a simulation study of a continuous two-phase dry digestion system

A Simulation Study of a Continuous Two-Phase Dry Digestion System

Joan Mata-Alvarez Department of Chemical Engineering, University of Barcelona, 08028 Barcelona, Spain

Accepted for publication November 10, 1988

In this article a two-phase system for the continuous digestion of wastes with a high solid content is simulated. The studied parameters are: 1) Recirculation f rom the methanizer to the hydrolyzer, 2) methanizer/hydrolyzer volume ratio, and 3) hydraulic retention time in the hydrolyzer (HRT). Results show that the recirculation ratio is an important operational factor with a large influence on the biodegradation yield, especially at low HRT. Opti- m u m levels of this parameter are established. Some runs of the program have been carried out to test the stabil ity of the system. This has proved t o be very stable, especially at low recirculation ratios. The results also show that volume ratio does not appreciably affect the performance of the system, provided it is over a critical value, dictated by the allowable methanizer load.

INTRODUCTION

A large amount of organic waste containing large amounts of cellulose is produced in our society mainly by agricultural, municipal, and food processing activities. Those with a total solid content (dry matter) over 20% are called “solid” wastes. An important waste included in this group is the organic fraction of municipal solid waste (OFMSW). The treatment of OFMSW by anaerobic digestion has received increasing attention, ’,* however a review of the literature indicates that there is a scarcity of infor- mation regarding the “dry” digestion processes . 3

Major benefits of anaerobic digestion are: the reduction of waste volume, smell, and pathogenic organisms, the production of biogas and the potential use of the digested residue as a soil conditioner or fert i l i~er.~ Dry digestion of- fers the possibility of drying the digested solids with the exhaust gas of a cogenerator set powered by the produced biogas. The produced electricity can be used as a power source for the drivers of the plant (shredders, conveyors, pumps, etc.). Excess can be sold or used in other processes.

As cellulose is a major constituent of solid wastes, the use of a two-phase fermentation system could allow a larger hydrolysis rate, which has been shown to be rate- limiting in overall digestion process.’ Phase separation in anaerobic digestion has been studied in a laboratory and at semitechnical scale by several However, in contrast with soluble substrates and liquid wastes, few

Biotechnology and Bioengineering, Vol. 34, Pp. 609-616 (1989) 0 1989 John Wiley & Sons, Inc.

studies have been reported on two-phase methanization of “solid” wastes.’ Phase separation appears to be warranted from the kinetic as well as from the process control point of view, because of the possibility of maintaining an optimum environment for each phase.

In the first digester of a two-phase system, the biodegradable matter is broken down and subsequently metabo- lized by fermentative bacteria to produce mainly volatile fatty acids (VFA). In the second, VFA are converted to acetate and hydrogen, which are removed by the methanogenic bacteria to form methane. A two-phase system to treat “solid” wastes can be operated semicontinuously or continuously. In the first mode, waste is loaded batchwise to the hydrolyzer. Then, recirculation of leachate from the hydrolyzer to the methanizer starts until almost all the biodegradable matter is converted to biogas. Finally, the digested solids are unloaded from the hydrolyzer. In a previous paper a simulation of the performance of such a system was studied, suggesting optimum levels of the operating parameters. lo The second possibility is to continuously operate the hydrolyzer, that is, the “solid’ waste is loaded continuously into the hydrolyzer, where it is partly solubilized and converted to volatile fatty acids (VFA). These are fed to the methanizer, where the second step of methane formation is accomplished. There is also a recirculation stream flowing between both digesters (Fig. 1). The degraded waste is discharged after a liquid fraction is separated.

The use of a continuous two-phase digestion system presents some advantages in front of the batch one described earlier.” Among them are the reduction of the required labor and a better utilization of the equipment (it is continuously used in the process and no time is spent in loading and unloading the waste). A major drawback is the increase of wastewater produced, but when large amounts of waste are involved, the use of a continuous system is defi- nitely more convenient.

The scope of the previous study did not cover the simulation of the continuous operation of the two-phase digesters because of substantial model differences with the batch operation. The aim of this article is to study the continuous system in order to find the optimum levels of oper-

CCC 0006-35921891050609-08$04.00

F-F ' R

B G 1 *

F

( A )

4 ( c ) I F '

Figure 1. Schematic of the simulated two-phase system for the continuous digestion of solid wastes. (A) Hydrolyzer, volume V h ; (B) Methanizer, volume V,; (C) separator; BG, , hydrolyzer biogas outlet; BG2, methanizer biogas outlet. Feed, recirculation, and drain flow rate are respectively F, R , and F ' .

ating parameters and to evaluate the extent of the effect of the variation of them. As a consequence, this study intends to complete some practical aspects concerning the operation of two-phase anaerobic digesters treating dry wastes. The model used here has been enlarged considering the acidogenic step as a separate one, and other minor changes presented in the next sections.

MODEL SET UP

The system is composed of a hydrolyzer of volume v h

and a methanizer of volume V, (Fig. 1). The volume ratio V,/Vh will be denoted by V,,,. A substrate flow-rate F en- ters to the system and goes to the hydrolyzer. The substrate has an initial total solid content, TS,; an initial total volatile solids, VS, and an initial biodegradable volatile solids content, BVS,. A given fraction of this biodegradable solid is considered soluble. After digestion in the hydrolyzer, nonsoluble solids are drained off with a flow-rate F' in a separator (filter, settler, or any other device) when they leave the hydrolyzer. Liquid leaving the methanizer is par- tially recirculated to the hydrolyzer, with a flow-rate R, in order to keep the VFA concentration level above critical values and to favor the buffer hydrolyzer capacity. The effluent of the system, with a flow-rate F-F' requires a fur- ther treatment, as their organic content is still too high.

The flow pattern of both digesters is considered com- pletely mixed because of the recirculation flow-rate. The methanizer is assumed to be a carrier reactor with biomass attached to some kind of support.

Kinetic Rate Equations

Kinetic rate equations for hydrolysis, acidogenesis , and methanogenesis are presented in Table I. Equations relating the constants' pH dependence are shown in Table 11. References substantiating these relations are given in reference 10. As can be seen, the acidogenic step-introduced

Table I. Two-phase digester simulation. Kinetic models used for the hydrolytic, acidogenic and methanogenic steps in the hydrolyzer and in the methanizer.

Hydrolytic step

Acidogenic step

Substrate removal rate

Yield equation


Yield equation

Methanogenic step


Yield equation


Yield equation

km, h s a , h X m . k

K s , m + so, h rm.h =

Table 11. Simulation of a two-phase digester. Kinetic constants as a function of the digester pH. Subindex i stands for the specific digester (h , hydrolyzer or m, methanizer).

pH Functions

First order kinetic constant for hydrolysis:

kh = kM(-0.5 pH: + 6.1 pHh - 17.6)

Maximum substrate removal rate for acidification:

ka,e = k,,,0(-0.497 pHf + 6.971 pH, - 22.303)

Maximum substrate removal rate for methanization:

km, , = km,fi(-0.501 pHf + 7.319 pH, - 25.701)

as an improvement over the previous model-has also been assumed to follow a Monod equation.".'* Another difference is the consideration of growth equations in the methanizer, for both, acidogenic and methanogenic step: Instead of assuming a constant microorganism concentration, this model computes its value in accordance with the degradation yield of the corresponding step and the solid retention time. This residence time is evaluated taking into account literature" and some of our own experimental data on down-flow stationary fixed film reactors. l3

Table I11 presents the maximum values of the kinetic constants, which occur at pH 6 , 7, and 7.5 for the hydrolytic, acidogenic, and methanogenic steps, respectively. ' '%'43'5

The pH of both digesters is a function of the VFA concentration. The equation relating both variables is based on experimental data and is also given in reference 10.

610 BIOTECHNOLOGY AND BIOENGINEERING, VOL. 34, AUGUST 1989

Table 111. Maximum kinetic constant values” used in the simulation of a continuous two-phase digester. Maximum constant values occur at pH 6.0, 7.5, and 7.5 respectively for the hydrolysis, acidogenesis and methanogenesis.

Methanization of Volatile Fatty Acids

VFA hydrolyzer balance:

Kinetic constant values

Hydrolysis:

kh ,o = 3 d-’

Acidification:

k a , m = ko,d = 7 kg/kg VSS micr d

K,, = 1.2 kg/m’

kdo = 0.02 d-’

Y, = 0.15 kg VSS micr/kg d

Methanization:

k,n,m = km,& = 7 kg VFA/kg VSS micr d

K,, = 0.4 kg/m3

kdm = 0.02 d-’

Y,,, = 0.04 kg VSS micr/kg VFA d

MATHEMATICAL MODEL

The dynamic model of a two-phase digester is based on the equations which express the mass conservation law. Mass balances for all the considered species have been carried out in both digesters. They are presented in the fol- lowing sections:

with

R,, = R / F (8)

Methanogenic microorganisms hydrolyzer balance:

Methanizer Balances

Acidification of Soluble Compounds

Soluble compounds methanizer balance:

with:

4rn = V,,,/(F + R - F ’ ) (11)

Acidogenic microorganisms methanizer balance:

Hydrolyzer Balances

Nonsoluble Compounds Hydrolysis

the hydrolyzer. As a consequence, only in that digester the balance is set up:

where Fa is a factor which takes into account the attachment of acidogenic bacteria to the methanizer support. A value of 100 has been selected to reach typical solids retention time appearing in the literature for carrier reactors4 and

Hydrolysis of nonsoluble compounds Only takes place in taking into account some of our own experimental data.13

Methanization of Volatile Fatty Acids d(S,,,) - Sins.0 Sin, - ( I ) VFA methanizer balance: rsol

dt $h,O $h

with,

4 h . O = V h / F ( 2 ) and

4 h = V h / ( F + R, (3) The term Sins /#h corresponds to the solids drained off in the separator. As a first order model is assumed, no microorganisms balance is necessary.

Acidification of Soluble Compounds

Soluble compounds hydrolyzer balance:

Acidogenic microorganisms hydrolyzer balance:

+ rXa,h - k d a X a , h ( 5 ) d ( x a , h ) - x a , h - - -- dt $h

Methanogenic microorganisms methanizer balance:

where F,,, is a factor which takes into account the attachment of bacteria to the methanizer support. A value of F, = 100 has also been selected.

Methane Production

If the system is fed F m3/d with a biodegradable matter concentration BVS,, assuming that all the biodegradable matter is transformed into acetic acid, the maximum potential methane production rate of the system is:

F . 10 - BVS, * 0.373 (m3STPCH,/d)

MATA-ALVAREZ: CONTINUOUS TWO-PHASE DRY DIGESTION SYSTEM 61 1

The actual methane production rates in both digesters are (m3 STP CH,/d):

Hydrolyzer: Vhrm,h 0.373;

Methanizer: Vmrm, 0.373

From these values, equations can be immediately derived to evaluate the percentage of the maximum methanization achieved in the digesters:

rm, m +m, 0 Methanizer yield: ~

10 BVS,

r h , m +h, 0 Hydrolyzer yield: ___ 10 BVS,

These parameters will later be used for comparison of the process performance, and will be expressed as a percentage of biodegradation.

PROCEDURE

A program has been written in FORTRAN 77 to solve the above set of nonlinear differential equations, using the in- tegration subroutine D03PGF from the NAG library. l6 The program has been run to simulate the fermentation of a slurry with the characteristics presented in Table IV, in both, steady-state and nonsteady-state conditions.

The process parameters studied have been the recirculation ratio, R,, (from 0 to loo), the hydraulic retention time in the hydrolyzer, $ h , , (from 3 to 100) and the volume ratio Methanizer/Hydrolyzer, V,, (from 0.05 to 1).

RESULTS AND DISCUSSION

In all the simulation runs of the program, methane production in the hydrolyzer was never observed. This is a consequence of the high level of VFA resulting from the large biodegradable organic matter feed concentration. When the hydrolyzer is operated in batch mode, methanization does occur’o when the VFA concentration reaches a low enough value. But when operated continuously, the VFA concentration is stationary at a high level, not allow- ing methanization to start.

Figures 2 and 3 present the percentage of maximum biodegradation obtained operating the system at different values of + h , O and at different recirculation ratios. As can be seen, the recirculation of the liquor exerts a large positive effect on the biodegradation yield. If the system is op-

Table IV. Initial values of the microorganisms concentration and feed characteristics used in the simulation of a continuous two-phase digester.

Initial conditions ~ ~

TS, = 90%

VS, = 80%

BVS, = 35%

S,ns,o = 350 kg/m’

Xa,m = 1.5 kg/m’ D,, = 0.1

Xa,& = 5.0 kg/m’

Xm,m = 1.5 kg/m’

Xm,& = 1.5 kg/m’

erated without recirculation, there is a dramatic fall affecting the level of biodegradation percentage.

Figure 2 clearly shows the presence of an optimal recirculation ratio, R , within the values 5-15, depending on the hydraulic retention time used (the larger the HRT, the larger the optimum R ) . Recirculation is responsible for two opposite effects: 1) a positive one, reducing the “effective” concentration of biodegradable matter entering the hy-. drolyzer and buffering its contents. As a consequence, the VFA concentration is lower and the pH higher. Otherwise the pH would be too low, even for hydrolytic bacteria. 2) The other effect acts against the methanization: At large recirculation flow-rates, the hydrolyzer HRT is reduced, promoting the wash-out of microorganisms, which are drained off in the separator, and reducing the contact time between substrate and bacteria, thus limiting the extent of the conversion. As a consequence, an optimum recirculation flow-rate exists for the operation of a two-phase system.

As can be seen (Fig. 2), even at optimal levels of R , the necessary hydrolyzer HRT to achieve a biodegradation over 80%, is rather large (over 15 days). This is due to the presence of insoluble compounds, which are difficult to de- grade. Besides, some of the biodegradable matter- such as soluble organics or VFA-is lost in the solids drain (which amounts for, in this case, 10% of the hydrolyzer incoming flow-rate). These results show the known fact that a large hydrolyzer HRT should be used when the nonsoluble fraction of the biodegradable feed is high. They also indicate that the influence of hydrolyzer HRT is stronger when large recirculation ratios are employed. Additionally, the convenience of an improved separation device, in order to minimize the amount of liquid in the drain, that is, in order to minimize the loss of soluble compounds, is also evidenced.

From Figure 3 it appears that large values of the HRT in the hydrolyzer, also have a negative effect on the biodegradation yield, when no recirculation is used at all. Such an effect is explained because of the low pH achieved, due to the increased VFA production. These pH values are detrimental for the microbial population, and are avoided when the methanizer effluent is recirculated.

A recirculation ratio over the optimum, makes it necessary to operate the system at a larger hydrolyzer HRT, in order to achieve the same level of solid reduction (see for instance, point A in Figure 3, with R = 10, which gives a biodegradation yield ca. 80%; to achieve the same yield with the system operated at R = 20, the hydrolyzer HRT should be around 28 days (point C) instead of 20 days as before. Otherwise a lower yield (point B) would be obtained). As a consequence, it is more economical to use reduced values of the recirculation ratio, not only because of the smaller pumping costs, but, more importantly, because of the reduction of the digesters investment costs. At this point it is interesting to look at Table V, which reflects the relative investment costs of the overall installed system. Assumptions have been made concerning the application of the cost factor method to estimate the capital investment. ’’ Additionally, maintenance and depreciation can be


0

12% J

20 LO €0

H Y D R O L Y Z E R HRT ( M Y S )

12% (D)

ao

Figure 2. retention times as a function of the recirculation ratio

Two-phase digester simulation results. Biodegradation profiles obtained at different hydrolyzer hydraulic

240

2 2 0

160-

120 -

80

LO.

0

(C)

24%

I I I 31 33 35

D A Y S

Figure 3. Two-phase digester simulation results. Biodegradation profiles obtained at different recirculation ratios as a function of the hydrolyzer hydraulic retention time. Points A and C (with R = 10 and 20, respectively) achieve the same biodegradation yield, but point C conditions require a hydrolyzer HRT about 8 days higher than point A).

assumed to be proportional to the investment cost. Thus, when studying the net cash flow of the plant, to operate the system close to the optimal recirculation rate values is again an important factor. In a practical situation, these can be obtained through experimental tests using the dry waste under consideration.

Considering the third parameter studied, the volume ratio V,/V,, it appears that approximately the same level of total methanization is observed whatever the value of V,,, is used, if it is over a certain critical value, which depends on the feed biodegradable matter content. That is, the more concentrated the substrate, the larger the critical metha-

MATA-ALVAREZ: CONTINUOUS TWO-PHASE DRY DIGESTION SYSTEM 613

Table V. plant, referred to the hydrolyzer volume (arbitmy units).

Relative investment cost of a two-phase anaerobic digestion

H ydrol yzer Relative volume cost

20 0.381 40 0.577 60 0.736 80 0.875

100 1 .Ooo 120 1.116 140 1.224 160 1.326 180 1.423 200 1.516 220 1 . a 5

nizer HRT to operate the system. The obvious reason is the higher methanizer load originated from the larger leachate solid contents. Figure 4 shows the soluble compounds concentration in the hydrolyzer, achieved at the critical values of the volume ratio, when the system is operated at a hydrolyzer HRT = 10 days, R = 5 and different feed concentrations (BVS,) . The concentration of insoluble compounds, at the hydrolyzer outlet, that is, the fraction not degraded which will be drained off the system, is also represented. Finally, presented in Figure 4 is the critical value of V,, used at different feed concentrations. The use of slightly lower values of the volume ratio, would cause the failure of the methanizer. The resulting methanizer organic load (kg biodegradable VS/dm3 d) is constant around 30 because of the critical conditions used to

obtain these results. This value is a function of the biomass present in the digester and also of the microorganism yield for the substrate fed to it. As for the system economy, the volume ratio V, /V, should be kept at the lowest allowable value, that is, the methanizer volume :hould be just large enough to prevent microorganisms to w,tsh out or acidification to occur.

Overloading Conditions

In order to assess the behavior of the iystem under shock load conditions, several runs were conducted, changing suddenly either the feed concentratio,i (BVS,) or the feed flow rate ( F ) so as to reduce the HRT. yonditions of the run were: Hydrolyzer retention time: 25 dars; Recir- culation ratio, R = 5 and 15; Volume ratio V,/V, = 0.1. Feed composition is the same as presented in Table IV. All the induced disturbances last for one day. The pulse mag- nitude, that is, the measure of how large the disturbance was from the steady conditions, was increased until a system failure was achieved. The system operated at a recirculation ratio of 5 was, in both overloading conditions, more stable than when operated at R = 15. Specifically, when R = 5, the two-phase digester could stand a feed flow-rate 150% larger than the value at steady conditions, and a feed concentration nearly five times that of steady state. On the other hand, when operated at a recirculation ratio R = 15, the maximum increases of flow-rate and feed concentration were 80% and loo%, respectively. This is due to the larger organic load reached in the methanizer when the recirculation rate ratio was 15. Although the

85

I

1 0 10 20 30

75

RECIRCULATION R A T I O

Figure 4. Two-phase digester simulation results. Values of some digester parameters: (curves A, total organic soluble compounds concentration entering the methanizer; curve B , critical volume ratio used; curve C, nonsoluble compound concentration leaving the hydrolyzer), when hydrolyzer HRT = 10 and R = 5. The system is operated at critical conditions regarding the methanizer organic load.


steady VFA concentration entering the methanizer is lower at R = 15, the higher flow-rate does not allow a much larger concentration. Thus, when a sudden increase in BVS, occurs, a system failure is more likely to happen. The same applies when the incoming feed flow rate increases suddenly: The methanizer load increases in the same extent, producing the reactor acidification.

It must be stated that in a real system, operated with a carrier reactor, the system failure would not be so severe as it is in the simulated digester. In this latter case, the microorganisms are washed out because of the continuous low pH. This would not happen in a real system because, in a carrier reactor, some of the microorganisms would re- main. As a result, a real system is more likely to recover once the disturbance is ceased. However, the performance of a real system will be always disturbed more severely at large recirculation ratios.

Finally, Figure 5 shows the evolution of some parameters when the two-phase digester operated at R = 5 is disturbed with an overloading of 100% of the feed concentration: The VFA level in both reactors is increased causing the rise in the methanizer load and also the rise in the VFA concentration of the liquid stream leaving the system.

CONCLUSIONS

After the results obtained simulating a continuous two- phase digester with a recirculation stream (Fig. 1) some qualitative conclusions can be drawn:

1. Given a hydrolyzer hydraulic retention time, an optimal recirculation flow rate exits, maximizing the per-

centage of biodegradation achieved. The system should be operated around this optimal value, which has to be determined by experimentation. The operation over this value is not recommended because 1) the system could give the same biodegradation yield at more economical conditions (i.e., at a recirculation rate under the optimal) and 2) the system is less stable at shock loading conditions. Moreover, the lower the hydraulic retention time is used, the larger are the effects of the recirculation ratio on the system performance. If no recirculation is used at all, the biodegradation yield experiences a dramatic descent.

2. The Methanizer/Hydrolyzer volume ratio should be selected in accordance with the feed concentration, so that the originated methanizer load can be main- tained under critical values. However, larger values of this ratio do not appreciably improve the biodegradation yield achieved and should not be used. The hydrolyzer retention time should be selected in accordance with the content of nonsoluble feed compounds.

3 . No methanization at all was observed in the hydrolyzer, even when working with very large recirculation ratios. This is a consequence of the large concentration of biodegradable compounds in the substrate. Methanization in the hydrolyzer can only be achieved by either working with dilute substrates-being then possible to use a single digester - or operating the hydrolyzer in a batch fashion, as was earlier described."

1 2 16 20 2L 20 32

BVSO ( % )

. 0 7

F 4 a >

0 I

- 0 . 5 '4

5 z w

J 0 >

- 03

figure 5. Two-phase digester simulation results. Evolution of hydrolyzer soluble components (curve A), methanizer VFA concentration (curve B) and gas production (curve C, in arbitrary units) after a pulse on the feed biodegradable matter concentration (curve D).

MATA-ALVAREZ: CONTINUOUS TWO-PHASE DRY DIGESTION SYSTEM 615

NOMENCLATURE

So,m VFA concentration in methanizer (kg/m’)

initial biodegradable Volatile Solids content (%) biodegradable Volatile Solids (%) drain ratio (F ’ / F , dimensionless) substrate flow-rate (m’/d) drain flow rate (m3/d) acidogenic attachment factor (dimensionless) methanogenic attachment factor (dimensionless) hydraulic retention time first order hydrolysis kinetic constant (d-I) first order hydrolysis kinetic constant at pH 6 (d-’) maximum substrate removal rate in the hydrolyzer for the acidogenic step (kg VFA/kg micr d) maximum substrate removal rate in the methanizer for the acidogenic step (kg VFA/kg micr d) maximum substrate removal rate in the hydrolyzer for the methanogenic step (kg VFA/kg micr d) maximum substrate removal rate in the methanizer for the methanogenic step (kg VFA/kg micr d) maximum substrate removal rate in the hydrolyzer at pH 7 for the acidogenic step (kg VFA/kg micr d) maximum substrate removal rate in the methanizer at pH 7 for the acidogenic step (kg VFA/kg micr d) maximum substrate removal rate in the hydrolyzer at pH 7.5 for the methanogenic step (kg VFA/kg micr d) maximum substrate removal rate in the methanizer at pH 7.5 for the methanogenic step (kg VFA/kg micr d) microorganism decay rate constant for acidification (d- I) microorganism decay rate constant for methanization (d-I) saturation constant for acidification (kg VFA/m3) saturation constant for methanization (kg VFA/m3) hydrolyzer pH (dimensionless) methanizer pH (dimensionless) recirculation ratio ( R / F , dimensionless) recirculation flow-rate (m3/d) acidification rate in the hydrolyzer (kg/m’ d) acidification rate in the methanizer (kg/m’ d) methanization rate in the hydrolyzer (kg/m3 d) methanization rate in the methanizer (kg/m’ d) hydrolysis rate (kg/m3 d) acidogenic microorganisms growth rate in hydrolyzer (kg VSS/ m3 d) acidogenic microorganisms growth rate in methanizer (kg VSS/ m3 d) methanogenic microorganisms growth rate in hydrolyzer (kg VSS/m’ d) methanogenic microorganisms growth rate in methanizer (kg VSS/m’ d) nonsoluble biodegradable compounds concentration in hydrolyzer (kg/m3) nonsoluble biodegradable substrate concentration (kg/m’) soluble biodegradable compounds concentration in hydrolyzer (kdm’) soluble biodegradable compounds concentration in methanizer soluble biodegradable compounds substrate concentration (kg/m3) VFA concentration in hydrolyzer (kg/m’)

x m . h

time (d) feed stream Initial Total Solids content (%) Volatile Fatty Acids hydrolyzer working volume (m3) methanizer working volume (m’) volume ratio (Vm/Vh, dimensionless) feed stream Initial Volatile Solid content (%) acidogenic microorganisms concentration in hydrolyzer (kg/m3) methanogenic microorganisms concentration in methanizer

methanogenic microorganisms concentration in hydrolyzer

methanogenic microorganisms concentration in methanizer

acidogenic yield kinetic constant methanogenic yield kinetic constant hydrolyzer retention time referred to substrate feed flow rate (d) hydrolyzer retention time referred to actual flow rate (d) methanizer retention time referred to substrate feed flow rate (d) methanizer retention time referred to actual flow rate (d)

(kg/m’)

(kg/m3)

(ks/m3)

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a simulation study of a continuous two-phase dry digestion system

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