43 7985.04 7981.03 23.12 22.87 22.79

    412.46 143.25 142.93 253.28 101.18 100.94

    15327.98 15862-54 15858.35 2205.61 5760.76 5761.64 570-60 735.54 733.79

    15123.33 15621.81 15623-21 2.62 2.24 2-24 0.26 0-63 0.63 0.27 0-23 0.23 0.03 0.06 0.06

    "In these computations the simple first-order kinetics of suspended solids degradation with KH=0.5 1/day was used for hydrolysis description.

    methanogenesis is going on, but for the 1 bar case the free-NH3 level inhibits the process substantially.

    3. When an NH3-inhibition process is excluded by an increase in the constants of NH3-inhibition func- tions (see Table 2) the system dynamics change substantially only at 1 bar total gas pressure (Fig. 6).

    Thus, increasing total gas pressure leads to decreasing inhibition from non-ionized ammonia.

    Synthet ic substrate: sulfate effect To reduce a hydrogen-sulfide concentration to non- critical levels, Olsson and Schleischer (1994) removed sulfide outside the reactor using the biogas. The biogas was led to a gas washing system, where H2S was absorbed in a scrubber liquid containing iron. We analysed the idea of lowering the total gas pressure to reduce H2S-concentration toxicity.

    A system of anaerobic microorganisms adapted to a low sulfate-load was studied by Parkin et al. (1990) for a step increased concentration in the input of

    sulfate. When the input sulfate concentration was increased from 200 mg S/1 to 625 mg S/I the system did not fail, but under the input sulfate concentra- tion of 1250 mg S/1 the high hydrogen sulfide concentration inhibited methanogenesis organisms.

    The quasi-steady-state of the model was obtained under the conditions shown in Table 4. After a step- wise increase in influent sulfate concentration from 200 mg S/I to 625 mg S/I at 0 days the system adap- ted to the new conditions, reaching a new steady-state where methanogenesis and sulfate- reduction were going on. However, at an influent sulfate concentration of 1250 mg S/1 the system failed and both sulfate-reduction and methane-pro- duction shut down (Fig. 7).

    This acetate/sulfate system was studied using the previous model of anaerobic digestion (Vavilin et al., 1994b). The model was able to provide a good com- parison with data. Some model coefficients are shown in Table 2.

  • Modelling of gas pressure effects on anaerobic digestion 29

    1400 ,.. Methanogens A, mg/I

    ~. . . . . . . . . . . . . . . . . . . . . . .

    0

    3594 : . - Acetate, mgll

    / 0 .~__-

    2456 ............. Propionate, rag/l z"

    8.1 ~- ........................... pR

    4 1033 -" ........................... Non-ionized NH 3, mgN/l

    :

    0 0.799 Methane

    flow rate, Ill*day - , . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    0 2.8 ~ Methane

    partial pressure, bar

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

    0.680 / Carbon dioxide ; partial pressure, bar

    0 120 days

    Fig. 2. Time-profiles of the main model variables (solid line) on computer display for the reactor after a step-wise increase at 0 days in the influent ammonium concentra- tion from 2.5 to 6.0 g N/1 and with a total gas pressure of 3 bar. Model dynamics presented in Fig. 1 are shown as

    the control (dotted line).

    L0 O

    o 0.5 t-

    0

    Fig. 3.

    \

    400 800 1200 1600 K I I K I2

    Non-ionized ammonia concentration (mgN/l)

    Inhibition

    2000

    effect of non-ionized ammonia con- centrations on the growth rate of methanogens.

    1400

    0 12000

    0 3000

    0 0.1

    4 1600

    0 0.840

    0 0.800

    0 0 .270

    (A)

    .............. Methanogens A,

    Acetate, mg/l

    Propionate, mg/l

    . . . . . . . . . . . . . . . . . . . . . . . . . . . '.pH

    mg/I

    ....................... Non-ionized NH3, mgN/l

    Methane ~ i . . . . . . . . . . . . . . . . . . . . . . flow rate, I / l ,day

    . . . . . . . . Methane partial pressure, bar

    ' - . . . . . . . . . . . . . . . . . . Carbon dioxide partial pressure, bar

    0 120 days

    1400

    0 12000 -

    0 3000,

    0 0.I

    4 1600

    0 0,290

    0 2.8

    0 0.663

    (a)

    x~ ......................... Methanogens A, mgll

    \

    _ Acetate, mall

    Propionate. mg/I / . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~

    .............................. pH

    ~ . ..................... Non-ionized NH 3, mgNll

    Methane " .................... f low rate, ill*day

    Methane partial pressure, bar

    / . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon dioxide partial pressure, bar

    120 days

    Fig. 4. Model dynamics (solid line) when the initial pH of 7.98 (see Table 3) was kept constant at a total gas pressure of 1 bar (A) and of 3 bar (B). Model dynamics with a variable pH in the reactor (Figs 1 and 2) are shown as the control

    (dotted line).

  • 30 V A. Vavifin, V.B. Vasiliev, S. V Rytov

    1400

    0 3594

    0 2456 -

    0 8.0-

    (A)

    / /

    - . . . . . . . . . . .

    4

    1.5

    0 0.667

    0 0.430

    '~. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Methanogens A, mg/I

    Acetate, m8/I

    Propionate, mg/l

    pH

    Non-ionized NH 3, mgN/I

    Methane flow rate, l/l.day

    Methane partial pressure, bar

    Carbon dioxide partial pressure, bar

    (B)

    1400 "~"-"--*-,~

    428

    0 260

    t...,

    0 8.0

    4 735

    oY 0.460 1

    0

    2.60 ["

    0.663

    Methanogens A, mg/I

    Acetate, mg/I

    Propionate, m8/I

    pH

    Non-ionized NH 3, mgN/I

    Methane flow rate, Ill*day

    Methane partial pressure, bar

    Carbon dioxide partial pressure, bar

    0 0 , 0 120 days 0 120 days

    Fig. 5. Model dynamics (solid line) when the pH was kept constant at a total gas pressure of 1 bar (pH=7.75:A) and of 3 bar (pH=7.60: B). Model dynamics with a variable pH in the reactor (Figs 1 and 2) are shown as the control (dotted

    line).

    (A) 1400

    1400 T'" Methanogens A, m8/I ~-.~. . . . . . . . . . . . . . . . . . . . . . .

    / 0/ 0

    , .- ................... i 420 3594 ] Acetate, mg/I

    2 Propionate, mg/I 260

    0 pn 8.1

    4 1200

    1,

    0 0.680

    0 0.310

    f . . . . . . . . . . . . . . . . . . . . . . . Non-ionized NH 3, mgN/I

    i Methane

    k. . f low rate, l/l.day

    Methane partial pressure, bar

    .............................. Carbon dioxide partial pressure, bar

    0

    Fig. 6.

    120 days

    (B)

    . . . . . . I . . . . . . . . . . . . . . . . . . . . . .

    735

    0 0.420 :

    2.8

    0 0.760

    Methanogens A, mgll

    pH

    Acetate, m8/I

    Propionate, mg/I

    . . . . . . . . . . . . . . . . . . . . . . . Non-ionized NH3, mgN/l

    Methane flow rate, Ill*day

    Methane partial pressure, bar

    ........................... Carbon dioxide partial pressure, bar

    0 120 days

    Model dynamics (solid line) without free ammonia inhibition at 1 bar (A) and at 3 bar (B) total gas pressures. Model dynamics with an ammonia inhibition (Figs 1 and 2) are shown as the control (dotted line).

  • Modelling of gas pressure effects on anaerobic digestion 31

    ol \ o X ,~.

    1300"

    0 0.400'

    0 13000

    o1"

    0

    o ~ 6.4 " - -~

    4

    Methanogens A. mg/I

    Sulfate-red. A. mg/l

    Sulfate, mgS/I

    Biogas flow rate. Ill*day

    Acetate, mg/l

    Sulfide. rngS/I

    Non-ionized H2S. mgS/l

    pH

    0 200 days

    Fig. 7. System failure after the step-wise increase at day 0 in the influent sulfate concentration from 200 to 1250 mg S/! and with a total gas pressure of 1 bar. Symbols: experimental data (Parkin et al., 1990); lines: model pre-

    dictions.

    500

    0 17

    0 1249

    0 0.410 -

    0 12470:

    Methanotgens A, mg/I

    Sulfate-red. A. mg/I

    Sulfate. mgS/l

    Biogas flow rate. Ill,day

    Acetate, mg/I

    [ OI " . . . . . . . . . . . . . . .

    41 ~~'~'~ , ................. Non-ionized H2S, mgS/I

    6.4 [ . . . . . . . . pH

    / , '\ 4 ~ i

    0 200 days

    Fig. 8. Simulation of system recovery (solid line) after a large step-wise increase at day 0 in the influent sulfate concentration from 200 to 1250 mg S/I and with a total gas pressure of 0"65 bar. Model dynamics at 1 bar pres-

    sure (Fig. 7) are shown as the control (dotted line).

    Table 4. Simulation conditions for acetate/sulfate system

    Chemical composition of the influent water dissolved OM 0.00 mg/l acetate 12500.00 mg/l propionate 0.00 mg/i sulfate 200.00 mgS/l carbonate 100.00 mgC/! sulfide 0.00 mgS/1 ammonia nitrogen 100.00 mgN/l calcium 250.00 mg/l sodium 10.00 mg/I phosphate 5.00 mgP/l

    Parameters of reactor: volume of reactor 1'50 ! volume of gaseous phase 0.50 1 temperature 33.00C pressure 1.00 bar feed flow rate 0.04 1/day

    In the present study the simulation was carried out under 0.65 bar of total gas-pressure conditions (Fig. 8). According to the model a decreasing CO2 partial-pressure increases the pH level, which, in turn, lowers the non-ionized hydrogen-sulfide con- centration. At these values of HzS and pH the system does not fail.

    CONCLUSION

    By changing the total gas pressure of an anaerobic digester toxicity effects can be avoided. An increase of COz partial-pressure decreases the pH value, which reduces the non-ionized ammonia concentra- tion. A decrease in CO 2 partial-pressure increases the pH level, which lowers the non-ionized hydro- gen-sulfide concentration.

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  • 32 V.A. Vavilin, V.B. Vasitiev, S. V. Rytov

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