Apparatus for controlling the methane fermentation of organic materials

An apparatus for controlling methane fermentation of organic materials including a main fermentor, sensors associated with the main fermentor for measuring physical-chemical measurements of organic material in the main fermentor, a computer for processing the measurements by using a fermentation model. An additional computer transforms the data of fermentation model into data which affects the operation of the main fermentor. A device is also provided to enable small-scale experimentation in order to adjust the characteristic parameters of the fermentation model incorporated in the computer. The installation allows to maintain the fermentor in optimum biological conditions with a minimum of measurements.

BACKGROUND OF THE INVENTION 
The present invention relates essentially to a method of controlling the 
methanic fermentation of various organic materials. 
It is also directed to an equipment for carrying out this method. 
There has already been proposed to operate fermentors by using mathematical 
models taking into account the diversity of the materials to be treated as 
well as the fluctuations of the catalytic behaviors of the 
micrc-organisms. 
It is also known for operating the fermentors to provide an anaerobic 
fermentation model for diluted substrates or substrates of simple nature 
but not for concentrated or complex organic materials. 
These models may integrate one or several populations of micro-organisms. 
They may or may not take into account the physical-chemical equilibriums, 
the inhibiting effect of the pH or of the volatile fatty acids such as the 
acetic acid which are essential metabolic intermediaries. 
There are further known purely biological mathematic models for diluted 
substrates in one operating step, based upon the acetate concentration or 
in two operating steps, namely acidogenesis and methanogenesis from simple 
substrates such as glucose. These models take into account an inhibition 
of the methanogeneous activity by the non-ionized volatile fatty acids but 
they do not integrate the pH as a state variable in relation to the 
physical-chemical balances. 
There are further known biological mathematical models for diluted 
substrates of the methanization taking into account the physical-chemical 
balances thus permitting to integrate the pH as a state variable and to 
contemplate its inhibiting part. 
These latter models, however, do not take into account the inhibiting 
effect of the non-ionized volatile fatty acids such as the acetic acid. 
But none of the models referred to hereinabove integrates the combination 
of physical-chemical balances, of the pH as a state variable and of the 
inhibiting effect of the non-ionized volatile fatty acids and/or takes 
into account the case of the concentrated or complex organic materials. 
It is further known that the operation of industrial methanization 
fermentors necessarily requires a great number of complementary 
physical-chemical measurements and analyses and in particular the 
measurement of: 
the amount of the biogas produced and its quality (percentage of methane 
and percentage of carbon dioxide), 
the pH and the temperature, 
the quality of the processed inputs and outputs, and 
the volatile fatty acids content (AGV) of the fermentation medium and/or 
the hydrogen percentage of the produced biogas. 
In a general manner the operation of the fermentors based upon earlier 
known models is not satisfactory in that the models are not adjusted to 
the specific operating conditions of the installation site of the 
fermentors. 
OBJECTS AND SUMMARY OF THE INVENTION 
Therefore the object of the present invention is to cope in particular with 
these inconveniences by providing a method which allows to control the 
state of the methanic fermentation while being based upon a special model. 
For that purpose, the subject of the invention is a method of controlling 
the methanic fermentation of organic materials in at least one fermentor 
and of the type consisting in effecting upon the fermentor 
physical-chemical measurements, processing these measurements in at least 
one computer permitting with the assistance of a fermentation model to 
obtain a variable characterizing the biological state of the fermentor and 
processing this variable to infer or derive therefrom an operation mode of 
the fermentor, characterized in that the aforesaid physical-chemical 
measurements are limited to two measurements such for example the pH and 
the volatile fatty acids content, the aforesaid variable characterizing 
the biological state is the methanogeneous biological activity and the 
aforesaid fermentation model is adjusted in accordance with the biomass 
present in the fermentor and with the organic materials to be treated by 
the latter. 
According to another characterizing feature of this method, the 
methanogeneous biological activity is expressed by the computer as a 
specific growth rate (.mu.) of the methanogeneous bacteria of the biomass 
of the fermentor. 
It should further be specified here that the aforesaid fermentation model 
is adjusted from an experimentation or testing on a reduced scale which is 
carried out upon a sample of material taken from the fermentor and which 
consists in automatically following the evolution of the pH with time and 
the volatile fatty acids concentration in the fermentor as well as the 
amounts of methane and carbon dioxide produced. 
It is thus already understood that under these conditions a quick and easy 
adaptation of the model to the operating conditions of the fermentor on 
its installation site is achieved. 
This fermentation model integrates the combination of the biological 
phenomena including the inhibiting effect of the non-ionized volatile 
fatty acids and of the physical-chemical balances in the fermentor with 
the pH as a state variable. 
The invention is also directed to an equipment for carrying out the method 
referred to hereinabove and of the type essentially comprising at least 
one fermentor, sensors associated with this fermentor to permit 
physical-chemical measurements to be performed and at least one computer 
processing these measurements with the assistance of a fermentation model, 
this equipment being characterized in that with the computer, providing 
owing to the fermentation model for the conversion of the 
physical-chemical measurements into a variable characterizing the 
biological state, is associated another computer providing for the control 
of at least one of the fermentors and in that the model is adjusted to the 
biomass present in the fermentor and to the substrate to be treated by an 
experimenting or testing device on a reduced scale essentially comprising 
a fermentor containing a sample of material taken from the first-named 
fermentor, sensors associated with the fermentor and connected to at least 
one computer providing the values of the parameters allowing to calibrate 
the aforesaid fermentation model. 
The functionalities of the aforesaid computers are gathered within one 
single and same computer. 
This experimenting or testing device may advantageously be located at 
another place than the installation site of the fermentor. 
It should further be specified that the sensors of the testing or 
experimenting device are adapted to continuously measure the pH, the 
amounts of CH.sub.4 and of CO.sub.2 produced and the concentration of 
volatile fatty acids in the fermentor whereas the parameters provided by 
the aforesaid computer are: 
the maximum methanogenous specific growth rate, 
the constant of inhibition of the methanogeneous bacteria, 
the saturation constant of the methanogeneous bacteria, and 
the yields or efficiency outputs expressed as the CH.sub.4 /biomass ratio, 
as the CO.sub.2 /biomass ratio and as the substrate/biomass ratio.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, there is seen an equipment for the control of the 
methanic fermentation of organic materials according to this invention, 
which essentially comprises a fermentor F1 with which are associated 
sensors 10, 11 to permit physical-chemical measurements to be made, namely 
in particular a measurement of the pH and a measurement of the content of 
volatile fatty acids. 
The sensors 10, 11 are connected to a computer C1 processing these 
measurements with the assistance of a fermentation model. 
The computer C1 which processes the aforesaid measurements and which 
provides a variable characterizing the biological state of the fermentor 
F1, is connected to another computer C2 which processes this variable to 
calculate a control mode to be applied to the fermentor F1. 
At F2 is shown a small laboratory fermentor which may be filled with 
organic material from the fermentor F1 as physically shown by the dotted 
line between F1 and F2. With this fermentor F2 are associated sensors 12, 
13, 14 and 15 themselves connected to one or several computers C3 
processing the data from the sensors and providing the computer C1 with 
the adjusted values of the characteristic parameters of the fermentation 
model incorporated into the computer C1. 
The F2-C3 assembly Forms an experimenting device on a reduced scale which 
as will be explained later in detail allows to automatically follow the 
evolution or development with time of the pH and of the volatile fatty 
acids concentration as well as the amounts of methane and of CO.sub.2 
produced. 
The small fermentor F2 is fitted with means necessary for performing the 
aforesaid experimentation or testing on a reduced scale. These means (not 
shown) are in particular a means for introducing a direct substrate of the 
methanogenesis such as the acetic acid, conventional stirring, heating 
means, etc. . . . 
The equipment which has just been described allows advantageously to follow 
the development of the methanic fermentation without the need of following 
the flow rate and the quality of the biogas produced by the fermentor F1 
and this while performing on this fermentor measurements in a limited 
number only, namely essentially a measurement of the pH and a measurement 
of the content with volatile fatty acids. Moreover owing to the 
experimentation device F2-C3 it will be possible to easily adjust the 
parameters of the fermentation model to the local conditions (substrate, 
bacteria, operating conditions) governing the fermentation within the 
fermentor F1. It should also be pointed out that the data issued from the 
computer C1 are data which reflect the global state of the fermentation 
and which therefore allow via the computer C2 to achieve a control of the 
fermentation within the fermentor F1. This control could not be that 
effective or efficient if it were obtained directly from physical-chemical 
measurements issued from the sensors 10, 11. 
Having described the equipment of FIG. 1, its operation will now be 
explained while recalling at first some principles of the methanic 
fermentation. 
The methanic fermentation comprises four stages: 
1. The hydrolysis of the substrate, the acidogenesis, the acetogenesis and 
the methanogenesis. 
The hydrolysis step permits to convert if need be the complex molecules 
into simpler molecules. 
The acidogenesis transforms these latter into fatty acids, alcohols, carbon 
dioxide and hydrogen. 
The acetogenesis provides the conversion of the products of the 
acidogenesis into immediate precursors of methane such as the acetic acid. 
The methanogenesis mainly provides the synthesis of methane from acetic 
acid according to the simplified formula: 
EQU CH.sub.3 COOH.fwdarw.CH.sub.4 +CH.sub.2 
According to the invention the fermentation model incorporated into the 
computer C1 and permitting to provide a characteristic variable of the 
biological state takes into account the coupling of the bacterial growth 
with the production of biogas, the inhibition of the growth of the 
methanogeneous bacteria by an excess of non-ionized acetic acid and 
acid-base equilibriums of the liquid and gaseous phases. 
There exists a close relationship between the instantaneous methanogeneous 
activity .mu., the maximum methanogeneous activity .mu..sub.max, the pH 
and the acetate concentration. 
This relationship is: 
##EQU2## 
where .mu. is the specific growth rate or factor of the methanogeneous 
bacteria consuming the acetic acid, 
.mu..sub.max is the maximum specific growth rate or factor of these same 
bacteria, 
K.sub.i is the inhibition constant, 
K.sub.s is the saturation constant, 
[HS] is the concentration (g/l) of non-ionized acetic acid, 
[S] is the concentration (g/l) of total acetic acid, 
[H.sup.+ ] is the pH value, 
[S.sup.- ] is the concentration (g/l) of ionized acetic acid. 
Therefore the above equations allow upon knowing the values measured in 
line or out of line of the pH and of the concentration of volatile fatty 
acids such as acetate to obtain the value of the methanogeneous activity. 
Thus with given fermentation conditions, the fermentation model permits to 
calculate the value 
##EQU3## 
as a function of the pH and of the content with volatile fatty acids or 
acetate. FIG. 2 illustrates the relationship between these three 
parameters for a given substrate. Therefore when knowing the value or the 
pH and the content with volatile fatty acids by means of the sensors 10 
and 11, the computer C1 determines the ratio. 
##EQU4## 
Then the information is conveyed to the computer C2 which contains 
mandatory instructions for the control of the fermentor F1, so that the 
computer C2 may act upon various members of the fermentor F1 such as a 
feed valve for instance. 
When the adjustment of the fermentation model is necessary in view for 
instance of the change of the substrate, of the change of the operating 
conditions of the fermentor or also at the start of a new equipment, the 
device for experimentation on a reduced scale F2-C3 will be used according 
to the following procedure. 
This procedure for the adjustment of the parameters of the model requires 
at most four tests by providing a range of acetic acid concentration and 
of pH ranging from little inhibiting conditions towards strongly 
inhibiting conditions of the methanogeneous bacteria. 
By way of example these four tests are run by varying the initial value of 
the pH from 6.5 to 7 and the initial concentration of acetic acid for 
instance from 1 g/l to 15 g/l. The variations of the initial pH are 
obtained by adding hydrochloric acid at 10% into the fermentor whereas the 
variations of the initial concentration of acetate are obtained by adding 
a solution of acetic acid or of calcium acetate. 
These additions are of course effected into the small fermentor F2 
previously filled with material taken from the fermentor F1. 
The table herebelow summarizes for instance the four tests carried out with 
a given substrate for the experimental enabling or validation of the 
model: 
______________________________________ 
Acetic acid 
initial pH 
concentration (g/l) 
State of the fermentation 
______________________________________ 
6.5 1 not inhibited 
6.5 10 inhibited 
7 10 not inhibited 
7 15 inhibited. 
______________________________________ 
For each one of these batch experiences, curves are then available, 
representing, versus time, the cumulated production of methane and of 
carbon dioxide within the small fermentor F2 as well as the variations in 
the pH and in the acetate content. 
FIG. 3 shows by way of example one of the curves recorded during these 
experiments. 
The dots represent the measured values. On the basis of the following 
equations describing the fermentation in the same experimental conditions, 
a method of adjustment is used, which permits to obtain the values of the 
characteristic parameters of this fermentation model while minimizing the 
deviation or difference between the measured values and the values 
calculated by the model. The solid line of FIG. 3 represents the simulated 
evolution or development for the experiment after this adjustment of the 
model. 
##EQU5## 
With the following intermediate equations: 
##EQU6## 
with H.sup.+ : the concentration of ions H.sup.+, 
S: the amount of substrate (acetate), 
HS: the amount of non-ionized acetate, 
S.sup.- : the amount of ionized acetate, 
B: the amount of bicarbonates, 
K.sub.a : the acidity constant of the acetic acid, 
K.sub.b : the dissociation constant of the bicarbonates, 
CO.sub.2d : the amount of dissolved carbon dioxide, 
IC: the amount of inorganic carbon, 
Z: the amount of cations, 
X: the amount of biomass, 
.mu.: the specific growth rate of the biomass, 
R.sub.3 : the output yield or efficiency expressed as the ratio 
substrate/biomass, 
CH.sub.4cum : the amount of cumulated produced methane, 
R.sub.1 : the yield output or efficiency expressed as the ratio 
methane/biomass, 
ICP: the amount of inorganic carbon evolved, 
R.sub.2 : the yield output expressed as the ratio carbon dioxide/biomass, 
P.sub.CP2 : the partial pressure of carbon dioxide in the gaseous volume, 
P.sub.t : the total pressure of the gaseous volume, 
CO.sub.2cum : the amount of cumulated carbon dioxide, 
.mu..sub.m : the maximum specific growth rate, 
K.sub.s : the saturation constant, 
K.sub.i : the inhibition constant, 
V: the volume of the fermentor of the experimental device, 
K.sub.H : the Henry constant for the carbon dioxide, 
CO.sub.2g : the amount of gaseous carbon dioxide, 
CO.sub.2ini : the amount of initial carbon dioxide in the gaseous phase, 
N.sub.2ini : the amount of initial nitrogen, 
IC: the amount of inorganic carbon. 
The four first algebraic equations reflect the acid-base equilibriums of 
the two following pairs of respective equilibrium constants K.sub.a and 
K.sub.b : 
acetic acid/acetate; 
produced dissolved carbon dioxide/bicarbonate. 
The fifth algebraic equation represents the electroneutrality of the medium 
and introduces an additional variable Z which is the whole amount of the 
cations present in the fermentor. 
The state variables of the model are the following: H.sup.+, HS, S.sup.-, 
B, CO.sub.2d, X, S, CH.sub.4cum, ICP and Z. 
The value of the output yield or efficiency R.sub.2 according to the ratio 
carbon dioxide/biomass is set to be equal to 1. The initial values of the 
pH, of the amounts of substrate S, methane and carbon dioxide, 
respectively, are measured. 
A calculation from the algebraic equations then permits to obtain the 
initial values of the amounts of acetic acid HS, acetate S.sup.-, 
bicarbonate B, dissolved carbon dioxide CO.sub.2d, inorganic carbon IC, 
respectively, as well as the whole amount of the dissolved cations within 
the fermentor. 
The acidity constants of the acetic acid K.sub.a, of the dissolved CO.sub.2 
K.sub.b and of the Henry constant K.sub.H are adjusted from the 
experimental data. 
In the example presented here, the values of these constants are the 
following: 
EQU K.sub.a =1.7.times.10.sup.-5 M 
EQU K.sub.b =1.7.times.10.sup.-7 M 
EQU K.sub.H =0.065M/atm 
The biological parameters of the model are the maximum growth rate 
.mu..sub.max, the saturation constant K.sub.S, the inhibition constant 
K.sub.i, the output yield of methane with respect to the biomass R.sub.1 
and the output yield of substrate with respect to the biomass R.sub.3. 
They are identified from the experimental data. The estimation of the 
parameters requires the integration of the model and the knowledge of the 
initial values of the state variables. The non-known and non measurable 
initial biomass is therefore considered as a parameter which is also 
identified and which is expressed as an arbitrary unit (UA). 
Still in the example presented here with a given substrate and in relation 
to FIG. 2 are hereinafter given the values of the characteristic 
parameters of this fermentation model which have permitted to set up the 
surface of FIG. 2: 
EQU .mu..sub.max =0.017 l/h 
EQU K.sub.S =2.18.times.10.sup.-5 M 
EQU K.sub.i =8.22.times.10.sup.-4 M 
EQU R.sub.1 =1M/M 
EQU R.sub.3 =350 mM/UA 
The invention is of course not at all limited to the embodiment described 
and illustrated which has been given by way of example only.